Open Collections

UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

Devices employing conductivity modulation in semiconductor films by ferroelectric polarization charging Teather, George Griffiths 1967

Your browser doesn't seem to have a PDF viewer, please download the PDF to view this item.

Item Metadata

Download

Media
831-UBC_1967_A7 T43.pdf [ 7.03MB ]
Metadata
JSON: 831-1.0104442.json
JSON-LD: 831-1.0104442-ld.json
RDF/XML (Pretty): 831-1.0104442-rdf.xml
RDF/JSON: 831-1.0104442-rdf.json
Turtle: 831-1.0104442-turtle.txt
N-Triples: 831-1.0104442-rdf-ntriples.txt
Original Record: 831-1.0104442-source.json
Full Text
831-1.0104442-fulltext.txt
Citation
831-1.0104442.ris

Full Text

DEVICES EMPLOYING CONDUCTIVITY MODULATION IN SEMICONDUCTOR FILMS BY FERROELECTRIC POLARIZATION CHARGING by GEORGE GRIFFITHS TEATHER B. Eng., McMaster U n i v e r s i t y , 1964 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n the Department of E l e c t r i c a l Engineering We accept t h i s t h e s i s as conforming to the required standard Research Supervisor Members of the Committee Head of the Department Members of the Department of E l e c t r i c a l Engineering THE UNIVERSITY OF BRITISH COLUMBIA June, 1967 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an a d v a n c e d d e g r e e a t the U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e Head o f my D e p a r t m e n t o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . D e p a r t m e n t o f The U n i v e r s i t y o f B r i t i s h C o l u m b i a V a n c o u v e r 8_ Canada ABSTRACT Two types of d e v i c e s employing f e r r o e l e c t r i c m o d u l a t i o n of a semiconductor t h i n - f i l m have been r e a l i z e d and s t u d i e d . The f i r s t c o n s i s t s of a cadmium s e l e n i d e f i l m w i t h e l e c t r o d e s d e p o s i t e d on a barium t i t a n a t e s u b s t r a t e t o g e t h e r w i t h a s w i t c h i n g e l e c t r o d e on the oth e r s i d e of the s u b s t r a t e . T h i s g i v e s a two-valued r e s i s t o r ; i n e f f e c t , a n o n d e s t r u c t i v e r e a d -out of the s t a t e of the f e r r o e l e c t r i c c r y s t a l w hich i s re g a r d e d as a s t o r a g e element. The second d e v i c e i s a t h i n - f i l m t r a n s -i s t o r (TFT) d e p o s i t e d on a barium t i t a n a t e c r y s t a l . A f o u r t h c o u n t e r e l e c t r o d e - on the o t h e r s i d e of the c r y s t a l a l l o w s changing between two o p p o s i t e p o l a r i z a t i o n d i r e c t i o n s i n the c r y s t a l , thus g i v i n g a TFT w i t h two s e t s of c h a r a c t e r i s t i c s , r o u g h l y e q u i v a l e n t t o a two-valued b u i l t - i n gate b i a s . The r e a d - i n , o r s w i t c h i n g t i m e , o f the d e v i c e i s s u b s t a n t i a l l y d etermined by the barium t i t a n a t e c r y s t a l and can be i n the m i c r o -second range for h i g h s w i t c h i n g f i e l d s . Readout of the d e v i c e s can be continuous or not, as desired. Characteristics of the TFT, which i s considered equi-valent to a two-gate device, are analyzed i n terms of the gradual channel approximation,- Experimental results of the two devices are presented and discussed i n r e l a t i o n to the predicted behaviour. i l TABLE OP CONTENTS Page TABLE OP CONTENTS ............................. i i i LIST OP ILLUSTRATIONS v i LIST OP TABLES ................................... v i i l LIST OP SYMBOLS ....................... ....... i x ACKNOWLEDGEMENT x i i i 1 . BASIC PRINCIPLE OP DEVICES ................... 1 2• THEORY" OF DEVICES a « o * « u o » * * « « « o o s v s o « « 3 2«1 FlSld. Eff©C"b o a o i f f a f t o o q o , a ? « o o o o « Q a 9 f f • 3 2.2 Ewo~Stat§ TPS - Structure and Operation . 6 2 . 3 Modified Il§m©ntary Theory of the T I T ... 10 2 . A theory of Resistance Modulation Due to Ferroelectric Substrate , 16 ; 2 , 5 Ifftot of £raps on Proposed Devices ..... 18 3 . IIRR01U0TRI0 AND RB1ATBD PR0P1R2I1S 01 BARIUM IITA1TATH . . . . . . . . . . . 2 2 3 . 1 Terroeleetrieity , ....... 2 2 3 . 2 Physical Properties of Barium Titanate .. 24 3 . 3 Polarization Reversal 2 5 3 . 3 . 1 Introduction 2 5 3 . 3 . 2 Mechanism of Polarization Reversal in Barium Titanate 2 6 3.4 Experimental Procedure and Results 30 3.4.1 Crystal Preparation 30 3.4.2 Conductivity of Crystals 31 3.4.3 Measurement of Remanent Polar-i i i Page 3 . 4 . 4 Slow Speed Switching .............. 35 3 . 4 . 5 High F i e l d P o l a r i z a t i o n Reversal .. 38 4. THE SEMICONDUCTOR FILM .................... 4 1 4 o 1 In*t X*OCLU.C "fc 1 O n 8 o o o o o o o e o o o o o o o o o o o o o e © o « o o Q 4 1 4 . 2 Properties of Cadmium Selenide and Cadmium Sulphide and ..Effects of Deposition Con-Cll"tlOIl .S e o o o o o a o o o o o o o o o o o o o o o o o o o o o o o o o o o 4"2 4 . 3 Thin Film Evaporation and Measurements ... 4 4 4 . 3 . 1 Evaporation Technique ............. 4 4 4 . 3 . 2 H a l l Effect Measurements .......... 46 4 . 3 . 3 Specimen and Specimen Holder . . . . . i , 4 8 4 . 3 . 4 Experimental Results .............. 4 9 5 . CADMIUM SULPHIDE AND CADMIUM SELENIDE FILMS ON BARIUM TITANATE 52 5 91 In"bx*ocLuc"fcion »<* o <> o o o o <* <> <> o o o <> o o o <> <> <> o o <> o <> <> <> 52 5 . 2 Specimen Preparation ..................... 52 5 . 3 E l e c t r i c a l Measurements .................. 53 5 . 3 . 1 H a l l Measurements 53 5 . 3 . 2 Two-State Resistor ................ 54 5 . 3 « 3 Temperature Dependence of Con-CL\lC"fcl"Vi"fcy o o e o o o o o o o o » o o 9 e * « » © « » » o « 57 5 . 3 « 4 S t a b i l i t y of Device ............... 62 5 . 3 . 5 Photoconductivity Measurements .... 6 3 5 . 4 Cadmium' Sulphide Device .................. 64 6. THE TWO-STATE THIN-FILM TRANSISTOR ............ 65 6 . 1 Fabrication Techniques ................... 65 6 . 2 E l e c t r i c a l Measurements .................. 65 6 . 3 Effect of S i l i c o n Oxide Layer ............ 70 iv rage 6.4 Discussion of Results 70 6.5 Comparison of Two Devices 74 7 . CONCLUSIONS . 75 APPENDIX I DEVELOPMENT OF MILLER-WE.INRE.ICH MODEL OP 180° DOMAIN WALL MOTION 77 APPENDIX I I MEASUREMENT OF FILM THICKNESS 81 REFERENCES 82 LIST OF ILLUSTRATIONS Figure Page 2-1 Modulation of Conductance by F i e l d E f f e c t 3 2-2 Cadmium Sulphide TFT on T r i g l y c i n e S u l -phate 5 2-3 Cadmium Selenide Two-State Thin-Film F i e l d E f f e c t T r a n s i s t o r 2-4 Metal-Insulator-Semiconductor Energy Level Diagram 7 2-5 I d e a l i z e d Geometry of Cadmium Selenide Two-State TFT . • 11 2-6 Representative Drain C h a r a c t e r i s t i c s f o r Two-State TFT 15 2-7 T y p i c a l C h a r a c t e r i s t i c s of Two-State R e s i s t o r .................. 17 2- 8 Demarcation Levels i n Semiconductor 19 3- 1 F e r r o e l e c t r i c H y s t e r e s i s Loop 23 3-2 . Cubic Perovskite Structure of Barium Titanate 23 3-3 P o l a r i z a t i o n Reversal of Barium Titanate . 27 3-4 Voltage Pulse Switching 28 3-5 Co n d u c t i v i t y Measurement of Barium Titanate C r y s t a l s . 31 3-6 Sawyer-Tower C i r c u i t to Determine Remanent P o l a r i z a t i o n 33 3-7 Experimental P-E Hy s t e r e s i s Curve of C r y s t a l #1 at Various Frequencies ... 34 3-8 Slow-Speed Switching C h a r a c t e r i s t i c Measurement C i r c u i t , 36 3-9 Slow. Voltage Ramp I-V Switching C h a r a c t e r i s t i c s 37 3-10 High F i e l d Response Measurement C i r c u i t 38 v i . F i g u r e Page 3- 11 H i g h F i e l d P u l s e Measurements 40 4- 1 I m p e r f e c t i o n Energy L e v e l s i n Cadmium Su l p h i d e and Cadmium S e l e n i d e 43 4-2 Veeco Vacuum D e p o s i t i o n Apparatus 45 4-3 H a l l Sample w i t h A r b i t r a r y P e r i p h e r y .... 47 .4-4 H a l l Sample o f Cadmium S e l e n i d e w i t h Aluminum C o n t a c t s 48 4- 5 H a l l Specimen H o l d e r ............... 50 5- 1 Semiconductor F i l m C o n d u c t i v i t y Measurement 54 5-2 O p e r a t i n g C h a r a c t e r i s t i c s of Two-State R e s i s t o r Sample #5 56 5-3 N-Type Semiconductor w i t h T r a p p i n g L e v e l s 58 5-4 Temperature Dependence of C o n d u c t i v i t y .. 59 5-5 L o S i o !j)»vs- I P l o t of Sample #8 ....... . 61 5-6 B a c k - S w i t c h i n g of R e s i s t a n c e Sample #10 62 5- 7 P h o t o c o n d u c t i v i t y Measurement 65 6- 1 Cadmium S e l e n i d e Two-State TFT 66 6-2 -'"D-^ C h a r a c t e r i s t i c °f Sample #14 i n Two S t a t e s w i t h Negative Gate B i a s 67 6-3 ^D~^D C h a r a c t e r i s t i c s of Sample $15 i n Two S t a t e s w i t h P o s i t i v e Gate B i a s 68 6-4 TFT I-Q-V-p C h a r a c t e r i s t i c M e a suring C i r c u i t . . 69 6-5 Output C h a r a c t e r i s t i c s of Sample #15 .... 71 6-6 Response of Sample #14 t o Square Wave Input on Gate 73 AI-1 N u c l e a t i o n of A n t i - p a r a l l e l Domain 77 v i i LIST OF TABLES TABLE Page I DEBYE LENGTH FOR CADMIUM SELENIDE AT VARIOUS ELECTRON CONCENTRATIONS ........ 9 I I "SUMMARY OF P-E HYSTERESIS LOOP MEASURE-MENTS AT VARIOUS FREQUENCIES ........... 32 I I I HALL MEASUREMENT RESULTS OF CADMIUM SULPHIDE AND CADMIUM SELENIDE ON GLASS „ 49 IV HALL MEASUREMENT RESULTS OF CADMIUM SELENIDE ON BARIUM TITANATE ...... . . 53 V SUMMARY OF DATA ON TWO-STATE RESISTOR .. 56 v i i i LIST OF SYMBOLS domain w a l l a r e a width, of a n t i p a r a l l e l domain s t e p magnetic f i e l d s t r e n g t h ( K i l o g a u s ) l a t t i c e c o n s t a n t p a r a l l e l t o the plane o f the c r y s t a l gate c a p a c i t a n c e per u n i t a r e a a n t i p a r a l l e l domain s t e p t h i c k n e s s normal component of e l e c t r i c d i s p l a c e m e n t (coulombs/cm.^) t r a p p i n g energy l e v e l (eV) t h i c k n e s s o f c r y s t a l energy l e v e l of c o n d u c t i o n band (eV) c o e r c i v e f i e l d s t r e n g t h E l e c t r i c f i e l d s t r e n g t h between source and d r a i n (assume c o n s t a n t ) F e r m i l e v e l energy (eV) normal component of e l e c t r i c f i e l d energy d i f f e r e n c e between t r a p p i n g l e v e l and Fermi l e v e l (eV) energy l e v e l of the v a l e n c e band (eV) f i e l d s t r e n g t h a t any p o i n t x between source and d r a i n e l e c t r o n i c charge t r a n s c o n d u c t a n c e t h i c k n e s s of the semiconductor l a y e r s o u r c e - d r a i n c u r r e n t maximum c u r r e n t l e v e l of s w i t c h i n g p u l s e s w i t c h i n g c u r r e n t a t time t e x p e r i m e n t a l l y determined c o n s t a n t o f the e q u a t i o n i x constant of the domain v e l o c i t y equation Boltzman constant length of source-drain channel reduced channel length due to "pinch-off" the Debye length height rpf a n t i p a r a l l e l domain step electron demarcation l e v e l the i n i t i a l charge concentration i n the semiconductor (el./cm. 2) concentration of traps electronic charge induced into the S-D channel at any point x~ (el./cm.2) unit vector normal to the surface of the capacitor electron concentration per unit area of the semi-conductor f i l m (assumed constant) bulk electron concentration i n the semiconductor (el./cm.5) concentration of conduction band electrons (el./cm. ) concentration of f i l l e d traps electron concentration in.Jhe S-D channel at a point x from the source (el./cm.2-f' nucleation rate of new a n t i p a r a l l e l domains hole demarcation l e v e l remanent polarization (coul./cm. ) normal component of remanent polarization (coul./cm. ) amount of P r effective i n .causing conductivity modul-ation H a l l constant capture crosssection of a trapping centre for electrons t t h i c k n e s s of i n s u l a t o r l a y e r ox J t l e n g t h of current pulse caused Toy p o l a r i z a t i o n . r e v e r s a l t ' experimentally determined constant AU energy change due to n u c l e a t i o n U* c r i t i c a l a c t i v a t i o n energy f o r n u c l e a t i o n U-^ * c r i t i c a l a c t i v a t i o n energy f o r n u c l e a t i o n of a step 1 unit., c e l l wide d e p o l a r i z i n g energy u thermal v e l o c i t y of c a r r i e r s V-p s o u r c e - d r a i n v o l t a g e Vo a p p l i e d gate voltage, V volume of nucleus n V " p i n c h - o f f " v o l t a g e necessary to s a t u r a t e the IJN-V-Q ^ c h a r a c t e r i s t i c s . V p o t e n t i a l d i f f e r e n c e between the surface and bulk of the semiconductor Vrp t h r e s h o l d gate v o l t a g e needed f o r the onset of S-D current V(x) p o t e n t i a l at any point x i n the S-D channel v reduced p o t e n t i a l , i n the semiconductor (eV/kT) v^ domain w a l l v e l o c i t y W width of S-D channel W Q a c t i v a t i o n energy of c o n d u c t i v i t y cr a c t i v a t i o n ' e n e r g y f o r p o l a r i z a t i o n r e v e r s a l y f r a c t i o n pf P e f f e c t i v e i n causing c o n d u c t i v i t y • modulation • e d i e l e c t r i c constant p a r a l l e l t o the plane of the c r y s t a l x i £ Q p e r m i t t i v i t y of f r e e space e ^ v p e r m i t t i v i t y of the SiO l a y e r OX X e r d i e l e c t r i c c o n s t a n t of m a t e r i a l b e i n g c o n s i d e r e d H n m o b i l i t y of e l e c t r o n s (cm, / v o l t - s e c ) p , charge d e n s i t y i n the semiconductor (el./cm.^) CT c o n d u c t i v i t y S-D c h a n n e l c o n d u c t i v i t y Q-Q e x p e r i m e n t a l l y determined c o n s t a n t of the e q u a t i o n (Xp c o n s t a n t of the e q u a t i o n CT w a l l energy per u n i t a r e a time c o n s t a n t of t r a p p i n g l e v e l x i i ACKNOWLEDGEMENT The a u t h o r i s i n d e b t e d t o Dr. L. Young f o r h i s h e l p and guidance throughout the course of t h i s i n v e s t i g a t i o n . G r a t e f u l acknowledgement i s a l s o g i v e n t o Dr. D. A k i t t f o r r e a d i n g the ma n u s c r i p t and f o r h i s h e l p f u l s u g g e s t i o n s . The a u t h o r i s g r a t e f u l t o Dr. C.A.T. Salama f o r s e v e r a l h e l p f u l d i s c u s s i o n s ; t o Messrs. G. Anderson, R. Proudlove. I . H u f f , N. Owen, and H. S t u b e r f o r t h e i r t e c h n i c a l h e l p ; t o Mrs. M„ Wein f o r t y p i n g the t h e s i s ; and t o Messrs. G„ Yan, C. D e l l ' O c a and P. W i l c o x f o r p r o o f r e a d i n g the m a n u s c r i p t . . G r a t e f u l acknowledgement f o r f i n a n c i a l s u p p o r t i s g i v e n t o the N a t i o n a l Research C o u n c i l ( B u r s a r y awarded 1964s. B l o c k Term Grant A-68) and t o the Defence Research Board ( c o n t r a c t s ECRDC T65A and T79). The work d e s c r i b e d i n t h i s t h e s i s was su p p o r t e d by the Defence Research Board under c o n t r a c t s ECRDC T65A and T79 and by the N a t i o n a l Research C o u n c i l . 1. BASIC PRINCIPLE OF DEVICES The purpose of the work d e s c r i b e d i n t h i s t h e s i s was the s t u d y of n o n d e s t r u c t i v e readout of the p o l a r i z a t i o n s t a t e : of f e r r o e l e c t r i c c r y s t a l s . P o s s i b l e methods i n c l u d e o p t i c a l t e c h n i q u e s ( A y e r s , 1967), (Yan, 1964), p y r o e l e c t r i c e f f e c t , (Chynoweth, I 9 6 0 ) , and c o n d u c t i v i t y m o d u l a t i o n of - a semi-c o n d u c t o r t h i n - f i l m d e p o s i t e d on the f e r r o e l e c t r i c c r y s t a l . The l a t t e r i s the method t o be i n v e s t i g a t e d h e r e . A f e r r o e l e c t r i c c r y s t a l has a remanent p o l a r i z a t i o n P , the d i r e c t i o n of which can be a l t e r e d by the a p p l i c a t i o n of an e l e c t r i c f i e l d . T h i s i m p l i e s t h a t , f o r a p a r a l l e l p l a t e c a p a c i t o r w i t h a f e r r o e l e c t r i c d i e l e c t r i c , the charge per u n i t a r e a on the e l e c t r o d e s i s non-zero f o r z e r o a p p l i e d v o l t a g e , and i s i n s t e a d e q u a l t o ^ » n > where n i s a u n i t v e c t o r normal t o the c a p a c i t o r s u r f a c e . The d i r e c t i o n of P , and thus the amount of s u r f a c e charge,can be r e v e r s e d by a p p l i c a t i o n of a s w i t c h i n g v o l t a g e p u l s e . I f a s e m i c o n d u c t i n g l a y e r w i t h ohmic c o n t a c t s i s used as one of the e l e c t r o d e s of the c a p a c i t o r , then the amount of charge i n the semiconductor w i l l v a r y as the f e r r o e l e c t r i c i s s w i t c h e d by an amount e q u a l t o 2 P r per u n i t area,, i f 7^ i s p e r p e n d i c u l a r t o the c a p a c i t o r s u r f a c e . Thus the d i r e c t i o n of p o l a r i z a t i o n ( i . e . the s t a t e of the f e r r o e l e c t r i c ) can be determined by measuring the c o n d u c t i v i t y of the semiconduct f i l m . I t i s p o s s i b l e t o f a b r i c a t e semiconductor l a y e r s w i t h a c o n c e n t r a t i o n of c a r r i e r s per u n i t a r e a small- w i t h r e s p e c t t o the f e r r o e l e c t r i c remanent p o l a r i z a t i o n of some c r y s t a l s . T h i s would seem t o ensure a l a r g e c o n d u c t i v i t y change. I n p r a c t i c e , t r a p p i n g of charge by s u r f a c e s t a t e s and i m p u r i t i e s i n the s e m i c o n d u c t o r , m o b i l i t y changes due t o changes i n charge c o n c e n t r a t i o n , and o t h e r e f f e c t s c o m p l i c a t e the b e h a v i o u r . I f an i n s u l a t i n g l a y e r and gate are added t o the semi-c o n d u c t i n g f i l m w i t h e l e c t r o d e s , a t h i n - f i l m t r a n s i s t o r i s formed. T h i s d e v i c e i s expected t o have two s e t s of c h a r a c t e r i s t i c s w hich d i f f e r i n z e r o gate v o l t a g e cur-rent l e v e l , depending on the p o l a r i z a t i o n d i r e c t i o n of the f e r r o -e l e c t r i c c r y s t a l . The d e v i c e s d i s c u s s e d p r o v i d e continuous." • non-destructiv-e readout of'-the s t a t e of the f e r r o e l e c t r i c s u b s t r a t e . The r e a d -i n or s t o r a g e time i s a f u n c t i o n of the s w i t c h i n g time of the • f e r r o e l e c t r i c c r y s t a l i t s e l f , and f o r h i g h s w i t c h i n g f i e l d s t r e n g t h s can be i n the microsecond range. 2. THEORY OF DEVICES 3 2.1 F i e l d E f f e c t C o n d u c t i v i t y m o d u l a t i o n of a sample of semiconductor by an e l e c t r i c f i e l d p e r p e n d i c u l a r t o i t s s u r f a c e i s known as f i e l d e f f e c t . E a r l y r e f e r e n c e s t o t h i s e f f e c t i n c l u d e those of L i l i e n f e l d ,• 1933 and H e i l , 1935. m o d u l a t i o n i n a germanium f i l m due t o a charged f i e l d p l a t e p a r a l l e l t o the germanium s u r f a c e , s i m i l a r t o the case shown i n F i g u r e 2-1. I n e f f e c t , the semiconductor a c t s as one p l a t e of a c a p a c i t o r . The e l e c t r i c d i s p l a c e m e n t D ^ of the c a p a c i t o r system can be d e s c r i b e d by e q u a t i o n 2-11 S h o c k l e y and P e a r s o n , 1948 observed f i e l d e f f e c t D = n e e E o r n (2-1) M i c a I n s u l a t i o n p-type G-e F i l m ^ / / ^ / / / s / / / / / / / / / / / / / / / / / / 7 ^ 7 A R ' / v M e t a l E l e c t r o d e J^-V „ meas F i g u r e 2-1. M o d u l a t i o n of Conductance by F i e l d E f f e c t 4 where i s the e l e c t r i c d i s p l a c e m e n t normal t o the s u r f a c e , e Q i s the p e r m i t i v i t y of f r e e space, e r i s the d i e l e c t r i c c o n s t a n t of mica, and E i s the e l e c t r i c f i e l d normal t o the s u r f a c e , n Thus f o r a g i v e n a p p l i e d f i e l d E n , t h e e l e c t r i c d i s p l a c e m e n t i s known i n terms of the s u r f a c e charge on the c a p a c i t o r p l a t e s . T h i s f i e l d - i n d u c e d charge, i f m o b i l e , w i l l t a k e p a r t i n the c o n d u c t i o n p r o c e s s . I n the p a r t i c u l a r case shown i n F i g u r e 2-1, c l o s i n g of the s w i t c h S w i l l r e p e l h o l e s from the p-type germanium, thus r e d u c i n g the c u r r e n t through i t . The c u r r e n t m o d u l a t i o n observed by Sho c k l e y and Pe a r s o n was o n l y a s m a l l f r a c t i o n of t h a t expected. The d i s c r e p a n c y was. i n t e r p r e t e d as e v i d e n c e f o r the e x i s t e n c e o f s u r f a c e s t a t e s c a pable of t r a p p i n g the induced charge. F i e l d e f f e c t m o d u l a t i o n has a l s o been observed by u s i n g a f e r r o e l e c t r i c i n s u l a t o r between the c a p a c i t o r p l a t e s . An e a r l y r e f e r e n c e i s t h a t of G-odefroy, 1952, who observed measureable f i e l d e f f e c t m o d u l a t i o n i n t e l l u r i u m e vaporated onto ceramic barium t i t a n a t e . F o r the case of a f e r r o e l e c t r i c i n s u l a t o r , e q u a t i o n 2-1 i s m o d i f i e d t o form e q u a t i o n 2-2J D = e £ E + P (2-2) n r o n r n where e i s the r e l a t i v e d i e l e c t r i c c o n s t a n t of the f e r r o -r e l e c t r i c , and P i s the remanent p o l a r i z a t i o n of the f e r r o e l e c t r i c r n * normal t o the s u r f a c e . F i e l d e f f e c t m o d u l a t i o n of c o n d u c t i v i t y can be observed even w i t h no a p p l i e d f i e l d s i n c e the d i r e c t i o n of P r can be changed by a v o l t a g e p u l s e a p p l i e d a c r o s s the f e r r o e l e c t r i c . S t a d l e r , 1965, observed a conductance change of 2% i n 100A° t h i c k g o l d f i l m s on s i n g l e c r y s t a l barium t i t a n a t e a f t e r r e v e r s i n g the p o l a r i z a t i o n d i r e c t i o n . M o l l , 1963 and Zu l e e g and Wieder, 1966, d e s c r i b e d an a c t i v e d e v i c e w i t h v a r i a b l e g a i n and "remanent memory. T h i s d e v i c e , shown i n F i g u r e 2-2, c o n s i s t e d of a cadmium s u l p h i d e i n s u l a t e d gate f i e l d , e f f e c t t r a n s i s t o r , s i m i l a r t o t h a t developed by Weimer, 1962, d e p o s i t e d Gate SiO I n s u l a t o r -Drain (Au) CdS T r i g l y c i n e S u l p h a t e C o u n t e r e l e c t r o d e (Au) F i g u r e 2-2. Cadmium S u l p h i d e TFT on T r i g l y c i n e S u l p h a t e ( M o l l ) on a f e r r o e l e c t r i c s u b s t r a t e of t r i g l y c i n e s u l p h a t e w i t h a c o u n t e r e l e c t r o d e on the o t h e r s i d e . M o l l , 1963. observed a r e s i s t a n c e v a r i a t i o n of 257^ as the p o l a r i z a t i o n was r e v e r s e d , f o r a r e s i s t a n c e of 60Kribetween source and d r a i n . F i n a l l y , Heyman and H e i l m e i e r , 1966, d e s c r i b e d a v a r i a b l e r e s i s t o r u s i n g p-type t e l l u r i u m between ohmic c o n t a c t s on t r i g l y c i n e s u l p h a t e , w i t h a c o u n t e r e l e c t r o d e on the o t h e r s i d e of the f e r r o e l e c t r i c substrate'. I n the work t o be r e p o r t e d h e r e , cadmium s e l e n i d e was used as the semic o n d u c t o r , and barium t i t a n a t e as the f e r r o e l e c t r i c s u b s t r a t e , t o form an a c t i v e d e v i c e w i t h memory s t o r a g e . P a s s i v e d e v i c e s w i t h v a r i a b l e r e s i s t a n c e a re a l s o d e s c r i b e d u s i n g cadmium s u l p h i d e and cadmium s e l e n i d e f i l m s d e p o s i t e d on barium t i t a n a t e . 2.2 Two-State T F T - S t r u c t u r e and O p e r a t i o n The b a s i c s t r u c t u r e of the a c t i v e d e v i c e t o be c o n s i d e r e d i s shown i n F i g u r e 2-3. I t c o n s i s t s of a t h i n f i l m of n-type cadmium s e l e n i d e d e p o s i t e d onto a barium t i t a n a t e s u b s t r a t e . Aluminum s o u r c e - d r a i n e l e c t r o d e s a re th e n evaporated onto the cadmium s e l e n i d e , f o l l o w e d by an i n s u l a t i n g l a y e r of s i l i c o n o x i d e and an aluminum gate e l e c t r o d e . The d e v i c e o p e r a t e s by m o d u l a t i o n of the cadmium s e l e n i d e f i l m c o n d u c t i v i t y by b o t h the gate and the f e r r o e l e c t r i c s u b s t r a t e . E s s e n t i a l l y , t h e r e -f o r e , the d e v i c e can be c o n s i d e r e d as a two-gate TFT. One of the g a t e s , the barium t i t a n a t e , has o n l y two p o s s i b l e s e t t i n g s , and the o t h e r i s c o n t i n u o u s l y v a r i a b l e . 7 SiO I n s u l a t o r (2000-4000 A ) A l Source (^1000 A A l G a t e ( ~ 1 0 0 0 A 0) A l D r a i n ( - V1000 A u) Evaporated ^Au C o u n t e r e l e c t r o d e n ^ I Q O O - 3 0 0 0 A 0) F i g u r e 2-3. Cadmium S e l e n i d e Two-State T h i n - F i l m F i e l d E f f e c t T r a n s i s t o r F i g u r e 2-4. M e t a l - I n s u l a t o r - S e m i c o n d u c t o r Energy L e v e l Diagram 8 C o n s i d e r - a o n e - d i m e n s i o n a l model of an n-type semi-c o n d u c t o r a t t h e r m a l e q u i l i b r i u m w i t h an i n s u l a t o r - m e t a l c o n t a c t as shown i n F i g u r e 2-4 (Many, 1965). The p o t e n t i a l V i n the semiconductor i s g i v e n by P o i s s o n ' s e q u a t i o n : v . V = - t ^ - (2-3) r o where jo i s the charge d e n s i t y i-n the semiconductor, . F o r the p l a n a r geometry shown and assuming n e g l i g i b l e h o l e c o n c e n t r a t i o n i n the semiconductor, e q u a t i o n 2-3 reduces t o the P o i s s o n - B o l t z m a n e q u a t i o n : = - ^ ^ ( l - e x p e f k T ) dx r o or f u r t h e r , = F~i~~Tcf *b ( 1 - e x * v ) ( 2 ~ 4 ) dx r 0 , where n^ i s the. semiconductor b u l k e l e c t r o n c o n c e n t r a t i o n and v = eV kT F o r v ^ e q u a t i o n 2-4'can be approximated by e q u a t i o n 2-5 ,2 d v _ y_ d x 2 I 2 (2-5) where e e kT ' r o D ~ \ / 2 e n . b With boundary c o n d i t i o n s : v. = v a t x = 0, and v = 0 a t x = oo , e q u a t i o n 2-5 can be. i n t e g r a t e d d i r e c t l y t o y i e l d e q u a t i o n 2-6: •. v = v e x p ( - x / l D ) (2-6) Thus Lp, the e f f e c t i v e Debye l e n g t h , i s the d i s t a n c e i n t o the semiconductor a t wh i c h t h e p o t e n t i a l v i s the l / e of i t s v a l u e a t the s u r f a c e , f o r s m a l l v. , The model j u s t developed i s analogous t o the g a t e -i n s u l a t o r - s e m i c o n d u c t o r l a y e r of the d e v i c e t o be c o n s i d e r e d . Thus, f o r s m a l l V,. m o d u l a t i o n of the c o n d u c t i v i t y of the semi-conductor by the gate i s . c o n f i n e d t o a depth i n the o r d e r o f the Debye l e n g t h from the s u r f a c e of the i n s u l a t o r . S i m i l a r l y , c o n d u c t i v i t y m o d u l a t i o n at the f e r r o e l e c t r i c - s e m i c o n d u c t o r i n t e r -f a c e ends at, a depth i n ' t h e o r d e r of the Debye l e n g t h i n t o the semiconductor. Table 1 g i v e s the value, of L^ f o r cadmium s e l e n i d e w i t h v a r i o u s e l e c t r o n c o n c e n t r a t i o n s . ELECTRON CONCENTRATION 1 0 1 6 el./cm.' 3 416 A° -1 0 1 5 . 1320 i o 1 4 " "... 4160 - i o 1 5 13200 '•: ' TABLE 1 Debye Length f o r Cadmium S e l e n i d e a t V a r i o u s E l e c t r o n C o n c e n t r a t i o n s 10 I t i s a l s o n e c e s s a r y t h a t the c a r r i e r c o n c e n t r a t i o n he. kept s m a l l i n o r d e r t o have a l a r g e amount of c a r r i e r m o d u l a t i o n due t o the gate and f e r r o e l e c t r i c s u b s t r a t e . I f the c a r r i e r c o n c e n t r a t i o n s a re too l a r g e , then the a d d i t i o n a l charges i n j e c t e d by the f i e l d e f f e c t w i l l be n e g l i g i b l e i n comparison. The s t r u c t u r e of the t w o - s t a t e r e s i s t o r i s s i m i l a r t o t h a t of the a c t i v e d e v i c e except the i n s u l a t i n g l a y e r and gate are m i s s i n g . C o n d u c t i v i t y m o d u l a t i o n i s caused o n l y by the f e r r o e l e c t r i c p o l a r i z a t i o n change., As f o r the a c t i v e d e v i c e , . m o d u l a t i o n i s g r e a t e s t when the semiconductor l a y e r has a s m a l l c a r r i e r c o n c e n t r a t i o n and i s r e l a t i v e l y t h i n . 2.3 Elementary Theory of the I n s u l a t e d Gate F i e l d E f f e c t T r a n s i s t o r M o d i f i e d t o I n c l u d e the F e r r o e l e c t r i c S u b s t r a t e I n t h i s s e c t i o n , the elementary t h e o r y of the t h i n - f i l m f i e l d e f f e c t t r a n s i s t o r (Borkan and Weimer, 1966), (Weimer, 1964), i s m o d i f i e d t o i n c l u d e the e f f e c t of the f e r r o e l e c t r i c s u b s t r a t e . I t i s seen t h a t a t w o - s t a t e TFT r e s u l t s from t h i s a n a l y s i s . The f o l l o w i n g assumptions are used i n t h i s t r e a t -ment of the model: 1. C a r r i e r m o b i l i t y i s c o n s t a n t , independent of the gate or s o u r c e - d r a i n f i e l d , and of the p o l a r i z a t i o n s t a t e of the f e r r o e l e c t r i c s u b s t r a t e . 2. The g r a d u a l c h a n n e l a p p r o x i m a t i o n h o l d s . That i s , the r a t e of change of the s o u r c e - d r a i n f i e l d i s s m a l l compared t o the r a t e of change of the gate f i e l d normal t o the c h a n n e l . P o i s s o n ' s e q u a t i o n may then be s o l v e d i n one dimension. n-type CdSe (a) Crossactional View (b) E l e c t r o d e S t r u c t u r e F i g u r e 2 - 5 . I d e a l i z e d Geometry of Cadmium S e l e n i d e Two-Stat T r a n s i s t o r 12 3. T r a p p i n g e f f e c t s a r e n e g l e c t e d . 4. Gate c a p a c i t a n c e i s assumed c o n s t a n t . 5» The work f u n c t i o n d i f f e r e n c e s between the e l e c t r o d e s and the semiconductor are n e g l e c t e d . Ohmic source and d r a i n c o n t a c t s a r e assumed. 6 . The c a r r i e r m o d u l a t i o n a t each s u r f a c e of the semi-cond u c t o r i s assumed independent of the o t h e r . The i d e a l i z e d d e v i c e s t r u c t u r e and the c o - o r d i n a t e axes used i n the a n a l y s i s are shown i n F i g u r e 2-5. V"Q. and "V^ r e p r e s e n t the p o t e n t i a l s a p p l i e d t o the. gate and d r a i n e l e c t r o d e s r e s p e c t i v e l y . V ( x ) i s the p o t e n t i a l of the semi-cond u c t o r a t any p o i n t w i t h r e s p e c t t o the s o u r c e . S o u r c e - d r a i n c u r r e n t 1 - ^ i s g i v e n by: XD = c r n E x where ar^ i s the ch a n n e l c o n d u c t i v i t y , E i s the f i e l d s t r e n g t h between s o u r c e and d r a i n , u i s the e f f e c t i v e m o b i l i t y i n the s o u r c e - d r a i n • j • c h a n n e l W i s the ch a n n e l w i d t h , n(x) i s the e l e c t r o n c o n c e n t r a t i o n per u n i t a r e a i n the semiconductor a t a d i s t a n c e ' x from the s o u r c e , and e i s the e l e c t r o n i c charge. 13 The charge per u n i t a r e a induced i n t o the semiconductor l a y e r a t any p o i n t x by the a p p l i e d gate v o l t a g e i s g i v e n by e q u a t i o n 2-7: C A N ( x ) = wfe ( VG " V ( x ) ) ( 2 ~ 7 ) where C i s the gate c a p a c i t a n c e p e r u n i t a r e a , C = e / t , g • g ox' ox C o n s i d e r a l s o the f e r r o e l e c t r i c s u b s t r a t e which has a permanent p o l a r i z a t i o n charge per u n i t a r e a of _+ P , which w i l l cause a compensating charge of + ? r t o be i n d u c e d i n t o the s e m i c o n d u c t i n g l a y e r . Thus, P n( x ) = AN(x) + N + — 1 e L ^ VV (x ) ) ] +1*o±T- ( 2 " 8 ) where N Q i s the i n i t i a l charge c o n c e n t r a t i o n p e r u n i t a r e a i n the semiconductor, n e g l e c t i n g the e f f e c t of the f e r r o e l e c t r i c s u b s t r a t e . S u b s t i t u t i n g f o r n(x) i n e q u a t i o n 2-6 and i n t e g r a t i n g from x = 0 t o x = L, y i e l d s e q u a t i o n 2-10: Ij, dx = / e a ¥ n(x) dV(x) D J c ^n 0 V. ff [ V V < X 3 + e N o ± P r ] dV(x) 0 (2-9) u c r Yj,2 D ~ L2 L>VG V V D 2 (2-10) 14 N eWL P WL where V T = - - g + (2-11) g g The t h r e s h o l d v o l t a g e i s the a p p l i e d v o l t a g e r e q u i r e d f o r the onset of d r a i n c u r r e n t . I t i s n e g a t i v e f o r d e p l e t i o n mode and p o s i t i v e f o r enhancement mode d e v i c e s . As i s i n c r e a s e d , a c o n d i t i o n occurs i n which the s u r f a c e c o n c e n t r a t i o n of c a r r i e r s a t a d i s t a n c e x from the sourc e i s z e r o : o - + U - ¥1 + V G - V(x) C WL K d ± d J T h i s c o n d i t i o n o c c u r s f i r s t a t x = L and the p i n c h - o f f v o l t a g e V i s g i v e n by e q u a t i o n 2-13• \ = \ - V T ( 2 " 1 5 ) F o r d r a i n v o l t a g e s g r e a t e r t h a n V , the g r a d u a l c h a n n e l a p p r o x i m a t i o n no l o n g e r h o l d s , s i n c e no cha n n e l e x i s t s over p a r t of the s o u r c e - d r a i n r e g i o n . I t i s assumed t h a t the g r a d u a l c h a n n e l a p p r o x i m a t i o n h o l d s up t o the p o i n t where V(x) = V . T h i s o c c u r s a t L', where L' < L. A l l charge which a r r i v e s a t L' i s assumed t o be swept a c r o s s the r e m a i n i n g h i g h f i e l d r e g i o n t o the d r a i n . Thus, f o r V-p zs- V , e q u a t i o n 2-10 becomes: = (vG - vT)2 (2-14) L' i s v e r y n e a r l y e q u a l t o L. T h e r e f o r e , the c h a r a c t e r i s t i c s of the d e v i c e s a t u r a t e f o r Y-rj> V • 15 / Yn= 6 VD ( b ) H i g h - S t a t e C h a r a c t e r i s t i c s F i g u r e 2-5. R e p r e s e n t a t i v e D r a i n C h a r a c t e r i s t i c s f o r Two-State TFT 16 . Figures 2-6(a) and (b) show representative IJJ^VJJ characteristics given by aquations 2-10 and 2-13 for the two regions of the device and the two states of the f e r r o e l e c t r i c c r y s t a l * Transconductance of the device below saturation i s given by equation 2-15: S m # •' - M> ( 2 - 1 5 ) m ,• oV G N I T VD In t h i s elementary analysis of the device, the reversal 1 of th© f e r r o e l e c t r i c p olarization charge gives two sets of - characteristics d i f f e r i n g only i n V^. A l l other parameter! of the device, including mobility and g , are constant. For' t h i s device, with V^s* V , and constant gate voltage, two current le v e l s may be observed, depending on the polar-i z a t i o n state of the f e r r o e l e c t r i c substrate. Thus, information about the state of the f e r r o e l e c t r i c c r y s t a l i s given by the • • current l e v e l of the device characteristics. 2*4 Shj_o,ry of Resistance Modulation Due to Ferroelectric Sub-. The following i s an elementary treatment of the theory o f the two-state r e s i s t o r . The device structure as described i n Figure 2-4 w i l l be assumed with the insulator and gate ' neglected. For a given voltage across the source-drain gap, assuming ohmic contacts, the drain current 1^ i s : 17 V. D = D V^Wh R L = neu, E-J/tf r n D (2-16) where E-^  = V-^/L i s the e l e c t r i c f i e l d s t r e n g t h between e l e c t -r o d e s , and n i s the e l e c t r o n c o n c e n t r a t i o n per u n i t a r e a of the semiconductor f i l m . f a c e o f the f e r r o e l e c t r i c i s compensated by an e q u a l and p o p p o s i t e m o b i l e charge i n the semiconductor. Then, n = n Q+ _ r e where n i s the e l e c t r o n c o n c e n t r a t i o n of the semiconductor assuming no p o l a r i z a t i o n charge. T h e r e f o r e , the c u r r e n t between the source and d r a i n w i l l be l i n e a r w i t h v o l t a g e a p p l i e d , as a r e s i s t o r , and w i l l hare two v a l u e s which depend on'the p o l a r i z a t i o n d i r e c t i o n of the f e r r o e l e c t r i c s u b s t r a t e as d e s c r i b e d by e q u a t i o n 2-17: Once a g a i n , assume the p o l a r i z a t i o n charge a t the s u r -d (n -P / e ) / L o r _ _ _ - i — -n +P / e ) / L 'Slope o f Two L i n e s F i g u r e 2 18 = n e ^ n ¥ E D P D (2-17) Note t h a t s i n c e P i s c o n s t a n t , the m o d u l a t i o n e f f i c i e n c y r V of the d e v i c e , d e f i n e d by y - 3 , i s i n v e r s e l y p r o p o r t i o n a l t o n . T h i s d e v i c e may a l s o be used t o g i v e n o n - d e s t r u c t i v e readout of the s t a t e of the f e r r o e l e c t r i c c r y s t a l . I n t h i s c a s e , the s t a t e of the c r y s t a l i s determined by the r e s i s t a n c e l e v e l of the d e v i c e . 2.5 E f f e c t of Traps on Pro-posed D e v i c e s B o t h w i t h i n the semiconductor and a t i t s s u r f a c e t h e r e e x i s t c e n t r e s which can c a p t u r e one or more e l e c t r o n s from the c o n d u c t i o n band. I f the c a p t u r e d e l e c t r o n has a h i g h p r o b a b i l i t y of r e t u r n i n g t o the c o n d u c t i o n band r a t h e r than r e c o m b i n i n g w i t h a h o l e , the c e n t r e s a c t as t r a p s (Bube, pg. 273)- I f t h e r e a re s e v e r a l l e v e l s of c e n t r e s between the c o n d u c t i o n and v a l e n c e bands, those c l o s e to. the c o n d u c t i o n band a c t as t r a p s w h i l e those f u r t h e r away are r e c o m b i n a t i o n c e n t r e s , s i n c e t h e r e i s a h i g h e r p r o b a b i l i t y of combining w i t h a h o l e t h a n of r e t u r n i n g t o the c o n d u c t i o n band. I t i s p o s s i b l e t o d e f i n e an e l e c t r o n d e m a r c a t i o n l e v e l JM^  between the two t y p e s of c e n t r e s (Rose, Pg. 2 8 ) . At t h i s l e v e l , t he p r o b a b i l i t y of an e l e c t r o n r e t u r n i n g t o the c o n d u c t i o n band and of r e c o m b i n i n g w i t h a h o l e i s e q u a l . There a l s o e x i s t s an e q u i v a l e n t d e m a r c a t i o n l e v e l f o r h o l e s , P~ . 19 The d e m a r c a t i o n l e v e l s shown i n F i g u r e 2-8 h o l d f o r o n l y one s p e c i f i c e q u i l i b r i u m c o n d i t i o n . As the Fermi l e v e l moves, N-g and must change a l s o . S u r f a c e s t a t e s o f t e n a c t as t r a p p i n g c e n t r e s and may be so numerous as t o s e r i o u s l y a f f e c t the performance o f f i e l d e f f e c t d e v i c e s such as a r e c o n s i d e r e d here (Weimer, 1964)• C o n s i d e r the case where the f e r r o e l e c t r i c p o l a r -i z a t i o n d i r e c t i o n i s r e v e r s e d suddenly t o i n c r e a s e the e l e c t r o n c o n c e n t r a t i o n i n the semiconductor. The charge i n d u c e d i n t o the semiconductor goes i n i t i a l l y i n t o the c o n d u c t i o n band. Over a p e r i o d of t i m e , some of t h i s charge i s removed from the c o n d u c t i o n band t o f i l l t r a p s . Thus,, the c a r r i e r c o n c e n t r a t i o n decreases u n t i l a s t e a d y s t a t e l e v e l i s reached where the number of t r a p s b e i n g f i l l e d e q u a l s the number emptying back i n t o the c o n d u c t i o n band. -t E l e c t r o n -Traps Re c omb i n a t i o n C e n t r e s f o r E l e c t r o n s E •D •-Re c o m b i n a t i o n C e n t r e s f o r Holes "-Traps f o r Holes F i g u r e 2-8. Demarcation L e v e l s i n Semiconductor 20 I f t he number of t r a p s i s l a r g e , almost a l l of the induced charge may be t r a p p e d . The k i n e t i c s of the t r a p p i n g p r o c e s s a re governed by an e q u a t i o n of the form: S nT = u S n c ( N m - n T ) - K n m exp(-E m/kT) (2-18) where u i s the t h e r m a l v e l o c i t y of the c a r r i e r (u = y 2kstn) , S i s the c a p t u r e c r o s s s e c t i o n of the c e n t r e f o r e l e c t r o n s , n i s the c o n c e n t r a t i o n of conduction.band e l e c t r o n s , ~C n m i s the c o n c e n t r a t i o n of f i l l e d t r a p s , N m i s the c o n c e n t r a t i o n of t r a p p i n g l e v e l s , E m i s the energy between the t r a p l e v e l and the c o n d u c t i o n band, and K i s a c o n s t a n t of p r o p o r t i o n a l i t y . A s i n g l e e l e c t r o n t r a p p i n g l e v e l w i l l g i v e an e x p o n e n t i a l time v a r i a t i o n i n c a r r i e r c o n c e n t r a t i o n , the time c o n s t a n t o f which i s g i v e n by: T T = ( u S N T ) _ 1 However, i n most m a t e r i a l s , s e v e r a l t r a p l e v e l s a r e p r e s e n t and the change i n c a r r i e r c o n c e n t r a t i o n w i t h r e s p e c t t o time becomes the sum of many e x p o n e n t i a l s . F o r the case of the proposed d e v i c e s , a l a r g e s u r f a c e s t a t e d e n s i t y a t the s e m i c o n d u c t o r - i n s u l a t o r i n t e r f a c e i s expected f o r s e v e r a l r e a s o n s . Tamm s t a t e s , (Tamm, 1932), 21 which are due to a termination of the p e r i o d i c p o t e n t i a l of the c r y s t a l l a t t i c e at the semiconductor surface, may be present. A l s o , there may be a l a r g e concentration of i m p u r i t i e s on the f e r r o e l e c t r i c c r y s t a l before the semiconductor f i l m i s deposited, due to e l e c t r o s t a t i c a t t r a c t i o n between the f e r r o -e l e c t r i c c r y s t a l and impurity atoms present i n the atmosphere. These contaminants, being i n intimate contact w i t h the f e r r o -e l e c t r i c surface,would have very f a s t trapping time constants, s i m i l a r to the f a s t states found i n germanium (Heiman and W a r f i e l d , 1965). 22 3. FERROELECTRIC AND RELATED PROPERTIES OF BARIUM TITANATE 3.1 F e r r o e l e c t r i c i t y A f e r r o e l e c t r i c c r y s t a l i s d e f i n e d as one which e x h i b i t s a spontaneous e l e c t r i c d i p o l e moment or p o l a r i z a t i o n , the d i r e c t i o n of which may be a l t e r e d by a p p l i c a t i o n of an e l e c t r i c f i e l d (Dekker, Pg. 184). T o t a l p o l a r i z a t i o n P i s r e l a t e d t o a p p l i e d e l e c t r i c f i e l d E by a h y s t e r e s i s l o o p as i n F i g u r e 3-1. I n g e n e r a l , the d i r e c t i o n of spontaneous p o l a r i z a t i o n , known as the p o l a r a x i s , i s not the same throughout the c r y s t a l . A number of i n d i v i d u a l domains may e x i s t w i t h d i f f e r e n t p o l a r i z a t i o n d i r e c t i o n s which depend on the c r y s t a l s t r u c t u r e of the m a t e r i a l . I f a macroscopic sample w i t h z e r o i n i t i a l n e t p o l a r i z a t i o n i s c o n s i d e r e d , a p p l i c a t i o n of an e l e c t r i c f i e l d w i l l cause the net p o l a r i z a t i o n t o i n c r e a s e as the v a r i o u s domains tend t o l i n e up w i t h the a p p l i e d f i e l d . T h i s p r o c e s s corresponds t o the curve OAB i n F i g u r e 3-1• When a l l domains are a l i g n e d w i t h the a p p l i e d f i e l d , f u r t h e r i n c r e a s e s i n P a r e s m a l l and due o n l y t o induced p o l a r i z a t i o n e f f e c t s , F e r r o e l e c t r i c s a re so named because of the analogy between the P-E h y s t e r e s i s l o o p of F i g u r e 3-1 and the B-H l o o p of f e r r o -magnetic m a t e r i a l s (Megaw, Pg. l ) . The permanent d i p o l e moment of f e r r o e l e c t r i c c r y s t a l s d i s a p p e a r s above a c e r t a i n temperature T^, which i s known as the C u r i e t emperature. F e r r o e l e c t r i c s are c h a r a c t e r i z e d by a n o n - c e n t r i c - c r y s t a l s t r u c t u r e w i t h a unique P polarization(coulombs/cm. ) E e l e c t r i c field(volts/cm.) E coercive f i e l d co P remanent polarization r iP F i g u r e 3-1• F e r r o e l e c t r i c H y s t e r e s i s Loop F i g u r e 3-2. Cubic P e r o v s k i t e S t r u c t u r e of Barium T i t a n a t e 24 p o l a r a x i s which can be r e v e r s e d by an e l e c t r i c f i e l d . Above the C u r i e temperature, the c r y s t a l c l a s s changes t o one of h i g h e r symmetry. I n t h e i r f e r r o e l e c t r i c phase, f e r r o e l e c t r i c c r y s t a l s e x h i b i t b o th p y r o e l e c t r i c and p i e z o e l e c t r i c : p r o p e r t i e s . 3.2 P h y s i c a l P r o p e r t i e s of Barium T i t a n a t e Barium t i t a n a t e , a l t h o u g h o n l y d i s c o v e r e d t o be f e r r o -e l e c t r i c i n 1945-46^ (Wul and Goldman, 1945), ("Von H i p p e l , 1946) has s i n c e been w i d e l y i n v e s t i g a t e d . I t s s t a b i l i t y , b o t h m e c h a n i c a l and c h e m i c a l , a l a r g e remanent p o l a r i z a t i o n , and a C u r i e temperature of over 100°C are some of the reasons f o r the i n t e r e s t shown barium t i t a n a t e , e s p e c i a l l y f o r p r a c t i c a l d e v i c e a p p l i c a t i o n s . The atomic s t r u c t u r e of the p e r o v s k i t e c r y s t a l s , of which barium t i t a n a t e i s the b e s t known, i s much s i m p l e r than t h a t of o t h e r f e r r o e l e c t r i c s . Thus, r e s e a r c h i n t o the b a s i c phenomenon of f e r r o e l e c t r i c i t y has i n c l u d e d much work w i t h barium t i t a n a t e ( D e v o n s h i r e , 1954). Barium ( t i t a n a t e i s c r y s t a l l i n e , c l e a r t o dark brown i n c o l o u r depending on the amount of i r o n dopant p r e s e n t , and has a h i g h r e f r a c t i v e i n d e x . The C u r i e temperature i s near 120°, the e x a ct t r a n s i t i o n temperature b e i n g dependent on the c r y s t a l i m p u r i t i e s . The n o n - p o l a r phase of barium t i t a n a t e i s c u b i c ( p o i n t group m3m) and has the c u b i c p e r o v s k i t e type s t r u c t u r e , a u n i t c e l l of which i s shown i n F i g u r e 3-2. Below the C u r i e t emperature, the c r y s t a l becomes t e t r a g o n a l ( p o i n t group 4mm). The t e t r a g o n a l a x i s may have t h r e e p o s s i b l e o r i e n t a t i o n s , which a r e a l o n g one of the t h r e e 25 pseudo-cubic axes. A l s o , the p o l a r a x i s , which i s p a r a l l e l t o the t e t r a g o n a l c a x i s , may be i n e i t h e r d i r e c t i o n a l o n g the a x i s . The t e t r a g o n a l c a x i s i s about 1% l o n g e r t h a n the o t h e r two. T h i s causes m e c h a n i c a l s t r e s s e s t o occur i n the c r y s t a l between d i f f e r e n t domains (Megaw, Pg. 65). I n an e q u i l i b r i u m c o n d i t i o n , t h e p o l a r a x i s l i e s i n a d i r e c t i o n t o m i n i m i z e s t r e s s e s , b o t h m e c h a n i c a l and e l e c t r i c a l , on the c r y s t a l . S m a l l a r e a s w i t h i n the c r y s t a l tend t o a l i g n themselves t o g e t h e r t o form domains, s i n c e , over a s m a l l a r e a of the c r y s t a l , s t r e s s c o n d i t i o n s a r e the same. Thus, the f e r r o e l e c t r i c domain s t r u c t u r e of barium t i t a n a t e can be' v e r y complex, w i t h v a r i o u s i n d i v i d u a l domains a l i g n i n g themselves i n any one of s i x p o s s i b l e d i r e c t i o n s . F u r t h e r c r y s t a l l o g r a p h i c phase changes occur i n barium t i t a n a t e a t 5°C and -90°C (Jona and S h i r a n e , Pg. 109). The c r y s t a l remains f e r r o e l e c t r i c but w i t h reduced remanent p o l a r -i z a t i o n . 3.3 P o l a r i z a t i o n R e v e r s a l 3.3«1 I n t r o d u c t i o n U n l i k e u n i a x i a l f e r r o e l e c t r i c s such as R o c h e l l e s a l t and t r i g l y c i n e s u l p h a t e (TGS), i t i s p o s s i b l e t o change the d i r e c t i o n of p o l a r i z a t i o n of a s i n g l e domain of barium t i t a n a t e by e i t h e r 90° or 180°. Of the two, 180° s w i t c h i n g or p o l a r -i z a t i o n r e v e r s a l has been s t u d i e d most, s i n c e many p r a c t i c a l a p p l i c a t i o n s of barium t i t a n a t e i n v o l v i n g i n f o r m a t i o n s t o r a g e make use of the e f f e c t of p o l a r i z a t i o n r e v e r s a l (Anderson, 1952), (Anderson, 1956), (Campbell, 1957). 26 Most barium t i t a n a t e s i n g l e c r y s t a l s are grown by the Remeika method (Remeika, 1954). The growth ha b i t of the c r y s t a l s i s such that t r i a n g u l a r p l a t e s w i t h one pseudo-cubic a x i s perpendicular to the plane of the p l a t e are formed. When the p o l a r a x i s of a domain i s normal to the c r y s t a l surface, the domain i s s a i d to be c-domain; when p a r a l l e l to the surface, the i domain i s a-domain (Hooton and Merz, 1954). One can d i f f e r -e n t i a t e between the two po l a r d i r e c t i o n s of a c-domain by using the n o t a t i o n c + and c , depending on the d i r e c t i o n of the polar a x i s w i t h respect to the top of the c r y s t a l . 180° swi t c h i n g u s u a l l y involves r e v e r s i n g the d i r e c t i o n of p o l a r i z a t i o n i n a c-domain c r y s t a l by a p p l i c a t i o n of an e l e c t r i c f i e l d opposite to the e x i s t i n g p o l a r i z a t i o n d i r e c t i o n . 3.3»2 Mechanism of P o l a r i z a t i o n Reversal i n Barium Titanate The thermodynamic theory of barium t i t a n a t e switching developed by Devonshire (Devonshire, 1949, 1951) predicted the general shape of the P-E r e l a t i o n s h i p found experimentally. However, the predicted coercive f i e l d was s e v e r a l orders of magnitude above that obtained experimentally. I t was therefore assumed that p o l a r i z a t i o n r e v e r s a l took place i n a small area of the c r y s t a l f i r s t , then spread out, r a t h e r than the whole c r y s t a l r e v e r s i n g i t s p o l a r i z a t i o n s t a t e at once as required by the thermodynamic theory, since the energy requirements are much lower f o r t h i s process than f o r a t o t a l instantaneous change. Merz, 1954 and L i t t l e , 1955 both reported p o l a r i z a t i o n r e v e r s a l to occur by the formation of small a n t i - p a r a l l e l domains 27 on t h e c r y s t a l s u r f a c e s which t h e n grew th r o u g h the c r y s t a l t o form s p i k e s as shown i n F i g u r e 3-3• E app , P o l a r A x i s of Main C r y s t a l A n t i p a r a l l e l Domains F i g u r e 3-3. P o l a r i z a t i o n R e v e r s a l of Barium T i t a n a t e The f i r s t p u b l i s h e d d a t a on the r a t e of p o l a r i z a t i o n r e v e r s a l and time o f . r e v e r s a l was p r e s e n t e d by Merz, 1954. The s w i t c h i n g c u r r e n t was re c o r d e d . a s a v o l t a g e drop a c r o s s a r e s i s t o r i n s e r i e s w i t h the s w i t c h i n g v o l t a g e source as. shown i n F i g u r e 3-4(a). M e t a l e l e c t r o d e s were used on the c r y s t a l s . F i g u r e 3-4(b) denotes a t y p i c a l response curve of a c r y s t a l i n the c~ s t a t e t o a p o s i t i v e v o l t a g e p u l s e . F i g u r e 3.4(c) r e p r e s e n t s the response of the c r y s t a l t o a second p o s i t i v e v o l t a g e p u l s e . At low f i e l d s t r e n g t h s , (.2 Kv./cm. or l e s s ) the e x p e r i m e n t a l r e s u l t s of Merz can be f i t t e d t o e q u a t i o n s of the form: l . 'max = i ^ exp ( -a/E) (3 -D and 1/t = 1 / ^ exp ( -a/E) (3-2) l o o : and co a r e the c u r r e n t and s w i t c h i n g time r e s p e c t i v e l y f or .an- i n f i n i t e f i e l d s t r e n g t h E, 28 + or - v o l t a g e p u l s e i ^ p j C r y s t a l • R (a) S w i t c h i n g C i r c u i t of Merz (c) Response C u r r e n t f o r F i r s t P u l s e i (d) Response t o Second P u l s e F i g u r e 3 - 4 . V o l t a g e P u l s e S w i t c h i n g (Merz) 29 and: a i s termed the. s w i t c h i n g a c t i v a t i o n energy and i s IV,./cm. f o r the samples of Merz. S w i t c h i n g times as f a s t as .2 jiseconds -"5 were observed f o r c r y s t a l s 5 x 10 c m . t h i c k . S t a d l e r , 1958, 1962 extended the work of Merz t o h i g h e r v a l u e s of f i e l d s t r e n g t h and found t h a t f o r f i e l d s up t o 100 Kv./cm. e q u a t i o n 3-3 was v a l i d : l / t s = l / t c o E 1 " 4 (3-3) A t h e o r e t i c a l model of p o l a r i z a t i o n r e v e r s a l which assumes motion of 180° domain w a l l s , developed by M i l l e r and W e i n r e i c h , I 9 6 0 , g i v e s r e s u l t s which a r e c o n s i s t e n t w i t h much of the p u b l i s h e d e x p e r i m e n t a l d a t a . The model assumes t h a t n u c l e a t i o n of a n t i p a r a l l e l domains c o n t r o l s domain w a l l motion. That i s , n u c l e a t i o n of new a n t i p a r a l l e l domains occurs a t the w a l l boundary p r e f e r e n t i a l l y and causes domain w a l l p r o p a g a t i o n . T h i s t h e o r y , which i s developed i n Appendix I , y i e l d s e x p r e s s i o n s dn f o r n u c l e a t i o n r a t e TJJ^ T, and domain w a l l v e l o c i t y v^, of the forms |a = ^ exp - o/E (3-4) and v d = K 2 exp - <x/E (3-5) where and are independent of E. These e x p r e s s i o n s h o l d f o r low f i e l d strengths,. (<3Kv./cm.). F o r h i g h e r f i e l d s t r e n g t h s , the d e r i v a t i o n s of M i l l e r and W e i n r e i c h were extended by S t a d l e r and Zachmandis, 1964 and y i e l d e d e x p r e s s i o n s of the form: = K E 1 ' 4 (3-6) 30 and v d = Z 4 E 1 ' 4 (3-7) where and are al s o independent of E, S t a d l e r and Zachmandis, 1964 al s o observed experimentally the high f i e l d n u c l e a t i o n r a t e and domain w a l l v e l o c i t y and found the same power dependence on, E as derived from the M i l l e r - W e i n r e i c h theory. While the theory of p o l a r i z a t i o n r e v e r s a l i s by no means complete, the mechanism of p o l a r i z a t i o n r e v e r s a l proposed by • M i l l e r and Weinreich i s promising as a basis f o r formulating a more complete theory of switching behaviour. 3.4 Experimental Procedure and Results 3.4.1 C r y s t a l Preparation The barium t i t a n a t e c r y s t a l s used were t h i n t r i a n g u l a r p l a t e s 0;2 to 0.4 mm t h i c k . They were commercially grown by a modified Remeika process. The c r y s t a l s were cleaned by etching f o r 10-15 minutes i n 85% by volume E^PO^ at a temperature of 140-160°C, then r i n s e d i n d i s t i l l e d and deionized water. Longer etching periods were used at times to f u r t h e r reduce c r y s t a l t h i ckness. The etching was c a r r i e d out above the Curie temperature i n order to prevent p r e f e r e n t i a l e tching of the various domains.. (Hooton and Merz, 1955). In most cases, no e f f o r t was made to obtain s i n g l e domain c r y s t a l s before electrode deposition.. However, the c r y s t a l s could be 'poled' to the "c" st a t e by p l a c i n g the c r y s t a l s between electrodes i n deionized water and applying a Semi-Elements Inc. P u r i t y of c r y s t a l s > 99.95% 31 l a r g e a l t e r n a t i n g v o l t a g e (Campbell, 1957)= There was a t e n d -ency f o l l o w i n g t h i s t r e a t m e n t f o r some of the c r y s t a l s t o r e v e r t t o a polydomain s t a t e because of s t r e s s e s w i t h i n the c r y s t a l . C i r c u l a r e l e c t r o d e s of aluminum or g o l d were d e p o s i t e d on the c r y s t a l s u s i n g s t a n d a r d vacuum d e p o s i t i o n t e c h n i q u e s i n a "Veeco -5 -6 vacuum system. D e p o s i t i o n p r e s s u r e s were i n the 1 0 - 1 0 t o r r . range. s i n c e they were more d u r a b l e . However, g o l d e l e c t r o d e s were used i n some i n s t a n c e s t o ensure good c o n t a c t s . 3.4.2 C o n d u c t i v i t y of C r y s t a l s aluminum e l e c t r o d e s was measured a t low f i e l d s t r e n g t h . The s i m p l e c i r c u i t used t o determine sample r e s i s t a n c e i s shown i n F i g u r e 3-5« The v o l t a g e was a p p l i e d so as not t o r e v e r s e t h e domain p o l a r i z a t i o n d i r e c t i o n . C o n d u c t i v i t y of the c r y s t a l s was found t o v a r y between 1.0 x 1 0 - ^ and 2.0 x l O - " ^ H-cm. f o r F o r most measurements, aluminum e l e c t r o d e s were used The c o n d u c t i v i t y of p r e p o l e d samples w i t h c i r c u l a r K e i t h l e y 600A E l e c t r o m e t e r 1.5 v o l t s \ \M Barium T i t a n a t e Sample F i g u r e 3-5. C o n d u c t i v i t y Measurement of Barium T i t a n a t e C r y s t a l s 32 samples .17 t o .32 cm. t h i c k . T h i s compares f a v o u r a b l y w i t h p r e v i o u s l y quoted v a l u e s (Jona and S h i r a n e , pg. 182). 3 « 4 . 3 Measurement of Remanent P o l a r i z a t i o n I n o r d e r t o observe the P-E h y s t e r e s i s l o o p of the c r y s t a l s , the m o d i f i e d Sawyer-Tower, 1930 c i r c u i t of E i g u r e /3-6 5was used. The v o l t a g e a c r o s s the c r y s t a l was measured d i r e c t l y by the X d e f l e c t i o n of the o s c i l l o s c o p e . The p o l a r -i z a t i o n of the c r y s t a l , p r o p o r t i o n a l t o the v o l t a g e a c r o s s the l i n e a r c a p a c i t o r C , i s measured by the Y d e f l e c t i o n of the o s c i l l o s c o p e . The c r y s t a l h o l d e r was des i g n e d t o mi n i m i z e s t r a y c a p a c i t a n c e . The P-E h y s t e r e s i s l o o p of c r y s t a l #1 i s shown i n F i g u r e 3-7 a t v a r i o u s f r e q u e n c i e s . I n Tab l e I I a r e g i v e n the v a l u e s of P^ and E C Q , c a l c u l a t e d f o r c r y s t a l s #1 and #4 a t v a r i o u s f r e q u e n c i e s . "^TAL ^ t h i c k n e s s : .21mm. E l e c t r o d e a r e a : 3.0mm. "^TAL ^ 4 t h i c k n e s s : ,.15mm. E l e c t r o d e a r e a : 3.0mm/ XTAL # 1 % #4 Frequency cps. 10 100 250 2 P i i c o u l . / c m . r ± 5 * 23.6 22.2 21.0 E^ v o l t s / c m . 0 0 ±3* 2000 2350 2750 P ucoul./cm.. ±5% 24.8 E v o l t s / c m . 0 0 ± 5 * 2400 T A B L E I I Summary of P-E H y s t e r e s i s Loop Measurements a t V a r i o u s F r e q u e n c i e s 3 3 G-eneral Radio U n i t O s c i l l a t o r and A m p l i f i e r : Sample 1 H o l d e r 1 X T e k t r o n i x lOx Probes t o O s c i l l o s c o p e Y C o a x i a l Cable C - G-eneral Radio 0 ,7'ufd. Standard C a p a c i t o r F i g u r e 3 - 6 . Sawyer-Tower C i r c u i t t o Determine Remanent P o l a r i z a t i o n 34 " 1 M M ' i i , l i i _ r | i • / ; s : " f . 1 1 i _ . — - \ 1 • • • . , • i . i i i i j . . . \ j (a) Measurement Frequency 10 c . / s . (b) Measurement Frequency 100 c./s.; (c) Measurement Frequency 250 c . / s . F i g u r e 3.7 E x p e r i m e n t a l P-E H y s t e r e s i s Curves of C r y s t a l #1 at V a r i o u s F r e q u e n c i e s V e r t i c a l S c a l e .7 amp./div. H o r i z o n t a l S c a l e 20 v o l t s / d i v . The commonly accepted low frequency value of P r f o r s i n g l e c r y s t a l BaTiO^ i s 26 |iCOul./cm. (Merz, 1953) at room temperature. The s l i g h t l y lower values obtained here i n d i c a t e that the c r y s t a l s used are not e n t i r e l y s i n g l e domain, but r a t h e r contain small domains which do not switch, p o s s i b l y because of c r y s t a l imperfections. As the measurement frequency increased, the value of decreased slowly as shown i n Figure 3-7(a), (b) and ( c ) . A p o s s i b l e reason f o r t h i s e f f e c t i s that the more h i g h l y s t r a i n e d areas of the c r y s t a l do not have time to reverse and thus do not contribute to the measured p o l a r -i z a t i o n . The coercive f i e l d i s a l s o seen to increase i n value as the frequency increases. This behaviour i s predicted by equation 3-1• At f a s t e r switching speeds, the switching current i must be l a r g e r i n order to complete p o l a r i z a t i o n 3 r e v e r s a l i n time,since t 2P r =J~J. i s ( t ) d t (3-8) where i (t) i s the switching current at time ( t ) . s Thus, higher switching voltages are required and E increases as t decreases, s 3.4.4 Slow Speed Switching Very slow speed switching c h a r a c t e r i s t i c s of the barium t i t a n a t e c r y s t a l s were recorded using the c i r c u i t of 36 F i g u r e 3-8. S w i t c h i n g o c c u r s a t a much b e t t e r d e f i n e d v o l t a g e a t low speeds. F i g u r e 3-9(a), (b) and ( c ) , show the I-V s w i t c h i n g c u r v e s f o r t h r e e d i f f e r e n t c r y s t a l s f o r a s l o w l y i n c r e a s i n g ramp v o l t a g e (30 V./min.). The c r y s t a l of F i g u r e 3-9(a) i s r e l a t i v e l y w e l l - b e h a v e d . S w i t c h i n g o c c u r s a t the same v o l t a g e i n b o t h d i r e c t i o n s and E i s w e l l d e f i n e d . The s w i t c h i n g curve i s a l s o v e r y s y m m e t r i c a l . T h i s i s t y p i c a l of an u n s t r a i n e d , s i n g l e domain c r y s t a l . F i g u r e s 3-9(b) and (c) show d e v i a t i o n s from the i d e a l . I n F i g u r e 3-9(b), the c o e r c i v e f i e l d i s i l l - d e f i n e d and the c r y s t a l appears t o be h i g h l y s t r a i n e d and p o l y c r y s t a l l i n e . The c r y s t a l of F i g u r e 3-9(c) seems t o be p r e f e r e n t i a l l y o r i e n t e d . That i s , i t would r a t h e r be one of the c s t a t e s than the o t h e r . I n a l l c a s e s , t h e areas under the curve a r e e q u a l i n both d i r e c t i o n s from z e r o v o l t s . T h i s i s n e c e s s a r y , s i n c e the a r e a under the curve i s a measure of the amount of charge 180 v o l t s Barium —, T i t a n a t e Sample S x K e i t h l e y 600A E l e c t r o m e t e r Moseley X-Y Recorder To Motor D r i v e F i g u r e 3-8. Slow-speed S w i t c h i n g C h a r a c t e r i s t i c Measurement C i r c u i t (c) F i g u r e 3 -9 . Slow "Voltage Ramp I-V S w i t c h i n g C h a r a c t e r i s t i c s being reversed by the applied voltage. If a voltage ramp is followed by a second ramp of the same polarity, negligible current w i l l flow during the second ramp, since the polar-ization charge has already been reversed and the resistance of the sample is very high (> 10%7) • The spontaneous polarization of the crystals may be calculated from this graph,and the total charge flowing has been found to be equal to 2? r within the l imits of experi-mental error (ry 20%). The coercive f i e l d , defined as the f i e l d at which the current is maximum, is between 310-550 V/cm for the samples shown. These values are much below those found at the higher switching speeds used to determine the hysteresis loops of Figure 3-7, as expected. 3.4.5 High Field Polarization Reversal Polarization reversal of the barium titanate crystals was performed using high voltage pulses. The magnitude of the switching current and the switching time were measured using the c ircuit shown in Figure 3-10. Sample Holder Hewlett-Packard Pulse Generator • (Rise-time <20 nanoseconds) 1 • 10x Probe to Oscilloscope and Camera Figure 3-10. High Field Response Measurement Circuit 39 R q must be s m a l l i n o r d e r t o keep the system time c o n s t a n t as s m a l l as p o s s i b l e . A v a l u e of 10 ft gave r e a s o n a b l e output v o l t a g e s f o r the c r y s t a l e l e c t r o d e a r e a used. The c r y s t a l s t e s t e d were p r e p o l e d by the slow speed s w i t c h i n g t e c h n i q u e t o ensure s i n g l e - d o m a i n s t r u c t u r e , then s w i t c h e d by a s i n g l e v o l t a g e p u l s e . F i g u r e 3-11(a) and 3-11(b) show the i n p u t v o l t a g e p u l s e and p o l a r i z a t i o n r e v e r s a l c u r r e n t f l o w r e s p e c t i v e l y f o r a w e l l behaved c r y s t a l . I t i s seen t h a t the output c u r r e n t p u l s e i s s i m i l a r t o t h a t o b t a i n e d by Merz, 1954'. F i g u r e 3 - l l ( c ) shows the c u r r e n t f l o w caused by a second i n p u t p u l s e of the same s i g n and magnitude. The a r e a under the second p u l s e i s l e s s than 10% of t h a t under the f i r s t . The c r y s t a l has been almost c o m p l e t e l y s w i t c h e d by the f i r s t p u l s e and the second p u l s e i s caused almost t o t a l l y by the normal c a p a c i t i v e a c t i o n of the barium t i t a n a t e c r y s t a l (Anderson, 1952). An i n s t a n t a n e o u s c u r r e n t l e v e l of .7 amps i s reached a c r o s s the c r y s t a l d u r i n g s w i t c h i n g . Care must be t a k e n not t o d e s t r o y the e l e c t r o d e s by r a p i d l y r e p e a t e d s w i t c h i n g a t h i g h speeds. I t i s a l s o p o s s i b l e t o determine P from the a r e a under * r the s w i t c h i n g curve a t t h e s e speeds. A rough c a l c u l a t i o n from F i g u r e 3-11 (b) y i e l d s a v a l u e f o r P^ of 22 ucoul./cm, , 2 i n good agreement w i t h the expected v a l u e of 26 ucoul./cm. . S w i t c h i n g time of 2 usee, f o r c r y s t a l t h i c k n e s s of .22mm. and s w i t c h i n g v o l t a g e of 170 v o l t s has been o b t a i n e d . F o r t h i n n e r c r y s t a l s , the s w i t c h i n g v o l t a g e may be reduced p r o p o r t i o n a t e l y t o m a i n t a i n the same s w i t c h i n g t i m e . 1 u s e e . / d i v . (a) Input V o l t a g e P u l s e i i > 1 > i i i 1 p.sec./div. (b) C r y s t a l Re sponse t o F i r s t P u l s e .5A./div. i i i i i i , i _ 1 [ i s e c . / d i v . (c) C r y s t a l Response t o Second P u l s e of the Same P o l a r i t y F i g u r e 3-11. H i g h F i e l d P u l s e Measurements 41 4. THE SEMICONDUCTOR FILM i 4.1 I n t r o d u c t i o n Important r e q u i r e m e n t s on the c h a r a c t e r i s t i c s of the semiconductor f i l m a re h i g h m o b i l i t y and low c a r r i e r con-c e n t r a t i o n (Weimer,' 1964). H i g h m o b i l i t y i s needed t o g i v e good d e v i c e g a i n and f r e q u e n c y r e s ponse. The c a r r i e r con-c e n t r a t i o n must be low enough t o a l l o w e f f e c t i v e m o d u l a t i o n by b o t h the gate and f e r r o e l e c t r i c s u b s t r a t e . Much of the work done w i t h t h i n - f i l m t r a n s i s t o r s has employed wide band-gap semiconductors i n o r d e r t o o b t a i n low c a r r i e r c o n c e n t r a t i o n s . V a r i o u s semiconductors have t h i s p r o p e r t y : s i l i c o n of the group IV, g a l l i u m a r s e n i d e and g a l l i u m a n t i m o n i d e of the group I I I - V , and cadmium s u l p h i d e and cadmium s e l e n i d e of the group I I - V I compounds. Of t h e s e , cadmium s e l e n i d e and. cadmium s u l p h i d e were chosen f o r the work d e s c r i b e d h e r e . The p r e p a r a t i o n of r e l a t i v e l y h i g h m o b i l i t y , low c o n c e n t r -a t i o n p o l y c r y s t a l l i n e f i l m s of cadmium s e l e n i d e and s u l p h i d e has been r e p o r t e d by many a u t h o r s . The amount of s u b s t r a t e h e a t i n g r e q u i r e d f o r d e p o s i t i o n of s i l i c o n would cause problems, and the t e c h n i q u e s of d e p o s i t i n g s e m i c o n d u c t i n g g a l l i u m a r s e n i d e a r e not w e l l d e f i n e d . Cadmium s e l e n i d e was found t o be b e t t e r s u i t e d than cadmium s u l p h i d e as the semiconductor f i l m f o r the d e v i c e s c o n s i d e r e d s i n c e the s i n g l e c r y s t a l m o b i l i t y of cadmium s e l e n i d e i s h i g h e r than t h a t of cadmium s u l p h i d e 2 2 (500 cm. / v o l t - s e c . v s . 200 cm. /voltrsec„, Bube, Pg, 269). 42 Another r e a s o n f o r the use of cadmium s e l e n i d e i s t h a t d e p o s i -t i o n of t h i s m a t e r i a l has been found t o be much more u n i f o r m and r e p r o d u c i b l e than cadmium s u l p h i d e . 4.2 P r o p e r t i e s of Cadmium S e l e n i d e and Cadmium S u l p h i d e and  E f f e c t s of D e p o s i t i o n C o n d i t i o n s Cadmium s e l e n i d e and cadmium s u l p h i d e are group I I - V I compounds w i t h an.hexagonal c r y s t a l l a t t i c e s t r u c t u r e . The energy band gap f o r s i n g l e c r y s t a l cadmium s e l e n i d e has been found t o be 1.7 ev. and f o r cadmium s u l p h i d e 2.4 ev. (Bube, Pg. 234). P r e p a r a t i o n of t h i n f i l m s of these compounds i s v e r y i m p o r t a n t i n d e t e r m i n i n g t h e i r e l e c t r i c a l p r o p e r t i e s . F i l m s d e p o s i t e d onto unheated s u b s t r a t e s are h i g h l y d i s o r d e r e d , of low m o b i l i t y , and n o n s t o i c h i o m e t r i c because of the no n e q . u i l i b r i u m p r o c e s s . The f i l m s a re u s u a l l y h i g h l y c o n d u c t i v e and cadmium-r i c h . D e p o s i t i o n onto heated s u b s t r a t e reduces these e f f e c t s but does not remove them. T h e r e f o r e , p o s t - d e p o s i t i o n t r e a t m e n t of the semiconductor f i l m s i s o f t e n d e s i r a b l e (Boer, E s b i t t , and Kaufman, 1966) t o o b t a i n f i l m s w i t h p r o p e r c h a r a c t e r i s t i c s . C r y s t a l d e f e c t s a r e i m p o r t a n t i n d e t e r m i n i n g the p r o p e r t i e s of the f i l m . Cadmium v a c a n c i e s a c t as donors and c a t i o n ( s u l p h u r , selenium) v a c a n c i e s a c t as a c c e p t o r s . C r y s t a l d e f e c t s a r e p r e s e n t i n a l l f i l m s and are e s p e c i a l l y p r e v a l e n t i n p o l y c r y s t a l l i n e f i l m s . I m p u r i t y atoms a f f e c t the e l e c t r i c a l p r o p e r t i e s of the f i l m s . a s w e l l . Group I I I and V I I I i m p u r i t i e s a c t as donors and group I and V atoms as a c c e p t o r s . F i g u r e 43 4-1 shows the energy l e v e l s of v a r i o u s donors and a c c e p t o r s of cadmium s u l p h i d e and cadmium s e l e n i d e as shown by Bube, Pg. 159. The a c c e p t o r . l e v e l s are so deep t h a t p-type c o n d u c t i v i t y i s not observed i n e i t h e r cadmium s u l p h i d e or cadmium s e l e n i d e . A l s o , Oxygen i m p u r i t y atoms may a c t as a c c e p t o r s i n b o t h cadmium s u l p h i d e and cadmium s e l e n i d e . P o s t - d e p o s i t i o n treatment of the f i l m s , i f n e c e s s a r y t o b r i n g the c a r r i e r c o n c e n t r a t i o n l e v e l t o the d e s i r e d v a l u e , may take many forms, but heat t r e a t i n g i n an a i r or s u l p h u r atmosphere a t 300-500°C has been found t o reduce the con-d u c t i v i t y of the m a t e r i a l by d i f f u s i o n of oxygen or s u l p h u r a c c e p t o r atoms i n t o the semiconductor (Bube, Pg. 175). Heat t r e a t i n g i n vacuum or i n cadmium vapour produces the o p p o s i t e e f f e c t . : A 2.4 Energy eV 0 C o n d u c t i o n Band \ ,03_eV_ 0 - 2 1.7 C o n d uction Band 1 l e V 0 "Valence Band .03eV Valence Band 1. Copper, Cadmium Vacancy 2. Group V I I , I I I I m p u r i t i e s CdS 1. Coppers, Cadmium Vacancy 2. Cadmium, Selenium Vacancy 3. Selenium Vacancy 4. Group V I I I m p u r i t i e s CdSe F i g u r e 4-1. I m p e r f e c t i o n Energy L e v e l s i n Cadmium S u l p h i d e and Cadmium S e l e n i d e 44 Ohmic contacts are formed by evaporating aluminum onto the cadmium sulphide or cadmium selenide f i l i n g or by d e p o s i t i n g gold electrodes on the substrate before the semiconductor f i l m i s deposited. 4•3 Thin F i l m Evaporation and Measurement 4.3*1 Evaporation Technique CdS and CdSe t h i n f i l m s were prepared by evaporating from a heated quartz c r u c i b l e i n the Veeco vacuum system onto a cold or heated substrate. Figure 4-2 shows the experimental set-up f o r the d e p o s i t i o n . Evaporation pressures were i n the -6 -5 10 - 10 t o r r . range. F i l m thickness was monitored using quartz c r y s t a l o s c i l l a t o r s (Chaurasia, 1964) i n s i d e the vacuum system close to the substrate. Further d i s c u s s i o n of thickness measurement i s given i n Appendix I I . The various f i l m c o n f i g u r a t i o n s were obtained by evaporating through photo-etched berylium copper masks held w i t h the substrate i n a mechanical j i g . Substrate heating, when required,, was accomplished by p l a c i n g a r e s i s t i v e heating element enclosed i n a copper holder i n thermal contact w i t h the mechanical j i g . Temperature was monitored by a thermocouple held i n the; centre of the mech-a n i c a l j i g which i s i n contact w i t h the barium t i t a n a t e c r y s t a l . The evaporation technique consisted of p l a c i n g the cleaned mechanical holder mask and substrate, already a l i g n e d , i n place i n the vacuum system, c l o s i n g i t and pumping the Cadmium Sulphide Powder UHP Eagle P i c h e r Co. Cadmium Selenide Powder 99.9995% pure A.D. Macay, Inc. 45 F i g u r e 4 - 2 . Veeco Vacuum D e p o s i t i o n Apparatus 46 —6 system u n t i l t he p r e s s u r e was i n the h i g h 10 t o r r . range. The r e s i s t i v e heated q u a r t z c r u c i b l e h o l d i n g the s o u r c e m a t e r i a l was then heated s l o w l y t o outgas the semiconductor evaporant. The source c u r r e n t was the n i n c r e a s e d u n t i l the source m a t e r i a l began t o evaporate a t the pr o p e r r a t e (1-10 A ° / s e c ) , th e n the s h u t t e r c l o s i n g o f f the s u b s t r a t e from the s o u r c e was opened and d e p o s i t i o n . began. Source c u r r e n t , p r e s s u r e , e l a p s e d t i m e , and t h i c k n e s s c r y s t a l m o n i t o r r e a d i n g were a l l r e c o r d e d . A f t e r e v a p o r a t i o n , the system was a l l o w e d t o c o o l b e f o r e e x p o s i n g the f i l m t o atmosphere. Ohmic c o n t a c t s were o b t a i n e d by e v a p o r a t i n g aluminum onto the semiconductor f i l m from a t u n g s t e n f i l a m e n t source- w i t h a _5 vacuum of 1 x 10 t o r r . 4.3-2 H a l l E f f e c t Measurements H a l l e f f e c t measurements t o determine t h i n - f i l m c a r r i e r c o n c e n t r a t i o n and m o b i l i t y were performed u s i n g a Van der Pauw, 1958 geometry. The advantage of t h i s geometry, i s t h a t sample shape i s not c r i t i c a l , , o n l y sample t h i c k n e s s "need be known. The measuring c i r c u i t used was d e s c r i b e d p r e v i o u s l y by T. ¥. Tucker, 1966 of t h i s l a b o r a t o r y . The c i r c u i t employs K e i t h l e y 6 0 0 A ' e l e c t r o m e t e r s and can measure specimen r e s i s t a n c e s , up to. 1 0 1 0 f t . w i t h < 5 % e r r o r . The r e s i s t i v i t y of a sample of a r b i t r a r y p e r i p h e r y as shown i n F i g u r e 4-3 i s " c a l c u l a t e d by s u b s t i t u t i n g i n e q u a t i o n 4-1: jth , RABCD + RBCDAN - /RABCDN /, , v ) L N D D BCDA where jD i s r e s i s t i v i t y of the sample i n fi-cm., h i s sample thickness i n cm., 4 7 ABCD - S j - f i , and R B C M = - f - f , where V A-V D i s the L AB BC p o t e n t i a l d i f f e r e n c e between A and D while current I £ C i s fl o w i n g between B and C. R ABCD When R A B C D - R B C D A , then f f g — Figure 4 - 3 . H a l l Sample with A r b i t r a r y Periphery The H a l l constant R^ i s c a l c u l a t e d from equation 4 - 2 : 1 0 8 A(V c-V A)h • RH = n7 = • J T T ^ ( 4 - 2 ) where B i s magnetic f i e l d across the c r y s t a l i n Kilogaus, and A(V C-V A) i s the change i n v"rj-v"A caused by the magnetic f i e l d , under constant current Ig-p. The m o b i l i t y i s c a l c u l a t e d from equations ( 4 - 1 ) and 48 (4-2), since (4-3) where u i s the m o b i l i t y i n cm. / v o l t - s e c . Measurement of p i s accurate to ±5f°, i f e r r o r s i n f i l m thickness are neglected, while the accuracy of R^ i s v a r i a b l e depending on the amount of noise superimposed on the H a l l voltage (A V Q - V^).. The accuracy of R^ i s u s u a l l y b e t t e r than ±15%. 4.3*3 Specimen and Specimen Holder Specimens were evaporated i n a four l e a f c l o v e r con-f i g u r a t i o n , used by Van Daal, 1964, to reduce contact e r r o r due to n o n - i n f i n i t e s i m a l contacts. Aluminum electrodes were deposited under vacuum on top of the semiconductor f i l m to form ohmic contacts. Figure 4-2 i s a photograph of an evaporated H a l l specimen w i t h contacts. Figure 4-2. H a l l Sample of Cadmium Selenide w i t h Aluminum Contacts 49 The specimen holder used had four pressure contacts of gold-plated berylium copper to contact the specimen. A f i f t h electrode consisted of the specimen holder base plate and was used to perform switching experiments on Hall samples of cadmium selenide deposited on barium titanate. This w i l l be discussed further in the next chapter. The specimen holder is pictured i n Figure 4-4(a) and (b). A l l measurements were performed with the light-proof casing i n place. 4.3.4 Experimental Results Hall specimens of cadmium sulphide and cadmium selenide, deposited onto unheated glass substrates, were measured immediately after preparation. The results of these measure-ments are shown in Table III. FILM THICKNESS RATE OF DEPOSITION (cm. 2 /volt-sec.) n (ely/cm.^) CdSe CdSe* CdS 3200 A 0 2600 A 0 3.3 A ° / s e c . 3.3 A ° / s e c . 63±10% 55+10% 40+10% 5.3xl018+10% 2.1xl0l6+10% 2.2xl0l6+10% TABLE III Hall Measurement Results of Cadmium Sulphide and Cadmium Selenide on Glass These'measurements show that the mobility of cadmium selenide is higher than that of cadmium sulphide, even i n the-After standard heat treatment in air at 300 C. for 15 min. (a) O v e r a l l View (b) Contacts Figure 4-5. H a l l Specimen Holder 51 p o l y c r y s t a l l i n e form as d e p o s i t e d . A l s o , the r e d u c t i o n i n c a r r i e r c o n c e n t r a t i o n by heat treatment i n a i r i s demonstrated. Cadmium s e l e n i d e t h i n f i l m s were found t o be much e a s i e r t o d e p o s i t and were much more r e p r o d u c i b l e than those of cadmium s u l p h i d e , e s p e c i a l l y on a barium t i t a n a t e s u b s t r a t e . T h e r e f o r e , cadmium s e l e n i d e was used almost e x c l u s i v e l y f o r the r e m a i n i n g e x p e r i m e n t s . 52 5. CADMIUM SULPHIDE AND CADMIUM SELENIDE FILMS ON BARIUM TITANATE 5.1 I n t r o d u c t i o n Both c o n d u c t i v i t y and H a l l measurements were performed on; cadmium s e l e n i d e f i l m s d e p o s i t e d on barium t i t a n a t e . . F i l m p r e p a r a t i o n i s d i s c u s s e d . The f i l m c o n d u c t i v i t y m o d u l a t i o n o b t a i n e d e x p e r i m e n t a l l y i s r e l a t e d t o t h a t which was p r e d i c t e d i n Chapter 2. I n i t i a l c o n d u c t i v i t y m o d u l a t i o n measurements were made w i t h cadmium s u l p h i d e as w e l l . 5•2 Specimen P r e p a r a t i o n The barium t i t a n a t e s u b s t r a t e was etched as d e s c r i b e d i n S e c t i o n 3*4.1 and c l e a n e d by a g i t a t i n g i n t r i c h l o r e t h y l e n e , i n a l c o h o l , and i n d i s t i l l e d , d e i o n i z e d water c o n s e c u t i v e l y i n the U l t r a s o n i c c l e a n e r . A l a r g e round g o l d or aluminum c o u n t e r or s w i t c h i n g e l e c t r o d e was then d e p o s i t e d onto the u n d e r s i d e of the s u b s t r a t e . No attempt was made t o a l i g n the c r y s t a l domain s t r u c t u r e . The semiconductor f i l m was th e n d e p o s i t e d , e i t h e r as a square f o r r e s i s t a n c e measurements, or i n a f o u r l e a f c l o v e r c o n f i g u r a t i o n f o r H a l l measurements. At t h i s s t a g e , the sample was u s u a l l y s u b j e c t e d t o a 15 minute heat t r e a t m e n t i n a i r a t 300°C t o reduce the f i l m c o n d u c t i v i t y . The mechanism f o r t h i s e f f e c t i s assumed t o be oxygen a b s o r p -t i o n as d e s c r i b e d i n Chapter 4. ' Aluminum e l e c t r o d e s were then d e p o s i t e d onto the f i l m . E l e c t r i c a l measurements were performed as soon as p o s s i b l e a f t e r removal from the vacuum system. F o r the narrow e l e c t r o d e gap r e s i s t a n c e m o d u l a t i o n 53 samples, a 75u w i r e was used t o form the s o u r c e - d r a i n gap, which was 0o6mm. l o n g . 5•3 E l e c t r i c a l Measurements 5«3°1 H a l l Measurements H a l l measurements were performed w i t h cadmium s e l e n i d e f i l m s on barium t i t a n a t e i n the same manner as d e s c r i b e d i n Chapter 4. Measurements were taken w i t h the c r y s t a l i n bo t h the c + and c s t a t e s . As d e s c r i b e d i n S e c t i o n 3°4°4? s w i t c h i n g was performed by a p p l y i n g a slow ramp v o l t a g e a c r o s s the c r y s t a l , between the c o u n t e r e l e c t r o d e , c o n t a c t e d t o the copper base--of the specimen h o l d e r , and the f o u r aluminum c o n t a c t s of the H a l l specimen combined. The top s w i t c h i n g e l e c t r o d e i s formed by the f o u r aluminum c o n t a c t s and the semi-cond u c t o r f i l m . C o n d u c t i v i t y m o d u l a t i o n i n the cadmium s e l e n i d e f i l m s was observed. V a l u e s of m o b i l i t y and e l e c t r o n c o n c e n t r a t i o n i n the two s t a t e s f o r s e v e r a l samples a r e g i v e n i n Table IV. SAMPLE THICKNESS STATE MOBILITY -(cm.2/volt-secTT) ELECTRON CON-CENTRATION' (el./cm.5) #6 1850A0 ±15% 66±10% 3.0xl0 1 8±10% low 12±15% 7oOxl0 1 3±20% h i g h 13±15% 6.0xlO-L5±20% #7 1700A0 low 33±15% 3oOxl0 1 5±15% ±15% h i g h 36±15% ' . . 1.5xl0 1 4±15% TABLE IV HALL'MEASUREMENT RESULTS OF CADMIUM SELENIDE ON BARIUM TITANATE — , , - •• • • - ^ ~~ B e f o r e Heat Treatment 54 I t can be seen t h a t the m o d u l a t i o n of the e l e c t r o n con-c e n t r a t i o n i s much below t h a t expected f r o m . t h e o r y . Assuming a one f o r one compensation of the p o l a r i z i n g charge by the semi-c o n d u c t o r f i l m , o n l y 0.03% of the e l e c t r o n c o n c e n t r a t i o n change expected o c c u r s f o r sample #6 and f o r #7, 6 . 5 x l O - 4 % . P o s s i b l e reasons f o r t h i s d i s c r e p a n c y a r e d i s c u s s e d f o l l o w i n g the next s e c t i o n . M o b i l i t y changes between the two s t a t e s are n e g l i g i b l e . I f t he charge c o n c e n t r a t i o n changes were l a r g e r , m o b i l i t y changes might occur ( H a e r i n g , 1964), Decreased m o b i l i t y due t o i n c r e a s e d s c a t t e r i n g a t g r e a t e r e l e c t r o n c o n c e n t r a t i o n l e v e l s i s po s s i b l e . , or the m o b i l i t y may i n c r e a s e due t o reduced s c a t t e r i n g by f i l l e d t r a p s . However, no changes were observed d u r i n g t h e s e e x p e r i m e n t s . 5.3.2 Two-State R e s i s t o r • C o n d u c t i v i t y measurements were performed on the t w o - s t a t e r e s i s t o r w i t h the gap formed by a 75j-i w i r e s w h i c h gave a gap w i d t h of 70|i. Measurements were performed u s i n g the s i m p l e c i r c u i t of F i g u r e "5-1. F o r sample #9, 1000A 0 t h i c k , heat 1.5 v o l t s _ = + V l _ K e i t h l e y U) §?0A+ + \ J E l e c t r o m e t e r s w i t c h F i g u r e 5-1. Semiconductor F i l m C o n d u c t i v i t y Measurement 55 t r e a t e d i n a i r , the r e s i s t a n c e change betweeen the e l e c t r o d e s f o r the two p o l a r i z a t i o n s t a t e s of the f e r r o e l e c t r i c was.from 9.5xl0 6 H to- l x l O ^ n . I f a m o b i l i t y of 20cm. 2/volt-sec. i s assumed, c a l c u l a t i o n shows t h a t o n l y 0.01% of the expected charge m o d u l a t i o n o c c u r s . Table V g i v e s changes i n r e s i s t a n c e f o r s e v e r a l r e s i s t o r s of v a r y i n g magnitude. F i g u r e 5-2 shows the two s t a t e s of sample % 5. As p r e d i c t e d , the r a t i o p — i n g e n e r a l i n c r e a s e s w i t h L i n c r e a s i n g r e s i s t a n c e . I f the charge. P r were a l l compensated by f r e e c a r r i e r s , then the r a t i o would i n c r e a s e l i n e a r l y w i t h i n c r e a s i n g r e s i s t a n c e . I t i s thus n e c e s s a r y t o modify e q u a t i o n 2-17 t o i n c l u d e the e f f e c t of uncompensated'charges as shown i n e q u a t i o n 5-1: • h = ( n o ± r - r ) e ^ i - W E D (5-1)' where *y i s the amount of f r e e charge modulated by the barium t i t a n a t e , as a f r a c t i o n of the t o t a l p o l a r i z i n g charge of the c r y s t a l . There are s e v e r a l p o s s i b l e reasons f o r the d i s c r e p a n c y i n magnitude between the a c t u a l and p r e d i c t e d c o n d u c t i v i t y m o d u l a t i o n e f f e c t . These w i l l be d i s c u s s e d i n t u r n . There i s a p o s s i b i l i t y t h a t the barium t i t a n a t e c r y s t a l i s not s w i t c h i n g f u l l y . However, i n every case t h a t the s w i t c h i n g charge was measured, i t was found t o be e q u a l t o t w i c e the remanent p o l a r i z a t i o n charge, w i t h i n the a c c u r a c y of measurement. P o s s i b l y some s m a l l a r e a s do not s w i t c h , b u t the d i s c r e p a n c y i s much too l a r g e t o be caused by t h i s e f f e c t . 56 SAMPLE # THICKNESS RL0W~ .^HIGH fo CHARGE** V R L 5 1400* 4.2xl0 5 6 . 0 x l 0 5 .1 1.33 9 1000 9 . 5 x l 0 6 l x l O 9 1.2x10"5 105 14 1700 5 x l 0 8 1 . 2 x l 0 9 2 . 3 x l O " 5 24 12 1700 5 x l 0 9 3X10 1 1 2.3xlO" 6 60 TABLE V Summary of Data on Two-State R e s i s t o r H a l l Sample, no Heat Treatment Assume |i:20cm. / v o l t - s e c . F i g u r e 5-2. O p e r a t i n g C h a r a c t e r i s t i c s of Two-State R e s i s t o r Sample #5 57 A h i g h s u r f a c e s t a t e d e n s i t y a t the f e r r o e l e c t r i c - s e m i -c o n d u c t o r i n t e r f a c e would reduce the e f f e c t of the remanent p o l a r i z a t i o n r e v e r s a l . The induced e l e c t r o n s would be t r a p p e d a t the i n t e r f a c e and unable t o take p a r t i n the c o n d u c t i o n p r o c e s s . A l s o , the semiconductor i t s e l f , because of the method of p r e p a r a t i o n , c o n t a i n s a l a r g e number of t r a p s and these would a c t t o reduce the change i n charge c o n c e n t r a t i o n (Weimer, 1964) due t o p o l a r i z a t i o n r e v e r s a l . Of these p r o c e s s e s , the l a s t two mentioned a r e the most l i k e l y t o cause the reduced e f f e c t . The s u r f a c e s t a t e problem e s p e c i a l l y i s a p p l i c a b l e . The barium t i t a n a t e c r y s t a l i s exposed t o the atmosphere b e f o r e d e p o s i t i o n of the semiconductor, and the e l e c t r o s t a t i c a t t r a c t i o n between the f e r r o e l e c t r i c domains and the m o l e c u l e s i n the atmosphere .could i n c r e a s e the s u r f a c e s t a t e d e n s i t y of t r a p s g r e a t l y . A s u r f a c e s t a t e t r a p d e n s i t y 14 2 i n the o r d e r of 10 /cm. would cause the r e d u c t i o n i n the e f f e c t observed. 5°3»3 Temperature Dependence of C o n d u c t i v i t y I t i s p o s s i b l e t o express the temperature dependence of the c o n d u c t i v i t y of the semiconductor by an e q u a t i o n of the form: • O- = o-0 exp(-W^kT) ( 5 - 2 ) where ¥ Q i s the t h e r m a l a c t i v a t i o n energy, and CT i s a c o n s t a n t of the e q u a t i o n . T h i s e q u a t i o n i s based on the s i m p l e harmonic o s c i l l a t o r 58 w i t h a p e r i o d i c p o t e n t i a l w e l l of energy W^, T h i s energy ¥ Q i s the e f f e c t i v e h a r r i e r which e l e c t r o n s have t o overcome t o e n t e r the c o n d u c t i o n band. The b a r r i e r may be due t o i n t e r -c r y s t a l l i t e p o t e n t i a l v a r i a t i o n (Tucker, 1966), the Permi l e v e l , or the l o w e s t energy d i f f e r e n c e between the con-d u c t i o n band and f i l l e d t r a p l e v e l s . I t i s assumed t h a t the l a t t e r i s the dominant p r o c e s s f o r the d e v i c e b e i n g c o n s i d e r e d h e r e . C o n s i d e r the case of an n-type semiconductor, as shown i n F i g u r e 5-3? w i t h v a r i o u s t r a p p i n g l e v e l s between the c o n d u c t i o n and v a l e n c e band. At a g i v e n temperature, the F e r m i l e v e l may be as shown. Donor l e v e l s 1 and 2 are empty and donor l e v e l 3 i s p a r t i a l l y f i l l e d , . Any i n c r e a s e i n temperature w i l l cause the e l e c t r o n s i n Donor l e v e l s t o e n t e r the c o n d u c t i o n E l e c t r o n Energy (eV) C o n d u c t i o n Band E-r 1 D. F i l l e d Trap O Empty Trap Valence Band F i g u r e 5 - 3 , N-Type Semiconductor w i t h T r a p p i n g L e v e l s 59* band a t a r a t e p r o p o r t i o n e d t o exp ( E C - E D ^ . ) / k T ) U n t i l t h i s l e v e l i s exhausted, i t w i l l c o n t r i b u t e the dominant e f f e c t t o -the c o n d u c t i o n band e l e c t r o n d e n s i t y i n c r e a s e . Thus, over a s m a l l temperature range, the a c t i v a t i o n energy ¥ Q of the con-d u c t i v i t y of the sample w i l l be E Q - E - ^ , F o r low c o n d u c t i o n band e l e c t r o n c o n c e n t r a t i o n l e v e l s the v a l u e E ^ , i s l e s s t h a n when the c o n c e n t r a t i o n i s h i g h e r . Thus,the h i g h e s t energy of f i l l e d t r a p s w i l l be l o w e r than when the c o n d u c t i o n band e l e c t r o n c o n c e n t r a t i o n i s h i g h , and the semiconductor w i l l have a l a r g e r a c t i v a t i o n energy. Cadmium s e l e n i d e H a l l sample #8 was used t o t e s t the temperature dependence of c o n d u c t i v i t y . The c i r c u i t used was as shown i n F i g u r e 5-4, The temperature range of the t e s t was r e s t r i c t e d t o 10°C - 40°C i n or d e r t o reduce the e f f e c t of the change i n P which o c c u r s w i t h temperature. (Jona and S h i r a n e , pg, 116), Over the temperature range g i v e n , the v a l u e "Temperature Test Chamber i r 1 1.5 v o l t s K e i t h l e y 600A E l e c t r o m e t e r F i g u r e 5-4, Temperature Dependence of C o n d u c t i v i t y 60 of v a r i e s from 26 t o 24 ucoul./cm. , which i s n e g l i g i b l e compared t o the e x p o n e n t i a l c o n d u c t i v i t y i n c r e a s e due t o t h e t h e r m a l a c t i v a t i o n energy. A l s o , f u r t h e r h e a t i n g would a f f e c t the p r o p e r t i e s of the semiconductor due t o d i f f u s i o n of oxygen a c c e p t o r atoms i n t o the f i l m . F i g u r e 5-5 g i v e s the l o g ^ I - p v s . 7g- p l o t f o r the c o n d u c t i v i t y of sample #8 i n b o t h c + and c~ p o l a r i z a t i o n s t a t e s . The a c t i v a t i o n energy i n e l e c t r o n v o l t s i s g i v e n by e q u a t i o n 5-3' d l o g - i n I - n V^^ 2.303 k (5-3) d 7p As p r e d i c t e d , the a c t i v a t i o n energy of the semiconductor f o r the low conductance s t a t e i s g r e a t e r (0.55ev) than t h a t f o r the h i g h conductance s t a t e (0.20ev.). I n the h i g h conductance s t a t e , t h e t r a p s between 0.55ev. and 0.20ev. of the conductance band have been f i l l e d . The e l e c t r o n s induced i n t o the semi-c o n d u c t o r by the p o l a r i z a t i o n charge and w hich have not e n t e r e d i n t o the c o n d u c t i o n p r o c e s s are most l i k e l y t o be found h e r e . Assuming t h i s t o be t r u e , one can s t a t e t h a t the number of t r a p s between 0.55 and,0.20ev. i s a p p r o x i m a t e l y e q u a l to t w i c e the remanent p o l a r i z a t i o n of the f e r r o e l e c t r i c s u b s t r a t e . The e x p e r i m e n t a l p l o t i s a s t r a i g h t l i n e over t h e temperature range g i v e n , and j u s t i f i e s the t h e o r y p r e s e n t e d . L i t t l e more can be s a i d about the r e s u l t s because of the l a c k of d e t a i l e d knowledge about the v a r i o u s t r a p p i n g l e v e l s and c o n c e n t r a t i o n s i n the cadmium s e l e n i d e f i l m . 61 -6 1.0x10 I D (amps.) 4 = 0 2.0 1.0x10 -7 4.0 2.0 1.0x10 -8 ' 3.1 1 T "'"xlO 5 3,3 J i 3.5 F i g u r e 5-5. L o g - ^ l p v s . ^ P l o t of Sample #.'8 6 2 5.3.4 Stability of Device Stability of the resistance of the device in either of the two states seems to depend mainly on the switching stability of the ferroelectric substrate. The switching stability in turn is dependent on the stresses within the crystal. If the crystal substrate is well-behaved and single domain, there 1will be no back-switching of the polarization after removal of the switching pulse, and the resistance w i l l be constant. The current across the source-drain of sample #9 of Table V was recorded for a period of one hour following switching to the highly conducting state. Resistance changes were found to be negligible on the scale used (less than 5%). Sample #10 was also tested and baok switching of the crystal to the highly conductive state from the low conducting state was observed as shown in Figure 5-6. A second effect was also observed over longer periods of time. . Resistance of the. device in the high state ,.lxlO"6 A./div. Time 10 see./div. Figure 5 -6 . Back-Switching- of fiesistaiioe Sample #10 63 decreased by a factor of 3 over three days. Also, the R^ /R^  ratio of resistance was somewhat reduced. This can be attributed to the diffusion of further oxygen impurity atoms into the film,which act as acceptors for the n-type cadmium selenide. Encapsulation of the devices in an i n e r t atmosphere would rimovt t h i s e f f e c t . 5.3.5 Photooenduotivltv Measurements Ehotoeoaductivity of the cadmium s e l t a i d e f i l m s was measured i a e r d t r t© demonstrate trappia§ e f f e e t s and te obtain an idea ©f the range ©f trap time ©©nstaats prest a t i n the m a t e r i a l . Sample #9,de§erihed i n £ahl§ V f was tested f o r response t© white ii§ht. She ligat»t©-da?k ©©aduetivity rati© was appre&imately 12-1 f o r the l i g h t i a t e a i i t y used. Figure 5=7 sh©ws the- ©©nduetivity, at measured ©n the Meseley X=¥ r§e©rder, at a r e s u l t of the switehiag of the l i f h t ©a and of f while the two st a t e resist©? i s i a the h i f h eeaduotaaoe s t a t e . She ©urreat l e v e l i s s t i l l ehaagiaf sl e w l y 40 see. 'ifclO" BA./div. Sifflt 10 §e©,/div. f i f u r t 5-7i ?hfcte.endueti"vity Measuremeat 64 ; a f t e r s w i t c h i n g the l i g h t . A l s o , the change i s not a s i n g l e e x p o n e n t i a l . A spectrum of t r a p p i n g time c o n s t a n t s e x i s t s f o r the f i l m , e x t e n d i n g from microseconds up t o 10 seconds or more. T h i s shows t h a t t r a p p i n g l e v e l s e x i s t between the c o n d u c t i o n and v a l e n c e bands over a l a r g e range of e n e r g i e s . 5.4 Cadmium S u l p h i d e Device ,A t w o - s t a t e r e s i s t o r was a l s o c o n s t r u c t e d u s i n g cadmium s u l p h i d e as the semiconductor. The f i l m was d e p o s i t e d -5 onto an unheated s u b s t r a t e a t a p r e s s u r e of 1x10 t o r r . F i l m t h i c k n e s s was c a l c u l a t e d from the c r y s t a l t h i c k n e s s m o n i t o r t o be 3000A 0. The f i l m was t h e n a i r - b a k e d a t 300°C f o r one hour and the aluminum e l e c t r o d e s added. F i g u r e 5-8 shows the r e s i s t a n c e of the d e v i c e f o r the two s t a t e s of the f e r r o e l e c t r i c s u b s t r a t e . F u r t h e r attempts t o o b t a i n cadmium s u l p h i d e f i l m s on barium t i t a n a t e were plagued by i n c o n s i s t e n c i e s . D i f f i c u l t i e s were e x p e r i e n c e d i n g e t t i n g the cadmium s u l p h i d e t o s t i c k t o the substrate.,. A l s o , f i l m s which were o b t a i n e d were non-uniform, i n c o l o u r and i n t h i c k n e s s . T h e r e f o r e , s t u d i e s o f cadmium s u l p h i d e on barium t i t a n a t e were d i s c o n t i n u e d . 2 V . / d i v . V-F i g u r e 5-8. Two-State R e s i s t o r of CdS 65 60 THE TWO-STATE THIN-EILM TRANSISTOR 6.1 F a b r i c a t i o n Techniques A t h i n - f i l m f i e l d e f f e c t t r a n s i s t o r was f a b r i c a t e d by e v a p o r a t i n g an i n s u l a t i n g l a y e r of s i l i c o n o x i d e 4000A 0 t h i c k onto the 75 p.-wide s o u r c e - d r a i n gap of sample #14 o f Table V, f o l l o w e d by an aluminum gate e l e c t r o d e 200 p-wide over the s o u r c e - d r a i n gap. The completed d e v i c e i s shown i n F i g u r e 6-1. The s i l i c o n o x i d e l a y e r was evaporated from p e l l e t s of s i l i c o n monoxide h e l d i n a t a n t a l u m boat.,, E v a p o r a t i o n p r e s s u r e was 1x10 t o r r . The i n s u l a t i n g f i l m was evaporated i n t h r e e l a y e r s , each 1000 A 0 t h i c k , a t a r a t e of 8 A°/sec. w i t h 1 minute between e v a p o r a t i o n s . The aluminum e v a p o r a t i o n f o r the gate e l e c t r o d e was then performed. A l l e v a p o r a t i o n s of d i f f e r e n t m a t e r i a l s ' were made on s u c c e s s i v e vacuum c y c l e s , s i n c e masking t e c h n i q u e s n e c e s s i t a t e d removal of the sample from the vacuum system. A second d e v i c e , #15, was f a b r i c a t e d i n the same manner, except t h a t the semiconductor f i l m was o n l y 900 A 0 t h i c k , and the a i r bake of the f i l m was f o r 30 minutes a t 400°C. T h i s was done t o o b t a i n a d e v i c e w i t h l o w e r f i l m c o n d u c t i v i t y , so t h a t the m o d u l a t i o n due t o the f e r r o e l e c t r i c s u b s t r a t e would be g r e a t e r . 6.2 E l e c t r i c a l Measurements A l l measurements r e p o r t e d here were c a r r i e d out i n the dark. E l e c t r i c a l c o n t a c t s were made t o the d e v i c e s by u s i n g t h r e e of the p r e s s u r e c o n t a c t s of the H a l l specimen h o l d e r of F i g u r e 4-3• S w i t c h i n g of the f e r r o e l e c t r i c was c a r r i e d " S e l e c t Grade" Kerne t Company, C l e v e l a n d , Ohio. 66 F i g u r e 6-1. Cadmium S e l e n i d e Two-State TFT out by a p p l y i n g a slow v o l t a g e ramp between the g o l d c o u n t e r -e l e c t r o d e and the source of the d e v i c e , which was grounded. The V-p- t r a n s i s t o r c h a r a c t e r i s t i c s of sample #14 were d i s p l a y e d on a T e k t r o n i x 575 curve t r a c e r w i t h a 1000 H r e s i s t o r between the e m i t t e r and base t o t r a n s f o r m the base c u r r e n t s t e p s of the curve t r a c e r t o the r e q u i r e d gate v o l t a g e s t e p s . The o p e r a t i n g c h a r a c t e r i s t i c s of the d e v i c e i n both p o s i t i v e and n e g a t i v e gate b i a s c o n d i t i o n s and f o r both s t a t e s of the f e r r o e l e c t r i c , c + and c , are shown i n F i g u r e 6-2(a), ( b ) , and F i g u r e 6-3(a) and ( b ) . To check the r e l a t i v e e f f e c t of each of the gate and the f e r r o e l e c t r i c s u b s t r a t e , the amount of gate v o l t a g e n e c e s s a r y t o change the c u r r e n t l e v e l of the d e v i c e by an amount eq u a l t o t h a t caused by the s u b s t r a t e s w i t c h i n g was measured f o r 67 .5V./div. V D (a) c + S t a t e .5V./div. V D (b) c~ S t a t e F i g u r e 6-2. ^-V-p C h a r a c t e r i s t i c s of Sample #14 i n Two S t a t e s w i t h N e g a t i v e Gate B i a s 68 V „ = 0 • 5 V./div. V D (b) c" S t a t e F i g u r e 6 - 3 . - ^ D -^) C h a r a c t e r i s t i c s o f Sample #14 i n Two S t a t e s w i t h P o s i t i v e Gate B i a s 69 sample #14. The d e v i c e c h a r a c t e r i s t i c s f a i l t o s a t u r a t e as the d r a i n v o l t a g e ' . i n c r e a s e s . A l s o , the z e r o gate b i a s c u r r e n t l e v e l changes as the d e v i c e i s operated i n the p o s i t i v e or n e g a t i v e gate b i a s c o n d i t i o n . Reasons f o r t h i s b e h a v i o u r w i l l be d i s c u s s e d l a t e r . The t r a n s c o n d u c t a n c e of the d e v i c e , c a l c u l a t e d from F i g u r e 6-2(a), i s 4 (imhos a t V^ = 4 v o l t s and v a r i e s l i n e a r l y w i t h V-p s i n c e the d e v i c e does not s a t u r a t e . Input c a p a c i t a n c e , measured a t l K e . / s e c . was 38 p f . D.C i n p u t r e s i s t a n c e was measured t o be g r e a t e r than 10"^f\ . The e f f e c t i v e m o b i l i t y f o r the TFT below s a t u r a t i o n i s g i v e n by e q u a t i o n 6-1: P-3m n C Vj, • g D (6-1) M o b i l i t y f o r the d e v i c e under c o n s i d e r a t i o n was c a l c u l a t e d t o be a p p r o x i m a t e l y 7 cm. / v o l t - s e c . A l s o , the g a i n bandwidth pr o d u c t of the p r e s e n t d e v i c e g i v e n by GBW = i s 16 Kc. a t V D = 4 v o l t s . C h a r a c t e r i s t i c s of sample 15 "were o b t a i n e d u s i n g the measuring c i r c u i t shown i n F i g u r e 6-4, s i n c e the c u r r e n t l e v e l rjvpm , 1 • s w i t c h +ve V o l t a g e Ramp V F i g u r e 6-4. ^ - L p C h a r a c t e r i s t i c Measuring C i r c u i t 70 of the d e v i c e i s too low t o be d e t e c t e d , on the curve t r a c e r . I n F i g u r e 6-5(a) are the I-^- V-^  c h a r a c t e r i s t i c s of the d e v i c e w i t h the f e r r o e l e c t r i c s u b s t r a t e i n the c + s t a t e . I n the c s t a t e , the d e v i c e c h a r a c t e r i s t i c s a r e c o m p l e t e l y c ut o f f . F i g u r e 6-5(b) shows the d i f f e r e n c e i n the z e r o gate v o l t a g e c u r r e n t l e v e l of the d e v i c e i n the two s t a t e s . T h i s d e v i c e does s a t u r a t e b e t t e r , than sample #14. c The g m of the d e v i c e i n the c + s t a t e i s 6 umhos a t = 10 v o l t s . D.C. i n p u t r e s i s t a n c e of sample #15 was measured t o be g r e a t e r than l O ^ H a l s o . 6.3 E f f e c t of S i l i c o n Oxide L a y e r The i n s u l a t i n g l a y e r as evaporated has a c h e m i c a l f o r m u l a , SiO , where l < x < 2 , because of a b s o r p t i o n of oxygen i n t o the m a t e r i a l from the vacuum system atmosphere. There i s a n et p o s i t i v e charge i n the i n s u l a t i n g l a y e r , caused by oxygen v a c a n c i e s . T h i s charge i s compensated by an e l e c t r o n accumula-t i o n l a y e r a t the semiconductor s u r f a c e , thus i n c r e a s i n g the o v e r a l l conductance of the f i l m . R e s i s t a n c e between source and d r a i n e l e c t r o d e s f o r sample #14 decreased t o 3 x l 0 4 n and 4 . 5 x l 0 4 n f o r the two s t a t e s of the f e r r o e l e c t r i c a f t e r the i n s u l a t i n g - l a y e r was added. T h i s i s a l a r g e d e c rease and must be a l l o w e d f o r i n the f a b r i c a t i o n of the d e v i c e . The c o n d u c t i v i t y b e f o r e e v a p o r a t i o n of the f i l m must be made v e r y s m a l l by heat t r e a t i n g or o t h e r means so t h a t the f i n a l e l e c t r o n c o n c e n t r a t i o n w i l l be s m a l l enough t o make e f f e c t i v e gate m o d u l a t i o n p o s s i b l e . 6.4 D i s c u s s i o n of R e s u l t s The change i n gate v o l t a g e n e c e s s a r y t o change 1-^  by an amount e q u a l t o t h a t caused by the f e r r o e l e c t r i c s w i t c h i n g was 71 XD 50ua / d i v c_ V 0 0 4 8 D 12 (b) D i f f e r e n c e i n Zero Gate B i a s C u r r e n t L e v e l s Between c+ and c~ P o l a r i z a t i o n S t a t e s F i g u r e 6-5. Output C h a r a c t e r i s t i c s of Sample #15 72 used t o c a l c u l a t e P e f f . , the amount of p o l a r i z a t i o n charge e f f e c t i v e i n i n f l u e n c i n g the d e v i c e c h a r a c t e r i s t i c s . At Y-^ = lOv the r e q u i r e d gate v o l t a g e was measured a t 2.5 v o l t s . P r e f f . 11 2 was c a l c u l a t e d and found t o be 1.8 x 10 el./cm. , a p p r o x i m a t e l y 0.1% of P r a c t u a l . T h i s amount of e f f e c t i v e change i s con-s i s t e n t w i t h t h a t c a l c u l a t e d f o r the t w o - s t a t e r e s i s t o r and can be e x p l a i n e d by the e f f e c t of s u r f a c e s t a t e s a t the barium t i t a n a t e semiconductor i n t e r f a c e and the t r a p d e n s i t y i n the semiconductor as w e l l . . The s u r f a c e s t a t e d e n s i t y a t the semiconductor i n s u l a t o r i n t e r f a c e was c a l c u l a t e d a t about 1x10 '/cm. , w h i l e t h a t a t the barium t i t a n a t e semiconductor i n t e r f a c e i s i n the o r d e r of 1 0 1 4 / c m . 2 The change i n V m under p o s i t i v e or n e g a t i v e gate b i a s c o n d i t i o n s may be a t t r i b u t e d t o charge m i g r a t i o n i n the i n s u l a t i n g l a y e r . I m p u r i t y i o n s caused by a d s o r p t i o n of water vapour and o t h e r i m p u r i t i e s i n t o the i n s u l a t i n g l a y e r may become mob i l e under a p p l i c a t i o n of an e l e c t r i c f i e l d a c r o s s the i n s u l a t i n g l a y e r . A p o s i t i v e i o n under the e f f e c t of a p o s i t i v e gate v o l t a g e w i l l , i f m o b i l e , m i g r a t e t o the semiconductor t o c o u n t e r a c t t h i s i n c r e a s e d p o s i t i v e charge a t the i n t e r f a c e . I n c r e a s e d c o n d u c t i v i t y w i l l r e s u l t i n the semiconductor f i l m . The h y g r o s c o p i c i t y of the S i O x l a y e r c o n t r i b u t e s t o t h i s e f f e c t . I t i s p o s s i b l e t o reduce the amount of d r i f t of V m by k e e p i n g the d e v i c e i n a vacuum or i n e r t atmosphere. A rough c a l -c u l a t i o n , which accounts f o r the e f f e c t of mobile i o n s i n the i n s u l a t i n g l a y e r , y i e l d s an i o n i c charge c o n c e n t r a t i o n of 73 18 2 1x10 el./cm. , which i s comparable w i t h the found by Salama, 1966, due t o water vapour a b s o r p t i o n . The s o u r c e - d r a i n c h a r a c t e r i s t i c s of sample #14 f a i l e d t o s a t u r a t e , p o s s i b l y because a p a r a l l e l conductance p a t h e x i s t e d i n the c e n t r e of the semiconductor l a y e r , as w e l l as the modulated s u r f a c e l a y e r s . T h i s unmodulated p a r a l l e l conductance p a t h , h a v i n g a r e l a t i v e l y l a r g e c o n c e n t r a t i o n of c a r r i e r s and being unaffected by the g a t e , d i s p l a y s a l i n e a r I-V c h a r a c t e r i s t i c , thus the o v e r a l l c h a r a c t e r i s t i c s of the d e v i c e do not s a t u r a t e . R e d u c t i o n of f i l m t h i c k n e s s , and/or : longer heat treatment of the f i l m would counteract t h i s e f f e c t . The response of sample #14 to a ± 2V step on the gate electrode i s shown i n F i g u r e 6-6. The slow r i s e t o the steady st a t e value' i s s i m i l a r t o t h a t found f o r p h o t o c o n d u c t i v i t y measurements i n F i g u r e 5-7, and can be e x p l a i n e d by the m i g r a t i o n of m obile i o n s i n the i n s u l a t i n g l a y e r as d i s c u s s e d p r e v i o u s l y . I n the case of sample #15, the semiconductor f i l m of w hich was t h i n n e r and was s u b j e c t e d t o a l o n g e r heat treatment t h a n t h a t of sample #14, I — 4 ' 10 s e c . / d i v . Time F i g u r e 6-6. Response of Sample #14 t o Square-Wave Input on Gate 74 the d e v i c e c h a r a c t e r i s t i c s a r e c o m p l e t e l y t u r n e d o f f w h i l e the c r y s t a l i s i n the c s t a t e , No measurable t r a n s c o n d u c t a n c e was o b t a i n e d . I n the c + s t a t e , the c h a r a c t e r i s t i c s tended t o s a t u r a t e and a g of 5 [amhos was. measured i n the s a t u r a t e d s e c t i o n of the c h a r a c t e r i s t i c s . As expected, f o r t h i n n e r f i l m s w i t h a l o w e r charge c o n c e n t r a t i o n , the e f f e c t of the gate and f e r r o e l e c t r i c s u b s t r a t e i n c r e a s e s . 6.5 Comparison of Two D e v i c e s The t w o - s t a t e r e s i s t o r o p e r a t e s as a t h r e e t e r m i n a l d e v i c e w i t h the f e r r o e l e c t r i c c r y s t a l v a r y i n g the s l o p e of a l i n e a r l y i n c r e a s i n g I ^ - V-^  c h a r a c t e r i s t i c . I n o r d e r t o o b t a i n a c e r t a i n c u r r e n t l e v e l , the d r a i n v o l t a g e must be a t a s p e c i -f i e d l e v e l as w e l l . F o r the case of the t w o - s t a t e TFT, w h i c h o p e r a t e s as a f o u r t e r m i n a l d e v i c e , and assuming s a t u r a t i n g c h a r a c t e r i s t i c s , a c o n s t a n t c u r r e n t l e v e l w i l l o c cur over a range of s o u r c e -d r a i n v o l t a g e s g r e a t e r t h a n Vp. The gate e l e c t r o d e can a l s o v a r y the c u r r e n t l e v e l i n the s a t u r a t e d r e g i o n of the c h a r a c t e r -i s t i c s . I t would be p o s s i b l e t o "gate" a s i g n a l a p p l i e d t o the gate of the d e v i c e , by s w i t c h i n g the f e r r o e l e c t r i c c r y s t a l back and f o r t h between i t s two s t a t e s . The a c t u a l c u r r e n t l e v e l s o b t a i n e d i n the two- -states are dependent on the c o n d i t i o n s of d e p o s i t i o n , t h i c k n e s s and p o s t - d e p o s i t i o n a l t r e a t m e n t of the s e m i c o n d u c t i n g f i l m . 75 7. CONCLUSIONS A cadmium s e l e n i d e t h i n - f i l m t r a n s i s t o r was d e p o s i t e d onto a barium t i t a n a t e s u b s t r a t e t o form an a c t i v e d e v i c e w i t h two s e t s of c h a r a c t e r i s t i c s . dependent on the. p o l a r i z a t i o n ' s t a t e of the f e r r o e l e c t r i c c r y s t a l . Cadmium s u l p h i d e and cadmium s e l e n i d e f i l m s d e p o s i t e d on barium t i t a n a t e were a l s o used t o produce t w o - s t a t e r e s i s t o r s , the r e s i s t a n c e of which was a f u n c t i o n of the f e r r o e l e c t r i c s u b s t r a t e p o l a r i z a t i o n s t a t e . The e x p e r i m e n t a l r e s u l t s were found t o agree q u a l i t a t i v e l y w i t h the t h e o r y d e r i v e d f o r both t y p e s of d e v i c e s . However, the amount of m o d u l a t i o n of the e l e c t r o n c o n c e n t r a t i o n i n the n-type semiconductor l a y e r was found t o be v e r y much l e s s than t h a t p r e d i c t e d by t h e o r y . A l a r g e s u r f a c e s t a t e d e n s i t y a t the barium t i t a n a t e - s e m i c o n d u c t o r i n t e r f a c e was p o s t u l a t e d as the r e a s o n f o r the d e v i a t i o n from t h e o r y . The cadmium s e l e n i d e s e m i c o n d u c t i n g f i l m s , as p r e p a r e d i n t h i s work, were found t o have a l a r g e number of slow t r a p p i n g c e n t r e s , as demonstrated by the p h o t o c o n d u c t i v i t y measurements. The s t a b i l i t y of the r e s i s t o r s was found t o be m a i n l y dependent on the a b i l i t y of the f e r r o e l e c t r i c s u b s t r a t e t o m a i n t a i n i t s e l f i n a s i n g l e domain c o n f i g u r a t i o n w i t h o u t b a c k s w i t c h i n g . An a d d i t i o n a l slow decrease i n o v e r a l l f i l m c o n d u c t i v i t y was assumed due t o a b s o r p t i o n of oxygen atoms from the a i r . The s t a b i l i t y of the TFT was a l s o dependent on the f e r r o e l e c t r i c s u b s t r a t e . A b s o r p t i o n of i m p u r i t y i o n s i n t o the 76 s i l i c o n o x i d e i n s u l a t i n g l a y e r from the a i r , e s p e c i a l l y water vapour, causes a decay i n d e v i c e c h a r a c t e r i s t i c s . O b j e c t i v e s of any f u r t h e r work on these d e v i c e s s h o u l d i n c l u d e improvement of the semiconductor f i l m i n terms of i n c r e a s e i n m o b i l i t y and decrease i n the d e n s i t y of t r a p s , as w e l l as an improvement i n the s t a b i l i t y of the i n s u l a t i n g l a y e r of the t r a n s i s t o r . Attempts s h o u l d a l s o be made t o reduce the s u r f a c e s t a t e d e n s i t y a t the f e r r o e l e c t r i c - s e m i c o n d u c t o r i n t e r f a c e . 77 APPENDIX I DEVELOPMENT OF MILLER-WEINREICH MODEL OF 180° DOMAIN WALL MOTION I t i s assumed t h a t n u c l e a t i o n of new a n t i p a r a l l e l domains oc c u r s o n l y a t 180° domain w a l l s , and i s the s o l e cause of domain w a l l movement. T h i s i s the dominant e f f e c t i n p o l a r i z a t i o n r e v e r s a l . Assume n u c l e a t i o n of a t r i a n g u l a r , a n t i p a r a l l e l s t e p a t a 180° domain w a l l , as shown i n F i g u r e A - l , inhere c i s the s t e p w i d t h and 1 » d. The energy change which occurs f o l l o w i n g the f o r m a t i o n of a n u c l e u s of volume V and domain w a l l a r e a A. >, i s g i v e n by e q u a t i o n A-I-ls P o l a r i z a t i o n D i r e c t i o n F i g u r e A I - 1 , N u c l e a t i o n of A n t i p a r a l l e l Domain 78 where 0"w i s the w a l l energy per u n i t a r e a , P i s the remanent p o l a r i z a t i o n of barium t i t a n a t e , i s the d e p o l a r i z i n g energy, and E i s the a p p l i e d f i e l d s t r e n g t h . C a l c u l a t i o n of by M i l l e r and W e i n r e i c h y i e l d e d e q u a t i o n A I - 2 a f t e r s u i t a b l e t r a n s f o r m a t i o n s t o compensate f o r d i e l e c t r i c a n i s o t r o p y i n barium t i t a n a t e were made: 2 2 U, = 8 P 2 — f - I n (AI-2) d r £ 1 . eb 1 1 1 1 i J a where e i s the d i e l e c t r i c c o n s t a n t p e r p e n d i c u l a r t o P , a * " r ' and b i s the l a t t i c e c o n s t a n t of the c r y s t a l i n the c d i r e c t i o n . The t o t a l energy change f o l l o w i n g n u c l e a t i o n a t the w a l l i s , from e q u a t i o n s AI-1 and. A I - 2 : l / 2 2 2 AU - -2 P E a l e + 2 o - c ( a 2 + l 2 ) + 8 P 2 ( ^ - f ^ ) l n r w r e 1 2 a eb a ( A I - 3 ) -= 0, and • = 0 determine the a 0.1 c r i t i c a l n u c l e u s dimensions a and 1 , as w e l l as the- c r i t i c a l a c t i v a t i o n energy U f o r n u c l e a t i o n t o o c c u r . The e x p r e s s i o n s i m p l i f i e s t o form e q u a t i o n A l — 4 i f the boundary c o n d i t i o n s are applied? .higher, order, terms: are :.• n e g l e c t e d , and. a . n u c l e a t i o n - s t e p one l a t t i c e , c o n s t a n t wide ..; 79 i s assumed: 1/2 3/2 3/T P r E 1 / 2 ^ 1/2 w i t h a = 2/3-p ^ , and 1 = _ w _ j - ^ — — , where 4 P r b 2a The n u c l e a t i o n r a t e i s p r o p o r t i o n a l t o e x p ( - p j O , Thus, assuming t h a t s i d e w i s e w a l l motion i s caused o n l y by n u c l e a t i o n s a t the boundary, the w a l l v e l o c i t y may be e x pressed by e q u a t i o n A I - 5 : 1/2 3/2 T T * -8b cr cr v d = K v exp (- j^ ) = K v exp( ? w ) = Ky exp(-a/E) 3 y i " P r E kT (AI-5) The t h e o r e t i c a l a c t i v a t i o n f i e l d ,a^. i s g i v e n by e q u a t i o n AI-6: -d I n . v 1 d U / . T r\ t - d 1/E - kT d l / E l A ± - b ; E q u a t i o n A-5 y i e l d s the f i e l d dependence of v e l o c i t y r e q u i r e d t o f i t e x p e r i m e n t a l r e s u l t s f o r low f i e l d s t r e n g t h s ( M i l l e r , 1958, 1959), t h a t i s , v CX exp -cc/E , A l s o , sub-s t i t u t i o n of e x p e r i m e n t a l l y determined v a l u e s of the p a r a -meters i n e q u a t i o n A I - 5 y i e l d s a 180° w a l l energy of 0*42 i ergs/cm. , which i s i n agreement w i t h p r e v i o u s l y determined 80 v a l u e s . F u r t h e r Development of the Model I f n u c l e a t i o n s t e p s nb w i t h n > 1 are a l l o w e d i n the model, t h e n f o r s m a l l n, e q u a t i o n A I - 7 h o l d s : U = n U x (AI-7) where i s the a c t i v a t i o n energy f o r a s t e p one u n i t c e l l wide, The e x p r e s s i o n f o r w a l l v e l o c i t y now becomes. * 3/2 * 3/2 * v d = K y exp(-U 1/kT) + 2exp(-2 I^/kT) + 3exp(-3 U-j/kT^ ... (AI-8) o r , i n c l o s e d form: 3/2 r vd:= VQJJ n exp(-n <x/E) (AI - 9 . ) S t a d l e r and Zachmandis, 1963 summed t h i s s e r i e s f o r various> v a l u e s of E, assuming a = 4 Kv./cm., as found by M i l l e r and W e i n r e i c L , and showed t h a t f o r low f i e l d s t r e n g t h s , ( < 2 Kv./cm.) e q u a t i o n AI-5 was v a l i d . At h i g h e r f i e l d s t r e n g t h s , m u l t i p l e s t e p w i d t h s are more l i k e l y , and the s e r i e s changes. W a l l v e l o c i t y becomes p r o p o r t i o n a l t o E , w i t h x v a r y i n g from 1.45 t o 1.33 as E v a r i e s between 3 Kv./cm. andQO • 81 . APPENDIX I I MEASUREMENT OF FILM THICKNESS The t h i c k n e s s of the semiconductor and o t h e r f i l m s d e p o s i t e d i n the Veeco vacuum system i s determined from measurements on a q u a r t z c r y s t a l o s c i l l a t o r ? the f r e q u e n c y of w h i c h v a r i e s ^ p r o p o r t i o n a l l y - t o the weight of m a t e r i a l d e p o s i t e d on i t , w h ich i s i n t u r n p r o p o r t i o n a l t o the t h i c k n e s s o f the m a t e r i a l . The t h i c k n e s s i s g i v e n e m p i r i c a l l y by t h e r e l a t i o n : ; D ^ '.. t h i c k n e s s of m a t e r i a l . = '.61 Af = ( Af, \ ' <x ( A I I - 1 ) where D i s the d e n s i t y of the m a t e r i a l . F o r cadmium s e l e n i d e , ^= .284 and f o r cadmium s u l p h i d e , J ~ .342. The r e l a t i o n has been found t o be a c c u r a t e f o r -aluminum f i l m s , ( C h a u r a s i a , 1964). E r r o r s do a r i s e i n the case of semiconductor f i l m s however, e s p e c i a l l y when the s u b s t r a t e i s heated. The s t i c k i n g c o e f f i c i e n t of the semi-c o n d u c t o r may be d i f f e r e n t on c o l d and heated s u r f a c e s , as w e l l as on d i f f e r e n t types o f s u r f a c e s , such as the q u a r t z c r y s t a l and' the barium t i t a n a t e s u b s t r a t e . The t h i c k n e s s of s e v e r a l f i l m s of cadmium s e l e n i d e d e p o s i t e d onto an unheated barium t i t a n a t e c r y s t a l was measured u s i n g the c r y s t a l o s c i l l a t o r , and checked u s i n g a S l o a n Angsrommeter, which has a r e s o l u t i o n of + 50 A 0. The t h i c k -i ness measured by the Angstrommeter was found t o be w i t h i n 200 A 0 of t h a t determined by the c r y s t a l o s c i l l a t o r f o r f i l m s 1000-2500 A 0 t h i c k . REFERENCES 82 Anderson, J.R., 1952, " F e r r o e l e c t r i c M a t e r i a l s as Storage Elements f o r D i g i t a l Computers", Trans. Am. I n s . E l e c . E n g r s . , V o l . 71, P t . I , pg. 395-401, 1952. Anderson, J.R., 1956, "A New Type of F e r r o e l e c t r i c S h i f t R e g i s t e r " , Trans. I n s , Radio E n g r s . , E.C.-5, No, 4, pg. 184-191, 1956. A y e r s , S., 1957, B r i t i s h P r o v i s i o n a l P a t e n t S p e c i f i c a t i o n No. 15645, 1957. Boer, K.W., E s b i t t , A.S., and Kaufman, W.M., 1966, "Evaporated and R e c r y s t a l l i z e d Cd S L a y e r s " , J o u r . A p p l , Phys,, V o l . 37, No. 7, pg, 2664-2678, 1966, Borkan, H., and Weimer, P.K., 1966, "An A n a l y s i s of the C h a r a c t e r i s t i c s of I n s u l a t e d Gate T h i n - F i l m T r a n s i s t o r s " , R.C.A. Rev.. V o l . 24, pg, 153-165, 1963. Bube, R.H., " P h o t o c o n d u c t i v i t y of S o l i d s " , J . W i l e y and Sons I n c . , New York, I960. Campbell, D.S., 1957, "Barium T i t a n a t e and I t s Use as a Memory S t o r e " , J o u r . B r i t . I.R.E., V o l . 17, pg. 385-395, 1957. C h a u r a s i a , H.K., 1964, " S t u d i e s on T h i n F i l m s of G o l d " , M.A.Sc. T h e s i s , U n i v e r s i t y of B r i t i s h Columbia, 1964. Chynoweth, A.G., I960, U.S. P a t e n t No. .2-,92.6».336 i s s u e d i n I960. Dekker, A . J . , " S o l i d S t a t e P h y s i c s " , P r e n t i c e - H a l l I n c . , New J e r s e y , 1963» D e v o n s h i r e , A.F., 1949, "Theory of Barium T i t a n a t e " , P t . 1, P h i l . Mag. V o l . 40, pg. 1040-1063, 1949. D e v o n s h i r e , A.F., 1951, "Theory of Barium T i t a n a t e " , P t , 1, P h i l . Mag.. V o l . 42, pg. 1065-1079, 1951. G o d e l f r o y , L.R., 1952, Diplome d'Etildes S u p e r i e u r e s , F a c u l t y des S c i e n c e s , P a r i s , 1952, H a e r i n g , R.R., 1964, ''Theory of T h i n F i l m T r a n s i s t o r O p e r a t i o n " , S o l i d S t a t e E l e c t r o n i c s . V o l . 7, pg.'. 31-38, 1964. H e i r , 0., 1935, B r i t i s h P a t e n t No. 439457 g r a n t e d 1935. Heiman, F.P., and W a r f i e l d , G.Si 1965, "The E f f e c t of Oxide Traps on the MOS Capacitance 1", I.E.E.E. Trans, on E l e c t r o n  D f i v i ^ . q . V o l . PGED-12, pg. 167-173 9 1965, As quoted i n " P r o g r e s s i n Semiconductors", V o l , 1, New York, Wiley," New York, 1956, pg. 216. 83 Heyman, P.M., and Heilmeier, G.H., 1966, "The F e r r o e l e c t r i c ..Field E f f e c t Device", Proc. I.E.E.E.. V o l . 5A, pg. 8,42-848., 1966.. Jona, F., and Shirane, G., " F e r r o e l e c t r i c C r y s t a l s " , Macmillan Co., New York, 1962. L i l i e n f e l d , J . E * . , U.S. Patent No. 1,900,018 issued i n 1933. l i t t l e , E.A., 1955,"Dynamic Behaviour of Domain Walls i n Barium T i t a n a t e " , Phys. Review. V o l . 98, pg. 978-986, 1955. Many, A., Go l d s t e i n , Y. and Grover, N.B., "Semiconductor Sur-faces" . John Wiley and Sons Inc., New York, 1965. : Megaw, H.D., " F e r r o e l e c t r i c i t y i n C r y s t a l s " . Methuen and Co. Lt d . , 1957. Merz, W.J., 1953, "Double Hy s t e r e s i s Loop of BaTiO,, P h y s . Review. V o l . 91, pg. 513-517, 1953- ^ Merz, W.J., 1954, "Domain Formation and Domain Wall Motions", Phys. Review. V o l . 95, pg.'690-698, 1954-M i l l e r , R.C., and Weinreich, G., I960, "Motion of 180° Domain Walls i n Barium Titanate", Phys. Review. Vol.• 117, pg. 1460- ! 1470, I960. M o l l , J.L., and T a r u i , Y., 1963, "A New S o l i d State Memory R e s i s t o r " , I.E.E.E. Trans, on E l e c t r o n Devices. V o l . ED-10, pg. 338-359, 1963. Remeika, J.P., 1954, "A Method of Growing Barium Titanate Single C r y s t a l s " , Jour. Am. Chem. S o c . V o l . 76, pg. 940-941, 1954. Rose, A., "Concepts i n Photoconductivity and A l l i e d Problems". John Wiley and Sons Inc., New York, 1963. Salama, C.A.T., 1966, "Evaporated S i l i c o n Thin-Film T r a n s i s t o r s " . Ph.D. Thesis, U n i v e r s i t y of B r i t i s h Columbia, 1966. Sawyer, C.B., and Tower, C.H., 1930, "Rochelle S a l t as a D i e l e c t r i c " , Phys. Review. V o l . 35, pg. 269-273, 1930. Shockley, W., and Pearson, G.L., 1948, "Modulation of Con-ductance of Thin Films of Semiconductors by Surface Charges", Phys. Review. V o l . 74, pg. 232-233, 1948. St a d l e r , H.L., 1958, " F e r r o e l e c t r i c Switching of BaTiO C r y s t a l s at High Voltages", Jour. Appl. Physics. V o l . 29, pg< 1485-1487, 1958. St a d l e r , H.L., 1962, "Thickness Dependence of Ba TiO* Switching Time", Jour. A P P I . Phvsics. V o l . 33, pg. 3487-3490, 1962. 84 Stadler, H.L., and Zachmandls, P.J., 1964. "Nucleation and Growth of Ferroelectric Domains i n BaTiO^ at Fields From 2-450 kV.-/cm.,", Jour. Appl. Physics. Vol. 34, pg. 3255-3260, 1964. Stadler, H.L., 1965, "Changing Properties of Metals by Ferro-e l e c t r i c P o l a r i z a t i o n Charging"/ Phys. Review Letters. Vol. 14, pg. 978-981, 1965. r ~ Tamm, I.E., 1933, "A Possible Binding of the Electrons of a Crystal Surface", Jour. Exp'tl. Theoretical Phys. (U.S.S.R.). Vol. 5, pg. 34-35, 1933. Tucker, T.W., 1966, "The E l e c t r i c a l Properties of Evaporated Silicon.Films". M.&.Sc. Thesis, University of B r i t i s h Columbia, 1966. Von Hippel, A., "High D i e l e c t r i c Constant Ceramics", Ind. Eng. Chem* Vol. 38, 1946, pg. 1097-2009, 1946. Van der Pauw, L.J., 1958, "A Method of Measuring Specific R e s i s t i v i t y and H a l l Effect of Discs of Arbitrary Shape", P h i l i p s Res. Rep.. Vol. 13, pg. 1-9, 1958. Weimer, P.K., 1962, "The TPT A New Thin Film Transistor", Proc. I.R.E.. Vol. 50, pg. 1462-1467, 1962. Weimer, P.K., 1964, "The Insulated Gate Thin-Film Transistor", Physics of Thin Films. "Vol. 2, Academic Press Inc. >;vNew'" York, 1964, pg. 147-190. Wul, B., and Goldman, I., 1946, " D i e l e c t r i c Hysteresis i n Barium Titanate"Oompt., Rend. Aca. Science. U.R.S.S.. Vol. 51, pg. 21-23, 1946.- : Yan, G., 1964, ."BaTiO^, A Memory Store?", U.B.C. Engineer. Vol. 5, pg. 14-17, 1965. Zuleeg, R., and Weider, H., 1966, "Effect of Ferroelectric P o l a r i z a t i o n on Insulated-Gate Thin-Film Transistor Parameters", Solid State Electronics. Vol. 9, pg. 657-661, 1966. 

Cite

Citation Scheme:

        

Citations by CSL (citeproc-js)

Usage Statistics

Share

Embed

Customize your widget with the following options, then copy and paste the code below into the HTML of your page to embed this item in your website.
                        
                            <div id="ubcOpenCollectionsWidgetDisplay">
                            <script id="ubcOpenCollectionsWidget"
                            src="{[{embed.src}]}"
                            data-item="{[{embed.item}]}"
                            data-collection="{[{embed.collection}]}"
                            data-metadata="{[{embed.showMetadata}]}"
                            data-width="{[{embed.width}]}"
                            async >
                            </script>
                            </div>
                        
                    
IIIF logo Our image viewer uses the IIIF 2.0 standard. To load this item in other compatible viewers, use this url:
http://iiif.library.ubc.ca/presentation/dsp.831.1-0104442/manifest

Comment

Related Items