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Optical absorption and photoconductivity in magnesium oxide crystals Peria, William Thomas 1957

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%[\t  of ^rtttslj Columbia  P«tlier0rtg  Faculty of Graduate Studies  PROGRAMME OF THE  FINAL  O R A L  E X A M I N A T I O N  FOR THE D E G R E E OF D O C T O R  O F  PHILOSOPHY  WILLIAM T H O M A S PERIA M. A. Sc., University of British Columbia, 1951  TUESDAY, AUGUST 6th, 1957,  IN ROOM 300,  at 2:30  PHYSICS BUILDING  COMMITTEE IN CHARGE D E A N G . M . SHRUM, A. K. R. J.  M. CROOKER C. MANN E . BURGESS B. GUNN  p.m.  Chairman  J. NORRIS F. NOAKES R. D. JAMES G. B. PORTER  External Examiner Dr. E U G E N E B. HENSLEY University of Missouri  OPTICAL IN  ABSORPTION  AND  MAGNESIUM  PHOTOCONDUCTIVITY  OXIDE  CRYSTALS  ABSTRACT  The purpose of this investigation was the determination of the nature of certain imperfections in magnesium oxide crystals.  Optical absorption  and photoconductivity spectra of specimens cleaved from a number of larger pieces were measured.  The effect of vacuum heating, of non-  stoichiometry and of ultraviolet and x-ray irradiation were investigated. The nature of the imperfections could not be inferred from the experimental results but an energy level diagram consistent with all the data has been deduced. A comparison of the present work with pertinent data from the literature is presented and a basic error in previous photoconductivity measurements is pointed out. A method for the determination of the sign of the charge carriers excited during photoconductivity measurements is described.  GRADUATE STUDIES Field of Study: Physics  Electromagnetic  Theory  Theory of Measurement Quantum Mechanics  ...  .•  ..... ...  Physics  -  A. M. Crook;', ..  ..  W. Opechowsh  _.  . .  Dielectrics and Magnetism  Chemical  ..  .. .  Nuclear Physics  Spectroscopy  . . .  .  ..  G. M. Volkoff K. C. Mam-,  A. J . Dekker and C. G. Eicholtz .  A . M . Crooker  ....  . . . A. J . Dekker  Other Studies: Advanced Quantum Mechanics Mathemetical Foundations of Statistical Mechanics  ....... .  L . Teng  P. C. Eosenbloom  OPTICAL ABSORPTION AND PHOTOCONDUCTIVITY IN MAGNESIUM OXIDE CRYSTALS  by W i l l i a r a Thomas P e r i a M.A. Sc. U n i v e r s i t y o f B r i t i s h Columbia, 1951  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE  REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY  i n the Department of Physics We accept t h i s t h e s i s as conforming t o the r e q u i r e d  THE  standard  UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1957  In p r e s e n t i n g the  this thesis in partial fulfilment  requirements f o r an advanced degree at the  of  University  of B r i t i s h Columbia, I agree t h a t the  L i b r a r y s h a l l make  it  study.  f r e e l y available f o r reference  and  I  further  agree t h a t permission f o r e x t e n s i v e copying o f t h i s t h e s i s f o r s c h o l a r l y purposes may  be granted by the Head o f  Department or by h i s r e p r e s e n t a t i v e .  my  I t i s understood  t h a t copying or p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not  be allowed without my  Department The U n i v e r s i t y of B r i t i s h Vancouver £, Canada. Date  Columbia,  written  permission.  ii  ABSTRACT The purpose o f t h i s i n v e s t i g a t i o n was the d e t e r m i n a t i o n of the nature o f c e r t a i n i m p e r f e c t i o n s i n magnesium oxide crystals.  O p t i c a l a b s o r p t i o n and p h o t o c o n d u c t i v i t y s p e c t r a  of specimens c l e a v e d from a number of l a r g e r p i e c e s were measured. The e f f e c t s of vacuum h e a t i n g , of n o n - s t o i c h i o m e t r y and o f u l t r a v i o l e t and X-ray  i r r a d i a t i o n were i n v e s t i g a t e d .  The nature of  the i m p e r f e c t i o n s c o u l d not be i n f e r r e d from the e x p e r i m e n t a l r e s u l t s but an energy  l e v e l diagram c o n s i s t e n t with a l l the  data has been deduced. A comparison o f the present work w i t h p e r t i n e n t data from the l i t e r a t u r e  i s p r e s e n t e d and a b a s i c e r r o r i n p r e v i o u s  p h o t o c o n d u c t i v i t y measurements i s p o i n t e d o u t . A method f o r the d e t e r m i n a t i o n o f the s i g n o f the charge c a r r i e r s e x c i t e d d u r i n g p h o t o c o n d u c t i v i t y measurements i s described.  iii TABLE OF CONTENTS Section ,  Introduction . . . . . . . . . . . . . . . .  1  A  Experimental D e t a i l s . . . . . . . . . . . . .  5  B  Results  : :  B. l *  10  O p t i c a l A b s o r p t i o n Measurements  2  A n a l y s i s of P h o t o c o n d u c t i v i t y Data  3  Dependence of Photocurrent on L i g h t I n t e n s i t y and E l e c t r i c F i e l d . . . .  4  Evidence t h a t P h o t o c u r r e n t s are a Bulk E f f e c t  C  . . .  10  . . 11  . ,  14  14  5  Space Charge Formation  . . . . . . . .  6  P r o p e r t i e s of C r y s t a l s as Received  7  Heat Treatment  8 9  Excess Magnesium C r y s t a l s The E f f e c t of U l t r a v i o l e t Irradiation . . . . .  15  . . 16  Studies  20 . . . . . .  21 22  10  The E f f e c t of X - I r r a d i a t i o n  23  11  S i g n of the Charge C a r r i e r s  26  Discussion C. l 2 3  4 5  . . . . . . . . . . . . . . . . . 2 9  O p t i c a l Absorption  29  D i s c u s s i o n of the F a c t o r s I n v o l v e d i n the Photoconductive Y i e l d . . . . .  30  The Dependence of the Photocurrent on L i g h t I n t e n s i t y and E l e c t r i c Field  33  Evidence that the Photocurrents are Due to a Bulk E f f e c t  37  Formation of Space Charge F i e l d s . . .  38  • S e c t i o n s B and C are d i v i d e d i n t o 11 s u b s e c t i o n s , each i n such a way t h a t the r e s u l t s d e s c r i b e d i n B . l are d i s cussed i n C . l and so on.  iv  TABLE OF CONTENTS  (continued)  Section  Page C„6  Properties  of C r y s t a l s  as Received  . . . . 43  7  Heat Treatment S t u d i e s  47  8  Excess Mg C r y s t a l s  50  9  Ultraviolet Irradiation  .•  52  10  X-Ray I r r a d i a t i o n  56  11  S i g n o f the Charge C a r r i e r s  64  D  Conclusions  E  Comparison with P r e v i o u s Work  .69 . . . . . . . . . 7 4  INDEX TO  v  FIGURES  Title  Figure  S c h e m a t i c Arrangement Apparatus  of the  Photoconductivity 1  Mounting of the C r y s t a l f o r P h o t o c o n d u c t i v i t y Measurements Development  2 and Removal  o f Space C h a r g e  3  T y p i c a l A b s o r p t i o n S p e c t r a o f MgO C r y s t a l s a s Received T y p i c a l P h o t o c o n d u c t i v i t y and A b s o r p t i o n S p e c t r a R e s o l u t i o n of a T y p i c a l P h o t o c o n d u c t i v i t y Spectrum Two G a u s s i a n B a n d s  4 5 into 6  E f f e c t o f Heat T r e a t m e n t on t h e B a c k g r o u n d A b s o r p t i o n  7  P h o t o c o n d u c t i v i t y and A b s o r p t i o n o f C r y s t a l s Heated i n Vacuum a t D i f f e r e n t T e m p e r a t u r e s  8  Absorption  9  S p e c t r u m Due  to Heating  i n Magnesium Vapor  The E f f e c t o f E x c e s s M a g n e s i u m on t h e B a c k g r o u n d A b s o r p t i o n o f MgO  10  The E f f e c t o f E x c e s s M a g n e s i u m on t h e B a c k g r o u n d A b s o r p t i o n o f MgO  11  The E f f e c t o f E x c e s s M a g n e s i u m on t h e A b s o r p t i o n o f MgO  12  Background  E f f e c t o f E x c e s s Mg on P h o t o c o n d u c t i v i t y  13  O p t i c a l A c t i v a t i o n of Photoconductivity  14  Effect of U l t r a v i o l e t t i v i t y i n MgO  15  I r r a d i a t i o n on  Photoconduc-  I n c r e a s e i n O p t i c a l D e n s i t y by I r r a d i a t i o n i n 5 ev Band Induced O p t i c a l Absorption  Due  16  to I r r a d i a t i o n i n  4 ev B a n d  17  Buildup  18  o f X-Ray I n d u c e d A b s o r p t i o n  Absorption  Spectra  o f X-Rayed C r y s t a l s  Decay o f X-Ray I n d u c e d A b s o r p t i o n P h o t o c o n d u c t i v i t y i n X - R a y e d MgO  19 20 21  No.  VI  INDEX TO FIGURES -(continued) Title  F i g u r e No,  P h o t o c o n d u c t i v i t y i n X-Rayed MgO  22  E f f e c t o f X - I r r a d i a t i o n on the P h o t o c o n d u c t i v i t y of U l t r a v i o l e t A c t i v a t e d MgO  23  B l e a c h i n g of Xr-Ray Induced A b s o r p t i o n at 4.2 ev by 2.3 ev Quanta  24  B l e a c h i n g of X-Ray Induced A b s o r p t i o n a t 2.3 ev by 2.3 ev Quanta  25  B l e a c h i n g of X-Ray Induced P h o t o c o n d u c t i v i t y by 2.3 ev Quanta  26  Determination i n 4 ev Band  of the S i g n o f Charge C a r r i e r s E x c i t e d 27  Determination i n 5 ev Band  of the Sign o f Charge C a r r i e r s E x c i t e d  Determination  of the S i g n of Charge C a r r i e r s i n an  28  X-Rayed C r y s t a l P o s s i b l e Mechanisms  29 f o r the Loss of Free C a r r i e r s  Proposed Energy L e v e l Schemes f o r MgO Thermal Decay of X-Ray Induced U l t r a v i o l e t A b s o r p t i o n at Room Temperature -Idealized F i e l d D i s t r i b u t i o n s I l l u s t r a t i n g the Method of Determining the S i g n o f the Charge C a r r i e r s  30 31 32 33  INDEX TO TABLES Title A b s o r p t i o n and P h o t o c o n d u c t i v i t y at 5.0 ev Photoconductive Y i e l d o f a Number o f Specimens F r a c t i o n a l Change i n Photoconductive by B l e a c h i n g w i t h 2.3 ev Quanta  Yield  C a l c u l a t e d Thermal I o n i z a t i o n E n e r g i e s o f the Shallow E l e c t r o n Traps  viii  ACKNOWLEDGEMENTS Support  f o r t h i s work was p r o v i d e d by the U n i t e d S t a t e s  Army S i g n a l Corps through c o n t r a c t s w i t h the U n i v e r s i t y o f Minnesota  E l e c t r o n Tube Research L a b o r a t o r y .  The author  wishes t o express h i s g r a t i t u d e t o P r o f e s s o r W. G. Shepherd, d i r e c t o r o f the L a b o r a t o r y , f o r the p r o v i s i o n o f f a c i l i t i e s and f o r h e l p f u l d i s c u s s i o n s .  Thanks are due a l s o t b o P r o f e s s o r  A. J . Dekker whose encouragement enabled the w r i t e r t o proceed t o graduate  study and who d i s c u s s e d a l l phases o f the work.  B. V. Haxby, R. G. Lye, R. W. Soshea and P. Wargo have p r o v i d e d a b e n e f i c i a l exchange o f i d e a s . A p p r e c i a t i o n t o Joan Theresa P e r i a f o r her encouragement and forbearance i s hereby a f f e c t i o n a t e l y  extended.  1 INTRODUCTION The  i n c r e a s i n g importance of semiconductors and i n s u l a t o r s  i n the modern technology  need h a r d l y be emphasized.  Solid  state d e v i c e s are f i n d i n g more and more a p p l i c a t i o n i n most deavors i n v o l v i n g the use of e l e c t r o n i c c i r c u i t s . the case,  en-  As i s always  i n c r e a s e d a p p l i c a t i o n c a l l s f o r i n c r e a s e d understand-  i n g of the p h y s i c a l p r o p e r t i e s of the m a t e r i a l s i n v o l v e d . Since many of the u s e f u l and  i n t e r e s t i n g p r o p e r t i e s of  s o l i d s stem from the presence of d e v i a t i o n s o f the  these  lattice  from s t r i c t p e r i o d i c i t y , a study o f such s o l i d s n e a r l y always c o n s i s t s of a study of the l a t t i c e i m p e r f e c t i o n s , whether they occur n a t u r a l l y o r are purposely  introduced.  D e v i a t i o n s from p e r i o d i c i t y can i n t r o d u c e , i n t o the bidden energy region c h a r a c t e r i s t i c of the unperturbed l o c a l i z e d energy l e v e l s ,  i . e . , the imperfect  e i g e n s t a t e s whose e i g e n v a l u e s  for-  lattice,  s o l i d has some  l i e i n the normally  forbidden  region of energy and whose e i g e n f u n c t i o n s are l o c a l i z e d i n the r e g i o n of the i m p e r f e c t i o n r a t h e r than extending out the whole l a t t i c e .  The  determination  such l e v e l s on the energy s c a l e i s a f i r s t the nature  o f the corresponding  through-  of the l o c a t i o n of step i n the  imperfections.  However, i t  i s p o s s i b l e t h a t the i n v e s t i g a t i o n of the energy l e v e l f o r a p a r t i c u l a r imperfect beyond the understanding s o l i d alone.  One  the d e t e r m i n a t i o n  s o l i d may  study  scheme  have some importance  of the p h y s i c a l p r o p e r t i e s o f  has o n l y to c o n s i d e r the v i t a l  this  role that  of atomic and n u c l e a r energy l e v e l schemes  has p l a y e d i n the development of p h y s i c s to r e a l i z e  the  of  2  s i g n i f i c a n c e o f t h i s statement.  Of course, a knowledge o f  the scheme alone does not permit the d e d u c t i o n o f the nature of  the i m p e r f e c t i o n s .  I n f e r e n c e s as to the c o n s t i t u t i o n o f  a p a r t i c u l a r i m p e r f e c t i o n may be made i n a number o f ways; (1)  By s t u d y i n g the p r o p e r t i e s o f t h i s i m p e r f e c t i o n  by i t s e l f and comparing the observed behavior w i t h the p r o p e r t i e s p r e d i c t e d by v a r i o u s t h e o r e t i c a l models, (2)  By o b s e r v i n g r e a c t i o n s o f the i m p e r f e c t i o n s among  themselves (3)  and w i t h o t h e r t y p e s , By i n t r o d u c i n g i m p e r f e c t i o n s i n such a way as t o  f a v o r the p r o d u c t i o n of a s p e c i f i c type. The  f i r s t group o f compounds t o r e c e i v e a  i n v e s t i g a t i o n of i t s structure s e n s i t i v e  thorough  ( i . e . , imperfection-  c o n t r o l l e d ) p r o p e r t i e s was the monovalent i o n i c a l k a l i h a l i d e group.  These m a t e r i a l s were s t u d i e d e x t e n s i v e l y by the  G o t t i n g e n school under P o h l , b e g i n n i n g about 1930. experiments tor  These  might be regarded as the b e g i n n i n g o f semiconduc-  physics.  Many o f the concepts employed i n c u r r e n t r e s e a r c h  on semiconductors a l k a l i halides.  arose i n connection w i t h the work on these Of course many o t h e r groups o f s o l i d s are  c u r r e n t l y r e c e i v i n g much a t t e n t i o n . i s probably;-most  Of these the group which  c l o s e l y r e l a t e d t o the a l k a l i h a l i d e s i s the  group o f a l k a l i n e e a r t h o x i d e s , i . e . , d i v a l e n t i o n i c compounds. In a d d i t i o n t o the i n t e r e s t  i n these compounds because o f t h e i r  analogy t o the b e t t e r known monovalent i o n i c compounds, they a l l have important•device a p p l i c a t i o n s .  Barium and s t r o n t i u m o x i d e s  are used i n t h e r m i o n i c cathodes while magnesium oxide i s a  3  very e f f i c i e n t  secondary e l e c t r o n e m i t t e r .  compounds magnesium oxide  Of these  three  i s by f a r the e a s i e s t to o b t a i n  and work with i n s i n g l e c r y s t a l form. A concerted e f f o r t to understand the secondary e m i t t i n g c h a r a c t e r i s t i c s o f t h i s m a t e r i a l has been underway i n the E l e c t r o n Tube Laboratory several years. emission  of the U n i v e r s i t y of Minnesota f o r  Because i t was  i s i n f l u e n c e d by  b e l i e v e d t h a t the  c e r t a i n types of  secondary  imperfections,  s t u d i e s of the e l e c t r i c a l and o p t i c a l p r o p e r t i e s of  these  i m p e r f e c t i o n s have been c a r r i e d out as an i n t e g r a l p a r t o f t h i s program. As mentioned p r e v i o u s l y , c r y s t a l l i n e i m p e r f e c t i o n s  lead  to l o c a l i z e d energy l e v e l s l y i n g i n the f o r b i d d e n energy  gap.  E x c i t a t i o n of e l e c t r o n s to or from these  course  l e v e l s l e a d s of  to o p t i c a l a b s o r p t i o n bands which would not be present perfect crystal. charge c a r r i e r s  The  a b s o r p t i o n p r o c e s s e s may  l e a d to f r e e  ( e l e c t r o n s or h o l e s ) e i t h e r d i r e c t l y o r by  subsequent thermal s t e p s . measurements may  In e i t h e r case  photoconductivity  l e a d to a d d i t i o n a l i n f o r m a t i o n which w i l l  a i d i n the c o n s t r u c t i o n of an energy l e v e l diagram. a l s o conceivable a b s o r p t i o n may versa)  t h a t i m p e r f e c t i o n s not d e t e c t a b l e  be d e t e c t e d i n p h o t o c o n d u c t i v i t y  since the l i m i t s o f d e t e c t i o n i n the two  governed by d i f f e r e n t p r o p e r t i e s o f the crystal. The  i n the  It i s in optical  (or v i c e cases  center and the  are host  T h i s w i l l be d i s c u s s e d i n more d e t a i l below. work to be d e s c r i b e d i n t h i s t h e s i s was  aimed at a  4  determination  of the nature o f c e r t a i n of the  commonly observed  i n MgO  color centers  c r y s t a l s , by the measurement o f  o p t i c a l a b s o r p t i o n and p h o t o c o n d u c t i v i t y o f specimens t r e a t e d i n v a r i o u s ways.  I n s o f a r as an unambiguous energy l e v e l  scheme has not been determined, not even the f i r s t the o r i g i n a l purpose has been achieved. by proceeding  On  step i n  the o t h e r hand,  i n each of the t h r e e f a s h i o n s enumerated p r e v i o u s -  l y , i t has been p o s s i b l e t o make c e r t a i n i n f e r e n c e s  concerning  the p r o p e r t i e s of some of the c o l o r c e n t e r s and the  relations  between them which at the v e r y l e a s t , suggest c r u c i a l experiments.  f u r t h e r , more  5  A.  EXPERIMENTAL  DETAILS  Unless otherwise noted the c r y s t a l s used i n these e x p e r i ments were o b t a i n e d from the Norton Company, Niagara F a l l s , New York.  Only those p i e c e s which showed no v i s i b l e  t i o n were used.  absorp-  From these l a r g e r p i e c e s t h i n s l a b s c o u l d  be e a s i l y c l e a v e d o u t .  These were u s u a l l y about  the f a c e s and from 0.2 t o 1.0 mm  5 x 10 mm on  thick.  Some of the c r y s t a l s were heated i n vacuum b e f o r e use. Such heat treatments were c a r r i e d out i n a small furnace operating inside a b e l l 5 x 10 ^ mm Hg.  jar.  The p r e s s u r e was u s u a l l y  To prevent contamination of the c r y s t a l f a c e s  d u r i n g the heat treatment, they were p l a c e d i n a boat from a l a r g e MgO  about  c r y s t a l and a MgO  ground  s l a b l i d was t i e d on w i t h  molybdenum wire. Specimens were a d d i t i v e l y c o l o r e d w i t h Mg by h e a t i n g them i n s t e e l bombs c o n t a i n i n g Mg metal. independent  temperature  temperature  o f the c r y s t a l s  Two-chamber bombs w i t h  c o n t r o l s were used.  In t h i s way the  ( i n the h o t t e r chamber) and the  vapor p r e s s u r e o f the metal c o u l d be v a r i e d independently. The bombs were assembled joint.  i n a i r by means o f a t a p e r e d c o n i c a l  E v a c u a t i o n was not necessary since the metal  readily  combined w i t h the oxygen and n i t r o g e n at the temperatures employed.  The c r y s t a l s were heated a t temperatures  i n the  range 1100°C t o 1350°C, i n Mg p r e s s u r e s from 1 t o 7500 mm and f o r times from 1 t o 50 hours.  Hg,  On some o c c a s i o n s the bombs  were quenched by dropping i n t o water, on o t h e r s they were a l l o w e d to c o o l s l o w l y .  6  The apparatus employed f o r the measurement o f photoconductivity  i s shown s c h e m a t i c a l l y i n F i g u r e I .  L i g h t from the  source A was f o c u s e d by the 12-inch diameter e l l i p t i c a l m i r r o r B onto the entrance s l i t  aluminum  o f a Bausch and Lomb- g r a t i n g  monochromator C, having a d i s p e r s i o n o f 33 angstrom/mm.  The  monochromator was normally used w i t h s l i t s o f 3 mm o r l e s s i n o  width so t h a t the band width was normally about At 5 ev t h i s corresponds t o an energy monochromator output.  100 A o r l e s s .  range o f 0.2 ev i n the  To remove the h i g h e r o r d e r d i s p e r s i o n s  from the monochromator output, sharp-cut g l a s s f i l t e r s c o u l d be i n s e r t e d a t D.  These were prevented from o v e r h e a t i n g when  necessary by 2 cm o f water between f u s e d quartz p l a t e s ( E ) . The output from the monochromator passed through the f u s e d quartz p l a t e F, from which a f r a c t i o n o f the beam was f o c u s e d onto a type 935 p h o t o c e l l , G, by the f r o n t s u r f a c e a l u m i n i z e d m i r r o r , H.  The main p o r t i o n o f the beam was focused onto the  c r y s t a l J by means o f a second f r o n t  s u r f a c e m i r r o r , K.  The  l a t t e r was p i v o t e d so t h a t i t c o u l d be moved i n a h o r i z o n t a l plane by means of micrometer screw, L. ing  the e x i t  A l l the p a r t s f o l l o w -  s l i t o f the monochromator were e n c l o s e d i n a  l i g h t - t i g h t box w i t h a f u s e d quartz entrance window.  The box  c o u l d be d e s i c c a t e d when necessary. Three range  l i g h t sources were employed t o cover the energy  from 1.7 t o about  5.6 ev.  A carbon a r c , u s i n g a  N a t i o n a l Carbon Company Type W cored anode, o p e r a t i n g at 4 0 , V  60^,  c o u l d be used o v e r the e n t i r e range.  More s t a b l e  output  was o b t a i n e d from a G e n e r a l E l e c t r i c Company Type AH6 h i g h  7  pressure mercury a r c (2.3 - 4.6 ev) o r a tungsten lamp (1.7 - 2.3 e v ) .  At e n e r g i e s g r e a t e r than 5.0 ev there was  an a p p r e c i a b l e amount o f s t r a y l i g h t output.  T h i s was accounted  photocurrent and l i g h t  i n the monochromator  f o r by measuring the decrease i n  i n t e n s i t y when a C o r n i n g 9700 f i l t e r  was p l a c e d i n the beam.  This f i l t e r  cut o f f the energy a t  which the measurement was b e i n g made but passed a l l the s t r a y  practically  radiation.  The p h o t o c e l l G was c a l i b r a t e d i n a separate- experiment by p l a c i n g a c a l i b r a t e d thermopile i n the sample p o s i t i o n and measuring the output o f b o t h p h o t o c e l l and thermopile as a f u n c t i o n o f wavelength.  The p h o t o c e l l output was d e t e c t e d by  a D.C. a m p l i f i e r c o n s t r u c t e d a c c o r d i n g t o the d e s i g n o f Lander  1  while a P e r k i n Elmer Model 53 breaker a m p l i f i e r measured  the thermopile  output.  The thermopile used had a time  constant o f approximately  1 s e c , so t h a t the c a l i b r a t i o n procedure r e q u i r e d s t a b i l i t y o f the l i g h t seconds.  source over p e r i o d s o f s e v e r a l  T h i s c o n d i t i o n c o u l d not be met when the carbon a r c  was employed. in  would n o r m a l l y have  T h i s d i f f i c u l t y was overcome, however, by p l a c i n g ,  the p h o t o c e l l c i r c u i t , a network having an e l e c t r i c a l  constant equal t o the thermal time  time  constant o f the t h e r m o p i l e .  Under these c o n d i t i o n s a comparison o f thermopile and photoc e l l outputs at a g i v e n time p r o v i d e s the r e q u i r e d c a l i b r a t i o n , d e s p i t e f l u c t u a t i o n s i n the l i g h t comparison,  source.  To f a c i l i t a t e the  the two o u t p u t s were recorded on a Sanborn Model  64-1300 A m u l t i - c h a n n e l r e c o r d e r .  8  Due t o the l a c k of s e n s i t i v i t y , the p h o t o c e l l c o u l d not be used a t quantum e n e r g i e s below 2.30 ev.  By u s i n g the  c a l i b r a t e d t h e r m o p i l e the v a r i a t i o n of the output of the tungsten lamp w i t h energy was determined.  Then under a g i v e n  set of c o n d i t i o n s the i n t e n s i t y c o u l d be measured at 2.3  ev  u s i n g the p h o t o c e l l and the i n t e n s i t y at lower quantum e n e r g i e s c a l c u l a t e d from t h i s . C r y s t a l s were p r e p a r e d f o r p h o t o c o n d u c t i v i t y measurements by p a i n t i n g e l e c t r o d e s of a i r - d r y i n g s i l v e r p a i n t No. 4817) on two of the edges.  (DuPont  They were h e l d between T e f l o n  b l o c k s , which were a l s o coated on one face w i t h the same s i l v e r paste  ( F i g . 2). The e l e c t r i c f i e l d was a p p l i e d to the c r y s t a l by a number  o f 300^ dry c e l l s , w h i l e the c u r r e n t was measured by a d i r e c t coupled feedback a m p l i f i e r whose f i r s t  stage was mounted near  19  the c r y s t a l .  An input r e s i s t o r o f 10  ohms c o u l d be  employed  and c u r r e n t s of 5 x l O " ^ amperes were r e a d i l y measurable. -  With t h i s input r e s i s t o r the time constant o f the system about 2 seconds.  The  was  s e n s i t i v i t y and l i n e a r i t y of the system  were checked by a p p l y i n g the a c c u r a t e l y known v o l t a g e s from a Rubicon p o t e n t i o m e t e r t o the i n p u t . In a l l the p h o t o c u r r e n t measurements, the exposure o f the c r y s t a l t o l i g h t , when the e l e c t r i c f i e l d was a p p l i e d ,  was  kept as small as p o s s i b l e i n o r d e r to minimize the f o r m a t i o n of space charge f i e l d s .  The l i g h t was a l l o w e d to f a l l on the  c r y s t a l i n s i n g l e " p u l s e s " v a r y i n g from .05 to about 5 seconds depending on the s e n s i t i v i t y  (and hence  response time) o f the  9  apparatus.  A m a g n e t i c a l l y o p e r a t e d s h u t t e r was  p r o v i d e the s h o r t e r l i g h t p u l s e s , but t h i s was found to be unnecessary to  set up to ordinarily  s i n c e the c u r r e n t s were s m a l l enough  r e q u i r e the use of the l a r g e s t  input r e s i s t o r s .  these circumstances the longer response time of the required light of  Under amplifier  f l a s h e s of d u r a t i o n s e a s i l y o b t a i n e d by means  a manually o p e r a t e d s h u t t e r .  The  f l a s h e s were s t i l l  s h o r t , however, t o permit the photocurrent and l i g h t to be read from meters.  Consequently,  too  intensity  both q u a n t i t i e s were  r e c o r d e d s i m u l t a n e o u s l y on the p r e v i o u s l y ; mentioned r e c o r d e r . Aside from the p r e v e n t i o n of space use of short p u l s e s o f l i g h t was  charge  f o r m a t i o n , the  d e s i r a b l e because of the  poor  long-term s t a b i l i t y of the carbon a r c and the e f f e c t o f p r o longed UV  r a d i a t i o n on the photoconductive  response.  10  B. B.l*  O p t i c a l Absorption Absorption  DU q u a r t z  RESULTS Measurements  s p e c t r a were measured with a Beckman Model  spectrophotometer.  i n terms of the a b s o r p t i o n T  I where I  0  and  a  „ I  Q  e  The  data are u s u a l l y  presented  c o e f f i c i e n t , K, d e f i n e d by — Kd  ., ^.2 (1-R&/  I are the i n c i d e n t and t r a n s m i t t e d  intensities,  r e s p e c t i v e l y , d i s the  specimen t h i c k n e s s and R i s the  reflec-  tion coefficient.  above e x p r e s s i o n  losses  at the f i r s t and  The  second p a r t i a l  a l l o w s f o r the  relfectionslonly.  Since R  i s s m a l l , t h i s approximation i s s u f f i c i e n t l y a c c u r a t e purposes.  f o r our  R was  c a l c u l a t e d from the index of r e f r a c t i o n data 2 of Strong and B r i c e , which was e x t r a p o l a t e d i n t o the short 3 wavelength region u s i n g the S e l l m a i e r equation , 2 u.2 - 1.945  X  X  - 1.251  2  x  10  b  where u. i s the index o f r e f r a c t i o n and X the wavelength o  expressed  i n Angstrom u n i t s .  Since the c l e a v e d s u r f a c e s of the  c r y s t a l s were not  p e r f e c t l y smooth but had a "wavy" appearance, these c a l c u l a t e d r e f l e c t i o n l o s s e s were no doubt too low. changes (AK)  are p r e s e n t e d ,  Where a b s o r p t i o n  t h i s l o s s , of course,  cancels  out.  • S e c t i o n s B and C are d i v i d e d i n t o 11 subsections each i n such a way t h a t the r e s u l t s d e s c r i b e d i n B . l are d i s c u s s e d i n C . l and so on.  11 On  some o c c a s i o n s , the  change i n o p t i c a l d e n s i t y (AD)  p l o t t e d r a t h e r than the change i n a b s o r p t i o n  is  coefficient.  Since I D  =  l o g  io  "I '  then 2.3 AE = — j B.2  AD  A n a l y s i s of the P h o t o c o n d u c t i v i t y Data For reasons which w i l l become e v i d e n t i n l a t e r s e c t i o n s ,  the p h o t o c o n d u c t i v i t y data had t o be analyzed i n a somewhat d i f f e r e n t f a s h i o n from that normally used i s presented  employed.  The a n a l y s i s  below:  Consider a c r y s t a l with an a b s o r p t i o n c o e f f i c i e n t where E d e s i g n a t e s the quantum energy.  K may  K(E),  be a composite  c o e f f i c i e n t which d e f i n e s the t o t a l a b s o r p t i o n , at a g i v e n energy, due  to s e v e r a l d i f f e r e n t a b s o r p t i o n p r o c e s s e s ,  K^(E),  i.e., K(E)  = £  YL {E) ±  i  I f the c r y s t a l absorbs a f r a c t i o n , a_, of the i n c i d e n t r a d i a t i o n , the f r a c t i o n absorbed by o p t i c a l t r a n s i t i o n s of K-i a the i t h k i n d i s — i — K  .  L e t p i be the p r o b a b i l i t y t h a t  a t r a n s i t i o n leads to a f r e e charge c a r r i e r .  Then the  of the i n c i d e n t r a d i a t i o n which produces f r e e charge by a l l p o s s i b l e types o f t r a n s i t i o n s , i s -  Z p,  K i  l  K i  such fraction  carriers,  12 Let the c a r r i e r s move an average d i s t a n c e x^ i n u n i t field  (we use the s u b s c r i p t here t o a l l o w f o r the p o s s i b i l i t y  that both e l e c t r o n and hole e x c i t a t i o n occur a t the same energy, i . e . , x^ t a k e s on e i t h e r one o f two p o s s i b l e  values).  Then each c o n t r i b u t e s a charge, _iZZZ.e, t o the e x t e r n a l w  circuit,  where w i s the d i s t a n c e between the e l e c t r o d e s and V i s the applied voltage.  Then i f N quanta p e r sec. f a l l on the c r y s t a l ,  the observed p h o t o c u r r e n t w i l l be  or i K w 2 x.p.K, = — . _ . — i  1  1  i  N  a  2  = Y, say.  (1)  v  eV  K was determined by o p t i c a l a b s o r p t i o n measurements on each c r y s t a l and a c a l c u l a t e d therefrom.  Thus, from  a'combination  of o p t i c a l a b s o r p t i o n and p h o t o c o n d u c t i v i t y measurements, the q u a n t i t y Y(E) c o u l d be determined. In the l i t e r a t u r e , p h o t o c o n d u c t i v i t y data i s u s u a l l y p r e s e n t e d by p l o t t i n g e i t h e r /N v e r s u s E o r /Na v e r s u s E. X  1  The former method i g n o r e s the f a c t t h a t some o f the r a d i a t i o n may be l o s t by t r a n s m i s s i o n through the sample, w h i l e the second does not a l l o w f o r the p o s s i b i l i t y that the major a b s o r p t i o n processes may not c o n t r i b u t e t o the p h o t o c o n d u c t i v i t y . That these assumptions may l e a d t o s e r i o u s e r r o r w i l l be shown below. An e x p r e s s i o n f o r a_ which accounts f o r the l i g h t on the f i r s t  absorbed  t r a v e r s a l through the c r y s t a l and a l s o f o r that  absorbed on the second t r a v e r s a l  (i.e., after partial  reflection  13 at the  second MgO-air i n t e r f a c e ) may  e a s i l y be  shown to be  a = (l-R)(l-e~ )(l+Re*" ) K d  where R(E)  i s the  crystal thickness.  (2)  K d  r e f l e c t i o n c o e f f i c i e n t of MgO Then the c o r r e c t i o n f a c t o r  and d i s the /a i n equation  (1) i s — ^ a (l-R)(l-e-^ )(l+Re-^ ) d  Now  f o r Kd s u f f i c i e n t l y small  to (3) i f R ^  0.1,  (3)  d  (say < 0.1),  a good approximation  is  £ = I a d  (4)  Hence, t h i s c o r r e c t i o n f a c t o r w i l l have no i n f l u e n c e on s p e c t r a l dependence of Y i n t h a t region of the the a b s o r p t i o n  the  spectrum where  c o e f f i c i e n t of the c r y s t a l i s small enough t o  s a t i s f y the i n e q u a l i t y , Kd<  0.1.  On the o t h e r hand, f o r Kd  s u f f i c i e n t l y l a r g e (say > 2.5)  a  good approximation to (3) i s  1a =  1-R  In t h i s case, the tance The  (5)  T^~  c o r r e c t i o n f a c t o r i s of the utmost impor-  i n the d e t e r m i n a t i o n  of the  s p e c t r a l dependence of  Y.  n e g l e c t of t h i s f a c t o r i s e q u i v a l e n t to the assumption t h a t  the p h o t o c o n d u c t i v i t y  i s due  to those  c e n t e r s which are  b l e f o r most of the o p t i c a l a b s o r p t i o n . j u s t i f i a b l e assumption and  i t may  responsi-  T h i s i s not always a  l e a d to erroneous r e s u l t s .  14  A l l the p h o t o c o n d u c t i v i t y y i e l d s quoted here were c a l c u l a t e d  i \  a c c o r d i n g t o equations ( 1 ) , and ( 3 ) , (4) o r ( 5 ) . B.3 Dependence o f Photocurrent on L i g h t  I n t e n s i t y and E l e c t r i c  Field In u n t r e a t e d c r y s t a l s the p h o t o c u r r e n t s were found t o be p r o p o r t i o n a l t o the e l e c t r i c f i e l d up to about In  6*000 v o l t s / c m .  t h i s range o f f i e l d s , the dark c u r r e n t was n e g l i g i b l e , i . e . ,  l e s s than 10 ^ critical  amperes.  5  However, f o r f i e l d s g r e a t e r than some  value ( c a . 6,000 v o l t s / c m . ) , the dark current rose  s h a r p l y t o 10""^ amperes o r more.  T h i s c u r r e n t was s u f f i c i e n t l y  unstable t h a t p h o t o c o n d u c t i v i t y measurements were i m p o s s i b l e . U s u a l l y , the p h o t o c u r r e n t s were small enough at f u l l  light  i n t e n s i t y t o prevent an i n v e s t i g a t i o n o f the dependence o f c u r r e n t on i n t e n s i t y .  However, because o f the l a r g e r c u r r e n t s a v a i l a b l e  i t was p o s s i b l e t o perform the'experiment u s i n g 2.3 ev quanta.  on an X-rayed  crystal,  In t h i s case, the p h o t o c u r r e n t was p r o p o r -  t i o n a l t o i n t e n s i t y f o r a 40 t o 1 v a r i a t i o n o f i n t e n s i t y . Because o f the i r r e g u l a r s p e c t r a l output o f both the carbon arc  and the mercury a r c , i t i s b e l i e v e d that any s i g n i f i c a n t  d e v i a t i o n s from p r o p o r t i o n a l i t y i n the cases o f o t h e r types of  c r y s t a l s would have l e d to corresponding i r r e g u l a r i t i e s i n  the c a l c u l a t e d y i e l d curves.  S i n c e the l a t t e r  irregularities  were not observed, we conclude t h a t i n the range of i n t e n s i t i e s used light B.4  (at l e a s t 1,000 t o 1) the photocurrent was p r o p o r t i o n a l t o intensity. Evidence t h a t P h o t o c u r r e n t s are a Bulk E f f e c t In  view o f the f a c t that the observed c u r r e n t s were so  s m a l l , the p o s s i b i l i t y that they were due t o a s u r f a c e e f f e c t  15 must be c o n s i d e r e d .  To i n v e s t i g a t e t h i s p o s s i b i l i t y the photo-  c o n d u c t i v i t y o f a t h i c k c r y s t a l was measured. c l e a v e d o f f and the remainder remeasured.  A section  was  T h i s procedure  was  repeated s e v e r a l times, measuring the p h o t o c o n d u c t i v i t y and a b s o r p t i o n at each step. ent  Table I shows Y and K f o r the d i f f e r -  s e c t i o n s of the same o r i g i n a l  crystal.  Table I A b s o r p t i o n and P h o t o c o n d u c t i v i t y at 5.0 ev T h i c k n e s s (cm)  i/ electrons N  x  1 Q  10  x  1 Q  10  _cm  K ( c m  -1)  .814  7.1  1.85  1.92  .498  4.5  1.61  2.27  .346  5.5  2.18  1.72  .211  3.3  1.83  1.30  .133  2.3  1.99  1.85  .101  1.3  1.45  2.03  As d i s c u s s e d i n S e c t i o n C.4 the  y  the approximate constancy of  t h i r d column r e l a t i v e t o the second, i m p l i e s t h a t the measured  p h o t o c u r r e n t s are the r e s u l t of o p t i c a l e x c i t a t i o n i n the b u l k of the c r y s t a l . B.5  Space Charge Formation As e x p l a i n e d p r e v i o u s l y , the exposure o f the c r y s t a l to  l i g h t was h e l d t o a minimum i n o r d e r to prevent the f o r m a t i o n of  space charge f i e l d s .  In o r d e r to demonstrate the d e v e l o p -  ment of such f i e l d s , the f o l l o w i n g experiment was  performed.  With an e l e c t r i c f i e l d a p p l i e d to a c r y s t a l , a p o r t i o n  16  i of  the volume between i t s e l e c t r o d e s was i r r a d i a t e d w i t h 4.4  ev quanta and the photocurrent measured as a f u n c t i o n o f time. At i n t e r v a l s the e l e c t r i c f i e l d was removed, the e l e c t r o d e s were brought  to the same p o t e n t i a l by connecting them through  the input r e s i s t o r , and the photocurrent measured under t h i s condition.  F i g u r e 3 ( a ) , curve A shows the v a r i a t i o n , w i t h  time of i r r a d i a t i o n , of the photocurrent w i t h a p p l i e d  field,  while curve B g i v e s the c u r r e n t when measured w i t h no a p p l i e d field.  The p h o t o c u r r e n t s o f curve B flow i n the o p p o s i t e  d i r e c t i o n t o those i n curve A. The  i r r a d i a t i o n was then continued w i t h no a p p l i e d  electric field  ( e l e c t r o d e s connected)  measured as a f u n c t i o n of time.  and the photocurrent  At i n t e r v a l s the e l e c t r i c  f i e l d was a p p l i e d and the p h o t o c u r r e n t measured under t h i s condition. 3(b).  The corresponding curves are p r e s e n t e d i n F i g u r e  In each case curve C i s the sum of curves A and B. It i s e v i d e n t from the curves that the f l o w of photo-  current i n the a p p l i e d f i e l d produces p e r s i s t a f t e r the i n i t i a l exposure of  space  charge  f i e l d s which  to l i g h t and a f t e r the removal  the p o t e n t i a l d i f f e r e n c e between the e l e c t r o d e s .  These  results  are d i s c u s s e d i n a more q u a n t i t a t i v e f a s h i o n i n S e c t i o n C.5.  B. 6  P r o p e r t i e s o f C r y s t a l s as Received In  view o f the reasonable agreement among the r e s u l t s  different  crystals  (Table I I ) l i t t l e attempt  to correlate  from optical  a b s o r p t i o n o r p h o t o c o n d u c t i v i t y data with i m p u r i t y content has  17 been made.  Spectrographic  a n a l y s e s of MgO  crystals  obtained  from the Norton Company have, however, been made i n t h i s laboratory .  These analyses  i s i r o n (.01  - .05%)  with  show that the p r i n c i p a l  somewhat s m a l l e r  of manganese, chromium, c a l c i u m and  impurity  concentrations  silicon.  As r e c e i v e d from the Norton Company, most of the c r y s t a l s employed i n t h i s i n v e s t i g a t i o n showed an  MgO  optical  absorption  spectrum as shown i n F i g u r e 4,  exceptions  to t h i s r u l e had a spectrum as shown by curve  The  curve A.  The B.  a c t u a l magnitude of the a b s o r p t i o n v a r i e d c o n s i d e r a b l y from  c r y s t a l to c r y s t a l but these two  shapes were n e a r l y always  found. The  photoconductivity  by a b s o r p t i o n  s p e c t r a of c r y s t a l s c h a r a c t e r i z e d  s p e c t r a of e i t h e r type A or type  were as shown i n F i g u r e 5, which g i v e s two t i v e y i e l d curves along w i t h the All  two  The  absorption  c r y s t a l s were of one  c o r r e l a t e d with any  photoconductivity  ev.  The  the  known d i f f e r e n c e among c r y s t a l s .  i n the  bands centered about 4.05  The  or  l a t t e r difference could  r e g i o n below 5.0  ev c o u l d be  accounted f o r i n most cases by a s u p e r p o s i t i o n of two  of a t y p i c a l  curves.  types, which d i f f e r i n shape p r i n c i p a l l y  in, the energy r e g i o n above 5.0 not be  4)  t y p i c a l photoconduc-  corresponding  s p e c t r a o b t a i n e d on u n t r e a t e d  o t h e r of these  B (Figure  ev and  5.05  spectrum i n t o these two  l o c a t i o n s of these peaks do not  ev.  The  well  Gaussian  decomposition  bands i s shown i n Figure correspond  to the p o s i t i o n s  of any o f the p r e v i o u s l y known o p t i c a l a b s o r p t i o n bands i n The  MgO.  r e p r o d u c i b i l i t y of the magnitude of these bands from  c r y s t a l to c r y s t a l i s i l l u s t r a t e d i n Table at 3.8  6.  and 4.6  I I , where the  ev and t h e i r r a t i o are presented  yields  f o r a number  18  of specimens.  These e n e r g i e s were chosen as b e i n g  representa-  t i v e of the magnitudes o f the low and h i g h energy peaks, tively.  respec-  C o n s i d e r i n g t h a t the measured y i e l d depends b o t h on the  d e n s i t y of p h o t o i o n i z a b l e c e n t e r s and on the range o f the f r e e c a r r i e r s , b o t h of which may it  depend upon i m p u r i t y  i s perhaps s u r p r i s i n g t h a t the magnitude  concentration,  of the y i e l d  not show a g r e a t e r v a r i a t i o n from c r y s t a l to c r y s t a l . magnitude  v a r i a t i o n s which d i d occur  with v a r i a t i o n s of magnitude spectrum.  c o u l d not be  does  The  correlated  nor type i n the o p t i c a l  absorption  19 Table I I Photoconductive Y i e l d o f a Number o f Specimens Type o f A b s o r p t i o n Spectrum 4.6 * (See F i g . 4) 10 (cm/volt) 4.6/ 3.8 Y  1  0  1  1  1 3  ,8  x  Y  Y  14.8  34  B  6.0  52  B  4.1  2.53  62  14.0  7.12  51  43 11.5  B  *  B  *  48  B  *  3.81 12.4  6.0  CO. 9  5.74  640  A  6.2  4.89  79  A  25  B  70.0  17.7 4.40  13.5  6.34  47  A 54  A  4.08  28  A  8.5  4.6  54  B  6.6  4.52  68  B  9.1  5.0  55  B  NOTES:  *  B  3.4  14.4  Lower P u r i t y  B  5.45 6.3  Comments  1.  Samples marked* were cut from the same l a r g e r p i e c e .  2.  The sample d e s i g n a t e d "lower p u r i t y " was impure  enough to be v i s i b l y c o l o r e d .  The p r i n c i p a l i m p u r i t y was probably  iron. The photoconductive y i e l d a t 4.8 ev ( i . e . i n the 5 ev band) was found t o be approximately twice as g r e a t a t 250°C as a t room  20  temperature.  With the e x p e r i m e n t a l arrangement  employed  the  ;  temperature v a r i a t i o n of the 4 ev band c o u l d not be measured. B.7  Heat Treatment S t u d i e s As has been p r e v i o u s l y mentioned  ( S e c t i o n B.6  and F i g . 4A)  most o f the c r y s t a l s as o b t a i n e d , p o s s e s s e d an u l t r a v i o l e t a b s o r p t i o n spectrum c o n s i s t i n g o f two obvious bands. of normal t h i c k n e s s (0.2 to 0.7 mm)  For c r y s t a l s  the two a b s o r p t i o n bands  c o u l d be removed by h e a t i n g i n vacuum at 1400°C f o r s e v e r a l hours. Such a treatment r e s u l t e d i n s p e c t r a l i k e that shown i n F i g u r e curve B.  4,  F u r t h e r h e a t i n g at the same temperature d i d not change  the shape o f the spectrum.  For convenience we w i l l  r e f e r to  c r y s t a l s w i t h t h i s type of a b s o r p t i o n spectrum as " s t o i c h i o m e t r i c " a l t h o u g h , a d m i t t e d l y , we  cannot be sure that t h i s spectrum i s  not c h a r a c t e r i s t i c o f a small d e v i a t i o n from s t o i c h i o m e t r y , i n e q u i l i b r i u m at the h i g h temperature. be termed the "background"  The  spectrum i t s e l f  will  absorption.  The e f f e c t of heat treatment a t v a r i o u s temperatures on the a b s o r p t i o n o f s t o i c h i o m e t r i c c r y s t a l s was detail.  s t u d i e d i n some  Since no c o n c l u s i o n s c o u l d be drawn from the d e t a i l e d  b e h a v i o r of the a b s o r p t i o n at the v a r i o u s temperatures, only the most important o b s e r v a t i o n s w i l l be g i v e n here. F i g u r e 7, curve C shows the spectrum of a specimen was  c o o l e d slowly from 1400°C.  T h i s type of spectrum was  c h a r a c t e r i s t i c o f specimens heated at 1000°C o r 1100°C quenched  therefrom.  also  and  The f o l l o w i n g o b s e r v a t i o n s summarize the  experiments performed at 1000°C and 1.  which  1100°C.  The change o f s p e c t r a l shape  from that of curve A to  21 t h a t o f curve C ( F i g . 7); was r e v e r s i b l e .  That i s , the curve A  c o u l d be regained by r e h e a t i n g a t 1400°C and quenching  from  this  temperature. 2.  The rate a t which the e q u i l i b r i u m shape C ( F i g . 7)  was approached  was e s s e n t i a l l y the same a t 1000°C as i t was  at 1100°C.  At both temperatures t h i s rate was found t o be  independent  of crystal thickness.  3.  The change i n shape o f the o p t i c a l a b s o r p t i o n curve  was not accompanied by a change i n the s p e c t r a l dependence o f the p h o t o c o n d u c t i v i t y . T h i s p o i n t i s i l l u s t r a t e d by F i g . 8 which shows the y i e l d curves f o r two c r y s t a l s , one heated a t 1400°C f o r 2 hours, the o t h e r a t 1000°C f o r 230 hours.  The  a b s o r p t i o n s p e c t r a were o f the types A and C ( F i g . 7) r e s p e c t i v e l y . B.8  Excess Mg C r y s t a l s A t y p i c a l a b s o r p t i o n spectrum f o r a c r y s t a l heated i n Mg  vapor i s shown i n F i g u r e 9, curve B.  S i n c e the shape o f the  curve d i f f e r e d somewhat f o r d i f f e r e n t  c o l o r i n g c o n d i t i o n s , two  experiments designed t o determine  the cause o f t h i s change o f  shape were c a r r i e d o u t , v i z . , 1,  A crystal  .068 cm t h i c k was heated f o r 1 hour at  1115°C i n a Mg p r e s s u r e o f approximately 2 atmospheres.  The  s p e c t r a b e f o r e and a f t e r t h i s treatment are shown i n F i g u r e 10. Figure 11 shows s i m i l a r curves f o r a c r y s t a l .074 cm t h i c k t r e a t e d i n a s i m i l a r f a s h i o n f o r 3 hours. were l i g h t l y 2.  Both of these  crystals  c o l o r e d compared t o t h a t shown i n F i g u r e 9.  A c r y s t a l .077 cm t h i c k was heated 4 hours a t 1115°C 4  22 i n about 2 atmospheres p r e s s u r e Is  shown by  of Mg.  curve A of F i g u r e 12.  ment of 5 hours was  c a r r i e d out.  The  absorption  A further similar  above two  v  .v-.v:  experiments show t h a t the t r u e  a b s o r p t i o n c o u l d not be  treat-  Curve B shows the e x t r a  a b s o r p t i o n induced by the second treatment o n l y . The  change  c a l c u l a t e d by simply  induced  s u b t r a c t i n g the  a b s o r p t i o n measured before h e a t i n g i n Mg vapor from that measured a f t e r h e a t i n g . i n S e c t i o n C.8.  The  T h i s p o i n t i s d i s c u s s e d more  photoconductivity  thoroughly  of therrmore s t r o n g l y  c o l o r e d c r y s t a l of F i g u r e 9 i s shown i n Figure 13,  curve  B.  T h i s specimen had been t r e a t e d f o r 48 hours at 1200°C i n a pressure  of 1 mm  Hg.  For comparison, the  Mg  spectrum of a t y p i c a l  s t o i c h i o m e t r i c or excess oxygen c r y s t a l i s a l s o i n c l u d e d (curve B.9  The  E f f e c t of U l t r a v i o l e t  The  e f f e c t o f u l t r a v i o l e t i r r a d i a t i o n on the  of stoicrrdometric c r y s t a l s was  Irradiation photoconductivity  s t u d i e d by i r r a d i a t i n g a c r y s t a l  with low energy quanta, measuring the y i e l d at fiwe  different  energies,  i r r a d i a t i n g at a higher energy, remeasuring, e t c .  F i g u r e 14  shows the r e s u l t s of t h i s experiment.  The  along the h o r i z o n t a l a x i s i n d i c a t e the energy of the l e a d i n g to the p h o t o c o n d u c t i v i t y  spectrum with the  numbers quanta corresponding  number. In view of the apparent presence o f two which l e a d to a p p r e c i a b l e p h o t o c o n d u c t i v i t y ultraviolet c r y s t a l s one  i r r a d i a t e d at 3.8  quanta/cm ) the other at 4.6 2  a b s o r p t i o n bands ( F i g u r e 6), the  i r r a d i a t i o n experiments were repeated on two of which was  A).  ev  (4 x 1 0  1 7  ev,  (1.6  x  quanta/cm ). 2  other IO  1 9  These  23  e n e r g i e s were chosen so t h a t the a c t i v a t i o n would i n each case be due  to a b s o r p t i o n  i n o n l y one  of the two  bands.  The  results  of these experiments are -shown i n F i g u r e 15, which a l s o shows f o r comparison a t y p i c a l The  absorption  changes produced by  shown i n F i g u r e s 16 and B.10  The The  spectrum before  irradiation. such i r r a d i a t i o n s  17.  E f f e c t of X - I r r a d i a t i o n e f f e c t of X - i r r a d i a t i o n on the o p t i c a l a b s o r p t i o n  p h o t o c o n d u c t i v i t y was  s t u d i e d by exposing  and b e r y l l i u m window.  The  samples were p l a c e d about 8  from the t a r g e t and the tube was  operated  and  stoichiometric  c r y s t a l s to the beam from an X-ray tube with a tungsten  During  are  at 50 kv and  target cm 15  ma.  the i r r a d i a t i o n the specimens were covered w i t h aluminum  f o i l - t o p r o t e c t them from o p t i c a l r a d i a t i o n . F i g u r e 18 shows the induced a b s o r p t i o n at two e n e r g i e s as a f u n c t i o n of the i r r a d i a t i o n time. were o b t a i n e d f o r a l l quantum e n e r g i e s approximation i t may absorption  be  s a i d that the  Similar  curves  so that to a good shape of the  induced  spectrum does not vary d u r i n g the p e r i o d of X-ray  irradiation.  The  s a t u r a t e d induced  a b s o r p t i o n spectrum at  room temperature i s shown i n F i g u r e 19, The  quantum  specimen was  curve  A.  s t o r e d i n the dark at room temperature  and the a b s o r p t i o n measured at i n t e r v a l s .  F i g u r e 19,  curve  B,  shows the induced a b s o r p t i o n a f t e r 95 hours of dark decay while the a b s o r p t i o n at two of time i n Figure  quantum e n e r g i e s  i s p l o t t e d as a f u n c t i o n  20.  F i g u r e 21 shows the e f f e c t of X-rays on the  photoconductivity.  24 Curve A corresponds to the u n t r e a t e d c r y s t a l .  The 5.0 ev  band i s obvious but the 4.0 ev band i s j u s t r e s o l v e d i n t h i s particular minutes  case.  The c r y s t a l was exposed  to X-rays f o r 40  and curve B measured immediately a f t e r t h i s i r r a d i a -  t i o n while curve C was measured 4 hours l a t e r .  A f t e r 50  hours the y i e l d had changed to curve D and changed o n l y very slowly  thereafter.  F i g u r e 22 shows s i m i l a r data f o r the same c r y s t a l i n the low energy  r e g i o n o f the spectrum.  immediately a f t e r  Curve A was o b t a i n e d  i r r a d i a t i o n ; curve B, 72 hours l a t e r .  The  change i n a b s o r p t i o n c o e f f i c i e n t d u r i n g the same p e r i o d i s shown by the d o t t e d curves i n the same By comparing  figure.  F i g u r e 15, curve C, with F i g u r e 21, curve D,  i t may be seen t h a t the p h o t o c o n d u c t i v i t y o f a specimen  activated  by i r r a d i a t i o n i n the 5.0 ev band i s s i m i l a r t o the photocond u c t i v i t y of specimens i r r a d i a t e d w i t h X-rays and allowed t o decay t h e r m a l l y .  The e f f e c t o f subsequent  X-irradiation  on a  p r e v i o u s l y U V - i r r a d i a t e d c r y s t a l i s shown i n F i g u r e 23. Curve A was o b t a i n e d by i r r a d i a t i o n with 4.4 ev quanta, i . e . , the i r r a d i a t i o n was i n the 5 ev band and the spectrum i s t h e r e f o r e s i m i l a r t o that o f F i g u r e 18, curve C.  Curve B  r e s u l t e d from the X-ray exposure, while- curve C was o b t a i n e d after  96 hours o f subsequent  thermal  decay.  The e x t r a a b s o r p t i o n and p h o t o c o n d u c t i v i t y induced by X-irradiation  can be reduced by o p t i c a l i r r a d i a t i o n .  A  thorough study o f t h i s e f f e c t has not been made but the e f f e c t o f 2.3 ev quanta has been s t u d i e d t o some e x t e n t . F i g u r e 24, curve A, shows the v a r i a t i o n  w i t h time o f the  25  o p t i c a l a b s o r p t i o n at 4.2  ev, when a f r e s h l y  i r r a d i a t e d w i t h 3 x 10  quanta/cm - s e c .  f o r a c o n t r o l sample X-rayed at the the dark at room temperature. at  2.3  The  variation  Curve B i s  same time but  stored i n  S i m i l a r curves f o r the  ev are shown i n F i g u r e 25.  quantum e n e r g i e s was  crystal  2  15  was  X-rayed  The  l i k e w i s e reduced  absorption  a b s o r p t i o n at a l l o t h e r by the  irradiation.  w i t h i n t e g r a t e d l i g h t f l u x , of the photo-  conductive y i e l d of a p a r t i a l l y decayed, X-rayed  crystal,  is  absorption  shown i n F i g u r e 26.  was  not measured d u r i n g nor a f t e r  on F i g u r e 26 was equation we  In t h i s case the o p t i c a l  the i r r a d i a t i o n .  p l o t t e d assuming t h a t the f a c t o r  The  data  /a i n  (1) remained constant d u r i n g the i r r a d i a t i o n .  Since  know from F i g u r e s 24 and 25 t h a t t h i s f a c t o r a c t u a l l y  decrease i t may  during i r r a d i a t i o n  be  (especially  f o r the h i g h e n e r g i e s ) ,  seen t h a t the l e f t hand s i d e s of the  F i g u r e 26 should be  curves i n  r a i s e d r e l a t i v e to the r i g h t  hand s i d e s .  Since the amount of t h i s i n c r e a s e i n l a r g e s t f o r the 3.5,  4.0  and 4.5  did  ev, i t would tend to make the  energies  shapes of a l l  curves more n e a r l y the same. Since the o p t i c a l a b s o r p t i o n at the b e g i n n i n g o f the i r r a d i a t i o n was  known, the t r u e y i e l d (Y) at t h i s time c o u l d  be  Then, by assuming t h a t the i r r a d i a t i o n  calculated.  the a b s o r p t i o n to a small v a l u e , c a l c u l a t e d from e q u a t i o n a lower l i m i t to / a , K  (4).  /a a f t e r the i r r a d i a t i o n  change i n y i e l d  c a l c u l a t e d therefrom w i l l be an upper l i m i t .  i n the two  was  S i n c e t h i s assumption g i v e s  the t o t a l f r a c t i o n a l  v a l u e s of t o t a l f r a c t i o n a l  reduced  Thus the true  change i n y i e l d l i e between those  columns of the t a b l e f o l l o w i n g .  Table I I I T o t a l F r a c t i o n a l Changes i n Y i e l d (%) Uncorrected Over-Corrected  Ene rgy (e.v.) 2.3  61  61  3.0  61  63  3.5  57  60  4.0  56  65  4.5  53  70  The  s i g n i f i c a n t p o i n t i l l u s t r a t e d by Table I I I i s  that the r e d u c t i o n i n photoconductive independent  o f energy.  yield  is-essentially  As d i s c u s s e d i n s e c t i o n C.10 t h i s  i m p l i e s t h a t the main e f f e c t o f 2.3 ev i r r a d i a t i o n i s to empty some l e v e l s which have been f i l l e d d u r i n g t h i s X-ray e x c i t a t i o n , thus reducing the e l e c t r o n i c B.11  range.  Sign o f the Charge C a r r i e r s In o r d e r t o determine  the s i g n of the o p t i c a l l y  induced  current c a r r i e r s , the f i e l d d i s t r i b u t i o n due t o the photoe l e c t r i c a l l y produced was s t u d i e d .  space  charge  r e g i o n s (see s e c t i o n C.5)  F o r each s p e c t r a l r e g i o n o f i n t e r e s t a c e n t r a l  r e g i o n (such as b i n F i g u r e 3(c) o f a c r y s t a l was i r r a d i a t e d w i t h the e l e c t r i c f i e l d a p p l i e d f o r p e r i o d s ranging from 20 minutes to s e v e r a l hours. to  When the photocurrent was reduced  a low value by t h e J f o r m a t i o n o f the space  charge  field,  the a p p l i e d v o l t a g e was removed, the two e l e c t r o d e s of the c r y s t a l were connected t o g e t h e r through the a m p l i f i e r i n p u t resistor  (see F i g u r e 1) and a narrow beam o f l i g h t o f the  27  same quantum energy was moved a c r o s s the c r y s t a l by means o f the  micrometer L (Figure 1 ) .  In t h i s way  the photocurrent  c o u l d be measured as a f u n c t i o n of the p o s i t i o n beam on the c r y s t a l . the  of the l i g h t  I f the photoconductive s e n s i t i v i t y o f  c r y s t a l were u n a f f e c t e d by the i r r a d i a t i o n , the p l o t o f  photocurrent versus p o s i t i o n  would a l s o be a p l o t o f space  charge f i e l d v e r s u s p o s i t i o n . induced changes  The e f f e c t s o f i r r a d i a t i o n -  in photosensitiviity  c o u l d be minimized i n  a number of ways and t h e r e f o r e approximate distributions  space charge  c o u l d be o b t a i n e d .  To determine the sign of the charge c a r r i e r s  produced  by quanta i n the 4.0 ev band, the f o l l o w i n g experiment performed:  field  An u n t r e a t e d s t o i c h i o m e t r i c specimen was  was  irradiated  over i t s whole volume f o r a p e r i o d o f 42 hours, w i t h no a p p l i e d electric field;  3.9 ev quanta were used f o r t h i s  to ensure that l i t t l e The  spectral  irradiation  a b s o r p t i o n o c c u r r e d i n the 5.0 ev band.  d i s t r i b u t i o n of p h o t o c o n d u c t i v i t y a f t e r  this  i r r a d i a t i o n has a l r e a d y been g i v e n i n F i g u r e 15, (curve B ) . An e l e c t r i c f i e l d was the  central  and  r e g i o n i r r a d i a t e d with 3.8 ev quanta to c r e a t e  a space' charge f i e l d . the  then a p p l i e d t o the c r y s t a l  same p o t e n t i a l  The e l e c t r o d e s were then brought t o  and the f i e l d d i s t r i b u t i o n was  determined  as d e s c r i b e d above, u s i n g a l i g h t beam 1/3 o f the width o f that used i n the i r r a d i a t i o n . shown i n F i g u r e 27, curve A.  This f i e l d d i s t r i b u t i o n i s The i r r a d i a t i o n was then con-  t i n u e d , u s i n g the wider beam, u n t i l the p h o t o c u r r e n t was again reduced to a small v a l u e .  The r e s u l t i n g  b u t i o n i s shown by curve B o f F i g u r e 27.  field  The most  distri-  28  significant  f e a t u r e of the l a t t e r curve i s the s h i f t of the  f i e l d minimum towards the n e g a t i v e e l e c t r o d e w i t h r e s p e c t to the minimum of curve A. of the  a reproducible feature  experiment.  A s i m i l a r experiment t h i s time u s i n g 4.6 a b s o r p t i o n band.  was  ev quanta  performed  on a d i f f e r e n t  crystal  i . e . i r r a d i a t i n g i n the 5.0  In t h i s case, however,  g i v e n an i n i t i a l o v e r a l l tivity.  T h i s was  the sample was  i r r a d i a t i o n to homogenize the  not  sensi-  For t h i s reason the photocurrent d i s t r i b u t i o n s  ob-  t a i n e d represented e l e c t r i c f i e l d d i s t r i b u t i o n s  o n l y when  the scanning beam was  irradiated  region.  w i t h i n the l i m i t s of the  The d i s t r i b u t i o n s  curve A a f t e r one  o b t a i n e d are shown i n F i g u r e 28,  i r r a d i a t i o n with e l e c t r i c f i e l d  curve B, a f t e r a subsequent  applied,  i r r a d i a t i o n w i t h no a p p l i e d  and curve C a f t e r a f u r t h e r i r r a d i a t i o n a l s o without ("No  ev  field,  field.  a p p l i e d f i e l d " always i m p l i e s t h a t the e l e c t r o d e s were  maintained at the same p o t e n t i a l . ) The  s i g n of the charge  the X-rayed The  crystal  c a r r i e r was  a l s o determined f o r  corresponding to curve D of F i g u r e 21.  f i e l d distributions  are shown i n F i g u r e 29.  In t h i s  case 4.4  ev quanta were used to form and to d e t e c t the  charge.  In t h i s experiment  the most s i g n i f i c a n t  space  feature i s  the appearance i n curve B of a minimum d i s p l a c e d toward positive  e l e c t r o d e from another minimum corresponding i n  position  t o the minimum i n curve  The  interpretation  in section G . l l .  the  A.  of these experiments  i s presented  29  C. C.l  DISCUSSION  O p t i c a l Absorption The u s u a l model used i n d i s c u s s i n g the p r o p e r t i e s o f  s o l i d s i n v o l v e s the i n t r o d u c t i o n of a p e r i o d i c p o t e n t i a l due t o a l l n u c l e i and a l l the e l e c t r o n s but one. The e i g e n v a l u e s and e i g e n f u n c t i o n s o f the remaining e l e c t r o n are then g i v e n by the s o l u t i o n s o f the S c h r o d i n g e r e q u a t i o n w i t h this potential.  The most important  f e a t u r e o f t h i s model i s  the occurrence o f q u a s i c o n t i n u o u s groups o f energy v a l u e s spearated by energy  eigen-  r e g i o n s which are f o r b i d d e n .  The d i s r u p t i o n of the p e r i o d i c p o t e n t i a l by some type of i m p e r f e c t i o n r e s u l t s i n the occurrence o f l o c a l i z e d l e v e l s i n the normally f o r b i d d e n r e g i o n .  energy  I t i s the e x c i t a t i o n  of e l e c t r o n s t o o r from such l e v e l s i n MgO t h a t i s the main concern o f t h i s  thesis.  Since the r e g i o n o f the c r y s t a l i n which the d e v i a t i o n from p e r i o d i c i t y o c c u r s ( h e r e a f t e r r e f e r r e d t o as the " c e n t e r " ) w i l l u s u a l l y have a s s o c i a t e d w i t h i t more than one energy 29 l e v e l i n the normally f o r b i d d e n band  , we may expect t h a t  t r a n s i t i o n s between these l e v e l s may be o f some  importance.  Thus one may expect t o f i n d a s s o c i a t e d w i t h the c e n t e r s c e r t a i n a b s o r p t i o n bands.  There are many examples o f t h i s O A  type o f a b s o r p t i o n i n the l i t e r a t u r e The  .  f a c t that bands occur r a t h e r than l i n e s ,  (as might  be expected f o r t r a n s i t i o n s between d i s c r e t e s t a t e s ) has been w e l l e x p l a i n e d as due t o i n t e r a c t i o n between the l o c a l 's  i z e d e l e c t r o n and the thermal v i b r a t i o n s of the l a t t i c e  "I  .  30  C a l c u l a t i o n s of the v a r i a t i o n of a b s o r p t i o n w i t h quantum energy have been made on a s i m i l a r b a s i s .  I t i s found that  under most circumstances the a b s o r p t i o n bands can be w e l l approximated by Gaussian curves. O p t i c a l t r a n s i t i o n s which take e l e c t r o n s between  local-  i z e d l e v e l s and energy bands of the p e r f e c t c r y s t a l are of course a l s o p o s s i b l e .  Numerous examples of t h i s type of 3?  a b s o r p t i o n have a l s o been r e p o r t e d  .  The a b s o r p t i o n i n  these cases has u s u a l l y been d e t e c t e d by p h o t o c o n d u c t i v i t y measurements.  Since the f i n a l s t a t e of the t r a n s i t i o n i s  not d i s c r e t e the a b s o r p t i o n may  be expected to extend over  a wider range of energy than i n the p r e v i o u s case and i n a d d i t i o n w i l l not be C.2  symmetrical about  some c e n t r a l  energy.  D i s c u s s i o n of the F a c t o r s I n v o l v e d i n the Photoconductive Yield In the a n a l y s i s of the p h o t o c o n d u c t i v i t y data d e s c r i b e d  i n s e c t i o n B.2,  a f a c t o r p was  i n t r o d u c e d t o represent the  p r o b a b i l i t y t h a t , f o l l o w i n g an e l e c t r o n i c e x c i t a t i o n , a f r e e charge c a r r i e r be formed.  Such a f a c t o r i s necessary i n o r d e r  to take account of the p o s s i b i l i t y that the e x c i t a t i o n may p l a c e , not to an energy band but r a t h e r t o another l e v e l b e l o n g i n g to the same a b s o r p t i o n c e n t e r . the formation of a f r e e charge p r o c e s s by which the p a r t i c l e  energy band.  discrete  In t h i s case,  carrier requires a  subsequent  i n the e x c i t e d s t a t e may  s u f f i c i e n t l y more energy t h a t i t may The e x t r a energy may  take  gain  be t r a n s f e r r e d to an  be o b t a i n e d by the a b s o r p t i o n  31  of l a t t i c e phonons, f o r example.  Under these  c o n d i t i o n s the  p h o t o c o n d u c t i v i t y may be expected t o c o n t a i n a strong temperat u r e dependence i n a c e r t a i n temperature i n t e r v a l . example o f such behavior  i s g i v e n by KC1 c r y s t a l s  A good containing  a s t o i c h i o m e t r i c excess o f potassium^. In c o n t r a s t t o the weak temperature dependence mentioned i n s e c t i o n B.6, Day^ has r e p o r t e d t h a t the p h o t o c o n d u c t i v i t y i n MgO decreased 90°K.  10^ times on lowering the temperature t o  Although we cannot determine d e f i n i t e l y from Day's  paper whether t h i s r e s u l t a p p l i e s to the u n t r e a t e d (as does the present  crystal  r e s u l t ) i t i s believed that i t a p p l i e s  to neutron i r r a d i a t e d samples so t h a t there may be not d i s p a r i t y between the two r e s u l t s . The  q u a n t i t y x i n t r o d u c e d i n equation  (1) was d e f i n e d  as the mean d i s t a n c e a charge c a r r i e r moves d u r i n g i t s l i f e time, when the e l e c t r i c f i e l d i s u n i t y . be  T h i s q u a n t i t y can  written x - u.T  where a. i s the m o b i l i t y o f the c a r r i e r s and T t h e i r mean lifetime.  The l a t t e r , o f course,  d e n s i t y and capture  i s determined by the  c r o s s - s e c t i o n o f l e v e l s i n which the  c a r r i e r s may be trapped.  T h i s t r a p p i n g may be temporary o r  permanent, i . e . , thermal r e l e a s e from the t r a p may  occur  w i t h i n the time o f a measurement o r o n l y a f t e r a time which i s l o n g compared t o the l a t t e r . photocurrent  In the former case, the  w i l l be observed t o i n c r e a s e d u r i n g the time  of the measurement.  Since no such "secondary" e f f e c t s were  32  noted i n these experiments, the t r a p p i n g i n MgO as permanent.  Indeed i t i s concluded  some e l e c t r o n s may  be trapped  In equation;(1)  we  may  be  regarded  i n a l a t e r section that  f o r many months.  have employed the f a c t t h a t a charge  e d i s p l a c e d a d i s t a n c e x i n the e l e c t r i c f i e l d w i l l be served e x t e r n a l l y as the passage of a charge ~ . f o l l o w s from simple be d e f e r r e d u n t i l  ob-  This fact  e l e c t r o s t a t i c s but the d e r i v a t i o n w i l l  s e c t i o n C.5  where the a p p r o p r i a t e  equations  are developed f o r another purpose. The e q u a t i o n s e c t i o n B.2 and  f o r the photoconductive  y i e l d developed i n  g i v e s f o r a g i v e n ( c r y s t a l , constant  s p e c t r a l r e g i o n of s t r o n g a b s o r p t i o n  electric  (equation  field,  ( 5 ) , the  proportionality Y~ —  K.  N Thus i f the ( /N) 1  r a t i o of photocurrent  to i n c i d e n t l i g h t  flux  i s p l o t t e d a g a i n s t quantum energy a d i s t o r t e d p i c t u r e  of the t r u e absorption, spectrum of the p h o t o i o n i z e d w i l l be o b t a i n e d  centers  since  L~ 1 N  It can e a s i l y be  seen that i n s p e c t r a l regions where  peaks i n the t o t a l a b s o r p t i o n ionization varies relatively minima i n /N X  may  be o b t a i n e d .  agreement between the present completely  due  K "  to t h i s cause.  (K) e x i s t and where the photoslowly w i t h energy,  spurious  I t i s b e l i e v e d t h a t the r e s u l t s and those of Day^  disare  A more d e t a i l e d comparison of  33  the two C.3  s e t s of r e s u l t s w i l l be g i v e n i n a l a t e r s e c t i o n .  The  Dependence of the Photocurrent  and E l e c t r i c The  on L i g h t I n t e n s i t y  Field  t h r e s h o l d e l e c t r i c f i e l d above which a l a r g e i n c r e a s e  i n dark c u r r e n t o c c u r r e d (see s e c t i o n B.3) by Day^  f  not  observed  although fieldsuupifeo 14,000 volts/cm were employed.  In the present work the c i r t i c a l The  was  f i e l d was  about 6,000 volts/cm.  reason f o r t h i s d i s c r e p a n c y i s not known although two  s i b i l i t i e s have been c o n s i d e r e d .  pos-  In the f i r s t p l a c e , the  e l e c t r o d e s used i n Day's work and the p r e s e n t work were Aguadag ( c o l l o i d a l g r a p h i t e a p p l i e d i n water suspension) s i l v e r paste, r e s p e c t i v e l y .  It i s d i f f i c u l t  f u n c t i o n f o r such m a t e r i a l s but  for  the two  were due  cases.  to d e f i n e a work  i t i s l i k e l y t h a t the  t i v e work f u n c t i o n " f o r s t r o n g f i e l d e m i s s i o n  "effec-  i s different  I f the sudden onset o f a l a r g e dairk c u r r e n t  to strong f i e l d e l e c t r o n e m i s s i o n from the  negative  e l e c t r o d e , the t h r e s h o l d f i e l d would be expected to be i n the two  and  different  cases.  Secondly,  although i n the work of Day  the c r y s t a l s were  measured i n dry a i r , i n most of the present measurements i t was  not found necessary to d e s i c c a t e the sample chamber d u r i n g  the measurement of the p h o t o c o n d u c t i v i t y s p e c t r a .  Consequently,  a s u r f a c e breakdown might occur at a s m a l l e r e l e c t r i c due  to a surface f i l m of e.g. In any  they  field  hydroxide.  case, whatever the cause of the h i g h dark c u r r e n t s  c o u l d be r e a d i l y a v o i d e d simply by m a i n t a i n i n g an e l e c -  tric field  l e s s than the c r i t i c a l v a l u e .  The  small dark  34  c u r r e n t which d i d f l o w under the l a t t e r circumstances d i d not cause measurable space  charge  development.  The experimental o b s e r v a t i o n o f the l i n e a r r e l a t i o n between photocurrent and e l e c t r i c f i e l d for  strength i s a j u s t i f i c a t i o n  equation (1) and the assumption  i m p l i c i t therein, viz.  that  none o f the e x c i t e d c a r r i e r s reach the e l e c t r o d e s , f o r i f they did, ing  a tendency  f o r the photocurrent t o s a t u r a t e with i n c r e a s -  f i e l d would have been observed.  Thus, knowing the d i s -  tance from the edge o f the i r r a d i a t i o n r e g i o n t o the e l e c t r o d e , an upper l i m i t on x may be o b t a i n e d . x < l(f for  untreated c r y s t a l s .  5  This l i m i t i s  cm /volt 2  S i m i l a r experiments  performed on X-rayed o r UV i r r a d i a t e d  have not been  specimens.  As mentioned above, x i s determined by the l i f e t i m e o f the e x c i t e d c a r r i e r s .  A c a r r i e r may end i t s l i f e by one o r  o t h e r of the f o l l o w i n g mechanisms: (see F i g . 30) (a)  Direct  recombination w i t h a f r e e c a r r i e r o f the  o p p o s i t e type, (b)  recombination w i t h a f r e e c a r r i e r o f the o p p o s i t e  type through some i m p e r f e c t i o n - c o n t r i b u t e d i n t e r m e d i a t e state, (c)  r e t u r n to a l e v e l o f the same type as that  from  which i t came o r , (d)  t r a p p i n g by a l e v e l o t h e r than t h a t from which i t  came. Process (a) ( F i g u r e 30(a) i s o f no importance  when e x c i t a -  t i o n occurs i n a s i n g l e a b s o r p t i o n band, assuming o f course,  35  t h a t the quanta are not s u f f i c i e n t l y e n e r g e t i c t o cause bandto-band t r a n s i t i o n s and thus create e l e c t r o n - h o l e p a i r s . the f o r b i d d e n energy r e g i o n i s 10.5 ev wide i s met  i n a l l the present measurements.  '  this  Since  condition  In the energy r e g i o n  of band o v e r l a p (Figure 6 ) , however, i t i s c o n c e i v a b l e t h a t both e l e c t r o n s and h o l e s may  be s i m u l t a n e o u s l y e x c i t e d and  direct  considered.  recombination must be  Such recombination,  however, l e a d s t o a n o n - l i n e a r r e l a t i o n s h i p between photoc u r r e n t and l i g h t B.2)  intensity  .  S i n c e we have  concludedi(section  t h a t , f o r the range of i n t e n s i t i e s used and the  spectral  r e g i o n i n v e s t i g a t e d , the p h o t o c u r r e n t was p r o p o r t i o n a l to light  i n t e n s i t y , we  conclude a l s o that d i r e c t  recombination  i s not a dominant mechanism. Recombination  through i n t e r m e d i a t e s t a t e s ( F i g u r e  30(b)  would a g a i n be important o n l y i n a s p e c t r a l r e g i o n where both f r e e e l e c t r o n s and f r e e h o l e s were b e i n g generated. For the low e x c i t a t i o n s o b t a i n e d i n these experiments,  a  l i n e a r dependence of photocurrent on l i g h t i n t e n s i t y would be expected i n t h i s case^.  Thus mechanism (b) cannot  be  r u l e d out by the same argument used t o e l i m i n a t e ( a ) . However, i f t h i s mechanism were important one would expect to f i n d an abnormal decrease i n photoconductive as the quantum energy t i o n band o v e r l a p .  response  reached the s p e c t r a l r e g i o n of absorp-  I t may  be seen from F i g u r e 6 t h a t such a  decrease was not observed i n u n t r e a t e d or vacuum heated crystals.  Therefore, i t i s b e l i e v e d that free hole-electron  recombination i s not an important mechanism i n such  crystals.  36  Since UV o r X-ray e x c i t a t i o n can h a r d l y be expected t o change the number o r type o f l e v e l s through which the recombination can occur the same c o n c l u s i o n may be a p p l i e d t o such  crystals.  The t h i r d mechanism l i s t e d above was the r e t u r n o f e x c i t e d c a r r i e r s t o the same type o f c e n t e r from which they came.  I f t h i s center had p r e v i o u s l y been emptied by o p t i c a l  excitation,  such a t r a p p i n g event would assure that the net  e f f e c t on the p r o p e r t i e s o f the c r y s t a l would be z e r o .  Since  the r e s u l t s o f s e c t i o n B.9 show that the photoconductive spectrum was a l t e r e d by prolonged UV i r r a d i a t i o n i n e i t h e r of  the two bands i t must be concluded t h a t t h i s type o f  t r a p p i n g event  i s not the o n l y one which can o c c u r .  Also,  simple c o n s i d e r a t i o n s show that t h i s mechanism l e a d s t o a n o n - l i n e a r dependence o f c u r r e n t o n l l i g h t  intensity.  For  t h i s reason one can make the s t r o n g e r c o n c l u s i o n that the t r a p p i n g o f c a r r i e r s i n a l r e a d y i o n i z e d c e n t e r s o f the type from which they came i s not the dominant mechanism.  I f on the o t h e r hand the e x c i t e d c a r r i e r s were t o  be t r a p p e d i n centers e x a c t l y s i m i l a r to  recombination  (i.e.  still  occupied)  those from which they came ( F i g u r e 3 0 ( c ) , a change i n  s p e c t r a l response would be expected and a linear^dependence on  i n t e n s i t y W o u l d be o b t a i n e d . T r a p p i n g by p r e v i o u s l y unoccupied l e v e l s ( F i g u r e 30(d)  would a l s o f u l f i l l mental  the two c o n d i t i o n s r e q u i r e d by the e x p e r i -  r e s u l t s and d i s c u s s e d above.  l e v e l s comprises a thermodynamically will  The occupancy  o f such  u n s t a b l e s i t u a t i o n which  r e v e r t t o the o r i g i n a l s t a t e , g i v e n enough time.  view o f the f a c t that  In  space charge d i s t r i b u t i o n s which l a s t  37  f o r c o n s i d e r a b l e p e r i o d s of time can be it  can be  concluded  formed ( s e c t i o n  t h a t at l e a s t some of the trapped  have r e l e a s e times much g r e a t e r than 0.5 cluded i n s e c t i o n C.10 remain t r a p p e d  recombination  empty, more shallow l e v e l s .  mechanisms are  c e n t e r s of the same other,  It i s t e n t a t i v e l y  that f o r untreated  con-  c r y s t a l s both  operative.  Evidence that the Photocurrents The  mechanisms seem to  from which they were e x c i t e d and/or by  cluded i n s e c t i o n C.6  C.4  I t i s con-  f o r s e v e r a l months.  be the t r a p p i n g of e x c i t e d c a r r i e r s by  normally  carriers  t h a t at l e a s t some of the e l e c t r o n s  Thus, the most l i k e l y  type as those  seconds.  B.5)  q u e s t i o n of p h o t o c u r r e n t s  are Due  to a Bulk E f f e c t  e x c i t e d i n surface f i l m s  (e.g. magnesium hydroxide) a r i s e s because of the low s e n s i t i v i t y of the MgO  c r y s t a l s to o p t i c a l i r r a d i a t i o n .  c u r r e n t s were primarily due  to surface conduction,  t i o n from the f r o n t s u r f a c e beam was  ( i . e . the  reduced.  contribu-  s i d e on which the  i n c i d e n t ) would remain constant  the specimen was  the  I f the  light  as the t h i c k n e s s of  I f the s u r f a c e l a y e r were to form  r a p i d l y ( i n c. 1 hour, say) then the c o n t r i b u t i o n from the back, f r e s h l y cleaved, face would be p r o p o r t i o n a l to the l i g h t i n c i d e n t on i t and would thus i n c r e a s e somewhat as successive p i e c e s of the specimen were c l e a v e d o f f . the q u a n t i t y /N X  Thus  would be expected to i n c r e a s e s l i g h t l y  the t h i c k n e s s of the specimen was  as  reduced.  On the other hand, i f the p h o t o s e n s i t i v e  surface layer  38  required a very by  the  first  l o n g time  line  of Table  c o n t r i b u t i o n from decrease in  by  less  t h i c k n e s s and If  both than  I w o u l d be  surfaces. a factor  constant,  Y was  the  ble  to  case  2 a f t e r the  X  contain a  /N  first  would reduction  thereafter.  c a l c u l a t e d y i e l d Y w o u l d be  independent q u a n t i t y /N 1  slightly  the o p t i c a l  d i d not  vary  i n e i t h e r of the  I).  The  g r e a t e r than the  for this variation.  crystal,  to  be  above  variation error.  as e v i d e n c e d  column, T a b l e  I t s h o u l d be  would not  observed  experimental  of the  absorption (last  volume n o n - u n i f o r m i t y  expected  a  of thickness.  However, t h e n o n - u n i f o r m i t y by  In t h i s  remain constant  d i s c u s s e d f a s h i o n s (see Table in  expected  represented  t h e m e a s u r e d p h o t o c o n d u c t i v i t y were e x c l u s i v e l y  bulk effect,  The  t o form, o n l y the t r i a l  I) may  be  emphasized t h a t  affect  also responsi  such  a  a surface photosensi-  tivity. It vides  i s b e l i e v e d t h a t the  convincing evidence  photocurrent  was  excited  further discussion w i l l C.5  Formation The  t h a t the major p o r t i o n of i n the  be  results presented  volume o f t h e  p r e s e n t e d on  o f Space C h a r g e  terms of a simple to  above d i s c u s s e d e x p e r i m e n t  this  pro-  the  crystal .  All  basis.  Fields  i n F i g u r e 3 can be  model, which w i l l  be  explained i n  d i s c u s s e d by  v  reference  Figure 3(c). The  YZ  planes  A p o r t i o n o f t h e XZ  of the plane  crystal was  constitute  the e l e c t r o d e s .  i l l u m i n a t e d w i t h a beam o f  39  l i g h t p a r a l l e l to the Y a x i s .  Upon i l l u m i n a t i o n , some e x c i t e d  c a r r i e r s t r a v e l t o the edge o f the i l l u m i n a t e d r e g i o n and even outside  it.  I f the range o f the c a r r i e r s i s small  compared  to the width o f the i r r a d i a t e d r e g i o n , the net r e s u l t o f the i r r a d i a t i o n with f i e l d  can be c o n s i d e r e d  as the formation o f  a sheet o f charge at e i t h e r side o f the i r r a d i a t e d r e g i o n .  2 Let the charge i n each sheet be + ne p e r cm . E(x)  Then i f  i s the e l e c t r i c f i e l d and e the d i e l e c t r i c constant, we  have from Poisson's e q u a t i o n AE  = + —  ne  (6)  e"  where AE i s the d i s c o n t i n u i t y i n the e l e c t r i c f i e l d at e i t h e r side o f the i r r a d i a t e d r e g i o n . % where; E regions,  &  •  a  l  +  E  b •  b  +  We a l s o have  \ •2 a  =  V  ( 7 )  and E^ are the e l e c t r i c f i e l d s i n t h e i r  respective  V i s the p o t e n t i a l d i f f e r e n c e between the e l e c t r o d e s  and  d i s the c r y s t a l t h i c k n e s s .  From equations ( 6 ) and (7)  and  the d e f i n i t i o n o f AE we o b t a i n  With the e l e c t r o d e s a t the same p o t e n t i a l  = - iHLSL e  b  When the c o n d i t i o n E  h  ( i - k)  ( 9 )  d  = - E ' has been a c h i e v e d , the photob  c u r r e n t s measured w i t h and without the a p p l i e d e l e c t r i c should be equal i n magnitude but opposite  i n sign.  field  The time,  40  T, at which t h i s c o n d i t i o n h e l d can be o b t a i n e d from F i g u r e and we have from  3(a)  (8) and ( 9 ) , n(T) =  ^ 8ire  For the experimental  (10) (d-b)  c o n d i t i o n s employed i t was  calculated  that n(T) = 8.7 Now  x 10  9  the f o r m a t i o n of t h i s space  electrons/cm . 2  charge  i s e q u i v a l e n t , as  f a r as the e x t e r n a l c i r c u i t  i s concerned,  of  s i d e of the i l l u m i n a t e d r e g i o n  a charge  n(T)Ae  from one  t o the  transfer  to the other, a d i s t a n c e b (A i s the c r o s s s e c t i o n a l a r e a of the c r y s t a l i n the YZ p l a n e ) . T  i.dt  The  = n(T) ^  (11)  d  reason f o r the f a c t o r — on the r i g h t hand s i d e of d  Equation 11 i s e x p l a i n e d l a t e r i n t h i s  section.  By i n t e g r a t i n g under the experimental the l e f t hand s i d e of Equation (11) was  curve A of F i g u r e  determined  and n(T)  c u l a t e d to be n(T) = 9.0 Thus the two  x 10  9 9  electrons/cm  v a l u e s of n(T) agree w i t h i n the  2  experimental  error. The  curves of F i g u r e 3 are approximately  character.  exponential i n  A simple a n a l y s i s would show t h a t they should be  3(a) cal-  41  p r e c i s e l y e x p o n e n t i a l i f the UV i r r a d i a t i o n had no e f f e c t on the c r y s t a l charge of  (except, o f course f o r the f o r m a t i o n o f the space  field).  I f the l a t t e r were the case the absolute v a l u e s  the p h o t o c u r r e n t s measured w i t h and without e l e c t r i c  field  should have a constant sum (see equations 8 and 9 ) .  Curves  C o f Figure 3(a) and (b) show t h a t t h i s i s not so.  From the  r e s u l t s of s e c t i o n B.9, however, an a c t i v a t i o n o f the c r y s t a l was t o be expected  i n t h i s experiment,  so that curves C o f  F i g u r e 3 should i n c r e a s e as a f u n c t i o n of time.  The maximum  i n F i g u r e 3(a) and the minimum i n F i g u r e 3(b) are not e x p l a i n e d . It  should be emphasized t h a t the presence  of t h i s  activa-  t i o n e f f e c t has ho i n f l u e n c e on the f o r e g o i n g c a l c u l a t i o n s of  space  charge d e n s i t y (equations (10) and (11).  comparison w i t h experiment  Indeed the  was done i n the above f a s h i o n i n  'order to e l i m i n a t e the e f f e c t o f the changing  photosensitivity  w i t h time of- i r r a d i a t i o n . In  the work o f Day^, i t was found t h a t when a narrow r e g i o n  i n the c e n t e r o f a c r y s t a l was i l l u m i n a t e d , the p h o t o c u r r e n t decreased i n i t i a l l y by a f a c t o r two, then remained constant for  many hours.  The i n i t i a l decrease may have been due t o the  development o f space  charge  f i e l d s as d e s c r i b e d above.  The  f a c t t h a t the current was not reduced t o zero i s i n agreement w i t h the r e s u l t s o b t a i n e d , i n c e r t a i n cases, i n the present experiments.  A p o s s i b l e explanation of t h i s non-reproducible  r e s u l t i s as f o l l o w s :  As the e l e c t r i c f i e l d  i n the i l l u m i n a t e d  r e g i o n i s decreased due t o the f o r m a t i o n o f the space t h a t i n the dark r e g i o n i s i n c r e a s e d (equation ( 7 ) .  charge, Thus, ,  42  depending on the magnitude o f the i n i t i a l e l e c t r i c f i e l d the r e l a t i v e magnitudes of b and d (equations the e l e c t r i c f i e l d c r i t i c a l value f i e l d was  i n the dark region may  ( s e c t i o n C.3)  (8),  i n c r e a s e to the  even though the  w e l l below t h i s v a l u e .  (7) and  and  i n i t i a l , uniform,  I f t h i s were the  case,  the  e q u i l i b r i u m c u r r e n t observed would c o n s i s t i n p a r t of "dark c u r r e n t " and the q u e s t i o n ,  r a i s e d by Day,  as t o the mechanism  a l l o w i n g the passage of continuous photocurrents r e v o l v e d i n t o the  same q u e s t i o n  concerning  would be  the passage of  continuous dark c u r r e n t s . From t h i s experiment i t may c o n d u c t i v i t y mechanism may  be  concluded t h a t the photo-  be the most obvious one,  v i z . , the  e x c i t a t i o n of c a r r i e r s to an energy band, t h e i r motion t h e r e i n , and t h e i r subsequent t r a p p i n g with  consequent formation  space charge l a y e r s at the boundaries of the  of  irradiated  region. From the above equations i t i s e a s i l y shown t h a t  the  motion of a charge between the e l e c t r o d e s i s measured by  the  e x t e r n a l c i r c u i t as a s m a l l e r charge by the  dis-  tance moved to the e l e c t r o d e i n equation  (1).)  To  separation.  see t h i s we  r a t i o of the  ( T h i s was  f i r s t apply Gauss' Theorem  to a r e c t a n g u l a r p a r a l l e l e p i p e d c o n t a i n i n g the l e f t e l e c t r o d e of the c r y s t a l  (see Figure  i t s f a c e s p a r a l l e l to t h i s e l e c t r o d e . f r i n g i n g of the e l e c t r i c f i e l d equations (8) and  (9) we eE^,  assumed  3(c) and having  hand two  of  Then i g n o r i n g the  (as i n the d e r i v a t i o n of  obtain  A = 4-n-q-^  (12)  43  where q^ i s the charge on the e l e c t r o d e .  A charge e i s  noxtf moved from x = a-^ t o x = a^ + b where a^ and b are arbitrary.  now  From (12) sA  (13)  and o f course the passage o f Aq-^ i s observed e x t e r n a l l y . Now  from (6) and  a  (7)  l  V +  d  4irneb e  and 4irne  b d  (14)  For the case under c o n s i d e r a t i o n the charge /cm be r e p l a c e d by e/A AE."  and combining  so t h a t  ali  , ne must  (14) becomes  4ire b_ eA ' d  (13) and  (15)  (15)  b Aq-^ = — . e d which i s the d e s i r e d C.6  (16)  result.  P r o p e r t i e s of C r y s t a l s as Received The two obvious a b s o r p t i o n peaks i n s p e c t r a such as  curve A, F i g u r e 4 were shown by Weber"^ to be  characteristic  of the presence of a s t o i c h i o m e t r i c excess of oxygen i n the crystal.  T h i s f a c t has been v e r i f i e d by Soshea^l and i n  a d d i t i o n he has shown t h a t such s p e c t r a a l s o c o n t a i n a peak  44  c e n t e r e d a t 4.8 ev which i s masked by those a t 4.3 and 5.7 ev. None o f these t h r e e bands were observed i n the photoconductivity  spectra.  Consequently i t must be concluded t h a t  the p r o b a b i l i t y p o f thermal i o n i z a t i o n o f the e x c i t e d  state  i s c o n s i d e r a b l y s m a l l e r than f o r the 4.0 and 5.0 bands which were observed i n p h o t o c o n d u c t i v i t y . x, i s s u f f i c i e n t l y  Since the mean range,  l a r g e for conduction by e i t h e r h o l e s o r  e l e c t r o n s t o be observed ( s e c t i o n C . l l ) i t cannot be argued t h a t t h i s i s the f a c t o r which prevents the o b s e r v a t i o n o f the three excess oxygen bands i n p h o t o c o n d u c t i v i t y measurements. Similarly  i n many of the specimens measured the t o t a l  t i o n was dominated by these bands.  absorp-  Thus as has a l r e a d y been  s t a t e d , p must be v e r y small f o r the excess oxygen c e n t e r s . Since i t i s reasonable t o expect t h a t the excess oxygen i s i n c o r p o r a t e d s u b s t i t u t i o n a l ^ as a d i v a l e n t  i o n , the  observed o p t i c a l t r a n s i t i o n s thus produced may be expected to r e s u l t from e l e c t r o n i c t r a n s i t i o n s t o l e v e l s which have been emptied t o complete the o u t e r s h e l l o f the i o n i c excess oxygen.  Thus, i f the t r a n s i t i o n s took p l a c e from the valence  band they might be v i s u a l i z e d as i n F i g u r e 31(a). it  However,  has been argued above t h a t the t r a n s i t i o n produces a f r e e  hole w i t h very small p r o b a b i l i t y .  Hence, we v i s u a l i z e  transi-  t i o n s such as t h a t i l l u s t r a t e d by F i g u r e 31(b). It has been observed by s e v e r a l workers"*"^'^'^  t h a t the  r a t i o o f the i n t e n s i t i e s o f the 4.3 and 5.7 ev bands i s  45  s t r i k i n g l y constant under r a t h e r w i d e l y v a r y i n g c o n d i t i o n s , while the 4.8 ev band has a d i f f e r e n t  intensity  r e l a t i v e to  the o t h e r two, depending on the c o n d i t i o n s of f o r m a t i o n o f the bands.  These f a c t s s t r o n g l y suggest t h a t the 4.3 and 5.7 ev  transitions  occur i n the same center while the 4.8 ev t r a n s i -  t i o n may be c h a r a c t e r i s t i c o f a d i f f e r e n t The  4.0 and 5.0 ev t r a n s i t i o n s  center.  observed  i n the photo-  c o n d u c t i v i t y measurements had not been observed p r e v i o u s l y . 13 Recently, however. Lye transition  has d e f i n i t e l y i d e n t i f i e d the 5.0 ev  i n the a b s o r p t i o n s p e c t r a o f s t o i c h i o m e t r i c  crystals  and has a l s o observed a s m a l l a b s o r p t i o n which may be i d e n t i f i a b l e w i t h the 4.0 ev t r a n s i t i o n .  The magnitude o f the  a b s o r p t i o n was very n e a r l y the same f o r a l l c r y s t a l s and had the approximate values  K  5 . 0 " °-  Q  7  c*'  1  .05 cm  These v a l u e s enable an e s t i m a t e o f the product xp to be made.  F o r t h i s the data o f F i g u r e 6 can be employed.  We o b t a i n  The  xp  = 2 x 10~  1 0  5 > 0  xp  3 x 10"  1 1  4 < 0  cm /volt. 2  (17)  cm /volt. 2  f a c t t h a t most o f the specimens l i s t e d i n Table I I show  v a l u e s o f the photoconductive  y i e l d o f the same o r d e r o f  magnitude i n s p i t e o f the f a c t t h a t they were cut from s e v e r a l  46  different  samples o b t a i n e d from two  very important  one.  The  different  sources, i s a  i m p l i c a t i o n s of t h i s f a c t w i l l  now  be d i s c u s s e d . In s e c t i o n C.2  i t was  concluded the most l i k e l y t r a p p i n g  mechanisms were (a)  Capture  of the f r e e c a r r i e r by a c e n t e r of the  same type as t h a t which i n i t i a l l y p r o v i d e d the (Figure 30(c) (b)  and,  Capture  of the f r e e c a r r i e r by p r e v i o u s l y unoccu-  p i e d , shallower l e v e l s Consider one.  We  carrier  ( F i g u r e 30(d).  f o r the moment t h a t mechanism (a) i s the dominant  have K<~  n  where n i s the d e n s i t y of the c e n t e r s i n q u e s t i o n . a c c o r d i n g to (a) above the same c e n t e r s dete range x, we  Since,  'the mean  have  1  x ^ **n T h e r e f o r e the y i e l d Y = xpK and  p  i s independent of the d e n s i t y of the c e n t e r s . I f , on the o t h e r hand, mechanism (b) were to dominate,  K and x would be independently  c o n t r o l l e d by the d e n s i t y of  a b s o r b i n g c e n t e r s and the d e n s i t i e s of the shallower respectively.  levels,  Thus the p o s t u l a t e of a constant number of  a b s o r b i n g c e n t e r s independent o f the c r y s t a l  source would not  47  s u f f i c e t o e x p l a i n the r e p r o d u c i b l e photoconductive  yield.  Since the p o s t u l a t e o f r e p r o d u c i b l e d e n s i t i e s of both absorbi n g c e n t e r s and shallow l e v e l s ("trapping  centers")  somewhat u n l i k e l y , we are l e d t o the h y p o t h e s i s t r a p p i n g mechanism i s ( a ) . by  That  seems  t h a t the main  (b) a l s o occurs  i s manifested  some v a r i a t i o n i n the y i e l d from c r y s t a l t o c r y s t a l , and  a l s o by the r e s u l t s o f the UV a c t i v a t i o n experiments d i s c u s s e d i n s e c t i o n C.9. F u r t h e r d i s c u s s i o n as t o the nature  o f the 4 and 5 ev  centers w i l l be d e f e r r e d u n t i l the s i g n o f the charge has been deduced from the experiments o f s e c t i o n B . l l .  carrier Lower  l i m i t s on the v a l u e s o f p can, however, be made, u s i n g eguation  (17) and the l i m i t x < 10  cm / v o l t g i v e n i n s e c t i o n  C.3. We o b t a i n  P .0 >  2  x  1  0  5  ~  5  (18) p  C.7  4 > 0  Heat Treatment  > 3 x 10"  6  Studies  O p t i c a l a b s o r p t i o n o c c u r r i n g near the fundamental edge 14 m  a l k a l i h a l i d e c r y s t a l s i s known  t i v e property.  t o be a s t r u c t u r e s e n s i -  The fundamental a b s o r p t i o n edge i t s e l f i s  b e l i e v e d t o be due t o e l e c t r o n i c t r a n s i t i o n s from ions to neighboring  p o s i t i v e i o n s , i n such a way t h a t the e l e c -  t r o n and i t s corresponding in interaction.  negative  " h o l e " on the negative  i o n remain  Thus the energy f o r t h i s t r a n s i t i o n i s lower  48  than that for complete removal of theselectron.  Since a l l  the ions of the lattice can contribute to this absorption, the absorption coefficient  is very high (10  - 10° cm  ).  However, at energies slightly less than that required for this "exciton" transition, the structure sensitive absorption referred to above is always found.  It has been suggested  that such absorption is due to exciton transitions in ions situated near crystalline imperfections such that a smaller energy is required to transfer the electron.  Experimentally  i t has been shown^ that the magnitude of the absorption is increased by plastic deformation which is believed to introduce dislocations and vacancies.  Thus a study of the optical  absorption near the fundamental edge may be considered useful in any investigation of imperfections in ionic crystals. The f i r s t fundamental absorption edge in MgO was located at about 7 . 5 ev by Johnson 16 formation by Krumhansl  .  15  and was later ascribed to exciton  In stoichiometric crystals a measur-  able absorption occurs, however, at energies as low as 4 ev (Figure 7, curve A) i . e . , at roughly one-half the exciton energy.  Thus the ultraviolet absorption begins much further  from the fundamental edge than i t does in typical alkali halide crystals. (a)  It can then be argued that either,  The ultraviolet absorption in stoichiometric MgO  is not due to "perturbed" exciton transitions but rather to absorption by impurity atoms, or, (b)  The absorption is due to perturbed exciton transi-  tions and crystalline imperfections have a much greater  49  i n f l u e n c e on the energy of the e x c i t o n t r a n s i t i o n i n than i n a l k a l i h a l i d e It does not two  MgO  crystals.  seem p o s s i b l e to disting<uisrfo between these  p o s s i b i l i t i e s on the b a s i s of the a v a i l a b l e e x p e r i m e n t a l  evidence. A comparison of curves A and B, F i g u r e  7 seems to  cate t h a t the magnitude of the background a b s o r p t i o n decreased by  c o o l i n g slowly  and thus a l l o w i n g the  indiwas  crystal  to  approach e q u i l i b r i u m at a temperature lower than 1400°C, the temperature of the  treatment.  t h a t the  shape of the a b s o r p t i o n  t u r e and  therefore  Curve C however  spectrum changes w i t h tempera-  i m p l i e s t h a t curve B was  e q u i l i b r i u m s t a t e between the T h i s i m p l i c a t i o n was  implied  merely a  non-  s t a t e s r e p r e s e n t e d by A and  f u r t h e r brought out by the  C.  subsequent  experiments, the d e t a i l s of which were o m i t t e d from s e c t i o n B.7. Such a change i n the a b s o r p t i o n f o r m a t i o n of new of the the  new  the  imperfections The  may  be  The  f a c t t h a t more  Since  formed from the o l d , e.g.  l a c k of i n f l u e n c e o f the  rate at which the new  with t h i s  not  t y p e s of i m p e r f e c t i o n s .  l a t t e r were formed at lower temperatures suggests t h a t  aggregation. on  spectrum i n d i c a t e s the  crystal  by  thickness  state i s achieved i s i n  accord  suggestion. the  s p e c t r a l dependence of p h o t o c o n d u c t i v i t y  change w i t h heat treatment i t must be  background a b s o r p t i o n photoconductivity.  did  concluded t h a t  the  makes a n e g l i g i b l e c o n t r i b u t i o n to  the  For t h i s reason we  have r e p r e s e n t e d  50  ( F i g u r e 31(d) the t r a n s i t i o n s i n v o l v e d i n such a way a b s o r p t i o n p r o c e s s w i l l l e a d t o no f r e e C.8  Excess Mg  that the  carriers.  Crystals  The experiments d e s c r i b e d i n s e c t i o n B.8  and  illustrated  by F i g u r e s 10, 11 and 12 show t h a t the o p t i c a l a b s o r p t i o n c h a r a c t e r i s t i c o f the s t o i c h i o m e t r i c c r y s t a l i s reduced by the  a d d i t i o n o f excess Mg.  The a c t u a l form o f the induced  o p t i c a l a b s o r p t i o n can t h e r e f o r e o n l y be deduced experiment the  from an  l i k e t h a t i l l u s t r a t e d by F i g u r e 12; assuming  a b s o r p t i o n decrease i s e s s e n t i a l l y complete  that  a f t e r the  first  treatment, the change on the second treatment, curve B, g i v e s the  a c t u a l s p e c t r a l dependence o f the o p t i c a l a b s o r p t i o n .  Thus i n F i g u r e 9, f o r example, the t o t a l a b s o r p t i o n spectrum a f t e r the h e a t i n g i n Mg vapor i s approximately the induced absorption It  spectrum.  i s then c l e a r from F i g u r e 9 t h a t the induced spec-  trum c o n t a i n s no obvious s t r n c t u r e and cannot be r e s o l v e d i n t o "bands".  For t h i s reason we p o s t u l a t e t h a t the spectrum  s i s t s of a s u p e r p o s i t i o n o f s e v e r a l bands; a s u f f i c i e n t in  f a c t , t h a t the composite  connumber,  a b s o r p t i o n curve shows none of the  extrema nor double i n f l e c t i o n s which would be expected f o r the  s u p e r p o s i t i o n of bands c e n t e r e d about e n e r g i e s s u f f i c i e n t l y  different  from one  another.  The most obvious manner i n which the excess metal can be accommodated i n t o the l a t t i c i s s u b s t i t u t i o n a l ^ as doubly charged p o s i t i v e  ions.  Thus the e x t r a a b s o r p t i o n may  be  a s c r i b e d t o the f i l l i n g of c e r t a i n unoccupied l e v e l s by the  51  extra electrons.  Such a set o f l e v e l s i s shown i n F i g u r e  Since the f i l l i n g of these  l e v e l s reduced the background absorp-  t i o n as d i s c u s s e d above, we with each of them.  The  31(d).  a s s o c i a t e a f i l l e d ground s t a t e  r e d u c t i o n of the background a b s o r p t i o n  then f o l l o w s i f t r a n s i t i o n s such as t h a t i l l u s t r a t e d i n F i g u r e 34(d)  are  supposed to account f o r t h i s a b s o r p t i o n ,  as  mentioned i n the p r e v i o u s s e c t i o n . It might be  concluded  from the above t h a t when the back-  ground a b s o r p t i o n has e s s e n t i a l l y disappeared  (i.e.,  the  empty l e v e l s have been f i l l e d ) the t o t a l a b s o r p t i o n w i l l  not  change f u r t h e r .  con-  Experimentally,  however, the a b s o r p t i o n  t i n u e s to i n c r e a s e a f t e r the i n i t i a l f o r e necessary  decrease.  It i s there-  to assume t h a t the process of adding excess  Mg a l s o p r o v i d e s the l e v e l s i n which the e x t r a e l e c t r o n s are to  be assommodated.  T h i s may  by the d i f f u s i o n of negative the  i o n vacancies  i n t o the body of  crystal. If the p h o t o c o n d u c t i v i t y  to  be accomplished, f o r example,  ( F i g u r e 13)  i s to be  ascribed  the centers r e s p o n s i b l e f o r the b u l k of the o p t i c a l absorp-  t i o n ( F i g u r e 9), i t can be absorption  concluded  t h a t the low energy-  c o n t r i b u t e s very l i t t l e to the  T h i s low energy a b s o r p t i o n then cannot be that d i s c u s s e d i n connection C.10).  The  l a t t e r absorption  photoconductivity. identified  with  w i t h X-rayed c r y s t a l s ( s e c t i o n c o n t r i b u t e d a measurable photo-  conductivity. . The ever,  major f r a c t i o n of the o p t i c a l a b s o r p t i o n may,  c o n t r i b u t e nothing to the p h o t o c o n d u c t i v i t y .  how-  In t h i s  52  case two  s e t s of l e v e l s such as t h a t shown i n F i g u r e  would have to be p o s t u l a t e d .  One  o f these would be  31(d) required  to .account f o r the a b s o r p t i o n spectrum as d i s c u s s e d above while the o t h e r would account f o r the s t r u c t u r e l e s s p h o t o c o n d u c t i v i t y spectrum. None of the evidence between the above two trarily  so f a r p r e s e n t e d a l l o w s one  alternatives.  s e l e c t the f i r s t p o s s i b i l i t y  i n t r o d u c t i o n of fewer l e v e l s .  t o choose  At t h i s p o i n t we  arbi-  s i n c e i t r e q u i r e s the  As mentioned above, the  low  energy a b s o r p t i o n then must be a s c r i b e d to t r a n s i t i o n s which do not  result in free carriers.  No attempt has been made  to i n d i c a t e such t r a n s i t i o n s on F g i u r e 31(d).  It i s believed  that the t r a n s i t i o n s g i v i n g the p h o t o c o n d u c t i v i t y (and  accord-  i n g to our p o s t u l a t e the a b s o r p t i o n ) at higher e n e r g i e s may i d e n t i f i e d with t r a n s i t i o n s observed o t h e r ways. C.9  be expected  corrsponding  l i g h t of an a p p r o p r i a t e  c a r r i e r s i n shallower l e v e l s or i n o t h e r ,  were d i s c u s s e d e a r l i e r  These two  ( s e c t i o n G.3).  of i r r a d i a t i o n may  e f f e c t o f the  A decrease  due  pro-  y i e l d at the energy  be  c e n t e r s which can be (b)  as  t r a p p i n g mechanisms  The  i r r a d i a t i o n on the photoconductive  (a)  C.10.  to empty c e r t a i n l e v e l s , p l a c i n g the  yet u n e x c i t e d , deeper l e v e l s .  longed  and  Irradiation  I r r a d i a t i o n with u l t r a v i o l e t energy may  in crystals treated in  These are d i s c u s s e d i n s e c t i o n C.9  Ultraviolet  be  to a d e p l e t i o n i n the number of  i o n i z e d by t h i s quantum energy o r  An i n c r e a s e , i n s p i t e of ( a ) , due  to the  fact  53  that the d e n s i t y of t r a p p i n g centers i s decreased and the  range  The  occurrence of  range be  hence  increased. (a) would r e q u i r e d , of course,  that  the  l i t t l e a f f e c t e d by the f i l l i n g of t r a p s , i . e . , t h a t  the d e n s i t y of i o n i z a b l e centers be much l e s s than the of t r a p p i n g for  ccent'ersj. ^Experimentally,  i r r a d i a t i o n i n e i t h e r the 4.0  can be  however, (b) was  or 5.0  ev bands.  concluded t h a t the d e p l e t i o n of the  i s more than o f f s e t by the i n c r e a s e  ionizable  (5 ev)  Thus i t  ionizable  centers the  t h i s i t follows that  centers i s l e s s than the d e n s i t y of  c e n t e r s and "therefore >ithat the l a t t e r  themselves provide  observed  i n range a f f o r d e d by  f i l l i n g of the t r a p p i n g c e n t e r s . i s F r o m the d e n s i t y of t r a p p i n g  density  cannot  the o n l y t r a p p i n g mechanism.  A s i m i l a r c o n c l u s i o n f o l l o w s from the e f f e c t of the  pro-  longed i r r a d i a t i o n on the photoconductive y i e l d at quantum energies  other than t h a t of the i r r a d i a t i o n .  r e l a t i v e i n c r e a s e t h a t o c c u r r e d at the 15)  lower e n e r g i e s  can be e x p l a i n e d by the i n c r e a s e d occupancy of  which would be empty i f the The 15,  For the  levels  simply be due  spectrum o f  The  be e x p l a i n e d by the  s u p e r p o s i t i o n of 4 or 5 bands w i t h  to 0.8  ev.  experimental  The  Figure  to presence of a number of  o v e r l a p p i n g bands.  widths o f 0.6  (Figure  c r y s t a l were i n thermal e q u i l i b r i u m .  l a c k of obvious s t r u c t u r e i n the  curve C, may  large  curves c o u l d  probably  low energy s e c t i o n of curve B,  however, does not appear to r e q u i r e the assumption of as many levels.  T h i s spectrum appears to show that no l e v e l s  are  f i l l e d which have o p t i c a l e x c i t a t i o n e n e r g i e s as low as some  54  of those which can be f i l l e d by the 5.0 ev i r r a d i a t i o n . According  t o F i g u r e 14, the e f f i c i e n c y o f the a c t i v a t i o n  i n c r e a s e d a b r u p t l y when the energy o f i r r a d i a t i o n was changed from 3.4 t o 3.9 ev.  T h i s , o f course,  i s i n agreement  with  the p r e v i o u s l y determined s p e c t r a l dependence o f p h o t o s e n s i tivity  (see F i g u r e 6 ) . The d i f f e r e n c e between curves  5 and  6 was not expected but i n view o f the f a c t t h a t curves 5 and  6 were brought about by i r r a d i a t i o n s i n the 5 ev band,  f o l l o w i n g p r e v i o u s i r r a d i a t i o n s i n the 4 ev band, we may expect the s i t u a t i o n t o be q u i t e complicated.  T h i s w i l l be  e s p e c i a l l y true i f e x c i t a t i o n i n the two bands does not l e a d to  the same type o f c a r r i e r .  tivity  In f a c t , since the photoconduc-  s p e c t r a induced by the two i r r a d i a t i o n s were not the  same, i t may be concluded  that the two e x c i t a t i o n s do not  produce the same type o f c a r r i e r .  T h i s c o n c l u s i o n i s strengthened  by the r e s u l t s o f s e c t i o n B . l l . From the a b s o r p t i o n change which o c c u r r e d under 5.0 ev irradiation  (Figure 16) i t i s e v i d e n t t h a t such i r r a d i a t i o n  i n t r o d u c e d the c e n t e r s c h a r a c t e r i s t i c o f excess oxygen ( c f . Figure 4 ) .  Now a c c o r d i n g t o the d i s c u s s i o n to be g i v e n i n i  s e c t i o n C . l l , the 5.0 ev t r a n s i t i o n l e a d s t o f r e e e l e c t r o n s , so t h a t the o n l y obvious t r a n s i t i o n which c o u l d l e a d t o the formation  o f the excess oxygen c e n t e r s i s t h a t i n d i c a t e d i n  F i g u r e 31(e).  T h i s model a s s o c i a t e s the:5 ev t r a n s i t i o n  a p o t e n t i a l excess oxygen center.  with  The e x c i t a t i o n o f t h i s  center by the a b s o r p t i o n o f a 5 ev quantum and i t s subsequent  55  thermal i o n i z a t i o n would r e s u l t i n the Figure  31(c)  which we  a s c r i b e d to the excess oxygen  It should be noted that the i n Figure region.  16 has  l e v e l c o n f i g u r a t i o n of  induced a b s o r p t i o n  c o n d u c t i v i t y i n t h i s part of the  the photo-  spectrum ( F i g u r e 15,  to s e c t i o n C . l l the  l a t t e r i s not v e r y  Moreover, the a b s o r p t i o n  17  In any  shows t h a t there  obtained  specific  i s some d i f f e r e n c e between the  vapor.  s a i d w i t h c e r t a i n t y i s t h a t the  produced by the excess centers One  other  be mentioned.  (Figure suffi-  case, a comparison of Figures  and t h a t induced by h e a t i n g  can be  the  i n these measure-  t i o n induced a b s o r p t i o n A l l that  origin  since  change on i r r a d i a t i o n was  c i e n t l y small t h a t poor accuracy was  C).  ev  It i s not p o s s i b l e to a s s o c i a t e the  absorption.spectrum of the  and  curve  i r r a d i a t i o n at 4.0  o f these holes w i t h p o t e n t i a l metal-excess c e n t e r s ,  ments (Figure 17).  with  It i s suggested t h a t i t can  be a s c r i b e d to those centers which a l s o provide  creates free holes.  visible  spectrum cannot be a s s o c i a t e d  the excess oxygen type c e n t e r s .  9).  shown  a long t a i l e x t e n d i n g w e l l i n t o the  T h i s p a r t of the  According  center.  9 irradia-  i n Mg holes  4 ev r a d i a t i o n are not t r a p p e d to form oxygen(Figure  17).  important f e a t u r e of curve B,  15,  should  I r r a d i a t i o n i n the 4 ev band reduced the  photo-  conductive y i e l d i n the  5 ev band.  Figure  It seems reasonable to  suppose that the h o l e s , trapped i n shallower  levels after  b e i n g e x c i t e d by 4 ev r a d i a t i o n , can then p r o v i d e t r a p p i n g mechanism f o r e l e c t r o n s e x c i t e d by  an a d d i t i o n a l  5 ev r a d i a t i o n .  56  Thus the e l e c t r o n range would be reduced and the magnitude o f the y i e l d i n the 5 ev band would decrease as observed. To summarize, we  suppose t h a t i r r a d i a t i o n i n the 5 ev  band e x c i t e s e l e c t r o n s to the conduction band and t h a t at least  some of these are subsequently trapped i n a number o f  p r e v i o u s l y unoccupied range  l e v e l s (see F i g u r e 31(e).  The electron  i s thereby i n c r e a s e d and i n a d d i t i o n , s i n c e o p t i c a l  e x c i t a t i o n of these shallow l e v e l s i s now p o s s i b l e ,  photo-  conduction i s o b s e r v a b l e at lower quantum e n e r g i e s .  At the  same time e x c i t a t i o n to the s t a t e s emptied by the i r r a d i a t i o n becomes p o s s i b l e and an i n c r e a s e i n o p t i c a l a b s o r p t i o n i n the excess oxygen bands i s observed  ( F i g u r e 16).  On the o t h e r hand, i r r a d i a t i o n i n the 4 ev band c r e a t e s f r e e h o l e s which are than t r a p p e d by l e v e l s p r e v i o u s l y o c c u p i e d by e l e c t r o n s .  ( F i g u r e 31(f)  The p o t e n t i a l  excess  oxygen type c e n t e r s do not, however, take p a r t i n the hole trapping process. C.10  X-Ray I r r a d i a t i o n The X-ray induced a b s o r p t i o n spectrum of F i g u r e 19,  curve A, shows c l e a r l y the 5.7, 4.3 and 2.3 ev bands. Weber  10  has shown that the 4.8 ev band i s a l s o p r e s e n t and  t h i s r e s u l t has been confirmed by S o s h e a . 1 1  The  latter  author has a l s o o b t a i n e d evidence t h a t there i s more absorpt i o n t o the h i g h energy  side of the 5.7 ev peak than can be  accounted f o r by a symmetrical band centered at 5.7 ev.  It  appears, t h e r e f o r e , that the 5.2 ev band, found by J o h n s o n  1 5  i n excess oxygen c r y s t a l s , may a l s o be produced by X - i r r a d i a t i o n .  57  Thus the spectrum of F i g u r e 19, but one,  curve A,  contains a l l , or a l l  of the bands c h a r a c t e r i s t i c of excess oxygen p l u s  some e x t r a a b s o r p t i o n l y i n g i n the v i s i b l e trum.  The band at 2.3  ev i s e a s i l y d e f i n e d but  between the l a t t e r and the 4.3 resolved structure.  r e g i o n of the i n the  ev band there i s no  remains i n the  obvious  same s p e c t r a l r e g i o n some  a b s o r p t i o n of r a t h e r small magnitude.  by  ev band  1 1  there  region  In f a c t , i t has been shown, again  S o s h e a , t h a t a f t e r the thermal decay of the 2.3  spec-  unresolved  Consequently, i t  appears t h a t the a b s o r p t i o n , over and above that a l s o found i n excess oxygen c r y s t a l s , c o n s i s t s of the 2.3  ev band p l u s  a small more or l e s s continuous  a b s o r p t i o n extending  about 2 ev to at l e a s t 3.6  T h i s may  ev.  be due  to the  p o s i t i o n of a number of r e l a t i v e l y broad bands as i n the p r e v i o u s mens ( c f . Figure As  from super-  suggested  s e c t i o n i n c c o n n e c t i o n with. UV e x c i t e d s p e c i 15).  shown i n F i g u r e 20 most of the v i s i b l e  absorption  decays r a t h e r r a p i d l y at room temperature while the v i o l e t a b s o r p t i o n decays r a p i d l y at f i r s t  ultra-  then more and more  slowly u n t i l the decay rate becomes immeasurably  small.  These bands were observed to the extent of 40 percent  of t h e i r  s a t u r a t i o n i n t e n s i t y 4 months a f t e r t h e i r f o r m a t i o n .  Thus  some of the  c e n t e r s appear to have higher thermal a c t i v a t i o n  e n e r g i e s than o t h e r s .  T h i s type  of o b s e r v a t i o n has been made  p r e v i o u s l y i n the sase of X-rayed a l k a l i hal'ide  crystals . 1 8  I t has been v a r i o u s l y assumed f o r the l a t t e r t h a t (a)  The  thermal b l e a c h i n g o c c u r s by the thermal r e l e a s e  58  of  a trapped electron- and i t s subsequent recombination w i t h a  trapped h o l e . be  Both the e l e c t r o n s and h o l e s are b e l i e v e d t o  connected w i t h c e n t e r s which evidence themselves  a b s o r p t i o n spectrum. different  i n the  Thus t h e decays o f the a b s o r p t i o n i n  s p e c t r a l regions must be c o r r e l a t e d .  An a l t e r n a t i v e decay mechanism i s (b)  The recombination  by the quantum mechanical  o f trapped e l e c t r o n s and h o l e s  tunneling process.  The decay o f  the v a r i o u s a b s o r p t i o n bands must a g a i n be c o r r e l a t e d . With h y p o t h e s i s (a) the l a r g e v a r i a t i o n of decay r a t e w i t h time has been taken t o imply t h a t the c e n t e r s have d i f f e r e n t thermal  ionization energies i n spite of t h e i r  common  18 o p t i c a l e x c i t a t i o n energy.  Seitz  , however, p r e f e r s hypo-  t h e s i s (b) and i n t h i s case e x p l a i n s the decay r a t e v a r i a t i o n by a non-uniform  d i s t r i b u t i o n o f trapped e l e c t r o n s and h o l e s .  Such a d i s t r i b u t i o n o f course, l e a d s t o a wide d i s t r i b u t i o n in  the d i s t a n c e s between c e n t e r s o f o p p o s i t e type and t h e r e -  f o r e a l s o t o l a r g e v a r i a t i o n s i n the t u n n e l i n g p r o b a b i l i t y . Postulate  ( b ) , however, does not seem t o e x p l a i n i n a  simple f a s h i o n the commonly observed dependence o f t h e thermal s t a b i l i t y o f tie X-ray  induced c e n t e r s on the temperature a t  which the i r r a d i a t i o n i s c a r r i e d out. for  e x a m p l e , that samples i r r a d i a t e d a t low temperature 19  a higher i n i t i a l  decay rate a t room temperature  i r r a d i a t e d a t room temperature. 20 o b t a i n e d by S t u r t z at  I t i s g e n e r a l l y found,  room temperature  than  hawe  those  A s i m i l a r r e s u l t was r e c e n t l y  f o r MgO c r y s t a l s .  The i n i t i a l  decay r a t e  was found t o be much g r e a t e r f o r c r y s t a l s  59  irradiated  at that temperature than f o r those  higher temperatures. qualitatively  irradiated  I t i s b e l i e v e d t h a t these  at  f a c t s may  be  e x p l a i n e d by the model developed l a t e r i n t h i s  section. The was  not  e f f e c t of X-rays on the p h o t o c o n d u c t i v i t y c o r r e l a t e d with the  o b t a i n e d by the  absorption  same means (Figure 19).  to p o s t u l a t e t h a t the p h o t o c o n d u c t i v i t y the u n r e s o l v e d  (Figure  change at h i g h I t seems  21)  energies  reasonable  i s associated with  absorption discussed e a r l i e r i n t h i s s e c t i o n .  fi's^shown i n F i g u r e 23, the p h o t o c o n d u c t i v i t y induced by prolonged  i r r a d i a t i o n i n the  5.0  spectrum  ev band i s  similar  to t h a t of an X-rayed specimen which had undergone a p a r t i a l thermal decay (by " s i m i l a r " we  imply  t h a t the s p e c t r a  o n l y by a f a c t o r which i s independent of quantum Since the former i r r a d i a t i o n trapped  The  direct  type  type of c a r r i e r  decay very r e a d i l y  determination  of c a r r i e r  Thus the p h o t o i o n i z a b l e trapped  ev  (Figure 22).  suppose t h a t these  trapped  due holes  resolved absorption  It then seems reasonable  trapped holes d i s a p p e a r by  w i t h e l e c t r o n s from the 2.3 activation  type  at room temperature, at a rate which i s  comparable to that of the decay of the band at 2.3  of  capable  ( s e c t i o n C . l l ) shows t h a t i n both cases the s p e c t r a are to t r a p p e d e l e c t r o n s .  a  t h a t the p a r t i a l l y decayed  X-rayed c r y s t a l a l s o c o n t a i n s o n i y one of p h o t o i o n i z a t i o n .  energy)  can o n l y introduce one  c a r r i e r i t i s concluded  differ  ev.centers.  to  recombination  Since the  optical  energy of the l a t t e r i s s m a l l e r than t h a t f o r the  holes  (Figure 15,  curve B)  i t i s a l s o reasonable  to  60  suppose t h a t the rate of decay of both types of c e n t e r s i s determined by the thermal i o n i z a t i o n rate of the 2.3 ev c e n t e r s . The disappearance of the t r a p p e d h o l e s can, on the b a s i s of  p r e v i o u s c o n s i d e r a t i o n s , be expected t o have two  consequences.  In s e c t i o n C.9  i t was  other  concluded that the p r o -  d u c t i o n of trapped h o l e s p r o v i d e d an a d d i t i o n a l t r a p p i n g mechanism which a p p r e c i a b l y reduced the e l e c t r o n range.  Thus  i n the present case the decay of the t r a p p e d h o l e s can be expected to i n c r e a s e the photoconductive y i e l d i n t h a t r e g i o n of  the spectrum where e l e c t r o n i c e x c i t a t i o n predominates, i . e .  above 4„ev.  Secondly, a decrease i n y i e l d can be  expected  at  lower e n e r g i e s because  the above d i s c u s s e d recombination  of  shallow t r a p p e d e l e c t r o n s w i t h the t r a p p e d h o l e s l e a v e s  fewer c e n t e r s which can be i o n i z e d by quanta region.  in this  energy  Both of the r e q u i r e d f e a t u r e s are shown by F i g u r e 21  (compare curves B and D). A f t e r the r e l a t i v e l y r a p i d p r o c e s s e s d i s c u s s e d above have taken p l a c e the UV a b s o r p t i o n bands can o n l y decay by  combina-  t i o n withhthermally r e l e a s e d e l e c t r o n s from the c e n t e r s g i v i n g r i s e to the remaining photoconduction. tivity  Since the  spectrum must be due t o a number of d i f f e r e n t types of  c e n t e r s , the decay be complicated.  curves f o r the UV bands may  be expected t o  In f a c t since each type of trapped e l e c t r o n  w i l l be expected t o decay monomolecularly constant, the decay of  photoconduc-  w i t h i t s own  time  curve should be r e s o l v a b l e i n t o a number  simple e x p o n e n t i a l terms.  An a n a l y s i s of t h i s type has  been g i v e n f o r the decay of X-ray induced luminescence  i n KBr.  61  by W i l l i a m s e t a l  , who  a p p a r e n t l y have l o c a t e d s e v e r a l l e v e l s  i n which c a r r i e r s can be t r a p p e d .  A s i m i l a r a n a l y s i s of the  decay of o p t i c a l a b s o r p t i o n i n MgO  i s shown i n F i g u r e 32 where  3 c h a r a c t e r i s t i c time  c o n s t a n t s are a l s o g i v e n .  In o r d e r t o  c a l c u l a t e thermal i o n i z a t i o n e n e r g i e s from these, the value of ~V  o  99  i n the e x p r e s s i o n 1  - = v  r  must be known.  °  -E/kT  e  Here T i s the time constant t o be a s s o c i a t e d  with a thermal  i o n i z a t i o n energy  E.  Table IV shows the  c u l a t e d v a l u e s of E f o r v a r i o u s assumed v a l u e s o f v  cal-  . o  Table  IV  C a l c u l a t e d Thermal I o n i z a t i o n E n e r g i e s of  ^ 1230  = 10  9  the Shallow E l e c t r o n  sec"  1  v »10 sec~ 1 2  o  1.07  ev  ev  1  Traps  "10  sec  hrs.  .90  1.19  44 h r s .  .82  1.00  1.10  5 hrs.  .76  0.93  1.05  ev  Thus the t r a p s which decay i n reasonable times at room temperature of  1 ev.  have thermal a c t i v a t i o n e n e r g i e s i n the  The  vicinity  o p t i c a l i o n i z a t i o n e n e r g i e s s h o u l d be somewhat  g r e a t e r than t h e s e .  We  may  estimate on the b a s i s of v a r i o u s  62  t h e o r e t i c a l and experimental  results  23  t h a t the o p t i c a l  energies  should be grouped around 2 ev f o r these, the s h a l l o w e s t  traps,  which can be f i l l e d by e x c i t a t i o n at room temperature.  Then  the p h o t o c o n d u c t i v i t y  i n an X-rayed ( i . e . , t r a p s  filled)  specimen should begin to drop s h a r p l y i n t h i s energy F i g u r e 22  shows t h a t t h i s i s approximately  region.  so.  Those t r a p s which have thermal i o n i z a t i o n e n e r g i e s bly  g r e a t e r than 1 ev  apprecia-  ( o p t i c a l e n e r g i e s g r e a t e r than about 2  ev)  do not decay to a measurable extent at room temperature. Consequently, n e i t h e r the magnitude nor the of the p h o t o c o n d u c t i v i t y  s p e c t r a l dependence  i n the h i g h energy r e g i o n would be  expected to change with time, once the t r a p p e d holes and shallow t r a p p e d e l e c t r o n s have e s s e n t i a l l y To  disappeared.  summarize, the proposed model a t t r i b u t e s the  i n c r e a s e i n photoconductive  the  y i e l d induced by  large  X-irradiation  to (a)  A l a r g e i n c r e a s e i n x (see  meaning of the  An  f o r the  symbols) because many of the e l e c t r o n and  hole t r a p s are f i l l e d (b)  s e c t i o n B.2  and  i n c r e a s e i n the  the newly f i l l e d  because e x c i t a t i o n from  l e v e l s becomes possible.  s p e c t r a l dependence (curves B, C, D,  The  change i n  Figure 21) w i t h  thermal  decay i s a t t r i b u t e d to the l i b e r a t i o n of trapped  h o l e s which  a l s o a n n i h i l a t e the e l e c t r o n s which gave the 2.3  ev band  absorption  (Figure 22).  The  f a c t t h a t the y i e l d at h i g h  e n e r g i e s a c t u a l l y i n c r e a s e s d u r i n g the thermal decay D, F i g u r e 21) may  be evidence  (curve  t h a t the trapped h o l e s act  63  as e l e c t r o n t r a p s as p r e v i o u s l y suggested.  An i n c r e a s e i n  x f o r e l e c t r o n s would then occur as the h o l e s decayed. The  experiment i l l u s t r a t e d by F i g u r e 23 i s i n a c c o r d  with the above model i n t h a t i t shows t h a t a specimen e x c i t e d i n the 5.0  ev band (curve A)  same s t a t e C (except X-irradiation, decay to take The  by  the  simply  a l l o w i n g a few hours of thermal  place.  y i e l d by approximately  at a l l e n e r g i e s  (Figure 26)  to a decrease i n x.  ev reduced the photo-  the  same f r a c t i o n a l amount  i m p l i e s t h a t the r e d u c t i o n i s due  T h i s decrease presumably occurs because  ev quanta i o n i z e  some of the  shallow f i l l e d  thus p r o v i d i n g more l e v e l s f o r t r a p p i n g . these  returned t o  f o r an i n c r e a s e i n x a f t e r a subsequent  f a c t t h a t i r a d i a t i o n at 2.3  conductive  the 2.3  can be  The  levels  e l e c t r o n s from  l e v e l s can re combine with the trapped h o l e s i n the  excess oxygen type  levels  (Figure 3 1 ( c ) , thus reducing  o p t i c a l a b s o r p t i o n i n the 5.7, 24).  Since the photoconductive  p r e f e r e n t i a l l y bleached,  4.8  and  4.3  ev bands (Figure  y i e l d at 2.3  i t must be  the  concluded  ev was  not  that only a  small f r a c t i o n of the f i l l e d l e v e l s o f t h i s energy were emptied in  the  course  of t h i s experiment.  must, of course,  The  few e x t r a empty  i n c r e a s e the t r a p p i n g p r o b a b i l i t y  to decrease the range by  60 percent  levels  sufficiently  (Table I I I ) .  Although F i g u r e 25 shows a r a t h e r l a r g e f r a c t i o n a l decrease in  a b s o r p t i o n at 2.3  ev,  i t does not  c o n t r a d i c t the  made above i n connection with F i g u r e 26. F i g u r e 25 was  The  statements  crystal of  f r e s h l y X-rayed and t h e r e f o r e c o n t a i n e d a prominent  a b s o r p t i o n band at 2.3 ev ( c f . F i g u r e 29, curve A ) .  Iti s  b e l i e v e d t h a t the a b s o r p t i o n decrease shown i n F i g u r e 25 i s due p r i n c i p a l l y t o the disappearance o f t h i s band.  In  the case o f the p a r t i a l l y decayed c r y s t a l , however, t h i s band has a l r e a d y decayed t h e r m a l l y to a very low l e v e l .  The  b l e a c h i n g o f the p h o t o c o n d u c t i v i t y by quanta o f t h i s energy i s then a t t r i b u t e d t o the e x c i t a t i o n o f e l e c t r o n s from l e v e l s which are evidenced unresolved  by the p r e v i o u s l y d i s c u s s e d  absorption  ( S e c t i o n C.9)  superimposed on the 2.3 ev a b s o r p t i o n  band (Figure 16). C.11  S i g n o f the Charge C a r r i e r s In order t o p r o v i d e an i n t e r p r e t a t i o n o f the r e s u l t s  presented  i n section B . l l ,  some d i s c u s s i o n o f the space  charge f i e l d d i s t r i b u t i o n w i l l be g i v e n f o r a very ized  case. Consider  the experimental  s i t u a t i o n as i l l u s t r a t e d by  F i g u r e 3(c) wherein the ZY planes o f a c r y s t a l w i t h the l i g h t and  ideal-  c o n s t i t u t e the e l e c t r o d e s  beam i n c i d e n t on the XZ plane  i l l u m i n a t i n g a f r a c t i o n of the volume, b wide as shown  i n the f i g u r e .  Assuming t h a t the d i s t a n c e moved by each  e x c i t e d c a r r i e r i s small compared t o a-^, ag and b, the r e s u l t o f an i r r a d i a t i o n , d u r i n g which the e l e c t r i c f i e l d i s a p p l i e d , i s t o g i v e the f i e l d d i s t r i b u t i o n shown by the s o l i d l i n e i n F i g u r e 33(a). before  irradiation,  The d o t t e d l i n e  i . e . , t h a t which would be c a l c u l a t e d  from the a p p l i e d voltage and the e l e c t r o d e The  shows the f i e l d  separation.  second step i n t h i s i d e a l i z e d experiment i s t o  65  remove the a p p l i e d v o l t a g e same p o t e n t i a l . i n Figure  and b r i n g the e l e c t r o d e s to the  The f i e l d d i s t r i b u t i o n i s then as shown  33(b) which i s simply  the p r e v i o u s  a v e r t i c a l displacement o f  diagram.  For d e f i n i t n e s s i t i s assumed at t h i s p o i n t t h a t the e x c i t e d c a r r i e r s are e l e c t r o n s .  The d i s c u s s i o n can be e a s i l y  a l t e r e d t o f i t the case o f hole e x c i t a t i o n . the  f i e l d d i s t r i b u t i o n s of Figures  In t h i s ease  33(a) and (b) are the  r e s u l t o f the space charges i n d i c a t e d on the diagrams.  The  important f a c t here i s that the e l e c t r o n excess l i e s  outside  the  lies  i l l u m i n a t e d r e g i o n while the e l e c t r o n d e f i c i e n c y  within this The  region.  t h i r d step o f the experiment c o n s i s t s i n the e x c i t a -  t i o n o f more c a r r i e r s ( w i t h i n the same volume as p r e v i o u s l y i r r a d i a t e d ) and t h e i r motion under the i n f l u e n c e o f the space charge f i e l d alone.  Under these c o n d i t i o n s the motion o f  the e x c i t e d e l e c t r o n s i s t o the r i g h t and the tendency i s for  the former e l e c t r o n d e f i c i e n c y to be n e u t r a l i z e d and a  new d e f i c i e n c y t o form at the .-left hand side o f the i r r a d i a t e d region.  (Notice t h a t the e l e c t r o n excess i s not d i s t u r b e d  because i t l i e s  outside  the i r r a d i a t e d region.)  If this  p r o c e s s could be c a r r i e d t o completion the space charge d i s t r i b u t ion would be as shown i n F i g u r e  33(c) w i t h the cor-  responding e l e c t r i c f i e l d having an a p p r e c i a b l e at  the l e f t  hand edge of the i l l u m i n a t e d r e g i o n .  photocurrent were c a r r i e d by h o l e s the  value  only  I f the  r a t h e r than by e l e c t r o n s  " d i p o l e " l a y e r would e x i s t a t the opposite  edge o f the  66  i l l u m i n a t e d r e g i o n ( i . e . , the edge nearest which was h e l d negative According  d u r i n g the f i r s t  the e l e c t r o d e  step o f the  experiment).  to the f o r e g o i n g d i s c u s s i o n the s i g n o f the  charge c a r r i e r s can be determined i f the d i p o l e l a y e r can be located.  T h i s can be done i n p r a c t i c e by moving a narrow beam  of l i g h t a c r o s s the c r y s t a l and measuring the photocurrent a f u n c t i o n o f the beam p o s i t i o n .  as  I f the e l e c t r o d e s are a t  the same p o t e n t i a l d u r i n g t h i s "scanning"  the photocurrent  observed w i l l be due to the motion of c a r r i e r s i n the p r e v i o u s l y developed space charge f i e l d and hence w i l l be a measure o f t h i s f i e l d .  The measured dependence o f photo-  current on beam p o s i t i o n w i l l , o f course, o n l y be a t r u e r e p r e s e n t a t i o n o f the e l e c t r i c f i e l d d i s t r i b u t i o n i f the s e n s i t i v i t y o f the c r y s t a l i s uniform;  t h a t i s i f the UV  i r r a d i a t i o n has not a p p r e c i a b l y a c t i v a t e d the c r y s t a l . T h i s c o n d i t i o n very l i k e l y  cannot be met i n p r a c t i c e but  n e v e r t h e l e s s t h i s e f f e c t does not i n t e r f e r e with the l o c a t i o n of the d i p o l e l a y e r . In an a c t u a l experiment the r e s u l t s may be expected t o d e v i a t e from those (c).  shown s c h e m a t i c a l l y i n F i g u r e s 33(a) to  There are s e v e r a l obvious reasons f o r t h i s ; (a)  The c a r r i e r range may not be small compared t o the  width o f the i r r a d i a t e d (b)  region.  The c a r r i e r range may vary d u r i n g the course o f  the experiment by means o f the a c t i v a t i o n e f f e c t  discussed  above and a l s o because the e l e c t r i c f i e l d i s not constant d u r i n g the experiment. (c)  The term, "range", as used above should a c t u a l l y  read "mean range" since the displacement  of a given e l e c t r o n  67  can vary between zero and a d i s t a n c e  comparable t o the d i s -  tance between the e l e c t r o d e s , (d)  The width o f the scanning beam i s not n e g l i g i b l e  compared t o the width o f the i r r a d i a t e d r e g i o n , (e)  It i s d i f f i c u l t  of the second i r r a d i a t i o n  t o estimate the optimum (no a p p l i e d f i e l d ) .  duration  If i n s u f f i c i -  ent time i s employed the f i e l d due t o the d i p o l e l a y e r may be masked by t h a t due t o the u n n e u t r a l i z e d edge o f the i l l u m i n a t e d r e g i o n .  charges a t e i t h e r  I f the second i r r a d i a t i o n i s  prolonged t o a v o i d the former e f f e c t , the d i p o l e i t s e l f be destroyed,  may  by the continuous dark c u r r e n t , f o r example.  In s p i t e o f the above l i s t e d d i f f i c u l t i e s ,  i t i s believed  t h a t the r e s u l t s o f s e c t i o n B . l l may be used t o determine the s i g n o f the charge c a r r i e r s e x c i t e d under v a r i o u s  conditions.  For, even i f a c l e a r l y d e f i n e d f i e l d minimum, c o r r e s p o n d i n g to the " d i p o l e " of F i g u r e  33(c),  cannot be d i s c e r n e d , the  l o c a t i o n o f the " d i p o l e " can be deduced from the asymmetry of the f i e l d d i s t r i b u t i o n .  Thus i n F i g u r e  27, curve B, the  f i e l d minimum was s h i f t e d towards the negative the formation  o f the d i p o l e l a y e r .  It i s therefore  that e x c i t a t i o n i n the 4.0 ev band c r e a t e s f r e e In F i g u r e  28, curve B, there  ponding t o that o f curve A. (curve C) b r i n g s out the l e f t  concluded  holes.  i s no obvious minima c o r r e s -  However, s i n c e f u r t h e r  irradiation  hand minimum p r e f e r e n t i a l l y , we  a s c r i b e the l a t t e r to the d i p o l e l a y e r . formed nearest  e l e c t r o d e by  Thus the d i p o l e was  t o the p o s i t i v e e l e c t r o d e and i t i s concluded  t h a t e x c i t a t i o n i n the 5.0 ev band c r e a t e s f r e e e l e c t r o n s .  68  The  i n t e r p r e t a t i o n o f F i g u r e 29 i s more obvious  since  the r i g h t hand minimum of curve B i s r e a d i l y a s s o c i a t e d w i t h t h a t o f curve A.  Thus, the l e f t hand minimum o f curve B i s  a s c r i b e d t o the d i p o l e and i t i s concluded that 4.4 ev quanta create  f r e e e l e c t r o n s i n a p a r t i a l l y decayed X-rayed  crystal.  by  D.  CONCLUSIONS  Much of the m a t e r i a l p r e s e n t e d here c o n s i s t s of p r e l i m i n ary  i n v e s t i g a t i o n s of the o p t i c a l a b s o r p t i o n  tivity  i n MgO  c r y s t a l s of i n f e r i o r p u r i t y .  of the  r e s u l t s r e q u i r e d the  and The  photoconducinterpretation  i n t r o d u c t i o n of a l a r g e number (or  even continua) of energy l e v e l s i n the normally f o r b i d d e n It  i s recognized  not very  t h a t i n t e r p r e t a t i o n s made on  satisfying.  gap.  such a b a s i s  However, i t should be p o i n t e d out  even i n some o t h e r , much more thoroughly e x p l o r e d  areas  are  that (e.g.  94 the p r o p e r t i e s o f CdS  and  s i m i l a r photoconductors  p o s t u l a t e s are found to be necessary. served p r o p e r t i e s were found to be among the  s e v e r a l l o t s o f MgO  ), s i m i l a r  Furthermore, the  reasonably  available.  ob-  reproducible  Thus i t i s b e l i e v e d  that these p r o p e r t i e s , although they are probably determined to a l a r g e extent apply  by the  to many of the  l a r g e impurity  content, should  also  specimens f o r which data appear i n the  literature. The  f o l l o w i n g are the more important t e n t a t i v e  conclusions  r e s u l t i n g from t h i s work. (1)  The  most prominent UV  absorption  bands —  t i c of excess oxygen and o c c u r r i n g at 4.3, have no p h o t o c o n d u c t i v i t y (2)  The  4.05  photoconductivity  result  ev and  5.05  ev.  and  5.7  ev  —  a s s o c i a t e d w i t h them ( s e c t i o n 6). i n s t o i c h i o m e t r i c o r excess oxygen  c r y s t a l s l i e s p r i n c i p a l l y i n two at  4.8,  those c h a r a c t e r i s -  Gaussian shaped peaks  centered  E x c i t a t i o n i n these peaks i s found to  i n f r e e h o l e s and .free e l e c t r o n s r e s p e c t i v e l y .  (Sections  70  6 and 11). (3)  A d d i t i o n o f an excess o f Mg t o s t o i c h i o m e t r i c c r y s t a l s  reduces o r e l i m i n a t e s the o p t i c a l a b s o r p t i o n the s t o i c h i o m e t r i c state-.  ( S e c t i o n 8)  For t h i s reason i t i s  t e n t a t i v e l y concluded t h a t the a b s o r p t i o n metric the  spectra of s t o i c h i o -  c r y s t a l s and of excess Mg c r y s t a l s are a s s o c i a t e d  same centers  absorption  t o provide  ( F i g u r e 31(d).  due t o the t r a n s i t i o n s shown  The a d d i t i o n o f excess Mg i s then  e l e c t r o n s which f i l l  with  With t h i s model the background  i s i n t e r p r e t e d as b e i n g  i n the f i g u r e .  ing  c h a r a c t e r i s t i c of  considered  the upper s t a t e s , thus e l i m i n a t -  the p o s s i b i l i t y o f the t r a n s i t i o n s shown and t h e r e f o r e  reducing the background a b s o r p t i o n .  At the same time t r a n s i -  t i o n s from the upper s t a t e s t o the conduction  band are made  p o s s i b l e , thus p r o v i d i n g the e x t r a a b s o r p t i o n  and photocon-  d u c t i v i t y observed i n excess Mg c r y s t a l s .  I t seems necessary  to p o s t u l a t e a near continuum of l e v e l s to e x p l a i n the absorpt i o n i n e i t h e r s t o i c h i o m e t r i c o r excess Mg c r y s t a l s and a l s o to e x p l a i n the p h o t o c o n d u c t i v i t y (4)  o f the l a t t e r .  E x c i t a t i o n of f r e e e l e c t r o n s r e s u l t s i n the f i l l i n g of  a number o f shallow  levels  (Figure 31(e) some o f which seem  to c o n t r i b u t e s t r o n g l y (when empty) t o the e l e c t r o n t r a p p i n g process  (section 9).  Those o f h i g h energy (with e x c i t a t i o n  e n e r g i e s g r e a t e r than say 3 ev) may be those which are a l s o f i l l e d by the a d d i t i o n o f excess Mg.  The lower energy ones  are b e l i e v e d to be r e s p o n s i b l e f o r the a b s o r p t i o n visible for  i n the  regions o f F i g u r e 16 ( i r r a d i a t e d i n 5 ev band) and  p a r t o f that i n the v i s i b l e  (X-rayed).  region of Figure  19, curve A  71  (5)  I r r a d i a t i o n i n the 5 ev band i n t r o d u c e s the a b s o r p t i o n  bands c h a r a c t e r i s t i c o f excess oxygen.  The 5 ev t r a n s i t i o n  i s b e l i e v e d t o be a s s o c i a t e d with a p o t e n t i a l ©enter o f the l a t t e r type. (6)  (Figure  31(e).  The e x c i t a t i o n o f f r e e holes  (e.g. by 4 ev band i r r a d i a -  t i o n ) causes a small i n c r e a s e i n UV a b s o r p t i o n . i s obvious i n t h i s spectrum. valence  No s t r u c t u r e  Some l e v e l s l y i n g nearer  band than the 4 ev l e v e l t r a p these  h o l e s , making  p o s s i b l e photoconduction a t somewhat lower e n e r g i e s . 9).  t o the  (Section  These trapped h o l e s can a c t as e l e c t r o n t r a p s so t h a t  as they are formed, the e f f i c i e n c y o f e l e c t r o n i c photoconduct i o n decreases ( F i g u r e 15) and as they are t h e r m a l l y i o n i z e d the e l e c t r o n i c y i e l d i n c r e a s e s ( F i g u r e 21, curve D). (7)  X - i r r a d i a t i o n e x c i t e s both e l e c t r o n s a n d h o l e s and pro-  v i d e s the f o l l o w i n g changes i n l e v e l occupancy. (a)  The upper f i l l e d l e v e l s o f F i g u r e 31(e) are emptied  thus p r o v i d i n g the excess oxygen a b s o r p t i o n bands a t 5.7, 4.8 and 4.3 ev and at the same time d e c r e a s i n g the number o f 5.0 ev e x c i t a t i o n s ( F i g u r e 21). (b)  The s e r i e s o f shallow l e v e l s o f F i g u r e 31(e) i s  f i l l e d with e l e c t r o n s thus i n c r e a s i n g the range o f f r e e e l e c t r o n s and p r o v i d i n g o p t i c a l a b s o r p t i o n i n t h e v i s i b l e of the spectrum.  region  Part of t h i s absorption l i e s i n a well  d e f i n e d band (Figure 19) while the remainder shows no obvious s t r u c t u r e and corresponds t o the p h o t o c o n d u c t i v i t y of the c r y s t a l a f t e r a p a r t i a l thermal decay ( F i g u r e (c)  21(d).  The s e r i e s o f low l y i n g l e v e l s o f Figure 3 1 ( f ) are  72  emptied thus p r o v i d i n g p h o t o c o n d u c t i v i t y at lower  energies  ( F i g u r e 21, curve B) and at the same time p r o v i d i n g a d d i t i o n a l electron traps. (8)  The thermal decay proceeds by: (a)  The thermal r e l e a s e o f e l e c t r o n s from the c e n t e r s  which g i v e r i s e t o the 2.3 ev a b s o r p t i o n band.  These e l e c -  t r o n s then combine with the t r a p p e d h o l e s i n the l e v e l s o f Figure 31(f).  By t h i s means the hole p h o t o c o n d u c t i v i t y i s  reduced and the e l e c t r o n range i n c r e a s e d  (compare curves B  and D, F i g u r e 21), while the v i s i b l e a b s o r p t i o n (b)  The UV a b s o r p t i o n  decreases.  c e n t e r s ( F i g u r e 31(c) decay by  combination w i t h e l e c t r o n s t h e r m a l l y r e l e a s e d from the s e r i e s of l e v e l s o f F i g u r e 31(e).  Since a number o f thermal a c t i v a -  t i o n e n e r g i e s are i n v o l v e d i n t h i s r e l e a s e , the dependence o f UV a b s o r p t i o n on time can be decomposed i n a number o f simple exponential (9)  terms (Figure 32).  B l e a c h i n g o f the UV a b s o r p t i o n may be accomplished by  u s i n g any wave l e n g t h which can e x c i t e e l e c t r o n i c photoconduct i o n , i . e . , any energy from about 2 ev t o about 6 ev o r h i g h e r . The  higher e n e r g i e s may however, produce c o m p l i c a t i n g  effects.  When the b l e a c h i n g  side  r a d i a t i o n l i e s i n the 2.3 ev  a b s o r p t i o n band e l e c t r o n s may be e x c i t e d e i t h e r from the l e v e l s r e s p o n s i b l e f o r t h i s band o r from those which a l s o l i e a t t h i s energy but are p a r t o f the quasi-continuous i n Figure 31(e).  d i s t r i b u t i o n shown  Thus, i n the b l e a c h i n g experiment  i n s e c t i o n B.10, no simple  described  r e l a t i o n s h i p between the a b s o r p t i o n  changes in the 2.3 ev band and i n the UV a b s o r p t i o n bands may  73  be  expected.  (10)  B l e a c h i n g of the X-ray induced p h o t o c o n d u c t i v i t y can  achieved by i r r a d i a t i o n w i t h 2.3 due  ev quanta.  The e f f e c t  to the emptying of e l e c t r o n t r a p s by o p t i c a l  is  ionization  as d i s c u s s e d above and the consequent r e d u c t i o n i n e l e c t r o n range.  be  74  E. The tivity of  COMPARISON WITH PREVIOUS WORK  only p r e v i o u s l y r e p o r t e d measurements of photoconduc-  i n MgO  c r y s t a l s were made by Day^.  Since the two  sets  r e s u l t s are i n c o n f l i c t on a number of p o i n t s Day's r e s u l t s  w i l l be d i s c u s s e d i n some d e t a i l .  The most important d i f f e r e n c e s  are: 1.  The peaks at 3.6  and 4.8 ev o b t a i n e d by Day  i n neutron  i r r a d i a t e d c r y s t a l s were not found i n X-rayed c r y s t a l s i n the present work.  T h i s f a c t alone i s not  does seem reasonable  to expect  cause f o r alarm, but i t  t h a t any means of e x c i t a t i o n  which i s capable of e x c i t i n g both e l e c t r o n s and h o l e s w i l l r e s u l t i n the formation of s i m i l a r a b s o r p t i o n and photoconduct i v i t y bands.  The apparent  d i s c r e p a n c y can, however, be  re-  s o l v e d by a c o n s i d e r a t i o n of the geometry used by Day  and  f a c t t h a t the c o r r e c t i o n f a c t o r  omitted  in his calculations.  The  /a ( e q u a t i o n (1) was  dimension  the  of h i s c r y s t a l i n the  d i r e c t i o n p a r a l l e l to the l i g h t beam was  about 1 cm.  Thus  the q u a n t i t y a_ (equation (2) a c h i e v e d a constant value at r e l a t i v e l y low quantum energy (say about 3.6 to  ev)  compared  the case of a t h i n n e r specimen where the constancy  a would not be' a c h i e v e d u n t i l the quantum energy was i n t o the 4.3 (Day,  ev o p t i c a l a b s o r p t i o n band.  The  of well  spectrum  r e f . 5, F i g u r e 1) should t h e r e f o r e be m u l t i p l i e d by a  f a c t o r p r o p o r t i o n a l to K f o r quantum e n e r g i e s g r e a t e r than 3.6  ev.  Since K i s expected to peak at 4.3  the " v a l l e y " at 4.3  ev  (Figure  22)  ev i n Day's spectrum would tend to be  75  e l i m i n a t e d and the 3.6  and 4.8  ev peaks thus d i s a p p e a r .  The  s p e c t r a l dependence of p h o t o c o n d u c t i v i t y would then e s s e n t i a l l y agree with the r e s u l t s of the present measurements. 2.  The  f a i l u r e t o recognize i n the e a r l i e r work  5  that  the p h o t o c o n d u c t i v i t y i n s t o i c h i o m e t r i c c r y s t a l s l i e s i n two bands.  The  UV a c t i v a t i o n experiments  quantum energy  of 4.0 ev.  t i o n o b t a i n e d by t h i s UV  were performed  with a  A c c o r d i n g t o F i g u r e 6 the  activa-  i r r a d i a t i o n s h o u l d have been due i n  p a r t to a b s o r p t i o n i n the 4 ev band and i n p a r t to 5 ev band absorption.  The  s p e c t r a l dependence of the p h o t o c o n d u c t i v i t y  i n the a c t i v a t e d c r y s t a l a c t i v a t i o n o b t a i n e d was  ( r e f . 5, F i g u r e 2) shows t h a t the more c h a r a c t e r i s t i c of 5 ev band  irradiation. 3.  The  irradiation.  s i g n of the charge Day attempted  r e l e a s e d by 3.7  c a r r i e r s r e l e a s e d by 4 ev  to determine  the s i g n of the  charge  ev r a d i a t i o n by d e t e c t i n g the displacement  by  the e l e c t r i c f i e l d of a narrow UV a c t i v a t e d r e g i o n of the crystal.  The  l o c a t i o n of the a c t i v a t e d r e g i o n was  determined  by scanning the a c t i v a t e d c r y s t a l w i t h a narrow l i g h t beam. The procedure  i n v o l v e d the i m p l i c i t assumption  photocurrent versus d i s t a n c e a c r o s s the c r y s t a l p a r a l l e l to the f i e l d )  comprised  f i l e of the p h o t o s e n s i t i v i t y . was  of n e c e s s i t y performed  to the c r y s t a l ,  space  (in a direction  a d e t e r m i n a t i o n of the p r o -  However, s i n c e the  irradiation  w i t h the e l e c t r i c f i e l d a p p l i e d  charge  as d i s c u s s e d i n s e c t i o n 5.  t h a t a p l o t of  f i e l d s were presumably  developed  T h e r e f o r e , the v a r i a t i o n of  current w i t h d i s t a n c e i s a measure of the p r o f i l e of the  photoproduct  76  of e l e c t r i c f i e l d and p h o t o s e n s i t i v i t y .  Thus the i n t e r p r e t a -  t i o n of t h i s type of experiment i s confused of space charge  by the  formation  fields.  In s p i t e of the above o b j e c t i o n Day's c o n c l u s i o n t h a t the in his  c a r r i e r s produced by 3.7  ev i r r a d i a t i o n are h o l e s i s  agreement with the r e s u l t o f c o n c l u s i o n t h a t the  section B . l l .  However,  c a r r i e r s excited during his a c t i v a -  t i o n experiment were t h e r e f o r e a l s o h o l e s i s not ( a c t i v a t i n g i r r a d i a t i o n was the 5 ev band) and  at 4.0  ev,  warranted  i . e . i n the t a i l  i n f a c t , a c c o r d i n g to the present  of  work,  incorrect. The  o p t i c a l absorption  spectrum of the excess Mg  crystal  shown by F i g u r e 9 i s s i m i l a r to t h a t o b t a i n e d by Weber ^. 1  The  l a t t e r author,  however, b e l i e v e d t h a t t h i s spectrum c o u l d  be decomposed i n t o 3 bands c e n t e r e d at 4.8, The  (a)  The  s e c t i o n G.8 the  ev.  m i s l e d on two  counts.  d e c r e a s i n g background a b s o r p t i o n d i s c u s s e d i n  makes the  c a l c u l a t e d induced a b s o r p t i o n f a l l  off  range of quantum e n e r g i e s where the background absorp-  t i o n has, F i g u r e 12,  before treatment, an a p p r e c i a b l e magnitude curve A ) .  I f the induced a b s o r p t i o n i s  l e s s c a l c u l a t e d i n t h i s way, 4.8  and 2.3  s t r u c t u r e i s , however, not obvious i n Weber's data and i t  i s b e l i e v e d t h a t he was  in  3.6  (see  neverthe-  i t i s p o s s i b l e to p o s t u l a t e a  ev a b s o r p t i o n band, whereas the t r u e induced  absorption  would show no such f a l l i n g o f f at h i g h e n e r g i e s and  would  t h e r e f o r e not i n d i c a t e "the presence of such a band. (b)  Weber was  a l s o convinced  t h a t the a b s o r p t i o n  spectrum  77  of an excess Mg  crystal  should c o n s i s t of those bands which are  found i n X-rayed c r y s t a l s but not i n excess oxygen Since he was  not aware t h a t the 4.8  ev band was  crystals.  present  excess oxygen c r y s t a l s he b e l i e v e d t h a t i t should be i n excess Mg  c r y s t a l s s i n c e i t was  in  present  q u i t e obvious i n X-rayed  crystals. Having a t t r i b u t e d the a b s o r p t i o n at h i g h quantum i n excess Mg  c r y s t a l s to the 4.8  decompose the  remainder of the  ev band, Weber c o u l d then  spectrum i n t o 3.6  bands, both of which he b e l i e v e d were present induced  spectra.  i n excess Mg  ev  i n the X-ray  ev i n X-rayed c r y s t a l s .  seems to be no good evidence  c r y s t a l s nor  fashion.  the erroneous assumption of a 4.8 c r y s t a l s would l e a d one  for i t s existence  f o r the e x i s t e n c e of a 3.6  i n c r y s t a l s t r e a t e d i n any  3.6  and 2.3  There seems to be no doubt about the e x i s -  tence of an a b s o r p t i o n peak at 2.3 However, t h e r e  energies  ev band  It i s b e l i e v e d that  ev band i n the excess  t o p o s t u l a t e the e x i s t e n c e of  only Mg  the  ev band. Hibben  9 5  found t h a t MgO  by exposure to 4.9  c r y s t a l s were v i s i b l y  ev r a d i a t i o n .  colored  T h i s i s presumably the  same e f f e c t as i l l u s t r a t e d by F i g u r e  16.  26 Eisenstein of MgO  crystals.  s t u d i e d the X-ray induced An e m i s s i o n band at 3.6  have a very long decay time.  phosphorescence  ev wasJTound to  T h i s band may  be  connected with  the decay of the excess oxygen type a b s o r p t i o n c e n t e r s . long .decaya'time v isoxsoiisistent with the decay shown by Figure  20.  The  l o n g term a b s o r p t i o n  78  Yamaka and Sarwamoto  have determined the  c a r r i e r s by measuring the H a l l e f f e c t .  s i g n of e x c i t e d  I t does not seem p r o f i t -  able to d i s c u s s t h e i r work i n terms of the present no attempt was were present  made to determine which p h o t o c o n d u c t i v i t y  i n t h e i r samples.  i n a "band" centered at the  same energy.  c o n d u c t i v i t y i n excess Mg  In t h i s manner they  3.6  and  2.3  crystals.  t h i s c o n c l u s i o n i s i n doubt i n view of the doubt  ev "bands" Although concerning  the presence of these bands i n such c r y s t a l s , the f a c t was  detected  bands  assumed t o provide e x c i t a t i o n  determined t h a t e x c i t a t i o n i n the 4.8,  hole conduction  since  R a d i a t i o n of a p a r t i c u l a r wave-  l e n g t h ( p r o v i d e d by f i l t e r s ) was  p r o v i d e d hole  results  i s i n c o n t r a d i c t i o n to the  that dis-  c u s s i o n of S e c t i o n C.l-1. 28 Yamaka  d e t e c t e d , by measurements of thermoluminescence,  the presence of a number of t r a p s whose thermal a c t i v a t i o n e n e r g i e s he e s t i m a t e d  to f a l l  i n the r e g i o n .56  to 1.58  ev.  Since the same r e s u l t s were o b t a i n e d both by X-ray and 4.9  ev  o p t i c a l i r r a d i a t i o n the t r a p s are a c c o r d i n g to the present  work  electron traps.  These are presumably the t r a p s which are  b l e f o r the p h o t o c o n d u c t i v i t y c r y s t a l s ( F i g u r e 22,  i n X-rayed and p a r t i a l l y  curve B, and Table  IV).  responsi-  decayed  BIBLIOGRAPHY 1.  Lander, Rev. S c i . I n s t . , 24., 331 (1953).  2.  S t r o n g and B r i c e , J . Opt. Soc. Am.,  3.  J e n k i n s and White, Fundamentals o f P h y s i c a l O p t i c s , (McGraw-Hill, 1937) p. 294.  4.  S e i t z , Rev. Mod.  5.  Day, Phys. Rev., 91_, 822 (1953).  6.  G e l l e r , Phys. Rev., 101, 1685 (1956).  7.  H a l l , Phys. Rev.,  8.  Rose, Proc. IRE, 43, 1850 (1955).  9.  PBube, Proc. IRE, 4_3, 1846 (1955.).  25_, 207 (1935).  Phys., 26., 44 (1954).  8_7, 387 (1952).  10.  Weber, Z e i t s f . P h y s i k , 130, 392 (1951).  11.  Soshea, M. S. T h e s i s , E l e c t r i c a l E n g i n e e r i n g U n i v e r s i t y o f Minnesota, (1956).  12.  Molnar and Hartman, Phys. Rev. 91, 1015 (1950).  13.  Lye, S c i e n t i f i c Report No. 1, U. S. A i r Force Contract No. 33(616)-3325 (1956).  14.  Blakney and Dexter, D e f e c t s i n C r y s t a l l i n e S o l i d s (The P h y s i c a l S o c i e t y , London, 1955), p. 108; Phys. Rev. 96, 227 (1954).  15.  Johnson, Phys. Rev. 94_, 845 (1954).  16.  Krumhansl, P h o t o c o n d u c t i v i t y Conference (Wiley, p. 455.  17.  S e i t z , Rev. Mod.  18.  I b i d , p. 68  19.  I b i d , p. 68, F i g . 34.  20.  S t u r t z , E l e c t r i c a l E n g i n e e r i n g Department, U n i v e r s i t y o f Minnesota; p r i v a t e communication.  21.  W i l l i a m s , U s i s k i n and Dekker, Phys. Rev. 92_, 1398 (1953).  22.  Mott and Gurney, E l e c t r o n i c Processes i n I o n i c C r y s t a l s , Oxford, 1948), p. 130.  23.  Lehovec, Phys. Rev. 92, 253 (1953).  Department,  1956)  Phys. 26., 53 (1954).  80  V a s i l e f f , Phys. Rev.  96_, 603  (1954).  24.  -Rose, Proc. IRE 43, 1857  25.  Hibben, Phys. Rev.  26.  E i s e n s t e i n , Phys. Rev. 9_4, 776  27.  Yamaka and Sawamoto, Phys. Rev.  28.  Yamaka, Phys. Rev.  29.  Mott and Gurney, E l e c t r o n i c Processes i n I o n i c C r y s t a l s , (Oxford, 1948) p. 82.  30.  S e i t z , r e f . 4. B u r s t e i n et a l . , r e f . 16, p.  31.  (1955).  51, 530  96, 293  (1937). (1954). 101, 565  (1956).  (1954).  384.  Lax, r e f . 16, p. 111. Pekar, Z. e x p t l . t e o r . phys., 20, 267  (1950).  32.  B u r s t e i n e t a l . , P h o t o c o n d u c t i v i t y Conference (Wiley, 1956) pp. 379-382.  33.  Bernard V. Haxby, E l e c t r i c a l E n g i n e e r i n g Department, U n i v e r s i t y o f Minnesota, p r i v a t e communication.  34.  L i p s o n et a l . , Phys. Rev.  99, 444  (1955).  MOUNTING OF THE CRYSTAL FOR PHOTOCONDUCTIVITY MEASUREMENTS  FIG. 2  SCHEMATIC ARRANGEMENT OF THE PHOTOCONDUCTIVITY APPARATUS  FIG. I  I2r  or < or or < UJ  or or O o  H O I CL  8 12 16 TIME OF IRRADIATION (min)  4Z -7—r  I  FIG. 3  (O  TYPICAL ABSORPTION SPECTRA OF MgO C R Y S T A L S A S RECEIVED  100  80  \  2  o  UJ  60  \  o u.  UJ  o o  x— "^x. x  40  * x  g  -^y-x^  r-  X  \  \  0-  tr o  oo  20  \ \  < 0 *  6.0  5.0  4.0  QUANTUM FIG. 4  ENERGY(ev)  1  5.05 e.v.  RESOLUTION OF A TYPICAL PHOTOCONDUCTIVITY. SPECTRUM INTO TWO GAUSSIAN BANDS  b  o  X*  .16'°  Q / Half •-width \ \ / 0.6 9 e.v.  o  /  a  >  N n >-  (Point s are expierimental, Curves are calcijlated.)  /  o  UJ y 10'  >-  4. 05e.v. /  111 > o  o/j  O z o o o o  <  v/ /  / /  7v  12  10  /Half - width ^ >7 e.v. \  /  \ \  /7  » /  \ \  \ \ \ \ \ \  13  10  3.0  3.5  4.0  4.5  5.0  QUANTUM ENERGY (e.v.) FIG. 6  5.5  6.0  EFFECT OF HEAT TREATMENT ON THE BACKGROUND ABSORPTION  QUANTUM  FIG. 7  ENERGY(eV)  IO  9  ~,  ,  -• -  "1  PHOTOCC)NDUCTIVPFY AND A N ^BSORPTIO OF CRY!5TALS HE ATED IN V 'ACUUM - A T D1FF ERENT TE:MPERATUF IES  /  Y^o^r c -  tjlflf  0  o  > o  XL  Q. K  w M >-  J  UJ >\b  10  a 1  UJ  1  1  —X"  >  V lOooo^c  >  r * /# *  r-  E o  O z o o o  & I  fa! a  UJ 1  Q_  g  K, I400*C  ^13  LiLi-  1.0  g O z g  01 o  CD <  3.5  4.0 FIG. 8  4.5 5.0 5.5 QUANTUM E N E R G Y (e.v.)  0.1 6.0  ABSORPTION SPECTRUM DUE TO HEATING IN MAGNESIUM VAPOR (THICKNESS .036  cm.)  B  A-BEFORE  HEATING  B - A F T E R HEATING  8  20  2.4  2.8  32  36 4.0 QUANTUM ENERGY(ev) FIG. 9  4.4  4.8  5.2  5.6  THE EFFECT OF EXCESS Mg ON THE BACKGROUND ABSORPTION OF Mg 0 A "BEFORE TREATMENT -  B - A F T E R IHR. AT I I I 5 ° C  \  C - C H A N G E DUE TO HEATING  5.4  /^5.0  'A,  i — •  i 4.6  QUANTUM  4.2 ENERGY(ev)  FIG.  10  1  3.8  i= 3.4  1—  THE EFFECT OF EXCESS Mg ON THE BACKGROUND ABSORPTION OF Mg 0  FIG.  II  EFFECT OF EXCESS Mg ON ABSORPTION OF MgO 3  A " CHANGE BY HEATING 4 HRS. AT 1115* C  20  UJ O  B - C H A N G E B Y HEATING AN ADDITIONAL 5HRS. AT 1115° C  LT 16 U. UJ O O  Z  12  o  IQ_ CC O C/) CD  8  < UJ  e> z <  5 o  5.8  5.4  5.0  4.6  QUANTUM  4.2  3.8  ENERGY (ev)  FIG. 12  3.4  3.0  2.6  2.2  T  T  EFFECT OF EXCESS MG ON PHOTOCONDUCTIVITY x  10  101  /  / B O  £  E o  X  /  -II  10  X  7  CL X  w  7 -12  >  A -  UNTREATED  B-  EXCESS Mg  10  /  -13  10  2.5  3.0  4.0 , . QUANTUM ENERGY (ev.) FIG. 13  5.0  \pPTICAL ACTIVATION OF PHOTOCONDUCTIVITY l ^ l ^  ~  r -  IN  MgO  ^  ^A^/  S ^ - - ^ ^ ^ ^ ^  4  "7 ill  l a  "'•I 7  1 ic  /  1  "  1 .0  2.5 FIG. 14  •  •  -  2  3  4  5  6  1  \  J  1  1  3.0 3.5 4.0 QUANTUM ENERGY (e.v.)  4.5  QUANTUM ENERGY FIG. 15  (EV)  INDUCED  OPTICAL  IRRADIATION  Q  ABSORBTION IN 4 ev.  DUE  TO  BAND  <3  t  .040  IRRADIATION ENERGY (3Bev)  z  THICKNESS (0.65 Cm )  UJ Q  <  .030  Q. O  UJ CO  .020 x—-  z < I  X  x  o  .010 X^  -x- -sr x  X'X  3.2  3.6  4.0  x  44  4.8  QUANTUM ENERGY (ev) FIG. 17  5.2  5.6  6.0  6.4  BUILD-UP OF X-lRAY INDUCED A B S O R P T I O N FOR 50 K.V. X - R A Y S (I5ma.) * I 3 6 ( 0 . 4 0 mm.)  TIME  (HOURS) FIG. 18  ABSORPTION SPECTRA OF X - R A Y E D CRYSTALS 0.8  "  A " A F T E R X-RAY (SEE FIG. 21) B - A F T E R 95HOURS THERMAL DECAY (d = .040CM)  >H CO  0.6  "  0.4  -  0.2  -  z  UJ  o _l < o  — I CL  o o UJ O Z) Q  0.0 6.0  5.0  4.0  3.0  QUANTUM ENERGY(ev)  FIG. 19  2.0  PHOTOCONDUCTIVITY IN X-RAYED MgO  'A- UNTREATED B-AFTER 40MIN. OF X-IRRADIATION C-AFTER 4HRS. THERMAL DECAY —| D-AFTER 50HRS. THERMAL DECAY 1.5  3.0"  3.5  4.0  I  I  4.5  5.0  QUANTUM ENERGY (e.v.) FIG. 21  •  J _ 5.5  6.0  2.0  2.2  2.4  QUANTUM E N E R G Y (ev.) F I G . 22  2.0  2.5  3.0  3.5  4.0  4.5  QUANTUM ENERGY (e.v.) FIG. 23  5.0  5.5  OPTICAL DENSITY AT4.2eV. VS BLEACHING TIME 1.000 -  (IRRADIATION ENERGY = 2.3ev.) d = .IICM  .800-  .600-  A - IRRADIATED B - THERMAL  .400  DECAY  .200  ONLY  OPTICAL DENSITY BEFORE X-IRRADIATION  .000 30  60  90  TIME IRRADIATED FIG. 2 4  120 (MINUTES)  OPTICAL DENSITY AT 2 3 e v VS BLEACHING TIME .200H  (IRRADIATION ENERGY = 2.3eV.) d = .IICM. A - IRRADIATED B - T H E R M A L DECAY ONLY  >  .160  C-DIFFERENCE  ro cvi <  .120  >-  CO UJ  o  .080  <  OPTICAL DENSITIES  A_  BEFORE  B  X - IRRADIATION  g CL  o  040  60 90 TIME IRRADIATED (MINUTES)  FIG. 25  120  150  0  I FIG. 26  2 QUANTA/CM  3 2  x|0~  19  (Approximate)  .  ^  4  DETERMINATION CARRIERS  OF THE EXCITED  POSITION F IG. 27  SIGN OF CHARGE IN  4 EV BAND  FIELD  DISTRIBUTIONS A - AFTER  IRRADIATION  B-AFTER  SUBSEQUENT  C-AFTER  FURTHER  AFTER WITH  IRRADIATION IN 5 EV BAND  APPLIED  IRRADIATION  ELECTRIC WITH  IRRADIATION WITH  POSITION FIG.  28  FIELD  N OAPPLIED  N O APPLIED  ELECTRIC  ELECTRIC  FIELD FIELD  DETERMINATION OF THE SIGN OF CHARGE CARRIERS IN AN X-RAYED  POSITION FIG. 29  CRYSTAL  POSSIBLE MECHANISMS FOR THE LOSS OF F R E E CARRIERS  CONDUCTION BAND  /  /  77TTTTT7JT77T777777 VALENCE BAND  (a)  (c)  FIG. 30  («0  CONDUCTION BAND  5.05 ev.  4.3 ev.  5.7 ev. 4.05ev  / / / / / / / / / / / / / / / / / / / / / / / / . / / / / / / / (a)  (b) PROPOSED  (c)  (d)  ENERGY L E V E L FIG. 31  SCHEMES  VALENCE B A N D  (e) FOR  (f) Mg 0  7  /  7  THERMAL DECAY OF X - R A Y INDUCED ULTRAVIOLET ABSORPTION AT ROOM TEMPERATURE  O  100  200  300  400  500  600  TIME A F T E R X-IRRADIATION (HRS.) 9  FIG. 3 2  I D E A L I Z E D F I E L D DISTRIBUTIONS ILLUSTRATING D E T E R M I N I N G T H E SIGN O F T H E C H A R G E  T H E METHOD OF CARRIERS  Q _J LU  + + X=0 X=Q POSITIVE ELECTRODE  x=a+b  x=2a+  b  x=o x=a x*a+b x«2a+b  NEGATIVE ELECTRODE  NEGATIVE POSITIVE ELECTRODE ELECTRODE  (a)  +  (b)  FIG. 33  x=o  x= a x=a+b x*2a+b  NEGATIVE ELECTRODE  POSITIVE ELECTRODE  (c)  

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