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Optical detection of paramagnetic and cyclotron resonance in semiconductors Booth, Ian 1985

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Optical Detection of Paramagnetic and Cyclotron Resonance in Semiconductors By IAN J . M. BOOTH B . S c . j M . S c , Lakehead  U n i v e r s i t y , 1980  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE  REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY  in THE  FACULTY OF GRADUATE STUDIES (Department  We a c c e p t t h i s to  THE  of Physics)  thesis  the required  as c o n f o r m i n g standard  UNIVERSITY OF BRITISH COLUMBIA March 1985 (§) I a n Jeremy M. B o o t h , 1985  In presenting t h i s thesis i n p a r t i a l fulfilment of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission.  P h  Department of  y  s i c s  The University of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T D a t e  DE-6  (3/81)  1Y3  A p r i l 1.  1985  ABSTRACT Optical used  Detection of Magnetic  to observe both paramagnetic  Resonance  (ODMR) has been  and d i a m a g n e t i c  resonance  o f p h o t o - e x c i t e d e l e c t r o n s and h o l e s i n GaP, ZnTe and AgBr. Paramagnetic been  resonance of conduction e l e c t r o n s  studied  dependence strength  and  the microwave  of the e f f e c t  analysed.  was o b s e r v e d t o p r o d u c e  luminescence  a t 1.6  frequency  K.  giving  power signal  a p p r o x i m a t e l y 1% change i n  The g v a l u e  the electron  and  T h e maximum  deduced  r e s o n a n c e was 2.000 +_ 0.005. The r e s o n a n c e broadened  i n GaP h a s  from  the  was h o m o g e n e o u s l y  l i f e t i m e as a p p r o x i m a t e l y 4  nanoseconds. Paramagnetic been d e t e c t e d The  background  cyclotron  of electrons  and h o l e s  signals  p r e s e n t i n ODMR e x p e r i m e n t s  and a r e shown  resonance  heating  of photoexcited  carriers.  o f 9.2 a n d 36.3 GHz  have been made on GaP,ZnTe and A g B r , and c y c l o t r o n electrons  light and mass  and h o l e s  and heavy h o l e s  0.626 was  shorter three,  +_ 0.06  most  effective  The e f f e c t i v e  i n GaP w e r e f o u n d while  0.36 _+ 0.10. T h e e l e c t r o n  than  nitrogen  observed.  respectively  that  f o r holes  likely  due  mass  values  scattering  to scattering  o f 0.30 ii  masses o f  effective time  was  of approximately by  isoelectronic  were o b s e r v e d + 0.20  resonance  t o b e 0.154 +_ 0.01  the e l e c t r o n  by a f a c t o r  i m p u r i t i e s . Resonances  have  t o be c a u s e d by d i a m a g n e t i c  Measurements a t microwave f r e q u e n c i e s  of  has a l s o  i n AgBr.  been i n v e s t i g a t e d or  resonance  i n ZnTe a t  a n d 0.76  + 0.20  corresponding ZnTe  to electrons  resonances  due  to  and heavy h o l e s . electrons  different  luminescence  different  recombination centres  type. Cyclotron observed valence  holes  i n d i c a t i n g the to heating  resonance of e l e c t r o n s  band  non-parabolicity.  in  sensitivity  of  of e i t h e r  and h o l e s  indicated  energies  enhanced  by e m i s s i o n  A feature trapping  i n the  carrier  was  also and  electron  of e l e c t r o n s  o f one o r more LO  iii  and  appeared  i n AgBr and showed t h e e f f e c t s o f c o n d u c t i o n  resonance certain  bands  and  In b o t h GaP  phonons.  with  TABLE OF CONTENTS Abstract. Chapter  1. 1.1 1.2 1.3 1.4 1.5  Introduction. Historical. Paramagnetic Resonances. A d v a n t a g e s o f ODMR. C y c l o t r o n Resonance. Overview o f t h e t h e s i s .  1 1 6 13 14 16  Chapter  2. 2.1 2.2 2.3  A p p a r a t u s and E x p e r i m e n t a l T e c h n i q u e s . Apparatus. Experimental Procedures. Commentary.  19 19 25 29  Chapter  3. G a l l i u m P h o s p h i d e and Z i n c T e l l u r i d e . 3.1 I n t r o d u c t i o n . 3.2 E x c i t o n f o r m a t i o n and l u m i n e s c e n c e i n G a P . 1.3 O t h e r work on G a P . 3.4 R e s u l t s . 3.5 D i s c u s s i o n . 3.6 C y c l o t r o n R e s o n a n c e . 3.7 ODMR i n Z n T e . 3.8 O t h e r M a t e r i a l s . 3.9 Summary.  30 30 32 42 43 53 58 71 78 79  Chapter  4. 4.1 4.2 4.3 4.4 4.5  S i l v e r Bromide. AgBr. ODMR i n A g B r . Results. Discussion. Summary.  Chapter  5. 5.1 5.2 5.3 5.4 5.5  C o n c l u s i o n s and S u g g e s t i o n s f o r F u r t h e r Work .102 Gallium Phosphide. 102 Zinc Telluride. 103 S i l v e r Bromide. 104 Curve f i t t i n g . 105 Comments and S u g g e s t i o n s f o r F u r t h e r Work. 106  Appendix  82 82 82 86 93 99  A. Curve F i t t i n g . Statistical Analysis.  Bibliography.  110 118 122  iv  L I S T OF Table  3.1.  Microwave i n d u c e d  Table  3.2.  GaP  TABLES  luminescence  changes.  C y c l o t r o n Resonance r e s u l t s .  Table A . l . Gaussian  L.S.  coefficients.  T a b l e A.2.  L o r e n t z i a n L.S.  T a b l e A.3.  Revised  Gaussian  T a b l e A.4.  Revised  L o r e n t z i a n L.S.  T a b l e A.5.  Correlation limits.  v  113  coefficients.  coefficients  coefficients. and  68 113  coefficients. L.S.  41  confidence  117 117 119  L I S T OF FIGURES Fig.  1 .1 B a s i c ODMR  Fig.  1 .2 L u m i n e s c e n c e p r o c e s s e s .  Fig.  2 .1 A p p a r a t u s .  20  Fig.  3 .1 GaP b a n d s t r u c t u r e .  31  experiment.  F i g . 3 .2 GaP-N and G a P - B i  5 8  luminescence.  35  Fig.  3 .3 E x c i t o n  Fig.  3 .4 ODMR i n GaP-N.  44  Fig.  3 .5 ODMR i n GaP-S.  45  states  i n GaP.  37  F i g . 3 .6 ODMR i n G a P - B i .  46  Fig.  48  3 .7 ODMR microwave power d e p e n d e n c e .  F i g . 3 .8 GaP-N Zeeman  spectrum.  50  Fig.  3 .9 ODMR s i g n a l a t 36.3 GHz.  52  Fig.  3 .10 GaP c y c l o t r o n  59  Fig.  3 .11 GaP-N c y c l o t r o n  resonance.  61  Fig.  3 .12 GaP-S c y c l o t r o n  resonance.  62  Fig.  3 .13  GaP-Bi c y c l o t r o n  Fig.  3 .14  Cyclotron  Fig.  3 .15  ZnTe  Fig.  3 .16  ZnTe ODMR a t 5220 A.U.  75  Fig.  3 .17  ZnTe ODMR a t 5290 A.U.  76  Fig.  4 .1  AgBr b a n d s t r u c t u r e .  83  Fig.  4 .2  AgBr  85  resonance.  resonance.  resonance  64  a t 36.3GHz.  luminescence.  65 73  luminescence.  F i g . 4 .3  ODMR i n A g B r .  87  F i g . 4 .4  ODMR i n A g B r .  88  Fig.  4 .5  Cyclotron  Fig.  4 .6  Low power c y c l o t r o n  resonance  vi  i n AgBr. resonance  89 signal.  91  Fig.  4. 7  Power d e p e n d e n t c y c l o t r o n  resonance  F i g . A. 1  The  Nelder-Mead m i n i m i z a t i o n  F i g . A. 2  The  convergence  F i g . A. 3  Gaussian  F i g . A. 4  Fits  from  and  signal.  procedure.  process.  Lorentzian f i t s  IA5.  92 112 114  to  data.  116 121  vii  ACKNOWLEDGEMENTS It  gives  me  supervisor,  much  pleasure  t o thank  Dr.C.F.Schwerdtfeger,  my  research  f o r h i s advice  and  instruction.  The  h e l p g i v e n b y D r . R. B a r r i e  criticizing gratefully  this  and Dr. J . E l d r i d g e i n  t h e s i s and i m p r o v i n g  i t s presentation i s  acknowledged.  Thanks  are also  due  t o Dr.W.Czaja  for his helpful  criticism  o f o u r p u b l i c a t i o n s . Our a p p r e c i a t i o n  Dr.J.Merz  and  Telluride  The which  Dr.W.Czaja  and S i l v e r  assistance  f o r donating  post-graduate Columbia  thanks  samples  of  Zinc  Bromide.  o f my p a r e n t s  was i n v a l u a b l e , i s g r e a t l y  Finally  i s due t o  i n the analysis of data, appreciated.  a r e d u e t o NSERC  Scholarship  f o r a Teaching  67/2228 f o r s u p p o r t i n g  f o r t h e award  and t o t h e U n i v e r s i t y  Assistantship,  part  of B r i t i s h  and t o NSERC G r a n t #  o f t h e work.  viii  of a  Chapter 1. INTRODUCTION  1.1 H i s t o r i c a l .  Precession observed by  by P.Zeeman  S i r Joseph  parallel  of the spin  i n 1896 a f t e r  Larmor.  When  to the spin axis  about the f i e l d  o f an e l e c t r o n  direction  was  theoretical  a magnetic  predictions  field  i s applied  of the e l e c t r o n , precession with  a frequency  first  occurs  g i v e n by:  (o = gu e B/tt  where B i s t h e m a g n e t i c f i e l d , electron, is  g t h e Lande f a c t o r  Planck's  Quantum of  n e t h e m a g n e t i c moment o f t h e ( = 2.0023 f o r f r e e  constant.  mechanically  the electron  this  i n a magnetic  means t h a t t h e e n e r g y field  will  corresponding  to spins p a r a l l e l  by  be t w o l e v e l s  and a n t i - p a r a l l e l  to the  field.  Transitions by  levels  be s e p a r a t e d  a m o u n t s M<y a n d , f o r t h e e l e c t r o n , t h e r e w i l l  magnetic  s p a c e ) , and Yi  between  electromagnetic  transitions  correspond  t h e two s p i n  fields  states  of frequency  c a n be w,  and  t o absorption or t o induced  1  induced these  emission  of  a photon. This  change since spin  i s known a s p a r a m a g n e t i c  i n the e l e c t r o n the  spin  spin  changes  resonance.  i n such a t r a n s i t i o n  from  +1/2  t o -1/2  and  i s unity  a photon  1 i s a b s o r b e d o r e m i t t e d . Thus t h e t r a n s i t i o n  The  of  conserves  b o t h e n e r g y and a n g u l a r momentum.  The  transition  electron  magnetic  i s caused  field  electromagnetic  field  applied  field.  magnetic  with which  Electrons at a f i n i t e will as  tend to r e d i s t r i b u t e to  produce  differences. at  and  Thus,  resonance,  electrons this  a  a  by  the  absorption  component  perpendicular  their  spin  energies  absorption  of  measured  which  in a variety  In  a  typical  investigation c a v i t y . The  EPR  i s placed microwave  way  applied  occur  energy  have been u s e d t o d e t e r m i n e m a g n e t i c  frequencies of e l e c t r o n s  the  field  of  This i s the b a s i s of e l e c t r o n paramagnetic resonance studies  of  energy  is  will  the lower to the higher c a n be  of  field  energy  the  i n such a  distribution  electromagnetic  of  to the  temperature i n a magnetic  when an  move f r o m  interactions  magnetic  m u s t be  Boltzmann  net  the  as  state, ways. (EPR)  resonance  i n semiconductors.  experiment in a  magnetic  the  material  field  frequency i s fixed  f r e q u e n c y o f t h e c a v i t y , and  the magnetic  under  in a  microwave  at the  resonant  field  i s s w e p t . At  r e s o n a n c e t h e i n c r e a s e d a b s o r p t i o n o f t h e m i c r o w a v e power by the  sample changes  the Q f a c t o r  of the c a v i t y . This  2  causes  an the  observable  change  1952  Brossel  paramagnetic  by  resonance  irradiation  ground their  in  state spin  with the  of  power  prior  populations polarization  The  of of  first  Geschwind  et  reflected  have been  In another charged field,  the  from  a magnetic type  of  B the the  f r e q u e n c y eB/m, quantized  wavelength. to  the  electrons  this  and  change was  material  thus  at  force  magnetic  an  that  of  from  substances  as  right  evxB  cyclotron  (3-10).  angles  where  e  resonance.  to  is  field  direction  A  a  magnetic  the  electron  magnetic f i e l d . This r e s u l t s i n  that  the  detected.  was  many o t h e r  a  relative  l u m i n e s c e n c e changes  then  on  charged p a r t i c l e s p a r t i c i p a t e i n  moving a  their  a p p l i c a t i o n of  r e s o n a n c e s have b e e n o b s e r v e d  where m i s the so  the  in a solid  phenomenon known  experiences  produced  decay  changed  light;  measured  field  particle,  about  of  in A1203. Since and  with  whose p o l a r i z a t i o n depends  states  who  vapour  appropriate  normally  signal  emitted  investigated  mercury  d e c a y p r o c e s s . The  observation  studied  c h a r g e , and motion  spin  in  of  state  resonance the  (1)  e x c i t a t i o n was  light  photons  the  a l . (2)  impurities  s t a t e . The  excited  to  Bitter  electrons  intense  emitting  paramagnetic  are  microwave  and  a t o m s i n an e x c i t e d  Electrons  Cr  the  cavity.  In  the  in  at  circular angular  e l e c t r o n mass. These o r b i t s  infinite  3  series  of  levels  exist  separated  by e n e r g y  M<a • T r a n s i t i o n s  between these  c a n be i n d u c e d by an e l e c t r o m a g n e t i c f i e l d interaction  o f the e l e c t r i c  component o f t h i s  e l e c t r o n . The a b s o r p t i o n o f e n e r g y  from  levels  because field  of the  with the  the a p p l i e d  field  c a n b e m e a s u r e d i n t h e same way a s t h a t d e s c r i b e d f o r E P R .  The e f f e c t was f i r s t o b s e r v e d  i n gaseous  et  a l . i n 1950 ( 1 1 ) . I t was l a t e r s e e n i n t h e  Ge  by D r e s s e l h a u s  observed  e t a l . i n 1 9 5 3 (12)  i n other s o l i d  Detection  plasmas  by Lax  semiconductor  and has s i n c e  been  materials.  of cyclotron  resonance  by i t s e f f e c t s  on  l u m i n e s c e n c e has o n l y r e c e n t l y been r e p o r t e d by Romestain and W e i s b u c h  It  (13)  i n CdTe and b y B a r a n o v e t a l . i n Ge ( 1 4 ) .  i s t o be n o t e d t h a t t h e e f f e c t s  resonance  have  background resonance  been  signals signals  observed  of cyclotron  i n ODMR  underlying  experiments  the desired  as  paramagnetic  (7,15-17).  A Typical Experiment.  The  essential  in  Fig.1.1.  be  taken  The m a t e r i a l i s generally  temperature luminescence affected  elements  range, since  o f an ODMR e x p e r i m e n t on w h i c h cooled  by t e m p e r a t u r e .  to the liquid resonances  helium and t h e  t o observe  them a r e  The sample  may b e d o p e d  4  shown  the measurements a r e t o  paramagnetic  p r o c e s s e s used  are  strongly with  FIG. L i CHOPPED MICROWAVES  BASIC ODMR EXPERIMENT  DATA COLLECTION  impurities  t o produce  centres. source, or  Free usually  x-rays  particular  carriers  a laser  types  of recombination  aregenerated  by an e x c i t a t i o n  although  a r c lamps,  used.  The c a r r i e r s  a r e sometimes  electron  beams  recombine i n  v a r i o u s w a y s , t h e m o s t common l u m i n e s c e n t p r o c e s s e s the decay  of electrons,  either  c e n t r e , and donor-acceptor enough  excitation  impurity  levels,  electromagnet resonances  field  and  are excited  by a  spins affects  i n observable  i n total  measured b y a l o c k - i n  the or  by a c o n v e n t i o n a l  microwave  Paramagnetic  Magnetic  field  after  Resonance  which i s  heating  the recombination  changes  intensity,  i s monitored  The  may be o b s e r v e d .  either  field of  the  processes  i n the luminescence  polarization, appropriate  m i c r o w a v e s a r e c h o p p e d t o p r o d u c e an  1.2  complexes a t  frequency w h i l e the magnetic  the resonances.  or their  results  which  multi-exciton  At high  o r by a s u p e r c o n d u c t i n g magnet.  through  either  bound  t o an i m p u r i t y  recombination.  i s generated  generally held a t a fixed  carriers  pair  o r bound  c e n t r e s and e l e c t r o n - h o l e d r o p s  A magnetic  sweeps  free  being  or  wavelength,  filtering.  a.c. s i g n a l  detector or similar  The which i s  device.  Resonances.  most common a p p l i c a t i o n o f ODMR t o d a t e h a s been i n  detection bound.  o f paramagnetic  resonances  of carriers,  free  A number o f t e c h n i q u e s have been d e v e l o p e d t o  enhance t h e s e n s i t i v i t y  o f t h e b a s i c ODMR  experiment.  The  effects  illustrated 1.2(a)  of electron  paramagnetic  resonance  may  be  b y t h e s i m p l e e x a m p l e s s h o w n i n F i g . 1.2. F i g .  shows  a  typical  donor-acceptor  allowed recombinations of a spin  pair  1/2 e l e c t r o n  system  with  and a s p i n  3/2  hole.  Allowed electron  s y s t e m c h a n g e s by u n i t y  compensating half  t r a n s i t i o n s a r e ones i n w h i c h t h e s p i n o f the  f o r the spin  or zero during  o f t h e p h o t o n e m i t t e d . On  of the donor-acceptor p a i r s  states  that  allow  optical  d e c a y by n o n - r a d i a t i v e between  optically versa.  disallowed  Since  pairs  have  spin  t r a n s i t i o n s , the other  half  will  spin  The EPR  states  will  transitions cause  t r a n s i t i o n s t o become with  average  populated w i l l  processes.  the the e l e c t r o n  the decay,  optically  allowed  shown  pairs  with  allowed,and  vice  transitions  will  recombine f a s t e r than those w i t h o u t , the net e f f e c t of the resonance  signal  will  expense o f n o n - r a d i a t i v e  The the spin  spin  states  magnetic f i e l d states  become  be t o i n c r e a s e  a n d an EPR  pairs  a spin-forbidden  with  recombine, thus the resonance  An  exciton  and h o l e s  are s p l i t i n  t r a n s i t i o n between  a l l o w e d . In t h e absence  electron-hole  at the  decays.  of the e l e c t r o n s  can cause  luminescence  recombination  o f t h e EPR  the  i s a bound s y s t e m  7  resonance  appropriate  increases  electron  spins  to  only can  the luminescence.  o f an e l e c t r o n  and a  hole,  FIG. 1.2  b)  LUMINESCENCE  PROCESSES  Exciton levels formed by spin 1/2 electrons and spin 3/2 holes. Transitions involving a change of 0 or +1 in Mj are allowed while those causing a change of +2 are forbidden as shown.  8  somewhat s i m i l a r exciton  states  holes.  An  formed  example  considered the  to a hydrogen  i n Chapter  populations  states.  of  In p a r t i c u l a r  forbidden electron theory  of  optical  atom.  from  spin  such  a  3. The  1/2  electrons  system  exciton  i s GaP  electron  and  the  |2,+2> e x c i t o n more  likely  s p i n s are heated thus r e d u c i n g  absolute  zero  discussed  for a  saturation  i n more d e t a i l  practice  50%  epr  changes  of  be  depend  on  with  t o form  spin dipole  when  the  t h e l u m i n e s c e n c e . In could  signal.  i n Chapter  3/2  will  hole  states,  the  spin  which formed  initial  decays, are  and  states  the  a change i n l u m i n e s c e n c e o f  In  F i g . 1.2(b) shows  be  observed  These  points  at are  3.  the  order  of  1%  are  to  be  e x p e c t e d . A s i m i l a r a r g u m e n t c a n be a p p l i e d  i n the case of  hole  processes  paramagnetic  present the  in a  practical  A  experiment  number which  of  reduce  the  so  that  polarized. higher  In  the  addition  relaxation  excitons  are formed  exciton  levels  it  affected  luminescence  paramagnetic broadened, different  will  carrier  and  free  out  should  resonance,  i.e. d i f f e r e n t  depending  may  the e f f e c t s  carriers  while  be  free.  i f this  is  A  be  the  spin  lifetimes.  Once  within  maximum  the  change  saturating  the  inhomogeneously  of the c r y s t a l  resonant frequencies, only a small  9  may  of the resonance i f  by  parts  completely  on  occur  produced  but  be  of  absolute  temperature  carrier  thermalization 1  not  spin  temperature  time  'washing the  spins  the  than the sample  lattice  only  carrier  are  size  observed s i g n a l . O b v i o u s l y the sample i s not at  zero  in  resonance.  have  slightly  packet of  spins  can  be  saturated  In  some  overcome  at  one  cases  or  time.  the  factors  to  advantage  used  just  mentioned  to  may  enhance  be  signal  detectability.  Conduction used  to  measure  Instead  of  the  produced  to in  radiation  emitted  will to  light  must be  best  be  in  In  free  as spin  magnetic  resonant  (CESR)  or  polarization in  to  state  the  polarized  a decrease polarized parallel field  with  the  where  the  magnetic the  electrons  magnetic  electrons, the  rapidly  the  magnetic  circular  are  not  depolarizes  band gap.  The  recombination  produced  polarization  l u m i n e s c e n c e , and  is  the  sometimes  10  (since  them),  time  and  works of  the  time.  population  excitation visible  e n h a n c e m e n t can  In  excitation  method  thermalization  by  a considerable  resonance  field  a c o n v e n t i o n a l ODMR e x p e r i m e n t where s p i n  differences  field  in this polarization.  to  i s l e s s than t h e i r s p i n  circularly  recombination  applied  a  electrons  using  The  and  selective  spins,  by  widely  (6,10,18).  spin  provided  spins,  is  electrons  excitation.  initial  applied  systems  electrons  of  thermalization  parallel  generate  transverse  should  for  show up  order  a  on  circularly  'remember' t h e i r signal  value  resonance  a p a r t i c u l a r spin  light  be  spin  produce a  polarized  will  g  relying  recombination are  electron  be  light, in seen  the in  an  ODMR s i g n a l  cases  egual  by m o n i t o r i n g  and o p p o s i t e  only  one p o l a r i z a t i o n .  signals  a r e seen  I n some  by l o o k i n g a t  o p p o s i t e l y p o l a r i z e d components o f the l u m i n e s c e n c e , thus the  paramagnetic  intensity  circular the  would n o t g i v e polarizations  obtained the  magnetic the  similar  that  single  only  corresponding  total  different  l i n e correspond to  with  different  components. be d i s t i n c t  t o microwave  spin  i n a magnetic  i n ODMR s i g n a l s  Zeeman  will  but only i t s  Since  Zeeman s p l i t t i n g  enhancements  the total  monitored  (17,19).  w i t h i n an e m i s s i o n  components  fields  a signal  exhibit  by o b s e r v i n g  Zeeman  significantly  of excitons  components, t h i s w i l l Thus  n o t change  and an e x p e r i m e n t  recombination  field.  does  of the luminescence  polarization, intensity  resonance  may b e  In general at  higher  frequencies i n  r e g i o n o f 30 GHz o r a b o v e .  An spin  ODMR s i g n a l  populations  resonance  corresponds  of electrons  excitation  t o t h e change i n r e l a t i v e or holes  i s turned  as t h e magnetic  on o r o f f .  resonance s i g n a l would e q u a l i z e the spins them  completely)  cases  and g i v e  the paramagnetic  broadened  thegreatest resonance  A saturation  (i.e. d e p o l a r i z e  signal.  I n many  i s inhomogeneously  so t h a t t h e microwave e x c i t a t i o n can o n l y a f f e c t a  f r a c t i o n o f t h e t o t a l s p i n s a t any t i m e as i t sweeps t h r o u g h the  resonance,  and thus  a  saturation  realized.  A method  sometimes  modulate  the f i e l d ,  (usually  although  used  signal  t o overcome  theapplied  the microwave frequency  11  c a n n o t be this  magnetic  c o u l d be m o d u l a t e d  is  to  field  instead)  over  a small  relaxation the  range  time,  at a frequency  allowing  the spin  to participate i n  (7,15,20,21).  In  luminescence  several  to identify  ODMR s i g n a l . field  spectra  recombination  desirable  This  a t resonance  which  bands  components e x h i b i t  a particular  by s e t t i n g  through  theactual  signals  microwave d i e l e c t r i c with true  t h e ODMR caused  the magnetic spectral  field  dependence  ratios  or signal—to—noise  b e t t e r t o do a s e r i e s o f m a g n e t i c  centres)  corresponding to salient  (such  as s t r o n g  emission  t o deduce t h e s p e c t r a l  broad due t o  and s u b t r a c t i n g , signal  may b e  resonances a r e  a r epoor,  i t i s often  scans a t d i f f e r e n t  features peaks  dependence.  12  This  By d o i n g a s e c o n d  several  field  have  changes  o f t h e ODMR  present,  this  luminescence  may s t i l l  o f f resonance  where  spectrum  that  heating.  obtained, buti n situations  wavelengths  spectrum.  by l u m i n e s c e n c e  or carrier  with a  and c o m p a r i n g  optical  signal  the magnetic  t h e spectrum  t h e ODMR s i g n a l  c a n be c o m p l i c a t e d by t h e f a c t  background  from  i t i s often  and s w e e p i n g  n o t showing  contributions  or processes  c a n be done  'ODMR s p e c t r u m ' w i t h method  having  centres  monochromator, measuring  the  more p a r t i c l e s  than  r e s o n a n c e a t a n y t i m e , a n d t h e r e b y i n c r e a s i n g t h e ODMR  signal  scan  higher  i n the  from  optical  different  1•3 Advantages o f ODMR.  ODMR  looks  principle,  with  techniques.  at resonances conventional  visible,  microwave  i n  absorption  D e p e n d i n g on t h e s y s t e m b e i n g Observation  at least  EPR  c o n s i d e r e d , ODMR  may have  s e v e r a l advantages.  o f an EPR s i g n a l  requires  t h e a b s o r p t i o n o f a m e a s u r a b l e amount o f m i c r o w a v e  p o w e r b y a weak m a g n e t i c d i p o l e t r a n s i t i o n dissipation particular, saturate power  the  The weak  EPR s i g n a l  transition  cause  (although  excitation i t may a l s o  conventional  EPR.  ODMR  signals'  recombination  unless  very  In  tends t o  low microwave t o noise  are photo-generated, t h i s  obviates  (except  for suitable  dipole  recombination  transitions  changes  and e f f e c t i v e l y  e x c i t e d by  i n the optical amplify  the  a saturation of the spins results  i n a maximum  EPR  by t h e  ODMR  signal  b r o a d e n i t ) and s o i s n o t a p r o b l e m as In a d d i t i o n  in appropriate circumstances, luminescence processes  t h e EPR s i g n a l  macroscopic  probability,  relaxation.  g i v e s a poor s i g n a l  magnetic  Furthermore  microwave  in  a r e u s e d , and t h i s  n e c e s s i t y f o r doping  signal.  v i a spin  a t low temperature  As ODMR c a r r i e r s  centres). the  power  and become u n o b s e r v a b l e  levels  ratio.  of this  and subsequent  to giving  better  ODMR s e r v e s a s a t o o l  as w e l l as p a r a m a g n e t i c  from  free  carriers  at a particular  spins o f the recombining  centre  particles,  13  signals t o study  resonances.  can indicate i saffected  by  how the  a n d ODMR o f p a r t i c l e s  trapped  a t luminescence  centres  gives  information  directly  a b o u t t h e n a t u r e and symmetry o f t h e c e n t r e s .  1.4 Cyclotron Resonance  Conventional semiconductors microwaves carriers come  detection  involves  measuring  by f r e e . c a r r i e r s  may b e t h e r m a l l y  from  donor  or o p t i c a l l y  or acceptor  of  electrons  and  resonance i n  the absorption  i n a magnetic  resonance measurements give mass  of cyclotron  field  The  g e n e r a t e d , o r may  impurities.  information  holes,  (22).  of  Cyclotron  on t h e e f f e c t i v e  and  their  mobility.  M e a s u r e m e n t o f c y c l o t r o n r e s o n a n c e i s i n some ways a n a l o g o u s to  that  of paramagnetic  resonance  i s e x c i t e d by t h e e l e c t r i c  microwaves and  r e s o n a n c e , however  rather  i t tends  field  t h a n by t h e m a g n e t i c  to  be  much  the c y c l o t r o n  component  of the  component a s i n EPR,  stronger  and  more  readily  observable.  Several  factors limit  however. determines and  The  the usefulness  scattering  the width  time  obtaining a  the width  distinct frequency  carriers.  I n many c a s e s resonance  than  o f EPR l i n e s . T h e  and  i n most  W c  tSl,  precludes  at reasonable  14  cases,  condition u  T the s c a t t e r i n g  this  carriers  the spin r e l a x a t i o n time  resonance i s  cyclotron  cyclotron  the free  of the c y c l o t r o n s i g n a l  i s g e n e r a l l y much s h o r t e r  which governs  of  of the technique,  c  being time  for the  o f the  the observation of  microwave f r e q u e n c i e s  and  magnetic  field  impurities cyclotron  strengths.  also  tends  resonance  t o reduce  measurements  samples o f h i g h p u r i t y . a problem power  Doping w i t h donor o r a c c e p t o r carrier  are generally  Saturation  probability  the carriers  causing  and  made o n  o f the resonance  a s i n EPR, b u t a p p l i c a t i o n  may h e a t  mobility,  i s not  o f e x c e s s i v e microwave  and change  their  scattering  asymmetry i n the resonance.  I f the  c o n d u c t i o n o r v a l e n c e band i n q u e s t i o n i s s i g n i f i c a n t l y nonparabolic, this effective  The  detection  only recently  resonances  observed  result  i n a change i n t h e a p p a r e n t  of cyclotron  of electrons  induced  produces  resonance  i n GaAs and CdTe and h o l e s  by m o n i t o r i n g c h a n g e s  lineshape heating  also  mass.  Optical reported  will  by c a r r i e r  has been  (13) and i n Ge (14).  i n GaAs and CdTe  i n luminescent  were  i n t e n s i t y and  h e a t i n g . I n Ge t h e r e s o n a n t  observable decreases  i n electron-hole  drop  l u m i n e s c e n c e . The m e a s u r e m e n t s a r e i n many ways a n a l o g o u s t o ODMR e x p e r i m e n t s changes i n t o t a l polarization  Optical direct  although they intensity  rather  tend than  to rely  on o b s e r v i n g  looking  a t changes i n  a s i s o f t e n done i n ODMR.  detection  advantages  over  of cyclotron  t h e more commonly u s e d  measuring  microwave c y c l o t r o n  carriers,  since  this  much s t r o n g e r t h a n  resonance  o f f e r s few technique of  a b s o r p t i o n by p h o t o - g e n e r a t e d  absorption i s readily  observable  i n EPR s i g n a l s . H o w e v e r , by u s i n g  15  being  optical  d e t e c t i o n , i n f o r m a t i o n may luminescence  processes  be o b t a i n e d a b o u t t r a p p i n g  and  their  sensitivity  to  and  carrier  temperature.  1.5 Overview of the thesis  2 describes  Chapter apparatus digital and  u s e d , and  photon  those  of  cyclotron results  relative  of our  in  paramagnetic  i n the  the  m e r i t s of  the  i n p a r t o f the work  p r e v i o u s w o r k on GaP  investigation  from  and  detection.  3 reviews  luminescence  reported  the  c o u n t i n g t e c h n i q u e used  resonances  In GaP in  discusses  lock-in  Chapter  the e x p e r i m e n t a l procedure  and  of these  resonance  B i and  literature  ZnTe  paramagnetic and  presents  in  o f e l e c t r o n s was  S impurities; previously  luminescence  calculated  and  experimentally frequencies  i t  homogeneously approximately  and  by  good  agreement  observed  signal.  was  determined  broadened,  4 nanoseconds  the resonances, a r e s u l t  Cyclotron  caused  resonance  and  this  observed  had  not  been  supports  the  work  was  the  was  that  the  two  of e l e c t r o n s  was  with  the  microwave  resonances  carrier  not p r e v i o u s l y  16  found  using  deduced  expected  resonance  By  a  the  materials.  o f C a v e n e t t on ODMR f r o m N c e n t r e l u m i n e s c e n c e . The change  and  from  were  lifetime the  widths  of of  reported.  and  light  and  heavy  h o l e s was  observed t o cause  N,  Bi centres.  S and  determined more  from  centres  were  electrons  Although  these  accurate  than  shown  t o be  already  sensitive  of  the  agreement w i t h t h e e x p e r i m e n t a l  observed  resonance  i n various  interpretation uncertainty  Paramagnetic  4  details  of  process  on t h e t h e o r e t i c a l  impurity  was  resonances  our  showed  good  a n d h o l e s was  also  ZnTe, however  difficult  because  iodine  centres  features  seen  on t h i s  luminescence an  certain  using  of  of  in  the  material.  some  resonance  an enhancement  shown i n p a r t o f t h e e l e c t r o n r e s o n a n c e  increased energies  p h o n o n s . The e f f e c t i v e  trapping due  to  probability interaction  line  f o r electrons with  masses o f hot e l e c t r o n s  17  from  interesting  cyclotron  In p a r t i c u l a r  to  Cyclotron  luminescence  and showed  i n conventional  AgBr.  ODMR were shown  those of other r e s e a r c h e r s .  isoelectronic not  investigation  observed  observed  with  heating  results.  the r e s u l t s  also  indicated  based  of electrons  r e s o n a n c e was  of  different  seen.  l u m i n e s c e n c e bands from  substantially  experiments  values  significantly  the trapping  N  from  i n the i m p u r i t y content of the c r y s t a l .  Chapter  resemble  of  not  to cyclotron  on  of the e f f e c t  mass  published,  not p r e v i o u s l y  cross-section  Cyclotron  the e f f e c t i v e  depending  i n v o l v e d , a phenomenon  trapping  i n luminescence  m e a s u r e m e n t s were those  or holes  A calculation  a decrease  optical  calculated  from  the  results  agreed  substantially  with  theoretical  predictions.  Chapter for  our  conclusions  and  some  suggestions  f u r t h e r work.  An fitting of  5 contains  data  a p p e n d i x d e s c r i b e s a new w h i c h was from  the  experiments. This  devised rather  method f o r n o n - l i n e a r  i n connection noisy  i s compared w i t h  been p r o p o s e d r e c e n t l y and to convergence to f a l s e  with  signals another  i s s h o w n t o be  minima.  18  curve  the e x t r a c t i o n  produced  in  our  method w h i c h  has  less susceptible  Chapter 2.Apparatus and Experimental Techniques.  2.1  Apparatus A  block  diagram  experiments used  i s shown  i n the i n i t i a l  being  smaller  magnetic  electromagnet.  differed  to the bulk  beam  field  originally  (out of  as  shown  rather  allowed  atmospheric liquid vapour  o f f into  supplied  a return  i t s lambda  point  pressure of the helium could manometer a t t a c h e d  the  optical  walls.  casing  of  window than  shown i n  a  Varian  luminescence angles to the  used  f o r the  parallel  to the  the  transferred, line,  magnet liquid  could  the Dewars.  The  be m e a s u r e d by means o f  provided  Dewar  by a p u m p i n g s y s t e m  the  K.). T h e  t o t h e Dewar. B o t h i n n e r  were  be  at approximately  (2.2 d e g r e e s  m e a s u r e m e n t s and t o r e d u c e  Q u a r t z windows  continuously  by  device  at right  ( c o n t a i n i n g t h e helium) were c o n s t r u c t e d allow  Dewar  p r e s s u r e , o r be pumped o u t , t h u s c o o l i n g  to below  a mercury  t h e one  jacket to contain  p r e - c o o l a n t . H e l i u m , once to boil  from  helium  f o r the superconducting  Dewar. B o t h Dewars had an o u t e r nitrogen  i n t h e ODMR  liquid  of this  t h e same  i n fact)  used  no s u p e r c o n d u c t i n g magnet. The  only i n a direction  field  magnetic  i n F i g . 2 . 1 . The work  was  Due  c o u l d be v i e w e d  excitation  the equipment  and c o n t a i n i n g  field  magnetic  of  from  Dewars  Pyrex g l a s s t o  h e a t c o n d u c t i o n up i n the outer  vacuum  maintained  of a  diffusion  pump a n d a r o u g h i n g pump. T h e v a c u u m was b e t t e r  t h a n 10~4  torr.  19  consisting  was  metal  FIG. 2,1  MICROWAVE SWITCH  APPARATUS  MICROWAVE DETECTOR TRAVELLING WAVE TUBE AMPLIFIER  ISOLATOR MICROWAVE  GUIDE  KLYSTRON MAGNET  MODE SWEEP MONITOR  POWER NOVA II  SUPPLY  COMPUTER PUMPED SQUARE  WAVE  LIQUID  H E  GENERATOR SUPERCONDUCTING MAGNET  LOCK - IN DETECTOR  A /D CONV.  LASER LIQUID DEWAR  SPECTROMETER  PHOTOMULTIPLIER  Apparatus showing 9.2 GHz microwaves and superconducting magnet.  HE  B o t h D e w a r s , when f i l l e d , for  between  two  and  dissipation  of  whether the  helium  The  the  was  being  electro-magnet fields  could  be  computer  field  strength,  between  of  up  or  which  i n the  15  to  of  was  on  the  l i g h t being lower  helium power  used,  initial  either a  The  to  measurements  field  by  a  supply  particular  which  was  diphenylpicry1 observed  set  field  and  temperature.  k i l o g a u s s , i t s power  produce  limits.  resonance  g=2.0037  to  the  pumped t o a  used  liquid  depending  and  controlled  preassigned  EPR  hours  microwaves  produced  the  three  would r e t a i n  was  swept  calibrated  hydrazyl  using  (DPPH)  conventional  at  microwave  absorption.  The  superconducting  provided of  this  f i e l d s up  device  The by  the  This  computer with  to  could  DewarJ  experiments  t o 50 k i l o g a u s s . A d e t a i l e d  description  been g i v e n  power s u p p l y  provided supply  has  magnet used i n l a t e r  be  a  or  turned  however,  Optical  for this by  a  O.Ziemilis  sweep  o f f once heating  since  our  magnetic f i e l d ,  excitation  of  (23).  magnet c o u l d be box.  The  ' p e r s i s t e n t switch' which  eliminate  sweeping the  by  the  the  field  current  experiments this  the  21  magnet allowed  desired  from  driven  facility  sample  either  was  also  the  power  was  reached.  leads  in  the  always  involved  was  used.  was  not  produced  by  a  Spectra of  Physics  power  at  wavelengths violet  185  a r g o n i o n l a s e r w h i c h g e n e r a t e d 1/2  4880  A.U.  i n the  range  excitation  provided  by  operating  a  at  and  4580 A.U.  required  Spectra 3250  A.U.  could  for  be  to  the  tuned  5145  to  A.U.  studies  other  The  of  ultra-  AgBr  Physics  285  helium-cadmium  or  PEK  500  by  a  Watt  Watt  was laser  mercury  arc  sample  was  lamp.  A  narrow  band  of  luminescence  s e l e c t e d by a Spex m o n o c h r o m a t o r and R928  p h o t o m u l t i p l i e r tube  r a n g e o f i n t e r e s t . The pulse  shaping  which  had  from  d e t e c t e d by a Hamamatsu a good  output of the  electronic  circuit  as  the  response  in  the  photomultiplier fed a a p r e l i m i n a r y to  photon  counting.  Microwave e x c i t a t i o n A t 9.2 GHz  approximately  Hughes model 1177  was  available  at  9.2  10 W a t t s o f p o w e r was  travelling  output of the k l y s t r o n  amplifier  a  diode  m i c r o w a v e s t o be  modulated  pulse  the  generator,  The quartz)  s a m p l e was rod  waveguide allow  on  and  36.3  p r o v i d e d by a  switch  (chopped) by  was  which  fed to  allowed  o f f times being  a  impedance  hole  and  a  matching.  22  the  moveable  a  equal.  m o u n t e d on a n o n - c o n d u c t i n g  small  a  the  a s q u a r e wave f r o m  (teflon  i n t h e c e n t r e o f a TE102 c a v i t y c o u p l e d  by  GHz.  wave t u b e a m p l i f i e r d r i v e n by  r e f l e x k l y s t r o n . The through  and  teflon  to plug  or the to  The  microwave  detected  i n one  power  arm  of  reflected  a  'magic  from  the  cavity  was  and  this  signal  was  Tee'  used to tune the k l y s t r o n  to resonance with  the c a v i t y  and  to  match.  tuning  the  adjust  klystron drift view  the  impedance  to the  was of  not the  chopping  cavity  large  not  used because the  of  frequency  enough t o c a u s e p r o b l e m s , e s p e c i a l l y  large  would  was  Automatic  widths  also  tend  of  the  resonances  to i n t e r f e r e  observed.  with  the  in The  automatic  tuner.  Optical holes so  access  i n the  placed  sides  as  to  current flow around  At  36.3  interfere  as  little  associated with  the  The no  although  and  amplify  the  value  when  varying with  a of  provided  achieved  square  wave  by  which  the of the  provided  available  properly  microwave  up  to  800  modulating  the  switched  the  i t s modes.  m o u n t e d i n t h e o p e n end  the  with  investigation.  resonator. This  i t reduced  resonator,  with  o f f one  s a m p l e was cavity  possible  a Varian reflex klystron  voltage  on  as  p l a t e . These were  T E 1 0 2 mode. Q f a c t o r s  exact  o f p o w e r . C h o p p i n g was  reflector klystron  GHz  p r o v i d e d by c i r c u l a r  s l o t s i n the bottom  sample u n d e r  milliwatts  A  and  2000 were o b t a i n e d , t h e  particular  with  t o t h e c a v i t y was  microwave  tuned power  23  good  to  the  density  of a waveguide optical field  strength.  source, by  access  a  should factor  approximately  equal  t o the resonant Q v a l u e . There  reasons for not using a resonator: optical the  access  w o u l d be d i f f i c u l t  collection  bottom  of luminescence  of the resonator;  some o f t h e s a m p l e s  used  f i r s t , because  adequate  to provide, especially f o r  which  requires holes  and s e c o n d , b e c a u s e would  were two  have  i n the  the size of  made them d i f f i c u l t  to  accommodate i n t h e s y s t e m . The n e t e f f e c t w o u l d have been t o reduce the Q f a c t o r was  of a resonator  t o s u c h an e x t e n t  that i t  n o t deemed w o r t h w h i l e u s i n g o n e .  It  should  be n o t e d  that  paramagnetic  resonances are  e x c i t e d by t h e m a g n e t i c component o f t h e m i c r o w a v e while  c y c l o t r o n resonances  c o m p o n e n t . The TE102 c a v i t y the  electric  field, the  field  respond  of  to the e l e c t r i c  a t i t s c e n t r e , a n d a maximum  the  was u s e d .  producing was  We  cavity  a variable  critical  field  i n cyclotron  that  of  luminescence,  o r an up-down c o u n t e r .  a digital  electronic  s i n c e no  method f o r  counter  synchronous with the  using  either  The l a t t e r which  24  of the c y c l o t r o n  3 and 4 .  microwave c h o p p i n g , were d e t e c t e d detector  resonance  existed  simple  the existence  i n chapters  i n detected  this  from  h a s n o t been d e s c r i b e d b e f o r e . I t  i n determining  resonances discussed  Changes  believe  magnetic  to d i s p l a c e the samples  m e a s u r e m e n t s . A t 36.3 GHz no s u c h p r o b l e m s cavity  field  u s e d a t 9.2 GHz h a s a n o d e i n  t h u s i t was n e c e s s a r y  centre  field,  could  device  a  lock-in  consisted  be s w i t c h e d  to  count  positively  or negatively  f r e q u e n c y . Thus o p t i c a l  counts  a t t h e microwave  generated w i t h t h e microwave  s o u r c e s w i t c h e d on w e r e s u b t r a c t e d the  chopping  from  those produced  m i c r o w a v e s o f f . S i n c e the o n / o f f t i m e s were e q u a l , any  microwave-induced net p o s i t i v e  change i n t h e l u m i n e s c e n c e  showed up a s a  o r n e g a t i v e c o u n t . I t was f o u n d n e c e s s a r y t o  suppress incoming p u l s e s f o r a few microseconds the  with  each  c o u n t e r was s w i t c h e d t o p r e v e n t t r a n s i e n t p u l s e  Data were c o l l e c t e d w i t h a N o v a l l could  sweep e i t h e r  a series  required,  photomultiplier  system  could  range, pausing a t each  record  directly  which  o r the spectrometer i n  I f the photoluminescence  t h e computer  errors.  minicomputer  the output o f t h e l o c k - i n d e t e c t o r  counter.  digital  field  o f steps over the d e s i r e d  step t o record down  the magnetic  time  o r t h e up-  spectrum  the output  by u s i n g  was  of  the  an a n a l o g t o  converter.  The  number o f s t e p s i n a s c a n , and t h e t i m e  collecting  d a t a a t each  s t e p , were s e t b y t h e  spent i n  operator.  2.2 Experimental P r o c e d u r e s .  In  m o s t ODMR e x p e r i m e n t s  t h e change  i n luminescence  c a u s e d b y t h e m i c r o w a v e r e s o n a n c e w a s l e s s t h a n 1% s o t h a t obtaining  adequate  signal  to noise  p r o b l e m . As an i l l u s t r a t i o n : 0.1%  change  i n luminescence  ratios  was o f t e n  f o r a n ODMR s i g n a l  w i t h a photon  25  count  a  causing a  rate  o f one  m i l l i o n p e r s e c o n d , and a d a t a c o l l e c t i o n  time  per  an o u t p u t  step,  t h e ODMR  signal  would  give  c o u n t s . T h i s was e q u a l t o t h e s t a t i s t i c a l one  million  photon  count  the square  root o f the t o t a l  linearly,  i t would  contained  50  steps  frequency  effects  fluctuation  fluctuations  to noise ratio  and took such  i n the  as  about drift  increases  100 s e c o n d s p e r  o f 10. A t y p i c a l 1.5  ratio  i n c r e a s e as  c o u n t , and t h e s i g n a l  be n e c e s s a r y t o s p e n d  to obtain a signal  o f 1000  so t h a t t h e s i g n a l t o n o i s e  was u n i t y . S i n c e t h e s t a t i s t i c a l  step  o f one s e c o n d  hours.  scan  Since  low  rate  and  i n the count  m i c r o w a v e f r e q u e n c y become s i g n i f i c a n t o v e r s u c h t i m e s , d a t a were g e n e r a l l y  acquired  w i t h a s t e p t i m e o f 5 seconds and  t h e c o m p u t e r was programmed t o r e p e a t t h e s c a n a s many t i m e s as d e s i r e d was  thus  and t o a v e r a g e  t h e r e s u l t s . Low f r e q u e n c y n o i s e  f i l t e r e d out.  Efforts  were made t o e x t e n d  by r e d u c i n g t h e h e a t  loss  from  the a v a i l a b l e  running  the Dewars, s i n c e  the signal  to n o i s e r a t i o  o f weak s i g n a l s c o u l d be i m p r o v e d  data c o l l e c t i o n  t i m e s . Because t h e l i q u i d  pumped  t o below  experiment.  already  cold  contaminated  more h e l i u m w i t h o u t  When m o r e h e l i u m  used  was  usually  Dewar,  the optical  w i t h a i r which  t o reduce  heat  windows  generally  leaks  caused  26  interrupting  was t r a n s f e r r e d often  i n t o an became  l e a k e d i n and f r o z e on  to the inner s u r f a c e s o f the f l a s k . B a f f l e s were  by l o n g e r  t h e lambda p o i n t t o r e d u c e b u b b l i n g , i t was  not p o s s i b l e t o t r a n s f e r the  helium  time  and i n s u l a t i o n  by c o n d u c t i o n  through  t h e h e l i u m gas and by r a d i a t i o n f r o m  The  maximum r u n n i n g t i m e s o b t a i n e d were l i m i t e d by  c o n d u c t i o n down t h e Dewar w a l l s , of  w a v e g u i d e , and  heat  i n the  case  t h e s u p e r c o n d u c t i n g magnet Dewar, t h e magnet s u p p o r t  rods  and  c u r r e n t l e a d s . T h i s h e a t c o n d u c t i o n was  approximately  5 Watts f o r the  w o u l d be s u f f i c i e n t t o b o i l (a  the t o p of the Dewar.  typical  due  to the  The  amount u s e d low  length  of l i q u i d  i n approximately  by  the  9.2  GHz  and  the  50%  l a s e r p o w e r was of  runs  was  duty c y c l e  less  even  t h a n one  more  of  helium  two  hours  helium.  for losses  due  full  i n the  to chopping.  The  Watt. Thus  the  half  severely  be and  microwaves at  a p p r o x i m a t e l y 2.5 W a t t s a l l o w i n g  waveguide system maximum  o f f 10 l i t r e s  i n a run)  generated  to  s u p e r c o n d u c t i n g magnet  l a t e n t heat of v a p o u r i z a t i o n  heat  power was  calculated  limited  in  some  experiments.  S i n c e t h e up-down c o u n t e r was  used  the l o c k - i n d e t e c t o r , a comparison order. in  The  up-down  the photon  the s i g n a l monitors optical  counter  c o u n t s by  count  analog  f l u c t u a t i o n s . The  which will  data  t i m e s , the  lock-in  i s proportional have t h e  d e t e c t i o n has time  fluctuations  number o f them  l o c k - i n detector reduces  after  devices i s in  statistical  i s i n c r e a s e d . The  this  low-pass f i l t e r collection  reduces  signal  r a t e , and  o f t h e two  summing a l a r g e  gathering time an  interchangeably with  same  as  detector to  the  statistical  these with  the  o c c u r r e d and, f o r longer  constant of  this  filter  can  be i n c r e a s e d t h u s r e d u c i n g t h e n o i s e . I n o u r a p p a r a t u s t h e c o m p u t e r was p r o g r a m m e d t o r e a d t h e l o c k - i n o u t p u t times  p e r s t e p and t o a v e r a g e t h e r e s u l t s . T h i s p r o d u c e d t h e  same  effect  signal  as a l o n g e r  from  step  time  performances  approximately  the  lock-in  sensitive with for  detector  Thus,  o f t h e two systems  luminescence  microwave s i g n a l  by a d j u s t i n g t h e phase a n g l e c o u l d be a c h i e v e d  up-down s w i t c h i n g signal,  electronics. range  relaxation readily  with  pulses  be  f o r example while  be s w i t c h e d  o f being  with  l o c k - i n . T h e same  respect  would  require  ODMR  i n theexamination digital  as r a p i d l y  t o t h e microwave  operate  t o study  up-down  without  additional  i n t h e 100 kHz  signals  a t such  of spin  counters  causing  unacceptable  low  In our  s w i t c h i n g speeds, however.  Hz were u s e d ,  lattice  could not  could operate  20 t o 5000  delayed  t h i s c a n be c o m p e n s a t e d  c o u n t i n g e r r o r s . The up-down c o u n t e r  from  phase  an up-down c o u n t e r b y s h i f t i n g  but this  desired  times;  frequencies  the  should  changes are  on the  Lock-in detectors could  i f i t were  frequencies:  smearing the  i n principle,  has the advantage  so t h a t i f the  switching  without  same.  r e s p e c t t o the  effect the  constant  to step.  signal/noise  The  twenty  work most  at very  switching experiments  b e i n g d o n e a t 500 H z . T h e q u a l i t y o f d a t a o b t a i n e d f r o m t h e lock-in  detector  and f r o m  t h e up-down c o u n t e r  was f o u n d t o  be a b o u t t h e same. The m a i n a d v a n t a g e o f t h e up-down was  that  i t gave  a numerical  value  28  for  t h e ODMR  counter signal  which,  by c o m p a r i s o n  indicated in  with  t h e magnitude  the observed  of the signal  optical  count  rate,  a s a p e r c e n t a g e change  luminescence.  2.3  Commentary.  It  should  be n o t e d  months was i n t r o d u c e d required the  to i n s t a l l  system  computing  was  into  that  a delay  of approximately 6  t h e e x p e r i m e n t s by b u i l d i n g  work  t h e s u p e r c o n d u c t i n g magnet. O p e r a t i o n o f  also  slowed  down  by  the nature  s y s t e m . T h e i n p u t was v i a p u n c h e d  of the  paper t a p e and  an o l d f a s h i o n e d t e l e p r i n t e r . T h u s , t o i n p u t t h e o p e r a t i n g system  required  about  20 m i n u t e s  many c a s e s , h o w e v e r , a s y s t e m  i f a l l went s m o o t h l y . In  f a u l t i n the l a s t stages of  i n p u t made a r e - r u n n e c e s s a r y .  A teleprinter terminal home  i s not a s a t i s f a c t o r y  i n p u t u s e d on even t h e l e a s t  computers.  29  substitute  f o r the  e x p e n s i v e o f modern  Chapter 3. Gallium Phosphide and Zinc T e l l u r i d e . 3.1. Introduction Gallium cubic  phosphide  lattice.  valence  of the  o f f by t h e s p i n  0.127  eV  below  The  orbit  the other  conduction  band  [1,0,0]  the  hole  interaction  third  of  interest  has  three  directions  measurements  The  most  equivalent  at  the  X  electronic  and  valence  a t the r p o i n t with  electrons  holes  band i s  i t s maximum  minima  points  lying  ( 2 5 ) . The  above t h e minima t h e minimum  band  and  luminescence  are  approximately  i s indirect.  conduction  spherical the  in  hole  two ( 2 4 ) .  t h e X p o i n t s b y 0 . 5 eV a s s h o w n , t h u s  gap  The  the d e n s i t y  and has  band minimum a t t h e r p o i n t l i e s  conduction  and  0 . 6 7 and  (27) r e s p e c t i v e l y ,  bands  effective  masses  0 . 1 7 f o r the  a l l values  0.13 (26) f o r  heavy  are i n units  and  light  o f mass o f  t h e f r e e e l e c t r o n . The X p o i n t m i n i m a a r e a n i s o t r o p i c longitudinal 1.5  a n d mt  The been  =  and °-  transverse 1 8 ( 2 8 )  g value  measured  a  p o i n t k = 0 , the  e i g h t times  b a n d . The  with  i n Fig.3.1.  i s shown  approximately  light  split  at  semi-conductor  band i s t w o f o l d d e g e n e r a t e a t t h e r  states  along  III-V  The band s t r u c t u r e  h e a v y h o l e band h a v i n g of  is a  effective  parameters  =  •  o f e l e c t r o n s bound  t o be  mass  with  1.9976  + 0.0008  30  to n e u t r a l donors  by  has  E P R experiments ( 2 9 ) ,  FIG.  3.1  GAP  BANDSTRUCTURE  Direct band gap Indirect band gap  2.878 EV 2.339 EV  0.127  EV  Valence bands  r  x 31  and  a  value  excited  of  1.996  electrons  +_ 0.002  was  determined  f o r photo  u s i n g ODMR (30 ) . The g v a l u e o f h o l e s has  been e s t i m a t e d f r o m  Zeeman m e a s u r e m e n t s o f e x c i t o n s bound a t  n i t r o g e n i m p u r i t i e s t o be 0.99 +_ 0.06 ( 3 1 ) .  3.2. Exciton formation and luminescence i n GaP  Optical holes  recombination  occurs  important  largely  class  impurities  which form  gap. N i t r o g e n  being  isoelectronic  generally  Exciton as  of  phosphorous  isoelectronic  the e x c i t o n  centres  and and  form  are  sites.  An  isoelectronic  of e x c i t o n s i n the  such  residual of  centres  impurity  one  occur  part  i n GaP  at neutral which  electron  million.  donors  such  substitute for  i s much  due t o t h e h i g h  i n GaP i n  in a  (29). The e f f i c i e n c y  donors  the donor  of o p t i c a l  lower  probability taking  than  at  o f Auger  the energy o f  and b e c o m i n g i o n i z e d ( 3 2 ) .  Donor-acceptor important  a  i n GaP  at neutral  recombination/with  states  and t e l l u r i u m  as d o n o r s  recombination  as  can a l s o  selenium  impurity  phosphorous. N i t r o g e n i n p a r t i c u l a r  the order  recombination  sulphur,  localized  with  and  centres  and b i s m u t h  present  concentrations  at defects  of recombination  band  is  o f p h o t o - e x c i t e d e l e c t r o n s and  process  pair  (DAP)  recombination  recombination  another  i n m a t e r i a l s w i t h a h i g h d e n s i t y o f donors  a c c e p t o r s , i n which e l e c t r o n s acceptor  is  impurities energy  and h o l e s t r a p p e d  recombine  depending  32  on  a t donor  radiatively, the  the  donor-acceptor  separation. A modification in  GaP  i n which  trapped  at  donor  of t h i s  bound  isoelectronic  process  electrons bismuth  been  recombine  centres  measurements, however, donor-acceptor not  has  observed  with  (33).  In  our  pair recombination  was  significant.  Excitons  may  centres  by  exciton  while  two  form  at  isoelectronic  m e t h o d s . An free  and  electron  the  or the  which the  s e c o n d becomes t r a p p e d  first  i m p u r i t y may  particle  bismuth  has  (30).  been  In  shown by  may  form  trapped  particle  first  the  isoelectronic  a hole  with  the o b s e r v a t i o n of d o n o r - l i k e e l e c t r o n  The although  recombination  (34) and  binding  energy  difficult  recombination  to prove. of  i n t e r p r e t a t i o n electronegativity phosphorous  of  Analysis  nitrogen (37)  bound as  differences  atoms w h i c h  of  8 meV  (36) h a s  the  a  phonon  made  this  assisted  supports  comparison  t h e y r e p l a c e (38).  trap  deduced  between n i t r o g e n , b i s m u t h ,  33  (33)  phonon  an e l e c t r o n  excitons  does  of  excited  of the  approximately  measurements  the  (35).  n i t r o g e n c e n t r e i s t h o u g h t t o be the  of  measurements  analysis  o f bound e x c i t o n s  from r a d i a t i v e decay time more  by  an  after  energy  as w e l l as by  assisted  luminescence  an  an  impurity  40  at bismuth centres  at  i n t h e Coulomb f i e l d  bind  DAP  hole  approximately  states  meV  recombination  become  b i n d one  GaP to  and  exciton  impurity,  the  holes  this of and  The (39),  f r e e e x c i t o n i n GaP  and  bismuth  on  the  isoelectronic  the t o t a l  respectively  h a s a b i n d i n g e n e r g y o f 10  binding  impurities  energies  (36). L u m i n e s c e n c e  t r a p p e d e x c i t o n s . The  107  meV from  luminescence spectrum observed  from  centres d i f f e r s  Fig.3.2.  luminescence  nitrogen  and  and  observed  n i t r o g e n and b i s m u t h The  nitrogen  a r e 21 meV  is primarily  meV  markedly  spectrum  as shown i n  i s dominated  t h e so c a l l e d A and B l i n e n o - p h o n o n t r a n s i t i o n s . The A B of  lines  are caused  1 and  2  by  excitons  respectively,  formed  parallel  alignment of a spin  h o l e . The  phonon a s s i s t e d  relatively electron  weak  due  to  to the n i t r o g e n  recombination  of  1/2  by  and  a n g u l a r momenta J  the  antiparallel  or  with a spin  3/2  electron  transition  o f A and  B excitons  are  the  binding  energy  the  small  atom  line, generally referred  with total  by  (40). A t  lower  energy  of a  third  t o as t h e C l i n e r e s u l t s f r o m  excitons  at  neutral  sulphur  the  donor  impurities.  The  by  phonon  a s s i s t e d t r a n s i t i o n s . A t low t e m p e r a t u r e t h e A l i n e  i s not  seen  due  case  lies  bismuth  center  luminescence  to t h e r m a l i z a t i o n 2.7  transition,  meV  lower  i s dominated  w i t h the  i n energy.  B  line  The  i s seen o n l y weakly s i n c e  B  which  line,  in  this  no-phonon  the decay of the  J=2  e x c i t o n i s f o r b i d d e n a n d c a n o n l y o c c u r by m i x i n g w i t h t h e A line  states  strains,  i n the presence of a magnetic  or w i t h the a s s i s t a n c e  no-phonon t r a n s i t i o n semiconductor  i s due  c a n be  field,  o f p h o n o n s . The  crystal  fact  that  s e e n a t a l l i n an i n d i r e c t  to the f a c t  34  that, being localized  a  gap  a t an  2.305 EV  i  2.315  EV  I  c B  LU U  IT U  J  o  l  5370 A CO  I CL.  <  5340 A  The A and B l i n e s due to N and the C l i n e caused by S i m p u r i t i e s are shown ,  2.PQ FV  2,20  EV  CJ3  a  i  < CD  CD  5000 A G A P - B  I  5500 A  The B e x c i t o n no phonon l i n e and numerous phonon r e p l i c a s are seen 35  impurity,  the  electron  or  hole wave-function  i n k s p a c e and  a finite  probability  i s spread  of a d i r e c t  out  transition  exists.  The  bulk of the luminescence  contained  i n the  the v a r i o u s  broad  (33). The  i n the B i spectrum the  stronger  results  the  greater  of  the  states  effect  of  The  electrons  and  3/2  expected  splitting  spin  shown  luminescence  by  electron  photoluminescence.  transitions,  from  bound  hole  to  the  Bi  at  B  bismuth  to nitrogen  Cavenett  states  (30)  in  field.  explaining  from  The  line  has  been  (41)  and  in  which  which  the  i n an  (41). However  c a n be c a u s e d  1/2 their  field  observed  in  multi-exciton  i s expected  impurity  36  zero  for single  splitting  a t the bismuth  on  spin  with  i s unobservable  more s t r o n g l y  shown t h a t s u c h s p l i t t i n g  to  impurities  resonance  formed  a t o m s . The  field  exciton  replicas  atom, which  isoelectronic  magnetic  the  his  i s due  h o l e s a r e shown i n F i g 3.3  to the c r y s t a l  electron  Dean and  o f t h e phonon  paramagnetic  exciton  in a for  by  different  to the N spectrum  c o m p l e x e s w i t h N c e n t r e s (42) b u t  attributed  centre i s  t o t h e phonon modes.  formed  have been d e s c r i b e d  excitons  assisted  identified  strength  as c o m p a r e d  binding  exciton  splitting  phonon  have been  i n stronger coupling  The i n GaP  of  the bismuth  peaks c o r r e s p o n d t o c o m b i n a t i o n s of  t y p e s o f p h o n o n s and co-workers  band  from  has  been  t o be  felt  i s repulsive Morgan  to  (43)  has  by c o u p l i n g o f  the  FIG, 3.3  EXCITON  STATES  1/2  HOLES  -1/1  n,  1.1  J  =  =  G A P  n:  ELECTRONS  J  IN  l  2  -1/2 11/2,1$3/2,1/2) + 3/2 |l/2>-l/2)f3/2^/2)  1.0  =  vJl\\ji y7)\i>riAii) -  1„-1  =  1/2 11/2,-172) (3/2,-1/2) - /3>2 Jl/2,l/2>/3/2,-3/2)  2.2  =  2.1  =  2,0  =  /J/2 |l/2,l/2)/3/2,l/?) 1//2 | l/2,l/2)|3/2,-1^> + l//?/l/2,-l/2>/3/2,l/?)  2,-1  =  1/2 |l/2,l/2)|3/2,-3/2> +/3Z2 / l/2,-17?>/3/2,-l/2>  2,-2  =  I Highly  Ol TT  =  r  vrrJi/2,i/2)/3/2,  172,172)13/^,3^) 1/2 11/2,-1/^ l3/2,3/2> +  l^,-l/2))3/2,-3^>  forbidden  0 , 0  Ol  The e x c i t o n s t a t e s formed by a spin 1/2 e l e c t r o n and a s p i n 3/2 hole are shown with t h e i r composition, s p l i t t i n g i n a magnetic f i e l d , and o p t i c a l t r a n s i t i o n s  exciton to  t o phonon modes w i t h  the  mode  axis)  tends  mode  ([1,1,1]  to r a i s e  corresponding transitions line.  thus A  representing luminescence  triplet  raises  the  the  |2,2>  and  contribute  be  the  with  right  and  emitted  parallel  angles  To the  A  to the  calculate and  B  the  lines  expected  with  resonance  formation  the  that  (ni  the  -  to  must be  to  the  of  field  B  are  a±  polarization  magnetic  change  in  for  w h i l e ir  emitted  luminescence  application  signal,  the  exciton  a  of  an  at  are  of e l e c t r o n  by  from  electron  relative  states  particular  populations  rates  of  calculated  and  hole  spin  assuming  exciton forming  o f e l e c t r o n s and  holes  is  with  states.  exciton state multiplied  the  shown,  ) before exciton formation, also  appropriate spin  The  the  as  no  forbidden  i n luminescence  distribution  probability  proportional the  ng  the  various  assuming a thermal states  with  field.  paramagnetic of  mixing  Zeeman c o m p o n e n t s  circular to  the  already  luminescence  polarized  left  As  have d i p o l e  various  circularly  [1,0,0]  |2,-2> e x c i t o n s h a v i n g  do  field  the  c o u p l i n g to  doublet.  t o mix  not  along  o n l y d e c a y by  represents linear polarization right  symmetries. Coupling  while  states  and  to  T$  and  e x c i t o n s can  In a m a g n e t i c  expected  the  axis)  states,  3  (corresponding to s t r a i n  m e n t i o n e d , t h e J=2 the J=l  r  spin their  wave  f u n c t i o n s shown  complex  38  conjugates  to  in  Fig.3.3  give  the  probabilities,  thus the c o e f f i c i e n t s  and  states  hole  spin  that  squared. For example,  make  of the various e l e c t r o n  up t h e e x c i t o n  the r e l a t i v e  number o f  levels  are  |l,l> excitons  f o r m e d w o u l d be:  P  Taking  =  l l  n  l«n4  the t o t a l  n  the v a l u e s  =  n  /4 + 3.n2-n3 /4  number o f e l e c t r o n s o r h o l e s  t o be:  l + n2 + n3 + n4 + ns + n6  o f n i - n6  m a  D  Y  e  c a l c u l a t e d assuming a  thermal  distribution: — a  n i  e  =n.e  — a  /(1+e  e  )  a  ~ e n2  = n/(1 + e  n3=n.e  ~  3 a  ) a  h,,, " h "2ah /(1+e +e +e  2  n4=n.e  - ah  /(1+e  -oth  +e  ~2ah  +e  - 3  ah  )  ~3ahv )  where:  ge  and  gn  magnetic  being field,g  ae  = g e $H/kT  ah  = gh&H/kT  the e l e c t r o n t h e Bohr  and h o l e  g values,  H the  m a g n e t r o n a n d kT t h e t h e r m a l  energy.  When a n EPR  signal  i s a p p l i e d , the populations of the  39  electron  spin  signal.  states  relative  have been 3.1.  calculated  respectively magnetic  rates  using  o f the various  t h i s procedure  and h o l e g v a l u e s are  field  strength  is  state  the  each  noting.  formed i n the  the  A line  non-radiative  i s greater  than  a l s o be s e e n t h a t the  positive  Several  a r e both  a s more  s t a t e r the  excitons change i n  i n the B line,  increasing  and  states  l u m i n e s c e n c e , and c a n be e i t h e r  so t h a t  i f t h echange  i n one Zeeman  resolution  spectroscopy or a c i r c u l a r polarization observed  the  temperature. I t can  of theA or B line  ODMR s i g n a l  u  expected to  component  the  by P  points are  changes i n i n d i v i d u a l e x c i t o n  than i n t h e t o t a l or negative  | 2,2>  that  e f f e c t decreases r a p i d l y with  The change i n  change i n i n t e n s i t y  i n i n t e n s i t y a t t h e EPR r e s o n a n c e  larger  i n Table  t a k e n t o be 2 a n d 1,  calculated.  The A and B l i n e s  are  are  states  o f microwaves i n d i c a t e d  and t h et o t a l  A and B l i n e s i s a l s o  decrease  a s shown  o f 3.4 k i l o G a u s s .  application  shown f o r  exciton  f o r t e m p e r a t u r e s o f 1.6 a n d 4 d e g r e e s K a n d a  i n t e n s i t y w i t h the  worth  a.saturation  = n 2 = n/2  formation  The e l e c t r o n  for  assuming  Therefore:  J"*!  The  are equalized  i s monitored  c a n be e i t h e r  positive  using  high filter  or negative  d e p e n d i n g upon w h i c h component i s u s e d , and s h o u l d be l a r g e r than  t h e ODMR s i g n a l  shown b y t h e  40  entire  (A o r B) l i n e .  TABLE  3.1  MICROWAVE INDUCED LUMINESCENCE CHANGES  1.6 K  4 K  0.5212  0.5077  1  0.4834  0.4931  10  0.4087  0.4599  Yo  0.4047  0.4591  0.3170  0.4158  0.3384  0.4275  0.4291  0.4715  *2  0.5000  0.5000  P2l  0.4217  0.4661  ^1  0.4501  0.4792  ?20  0.4087  0.4599  2o  0.4047  0.4591  P2-1  0.3917  0.4529  2-l  0.3633  0.4399  P2-2  0.3720  0.4453  2-2  0.3258  0.4213  A line  change  1.636%  0.267%  B  change  0.327%  0.051%  Pll PU  p  p  Pl-l p  ¥-l  P22 p  P  P  P  The  line  numbers i n t h i s t a b l e r e p r e s e n t t h e r e l a t i v e populations of the various e x c i t o n states w i t h and w i t h o u t a r e s o n a n c e s i g n a l . They a r e n o t n o r m a l i z e d .  41  A number o f p r o c e s s e s observed  ODMR s i g n a l s  theory.  Non-saturation  microwave  signal,  may r e d u c e  below  either  the levels  reduce  predicted  of the electron due t o i n a d e q u a t e  or t o inhomogeneous b r o a d e n i n g would  the experimentally  the effect  spins  the  by t h e  microwave  o f the e l e c t r o n  seen  by  power  resonance,  i n a l l exciton  states.  T h e r m a l i z a t i o n b e t w e e n t h e d i f f e r e n t Zeeman components w o u l d diminish Zeeman  the larger  lines  ODMR s i g n a l s  and t h e s i g n a l s  expected  from  i n individual  thee n t i r e  A or B lines  w o u l d be r e d u c e d a s w e l l . T h e r m a l i z a t i o n b e t w e e n t h e A and B line in  states  either  signal. before  would line,  exciton  electron  would  of theelectron  reduce  on t h e r m a l l y i n d u c e d  spin  seen  the A spins  the observed  effect  polarization  of the  O t h e r work o n GaP  resonance  and h i s co-workers of electrons  i n GaP  0.0008 w h i c h as  t h e B and d e c r e a s i n g  thermalization  formation  o f t h e ODMR s i g n a l  population.  Title  Te  the size  increasing  Incomplete  w h i c h depends  3.3  equalize  (29,44). they  the g value  Cavenett  have o b s e r v e d  at neutral  They  predict  donor  obtained should  o f an e l e c t r o n  paramagnetic  impurities  a g value  S,Se and  o f 1.9976 _+  be a p p r o x i m a t e l y  t h e same  i n the conduction  band.  (30) h a s m e a s u r e d t h e c o n d u c t i o n e l e c t r o n g v a l u e  directly  u s i n g ODMR m e a s u r e m e n t s o f l u m i n e s c e n c e  centre,  obtaining  the value  g=1.996  42  from  the N  +_ 0.002. T h e s i g n a l  observed  by  Cavenett  theoretically,  as  strong  no  dependence  a signal  as t h e A  predictions.  Cu doped GaP have a l s o microwave  defects  3.4  only  which  line  been o b s e r v e d transitions  involve  pairs  a  predicted  0.1% c h a n g e  appears  signal  on  to  i n luminescence  the from  (45) and a r e a t t r i b u t e d  between  exciton  states at  o f Cu a t o m s .  Results ODMR s i g n a l s  GaP  were  The  g values  agree  i n luminescence  measured  and a r e shown  derived  to within  from  f o r the three  i s also  N,S and B i c e n t r e s i n  i nFigs.  the experimental  g = 1.9993 +_ 0.005 w h i c h  3.4 t o 3.6  luminescence error  isotropic.  giving  Attempts  to observe resonances o f h o l e s o r t o t o observe between e x c i t o n  states;  (46).  centres a  value  were made resonances  these were, however, u n s u c c e s s f u l .  Some e f f o r t was made t o a n a l y s e t h e o b s e r v e d in  i n  t o have g i v e n  i n contrast  ODMR s i g n a l s  induced  than  o f t h e ODMR  was f o u n d , and t h e B l i n e  theoretical  to  smaller  representing  luminescence, polarization  was  resonances  more d e t a i l . The N and S c e n t r e s were b o t h p r e s e n t i n t h e  same  sample  used  t o measure t h e B i c e n t e r l u m i n e s c e n c e ,  fairly  as r e s i d u a l  Different  samples  were  and t h e s e had a  h e a v y a n d somewhat i n h o m o g e n e o u s d o p i n g o f B i . The  ODMR s i g n a l s than  impurities.  those  from from  t h e B i s a m p l e s were o b s e r v e d the purer  samples  measurements.  43  used  t o be weaker  f o r t h e N and S  The e l e c t r o n ODMR s i g n a l seen as a 1% change i n the N luminescence  F I G . 3.5  MAGNETIC  ODMR  FIELD  IN  6 A P -  KG  ODMR s i g n a l from the S luminescence  FIG. 3.6  ODMR  IN  GA P - B I  3.5 MAGNETIC  The  ODMR  FIELD  KG  siqnal seen s<= a r , n i a seen as an 0.1% change i n the Bi luminescence  The  microwave  strength results  from  power  the N  dependence  luminescence  o f t h e ODMR  was m e a s u r e d  signal and  the  a r e shown i n F i g . 3 . 7 . The f i t shown by t h e s o l i d  l i n e i s c a l c u l a t e d on t h e b a s i s o f a two l e v e l s y s t e m a t t h e sample radio  temperature  (1.6 d e g r e e s  K) b e i n g  saturated  by a  f r e q u e n c y f i e l d . The e x p e r i m e n t a l p o i n t s were o b t a i n e d  by s c a n n i n g t h e r e s o n a n c e  at a series  o f microwave  settings, the scatter being primarily  due t o d r i f t  optical  system  between  results  indicate  achieved  and sample t e m p e r a t u r e that  saturation  i n the  s c a n s . The  o f the resonance  a t t h e maximum m i c r o w a v e p o w e r  ODMR s i g n a l  power  was  l e v e l s , g i v i n g an  t h a t r e p r e s e n t e d a p p r o x i m a t e l y a 1% c h a n g e i n  luminescence.  Similar  measurements  on t h e B i l u m i n e s c e n c e  ODMR s i g n a l , a l t h o u g h h a m p e r e d b y t h e p o o r s i g n a l t o n o i s e ratio,  indicated  that this  resonance  was a l s o  saturated.  T h e r e l a t i v e ODMR s i g n a l s t r e n g t h f r o m t h e A a n d B l i n e s o f t h e N l u m i n e s c e n c e was m e a s u r e d b y s e l e c t i n g t h e A o r B line  with  Both  lines  the spectrometer  and s c a n n i n g  were  t h e same ODMR s i g n a l  The B i c e n t r e ODMR s i g n a l  found  to give  luminescence  equally  was a l s o  the resonance.  observed  strength.  t o show t h e  i n t h e z e r o phonon l i n e and t h e phonon  replicas.  Attempts  were  made t o o b s e r v e t h e p r e d i c t e d  o f t h e ODMR s i g n a l b y s e l e c t i n g p a r t i c u l a r using  a  circular  polarization  47  enhancement  Zeeman components  analyser,  taking the  •3,7  ODMR  MICROWAVE  POWER  DEPENDENCE  SIGNAL PERCENT  •0016  . 008  . 04  MICROWAVE  0,2 POWER  1,0  WATTS  The N luminescence ODMR s i g n a l s t r e n g t h as a f u n c t i o n o microwave power. The resonance i s seen t o be saturated maximum power  48  luminescence p a r a l l e l t o the magnetic the  ODMR  circular  could  be s e e n  for either  prompted  an i n v e s t i g a t i o n  the A and B l i n e s ; a t y p i c a l  3.8.  with  lines  the exciton  shown.  <2,+_l|  states  Circular  a n d <l,+_l|  extinction  when  polarization  i splaced  analyzer  Thus  a  magnetic  fields  field  rather  strength  used  dependent  limited  in  the  i n their  of the appropriate system.  T h i s was  was s e e n f o r circular produced  ( A polarization have b e e n  ofa  too small  just described ) .  the s p l i t t i n g s  Temperature were  result  b u t would  ODMR m e a s u r e m e n t s o f i n d i v i d u a l  magnetic  i s expected  significant  used.  i n Fig.  for the various  i n t h e Zeeman s p e c t r u m  t o be s e e n b y t h e method  since  i s shown  responsible  i n theo p t i c a l  p e r c e n t may have e x i s t e d  made  left  splitting  o f t h e Zeeman c o m p o n e n t s was n o t b e i n g  a t the high  few  spectrum  and should  a circular  polarization.  polarization even  or  o f t h e Zeeman  polarization  lines  d o n e a n d no v a r i a t i o n either  right  polarization.  This of  signal  f i e l d . No c h a n g e i n  could  components  n o t be r e s o l v e d  n o t be at  the  i n t h e ODMR e x p e r i m e n t .  measurements  since  could  the samples  o f t h e ODMR were  signals  immersed i n  l i q u i d h e l i u m , g e n e r a l l y pumped t o b e l o w t h e lambda p o i n t t o eliminate  bubbling.  a  area  small  By f o c u s s i n g  o f the sample  the laser  excitation  and i n c r e a s i n g  49  onto  t h e power  FIG.  3.8  GAP-N  ZEEMAN  2,310  2.315  EV  5355 A  The  SPECTRUM  EV  5345 A  spectrum o f the N f p n t r a i , °f  25  k  Gauss  S^tS S y  50  «S:"S"2=?J  3  u i r r e r e n t exciton  " '  a  9  n  e  t  l  C  f  i  components  e  l  d  significant  heating  could  be p r o d u c e d .  F o r t h e GaP- N  s a m p l e t h e r a t i o o f t h e A and B l i n e l u m i n e s c e n c e p r o v i d e s a rough  thermometer  favours  the B  Comparing excitation, a  factor  a t low t e m p e r a t u r e s  t h e ODMR r e s u l t s  t h e ODMR s i g n a l  2 degrees  f o r low and h i g h  level  s t r e n g t h was o b s e r v e d  of approximately  2 a t high  the luminescing  laser  laser  t o d r o p by  power  which  r e g i o n o f the sample t o  K.  In t h e B i d o p e d s a m p l e h i g h observed  thermalization  line.  apparently heated roughly  since  to bring  up  luminescence  spectrum  the B l i n e .  Obviously  a  l a s e r power d e n s i t i e s  series  including these  o f new  lines  one a t 5555A.U.  were  i n  just  the  above  were t h e A l i n e and i t s phonon  r e p l i c a s w h i c h a r e n o t seen a t a l l a t low t e m p e r a t u r e due t o the g r e a t e r s p l i t t i n g between t h e A and B l i n e s . signals  c o u l d be d e t e c t e d  i n t h e new l i n e s ; p r e s u m a b l y t h e  i n c r e a s e d t e m p e r a t u r e made t h e e f f e c t  The  results  mentioned  unmeasurable.  so f a r were  m e a s u r e m e n t s a t 9.2 GHz m i c r o w a v e f r e q u e n c y . of  t h e ODMR  signal  No ODMR  was a l s o  made  from  A measurement  a t 36.3 GHz  f o r the  centre  luminescence with  signal  was c o n s i d e r a b l y w e a k e r t h a n a t 9.2 GHz d e s p i t e t h e  fact  that according to theory,  stronger to  results  obtained  at higher  magnetic  the lower a v a i l a b l e  s h o w n i n F i g . 3.9 .  t h e ODMR e f f e c t  fields.  This  m i c r o w a v e power  51  The  should get  was p r e s u m a b l y due  level  and a b s e n c e o f  N  FIG. 3.9  ODMR  SIGNAL  AT  36.3  GHZ  a resonant level  a t the  By 36.3  cavity  s a m p l e by  comparing GHz  ( t h e 9.2  the  i t can  GHz  a factor  widths  be  seen  i n frequency  t o homogeneous  amplified  of the order of  of  the  that  a p p r o x i m a t e l y t h e same w i d t h narrower  cavity  the  power  1000).  resonances  at  9.2  and  the  resonance  retained  in field  i . e . became  relatively  indicating  that  the  line  width  i s due  broadening.  3.5 Discussion  Our  o b s e r v a t i o n s o f ODMR o f e l e c t r o n s  following for  remarks.  several  electron effect band,  and  i s due  in  suggested  that  N  before  resonance. and  f o r the  of  indicated giving  the  case  of  Bi.  a  hole  nanoseconds  from  the s p i n - l a t t i c e before  trapping  exciton  the  line  was  time  a hole, which  decay  time  (36)  as  an the  (30)  has  at  the  by  the  same g v a l u e to w i t h i n at  the  36.3GHz  broadened,  approximately  Whether t h i s or the e l e c t r o n  i s not  as  that  trapped  homogeneously  i s k n o w n t o be  53  acts  affected  electrons  time  observed  Cavenett  is  the  i n the c o n d u c t i o n  Measurements  widths.  relaxation  was  shows  i s s o , i t must have the  relaxation  suggest  which  N c e n t r e the e l e c t r o n  experiments.  spin  N  of e l e c t r o n s  that the resonance  the  signal  i s a hole trap  capturing  If this  the  s p i n r e l a x a t i o n t i m e as t h e f r e e  accuracy  the  the  that  including  B i which  to resonance  least  centre  fact  luminescences  trap  at  The  i n GaP  clear.  represents lifetime  shorter The  4  than  linewidth  observed  by C a v e n e t t  (30) was c o m p a r a b l e  linewidths  observed  77  degrees  K were a p p r e c i a b l y  as  a factor  impurity  by T i t l e  to lifetime  s i n c e our  u s i n g c o n v e n t i o n a l EPR a t  s m a l l e r than  ours  o f 4) a l t h o u g h t h e l i n e w i d t h s  concentration.  subject  (29)  t o o u r r e s u l t s . The  Since  increased  t h e EPR r e s u l t s  broadening  (by a s much  would  with  n o t be  due t o r e c o m b i n a t i o n , and  s a m p l e s were o f h i g h e r p u r i t y t h a n T i t l e ' s  material,  i t i s reasonable, to conclude  determined  from  that  doped  the lifetime  our r e s u l t s r e p r e s e n t s the e l e c t r o n  lifetime  against hole capture.  An  attempt  broadening The  conclusively  The  shown w h e t h e r  the data  resonance  line  was o b s e r v e d  which  i s larger  t o be r a p i d  observed  sets o f data  fitted  by a  seen  t o be e q u a l  from  the N c e n t r e  f o r t h e A and B  line,  a p p r o x i m a t e l y a 1% c h a n g e i n l u m i n e s c e n c e a t  but s m a l l e r than  induced  shapes.  profile.  than  the predicted  f o r the A l i n e .  changes signal  (36) , a n d w o u l d  i n t h e two l i n e s  signal  This  t o t h e r m a l i z a t i o n between A and B l i n e  known  line  were b e t t e r  s t r e n g t h o f t h e ODMR s i g n a l  representing  due  the resonance  the line  l u m i n e s c e n c e . However, i t c o u l d n o t be  o r by a L o r e n t z i a n  luminescence  B line  to determine  are g i v e n i n Appendix A f o r s e v e r a l  the N centre  Gaussian  made  mechanism by a n a l y s i n g  results  from  was a l s o  for the  i s presumably  states which i s  result  i n t h e ODMR  being averaged.  Thus t h e  i s t h e same a s t h a t w h i c h w o u l d be o b t a i n e d  54  by  monitoring  t h e A and B l i n e s  theory  i s then  signal  approximates the  The  weaker  thermalize  The  reason  factor more  into  for  recombination  than  observed  i n t h e B i l u m i n e s c e n c e was  that of the N centre  signals  signal.  The  t h e weaker  than  within  the B  t o observe  by d e t e c t i n g c i r c u l a r l y  polarization even a t h i g h  was e x p l a i n e d  splitting. the longer  ( reportedly a  ) which  would  allow  exciton  s t a t e s , thus  enhancement  o f t h e ODMR  polarized  components  by t h e absence  of  the  of circular  f i e l d s . Zeeman s t u d i e s o f t h e  i n GaP r e p o r t e d  i nthe literature  theobservation  components, although  for  formed  i n t h e Zeeman c o m p o n e n t s o f t h e A a n d B l i n e s magnetic  described  looked  i s probably  f o r N (36)  of the  still  due t o t h e l a r g e r  time f o r excitons a t B i centres  inability  centres  be t h e a v e r a g e  A e x c i t o n s are  signal  the B  the signal.  luminescence  looked  still  B excitons  thermalization  Our  should  s i n c e the  o f 30 g r e a t e r  weakening  the  observed  signals  but  was  and t h e e x p e r i m e n t a l l y  with  maximum p r e d i c t e d t h e o r e t i c a l l y .  b u t t h e ODMR s i g n a l  A and B l i n e  not  Agreement  l u m i n e s c e n c e a t 1.6 d e g r e e s K d e r i v e s e n t i r e l y f r o m  line  Bi  good,  ODMR s i g n a l  considerably Bi  quite  combined.  t h e r e i s no s t a t e m e n t t h a t  f o r and not polarization  N centre with  of p o l a r i z a t i o n  found. Cavenett  (30)  N, S and  (31,41)  i n t h e Zeeman polarization  reports  having  dependence o f t h e ODMR s i g n a l  negative  been seen i s not c l e a r but  have  r e s u l t s . Why p o l a r i z a t i o n  from  has n o t  may b e d u e t o c r y s t a l s t r a i n s o r  55  defects.  The A line  B line  states  optical by  our  experiments  was  not  strains i t was  significantly  indicating  that  responsible  f o r the  observation  of  alignment presence excitons  of  also  disorient  the  Zeeman  negative  seeing  either  resonances B excitons  b u t we  due  inadequate  the  A  them  lines  transitions, resonance  and  the  and  fields  used,  levels.  magnetic  depends field,  strongly  reduce  Since  an  result  was  the  failure  a t g = 1.35  sensitivity.  thus  no  The  i t causes  a l l of  would along  which  n e t change  would m a n i f e s t i t s e l f  56  to  observe  states.  Inter-  g=1.25 f o r t h e  (8). Cavenett  (30)  reports  corresponding to  were u n a b l e t o d e t e c t t h i s  states  the  circular  investigation  a t g=0.75 and  respectively  since  the  with  the  on  question.  are expected  to observe  line  intensity  polarization  interacting  a v e r y weak r e s o n a n c e  difficult  exciton  the  In  predominantly  o f h o l e s or between e x c i t o n  B line, to  magnetic were  the  with  defects  Another  and  the  circular  excitons  might r e s o l v e  exciton  by  of  field.  that the B l i n e  defects  mixing  lines  resonances  A  affected  magnetic  o f t h e e m i t t e d l u m i n e s c e n c e . Such e f f e c t s  broaden  these  o r an a p p l i e d  observed  magnetic  o f random  polarization  o c c u r s through m i x i n g w i t h the  crystal  the  will  decay  A  signal, line  presumably  i s even  a redistribution have  allowed  i n luminescence as a c h a n g e  the  more within  optical  o c c u r s . The  i n the A  line  luminescence cannot  polarization  but,  as j u s t  mentioned,  be s e e n .  Resonance o f h o l e s s h o u l d m a n i f e s t i t s e l f in  this  luminescence  as f o r t h e e l e c t r o n  as a d e c r e a s e  resonance,  however t h e  e f f e c t w o u l d be s m a l l e r b e c a u s e o f t h e l o w e r h o l e g v a l u e . A calculation predicts  o f the A l i n e  a change  f o r a temperature  i n luminescence  p r i n c i p l e , a change o f t h i s signal  would  o f 1.6 % . A l t h o u g h , i n  m a g n i t u d e s h o u l d be v i s i b l e , t h e  be weakened b y inhomogeneous b r o a d e n i n g o f t h e  hole resonance  from  larger  of thecrystal  than  o f 1.6 d e g r e e s K  effect  c r y s t a l defects. This  on t h e e l e c t r o n  lattice  g value through  i s due t o t h e much  on t h e h o l e g v a l u e spin-orbit  coupling  (29) .  The  number o f e f f e c t s  be  attributed  on  carefully  available effect  of  Finally ODMR s i g n a l  K  samples  externally could  suggests  that  ODMR  of higher quality  studies  than  those  the temperature i n GaP - N a g r e e d  somewhat  estimation  applied  strains  on  ODMR  and  be s t u d i e d .  o f 2 decrease  being  limited  prepared  defects  o r not observed that can  t o us s h o u l d be i n t e r e s t i n g . A l t e r n a t i v e l y , t h e  luminescence  factor  to crystal  observed  dependence  approximately with  i n the signal greater  o f temperature  a c c u r a c y because  than  from  observed  theory, a  f o r h e a t i n g t o 2 degrees expected.  However, t h e  theA t o B l i n e  o f the unequal  57  forthe  r a t i o was o f  heating of different  parts  3.6  o f the sample.  Cyclotron  Resonance  ODMR e x p e r i m e n t s background changes other  i n l u m i n e s c e n c e . I n some b r o a d  magnetic  field  investigate  this  background  dependent.  t h e background  signal.  luminescence  falls  this  of several  signal  was o b s e r v e d  microwave to  resonance  which  suggests  i s expected  that  into  field  power d e p e n d e n c e  the signal  power. A l l p a r t s  be a f f e c t e d  equally  Luminescence  field.  a very  i f microwave  i s r e s p o n s i b l e . To t e s t  was moved  where t h e e l e c t r i c  The m i c r o w a v e  ODMR  the application of  a region  increased  of  the  was s t r o n g e r , t h e  was o b s e r v e d t o i n c r e a s e a s e x p e c t e d  resonance. it  o f a broad  zero a t high  of thesignal  of photo-excited carriers  microwave c a v i t y  decided to  represents a decrease i n  percent with  h y p o t h e s i s a sample  signal  I t was t h e r e f o r e  t o be  shows up on t h e h i g h s i d e o f t h i s  The b a c k g r o u n d  cyclotron  heating  was o b s e r v e d  o f f t o near  m i c r o w a v e s . The a p p e a r a n c e broad  non-resonant  scans, looking for  i t . F i g 3.10 s h o w s t h e r e s u l t  The e l e c t r o n ODMR s i g n a l large  showed  s i g n a l s , a p p a r e n t l y microwave h e a t i n g induced  resonances,  sweep,  o n GaP-N o f t e n  for cyclotron  was measured and linearly  o f the N luminescence  with the were  seen  by t h e r e s o n a n c e .  from  S centres  58  was o b s e r v e d  and a  3.10  GAP  CYCLOTRON  RESONANCE  •  «• • • •  different signals  signal  A  Figs.  3.11 a n d 3.12 show t h e  o b s e r v e d i n N and S l u m i n e s c e n c e w i t h f i t s f o r t h e  cyclotron  P  was s e e n .  resonance  a  1 2  (1 + w x  where P A i  s  equation:  +O)CT 2  -  2  W  +  O J  2  T  2  2  T  2  )  x  +  t h e power a b s o r b e d  u i s the microwave  2  2  4OI T  Ne x  2  m*  2  f r o m t h e microwave  f r e q u e n c y ,  t  n  e  cyclotron  field, frequency  given by: u  c  e being the e l e c t r o n i c  = eH/m*  charge, H the magnetic  field,  m* t h e  c a r r i e r e f f e c t i v e mass, and T the c a r r i e r s c a t t e r i n g Inhomogeneous b r o a d e n i n g dependent as i s l i k e l y phonon  Appendix  curve  i f u o r m*  a r e energy  a t h i g h e r c a r r i e r t e m p e r a t u r e s due t o  fitting  A. I t w i l l  procedure  be s e e n  that  used  i s described  cyclotron  Assuming  that  in  the S luminescence gives a  suggesting a resonance a t higher f i e l d  luminescence.  0.89  result  scattering.  The  signal  will  time.  the signals  than f o r the N a r e caused  by  r e s o n a n c e , t h e e f f e c t i v e mass v a l u e s i n d i c a t e d a r e  +_ 0.25 f o r t h e S a n d 0.36 + 0.1 f o r t h e N. T h e q u o t e d  error  bounds were o b t a i n e d by v a r y i n g t h e f i t p a r a m e t e r s and  comparing effective  the residuals mass  parameter  the  f i t . Several  and  the v a r i a t i o n  sets  t o s e e how was needed  much  change to  worsen  o f d a t a were o b t a i n e d f o r e a c h  centre,  i n the r e s u l t s  60  appreciably  i nthe  f o r t h e s e was a l s o  used t o  s i g n a l from the N luminescence showing e l e c t r o n resonance  cyclotron  FIG.  3.12 G A P - S  CYCLOTRON  RESONANCE  ODMR s i g n a l from the S luminescence showing hole resonance  cyclotron  help  arrive  at the values  g i v e n a b o v e . The v a l u e s  the two c e n t r e s were o f t h e o r d e r for  t h e N c e n t r e , t h e l o w O>T  for  the large error  The  o f 0.9 f o r t h e S a n d 0.3  f o r the N resonance  i n the e f f e c t i v e  effective  o f &JT f o r  mass.  masses deduced from  s u g g e s t e l e c t r o n s and heavy h o l e s .  accounts  t h e two r e s o n a n c e s  I t was h y p o t h e s i z e d  that  t r a p p i n g a t l u m i n e s c e n c e c e n t r e s was r e d u c e d b y c y c l o t r o n heating carrier the  of the free type  trapped  luminescence  carriers, first  from  and t h a t  heating  w o u l d have t h e g r e a t e s t e f f e c t on  that  weakly  and a n e u t r a l S donor  before  another  centre. should  N  binds  an  capture  first  electron.  the r e s u l t s  shown  that B i i s a hole  as s u g g e s t e d  t r a p , but the value  due t o i n c r e a s e d  resonance i n  i n F i g 3.13. T h e r e s o n a n c e  a p p e a r s t o be c a u s e d by h e a v y h o l e s  sure  electron  a hole  A t t e m p t s w e r e made t o o b s e r v e c y c l o t r o n GaP-Bi w i t h  o f the  by t h e f a c t  o f wx i s t o o low t o be  impurity  scattering  i n t h e doped  and l i g h t  and h e a v y h o l e  crystals.  Since  the e l e c t r o n  s h o u l d a l l be p r e s e n t could  n o t be  t o some e x t e n t  resolved  because  resonances  i n the luminescence but  o f the low v a l u e s  m e a s u r e m e n t s were made a t 36.3 GHz w i t h t h e r e s u l t s Fig. the  3.14. T h e s i g n a l  to noise ratio  was p o o r e r  low m i c r o w a v e power and a b s e n c e o f a c a v i t y ,  63  of&T  ,  shown i n  because o f but i t can  FIG.  3.13  G  A  P-B i  CYCLOTRON  RESONANCE  Al  MAGNETIC  FIELO  ODMR signal from the Bi centre showing a very broad resonance attributed to holes  FIG. 3.14  CYCLOTRON  RESONANCE  AT  36.3  GHZ  ODMR s i g n a l from N luminescence showing l i g h t and heavy hole resonances  be  seen  found  that  that  the  resonances  t h e N and  t h e h i g h e r f r e q u e n c y , two  to  effective  to  the  in intensities  resolved.  gave  similar  It  +_ 0.015  light  between  and the  was  resonances  r e s o n a n c e s were s e e n w h i c h  m a s s v a l u e s o f 0.154  correspond  difference  better  S luminescence  at  These  are  lead  and  0.626 _+  0.03.  heavy  holes.  The  two  resonances  i s to  be e x p e c t e d s i n c e t h e l i g h t h o l e b a n d h a s a l o w e r d e n s i t y o f states  than the heavy h o l e band.  not r e s o l v e d holes  indicated  underlying fitting fitted take the  but the f i t s  and  individually. of  resonance  resonances), cyclotron  field being  markedly,  indicating  electron's resonance  lower due  parameter  baseline  slopes  were  would  signals  (such not  be  formula.  light  between  1  and  as  The  and  used  in  each to  underneath of  other  f o r by  baseline  the  slopes differed  would have been but  splitting  to the a n i s o t r o p i c  an  signal.  resonance peaks,  as  included  tail  accounted  was  heavy  were  heavy h o l e resonances  hole  WT value  was  present the  another underlying  the  signal  which  peak o f the e l e c t r o n  valley  l i g h t and  peaks  dependent  resonance f o r the  the  f o r the  resonance  hole  fitted  which  slope  heavy The  calculated  The  calculated  A 'baseline  light  account  electron  the presence of the e l e c t r o n  effect.  the  The  because of  effective  the  of  at the  electron  m a s s , i t was  not  resolved.  Cyclotron using  resonance  microwave  measurements  absorption  of  66  by  Schwerdtfeger  photo-excited carriers  (27) at  35GHz g a v e s i g n a l s s i m i l a r t o t h o s e s h o w n i n F i g . 3.14; electron 'filling  GaP-Bi  resonance  GaP  samples  thought  a useful  The in  was  are  t o be  of  shown  ui  due  our  the  also  as  hole  run  at  t o h o l e s was  because  3.2  of  which  36.3GHz  still  contributions  the  9.2GHz and  i n the l i t e r a t u r e .  the S luminescence unresolved  t o the s i g n a l  but  the  too broad  cyclotron gives  from  d i s c r e p a n c y between  of  possible  to  mass.  values determined  from  a  resonances.  the  which  36.3GHz  and  results 36.3GHz  9.2GHz v a l u e  i s probably too  hole  would  effective  Our  The  signal  light  resonance  9.2GHz and  mass v a l u e s o f t h e h e a v y h o l e s .  determined high  except  measurements  i n Table  apparent  effective  between  were  the range of v a l u e s g i v e n an  seen  value of e f f e c t i v e  results  mass and  show  not  i n ' of the v a l l e y  The  give  resonance  the  and  electron  tend to d i s t o r t  the  fit.  We heating  next of  consider carriers  recombination observed reduction  centre.  t o be a f f e c t e d i n the  the  reduces  by  entire  probability  67  cyclotron from  luminescence must be due  of heated  ionization  recombine.  which  luminescence  e q u a l l y , the e f f e c t  to impact  trapped b e f o r e they can  the  Since the  trapping  the c e n t r e , or e l s e  mechanism  was to a  carriers  of c a r r i e r s  a  at  already  TABLE 3.2 GaP CYCLOTRON RESONANCE RESULTS  9.2  GHz  3 6.3  GHz  Literature  ITlj = 1.5  0.36+0.1  mt  = 0.18  average 6JT  0.3  m  0.89+0.25  0.626+0.03  0.8  2.6  nh  m Ih  0.154+_0.015  0) T  2.4 A. B a n d s t r u c t u r e  calculations  B. C y c l o t r o n r e s o n a n c e C. B a n d s t r u c t u r e  (78)  (27)  measurements  (78)  68  = 0.35  0.52(A) 0.67+.04(B) 0.88(C)  0.16(A) 0.17+.01(B) 0.132(C)  The  expected  microwave  power  scattering  time  values  heating levels  of electrons  used  t o be g i v e n  was  inside  of  V/Cm  100  microwave  t h e s a m p l e was c a l c u l a t e d giving  approximately  estimated  by t h e e l e c t r o n  a t 9.2 GHz. T h e maximum  strength  at the  an  average  0.5 meV. S i n c e  maximum  taking  the  resonance UT  electric  field  t o be o f t h e o r d e r  electron  the N centre  heating  binds  an  of  electron  w i t h a n e n e r g y o f 8 meV, a n d t h e b i n d i n g e n e r g y a t S a n d B i centres by  i s even  l a r g e r , the p o s s i b i l i t y  microwave h e a t e d e l e c t r o n s  Experimental trapping Dean  evidence  probability  (47)in  showed i n c r e a s e d  ionization  c a n be r u l e d o u t .  f o r the energy  of c a r r i e r s  excitation  of impact  dependence o f t h e  i n GaP h a s been o b t a i n e d by  spectrum  measurements.  H i s work  luminescence from the i s o e l e c t r o n i c  centres  N and B i when t h e p h o t o - e x c i t e d c a r r i e r s were g e n e r a t e d to  t h e band  carriers by  edges  could  phonon  using  thermalize  rapidly  excitation,  t o t h e bottom  and  analysis  at ionized  made by M.Lax (77).  o f t h e band  of trapping  impurities  cross  sections  i n semiconductors  He shows t h a t t h e t r a p p i n g  cross  trapped  into  increasingly  impurities.  Lax a l s o  for  impurities  neutral  calculates and f i n d s  69  large  orbits  the trapping  of  has been section  s h o u l d i n c r e a s e w i t h d e c r e a s i n g t e m p e r a t u r e as e l e c t r o n s be  when  emission.  A theoretical electrons  resonant  near  can  about the  cross  a 1/T d e p e n d e n c e .  section  Trapping impurity state and  o f an e l e c t r o n  at a neutral  i n GaP i s more c o m p l i c a t e d  for this  centre w i l l  scattering  produce  effects  since  isoelectronic the shallow  strong resonant  (38). A  N  bound  trapping  calculation  of the  p s e u d o p o t e n t i a 1 f o r t h e N i m p u r i t y i n GaP b y F a u l k n e r (790 shows a deep n a r r o w the  scattering  potential the  potential  and t r a p p i n g  I  E e  has c a l c u l a t e d  sections  f o r this  an e n e r g y d e p e n d e n c e o f  form:  + Ee)  i s t h e b i n d i n g e n e r g y o f t h e e l e c t r o n on t h e N a t o m , and the electron  measurements microwaves  kinetic  energy  t h e maximum  was  0.5  meV  centre  i s 8 meV,  decrease power.  was  the thermal  linearly  observed.  r e p r e s e n t an a p p r o x i m a t e l y  I t may b e q u e s t i o n e d  electrons  i n cyclotron  cyclotron orbit  with  from  energy  microwave h e a t i n g  in  electrons  r  the N  should  microwave  luminescence  signals  which  i n intensity.  whether t r a p p i n g c r o s s  wave  o  i s estimated to  the observed  5 % decrease  the  of the  section  increasing  The d e c r e a s e  5.8 % , i n good a g r e e m e n t w i t h  f o r plane  expected  K o n l y 0.13 meV w h i l e E j f  f o r t h e maximum  calculated  t o t r a p p i n g . In our  the trapping cross  approximately  This  expected  thus  prior  heating  and  e l e c t r o n s a t 1.6 d e g r e e s  be  Faulkner  cross  f o r e l e c t r o n s and f i n d s  l/(Ej E  well.  sections  are applicable  o r b i t s , f o r e x a m p l e , an e l e c t r o n  to ina  a b o u t a n i m p u r i t y m i g h t h a v e an i n c r e a s e d  70  trapping  probability.  impurities  are short  Faulkner  for N  spacings  or about  electrons magnetic  The p s e u d o p o t e n t i a l s range  i n GaP  field  10 A.U. T h e c y c l o t r o n Landau  trap  between  should  level  not  the cyclotron  isoelectronic  calculated  beyond  be  two  orbit  by  lattice  radius of  f o r the resonance  strength i s approximately  isoelectronic difference  effects,, that  isnegligible  i n the lowest  at  100 A.U., s o t h e  able  orbit  to  'see' the  and a p l a n e  wave  electron.  3.7  ODMR i n ZnTe  Samples Bell  telluride  L a b o r a t o r i e s . These  oxygen,  one s a m p l e  impurities acceptor  Zinc the  of zinc  such  level  as  were  containing  band  doped  which  f r o m J.Merz a t isoelectronic O^g.  forms  Stradling  measurements frequency  (49) u s i n g  d e g e n e r a t e . The  cyclotron  of thermally  excited  o f 1556 H z . The e f f e c t i v e  71  shallow present.  b a n d g a p o f 2.38 eV ( 4 8 ) , effective  m a s s e s o f l i g h t and h e a v y h o l e s i n ZnTe have b e e n by  Residual  a  f o r T e , were a l s o  has a d i r e c t  maximum b e i n g  with  the isotope  phosphorous,  when s u b s t i t u t i n g  telluride  valence  were o b t a i n e d  resonance holes  masses  at a were:  measured  absorption microwave  0.154 hh[100]  0.64 0.69  'hhtHO]  the  heavy  0.69  hole  band  being  non  spherical.  The  electron  e f f e c t i v e mass has b e e n e s t i m a t e d a t b e t w e e n  0.12  from  from  magneto-optical  acceptors  and  donors  studies  Dean  paramagnetic  (14) who  pair  v a l u e o b t a i n e d was electrons  on  on  resonance  donor-acceptor  ZnTe have observed  luminescence  +0.401 and  f o r Zn. L a r g e  reported  were  and  been the  made by  effects  was  in P  background  dependent  on  the p o l a r i z a t i o n  doped  attributed  shallow donors,  substituting  wavelength  shallow  Killoran,  of  electron  as a c h a n g e i n t h e p o l a r i z a t i o n  unidentified  and  luminescence  0.22  (50,51).  ODMR m e a s u r e m e n t s C a v e n e t t and  of  and  the  ZnTe.  The  to resonance  possibly signals  Al  g of  atoms  were  magnetic  of  also  field,  the  of the luminescence b e i n g  measured.  The in  luminescence  F i g . 3.15  peak  at  5220  . The A.U.  near This  attributed  to exciton  acceptors  including  substituting  for  spectrum  of  band gap  our  ZnTe  sample  luminescence  i s commonly  seen  in  has ZnTe  r e c o m b i n a t i o n a t a number P substituting  Zn  (14). Other  72  f o r Te, lines  i s shown  and  of  a  sharp and  is  shallow  L i and  Cu  correspond  to  ZnTe luminescence spectrum showing near band gap luminescence and the deeper 0 i s o e l e c t r o n i c centre spectrum  recombination spectrum phonon  a t a c c e p t o r s and  donors.  o f t h e o x y g e n c e n t r e s shows replicas  already observed  The  luminescence  the expected  for this  series of  isoelectronic  i m p u r i t y (52).  Attempts  were made t o o b s e r v e  the e l e c t r o n resonance i n  v a r i o u s p a r t s o f t h e l u m i n e s c e n c e , b o t h by m o n i t o r i n g t h e total  intensity  and  a n a l y s e r . No s i g n a l expected  value  background  by  was  of  a circular  polarization  seen anywhere i n the v i c i n i t y  g=0.401.  signals  luminescence,  using  were  Magnetic  seen  in  field  of the  dependent  a l l parts  of  h o w e v e r , and t h e s e were i n v e s t i g a t e d  the  using  9.2GHz m i c r o w a v e s w i t h t h e r e s u l t s  s h o w n i n F i g s . 3.16 a n d  3.17.  observed  Two  distinct  l i n e s h a p e s were  which p a r t of the luminescence  The  5220 A.U. l i n e  both gave r e s o n a n c e s  was  depending  monitored.  and t h e o x y g e n c e n t r e  which suggested  luminescence  an e f f e c t i v e  t h e o r d e r o f 0.3, w h i l e t h e 5290 A.U. p e a k g a v e a with  an  unusual  effective feature  mass  of these  value  decrease resonance  of the type  seen  resonance  approximately i s that  they  at resonance  mass o f  0.8.  An  represent  r a t h e r than a  i n most o f t h e o t h e r  cyclotron  results.  Measurements results  of  resonances  an i n c r e a s e i n t h e l u m i n e s c e n c e  upon  were  were a l s o  obtained  made a t 36.3GHz b u t no  but t h e low  74  power  level  useful  made  the  FIG. 3.16  ZNTE  ODHR  AT  5220 A  ODMR s i g n a l showing an increase i n luminescence caused by e l e c t r o n c y c l o t r o n resonance  FIG. 3.17 Z N T E  i  —  1  ODMR  AT  —  10 MAGNETIC  5290 A  20 FIELD  KG  Hole resonance i n 5290 A.U.  luminescence  signal  t o o weak t o  The  observe.  effective  resonances  mass  w e r e : 0.8  +_ 0.2 a n d  t h e o r d e r o f 0.9 a n d heavy  h o l e s and  low  by  rather  Stradling  heavy  0.3  from  the  9.2  mass  or p o s s i b l y  t h a n by  signal the  GHz  +_ 0.2 h a v i n g D T v a l u e s o f  light  was  l i g h t holes which  caused  were  same WT  In  view  resonances,  probably  t o have a p p r o x i m a t e l y t h e  suggest  holes.  i n t h e cox v a l u e s f o r t h e t w o  effective  electrons  deduced  0.2 r e s p e c t i v e l y . T h e s e v a l u e s  electrons,  of the d i f f e r e n c e the  values  by  observed  v a l u e as  the  holes.  The  fact  increase  that  luminescence  decay  processes  which  are  different bands  radiative  processes  be  are  this  resonances  there are  for electrons carrier  and  to  and  However,  luminescence  since,  a l l the  the  holes,  heating.  hypothesis  was  non-radiative  and  in different  inhibited,  and  to  i s that  exciton  i f  non-  luminescence  observed  the  observed  recombination  d o n o r s . E v i d e n c e has  m e a s u r e m e n t s t h a t t h e 5220 A.U. ionized  that  observed  in proportion  possibility  correspond  acceptors  are  the  resonance  t h e same i n a l l b a n d s .  Another bands  of  microwave  contradicts  increase  should  compete  by  resonances  effect  suggests  which  inhibited  which  should  the  luminescence at  been o b t a i n e d f r o m  luminescence  Zeeman  originates  a c c e p t o r s (51). I o n i z e d i m p u r i t i e s w i l l  77  ionized  from  t e n d t o be  neutralized cyclotron  by  photo-generated  heating  i s expected  ionized  atoms  trapping  probability  ionization centres  of  will  for  be  be  much  t h a t an  result  Our  luminescence  pointed  less  out  and  which  reducing o r by  number o f  would  be  that o p t i c a l  increase  i n an  the  impact ionized  increased,  probably  i m p u r i t i e s needed  due  already  to  i s also  the  in  expected  non-radiative  discussed  f o r GaP.  to  to  centres  the the  produce  electron absence the  of  paramagnetic appropriate  donor-acceptor  pair  l u m i n e s c e n c e r e p o r t e d by K i l l o r a n e t a l (14) w h i c h was visible  i n our  It  luminescence.  observe due  the  recombination  i n the number o f i o n i z e d  increase  to  acceptors  efficient  process  inability  r e s o n a n c e was donor  by  the  (13)  rate at  o b s e r v e d i n GaP,  a t n e u t r a l d o n o r s and  Auger r e c o m b i n a t i o n  should  either  n e u t r a l i z e d a t o m s . Thus  I t should  of e x c i t o n s  follows  was  holes  to reduce the  neutralized as  and  t h e e x c i t o n t r a p p i n g r a t e m i g h t be r e d u c e d by  heating.  but  are  available  although  electrons  not  sample.  3.8 Other M a t e r i a l s .  ODMR m e a s u r e m e n t s were made on S4  and  magnetic f i e l d  o b s e r v e d . The to give even  any  cox  dependence background  values  of these  measure of c a r r i e r  possible  to  s a m p l e s o f CdS  determine  78  Cdln2  s i g n a l s were  r e s o n a n c e s were too  effective  whether  and  the  m a s s , i t was  resonances  were  low not due  t o e l e c t r o n s o r h o l e s . I t was e v i d e n t results,  that, to obtain  c y c l o t r o n ODMR e x p e r i m e n t s  need  high  purity materials  and a t high  This  i sgenerally  for conventional  true  useful  t o be done  microwave  with  frequencies.  cyclotron  resonance  work.  3.9  Summary  ODMR m e a s u r e m e n t s w e r e u s e d t o o b s e r v e t h e paramagnetic S,  resonance  of electrons  and B i i m p u r i t i e s  been p r e v i o u s l y results  reported  confirm  on l u m i n e s c e n c e  i n G a P . ODMR from  effect of  signals  the i n t e r p r e t a t i o n that  electrons.  luminescence found  taken was  very  with  into  conduction  o f theresonance  the observed  a n d was  signal  strength  A and B l i n e s was  a c c o u n t . The t e m p e r a t u r e d e p e n d e n c e o f t h e  measured over a l i m i t e d decrease  for  of the expected  r a p i d t h e r m a l i z a t i o n between the  sample and the was  well  and o u r  t h e s i g n a l seen  was made  change as a r e s u l t  t o agree  when t h e  A calculation  N,  had n o t  S and B i l u m i n e s c e n c e  N i m p u r i t i e s i s due t o r e s o n a n c e o f p h o t o - e x c i t e d band  from  r a n g e by l a s e r  heating  i nsignal predicted  effect of the  theoretically  o b s e r v e d . T h e r e s o n a n c e was s h o w n t o b e h o m o g e n e o u s l y  broadened considering inferred  giving the  that  time. Circular  a  lifetime  results  this  of conventional  lifetime  polarization  luminescence, t h i s d e f e c t s on e x c i t o n s  of 4  nano  seconds.  EPR o n GaP i t was  was t h e e l e t r o n - h o l e effects  By  were  absent  was a t t r i b u t e d t o t h e e f f e c t s  capture from the  of crystal  w h i c h were shown t o be c o n s i d e r a b l e . Our  79  inability also  to seeparamagnetic  thought  t o be  hole resonance  due t o c r y s t a l  homogeneous b r o a d e n i n g o f t h e h o l e  Cyclotron  resonance  by ODMR, t h e f i r s t sensitivity  defects  causing  signal.  of electrons  and h o l e s was o b s e r v e d  such measurement i n t h i s  of different  e f f e c t s was  luminescence  bands  m a t e r i a l . The  to electron or  h o l e r e s o n a n c e was shown t o be d e t e r m i n e d b y t h e processes trapped  trapping  f o r the c o r r e s p o n d i n g i m p u r i t i e s , the c a r r i e r  first  by a n e u t r a l  impurity  had t h e g r e a t e s t  type  effect  when h e a t e d . The m a g n i t u d e a n d t e m p e r a t u r e d e p e n d e n c e o f t h e effect  was  predictions  measured  and compared  f o r thetrapping  cross  with  section  theoretical  o f the N i m p u r i t y  a n d g o o d a g r e e m e n t was f o u n d . A c c u r a t e v a l u e s o f l i g h t a n d heavy h o l e e f f e c t i v e 36.3  masses were o b t a i n e d by m e a s u r e m e n t s a t  GHz a n d a g r e e d  cyclotron  resonance  c o u l d n o t be r e s o l v e d to  the anisotropic  scattering  with  the results  experiments.  of conventional  The e l e c t r o n  p r o p e r l y a t 36.3 GHz, electron  effective  t i m e , probably caused  resonance  p r e s u m a b l y due  mass  and s h o r t e r  by r e s o n a n c e  scattering  from N i m p u r i t i e s .  ODMR s t u d i e s resonances signals  o f ZnTe showed e l e c t r o n and h o l e c y c l o t r o n  a n d , a s i n GaP,  different  corresponding to electrons  results, cyclotron  t h e Z.nTe  luminescence  resonances.  l u m i n e s c e n c e bands gave or holes.  was i n c r e a s e d  I t was h y p o t h e s i z e d  80  Unlike  that  t h e GaP by t h e ionized  impurity gap  centres  were r e s p o n s i b l e  luminescence,  an  measurements r e p o r t e d heating  increased  Recombination correspondingly  increase  oxygen  i n the  but  number  such due  i n the f r e e c a r r i e r  trapping  at  tool  centres.  i s correct, this  for  identifying  originating especially  from  do  not  work  would  Auger  be  isoelectronic  provide bands  (e.g. due  to  net  of  the  valuable  in  ZnTe  or  such  a  reduced  a  acceptors  techniques,  less  processes.  interpretation  luminescence  other  reduced  c a u s e d by  technique could  where  cyclotron  represented  population I f our  Zeeman  be  f o r the  i o n i z e d or n e u t r a l  i n cases  spectroscopy,  observed  band  impurities.  could  presumably  increase  results  centres  by  that  ionized  to non-radiative  luminescence  other  of  and  recombinations  i n luminescence  centre  supported  literature,  neutral  efficient optically The  assumption  the  at  f o r most o f t h e n e a r  as  donors,  as  Zeeman  unresolvable  splittings).  Attempts in  ZnTe  by  appropriate  to observe the e l e c t r o n paramagnetic ODMR  were  donors  unsuccessful to  l u m i n e s c e n c e band i n w h i c h  produce  due  the  to  lack  of  donor-acceptor  the e f f e c t m a n i f e s t s  81  a  resonance  itself.  Chapter 4.  4.1  Silver  Bromide.  AgBr  Silver indirect  bromide  band  gap o f  i s a highly 2.69  eV  polar  material  ( 5 3 ) . The  with  an  bandstructure  is  s h o w n i n F i g . 4.1 a n d i s , i n some r e s p e c t s , t h e i n v e r s e o f band minimum i s c e n t r e d a t t h e r  t h a t i n GaP. The c o n d u c t i o n  p o i n t and i s a p p r o x i m a t e l y s p h e r i c a l , w h i l e  the four  valence  band maxima l i e a t t h e X p o i n t s and a r e e l l i p t i c a l , g i v i n g a hole  an a n i s o t r o p i c e f f e c t i v e  valence strong  bands  mass  are appreciably  electron-phonon coupling  Several W.Czaja, crystal  pure  crystals  EPF, Lausanne, with  identified  (54). B o t h c o n d u c t i o n  non-parabolic t o be d e s c r i b e d  of  AgBr  were  Switzerland. crystal  axes.  obtained  These  The c r y s t a l s ,  The  main i m p u r i t y was i o d i n e w h i c h , b e i n g c h e m i c a l l y  concentrations  4.2  i s generally of the order  from  included  intentionally  bromine,  to the  later.  not  to  doped, c o n t a i n e d  due  and  one  though  some r e s i d u a l i m p u r i t i e s .  present  i n undoped  similar AgBr  in  o f one p a r t p e r m i l l i o n .  ODMR i n A g B r . The  principal  luminescence  spectrum  f e a t u r e s . A weak  multi-phonon isoelectronic  replicas  zero from  iodine centres  of  phonon exciton  forms  82  AgBr line  contains  two  and a s e r i e s o f  recombination  a broad  band w h i c h  on  peaks  FIG. 4.1 A G B R BANDSTRUCTURE  r  k  € = O.I5eV 8 = 0.58eV  83  L  at  approximately  energy A.U.  a broad  This  4900  A.U. a s s h o w n  f e a t u r e l e s s continuum  latter  band i s t h o u g h t  recombination a t i n t e r s t i t i a l silver the  specks  crystal  cycling  cooling  to light  t o room  temperature  first  slowly  on  single  resonance  using  a  either  by e x p o s i n g  induced  i n thermal  I t c a n be r e d u c e d by C for  have  been  frequency  (56), u s i n g  reported  (58), and by  (56,59). Hayes e t a l o b s e r v e d a  a t g=1.8 i n t h e 5700 A.U.  luminescence  of approximately  band  20 G H z .  10 W a t t s o f m i c r o w a v e power a t 35 GHz,  b a n d a t g v a l u e s o f 1.49,  1.75,  He a t t r i b u t e d  electron,  24 h o u r s a n d  (53,57).  AgBr  has o b s e r v e d a s e r i e s o f r e s o n a n c e s  free  electron-hole  b y H a y e s , Owen a n d W a l k e r  microwave  isotropic.  from  5700  atoms o r m u l t i - a t o m  a t 200 d e g r e e s  M a r c h e t t i and h i s c o - w o r k e r s  Marchetti  silver  temperatures.  measurements  previously,  to arise  o r by s t r e s s e s  helium  t h e sample  ODMR  peaks a t around  (55,56), and i s enhanced  to liquid  annealing  i n Fig.4.2. A t lower  i n t h e same  luminescence  1.81 a n d 2.08 a l l a p p a r e n t l y  t h e g=1.49  and t o hole  a n d g=2.08  paramagnetic  values to resonances  r e s p e c t i v e l y . He a s s i g n e d t h e t w o i n t e r m e d i a t e r e s o n a n c e s t o electrons the by  t r a p p e d on t h e d e f e c t o r c e n t r e r e s p o n s i b l e f o r  5700 A.U. comparing  various (59)  theeffects  carrier  have  also  luminescence of  luminescence. This  trapping studied  using  AgBr  interpretation  on s i g n a l s impurities. ODMR  from  doped  iodine. At intermediate doping  84  s a m p l e s doped  with  M a r c h e t t i and B u r b e r r y  signals  crystals  was v e r i f i e d  from  iodine  centre  with  various  levels  levels  (of t h e o r d e r o f  FIG. 4.2 2.1 EV  6000  AGBR  LUMINESCENCE  2.4 EV  2.5 EV  5500 5100 4900 WAVELENGTH A  2.6 EV  4700  100  ppm)  a  single  broad  resonance  o b s e r v e d . They a t t r i b u t e d  this  angular  the  4.3  momentum s t a t e s  of  to  at  g=5.65  was  transitions within  the  iodine  about  bound  exciton.  Results.  The  r e s u l t s of  our  luminescence band, at 4.3.  and  +_ 0.02  4.4.  and  At  g = 1.81  experimental visible high  9.2  9.2  GHz  low  was  were  some o f  the  g  large  value,  seen  36.3  at  (75)  GHz  on  are  the  5700  shown  in  A.U. Figs.  r e s o n a n c e s w e r e s e e n a t g = 2.07  and  e r r o r s . In  field,  and  two  +_ 0.02  merged w i t h  resonance  measurements  i s o t r o p i c to the  data a t h i r d  signal  side.  g = 1.708  within  at  At +_  peak  g=1.81 and  36.3  0.01  GHz  which  the  on  was the  only  one  was  also  isotropic.  The  4900  A.U.  iodine  emission  d i f f e r e n t ODMR s p e c t r u m . No sharp  EPR  using  either  two  seen  9.2  or  from  36.3  t r a c e o f any the  GHz  5700 A.U.  microwave  we  holes. results  attribute  This to  resonance  to  cyclotron  i s s u p p o r t e d by those  reported  differences  obtained by  the by  other  which w i l l  be  very  detected Instead,  shown i n F i g . of  electrons  similarity  detection  of  of  later.  4.5. and  these  cyclotron  (60-65). However,  discussed  86  a  relatively  stimulation.  striking  workers  the  b a n d was  resonance  direct  yielded  of  b r o a d e r r e s o n a n c e s w e r e o b s e r v e d as  These  are  lines  band  there  FIG. 4.3 ODMR IN A G B R  •• • g • 2.07  9.2 GHz  g - 1.81  2.9  _L  3.5  1  4.1  MAGNETIC FIELD kG ODMR of 5700 A.U. band showing e l e c t r o n and hole resonances  FIG. 'l.'l  12  J  14  ODMR  IN A G B R  L  16  18  MAGNETIC FIELD kG E l e c t r o n resonance at high  frequency  20  FIG. \,5 L  CYCLOTRON  RESONANCE  IN A G B R  T  m* • 0.29 m*«l.l  00  •• •  36.3 GHz  10  1  20  MAGNETIC FIELD kG Cyclotron  resonance of e l e c t r o n s and holes i n I luminescence  Unlike emission, at  t h e EPR r e s o n a n c e s ,  the signals  which  caused  represent a decrease  increased  i n luminescence  r e s o n a n c e . The l o w e r r e s o n a n c e , a t t r i b u t e d t o e l e c t r o n  cyclotron  r e s o n a n c e , was i s o t r o p i c . The h i g h e r r e s o n a n c e was  broadened  i ncertain  directions  resonance,  and t h e poor  signal  impossible  t o determine  whether  unresolved  splitting  hole  cyclotron  A t 9.2 GHz, the  i nthis  this  effective  mass  microwave  power  values level  (i.e. luminescence  The s p e c t r a l  increased  dependence  should  electron background  so  As t h e  resonance resonance  a t resonance) and  spectrum  w i t h the  magnetic  and h o l e r e s o n a n c e s .  obtained  resembled  the  spectrum.  be n o t e d  that  and h o l e resonances signal  t o be  o f t h e ODMR s i g n a l s was  s e t a t the peak o f the e l e c t r o n 'spectra*  the hole  while the electron  c h e c k e d by s c a n n i n g t h e i o d i n e  photoluminescence  both appear  a p p r o x i m a t e l y t h e same  was i n c r e a s e d  changed  It  for a  r e s o n a n c e s was s t u d i e d  a s t h e 36.3 GHz r e s u l t s .  and broadened,  ODMR  the  4.6 a n d 4.7. A t l o w p o w e r  lines giving  strengthened  field  by  i sexpected  and h o l e r e s o n a n c e s  homogeneously broadened  broadened.  made i t  material.  shown i n F i g s .  the electron  sign  ratio,  the  w h e r e more m i c r o w a v e p o w e r was a v a i l a b l e ,  the r e s u l t s  levels  of  was c a u s e d  which  power d e p e n d e n c e o f t h e c y c l o t r o n  with  The  to noise  of the line  resonance  but the width  presumably  t h e ODMR  signals  s i t on a l a r g e caused  90  from t h e  non-resonant  by d i e l e c t r i c  heating.  FIG, 4.6  LOW POKER CYCLOTRON RESONANCE SIGNAL  E l e c t r o n c y c l o t r o n resonance at low frequency showing homogeneous l i n e shape  FIG. 4.7  POWER  DEPENDENT  CYCLOTRON  RESONANCE  SIGNAL  ••••  •  • • •••  *  ' •V .  • 10 M A G N E T I C  Electron  and hole resonance  F I E L D  s as a f u n c t i o n  kG  o f microwave power  I  FULL  II I  U  IV  POWER  - 10 DB _20 nB  - 30 DB  A l s o no t r a c e o f t h e c y c l o t r o n r e s o n a n c e s o r b a c k g r o u n d i s seen  4.4  i n t h e 5700 A.U. l u m i n e s c e n c e  band.  Discussion.  Our  ODMR  results  from  t h e 5700  A.U.  band  agree  s u b s t a n t i a l l y w i t h t h o s e o f Hayes e t a l and M a r c h e t t i , t h e 9.2 GHz d a t a  showing  two  electron  trapped  resonance seen,  that  at higher  lower temperature cases  field  merged  This  signal agrees  of spin  respect  of  the other l i n e s  i n the  at high  frequency indicates  i n the iodine  the f a c t times  spins  Marchetti's  to the thermal  magnetic  energy.  Our  f r e q u e n c y may be  are almost completely that the lines  l e a s t i n part homogeneously broadened.  decay  with  relatively  t o t h e low m i c r o w a v e power a v a i l a b l e . The f a c t  merged a t l o w e r  to  becomes  states  t h a t t h e g=1.81 a n d g=1.7 r e s o n a n c e s  lines  t h e g=1.81  (4.2 d e g r e e s K t o 1.7 d e g r e e s K) s i n c e i n  with  to observe  attributed  EPR  with  resonance decreases i n i n t e n s i t y at  the s p l i t t i n g  increases  failure  t h e g=1.81  field.  observation that this  both  resonances  a t g = 2.07 a n d t h e  s t r o n g e r . A t 36.3 GHz o n l y t h e g=1.7 r e s o n a n c e i s  indicating  weaker  the hole resonance  centre  are longer  which  F a i l u r e t o observe  luminescence  t h a t , b e i n g an i s o e l e c t r o n i c  are at  i s p r o b a b l y due  centre, the exciton  allows greater  thermalization  before recombination.  Cyclotron  resonance  of  electrons  o b s e r v e d by s e v e r a l r e s e a r c h e r s u s i n g  93  i n AgBr  conventional  has  been  microwave  absorption first  techniques  measurement was  observed signal  an  indicated  and  photo  made by  asymmetric  which  Baxter  and  and an  generated  Ascarelli  strongly  effective  Ascarelli  (61),  and  carriers.  The  Brown  who  (60)  temperature  mass o f  and  dependent  0.27.  Tamura  and  Masumi  (62,63,64) h a v e s t u d i e d t h e t e m p e r a t u r e d e p e n d e n c e o f r e s o n a n c e i n more d e t a i l by c h a n g i n g t h e l a t t i c e and  increasing  fields.  Tamura  mass o f up which Low  the c a r r i e r  to  and  temperature  Masumi o b s e r v e d  15%  with high  levels  an  using  workers  Pines  (65) have  effective  excited  and  GHz. field split  magnetic  and  They  an  increase  predictions  of  Lee,  ( 6 7 ) . Hodby  and  co-  i n apparent  electron  monitored  resonance  Masumi  (54)  observed  two  to lines  strengths  who  microwave  at which  i s approximately  changes i n sample  parallel into  Larsen  field  level  e n e r g y . They  Cyclotron Tamara  and  observed  cyclotron  measuring  (66)  effective  excitation,  m a s s o f a p p r o x i m a t e l y 5% a t v e r y h i g h  frequencies  phonon  (LLP)  microwave  in  of microwave  they compared w i t h the t h e o r e t i c a l and  temperature  high  increase  this  20%  the c y c l o t r o n  the of  first  the  LO  resonance  by  seen  by  photoconductivity.  of  h o l e s has  used  been  microwave  a resonance  at  the  direction.  [1,0,0]  when t h e  field  axes.  94  absorption  m*=0.99 f o r a  was  only  This  applied  at  34  magnetic resonance  along other  Both  electron  strongly  energy  electron  and  coupling  and h o l e  e f f e c t i v e masses  d e p e n d e n t due t o n o n - p a r a b o l i c i t y  valence  between  bands.  This  i s due  c a r r i e r s a n d LO p h o n o n s  (54). a i s t h e c o n s t a n t f o r c o u p l i n g to  i n AgBr a r e of the  to the  strong  ( a e = i . 6 , ah = 2.8)  of electrons  and h o l e s  t h e l o n g i t u d i n a l o p t i c a l phonon mode and i s d e f i n e d  as:  a = < e 2 / j l ) ( i / e < B - l / e s )( m b / 2 n c o L O ) l / 2 where  es  em  and  dielectric  a r e t h e low and h i g h  constants,  o p t i c a l p h o n o n , a n d mD is  MwL0 t  n  the energy  e b a n d  frequency of a  lattice  longitudinal  mass o f t h e e l e c t r o n  t h e e f f e c t i v e mass i n t h e a b s e n c e  which  of the electron-phonon  interaction.  The  lattice  electron alpha  than  potential well  the  mass  o f an  t o aw L 0 )A, and f o r v a l u e s o f  6 the e l e c t r o n  becomes t r a p p e d  i n the  analysis  i n strongly  of  polar  the  po1aron-e1ectron  materials  by L e e , Low  and  (66). i s c a r r i e d o n l y t o t h e f i r s t o r d e r ; i t p r e d i c t s  increase  interaction  i n e f f e c t i v e mass c a u s e d without  work o f L a r s e n d e p e n d e n t mass. the  i n the presence  so p r o d u c e d ( 6 5 ) .  theoretical  effective Pines  energy  i s approximately equal  greater  A  relaxation  calculating  by  the energy  (67), however, i n d i c a t e s  electron-phonon dependence.  a strongly  In p a r t i c u l a r , when t h e e l e c t r o n  LO p h o n o n e n e r g y o f 17.2 mev  95  the e l e c t r o n  The  energy  approaches  mass  should  increase than  rapidly  that  for a cold  Our some The  results  respects  line  of  (54) was  the  and,  Splitting  magnetic  field  orientation  detected  obtained  hole  at  along  used  36.3  the  by  GHz,  be  was  nature  of  valence  the  The showed  asymmetry  unusual  experiments the  to  high  the  level  effective  be  the  electron  from  electron  conventional  non  signal  s i d e due results  showed an  to hot  by  a t 36.3  GHz  microwave  ascribed  to  tail  their  an  powers  greater  i n luminescence  trapping  used,  96  in  of  rate at iodine probability, recombination  the  heating  t e m p e r a t u r e so t h a t t h e increase  on  s h o w e d an i n v e r t e d  recombination  reducing  s m a l l compared t o the c a r r i e r be  in  resonance  electrons with  w h i l e h e a t i n g of hot e l e c t r o n s i n c r e a s e d t h e i r  not  the  resonance  i n c r e a s e . Thus m i c r o w a v e h e a t i n g  cold e l e c t r o n s reduced t h e i r  can  shift  usually displays a  tail  low  was  parabolic  electron  the  the  noise  microwave  cyclotron  while  For  and  applied  which  the  m a i n peak a p p e a r e d as a d e c r e a s e  rate.  to  the  mass a t h i g h  the  presumably  Tamura  signal  with  tail:  centres,  methods.  e x p e r i m e n t s . The  expected  f e a t u r e s . In  m a s s . Our  poor  in  band.  of  field  agree  t o homogeneous  [1,1,1] d i r e c t i o n  hole resonance to higher e f f e c t i v e levels  by  w o r k , due by  =1.6.  resonance  seen  apparent  i n many o f o u r  power  greater  conventional  resonance  should  40%  f o r a c o u p l i n g of a  optically  those  approximately  not r e s o l v e d i n our  broadening  ratio.  a value  electron  with  with  splitting  Masumi  to reach  the  hot  is  effect  electron  p o p u l a t i o n a t t h e expense o f c o l d  electrons  due t o m i c r o w a v e  heating.  A p o s s i b l e e x p l a n a t i o n i s that hot e l e c t r o n s can drop quickly  onto  traps  with  the emission  o f one  p h o n o n s . Such e f f e c t s  have been o b s e r v e d  Malm  (68)  and  Haering  photoluminescent as  a  function  recombination  intensity  measured  the  wavelength  i s e n h a n c e d when e l e c t r o n s  s t a t e w i t h t h e e m i s s i o n o f an i n t e g r a l Dean  i n other  (47) h a s  spectra  o f GaP  similar  effect  observed  the  f o r both  same  been  of  c e n t r e s i n CdS  and  showed  that  c a n d r o p t o a bound  number o f LO p h o n o n s .  process  in  excitation  N and B i l u m i n e s c e n c e  has r e c e n t l y  LO  materials,  dependence  from r e c o m b i n a t i o n  of excitation  o r more  observed  centres. A  in excitation  s p e c t r a o f AgBr doped w i t h v a r i o u s i m p u r i t i e s  (76). In t h e  present  near  case, resonant  phonon e n e r g y would rapid  transition  s h o u l d be n o t e d hole  first,  centre Thus  increase i t s probability  t o a bound  state  then  an e l e c t r o n  of making  a t an i o d i n e  that the i s o e l e c t r o n i c  t h e LO a  impurity. It  iodine centre traps a  i s attracted  to the  charged  and e n t e r s one o f a s e t o f h y d r o g e n - l i k e o r b i t s ( 6 9 ) .  there  phonon  will  energy  assisted  effective  be a b a n d from  transition  mass p r e s e n t e d  due  h e a t i n g o f an e l e c t r o n  which  of energies electrons  t o a bound  by L a r s e n  (67)  mass as t h e e l e c t r o n  to the strong  state.  can  e1ectron-phonon  97  below  make  a  t h e LO phonon  The t h e o r y o f p o l a r o n  predicts reaches  just  a rapid  increase i n  t h e LO phonon interaction.  energy, For a  coupling 40%  o f a=1.6  the theory  in effective  although  mass  the accuracy  r a n g e . The n e g a t i v e data  shows g e n e r a l  this feature than  tail  that  an i n c r e a s e  f o r a cold  o f about electron,  i s questionable  of the electron  agreement w i t h  this  i n this  resonance  theory,  i nour  t h e peak o f  a p p e a r e d i n f i e l d s b e t w e e n 20% a n d 50% h i g h e r  t h e main e l e c t r o n  t h e LO phonon  At  over  of the theory  s i g n a l . The t a i l  b r o a d e n e d due t o r a p i d near  predicts  was  inhomogeneously  variation i n electron  effective  mass  energy.  9.2 GHz t h e e l e c t r o n  r e s o n a n c e showed no a p p r e c i a b l e  a s y m m e t r y , homogeneous b r o a d e n i n g h a v i n g become d o m i n a n t . A t sufficiently  high  levels  o f microwave  inverted  and b r o a d e n e d . T h i s  had  heated  been  negative  tail  electron  caused  by  An from  peak.  increased  and o p t i c a l  that  t h e LO p h o n o n  of the resonance  energy  acoustic  to near  indicates  power  the electrons  energy  was s t r o n g e r  The b r o a d e n i n g  scattering  the signal  so t h a t t h e than  t h e low  of the s i g n a l  of the hot electrons  was by  phonons ( 6 4 ) .  a t t e m p t was made t o e s t i m a t e  t h e m i c r o w a v e s . The rms e l e c t r i c  the heating field  expected  strength  i n the  m i c r o w a v e c a v i t y i n t h e v i c i n i t y o f t h e s a m p l e a t maximum excitation  was c a l c u l a t e d  s a m p l e b y an e l l i p s o i d sample  a s 90 V/Cm  calculated resonance  from  (70).  t o be 180 V/Cm. A p p r o x i m a t i n g t h e  gave t h e f i e l d The e l e c t r o n  t h e CUT v a l u e  a t 9.2 GHz, g i v i n g  strength  i n s i d e the  scattering  o f t h e low power  the average  98  energy  time  was  electron  acquired  by  an  e l e c t r o n between  This  figure is obviously  increases  due  electrons  the  free electrons  occur at  4.5  an  the  are  of  the  and  meV.  phonons  as  i t indicates that heating  of  o r d e r o f one  levels  28  of  optical  LO  phonon e n e r g y  does  microwave e x c i t a t i o n .  Summary  been  resonance  studied  recombination  using at  exciton  of  electrons  ODMR.  of  heating.  luminescence  were  that  the  of  holes  recombination near  the  This  i s explained  the  LO  at  shown  microwave the  showed  shown  holes of  be  as  the  has that  the  of  an  centres,  at the  while  increased  with  phonon.  approximately  considerable  enhancement  substantiating this  iodine  their  of  LO  the  explanation.  99  rather  their  electrons  trapping  to  the  Cyclotron  o f a hot  power the  bound  reduced of  by  affected  centres  Measurements  sufficient  as  changes i n  heating  enhanced t r a p p i n g LO  well  equally  electrons  e l e c t r o n s 'to  resonance,  i n AgBr  decay processes.  cold  energy  by  frequency  to  holes  exciton  and  iodine  phonon  emission  has  e f f e c t s w e r e c a u s e d by  changes i n the  heating  and  A l l parts  t r a p p i n g r a t e o f e l e c t r o n s and t h a n by  holes  work  electrons  dielectric  indicating  Our  and  iodine isoelectronic impurities i s affected  cyclotron heating  microwave  of  approximately  overestimate since scattering  h e a t e d , but  highest  Cyclotron  by  as  to e m i s s i o n of a c o u s t i c  the  by  scatterings  rate.  electron at  heat phonon  lower  the  bulk  energy  luminescence  at  This  technique  provided  a  effective  mass o f h o t e l e c t r o n s  directly.  A  comparison  electrons  determined  predicted  by  the  by  the  their  near  resonance  splitting  resonance  and  phonon  mass  energy  of  resonance  the  the  with  hot that  agreement.  o f h o l e s was  also  resolved  insufficient  measuring  t h e LO  cyclotron  c o u l d n o t be  of  effective  theory gives reasonable  Cyclotron expected  of  means  observed  due  but  the  to the w i d t h  microwave  power  of  at  high  5700  A.U.  frequency.  The  ODMR  results  obtained  from  the  luminescence  band c o n f i r m t h o s e o f Hayes e t a l and  (58,56).  absence  The  luminescence responsible carriers.  The  d e f e c t . The  cyclotron  indicates  i s much  understood,  the  of  less  nature  although  of  centre  trapping  sensitive these  they  paramagnetic  trapping  that  resonance  to  appear  to  resonances  were  shown  effects at  the  cyclotron  centres  Marchetti  i s not  involve  in  this  centres  heating of completely  some t y p e  of  of e l e c t r o n s bound  to  to  be  homogeneously  broadened.  It i s worth noting that although o p t i c a l d e t e c t i o n c y c l o t r o n resonance such resonances The  resonance  luminescence  may  has  been r e p o r t e d i n v e r y few  materials,  have been o b s e r v e d but not r e c o g n i z e d .  r e p o r t e d by of  of  iodine  M a r c h e t t i and  doped  AgBr  100  Burberry  a t g=5.65, and  (59)  i n the  attributed  to was  a t r a n s i t i o n between almost  certainly  s i g n a l . The g r e a t e r those  reported  presumably  due  states the  width  of the i o d i n e  electron  cyclotron  of t h e i r resonance  h e r e , and i t s l a c k o f s t r o n g to the higher  consequent increase  bound  i n impurity  level  of  101  resonance  compared  to  asymmetry, i s  iodine  scattering.  exciton,  doping  and  Chapter 5. Conclusions and Suggestions f o r further work.  5.0 Introduction.  Sections define this  5.1 t o 5.4 c o n t a i n  t h e new r e s u l t s  thesis.  further  Section  which  brief  statements  which  a r e r e p o r t e d i n t h e body o f  5.5 c o n t a i n s  some  suggestions f o r  work.  5.1 Gallium Phosphide.  1)  ODMR  signals  from  B i and S i m p u r i t i e s  observed, they c o n f i r m t h a t the N s i g n a l conduction to  electron  be s a t u r a t e d  a lifetime  resonance.  of approximately  4 nano  2) The maximum s i g n a l e x p e c t e d to  give  The  good  agreement  comparison  showed  thermalization  between  temperature  dependence  a  range  limited  temperature  with  were  broadened seconds.  and f o u n d  the experimental that  there and B  o f the signal  a n d was f o u n d  shown  indicating  was c a l c u l a t e d  the A  been  corresponds to  The s i g n a l s  and h o m o g e n e o u s l y  have  results.  i s  rapid  lines.  The  was measured  over  to decrease  with  as p r e d i c t e d .  3) P o l a r i z a t i o n  effects  expected  i n t h e ODMR  signal  w e r e n o t o b s e r v e d . T h i s was shown t o be due t o a l a c k of  circular  polarization 102  and c o n f i r m e d  b y Zeeman  m e a s u r e m e n t s . The h o l e p a r a m a g n e t i c r e s o n a n c e was  also  crystal  4)  absent.  strains  Cyclotron  observed  Both  effects  were  signal  attributed  and d e f e c t s .  resonance  of electrons  and h o l e s  t o c y c l o t r o n h e a t i n g o f the c a r r i e r  first  by t h e n e u t r a l  trapped  power d e p e n d e n c e  centre.  The m i c r o w a v e  with  agreement  theory  f o r the N  was a l s o  obtained  centre.  f o rtheS  c e n t r e . M e a s u r e m e n t s a t 36.3 GHz g a v e a c c u r a t e light  with  and h e a v y h o l e  cyclotron  type  o f t h e e f f e c t was m e a s u r e d and showed  agreement  Qualitative  for  was  by ODMR. L u m i n e s c e n c e c e n t r e s were shown t o be  most s e n s i t i v e  good  to  effective  resonance  masses, these  results  obtained  by  values agreed other  workers.  5.2 Zinc T e l l u r i d e .  1)  ODMR  measurements  resonances resonances  were  luminescence.  heating The  which  observed  i n different  cyclotron  luminescence  t h e r e s o n a n c e s c a u s e d an i n c r e a s e i n  T h i s was a t t r i b u t e d  of ionized  signal  revealed  o f e l e c t r o n s and h o l e s . As i n GaP d i f f e r e n t  b a n d s . U n l i k e GaP,  number  on ZnTe  t o an i n c r e a s e i n t h e  i m p u r i t i e s as a r e s u l t  resulted  to noise  allow useful results  i n more e f f i c i e n t  ratio  of carrier luminescence.  a t 36.3 GHz w a s t o o l o w t o  t o be o b t a i n e d  103  for this  frequency.  2)  The  electron  seen.  This  acceptor  5.3  Silver  1)  explained  signal  could  resonance  by  luminescence  of a p p r o p r i a t e  resonance exciton  was  pair  to a lack  paramagnetic  the  bands  from  our  be  seen  was  of  not  donor-  samples  due  paramagnetic  i n any  of  the  bound  luminescence.  Bromide.  Paramagnetic  resonances  were o b s e r v e d and  shown  be  to  of  holes  homogeneously  Cyclotron  observed  by  broadened.  ODMR  the  iodine  bound  but  corresponding  hot  from  and  a  bound  the  rate  state  at  did,  the  trapping  the  9.2  in  iodine t o be effect  GHz  104  decrease  increased  with  adequate showed  enhance The  the  the  explained electrons emission  the  heating  of the o r d e r of was  in  resonance  population  centre.  was  exciton  showed  fact,  f o r those with  a  results  holes  electron  the e l e c t r o n  were c a l c u l a t e d energy  on  caused  electrons  to heat  electrons  increased to  tail  Experiments  m i c r o w a v e power  phonon  a  to  luminescence.  effects  resonances  was  literature.  and  luminescence,  luminescence  i n the  electrons  The  hot  Other  of  in  trapped  resonance  resonance  luminescence.  that  and  the e l e c t r o n  agreed w i t h those a l r e a d y r e p o r t e d  drop  absence  i m p u r i t i e s . No  electrons  2)  signal  by able  of  an  LO the to LO  phonon.  The  estimated found  effective  from  with  to noise  a  comparison  r e s o n a n c e was effective  observed  mass  as  resonance  ratio  to permit  was  results  and  predictions  a t 36.3 GHz  with  to  theory.  prevented The  hole  t o b r o a d e n and s h i f t t o h i g h e r  expected  at high  l e v e l s , but the hole resonances properly  electrons  error.  3) The p o o r s i g n a l accurate  hot  the t h e o r e t i c a l  the experimental  more  of  the c y c l o t r o n  to agree  within  mass  microwave  power  c o u l d n o t be r e s o l v e d  a more d e t a i l e d  analysis.  5.4 Curve f i t t i n g .  1) An o r i g i n a l  method o f n o n - l i n e a r c u r v e  developed  allow  to  the  best  fits  of  fitting  was  theoretical  p r o f i l e s t o n o i s y d a t a t o be made. T h i s was c o m p a r e d t o t h e o t h e r a v a i l a b l e m e t h o d s a n d was rapidly  and  to  be  relatively  found  to converge  insensitive  to  false  minima.  2) The use  method  was  statistical  various  a p p l i e d t o our data criteria  i n an a t t e m p t t o  in discriminating  mechanisms.  105  between  5.5  Comments a n d S u g g e s t i o n s f o r F u r t h e r  1)  I t h a s been  present  shown t h a t  Work.  t h e background  i n ODMR e x p e r i m e n t s  a r e caused  signals  often  by m i c r o w a v e  h e a t i n g and, i n p a r t i c u l a r , c y c l o t r o n h e a t i n g o f f r e e c a r r i e r s . I n some c a s e s c y c l o t r o n r e s o n a n c e s may h a v e been o b s e r v e d signals,  a s was m e n t i o n e d  cyclotron field  i n ODMR e x p e r i m e n t s  heating  i s caused  by t h e m i c r o w a v e  a n d EPR b y t h e m a g n e t i c  nodes  o f a microwave  f o r EPR  i n the case o f AgBr. S i n c e  p r o c e s s e s c a n be d i s t i n g u i s h e d the  and m i s t a k e n  component  by p l a c i n g  resonator.  electric the  two  the sample a t  Some  convenient  means o f m o v i n g t h e s a m p l e i n t h e r e s o n a t o r d u r i n g a n experiment,  and w i t h o u t d i s t u r b i n g  resonator, i s desirable  t o t h e same  cyclotron  resonance  scattering broad this  times  in  limitations  measurements  cyclotron  conventional carrier  and c o n s e q u e n t l y  mass v a l u e s r e p o r t e d i n  w i t h t h o s e d e t e r m i n e d by  e x p e r i m e n t s . The m a i n  advantage  u s i n g ODMR i s t h a t i n f o r m a t i o n may b e g a i n e d and l u m i n e s c e n c e  processes which  t h e r e s o n a n c e s . The r e s u l t s  b o t h r e d u c t i o n s and i n c r e a s e s as  as  resonance i s  due t o s h o r t  i n many m a t e r i a l s  t h e s i s were comparable  trapping by  cyclotron  r e s o n a n c e s . The e f f e c t i v e  conventional  i n the  and s h o u l d be d e v e l o p e d .  2) T h e u s e o f ODMR t o o b s e r v e subject  the f i e l d  a result  of cyclotron  106  reported  about  are affected here  showed  i n trapping a t impurities heating,  i n some  cases  allowing  theoretical  3) One o f t h e  p r e d i c t i o n s t o be t e s t e d .  factors  limiting  t h e u s e f u l n e s s o f ODMR  i s t h e q u a l i t y o f t h e s i g n a l s t h a t can be o b t a i n e d . I n cases in  where t h e r e s o n a n c e  produces o n l y  a small  change  l u m i n e s c e n c e , o r where t h e l u m i n e s c e n c e i t s e l f i s  weak, t h e s t a t i s t i c a l obscure  noise  the desired signals.  I n some c a s e s  remedied  by u s i n g  stronger  excitation  sources,  possible  technically.  Signal  this  thesis  high  i n t h e l u m i n e s c e n c e may  c a n be u s e d  microwave  power  butthis  averaging  t o improve  this  may be  levels  or  i s not always as d e s c r i b e d i n  the signal  to noise  r a t i o a s much a s i s d e s i r e d i f s u f f i c i e n t t i m e i s s p e n t in  collecting  time  data.  o f t h e Dewars  could  be s p e n t  In our experiments, imposed  on s u c h  explained  i nChapter  run  generally  posed  efficient even  alternative, running the  time,  factor  being  defects.  collection  2, r e f i l l i n g insurmountable a Dewar  pumped  on t h e t i m e  that  because, as  t h e Dewar d u r i n g a difficulties. with  on seems  liquid  Some helium  d e s i r a b l e . The  o f c o n s t r u c t i n g a Dewar w h i c h h a s a l o n g e r m i g h t p r o d u c e some i m p r o v e m e n t b u t  o f 10 o r so w h i c h  4) T h e r e s u l t s negative  data  means o f f i l l i n g  while  a limit  the running  i s needed.  o f ODMR s t u d i e s o n GaP s h o w e d  results  which  Measurements  were  on h i g h  107  hardly  attributed quality,  several  to crystal strain  free,  crystals the  w o u l d t h e r e f o r e be o f i n t e r e s t .  effects  studied  of strains  on o b s e r v e d  by a r t i f i c i a l l y  absence  stressing  of polarization  Alternatively,  effects  c o u l d be  t h e c r y s t a l s . The  reported  here  was  only  a p p r o x i m a t e , c i r c u l a r p o l a r i z a t i o n o f a few p e r c e n t may have e x i s t e d  a n d b e e n u n d e t e c t a b l e . A more  measurement o f the magnetic c i r c u l a r the  Zeeman c o m p o n e n t s o f e x c i t o n s  more  definite  analysis  sensitive  polarization of  i n GaP c o u l d  of the depolarizing  allow a  effects i n  GaP. The in  predicted  GaP-N  Zeeman these  polarization  luminescence  components  should  be o b s e r v a b l e  i n  lines.  the  To s e p a r a t e  i n a n ODMR e x p e r i m e n t  would  m i c r o w a v e f r e q u e n c i e s o f t h e o r d e r o f 70 G H z .  An  interesting  resonance  results  feature  o f t h e ODMR  was t h e d i f f e r e n c e  o b s e r v e d a t 9.2 a n d 36.3 GHz f r o m broad e l e c t r o n the  signals  of theexciton  spectroscopica1ly  require  d e p e n d e n t ODMR  hole  signal  resonances  cyclotron  i nthe signals  theN luminescence, a  b e i n g d o m i n a n t a t 9.2 GHz w h i l e were  more  apparent  a t 36.3  GHz.  M e a s u r e m e n t s a t an i n t e r m e d i a t e f r e q u e n c y s u c h a s 20GHz should  help  t o show  whether  the electron  d i s a p p e a r s due t o u n r e s o l v e d  splitting  5)  a means  The ZnTe r e s u l t s  ionized centres A test  or neutral  indicate donor  by t h e r e s o n a n c e s  a s we s u g g e s t . .  of distinguishing  or acceptor shown  o f our i n t e r p r e t a t i o n  108  i n their  of these  signal  recombination luminescence.  resonances  could  be  made b y u s i n g a ZnTe s a m p l e d o p e d w i t h  or  donor  with  resolution  luminescence  o f the observed  be o b t a i n e d at higher  known  i fsufficient  cyclotron  resonances  could  f r e q u e n c i e s s u c h a s 36.3 GHz.  interesting  enhancement  microwave  asymmetry o f t h i s effects  properties. Better  m i c r o w a v e power was a v a i l a b l e  6) T h e e l e c t r o n c y c l o t r o n  A higher  an a c c e p t o r  r e s o n a n c e i n AgBr showed an  of trapping  power  level  f o r hot electrons.  a t 36.3 GHz w h e r e t h e  resonance i s v i s i b l e  of s i g n i f i c a n t  would  microwave h e a t i n g  allow the  t o be s t u d i e d  as was d o n e a t 9.2 G H z . B e t t e r s i g n a l  to noise  ratios  could  allow  better  also  be  obtained  which  c o m p a r i s o n t o be made w i t h splitting similar other  of the hole effects  would  a  the theory. Resolving the  resonances  should  m i g h t be v i s i b l e  be p o s s i b l e and  i n them. A study o f  p o l a r m a t e r i a l s s u c h as A g C l and KBr u s i n g  technique  i s also  indicated.  109  this  APPENDIX 1 .  To signal  Curve  determine  fits  of  was  the  two  hypotheses  c o r r e c t , i t was  f o r the  y(i)  = A + E.i +  i s t o be the base  fitting  author the  2  o u t p u t i s known t o have a  positive  methods.  to other  described  curve  to  (73), i n somewhat m o d i f i e d  Meade p r o c e d u r e had  thought worth  and  Unfortunately was and  written was  written  t o be  testing  the  method  for  experimental  been t e s t e d  i n c o n n e c t i o n w i t h o u r p r e v i o u s work and had slowly  a  data  (72) i t h a s b e e n s u g g e s t e d t h a t  is  converge  A.2  included  Lorentzian  and  2  A + E.i are  Meade  Nelder  2  the terms  method o f N e l d e r and  The  2  previously  However, r e c e n t l y  superior  2  frequency.  has  plain  2  that  signal  find  A.l  ( l + C D z + i D z) ( 1+C D )(B -ZA ) z Z (l-C D*+i D ) + 4C D  noted  for increasing  The  necessary to  2  = A + E . i + B.exp(-D(C-i) )  because  regarding  expressions:  y(i)  It  (71).  which  output p r o f i l e  least-squares  slope  Fitting.  difficult  the proposed  printed  incomprehensible.  A  new  already  been found  o f t h e new  thus  procedure  o f t h e PASCAL version  was  language  therefore  i n B A S I C t o r u n on t h e c o m p u t e r a v a i l a b l e w i t h  110  to  variant.  version  i n a non-standard v e r s i o n  form,  t o s t a r t . I t was  new  the  the  experimental FORTRAN77  equipment  described  above,  and  t o r u n o n a much f a s t e r c o m p u t e r w h i c h  later was  in then  available.  In  essence,  follows: Figure trial  suppose  f o r two that  variables,  the contours  A.l represent equal function  three non-col1inear  (xl,x2)  space,  al,a2  are t o the a c t u a l converge.  values  and t h e e x p e r i m e n t a l  R = (y(xl,x2). - y  Any  the procedure  o b s  of the  data f o r :  2  points  are taken  and a3 s a y . N a t u r a l l y ,  minimum  e t c .i n  of the r e s i d u a l s  (xl,x2) )  initial  R = . l , R=.05  i s as  t h e more r a p i d l y  The a l g o r i t h m t h e n p r o c e e d s  i n the  the c l o s e r will  these  the process  as f o l l o w s :  1) E v a l u a t e t h e r e s i d u a l s a t a l , a 2 , a 3 . 2) S u p p o s e t h a t a i h a s t h e l a r g e s t r e s i d u a l . T h e n (in N dimensions) evaluate the r e s i d u a l at a new point a i ' which i s anti-symmetric with respect to a i through the point which i s the c e n t r o i d of the remaining p o i n t s . 3) I f t h e r e s i d u a l a t a i ' i s s m a l l e r t h a n t h a t a t ai but not smaller than the l e a s t r e s i d u a l a t one o f t h e r e m a i n i n g points, a i ' replaces a i and t h e p r o c e s s i s r e p e a t e1 d . 4) I f t h e r e s i d u a l a t a i i s l e s s t h a n any o t h e r r e s i d u a l , a n o t h e r new p o i n t a i " i s defined at double the d i s t a n c e from the c e n t r o i d . I f the r e s i d u a l a t t h i s p o i n t i s a g 1a i n t h e l o w e s t , a i " i s accepted, otherwise a i i s accepted. The process then r e p e a t s . 5) I f a i ' has a h i g h e r r e s i d u a l t h a n a i , a new point a i ' "1 i s t e s t e d , w h e r e a i ' " i s h a l f way between ai and t h e c e n t r o i d . I f t h e r e s i d u a l i s now less t h a n t h a t a t a i t h e new p o i n t i s accepted. I f not, a l l points except t h e one w i t h the lowest residual a r e moved h a l f way t o w a r d s the c e n t r o i d and t h e p r o c e s s r e p e a t s . Fig. A . l .  Ill  FIG. A l THE  NELDER - MEAD  MINIMIZATION 4  PROCEDURE  i  ».  The three starting points a^ to a^ are shown on a conour map with the three new t r i a l points marked by *  112  A graphic  illustration  works i s g i v e n i n F i g u r e  The from  new  method  input  ( derived  evaluating width)  by  base-slope  was  fairly  FORTRAN v e r s i o n was data  i n which  a p p l i e d to the data  Convergence  from  inspecting  the  and h e i g h t ,  slow  ( c a . 20  faster  peak  data  minutes)  IA6  Table  Data Set # A B C D E Residual  IA6  251.655 3012.29 18.7898 .159806 -56.8243 5.294E7 Table  4638.04 11991.2 20.2828 .239845 -.099438 1.279E7  A . l . Gaussian  IA5  derived  254.673 3031.71 20.1411 .148402 -6.47207 7.583E7  trial  roughly  height  and  i n B A S I C . The  ( c a .2 minutes). Results  Data Set #  5489.36 11082.3 19.0711 .222158 .348464 6.039E6  and  position  A . l and A.2.  IA5  sets  a plausible  s e t s a r e shown i n T a b l e s  A B C D E Residual  the process  A.2  was  our experiments.  o f t h e way  for 5  IA7  IA8  1015.07 5075.37 18.2713 .249994 .195465 5.339E6  1874.05 10622.52 23.8522 .108304 .021466 3.893E7  L.S. c o e f f i c i e n t s .  IA7 -319.256 179.129 18.0421 .264559 -27.6918 8.980E6  IA8 223.065 2175.93 23.6111 .143477 -21.7666 4.511E7  A.2. L o r e n t z i a n L . S . c o e f f i c i e n t s .  In t e r m s o f t h e r e s i d u a l s i t i s c l e a r t h a t t h e G a u s s i a n curve  provides the best  f i t t o the d a t a . However, i n s p e c t i o n  113  FIG, A2  THE  MINIMIZATION PROCESS  114  of  the  plots  suggested false  of  calculated  t h a t the  minimum. A  where  i t is  estimate  A  the  seen  number  starting  that  example  the  a t the  of  further  conditions  essence seeks  was  next  this  takes  the  residual. repeated  value  in  determination quadratic  the of  the  of  that  were  hope  the  the  made  t r y our trial  to  with  a  A.3.  under-  different  alternative,  until  no  own  method  parameters  parameter  best  Figure  appears  that  which  through the  start  to  and  o b t a i n e d . T h e s e were u n a v a i l i n g .  to  each of  interpolation  x(i+l)  points  end.  runs  decided  After cycling from  data  i s shown i n  Lorentzian  lower  b e t t e r , p a r a m e t e r s would be  It  observed  L o r e n t z i a n f i t m i g h t have c o n v e r g e d typical  data  and  in turn  gives  the  v a r i a b l e s the  f u r t h e r change  variable  values  (71).  In and  least  process occurs.  i s achieved  is The by  using:  = x(i) +  dx(fm -  fp)/(2fm-4f0+2fp)  where:  Since  fm  = f(xi-dx)  fO  = f(xi)  fp  =  f(xi+dx)  i t i s assumed  functions  o f position  condition  i s not  t h a t the  this  satisfied.  r e s i d u a l s are  may  lead  To  avoid  115  to  divergence  this,  the  quadratic when  the  programme i s  FIG. A3  GAUSSIAN  AND  LORENTZIAN  FITS  TO  DATA  A comparison of two p o s s i b l e l i n e shapes f i t t e d to the ODMR s i g n a l from GaP-N showing the d e v i a t i o n of the L o r e n t z i a n f i t at low f i e l d  so a r r a n g e d as t o t a k e as t h e n e x t v a l u e of  x(i+l),  xi-dx,  x i , or  xi+dx  of x i that  which  gives  member  the  least  residual.  The  results  programme, u s i n g  derived  as s t a r t i n g  A . l . and A . 2 . a r e shown  Table  i s  coefficients  those given  of  i n t a b l e s A . 3 . and A.4.  IA8  5630.67 11229.3 19.0879 .222943 -8.78307 5.668E6  4379.02 11960.6 20.2518 .241384 15.0888 1.195E7  1019.99 5054.56 18.2207 .258487 4.72859 5.123E6  1662.56 10428.8 23.6277 .111812 19.1017 3.558E7  A.3. R e v i s e d  IA5  4295.33 5181.97 19.0892 .279086 3.40091 4.856E6  have  that  changed  Gaussian L.S. c o e f f i c i e n t s  IA6  IA7  3015.40 3796.65 20.0000 .290409 -19.4032 1.650E7  A.4. R e v i s e d  evident  this  i n Tables  IA7  A B C D E Residual  It  values  operation  IA6  Data S e t #  Table  the  IA5  Data Set #  A B C D E Residual  from  IA8  497.539 804.270 18.1076 .334848 -8.86383 5.801E7  -1562.43 1298.64 23.4852 .121133 -44.7434 3.826E7  L o r e n t z i a n L.S. c o e f f i c i e n t s .  the  IA5  completely.  and Also  IA6  Lorentzian  i t will  t h a t w h i l s t t h e r e s i d u a l s f o r IA6 t o IA8 s t i l l  be  seen  show t h a t t h e  G a u s s i a n v e r s i o n i s s u p e r i o r , t h e d i f f e r e n c e i s n o t so g r e a t a s b e f o r e . I n a d d i t i o n , f o r I A 5 , t h e L o r e n t z i a n now  117  gives a  l o w e r r e s i d u a l a l t h o u g h by a v e r y  STATISTICAL  The  standard  they  decided  Chi-square  where  do  of the type i n v o l v e d i n our experiments  not involve  =  frequencies.  y)//z(xi  - x)(yi -  the  x^  are c a l c u l a t e d  observed v a l u e s . r  t e s t f o r goodness o f f i t i s n o t  t o compute t h e c o r r e l a t i o n  r  amount.  ANALYSIS  a p p l i c a b l e t o data since  small  =  I t was  c o e f f i c i e n t s , defined  - x)2.l(yi  values  and  In p r a c t i c e A.3. i s used  - y)2  t h e y^  .3.  are the  i n the form:  x  -  (^YO  z  (£xi) /N) ( % i -  reduction  that  cancellation results  i n loss of accuracy.  The  limits  95%  i s satisfactory  confidence  transformation  z =  results  so l o n g  Z/N)  This  The  A  by:  £ i . y i - (Exj.) ( l y i ) /N  J(txi  Fisher's  therefore  as r i s n o t so  on r c a n be d e r i v e d  (74) t o n o r m a l  form:  log((1+r)/(1-r))/2  a r e shown  i n Table  118  A.5.  small  using  Gaussian  IA5  IA6  IA7  IA8  .996 .994 .992  .992 .989 .984  .981 .973 .961  .980 .973 .964  Lorentzian  IA5  IA6  IA7  IA8  Upper 95% r Lower 9 5%  .997 .995 .993  .992 .985 .970  .984 .969 .939  .984 .971 .949  and c o n f .  limits.  Upper 9 5% r Lower 95%  T a b l e A.5. C o r r e l a t i o n  It curve  i s clear  The  however  of  t h a t i n the cases  gives superior c o r r e l a t i o n ,  small.  not  difference  the  This  the hypothesis  H  a  z  t  0  _ o  =  i s i n the  i s so  small  statistic  ( z i - zn)/ a  Gaussian  the d i f f e r e n c e i s reverse  that  c o n c l u s i o n i s supported  test  =  o f IA5-IA8 the  although  f o r IA5  difference  significant.  where  coefficients  the  sense,  result  is  by t h e v a l u e s  (74):  z  i  _  z  •  / l / (N-3) +1/ (N-3) "*  The v a l u e s a r e : IA5 IA6 IA7 IA8  which  supports  Lorentzian  fits  the  -.316 .661 .256 .175  assertion  that  are not s i g n i f i c a n t l y  119  the  Gaussian  different  and  a t t h e 95%  level  which  statistic  is  is  accepted  This  IA5  i s negative  i s contrary  least  which  this  to  remarked t h a t the b a s e - l i n e  slope  established  wide d i s p e r s i o n of  end  of  c u r v e as  be  f o r b o t h G a u s s i a n and  the  the  the  for  valid.  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T a m u r a , H., S o l i d  S t a t e Comm. 1 0 , 297 (1972)  124  6 5 . Hodby, J.W., C r o w d e r , J . G . , and B r a d l e y , C C , L a b o r a t o r y R e p o r t r e f . 4 7 (1973)  Clarendon  66. L e e , T.D., Low, F . E . , and P i n e s , D., P h y s . R e v . (1953)  9 0 , 297  67. L a r s e n , D., P h y s . R e v . 1 4 4 , 697 (1966) 68. Malm, H.L., and H a e r i n g , R.R., C a n a d i a n J . o f P h y s i c s 4 9 , 2970 (1971) 69. C z a j a , W.,  and B a l d e r e s c h i , A., J . P h y s . C . 1 2 , 406 (1979)  70.Stratton,J.A.,Electromagnetic Theory ( M c G r a w - H i l l , N.Y.) (1941) 207 7 1 . B o o t h , I . J . , a n d Booth,A.D.,  J . Comp. P h y s . 5 3 , 72  72. C a c e c i , M.S., and C a c h e r i s , W.P.,  BYTE 9 , 340 (1984)  7 3 . N e l d e r , J . A . , and Mead, R.,The Computer 74. F i s h e r , R.A., METRON 1, 3  J . 7 , 308 (1965)  (1921)  75. B o o t h , I . J . , a n d S c h w e r d t f e g e r , (Submitted)  C.F., P h y s . S t a t . S o l .  7 6 . S l i w c z u k , U., N a k a m u r a , K., a n d v o n d e r O s t e n , W., S t a t e Comm. 4 5 , 1013 (1983) 77. L a x ,  (1984)  Solid  M., P h y s . R e v . 1 1 9 , 1502 (1960)  78. C z a j a , W.,  P h y s . d . k o n d e n s i e r t e n M a t e r i e . 1 2 , 226 (1971)  79. F a u l k e n e r , R.A., P h y s . R e v . 1 7 5 , 991 (1968)  125  PUBLICATIONS I. Booth, M. Hawton, and W. Keeler "Pressure Dependent Compensation in InSb" Phys. Rev. B. Vol. 25, pg. 7713 (1982) I. Booth, C. Schwerdtfeger, and W. Czaja "ODMR of Bismuth and Sulpher in GaP" Solid State Comm. Vol. 45, pg. 677 (1983) I. Booth and A.D. Booth "Fisher's Exact Probability Test: the calculation", Math. Gaz. Vol. 67, pg. 131 (1983) I. Booth and A.D. Booth "On a class of least squares curve fitting problems." J . Comp. Phys. Vol. 53, pg. 72 (1984) I. Booth and A.D. Booth "PET revisited, a 40 year overview" Physics in Canada Vol. 40, pg. 57 (1984) I. Booth and A.D. Booth "An interesting determinant", Math. Gaz. Vol. 68, pg. 281 (1984) I. Booth "Non-equilibrium effect in electromagnetic resonators". Speculation in Science and Technology (Accepted 1984) I. Booth and A.D. Booth "A practical problem in Coordinate Transformation" J . Oceanography (accepted 1984) I. Booth and C. Schwerdtfeger "Optically detected cyclotron resonance in AgBr", Phys. Stat. Sol. (submitted) S. Fong, W. Keeler, and I. Booth "Pressure dependent Hall effect measurements in InSb", 1979 Congress of the CAP I. Booth and A.D. Booth "Computer development and construction in Saskatoon" CIPS Conference proceedings (1984)  

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