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The effects of mobile sodium contamination on MOS structures Bouthillier, Thomas Michael 1982

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THE EFFECTS OF MOBILE SODIUM CONTAMINATION ON MOS STRUCTURES by THOMAS MICHEAL BOUTHILLIER B . S c , H o nours P h y s i c s , U n i v e r s i t y o f A l b e r t a , 1976 B.Sc., E l e c t r i c a l E n g i n e e r i n g , U n i v e r s i t y o f A l b e r t a , 1978 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTERS OF APPLIED SCIENCE i n THE FACULTY OF GRADUATE STUDIES (Department o f E l e c t r i c a l E n g i n e e r i n g ) We a c c e p t t h i s t h e s i s a s c o m f o r m i n g t o t h e r e q u i r e d s t a n d a r d THE UNIVERSITY OF MARCH © Thomas M. BRITISH COLUMBIA 1 982 B o u t h i l l i e r In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. I t i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of t^l£ £ 'ff/Cf^ ' J^i^Cj / fi e-et'l The University of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date Ma*A 3o /9fc DE-6 (3/81) ABSTRACT As the • present trend t o l a r g e r s c a l e s of i n t e g r a t i o n of ch i p s continues an i n c r e a s i n g demand on r e l i a b i l i t y must be met. One source of premature device f a i l u r e i s contamination by mobile sodium i o n s . T h i s t h e s i s i s a study of some of the e f f e c t s of sodium contamination i n s i l i c o n d i o x i d e using i n t e r n a l photoemission and s e l f h e a l i n g breakdown. L a t e r a l inhomogeneities i n the b a r r i e r height at the S i -S i 0 2 i n t e r f a c e were probed by scanning a focussed beam of u l t r a v i o l e t l i g h t along a MOS c a p a c i t o r . I t was found that b a r r i e r height was uniform unless h i g h l y contaminated samples were subjected both to a high temperature and a l s o to a p o s i t i v e b i a s on the f i e l d p l a t e f o r a short time at moderate temperatures. Q u a l i t a t i v e f e a t u r e s of l a t e r a l inhomogeneities i n the surface p o t e n t i a l were c h a r a c t e r i z e d by high frequency c a p a c i t a n c e - v o l t a g e measurements showing s i m i l a r e f f e c t s . A model f o r the photoemission process was proposed. The e f f e c t of i l l u m i n a t i o n w h i l e measuring photoemission was found to a f f e c t the band bending s u b s t a n t i a l l y so that a r e l a t i v e l y simple model n e g l e c t i n g band bending i n the s i l i c o n was used. B a r r i e r height lowering caused by sodium contamination was measured on samples having a uniform photocurrent. By measuring samples at d i f f e r e n t contamination l e v e l s an experimental r e l a t i o n s h i p between sodium c o n c e n t r a t i o n and b a r r i e r height lowering was found. A simple model t a k i n g i n t o account Schottky b a r r i e r lowering r e s u l t i n g from the d i p o l e f i e l d was found to be i n good agreement with the r e s u l t s f o r low l e v e l s of surf a c e c o n c e n t r a t i o n . Good experimental agreement was found w i t h i i i p r evious work. S e l f h e a l i n g breakdown of the sample was a l s o performed showing a c o n v i n c i n g c o r r e l a t i o n between photoemission peaks and areas of e a r l y breakdown. i v Table of Contents A b s t r a c t .... i i Table of Contents i v L i s t of F i g u r e s v L i s t of Tables v i i i Acknowledgement i x Chapter 1 I n t r o d u c t i o n 1 Chapter 2 A Review of Previous Work 2 Chapter 3 Theory of Photoemission and B a r r i e r Height Lowering 6 3.1 I n t e r n a l Photoemission 6 3.2 B a r r i e r Height Lowering by Na+ 11 Chapter 4 Experimental D e t a i l s 16 4.1 Sample Design 16 4.2 Sample Pr e p a r a t i o n 17 4.3 Experimental Procedure 20 Chapter 5 Experimental R e s u l t s 23 5.1 Photoemissive Peaks 23 5.2 B a r r i e r Height Lowering 63 5.3 S e l f - h e a l i n g breakdown 71 Chapter 6 Summary and Conclusions 76 Appendix A The E f f e c t of I l l u m i n a t i o n on Band Bending ... 78 Appendix B A L i s t of Programs Used 81 V L i s t of Fi g u r e s 3.1(a ) £ t(b) The band diagrams of p-type s i l i c o n w i t h and without i l l u m i n a t i o n 8 3.2 E f f e c t of sodium ions at the S i - S i 0 2 i n t e r f a c e 13 3.3 P l o t of A* vs « f o r sodium at the i n t e r f a c e 14 4.1 Sample design 16 4.2 Steps i n sample f a b r i c a t i o n 19 4.3 I l l u s t r a t i o n of the apparatus used f o r photoemission experiments 21 4.4 Scanning patern used 22 5.1(a) Photocurrent map of sample DTI200 25 5.1(b) R e f l e c t a n c e map of sample DT1200 26 5.2 Photocurrent map of sample DT1200 under d i f f e r e n t b i a s 27 5.3 Photocurrent t r a n s i e n t f o r sample DT1500 28 5.4(a) Photocurrent map of sample DT1250 29 5.4(b) R e f l e c t a n c e map of sample DT1250 30 5.5(a)&(b) Photographs of sample DT1250 31 5.6 Photocurrent map of sample DTI200 a f t e r a n n e a l i n g 32 5.7(a) Photocurrent map of sample DTI 200 a f t e r a n n e a l i n g .. 33 5.7(b) Refl e c t a n c e map of sample DT1200 34 5.8 Photocurrent map of sample DTI000 37 5.9 Photocurrent map of sample DTI000 a f t e r a n n e aling 38 5.10 Photocurrent map of sample DT0050 39 5.11 Photocurrent map of DT0050 a f t e r annealing 40 5.12 C-V curve of DT0050 41 5.13 Photocurrent map of sample FG1000 42 5.14 Photocurrent map of sample FG1000 43 5.15 Photocurrent map of EV1000 45 v i 5.16 Photocurrent map of EV1000 a f t e r a n e a l i n g 46 5.17 Photocurrent map of SL2050 49 5.18 Photocurrent map of SL2050 a f t e r annealing 50 5.19 Photocurrent map of SL2050 a f t e r annealing 51 5.20 C-V curves f o r sample SL2050 52 5.21 Photocurrent map of sample NI1000 a f t e r annealing i n 10%H 2-N 2 at 470°C f o r 15 min 53 5.22 Photocurrent,map of sample FG1000 a f t e r annealing i n N 2 at 470°C f o r 15 min 54 5.23 C-V curves of sample FG1000 55 5.24 Photocurrent map of DAI000 57 5.25 Photocurrent map of DA4000 58 5.26 Photocurrent map of FG1000 * 61 5.27(a) to (c) Photographs of sample EF1000 a f t e r e t c h i n g . 62 5.28(a) P l o t of I 1 / 3 vs V 1/ 2 f o r sample BL1001 64 5.28(b) P l o t of I 1 / 3 vs V 1/ 2 f o r sample BL1002 65 5.28(c) P l o t of 1 1 / 3 vs V 1/ 2 f o r sample BL1004 66 5.28(d) P l o t of I 1 / 3 vs V 1/ 2 f o r samples BL2003 & 4 67 5.28(e) Photocurrent map of DA4000 68 5.28(f) P l o t of I 1 / 3 vs I 1 / 2 f o r two p o i n t s on DT4000 69 5.28(g) Measured • vs a f o r samples 70 5.29 Photocurrent map of EV1000 72 5.30 Shadow map of photocurrent map shown i n f i g u r e 5.29 .. 73 5.31 Shadow map of r e f l e c t i v i t y f o r sample EV1000 a f t e r s e l f h e a l i n g breakdown 74 5.32 Photograph of sample EV1000 a f t e r s e l f h e a l i n g breakdown 75 A1 I l l u s t r a t i o n of sample i l l u m i n a t i o n 84 A2 C a l c u l a t e d p l o t of * g vs i l l u m i n a t i o n a • • V l l l L i s t of Tables 4.1 Times and ambients i n furnace c l e a n i n g 17 4.2 Times and ambients f o r t h i n oxide o x i d a t i o n 18 ix ACKNOWLEDGEMENT I w o u l d l i k e t o t h a n k D r . L . Young f o r h i s h e l p and g u i d a n c e d u r i n g t h e c o u r s e o f t h e r e s e a r c h , and NSERC f o r t h e f u n d i n g w h i c h s u p p o r t e d t h i s work. The p r o o f r e a d i n g p e r f o r m e d by Dr.D. S m i t h i s a l s o g r e a t l y a p p r e c i a t e d . 1 Chapter 1 I n t r o d u c t i o n During the l a s t decade semiconductor devices have grown i n complexity to the s t a t e that as many as 10 s components can be found on a s i n g l e i n t e g r a t e d c i r c u i t . Chips of t h i s c o mplexity, which f a l l i n the category of very l a r g e s c a l e i n t e g r a t i o n (VLSI), place an i n c r e a s i n g importance on the r e l i a b i l i t y of each component. As VLSI f o r c e s s c a l i n g down of dimensions, i n c l u d i n g gate and f i e l d oxide t h i c k n e s s , the i n t e r f a c i a l region between semiconductor and oxide becomes a s i g n i f i c a n t p o r t i o n of the t o t a l oxide t h i c k n e s s . An increased knowledge of the i n t e r f a c i a l region i s t h e r e f o r e d e s i r e d . I t i s to these ends that t h i s t h e s i s i s devoted and s p e c i f i c a l l y to the study of contamination of s i l i c o n d i o x i d e by mobile sodium io n s . S i l i c o n d i o x i d e i s the m a t e r i a l commonly used as a gate d i e l e c t r i c f o r IGFETs and sodium i s known to be the major source of mobile ion contamination. Mobile ion contamination i s known to produce premature breakdown of the o x i d e , unstable device c h a r a c t e r i s t i c s , and i n c r e a s e d 1/f n o i s e . The m a j o r i t y of the sodium can be d r i f t e d next to the i n t e r f a c e thereby making i t p o s s i b l e to examine some of the p r o p e r t i e s of the i n t e r f a c i a l r e g i o n . The chapters are arranged i n the f o l l o w i n g o r d e r . Chapter 2 presents a review of previous work while chapter 3 covers the theory i n v o l v e d i n the photoemission process. Chapters 4 and 5 d e s c r i b e the experimental procedure and r e s u l t s r e s p e c t i v e l y . D i s c u s s i o n s and c o n c l u s i o n s based on the experimental f i n d i n g s and sugestions f o r f u r t h e r work are presented i n chapter 6. 2 Chapter 2 A Review of Previous Work 2.1 Mobile Sodium Transport The d r i f t of mobile sodium ions i n t h e r m a l l y grown S i 0 2 has been a t o p i c of much research because of i t s importance i n MOS d e v i c e s . F i r s t s t u d i e d by Snow et a l 1 samples were d e l i b e r a t e l y contaminated i n a s o l u t i o n of NaCl i n d e i o n i z e d water or by thermal evaporation of NaCl under vacuum. T h i s contamination i s u s u a l l y done before a f i e l d p l a t e i s added to the d e v i c e s . A f t e r a p p l y i n g a p o s i t i v e b i a s on the f i e l d p l a t e f o r some time at e l e v a t e d temperatures (known as p o s i t i v e b i a s - temperature s t r e s s "+BT") the f l a t b a n d v o l t a g e i s seen to s h i f t t o more negative v o l t a g e s when performing a C-V measurement. Studies i n v o l v i n g secondary ion mass sp e c t r o m e t r y 2 , neutron a c t i v a t i o n a n a l y s i s 3 and r a d i o a c t i v e ' " 6 N a 2 2 have shown that t h i s s h i f t i s due to Na* motion i n the oxide. I t has a l s o been shown that mobile Na* ions are found w i t h i n 5 nm of e i t h e r the m e t a l - S i 0 2 or S i - S i 0 2 i n t e r f a c e 8 , 9 so that the motion i s thought to be i n t e r f a c e l i m i t e d that i s , the i o n i c motion i s c o n t r o l l e d by t r a p s at the i n t e r f a c e s . A c t i v a t i o n energies f o r these t r a p s have been s t u d i e d 1 0 ' 1 1 and t h e i r energies seem to be d i s t r i b u t e d over a range of values so t h a t ion d r i f t can occur even at room t e m p e r a t u r e s 7 " 8 . With modern f a b r i c a t i o n techniques mobile ion d e n s i t i e s as low as 10 s cm - 2 can be a c h i e v e d 1 2 . Such low average values w i l l g e n e r a l l y have l i t t l e impact on device c h a r a c t e r i s t i c s provided the contamination i s not l o c a l i z e d i n very small areas. I n d i c a t i o n s a r e 2 6 that i f t h i s were the case the oxide may be unacceptably prone to breakdown i n these areas. I n s t a b i l i t i e s 3 due t o s o d i u m c a n be e l i m i n a t e d by s t r i n g e n t c l e a n l i n e s s d u r i n g f a b r i c a t i o n , t h e a d d i t i o n of H C l g a s t o t h e a m bient d u r i n g o x i d a t i o n , o r t h e a d d i t i o n o f a l a y e r o f p h o s p h o s i l i c a t e g l a s s w h i c h c a n t r a p s o d i u m 1 5 " 1 6 o r s i l i c o n n i t r i d e w h i c h can b l o c k s o d i um m o t i o n 1 7 * 1 8 . However s i l i c o n n i t r i d e c a n s t o r e c h a r g e and p h o s p h o s i l i c a t e g l a s s i s l i m i t e d i n t h e amount of Na* i t can g e t t e r . A l s o o n l y s m a l l amounts of H C l c a n be u s e d w i t h o u t phase s e p a r a t i o n o c c u r i n g 1 3 ' 1 * so t h a t m o b i l e Na* r e m a i n s an i m p o r t a n t p r o b l e m i n s e m i c o n d u c t o r t e c h n o l o g y . ~ 2.2 I o n i c C o n t a m i n a t i o n a t t h e S i - S i 0 2 I n t e r f a c e c P e r h a p s t h e most p o w e r f u l t o o l i n d e t e r m i n i n g t h e e f f e c t o f m o b i l e s o d i u m on t h e S i - S i 0 2 i n t e r f a c e i s i n t e r n a l p h o t o e m i s s i o n . F i r s t o b s e r v e d by W i l l i a m s 1 9 , when i l l u m i n a t i n g a MOS c a p a c i t o r w i t h UV l i g h t , a p h o t o c u r r e n t was seen t o f l o w t h r o u g h t h e S i 0 2 . The f i e l d d e p e n d e n c e of t h e c u r r e n t showed t h a t t h e p r o c e s s was n o t one o f p h o t o c o n d u c t i o n . P o w e l l 3 3 l a t e r showed by u s i n g o p t i c a l i n t e r f e r e n c e w i t h i n t h e o x i d e l a y e r and t h e r e b y c a u s i n g i n t e n s i t y maxima t o o c c u r a t e i t h e r i n t e r f a c e , t h a t t h e p r o c e s s was one of e l e c t r o n s f l o w i n g f r o m t h e s i l i c o n . A t h r e s h o l d e n e r g y o f an e l e c t r o n a t t h e t o p o f t h e S i v a l e n c e band t o t h e b o t t o m o f t h e S i 0 2 c o n d u c t i o n band ( s e e f i g u r e 4.1) was m e a s u r e d t o be between 4.2 and 4.3 e V " 9 " 2 3 f o r u n c o n t a m i n a t e d s a m p l e s . However when Na* was c a u s e d t o d r i f t t o t h e S i - S i 0 2 i n t e r f a c e t h i s b a r r i e r h e i g h t was seen t o be l o w e r e d 2 0 " 2 5 . P h o t o e m i s s i o n p e a k s were a l s o o b s e r v e d when s c a n n i n g a f o c u s s e d beam o v e r s a m p l e s 2 6 " 2 7 . V a r i o u s m o d e l s have been s u g g e s t e d t o e x p l a i n t h i s and w i l l be d i s c u s s e d i n c h a p t e r 5. 4 L a t e r , i t was a l s o f o u n d t h a t s u f f i c i e n t c l e a n l i n e s s b e f o r e t h e i n t e n t i o n a l c o n t a m i n a t i o n s t e p w o u l d m i n i m i z e t h e s e non-u n i f o r m i t i e s 2 5 . B a r r i e r h e i g h t d e t e r m i n a t i o n has a l s o been c a r r i e d o u t u s i n g e l e c t r i c a l c o n d u c t i o n m e a s u r e m e n t s 2 8 on S i 0 2 . In t h i s method, c u r r e n t a c r o s s t h e S i 0 2 i s m easured a s a f u n c t i o n o f a p p l i e d f i e l d and, by a s s u m i n g F o w l e r - N o r d h e i m e m i s s i o n , a b a r r i e r h e i g h t can be d e d u c e d . R e s u l t s c o n f i r m e d t h a t Na c o n t a m i n a t i o n a t t h e S i - S i 0 2 i n t e r f a c e would l o w e r t h e b a r r i e r h e i g h t a l t h o u g h not by t h e same amount as f o u n d by D i S t e f a n o 2 5 . E v i d e n c e o f l o c a l i z e d b a r r i e r h e i g h t l o w e r i n g 2 8 " 2 9 a t t h e i n t e r f a c e a l s o a g r e e d w i t h p h o t o c u r r e n t measurements. Of p a r t i c u l a r i n t e r e s t i s t h a t t h i s was seen t o o c c u r even f o r s a m p l e s h a v i n g a low l e v e l ( < l 0 1 0 cm" 2) of s o d ium c o n t a m i n a t i o n . 2.3 E f f e c t s o f E l e v a t e d T e m p e r a t u r e s D u r i n g f a b r i c a t i o n a d e v i c e may be s u b j e c t e d t o v a r i o u s h e a t t r e a t m e n t s . Commonly, a d e v i c e w i l l go t h r o u g h an a n n e a l i n g s t e p i m m e d i a t e l y a f t e r o x i d a t i o n t o r e d u c e s u r f a c e s t a t e d e n s i t y a n d / o r s u r f a c e c h a r g e . M o n t i l l o a n d B u l k 3 0 i n v e s t i g a t e d t h e e f f e c t s o f v a r i o u s a m b i e n t s and t e m p e r a t u r e s from 600°C t o 1000°C . A n n e a l i n g i n n e u t r a l a m b i e n t s s u c h a s n i t r o g e n , h e l i u m o r a r g o n was f o u n d t o r e d u c e t h e amount o f o x i d e c h a r g e . A n n e a l i n g a t 1000°C i n n i t r o g e n a l s o r e d u c e s s u r f a c e s t a t e d e n s i t y ( N s s ) s u b s t a n t i a l l y and i s t h e r e f o r e commonly u s e d . A n n e a l i n g i s a l s o p e r f o r m e d a t l o w e r t e m p e r a t u r e s , g e n e r a l l y f r o m 300°C t o 5 0 0 ° C 3 1 , f o r r e m o v i n g c r y s t a l and o x i d e damage f r o m d i f f u s i o n of i m p u r i t i e s o r r a d i a t i o n damage i n c u r r e d d u r i n g e-beam m e t a l i z a t i o n . An i n - d e p t h i n v e s t i g a t i o n of low 5 t e m p e r a t u r e a n n e a l i n g was p e r f o r m e d by K o o i 3 . T h e s e r e s u l t s a r e o f i n t e r e s t i n v i e w o f t h e f a c t t h a t c o n s i d e r a b l e d i f f u s i o n o f i m p u r i t i e s o c c u r s i n s i l i c o n a t t e m p e r a t u r e s above 4 0 0 ° C 3 2 and i r r e v e r s i b l e c h a n g e s i n t h e r m a l l y s t i m u l a t e d i o n i c c o n d u c t i v i t y ( T S I C ) c u r v e s o c c u r s 1 1 . Chapter3 Theory of Photoemission and B a r r i e r Height Lowering In the f o l l o w i n g s e c t i o n b a s i c equations f o r the i n t e r n a l photoemission process are presented. Equations are then d e r i v e d showing a r e l a t i o n s h i p between incre a s e s i n photocurrent and small decreases i n b a r r i e r h e i g h t . The f i n a l s e c t i o n d e s c r i b e s how the b a r r i e r height may be expected t o be i n f l u e n c e d by Na* contaminat i o n . 3.1 I n t e r n a l Photoemission The i n t e r n a l emission process i s perhaps best d e s c r i b e d i n terms of a three step process, t h a t i s 1) p h o t o e x c i t a t i o n of e l e c t r o n s from the S i valence band t o the S i conduction band 2) t r a n s p o r t of e l e c t r o n s t o the S i - S i 0 2 i n t e r f a c e 3) t r a n s p o r t over the b a r r i e r In 1) only t r a n s i t i o n s from the S i valence band to the S i conduction band w i l l be considered s i n c e f o r the purposes of t h i s experiment the conduction band s t a t e s w i l l be n e a r l y empty. Only when the s i l i c o n near the i n t e r f a c e i s made s t r o n g l y degenerate and n-type w i l l t r a n s i t i o n s from the conduction band become i m p o r t a n t 2 8 . F o l l o w i n g P o w e l l 2 3 the quantum y i e l d may be w r i t t e n as 3.1 Where N(E,hv) i s the energy d i s t r i b u t i o n of e x c i t e d e l e c t r o n s at 7 t h e e m i t t e r s u r f a c e , P ( E ) i s t h e p r o b a b i l i t y o f t r a n s m i s s i o n o v e r t h e b a r r i e r , Y(hi/) i s t h e quantum y i e l d , E i s t h e e n e r g y w i t h r e s p e c t t o t h e s i l i c o n v a l e n c e band hv i s t h e p h o t o n e n e r g y . N(E,hi/) w i l l d epend on s e l e c t i o n r u l e s g o v e r n i n g o p t i c a l t r a n s i t i o n p r o b a b i l i t i e s . N o n - d i r e c t t r a n s i t i o n s w h i c h i n c l u d e , 1) t r a n s i t i o n s i n v o l v i n g phonon e m i s s i o n o r a n n i h i l a t i o n , and 2) t r a n s i t i o n s where i n i t i a l o r f i n a l s t a t e w a v e f u n c t i o n s a r e l o c a l i z e d so t h a t a sum of B l o c h f u n c t i o n s i s r e q u i r e d t o d e s c r i b e t h e t r a n s i t i o n a d e q u a t e l y , a r e e x p e c t e d t o d o m i n a t e W i t h t h i s a s s u m p t i o n N(E,hi/) i s e x p e c t e d t o be a f u n c t i o n o f E-hv o n l y 3 5 - 3 6 o r N(E,hi/)=N(E-hi/) 3.2 U s i n g t h e model f i r s t p r o p o s e d by F o w l e r 3 5 i n w h i c h a e l e c t r o n must have i t s n o r m a l component o f g r o u p v e l o c i t y g r e a t e r t h e n some c r i t i c a l v a l u e i n o r d e r t o e s c a p e , i t i s f o u n d t o a good a p p r o x i m a t i o n Where E ^ i s shown i n f i g u r e 3.1 and T i s a c o n s t a n t . S u b s t i t u t i n g e q u a t i o n s 3.2 and 3.3 i n t o e q u a t i o n 3.1 one g e t s P ( E ) = T- (E-Eb) 3.3 3.4 W i t h t h e r e l a t i o n s I = ( P - Y ) / h v 3.5 8 L t a L f t L ' ! 1 6 band dla8raa,S ° f s 1 1 1"" W » " » o u t a n d ( b ) w i t h 9 Where I i s the measured p h o t o c u r r e n t , P i s the absorbed l i g h t power and hv i s the photon energy and Y i s the quantum y i e l d A l l o w i n g f o r S c h o t t k y b a r r i e r l o w e r i n g by the r e l a t i o n E b'=E+KV 1/ 2 3.6 Where K= ( q / 4 i r c t ) 1 / 2 , V i s the a p p l i e d v o l t a g e , e i s the h i g h f r e q u e n c y d i e l e c t r i c c o n s t a n t and t i s the e l e c t r o d e s e p a r a t i o n U s i n g e q u a t i o n 3.5 and 3.6 and making the change of v a r i a b l e E'=hi/-E g i v e s hv-E^+KV 1/ 2 I (hi/, V) = A (hi/) JN ( -E' ) • ( h i / - E b + K l / 2 V - E * ) dE* 0 N(-E*) can be ap p r o x i m a t e d t o be l i n e a r i n E' f o r many s e m i c o n d u c t o r s and s p e c i f i c a l l y f o r s i l i c o n s i n c e t h i s has been e x p e r i m e n t a l l y o b s e r v e d i n vacuum p h o t o e m i s s i o n t o be v a l i d w i t h i n an e l e c t r o n v o l t of the h i g h e s t energy of t h e v a l e n c e b a n d 3 6 and has been shown t h e o r e t i c a l l y t o be a good a p p r o x i m a t i o n over 1 e V . 3 7 so t h a t N(-E')=*E' where fi i s some c o n s t a n t . U s i n g t h i s and c a r r y i n g out the i n t e g r a t i o n I (hi/, V) = pA (hi/) • (hi/-E b+KV*/ 2 ) 3 3.7 Where A(hi/) i s some f u n c t i o n of hv and V i s the a p p l i e d v o l t a g e . In c a l c u l a t i n g N(-E') , band bending was not t a k e n i n t o a c c o u n t . T h i s i s f o r two reaso n s 1) E x p e r i m e n t a l l y t h i s a p p r o x i m a t i o n i s v a l i d f o r moderate 10 doping l e v e l s and as a c a l c u l a t i o n t a k i n g t h i s i n t o account w i l l show 8 2, f o r the b i a s v o l t a g e s and doping l e v e l s used there i s l i t t l e d iscrepancy. 2) There i s a net charging at the i n t e r f a c e due to e l e c t r o n s which are not e n e r g e t i c enough to t r a v e r s e the b a r r i e r . Appendix A shows t h i s e f f e c t can be q u i t e l a r g e and r a i s e s the surface p o t e n t i a l * s t o a value nearer f l a t b a n d as shown i n f i g u r e 3.1. So f a r I (hi/,V)=C(hi/) • (hiz-F^+KV 1/ 2) 3 From t h i s i t can be seen t h a t a p l o t of V •/2 v s . I 1 / 3 should give a s t r a i g h t l i n e with t h i s v o l t a g e a x i s i n t e r c e p t being equal to (hv-E^J/K . This method, f i r s t used by P o w e l l 2 3 , i s used i n the present work f o r determining b a r r i e r h e i g h t . Equation 3.7 can a l s o be w r i t t e n as I (hv,E)=C(hi/) ( h i / - E b + M . E i l / 2 ) 3 3.8 Where c(hv)=fih(hv), M = K ( t ) 1 / 2 , t i s the oxide t h i c k n e s s and E ± i s the e l e c t r i c f i e l d i n the oxide. Over the area of a sample there may e x i s t s m a l l f l u c t u a t i o n s i n e i t h e r E f e or the e f f e c t i v e f i e l d near the i n t e r f a c e E. Changes i n E^ c o u l d be due to a v a r y i n g d i p o l e l a y e r and changes i n trapped charge along the oxide w h i l e changes i n E could be caused by non-uniform charge d i s t r i b u t i o n near the i n t e r f a c e or from sur f a c e d e f e c t s . These s m a l l changes can b r i n g about a l a r g e change i n photocurrent near t h r e s h o l d as can be seen by t a k i n g p a r t i a l s of eq. 3.8 which g i v e s AI/I = (-3AEfc+(3/2)MAE V2) (hi/-E +ME1 2) 11 J u s t how much the b a r r i e r height may be expected t o change i s the t o p i c of the next s e c t i o n . 3.2 B a r r i e r Height Lowering by Na* The amount of b a r r i e r height lowering induced by sodium ions may be c o n s i d e r a b l e . This i s because sodium ions are r e l a t i v e l y f r e e to d r i f t i n S i 0 2 and when temperatures of above 100°C and a p o s i t i v e b i a s are a p p l i e d to the f i e l d p l a t e f o r some time the m a j o r i t y of sodium are found w i t h i n 5 nm of the i n t e r f a c e 8 1 9 and may be i o n i z e d to a l a r g e degree The t r a n s i t i o n l a y e r between S i and S i 0 2 i s l e s s than 3 nm 3** 3 8 as measured by v a r i o u s methods. S p e c i f i c a l l y photoemission experiments show that t h i s l a y e r may be as small as 0.5 nm , 0 . Exp e r i m e n t a l l y i t i s not known whether the t r a n s i t i o n l a y e r i s made up of submicron S i i n c l u s i o n s or an SiO^ l a y e r or a mixture of b o t h 3 8 ' * 1 . To see how sodium may be expected t o change the b a r r i e r height I s h a l l make the f o l l o w i n g assumptions 1) T h e i r e x i s t s a 3 nm t r a n s i t i o n l a y e r of SiC»x (0<x<2) 2) A l l sodium ions d r i f t e d l i e at the edge of t h i s t r a n s i t i o n l a y e r 3) SpOt the S i O x i s given the value of 6. This i s a compromise between Si(« r =11.8) and S i 0 2 U r =3.8). The q u a n t i t a t i v e features of the b a r r i e r lowering w i l l not be in f l u e n c e d g r e a t l y by c r 4) The sodium e x i s t s e n t i r e l y i n i o n i c form 5) Under the c o n d i t i o n s of strong i n v e r s i o n and i l l u m i n a t i o n the f i e l d w i l l penetrate only a very s m a l l d i s t a n c e i n t o the s i l i c o n and the e f f e c t of band bending i n the S i w i l l 12 be n e g l e c t e d . The s i t u a t i o n i s d e p i c t e d i n f i g u r e 3.2. W i t h t h e s e a s s u m p t i o n s t h e c a l c u l a t i o n i s c a r r i e d o ut by l e t t i n g <s be t h e Na+ s u r f a c e c o n c e n t r a t i o n , t t h e e l e c t r i c p e r m i t t i v i t y o f f r e e s p a c e and u s i n g *=€ rc E and a c c o u n t i n g f o r t h e image f o r c e l o w e r i n g g i v e n by A*=(q-E/4»r£ K) '/ 2 where K i s t h e h i g h f r e q u e n c y d i e l e c t r i c c o n s t a n t o f S i 0 2 . Or A * ( e V ) = l 4 . 0 X10" 8(*)V 2 ( i o n s / c m 2 ) F i g u r e 3.3 i s a p l o t of A* vs <r f o r i o n s u r f a c e c o n c e n t r a t i o n s from 10 9 t o 10 1 * i o n s / c m u s i n g t h i s r e l a t i o n a s w e l l a s t h e r e s u l t s of O s b u r n 2 8 . Above a c o n c e n t r a t i o n of 1 0 1 3 t h e model w i l l f a i l f o r two r e a s o n s . F i r s t l y i f « y = 5 X l 0 1 3 i o n / c m 2 t h e n E= 1.5X10 7 V/ctn w h i c h i s above t h e breakdown f i e l d o f a good o x i d e . S e c o n d l y t h e r e i s t h e p r o b l e m t h a t t h e d i s t a n c e f r o m t h e i n t e r f a c e of t h e p o t e n t a l maximum i s of t h e o r d e r of a t o m i c d i s t a n c e s and t h e r e f o r e t h e c o n c e p t of e l e c t r i c p e r m i t t i v i t y i s l o s t . Of c o u r s e t h i s model w o u l d f a i l i f t h e t r a n s i t i o n r e g i o n had a t h i c k n e s s of 1 nm o r l e s s a s measured i n d e p e n d e n t l y by D i S t e f a n o * 0 and S t e r n * 3 . T h i s i s b e c a u s e t h e d i s t a n c e of minimum p o t e n t i a l due t o t h e image f o r c e i s a t a d i s t a n c e g r e a t e r f r o m t h e i n t e r f a c e t h a n t h e p o s i t i o n of t h e i o n s * 2 . C o n c e r n i n g a s s u m p t i o n 4 ) , an a p p l i e d f i e l d c o u l d a f f e c t t h e d e g r e e of i o n i z a t i o n of t h e s o d i u m . I f t h e f i e l d p l a t e were b i a s e d more n e g a t i v e l y , i n o r d e r t o r e a c h f l a t b a n d , t h e f i e l d i n t h e t r a n s i t i o n l a y e r would be r e d u c e d and e l e c t r o n s would t e n d t o f a l l i n t o t h e c o n d u c t i o n band of t h e s i l i c o n . T h i s i s b e c a u s e F i g u r e 3.2 E f f e c t of sodium ions at the S i - S i 0 2 i n t e r f a c e calculated curve Osburn's result 10 10' F i g u r e Na surface concentration (cm 3.3 P l o t of A# vs 0 f o r sodium a t the i n t e r f a c e 1 5 t h e e n e r g y l e v e l of t h e sodium r e l a t e d t r a p s i s s e v e r a l eV h i g h e r t h a t t h e s i l i c o n c o n d u c t i o n band ( s e e f i g u r e 3 . 1 ) . Thus t h e m easured s h i f t i n f l a t b a n d would show t h e sodium a s i o n i c s i n c e a l a r g e n e g a t i v e v o l t a g e would be r e q u i r e d t o r e a c h f l a t b a n d c o n d i t i o n s . In any e v e n t , as more and more sodium i o n s a r e d r i f t e d t o t h e i n t e r f a c e one would e x p e c t t o o b t a i n a l i m i t i n g v a l u e f o r t h e b a r r i e r h e i g h t . E x p e r i m e n t a l l y D i S t e f a n o 2 7 f o u n d a b a r r i e r h e i g h t l i m i t of 2.6 eV. w i t h a b o u t 1 0 1 5 i o n s / c m 2 w h i c h i s a p p r o x i m a t e l y e q u a l t o one m o n o l a y e r of S i . By c a l c u l a t i o n , t h i s l i m i t i s e x p e c t e d t o be r e a c h e d when t h e S i i s c o v e r e d by a b o u t one m o n o l a y e r * * and t h e v a l u e d o e s compare w e l l w i t h t h e p r e d i c t i o n of 2.73 e V * 5 . However t h e m e a s u r e d amount of b a r r i e r l o w e r i n g 2 7 ' 2 8 a t l o w e r c o n c e n t r a t i o n s of Na* i s c o n c i d e r a b l y more t h a t p r e d i c t e d by H i r a b a y s h i * 5 o r L a n g " * . A l s o v a l u e s m e a s u r e d by O s b u r n 2 8 do n o t a g r e e w i t h t h e m e a s u r e d v a l u e s of D i S t e f a n o 2 7 . Hence, i t would a p p e r t h a t f u r t h e r e x p e r i m e n t a l work i s d e s i r a b l e . 16 Chapter 4 Experimental D e t a i l s 4.1 Sample Design F i g u r e 4.1 shows the design of the c a p a c i t o r s used i n studying photoemission and s e l f - h e a l i n g breakdown. The t h i c k aluminum i s used f o r an e l e c t r i c a l contact to a probe and the t h i c k oxide i s used to f a c i l i t a t e C-V measurements and to a l l o w fo r a higher e l e c t r i c f i e l d over the t h i n oxide l a y e r . The t h i n aluminum f i e l d p l a t e a l l o w s approximatly 25% UV l i g h t t r a n s m i s s i o n as w e l l as a l l o w i n g f o r s e l f h e a l i n g breakdown to occur. A l l samples used were <100> p-type s i l i c o n w i t h r e s i s t i v i t i e s between 2 and 8 ohm cm. 15nm Si F i g u r e 4.1 Sample design 17 4.2 Sample P r e p a r a t i o n S t e p s i n v o l v e d i n f a b r i c a t i o n o f t h e s a m p l e s a r e shown i n f i g u r e 4.2 . They a r e i ) and i v ) R.C.A. C l e a n i n g T h i s i s a p r o v e n m e t h o d 5 3 o f r e m o v i n g b o t h i n o r g a n i c and o r g a n i c c o n t a m i n a t i o n t o a h i g h d e g r e e i i ) T h i c k o x i d e g r o w t h T h i c k o x i d e s were steam grown i n a q u a r t z t u b e a t 1100 °C a c c o r d i n g t o t h e t i m e s and a m b i e n t s f o l l o w e d by T s o i 9 2 i i i ) and x) P a t t e r n g e n e r a t i o n T h i s c o n s i s t e d o f s p i n n i n g a l a y e r o f p h o t o r e s i s t on t h e S i w a f e r , e x p o s u r e t o t h e d e s i r e d p a t t e r n , d e v e l o p m e n t and e t c h i n g t h e e x p o s e d o x i d e or aluminum w i t h a b u f f e r e d HF s o l u t i o n or p h o s p h o r i c a c i d s o l u t i o n r e s p e c t i v e l y v) G a t e o x i d e p r e p a r a t i o n G a t e o x i d e s of v a r y i n g t h i c k n e s s and H C l c o n c e n t r a t i o n s were grown a t 1100 °C. In g r o w i n g d r y o x i d e s t h e q u a r t z t u b e f u r n a c e was f i r s t s u b j e c t e d t o t h e f o l l o w i n g t r e a t m e n t i n o r d e r t o r e d u c e t h e amount of m e t a l c o n t a m i n a t i o n . T a b l e 4.1 T i m e s and a m b i e n t s f o r i n i t i a l f u r n a c e c l e a n i n g gas f l o w r a t e a m b i e n t t i m e d e s c r i p t i o n 1 l / m i n + 1 5 c c / m i n 0 2+15%HC1 1 h r c y l i n d e r w a l l c l e a n 1 1/m i n 1 h r H C l p u r g e 18 Table 4.2 Times and ambients f o r t h i n oxide growth Ambient 0 2 N 2 Dry Oxides time(min) 50 nm 100 nm 10 36 20 20 d e s c r i p t i o n O x i d a t i o n Anneal Ambient 0 2 o 2+5%HCl 0 2 N 2 HCl Oxides time(min) 50 nm 100 nm 2 2 8 29 5 5 20 20 d e s c r i p t i o n p r o t e c t i v e oxide growth HCl o x i d a t i o n HCl purge annealing v i i i ) Oxide etch This was done by ap p l y i n g drops of HF on the back s i d e of the wafer or by vapor e t c h i n g v i ) and v i i i ) M e t a l l i z a t i o n M e t a l l i z a t i o n s were performed using e i t h e r an e-beam evaporator or a tungsten f i l a m e n t at a vacuum pressure of about 5 X10" S t o r r or l e s s . Both the f r o n t contact pads and back s i d e ohmic conta c t were made 500 nm t h i c k or greater i x ) S i n t e r / A n n e a l T h i s was done at a temperature of 470°C f o r 30 minutes i n a n i t r o g e n ambient. This i s done to anneal r a d i a t i o n damage from i ) R.C.A. Cleaning i i ) Thick oxide growth v i i i ) Oxide e t c h and i v ) R.C.A. Cleaning v) Gate oxide p r e p a r a t i o n i r T r v i ) M e t a l l i z a t i o n v i i i ) Oxide etch v i i i ) M e t a l l i z a t i o n and i x ) S i n t e r / A n n e a l x) P a t t e r n generation x i ) Thin aluminum d e p o s i t i o n and p a t t e r n generation Sure 4.2 Steps i n Sample f a b r i c a t i o n 20 e-gun e v a p o r a t i o n a s d i s c u s e d i n s e c t i o n 2.3 and a l s o t o s i n t e r an ohmic c o n t a c t on t h e back s i d e o f t h e waf e r x i ) T h i n aluminum d e p o s i t i o n and p a t t e r n g e n e r a t i o n The p a t t e r n f o r t h e t h i n a luminum f i e l d p l a t e was d e f i n e d e i t h e r by d e p o s i t i n g t h e aluminum w h i l e t h e w a f e r was i n c o n t a c t w i t h a m e t a l mask o r by u s i n g t h e same t e c h n i q u e u s e d i n f o r m i n g t h e c o n t a c t p a d s . The sample was t i l t e d s l i g h t l y f o r b e t t e r c o v e r a g e o f aluminum a l o n g t h e edge of t h e t h i c k o x i d e . 4.3 E x p e r i m e n t a l P r o c e d u r e The b a s i c a p p a r a t u s u s e d f o r p h o t o e m i s s i o n e x p e r i m e n t s t o f o l l o w i s shown i n f i g u r e 4.3. I t was d e s i g n e d by T s o i * 2 and f o r a f u r t h e r d e s c r i p t i o n r e f e r e n c e i s made t o h i s t h e s i s * 2 . Some s l i g h t m o d i f i c a t i o n s were made w h i c h i n c l u d e d 1) The a d d i t i o n o f a t i m e a v e r a g i n g s u b r o u t i n e t o t h e computer c o n t r o l p r o g r a m . T h i s a l l o w e d f o r m u l t i p l e r e a d i n g s t o be t a k e n and a v e r a g e d a t e a c h p o i n t . 2) S c a n n i n g i n a p a t t e r n as shown i n f i g u r e 4.4 was done so a s t o remove t h e e f f e c t s due t o t h e b a c k l a s h i n t h e t r a n s l a t i o n g e a r s . 3) The o r i g i n a l f o c u s s i n g l e n s was r e p l a c e d by a q u a r t z one w h i c h made t h e i l l u m i n a t i o n on t h e sample a b o u t 4 t i m e s more i n t e n s e w i t h s l i g h t l y l e s s r e s o l u t i o n as m e a s u r e d by s c a n n i n g o v e r an edge o f t h e aluminum as d e s c r i b e d i n r e f e r e n c e 46. T h i s a l l o w e d f o r an i m p r o v e d s i g n a l . 4) B e t t e r s h i e l d i n g was added f o r a r e d u c e d n o i s e l e v e l . 5) A 12 b i t A/D c o n v e r t e r was add e d t o measure t h e o u t p u t o f t h e l o c k - i n a m p l i f i e r . T h i s a l l o w e d f o r t h e p o s s i b l e p o l a r i t y change of t h e l o c k - i n a m p l i f i e r ' s o u t p u t a s w e l l a s g i v i n g g r e a t e r He-Cd laser light chopper beam splitter mirror scan stage & heater \ temp, control photo-detector lock-in amp. electrometer motor control interface S Z 5 PDP8 / e F i g u r e 4 .3 I l l u s t r a t i o n of the apparatus used f o r photoemission experiments 10 2 2 accuracy in reflectance measurements. The high frequency C-V analysis was performed using a program written by Graeme Boyd and the time dependence of the photocurrent was examined by using a program written by David Smi th (f igure 5.3). Figure 4 . 4 Scanning pattern used 23 Chapter 5 Experimental R e s u l t s The r e s u l t s to f o l l o w are d i v i d e d i n t o three main t o p i c s ; 1) photoemissive peaks and t h e i r cause, 2) b a r r i e r height lowering caused by the sodium and 3) the r e l a t i o n s h i p between photoemissive peaks and e a r l y breakdowns. 5.1 Photoemissive peaks Photocurrent and r e f l e c t i v i t y maps of a 44 nm dry S i 0 2 f i l m are shown i n f i g u r e s 5.1(a) and 5.1(b) r e s p e c t i v e l y . These maps were seen a f t e r a p p l y i n g a p o s i t i v e b i a s temperature s t r e s s (+BT) to the sample. The u n i f o r m i t y of photocurrent maps was observed to be a property of " c a r e f u l l y prepared" samples by D i S t e f a n o 2 6 . In the present work u n i f o r m i t y was observed f o r samples not annealed a f t e r evaporation of the t h i n aluminum f i e l d p l a t e . This annealing step has not been mentioned i n previous w o r k 2 7 . Figure 5.2 i s a photomap of the same sample as i n f i g u r e s 5.1(a) and 5.1(b) but w i t h a b i a s of 1 MV/cm a p p l i e d to the f i e l d p l a t e during scanning. A b i a s of 1 MV/cm was used unless a very low p h o t o y i e l d made a higher b i a s d e s i r a b l e . A p l o t of the cur r e n t vs time when the focussed beam was i n i t i a l l y turned on i s shown i n f i g u r e 5.3. The s e t t l i n g time was l a r g e r than the RC of the c i r c u i t and was probably due t o the change i n • as discussed i n appendix A. In the scans reported below a step time of 300 ms or more was used. O c c a s i o n a l l y a solution-contaminated sample was observed to be non-uniform i n p h o t o y i e l d a f t e r the i n i t i a l d r i f t as seen i n f i g u r e 5.4(a) and 5.4(b). Photographs 5.5(a) and 5.5(b) show that t h i s non-uniformity i s probably due to s t r e a k i n g or beading 24 of the NaCl/H 20 s o l u t i o n when i t was being blown o f f . A d i f f e r e n t photomap was observed a f t e r the sample, whose o r i g i n a l photocurrent maps are shown i n f i g u r e 5.2, was annealed at low temperatures. This i s shown i n f i g u r e 5.6 and f i g u r e 5.7(a) w i t h the r e f l e c t a n c e map shown i n f i g u r e 5.7(b). In the s e c t i o n s to f o l l o w i n v e s t i g a t i o n s are made to b e t t e r understand the reasons f o r the appearance of the peaks i n the photocurrent maps. u. uu • r ' 1 ' I " 1 1 — r 160 320 480 640 800 960 1120 DISTANCE(MICRONS) Figure 5.1 Photocurrent(a) and R e f l e c t i v i t y ( b ) maps of sample DT1200. t-44 nm E=2MV/cm Qd=6 X10 _ a coul 0£HC1 • M REFLECTANCE (ARBITRARY UNITS) © © .-O Ol o I 1 —» CO o .Rel=107ohms = i n 8 ohms • •# ••••••• • 100 200 300 Time (ms) 400 500 600 700 F i g u r e 5.3 Photocurrent t r a n s i e n t f o r sample DT1500 800 960 T 320 480 640 800 960 DISTANCE(MICRONS) 1120 Figure 5.4 Photocurrent(a) and reflectance(b) maps of sample DT1250.t=44 nm, 0#HC1, Qd=5 XHT 8coul. fO F i g u r e 5.4(b) R e f l e c t a n c e map of sample DTI250 o F i g u r e 5.5(a)t(b) Photographs of sample DT1250 960 DlSTANCE(MICRONS) Figure 5.6 Photocurrent map of sample DT1200 a f t e r an anneal of 15 minutes In N2 at 470 C f o r 15 minutes CO Figure 5.7 Photocurrent(a) and r e f l e c t i v i t y ( b ) maps of sample DT1200 a f t e r a further anneal of 15 minutes CO CO DISTANCE(MICRONS) F i g u r e 5.7(b) R e f l e c t a n c e map of sample DT1200 co ilk 35 I) Oxide t h i c k n e s s , HCl c o n c e n t r a t i o n , and Na l o c a t i o n before annealing F i g u r e 5.8 shows a photocurrent map of a 10% HC1-100 nm f i e l d oxide a f t e r an i n i t i a l d r i f t . I t can be seen that most of the sodium has been p a s s i f i e d . However a f t e r annealing the photocurrent map looked l i k e f i g u r e 5.9. The a d d i t i o n of HCl was seen to give peaks more l o c a l i z e d and g r e a t e r i n number than oxides without HCl as seen w i t h a 0%HC1-100 nm oxide sample whose maps are shown i n f i g u r e s 5.10 and 5.11. S i m i l a r r e s u l t s were found with 50 nm and 170 nm oxides. In o b t a i n i n g f i g u r e 5.11 the sample was subjected to a negative b i a s temperature s t r e s s (-BT) before annealing so that the l a t e r a l l o c a t i o n of the sodium before annealing has l i t t l e e f f e c t on the peaks. This seems to c o n t r a d i c t an e x p l a n a t i o n f o r the peaking f i r s t proposed by W i l l i a m s and Woods 5 0. Their e x p l a n a t i o n f o r the observed peaks was th a t areas of high c o n c e n t r a t i o n s of sodium were formed as a r e s u l t of image f o r c e lowering at the S i - S i 0 2 i n t e r f a c e c r e a t e d by the sodium i o n s . As the c o n c e n t r a t i o n of the sodium r i s e s the S i s u r f a c e becomes more m e t a l l i c i n nature (approaching degeneracy) and t h e r e f o r e the e f f e c t i v e « becomes higher and the p o t e n t i a l lowering of the image fo r c e becomes g r e a t e r which would then a t t r a c t more ions causing an i n s t a b i l i t y . However the peaks were a l s o observed when the major contamination was at the A l - S i 0 2 i n t e r f a c e d u r i n g annealing and one would expect t h i s i n s t a b i l i t y t o not o c c u r 5 3 with t h i s model. Figure 5.12 shows the change i n the C-V curve a f t e r a n n e aling the sample. The downward slope a t v o l t a g e s s l i g h t y 36 negative of the fl a t b a n d v o l t a g e (sect A-B i n f i g u r e 5.12) i s an i n d i c a t i o n of l a t e r a l n o n - u n i f o r m i t i e s i n the Na+ d i s t r i b u t i o n as shown i n reference 51 . I I ) M o b i l i t y Figures 5.13 and 5.14 i l l u s t r a t e how peaks could be seen to reappear a f t e r applying a -BT to o b t a i n a very low photocurrent f o l l o w e d by a +BT. This was observed i n other samples as w e l l and shows that the Na+ i s s t i l l mobile and i n a s t a b l e d i s t r i b u t i o n . I I I 160 320 480 640 800 960 1120 DISTANCE(MICRONS) Figure 5.8 Photocurrent map of sample DT1000. t=100 nm, 10#Hcl, 0^=7 X10" 8coul. F i g u r e 5 . 1 2 C-V c u r v e o f D T 0 0 5 0 43 44 I I I ) NaCl evaporation The evaporation of NaCl on to the f i e l d oxide under vacuum has been shown to produce a uniform c o n t a m i n a t i o n 5 2 . Samples were contaminated at 10" 3 t o r r and had a f i e l d oxide 100 nm t h i c k grown i n 5%HC1. The r e s u l t s a f t e r a +BT and annealing are shown i n f i g u r e s 5.15 and 5.16. The photomap was uniform showing s i m i l a r i t y w i t h the previous r e s u l t s . This confirms t h a t the s o l u t i o n contamination was uniform when uniform photocurrent maps where observed and that the increased H 20 c o n c e n t r a t i o n , when s o l u t i o n contaminating, has no e f f e c t . 160 320 480 640 800 960 1120 D ISTANCE(MICRONS) Figure 5.15 Photocurrent map of EV1000. t=100 nm, HCl, Q.= 1.5 X10"8 01 47 IV) Annealing temperatures and Ambients Figure 5.17 shows a 0% HCl oxide a f t e r an i n i t i a l d r i f t . The photocurrent shows non-uniformity which i s again a t t r i b u t e d to an uneven contamination as discussed i n s e c t i o n 5.1 . F i g u r e 5.18 shows the photocurrent map a f t e r an anneal i n N 2 f o r 5 minutes at 425°C. Only a s l i g h t change i n the photocurrent map and i n the C-V curve was observed. Samples annealed at 300°C f o r 20 minutes a l s o showed no change. Small spikes appear i n the photocurrent map. These spikes were seen under a microscope to be l o c a t e d where there were p i n h o l e s i n the aluminum. When the sample was annealed at 470°C f o r 15 minutes i n N 2 the map and C-V curve as shown i n f i g u r e s 5.19 and 5.20 were observed so that the peaking and change i n C-V were observed only at the higher annealing temperatures. I t a l s o appears i a f i g u r e 5.19 that i f the peaking were due to sodium, that the c o a l e s c i n g of the sodium occurred i n areas of already high c o n c e n t r a t i o n . This r e l a t i o n s h i p w i l l be s t u d i e d i n the next s e c t i o n . Another commonly used ambient i n annealing i s a mixture of N 2 and H 2 c a l l e d forming g a s 3 . Comparison of the photocurrent maps of a sample annealed i n a 10%H 2-N 2 ambient ( f i g u r e 5.21) and a sample annealed i n a N 2 ambient ( f i g u r e 5.22) shows th a t H 2 c o n c e n t r a t i o n i n the annealing ambient has l i t t l e e f f e c t . F igure 5.23 again shows the t y p i c a l C-V change as observed f o r the sample whose photomap i s shown i n f i g u r e 5.22. At these e l e v a t e d temperature there i s s t i l l very l i t t l e d i f f u s i o n of aluminum i n the S i 0 2 1 1 so that i t i s not p o s s i b l e f o r aluminum from the f i e l d p l a t e to d i f f u s e t o the S i - S i 0 2 i n t e r f a c e . H i c k m o t t 1 1 has shown that sodium i s the i o n i c element 48 m aking up t h e major p o r t i o n o f t h e i o n i c c o n d u c t i v i t y a t t h e s e t e m p e r a t u r e s . One s e e s by e x a m i n i n g t h e photomaps t h a t when p e a k i n g of t h e p h o t o c u r r e n t e x i s t s t h a t t h e s u r r o u n d i n g p h o t o c u r r e n t i s r e d u c e d so t h a t i t i s p r o b a b l e t h a t t h e sodium i s r e d i s t r i b u t i n g i t s e l f . A c c e p t i n g t h i s , one i s l e f t w i t h two p o s s i b i l i t i e s ; e i t h e r an i n s t a b i l i t y i n t h e sodium c o n c e n t r a t i o n i s o c c u r r i n g o r g e t t e r i n g t o an e l e c t r i c a l l y a c t i v e s t a c k i n g f a u l t i s o c c u r r i n g . The l a t t e r i s p o s s i b l e s i n c e i t i s known t h a t d i f f u s i o n of i m p u r i t i e s i n s i l i c o n d o e s t a k e p l a c e t o a s m a l l d e g r e e a t t h e s e t e m p e r a t u r e 3 2 . The r e s u l t s t o f o l l o w t e s t t h e s e p o s s i b l e e x p l a n a t i o n s . 800 960 • • i i 320 480 640 800 DISTANCE(MICRONS) Figure 5.18 Photocurrent map of SL2050 a f t e r annealing f o r 5 min. at 425 C showing high spikes due to pinholes i n the aluminum gate cn o 160 320 480 640 800 960 1120 DISTANCE* MICRONS) Figure 5.19 Photocurrent map of SL2050 after a further anneal of 15 min. at 470 C before annealing i 1 1 1 1 1 1 1 i 1 1 1 0 . B B - 9 . BB - 8 . B B - 7 . B B -6.BB -S.BB -^.BB - 3 . B B - 2 . B B - 1 BH B . B-B GATE VOLTflCe Figure 5.20 C-V curves f o r sample SL2050 DISTANCE(MICRONS) F i g u r e 5.22 Photocurrent map of sample FG1000 a f t e r a n n e a l i n g i n N 2 a t 470°C f o r 15 min. F i g u r e 5.23 C-V curves of sample FG1000 cn 56 V) Contamination l e v e l s W o j t o w i c z 5 3 has c a l c u l a t e d f e a t u r e s of the image f o r c e i n s t a b i l i t y model of W i l l i a m s . One of h i s c o n c l u s i o n s was t h a t the i n s t a b i l i t y should occur even f o r low c o n c e n t r a t i o n s . F i g u r e 5.24 shows that t h i s was not observed i # e , photoemissive peaks d i d not occur i n samples w i t h low contamination l e v e l s . The r e s u l t of annealing a h i g h l y contaminated sample i s shown i n f i g u r e 5.25 and has s i m i l a r c h a r a c t e r i s t i c s as moderately contaminated samples. The r e s u l t s of the next s e c t i o n look i n t o the p o s s i b i l i t y that d e f e c t s i n the s i l i c o n may be r e s p o n s i b l e . DISTANCE(MICRONS) Figure 5 .24 Photocurrent map of DAI000 ' The high spikes are due to pinholes i n the aluminum f i e l d plate DISTANCE(MICRONS) Figure 5.25 Photocurrent map of DA4000. t=84 nm, 0 9&HC1, Qd=3.5 X10"7 ooul. cn oo 59 VI) E t c h i n g S t a c k i n g f a u l t s may e x i s t near the s u r f a c e of the s i l i c o n and are a t t r i b u t e d to the presence of o x y g e n 5 * * 5 5 . These f a u l t s can be viewed by using a modified Secco etch using a s o l u t i o n of 0.15M K 2 C r 2 0 7 and HF a c i d which i s known to etch d i f f e r e n t c r y s t a l l o g r a p h i c planes of the c r y s t a l at d i f f e r e n t r a t e s 5 6 . Figure 5.26 shows the photocurrent map of one sample and p i c t u r e s i n f i g u r e s 5.27(a) and 5.27(b) show the same sample a f t e r being etched i n a modified Secco etch f o r d i f f e r e n t times. Many small e t c h p i t s can be seen and f i g u r e 5.27(c) shows s t a c k i n g f a u l t s seen at a s i t e removed from the sample on the same wafer. Other etches on other samples were a l s o t r i e d such as a modified S i r t l e t c h 5 8 and anodic e t c h i n g 5 7 with s t i l l no observed c o r r e l a t i o n between the photoemissive peaks and s t a c k i n g f a u l t s . S tacking f a u l t s are the only non-uniformity i n the s i l i c o n of l a r g e enough s c a l e to account f o r the p e a k s 5 9 . I t appears from these r e s u l t s that the sodium i s unstable i n a uniform d i s t r i b u t i o n at high temperatures and c o n c e n t r a t i o n s . Although the model of W i l l i a m s and Woods cannot be completely r u l e d out i t i s more probable t h a t t h i s behaviour of c o a l e s c i n g i n t o patches of high c o n c e n t r a t i o n i s a property of sodium ions i n the oxide and not a r e s u l t of sodium ions i n t e r a c t i n g w i t h e i t h e r of the i n t e r f a c e s . Phosphorus and boron are known to separate i n t o regions of high c o n c e n t r a t i o n s i n s i l i c o n at h i g h temperatures and c o n c e n t r a t i o n s S 0 " 6 , . T h e l a r g e d i s t a n c e s that the m i g r a t i n g sodium must t r a v e l ( t y p i c a l l y 100 um) i s unusual and i s a t t r i b u t e d t o the r e l a t i v e ease at which the sodium can t r a v e l i n S i . The c o n c e n t r a t i o n g r a d i e n t 60 i n i t i a l l y set up near the s i t e of the i n s t a b i l i t y would act as the d r i v i n g f o r c e f o r the d i f f u s i o n and detrapping at annealing temperatures would be very h i g h . Therefore i t i s not unreasonable to expect c o a l e s c i n g may occur w i t h sodium i n S i 0 2 . DISTANCE(MICRONS) F i g u r e 5.26 Photocurrent map of FG1000 63 5.2 B a r r i e r height lowering In order to a r r i v e at an idea of the amount of non-u n i f o r m i t y of the charge, samples were subjected to v a r i o u s amounts of contamination and then given a +BT of 1/2 hour at 130°C with a b i a s of 1MV/cm. Almost a l l the sodium had d r i f t e d by t h i s time as was a l s o found p r e v i o u s l y 2 9 . B a r r i e r measurements were then made using a focussed beam. Even though the photocurrent was uniform along the surface of the sample a focussed beem was used so as to be a more d i r e c t comparision f o r b a r r i e r height determinations done on samples having a non-uniform photocurrent map. F i g u r e s 5.28(a) to (d) are r e s u l t s using the photo I-V method of P o w e l l 2 3 and f i g u r e 5.28(g) i s a p l o t of the measured b a r r i e r height vs c o n c e n t r a t i o n . The r e s u l t s are expected to be somewhat low compared wi t h the r e s u l t s of others because of the cube root f i t used (as opposed to a square root f i t ) and the d i p o l e l a y e r ' s e f f e c t on the Schottky b a r r i e r l o w e r i n g 2 5 . The p l o t , however, agrees w e l l w i t h the determination done by O s b u r n 2 9 and was b a s i c a l l y used to a r r i v e at an . approximate value of the amount of v a r i a t i o n i n c o n c e n t r a t i o n of sodium f o r samples having a non-uniform photocurrent map. I t can be seen from measuring the b a r r i e r heights at p o i n t s i n a sample having a non-uniform photomap ( f i g u r e 5.28(e) and f i g u r e 5.28(f) ) and comparing t h i s to known conce n t r a t i o n s ( f i g u r e 5.28(g)) that the amount of v a r i a t i o n i n sodium c o n c e n t r a t i o n i s as much as a f a c t o r of one hundred. The change i n p h o t o y i e l d w i t h a p p l i e d v o l t a g e as d i s c u s s e d i n s e c t i o n 3.1 a l s o i s f o l l o w e d w i t h i n experimental e r r o r . 64 F i g u r e 5 . 2 8 ( a ) P l o t o f I « / 3 v s V ' / * f o r s a m p l e B L 1 0 0 1 o o< >< O t o sr II II oi 9e* o r + — r 7 2 ' 8f + "917 LO U Q LO O LU LO O CM < o o LO If) J— 9 LO O e/i ( S d W V 6 Q l / i N 3 y y n 3 ) Ns= 2.1X10 t = 1740 PM + + 4 1/2 0 2 (VOLTAGE/ V) Figure 5 . 2 8 ( b ) P l o t of I 1 / * vs V 1/* f o r sample BL1002 "T~ 6 e 0 l X E / | ( V / l N 3 c J c i n O ) F i g u r e 5.28(d) Plot of I 1 / 3 v s V 1/ 2 f o r s a m p l e s BL2003 & 4 160 320 480 640 800 960 1120 DISTANCE* MICRONS) Fig u r e 5.28(e) Photocurrent map of DT0050 a f t e r a n n e a l i n g 00 4.0 > JZ cn JZ + + measured points from samples with uniform contaimination O points using measured barrier height values from a sample having a non-uniform photocurrent + as h j i i I I I Au I I I I i l l J I L J I M i l J I t i l l 1011 i12 10 ,13 «-2 Na surface concentration ( c m ) F i g u r e 5.28(g) Measured • vs * f o r samples 10 o 71 5.3 S e l f - h e a l i n g Breakdown The c o r r e l a t i o n between areas of high p h o t o y i e l d and areas of f i r s t breakdown was performed using an apparatus f o r observing s e l f - h e a l i n g breakdown which was designed by T s o i * 2 . Figure 5.29 shows a photomap of a sample t e s t e d w h i l e f i g u r e 5.30 i s a shadow map of f i g u r e 5.29. A f t e r s e l f - h e a l i n g breakdown was performed the sample appeared as i n the p i c t u r e shown i n f i g u r e 5.32 and a corresponding shadow map of the r e f l e c t i v i t y i s given i n f i g u r e 5.31. A high degree of matching between areas of e a r l y breakdown and increased p h o t o y i e l d i s apparent. This gives strong reinforcement to the e a r l i e r f i n d i n g of DiStefano and W i l l i a m s . D I S T A N C E ( M I C R O N S ) F i g u r e 5.29 Photocurrent map of EV1000 180 DISTANCE(MICRONS) 320 480 640 800 960 160-320 1 480 i 640-j 800-3 960 it t i i i i i i ! t i i • i i i i i I i • i • • • • • • I • • . t . . . , . [ . . . . . . . . . I .... . 1120 "I I I I I I I I I I \ I I I 1 I I I I I | I I I | I | | | | | | | | | | | i i t | | i | i i i j i i | | | | t 4 | i | t | 5.30 Shadow map of photocurrent map shown i n f i g u r e 5.29 74 D I S T A N C E ( M I C R O N S ) 160 320 480 640 800 960 1120 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 • 1 1 1 • 1 1 1 • 1 1 1 1 • 1 1 i . . , i . • • • • • • l> • • • • • • • • • • • • • • * • 160 T 1 . . I I I 320 A 4 • • • • • 480"i t •* • • • • • 640 A 800-1 960 i 1120H  ~ 111111111 p 11111111111111, t, 11,,,,,,,,, |,,,, t,,,, 5.31 Shadow map of r e f l e c t i v i t y f o r sample EV1000 a f t e r s e l f h e a l i n g breakdown 7 5 5.32 P h o t o g r a p h o f sample EV1000 a f t e r s e l f h e a l i n g breakdown 76 Chapter 6 Summary and Conclusions The f o l l o w i n g was found on the e f f e c t of sodium contamination on MOS s t r u c t u r e s using photoemission and s e l f h e a l i n g breakdown. 1) A c a l c u l a t i o n was c a r r i e d out to show that a model f o r the photemission process, which i n c l u d e s the e f f e c t of band bending i n the s i l i c o n on the p h o t o y i e l d , should a l s o take i n t o account the increased e l e c t r o n d e n s i t y i n the conduction band as a r e s u l t of the i l l u m i n a t i o n . The e f f e c t of the increased e l e c t r o n d e n s i t y on the p h o t o y i e l d i s so l a r g e the a model n e g l e c t i n g band bending was used. 2) Photoemission peaks reported by e a r l i e r i n v e s t i g a t o r s were found to occur only a f t e r h i g h contamination l e v e l s of sodium were subjected to e l e v a t e d temperatures so that the peaking found e a r l i e r was thought to be due to poor c l e a n l i n e s s d u r i n g f a b r i c a t i o n as i m p l i e d by the work of DiStefano. The cause f o r the peaking was then i n v e s t i g a t e d and i t was found that i t i s probably a property of high c o n c e n t r a t i o n s of sodium i n S i 0 2 at high temperatures and not a r e s u l t of the i n t e r a c t i o n with the S i - S i 0 2 i n t e r f a c e proposed by W i l l i a m s . The peaking was seen not to occur f o r low contamination l e v e l s and lower annealing temperatures so that with modern c l e a n l i n e s s standards and low temperature annealing t h i s e f f e c t may be avoided. 3) Using the photo I-V method of P o w e l l , measurement of how the b a r r i e r height i s a f f e c t e d by sodium surface c o n c e n t r a t i o n was performed. Good agreement was found both w i t h the work of Osburn and w i t h i n experimental e r r o r of a simple model f o r low c o n c e n t r a t i o n s . 77 4) A h i g h c o r r e l a t i o n was f o u n d between a r e a s w i t h p h o t e m i s s i v e p e a k s and a r e a s of e a r l y breakdown w h i c h r e i n f o r c e d p r e v i o u s work. S u g g e s t i o n s f o r f u r t h e r work 1) The use o f d i f f e r e n t f r e q u e n c i e s o f t h e i l l u m i n a t i n g l i g h t i n d e t e r m i n i n g t h e b a r r i e r h e i g h t w o u l d g i v e more i n f o r m a t i o n and € of t h e o x i d e n e a r th e i n t e r f a c e c o u l d be d e t e r m i n e d by a s s u m i n g t h e S c h o t t k y e f f e c t . 2) I t a p p e a r s t h a t t h e g r a p h s o b t a i n e d i n d e t e r m i n i n g t h e b a r r i e r h e i g h t show a t a i l i n g o f t h e p h o t o y i e l d . T h i s may be due t o t h e i l l u m i n a t i o n w h i c h c r e a t e s a l a r g e i n c r e a s e i n t h e o c c u p a n c y of t h e i n t e r f a c e s t a t e s . By t h e use of d i f f e r e n t i n t e n s i t i e s and w a v e l e n g t h s o f i l l u m i n a t i o n as w e l l as v a r y i n g t h e a p p l i e d v o l t a g e t h i s s u p p o s i t i o n c o u l d be t e s t e d as w e l l as t e s t i n g T s o i ' s c a l c u l a t i o n s . 3) Added d a t a p o i n t s i n t h e g r a p h of sodium c o n c e n t r a t i o n v s b a r r i e r h e i g h t o r i n c r e a s e d a c c u r a c y as d e s c r i b e d i n 1) would t e s t t h e m o d e l s f o r t h i s r e l a t i o n s h i p 4) The p h o t o c u r r e n t t e c h n i q u e i s a v e r y v e r s a t i l e one and c a n be u s e d t o d e t e r m i n e s u r f a c e s t a t e d e n s i t y and m i n o r i t y c a r r i e r 1 i f e t imes. 5 ) The e f f e c t of H C l on sodium c o u l d be i n v e s t i g a t e d more f u l l y t o d e t e r m i n e t h e r e l a t i o n s h i p between t h e amount of H C l added d u r i n g o x i d a t i o n an t h e amount o f Na t h a t can be p a s s i f i e d REFERENCES E.H. Snow,A.S.Grove,B.E.Deal and C.T.Sah, J . A p p l . Phys 3_6, 1644 ( 1 9 6 5 ) C. W. Magee and W.L. H a r r i n g t o n , A p p l . P h y s . L e t t . 3 3 , 193 ( 1 9 7 8 ) E. K o o i , 'The S u r f a c e P r o p e r t i e s o f O x i d i z e d S i l i c o n ' New Y o r k , S p r i n g e r - V e r l a g (1967) D. V. McCaughan ,C.W.White, R.A.Kushner and D.L.Simms, A p p l . P h y s . L e t t . 3_5, 4 0 5 ( 1 9 7 9 ) T . H . D i S t e f a n o , J . Vac . Sc i . T e c h n o l . J_3, 8 5 6 ( 1 9 7 6 ) F. M.Fowkes and T . E . B u r g e s s , S u r f . S c i . j_3, 184 ( 1 9 6 9 ) D.R.Keer, I . E . E . E . T r a n s . E l e c t . Dev. , 630 (1967) D.J . D i M a r i a , J . A p p l . P h y s . 48, 5149 (1977) D.J. D i M a r i a , Z.A.Weinberg and J . M . A i t k e n ., J . A p p l . P hys. 48, 898 (1977) M.R. Boudry and J . P . S t a g g , J . A p p l . P h y s . -50, 942 (1979) T.W. H i c k m o t t , J . A p p l . P h y s . 46, 2583 (1975) J . M a n k o w s k i , R . E . T r e s s l e r and J . S t a c h , J . E l e c t r o c h e m . S o c . 1 2 5 ( 1 1 ) , 1867 (1978) J . Mankowski, " S e m i c o n d u c t o r S i l i c o n 1977", P r o c e e d i n g of t h e T h i r d I n t e r n a t i o n a l Symposium on S i l i c o n M a t e r i a l s S c i e n c e and T e c h n o l o g y , The E l e c t r o c h e m i c a l S o c i e t y 324 D.R. K e r r , J . S . L o g a n , P . J . B u r k h a r d t and W . A . P l i s k i n , IBM J . Res. Dev. 8, 376 (1964) D.R. K e r r and J . M . E l d r i d g e , J . E l e c t r o c h e m . Soc 118, 986 ( 1971) S.M. Hu, J . E l e c t r o c h e m . S o c . 113, 694 (1965) T.E. B u r g e s s , J.C.Baum, F.M.Fowkes, R.Holmstrom and G . A . S h i r n , J . E l e c t r o c h e m . S o c . 116, 1005 (1965) R. W i l l i a m s , P h y s . Rev. 140, A569 (1965) A. M. Goodman, P h y s . Rev. J_44, 588 ( 1 9 6 6 ) B. E. D e a l , E.H. Snow and C A . Mead, J . P h y s . Chem. S o l i d s 27, 1875 ( 1 9 6 6 ) A.M. Goodman , J . A p p l . P h y s . 37, 3580 (1966) R. J . P o w e l l , J . A p p l . P h y s . 4J., 2424 (1970) R. W i l l i a m s , J . A p p l . P h y s . 3J7, 1491 (1966) T . H . D i S t e f a n o and J . E . L e w i s , J . V a c . S c i . T e c h n o l . J J _ , 1020 (1974) T . H . D i S t e f a n o , J . A p p l . P h y s . _44, 527 (1973) R . W i l l i a m s and M.ff.Woods, J . A p p l . P h y s . 43, 4143 (1972) C. M.Osburn and D.Wormond, J . E l e c t r o c h e m . S o c . 121, 1195 (1974) C.M.Osburn, J . E l e c t r o c h e m . S o c . 120, 1377 (1974) F . M o n t i l l o and P . B a l k , J . E l e c t r o c h e m . S o c . 118, 14 6 3 ( 1 971) T.P.Ma, ' S e m i c o n d u c t o r S i l i c o n 1981', P r o c e e d i n g s o f t h e F o u r t h I n t e r n a t i o n a l Symposium on S i l i c o n M a t e r i a l s S c i e n c e and T e c h n o l o g y , The E l e c t r o c h e m i c a l S o c i e t y , p427 J . B e s t and J . O . M c C o l d i n , J . A p p l . P h y s . 46, 4071 (1975) R . J . P o w e l l , J . A p p l . P h y s . 40, 5093 (1969) C . N . B e r g l u n d and W . E . S p i c e r , P h y s . r e v . 136, A1030 (1964) R.H. F o w l e r , P h y s . Rev. 3_8, 45 (1931) W . E . S p i c e r and R.C.Eden, ' P h o t o e m i s s i o n I n v e s t i g a t i o n o f t h e band s t r u c t u r e of S e m i c o n d u c t o r s ' , I n t . C o n f . P h y s . S e m i c o n d . 9 t h L e n i n g r a d , p65, 1968 and r e f e r e n c e s c o n t a i n e d t h e r e i n E.O.Kane, Phys. Rev. 146, 558 (1966) R . W i l l i a m s , J . V a c . S c i . T e c h n o l . j_4, 1106 (1977) A.Rose ' C o n c e p t s i n P h o t o c o n d u c t i v i t y ' , New Y o r k , I n t e r s c i e n c e P u b l i s h e r s , 1963 T.H. D i S t e f a n o 'The P h y s i c s o f S i 0 2 and i t s I n t e r f a c e s " , 1 9 6 7 , Pergamon P u b l i s h e r s , 362 C R . H e l m s , ' S e m i c o n d u c t o r S i l i c o n ' , p455 (1981) H . Y . T h o i , Ph.D. T h e s i s , U n i v e r c i t y of B.C. (1*79) A . S t e r n , S o l i d S t a t e Comm. 2J_, 163 (1970) N.D.Lang, Phys. Rev. B 4, 4234 (1971) K . H i r a b a y s h i , P hys. Rev. B 3, 4023 (1970) S.M.Sze, 'The P h y s i c s of S e m i c o n d u c t o r D e v i c e s ' , J o h n W i l e y and Sons, I n c . 1969 S . H o f s t e i n , IEEE T r a n s . E l e c t . Dev., 785 (1967) T.C.Poon and H.C.Card, J . A p p l . P h y s . 5 1 ( 1 2 ) , 5880 (1967) W.Kern and D . P u o t i n e u , RCA Review 187 (1970) R . W i l l i a m s and M.H.Woods, A p p l . P h y s . L e t . 22(9) , 458 (1973) N.Zamani and J , M a s e r j i a n , 'The P h y s i c s of S i 0 2 and i t s I n t e r f a c e s ' , Pergamon P u b l i s h e r s , 443 T . H . D i S t e f a n o , J . Vac . Sc i . T e c h n o l . J_3, 856 (1976) P . J . W o j t o w i c z , RCA Review 36, 132 (1975) S.M.Hu, J . A p p l . P h y s . 51(7) , 3666 (1980) C . J . V a r k e r and K . V . R a v i , J . A p p l . P h y s . 4 5 ( 1 ) , 263 and p272 (1974) A . A r m i g l i a t o e t a l ' S e m i c o n d u c t o r S i l i c o n 1977' P r o c e e d i n g s of t h e T h i r d I n t e r n a t i o n a l Symposium on S i l i c o n M a t e r i a l s S c i e n c e and T e c h n o l o g y , The E l e c t r o c h e m i c a l S o c i e t y , 638 H . F o l l , J . E l e c t r o c h e m . S o c . 1 2 7 ( 9 ) , 1925 (1980) J.T.Yue ' S e m i c o n d u c t o r S i l i c o n 1981', 596 A.Bunde and W . D i e t e r i c h , S o l i d S t a t e Comm. 3 6 ( 1 1 ) , 935 (1980) See f o r example E . A . I r e n e "The P h y s i c s of S i 0 2 and i t s i n t e r f a c e s ", 1967, Pergamon P u b l i s h e r s , p205 G . M a s e t t i e t a l ' S e m i c o n d u c t o r S i l i c o n 1977' on S i l i c o n M a t e r i a l s S c i e n c e and T e c h n o l o g y , The E l e c t r o c h e m i c a l S o c i e t y , 648 A.S. Grove ' P h y s i c s and T e c h n o l o g y of Semiconductor d e v i c e s ' John W i l e y and Sons, I n c . (1967) G.Schwab, 'Semiconductor S i l i c o n 1977' P r o c e e d i n g s of the T h i r d I n t e r n a t i o n a l Symposium on S i l i c o n M a t e r i a l s S c i e n c e and T e c h n o l o g y , The E l e c t r o c h e m i c a l S o c i e t y p48l J.D.Kamm, 'Semiconductor S i l i c o n 1977' P r o c e e d i n g s of the T h i r d I n t e r n a t i o n a l Symposium on S i l i c o n M a t e r i a l s S c i e n c e and T e c h n o l o g y , The E l e c t r o c h e m i c a l S o c i e t y , p49l J . G r o s v a l e t and J u n d , IEEE T r a n s . E l e c t . Dev. EDI 4, 777 (1967) J.D. Kamm 'Semiconductor S i l i c o n ' ,p49l (1977) L . J a s t e z e b s k i , J . L a g o w s k i and H.Gatos, A p p l . P h y s . L e t t . 27(10), 537 (1975) W.H.Hackett, J . A p p l . P h y s 43(4), 1649 (1972) 82 APPENDIX 1 The E f f e c t of I l l u m i n a t i o n on Band Bending Consider the case of i l l u m i n a t i o n under strong i n v e r s i o n , which would be the case f o r the bi a s v o l t a g e s a p p l i e d d u r i n g photoemission. The a p p l i e d v o l t a g e f o r an i d e a l MIS c a p a c i t o r i s given by V=* s+dEi where d i s the oxide t h i c k n e s s , E^ i s the f i e l d i n the oxide, and * s i s the surface p o t e n t i a l Also where E-, i s the f i e l d i n the s i l i c o n near the s u r f a c e , c „ , c. -s s i are the low frequency d i e l e c t r i c constants of the s i l i c o n and s i l i c o n d i o x i d e r e s p e c t i v e l y . I t can be shown 5 0 that under strong i n v e r s i o n . * reaches a l i m i t of approximately 2*g w i t h *g= (kT/q) In (N^/n^ ). Samples used were between 2 and 10 ohm-cm so tha t • ^ 0.8 V o l t s and t h e r e f o r e v = ' f e " + d 6 s E s / t i - d 6 S / € i ) E s A' 1 50 Now5 e sE s= 2€ s ( k T / L D ) -P( , n p Q / p p o ) A.2 Where 1^= (2kTc/p^- ) ^ 2 , p=q/kT and F ( M , n p o / p p Q ) =[(exp(-**) +0*-1) + n p o / p p Q (exp( **) - * * - l ) ] l / 2  npo'Ppo a r e t"*e e q u i l i b r i u m d e n s i t i e s of e l e c t r o n s and holes r e s p e c t i v e l y i n the bulk of the s i l i c o n . In strong i n v e r s i o n F ( * * s , n p o / p p o ) " ( n p o / P p o e x P ( p * s ) 1 / 2 A - 3 83 So that equations A.1,A.2 and A.3 y i e l d V - d 6 s A i ( 2 k T n p o A s ) , / 2 e x p ( M s / 2 ) When the sample i s i l l u m i n a t e d w i t h u l t r a v i o l e t l i g h t e l e c t r o n - h o l e p a i r s w i l l be created l a r g e l y w i t h i n 5 nm of the S i - S i 0 2 i n t e r f a c e . P h o t o y i e l d s are t y p i c a l l y 10" 3 electrons/photon or l e s s so that the m a j o r i t y of c a r r i e r s must e i t h e r recombine or d i f f u s e which w i l l i n turn a f f e c t • . In computing n^ Q there w i l l e x i s t a balance between ^ p n _ the c u r r e n t of the e x c i t e d c a r r i e r to the i n t e r f a c e , J -the sc e f f e c t of recombination at the s u r f a c e , - the cur r e n t d e n s i t y of c a r r i e r d i f f u s i n g away from the i n t e r f a c e . Since the e x c i t a t i o n of c a r r i e r s occurs w i t h i n about 5 nm of the i n t e r f a c e " 2 i t i s assumed tha t most of the e l e c t r o n s w i l l d r i f t to the i n t e r f a c e due to band bending; t h i s assumption w i l l be checked l a t e r . J i s given by ph J , -q G(x)dx P ( l - l o s s ) / h i / A ph input where A i s the area of i l l u m i n a t i o n as shown i n f i g u r e A.I and l o s s i s the f r a c t i o n of l i g h t l o s t i n reaching the s i l i c o n . Using the values of a 3 mW l a s e r w i t h hi/=3.8l eV, a 99% l o s s and a spot diameter of 18 um g i v e s a value f o r J of 3.9 amp/cm2 i s given b / 8 CfcL =(-qD /LJJ) [n(0)exp(-q* s/kT) - n p £ )] A.4 And J s c i s given b y 6 1 J =S-(p(0)n(0)-n i 2)/(n(0)+p(0)+2n i) Knowing that n(0)=np 0exp(A# s) and 84 p(0)=Pp O exp(-A* s) A.5 I t can be seen that n(0)>>p(0)»n i so that J 8 C =-q*s«p(0) A. 5 As w i l l be shownjthe sur f a c e recombination at the i n t e r f a c e w i l l dominate and using a value of 10 s cm/sec f o r s s * 6 3 one a r r i v e s at p ( 0 ) ^ 1 0 1 4 cm" 3 and t h i s g i v e s a value f o r p by usin g equation A.5. The m i n o r i t y c a r r i e r c o n c e n t r a t i o n i s a l s o expected to have t h i s value at a d i s t a n c e between the edge of the space charge region and the d i s t a n c e small compared t o Ln (-10 um) from the i n t e r f a c e because of charge n e t u r a l l i t y i n the bulk so the value of n ( 0 ) i n equation A.4 w i l l be replaced by t h i s v a l u e . Figure A.1 i s a p l o t of * g vs f o r d i f f e r e n t v alues of n T /n . I t can be seen that the change i n • can be J J po s u b s t a n t i a l . By using A.4 and A.5 E- c /c. (2kT/«) 1/ 2p(0)expU* ) 8 x 3 S u s i n g E= 1 X10 6 V/cm g i v e s a value of .15 eV f o r • Using the value of • =.3 eV can be seen to be about 10" 7 amp/cm2 so that t h i s component of the c u r r e n t w i l l have l i t t l e e f f e c t . F igure A.I diagram of i l l u m i n a t e d region A2 Calculated plot of * $ vs i l l u m i n a t i o n 8 6 Main Program PHOTO APPENDIX B A L i s t of Programs Used S u b r o u t i n e s Used D e s c r i p t i o n MV0RG,ADC12, RANDAL,STEP, Main S c a n n i n g Program Documention Memol Re f l e e MV0RG,ADC12 DDMAP Tr a n s C-V q u i c k s c a n n i n g Memol f o r p l o t t i n g of Memo2 d a t a f o r m e a s u r i n g t h e p h o t o c u r r e n t t r a n s i e n t s measures and p l o t s C-V c h a r a c t e r i s t i c s 

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