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CLOSING ADDRESS
W. Paul
To cite this version:
W. Paul. CLOSING ADDRESS. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-1165-C4-1171.
�10.1051/jphyscol:19814247�. �jpa-00220879�
CLOSING ADDRESS
W. Paul
Division o f Applied Sciences, Harvard U n i v e r s i t y , Cmbridge, Massachusetts 02138, U.S.A.
I n t r o d u c t i o n . - The t a s k g i v e n me f o r t h i s C l o s i n g Address was n o t c l o s e l y d e f i n e d . I suppose it was accepted t h a t I had t o o f f e n d t w o - t h i r d s of t h e p o p u l a t i o n by n o t a t t e n d i n g t h e i r s e s s i o n s and t h a t , i n my r e p o r t , I might o f f e n d t h e r e s t by d i s p l a y - i n g misinformation o r misunderstanding on t h i s o r t h a t s p e c i f i c p o i n t which had been e x 2 l a i n e d i n d e t a i l adequate f o r even a c h i l d t o understand. I ' m a f r a i d I cannot develop o v e r n i g h t u n i v e r s a l l y a c c e g t a b l e new e x p l a n a t i o n s t o problems good minds have considered a t some l e n g t h . And s o , I a m condemned t o d i s a p p o i n t some and p r - haps enrage o t h e r s . Perhaps t h a t i s my f u n c t i o n a f t e r a l l . I a m reminded of a con- f e r e n c e I went t o a t S t . Andrews i n Scotland on Polarons and E x c i t o n s , i n 1962.
A f t e r a most e n j o y a b l e banquet - t h e y c a n , you know, d i n e very w e l l i n t h e s e u n c i v i - l i z e d p a r t s , helped by t h e s s c i a l medicinal q u a l i t i e s of t h e l o c a l w a t e r s
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a Scot-t i s h clergyman rose t o e x p l a i n t h e f u n c t i o n o f h i s church i n S c o t t i s h l i f e . I c a n ' t t e l l why now he gave such a sermon, although it may be r e l e v a n t t h a t he had been l i s t e n i n g most of t h e evening t o a d i s s e r t a t i o n on p o l a r o n s from an I l l i n o i s c o l - league. The S c o t t i s h m i n i s t e r f e l t he had t o e x p l a i n t h a t h i s f u n c t i o n was n o t t o p r e v e n t t h e S c o t s from s i n n i n g ; t h a t was g r a n t e d t o be i m p o s s i b l e ; t h e aim was t o p r e - v e n t them e n j o y i n g i t . Now, I ' m n o t t a k e n a l l t h a t o f t e n f o r a churchman, although some of you may remember how I stopped t h e r a i n i n Cambridge two y e a r s ago. Never- t h e l e s s , i f you s l e e p through my o p i n i o n of your s c x e n t i f i c work, may you n o t e n j o y it and may you be rewarded with a niqhtmare of a cascade of i r r e v e r s i b l e , i r r e p r o - d u c i b l e d a t a .
General Remarks.- The composition 0.f t h e conference showed a continued t r e n d toward hydrogenated a - S i , with o v e r h a l f of t h e s e s s i o n s devoted t o t h a t g e n e r a l t o p i c . Tilere were, of c o u r s e , s t i l l many s e s s i o n s on c h a l c o g e n i d e s , and t h e r e were s e s s i o n s on o x i d e s and s u l f i d e s and, h a p p i l y , l i q u i d s . I have l i t t l e t o s a y on any of t h e s e . F r i e n d s t e l l me of advances i n accumulation of f a c t s b u t no s i g n i f i c a n t new develop- ment of t h e impact o f t h e Anderson paper on n e g a t i v e U o r t h e subsequent work by Mott and S t r e e t , and by K a s t n e r , Adler and F r i t z s c h e on n e g a t i v e U d e f e c t s .
Nor s h a l l I say much about t h e o r y , e x c e p t t o remark t h a t l o c a l i z a t i o n t h e o r y seems t o be s e t t l i n g down t o a n e a r consensus. P r o f e s s o r Mott, however, h a s ex- p r e s s e d h i s c o n t i n u i n g r e s e r v a t i o n s about t h e a p p l i c a t i o n s of t h e B e l l Labs t h e o r i e s t o t h e minimum m e t a l l i c c o n d u c t i v i t y ; we have enough e x p e r i e n c e of h i s u n e r r i n g p h y s i c a l sense t o withhold f i n a l judgment on t h e m a t t e r . The o n l y o t h e r comment I would make h e r e i s t h a t we need t o encourage more t h e o r y o f a b a s i c s o r t a t t h e s e c o n f e r e n c e s , b u t we s h a l l o n l y g e t t h a t i f e x p e r i m e n t e r s determine t h a t t h e y a r e going t o d e s i g n experiments t h a t w i l l t r y t o answer some of t h e d i f f i c u l t fundamen- t a l q u e s t i o n s , on systems amenable t o b a s i c t h e o r y unburdened by u n r e a l i s t i c assump- t i o n s .
The r e a s o n s f o r t h e dominance o f a-SiH l i e i n i t s p r o v i d i n g a m a t e r i a l with a low gap d e n s i t y of s t a t e s which p e r m i t s , on t h e one hand, fundamental s t u d i e s and, on t h e o t h e r , t h e p o t e n t i a l f o r t e c h n o l o g i c a l a p p l i c a t i o n s . T h i s b r i n g s me t o t h e f o l l o w i n g conclusion: t h e impressive p r o g r e s s i n a p p l i c a t i o n s h a s not been matched s o f a r b y improvements i n fundamental u n d e r s t a n d i n g . Need I even l i s t t h e s e a p p l i c a - t i o n s
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t o s o l a r c e l l s of 7 k % e f f i c i e n c y , t o xerography, t o v i d i c o n s , t o FET's, t o u l t r a f a s t r a d i a t i o n d e t e c t o r s . These advances have been very r a p i d ( s a y , on t h e t i m eArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814247
C4- 1 166 JOURNAL DE PHYSIQUE
s c a l e of development o f e a r l y c r y s t a l l i n e t r a n s i s t o r d e v i c e s ) , and e x c i t i n g , even e x h i l a r a t i n g ; o u r s u b j e c t i s a l i v e and developing r a p i d l y i n many d i r e c t i o n s . But much of t h e advance h a s been t h e r e s u l t of improvements i n d e v i c e technology by sound e n g i n e e r i n g methods, r a t h e r t h a n t h e r e s u l t of fundamental understanding. U 1 - t i m a t e l y , i n my view, t h e s c i e n c e and t h e technology w i l l s t a y v i b r a n t and o r i g i n a l o n l y i f i d e a s flow t o , and back from, b a s i c p h y s i c a l i n v e s t i g a t i o n . Sometimes,
"The world i s t o o much w i t h u s Late and soon, g e t t i n g and spending We l a y waste our powers"
Wordsworth d i d n ' t have t o w r i t e p r o j e c t a p p l i c a t i o n s o r r e p o r t s , b u t h i s words c a s t a l o n g shadow forward. Our e x p e r i m e n t a l and t h e o r e t i c a l s t u d i e s on fundamental pro- c e s s e s which have no immediately e v i d e n t a p p l i c a t i o n have been s l i g h t e d , b u t t h e y must be n u r t u r e d . My focus i s t h e r e f o r e on t h e s t a t u s o f o u r u n d e r s t a n d i n g o f b a s i c p r o c e s s e s . I s h a l l say l i t t l e more about d e v i c e s , e x c e p t t o mention t h a t I e x p e c t we s h a l l be h e a r i n g a good d e a l more about new m a t e r i a l s , such a s a l l o y s of Si-Ge-C-Sn, which w i l l be u s e f u l t o t a i l o r t h e s i z e of t h e energy gap and a d j u s t t h e gap d e n s i t y of s t a t e s f o r s p e c i f i c a p p l i c a t i o n s . We s h a l l a l s o , I e x p e c t , h e a r more about t h e e f f e c t s o f a d d i t i v e s such a s H , F and 0. These s t u d i e s on m a t e r i a l s w i l l s u r e l y g e n e r a t e new c o n c e p t s and new q u e s t i o n s , such a s t h e p r e f e r e n t i a l attachment o f H t o d i f f e r e n t a l l o y components, t h e randomicity of d i s t r i b u t i o n of a l l o y components, t h e p o s s i b i l i t y o f phase s e p a r a t i o n , and t h e p o s s i b i l i t y of g e n e r a t i n g mixed microcrys- t a l l i n e and amorphous phases of very s m a l l (and v e r y i n t e r e s t i n g ) dimensions.
Our Understanding of Fundamental P r o c e s s e s . - What a r e t h e s e fundamental t o p i c s on which I could s e e r e l a t i v e l y slow p r o g r e s s ? I n p o s i n g t h e q u e s t i o n , I wonder i f some o f t h o s e i n t h e audience who have r e c e n t l y e n t e r e d t h e f i e l d r e a l i z e how few of t h e i d e a s o r c o n c e p t s o r models we r e f e r t o a r e r e a l l y w e l l e s t a b l i s h e d ?
T r a n s p o r t p r o c e s s e s i n t h e bands
above/below t h e m o b i l i t y edge ( m o b i l i t y edge ? ) through gap s t a t e s
Recombination p r o c e s s e s Nature of t h e d e f e c t s t a t e s Source o f t h e t a i l s t a t e s
Thermalization of h o t c a r r i e r s i n t h e bands Modes of d e f e c t compensation
P o l a r o n i c e f f e c t s and s t o k e s s h i f t s
F i r s t Viewgraph
L 4
Under t h e f i r s t heading a r i s e t h e unresolved q u e s t i o n s of d i f f u s i o n , hopping, pola- r o n s and shallow t r a p p i n g and r e l e a s e . A l l i e d with t h e s e q u e s t i o n s a r e t h o s e of d i s - p e r s i v e / n o n d i s p e r s i v e t r a n s p o r t and which a s p e c t s o f f i l m s ' s t r u c t u r a l and chemical p r o p e r t i e s may be c o r r e l a t e d with t h e mode of t r a n s p o r t . Under t h e second heading we have t o c o n s i d e r when recombination p r o c e s s e s a r e geminate o r nongeminate, and which n o n r a d i a t i v e recombination p r o c e s s e s t h e r e a r e , and what t h e recombination p a t h s may be f o r d i f f e r e n t gap d e n s i t i e s of s t a t e s (GDOS)
.
Under t h e t h i r d heading we should note t h a t , although t h e r e a r e only a few g- v a l u e s found i n e l e c t r o n s p i n resonance and l i g h t - i n d u c e d e l e c t r o n s p i n r e s o n a n c e , and t h e r e a r e only two n.m.r. l i n e s , t h e s i m p l e , u b i q u i t o u s , d e f e c t s t a t e s have n o t been r e l i a b l y and u n i q u e l y i d e n t i f i e d . Nor h a s t h e r e been any d e f i n i t i v e e v i d e n c e , a s opposed t o s p e c u l a t i o n , on m u l t i p l e d e f e c t s o r d e f e c t - i m p u r i t y complexes.
T a i l s t a t e s have been i n v e s t i g a t e d f o r some t i m e , b u t whether t h e y ( o r t h e phenomena such a s e x p o n e n t i a l a b s o r p t i o n edges l i n k e d t o them) stem from l o n g range Coulomb p o t e n t i a l s from randomly d i s t r i b u t e d c h a r g e s , o r f l u c t u a t i n g gaps produced by chemical o r s t r u c t u r a l inhomogeneity, o r d i s o r d e r on a more l o c a l s c a l e of ang- s t r o m s , i s n o t known.
I make no bones about s t a t i n g t h e main p a r t of my answer, which i n f a c t a l s o formed t h e end of a review p a p e r on amorphous semiconductors I gave a t t h e Warsaw Conference i n 1972. There, I r e f e r r e d t o t h e p r e f a c e of S h o c k l e y ' s t e x t i n which he i n s t r u c t e d t h e i n v e s t i g a t o r t o ask t h e q u e s t i o n s on t h e second viewgraph.
( 1 ) What a r e t h e atoms involved and how a r e t h e y a r r a n g e d ? ( 2 ) How d i d t h i s arrangement come i n t o b e i n g ?
( 3 ) How does t h i s arrangement l e a d t o c e r t a i n mechanisms of e l e c t r o n i c and atomic motion?
( 4 ) How do t h e s e mechanisms g i v e r i s e t o t h e observed p r o p e r t i e s ? Second Viewgraph
b
I t seems almost t r i t e and obvious. The e a r l y work d i d c o n c e n t r a t e on X-ray and e l e c t r o n d i f f r a c t i o n s t u d i e s and on s t r u c t u r a l modelling. Continuous random networks were one r e s u l t . We a l s o h e a r d about d e n s i t y d e f i c i t s from t h e c r y s t a l and r e g i o n s o f low a t o m i c / e l e c t r o n i c d e n s i t y . But, when it came t o analyze p r o p e r t i e s - a grow- i n g wealth o f e s o t e r i c p r o p e r t i e s , e s p e c i a l l y a f t e r t h e Dundee breakthrough and t h e r e a l i z a t i o n o f t h e r o l e o f H
f i e l d e f f e c t , DLTS, C(V)
,
C(U) SCLC PL i n a l l i t s a s p e c t sTRS i n a b s o r p t i o n ,
no,
PLH a l l e f f e c t , T .E .P.
,
c o n d u c t i v i t y Magnetic e f f e c t swhat d i d we do? We proceeded t o use t h e a v a i l a b l e t h e o r y which was f o r a homogeneous d i s o r d e r e d network w i t h o u t a s k i n g how it might be modified f o r t h i n f i l m s d e p o s i t e d from vapors.
T h i s conference marks, I t h i n k , a r e t u r n t o t h e r e a l i z a t i o n t h a t t h e f i l m h e t e r o s t r u c t u r e may be i m p o r t a n t . Atoms i n f i l m s d e p o s i t , n u c l e a t e i n c l u s t e r s , grow t o g e t h e r , and t h e n u c l e i must u n i t e . How t h e y do s o d e t e r m i n e s t h e m i c r o s t r u c t u r e , and we can imagine many p o s s i b i l i t i e s , a s c r u d e l y d e p i c t e d on t h e t h i r d viewgraph.
1. What i s A? 0 a c r y s t a l ?
a CRN of one component, few D.B.?
a modified CRN with a few t e r m i n a t i n g atoms such a s H?
a random a l l o y of one o r more c o o r d i n a t i o n s ? 2. What i s B? Connective t i s s u e of an undefined s o r t b u t
d i s o r d e r e d
*
r i c h i n D.B.*
low d e n s i t yi n SiH, r i c h i n H , r i c h i n m u l t i p l e H-bonds 3 . Morphology?
4. Two p h a s e s o f d i f f e r e n t s t r u c t u r e and c h e m i s t r y band s t r u c t u r e and GDOS
t r a n s p o r t , o p t i c a l , recombination p r o p e r t i e s T h i r d Viewgraph
C4-1168 JOURNAL DE PHYSIQUE
M a t e r i a l A may p o s s i b l y be a c r y s t a l . We h e a r d an account of t h e important work of Veprek, based on t h e n o t i o n t h a t t h e d e c i s i v e parameter c o n t r o l l i n g t h e s t r u c t u r e i s t h e e x t e n t o f t h e d e p a r t u r e of t h e system from chemical e q u i l i b r i u m a t t h e plasma - s o l i d i n t e r f a c e . T h i s was backed up by f u r t h e r r e p o r t s from Spear, R i c h t e r , Uchida, Tsu and o t h e r s which e x p l o i t e d t h e s e i d e a s o r i n v e s t i g a t e d t h e consequences of micro- c r y s t a l l i n i t y . M i c r o c r y s t a l s , we h e a r d , could be produced by t h r e e p r o c e s s e s : (i) h e a t i n g , ( i i ) adjustment downward of t h e s ~ H ~ / H ~ r a t i o and p o s s i b l y o t h e r parameters o f t h e glow-discharge method, and ( i i i ) s p u t t e r i n g of c-Si i n A r - H 2 plasmas of h i g h H p a r t i a l p r e s s u r e .
However, m a t e r i a l A could a l s o be any one of t h r e e o t h e r p o s s i b i l i t i e s . Mate- r i a l B was a l s o n o t c l o s e l y d e f i n e d , b u t t h e g e n e r a l d e s c r i p t i o n given seemed t o be a c c e p t e d . The dimensions of A and B were u n c e r t a i n ; o f A , perhaps 50-100
W,
based on evidence from T .E .M. s t u d i e s and more i n d i r e c t e v i d e n c e from p r o p e r t y measurements.The dimensions and c o n f i g u r a t i o n s of B were more u n c e r t a i n , with some s t u d i e s sug- g e s t i n g f i n e f i l a m e n t s , o t h e r s t i s s u e o f about 10-20 a c r o s s . The c o n n e c t i v i t y of A and B was o n l y c o n j e c t u r e d .
In sum, though, a r e a l i s t i c model f o r 1981 t h a t w i l l need more i n v e s t i g a t i o n by 1983 would seem t o i n v o l v e two phases o f d i f f e r e n t s t r u c t u r e and c h e m i s t r y , band s t r u c t u r e and d e f e c t band s t r u c t u r e , and t r a n s p o r t , o p t i c a l and recombination proper- t i e s . Such a two-phase model h a s t h e p o t e n t i a l t o d i s p l a y many o f t h e anomalous e f f e c t s we s t r u g g l e t o e x p l a i n on t h e b a s i s o f t h e o r y f o r a homogeneous s u b s t a n c e ,
f o r example, t h e e f f e c t s of water v a p o r , t h e e f f e c t s of i n t e n s e i l l u m i n a t i o n , a l l manner of i n s t a b i l i t i e s , and a l s o a group of t r a n s p o r t anomalies of l o n g s t a n d i n g , such a s t h e famous d i f f e r e n c e between E, and Eo, and k i n k s i n t h e high t e m p e r a t c r e dependence of c o n d u c t i v i t y on t e m p e r a t u r e . Of c o u r s e , t h e r e may, a s h a s been sug- g e s t e d , be o t h e r e x p l a n a t i o n s ; b u t t h e consequences of h e t e r o s t r u c t u r e must be ex- p l o r e d a t t h i s j u n c t u r e .
Theory and Experiment f o r Homogencous Substances.- I s h a l l ignore how one s k i r t s around t h e d i f f i c u l t y of h e t e r o s t r u c t u r e i n o r d e r t o still o b t a i n qood d e v i c e s . Ob- v i o u s l y t h i s h a s a l r e a d y been done e m p i r i c a l l y and I e x p e c t t h i n g s w i l l c o n t i n u e t o improve. I n s t e a d I a s k : how can we c o n s t r u c t a d i s o r d e r e d semiconductor o f reason- a b l y e s t a b l i s h e d s t r u c t u r e , homogeneous on a s c a l e t h a t t h e p r o p e r t i e s of t h e homo- geneous phase may be i d e n t i f i e d and c o n f r o n t e d with t h e o r y ? Some p o s s i b l e ways, a number o f them very s p e c u l a t i v e i n d e e d , a r e shown on t h e f o u r t h viewgraph.
How do we f i n d t h e p r o p e r t i e s of t h e more-or-less homogeneous
" i s l a n d " phase A ?
e x t r a p o l a t i o n from m i c r o c r y s t a l s
d i s c o v e r y of s p e c i a l p r e p a r a t i o n c o n d i t i o n s minimizing t i s s u e
CVD m a t e r i a l , l i g h t l y p o s t hydrogenated laser-amorphized c r y s t a l s
c r y s t a l s d i s o r d e r e d by bombardment
e x t r a p o l a t i o n from 4-6 g l a s s e s of v a r i a b l e composition What p r o p e r t i e s would it be b e s t t o s t u d y ?
Fourth Viewgraph
I
It i s n ' t c l e a r what p r o p e r t i e s should be s t u d i e d . T r a n s p o r t may be h a r d e s t o f a l l . Perhaps photoluminescence a t low t e m p e r a t u r e s , p e r h a p s AC t r a n s p o r t , p e r h a p s t h e low energy (one-phase?) p a r t o f t h e a b s o r p t i o n edge. I have more q u e s t i o n s t h a n answers h e r e .
The Noah ' S Ark E f f e c t
.-
Our problems seem t o come i n p a i r s .5 . bound/"unbound" H 6. two PL l i n e s 7. i s l a n d s / t i s s u e
No f i n a l l y e s t a b l i s h e d ( t e n t a t i v e l y e s t a b l i s h e d ? ) one-to-one c o r r e l a t i o n between d i f f e r e n t p a i r s .
F i f t h Viewgraph
I
The. v i b r a t i o n a l o p t i c a l a b s o r p t i o n n e a r 2090 cm-' seems now t o be a s s i g n e d b o t h t o Si-H2 and t o a n o t h e r u n i d e n t i f i e d , b u t b a s i c a l l y "monohydride" e n t i t y , s i n c e t h e l i n e i s seen i n t h e absence of bending modes n e a r 800-900 cm-'.
Less w e l l recognized i s t h e f a c t t h a t t h e 2000 cm-' l i n e must a l s o correspond t o H i n two f a i r l y d i s t i n c t environments, s i n c e f i l m s w i t h p u r e 2000 cm-' mode ab- s o r p t i o n , and no bending modes, s t i l l g i v e a broad and a narrow NMR l i n e .
There a r e two NMR l i n e s i n most samples, and a p p a r e n t l y no more t h a n two. What s o r t s of H environment l e a d t o such d i f f e r e n t l i n e s , and n o t a s u p e r p o s i t i o n of l i n e s o f s e v e r a l w i d t h s ?
When many ( n o t a l l ) samples of a-SiH a r e h e a t e d , t h e H e v o l v e s i n two s t a g e s , one n e a r 350°c, and t h e second n e a r 6 5 0 ' ~ . The e v o l u t i o n s p e c t r a have d i f f e r e n t k i n e - tics. C o r r e l a t i o n o f t h e e v o l v i n g H - e n t i t y u n i q u e l y with o t h e r p r o p e r t i e s h a s n o t been achieved.
When H i s evolved from a-SiH n e a r t h e 350°c peak, some of it comes o u t with l i t t l e o r no o b s e r v a b l e change i n t h e i n f r a r e d v i b r a t i o n a l spectrum. The s o u r c e o f t h e "unbound" H h a s n o t been e s t a b l i s h e d .
There a r e two photoluminescence l i n e s i n many r e p o r t s , an " i n t r i n s i c " one n e a r 1.4 eV and a d e f e c t - r e l a t e d one n e a r 0.9 eV. A t Grenoble t h e f a m i l y h a s r e c e n t l y been expanded a s we h e a r d a t t h i s c o n f e r e n c e .
And, o f c o u r s e , t h e r e i s i s l a n d and t i s s u e , sometimes demonstrated d i r e c t l y ( c e r t a i n TEM e x p e r i m e n t s ) , o t h e r w i s e modelled. I t i s t o be noted t h a t t h e c o n n e c t i v e t i s s u e i s a phase whose p r o p e r t i e s may vary from l o c a t i o n t o l o c a t i o n i n t h e f i l m .
A s f a r a s I know, t h e r e i s no e s t a b l i s h e d one-to-one c o r r e l a t i o n between t h e d i f f e r e n t p a i r s . Some of t h e s e q u e s t i o n s a r e o l d o n e s , b u t t h e y remain w i t h u s , un- r e s o l v e d .
S p e c i a l l y Notable New Techniques
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2. TRS of a l l forms induced a b s o r p t i o n PL
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3 . Measurement o f s m a l l a A 0
guided wave PAS
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PHYSIQUEI have time t o make s p e c i a l comment on o n l y a few of t h e s e . The NMR experiments have been w i t h us f o r some t i m e , b u t t h e y c o n t i n u e t o d i s p l a y a g r e a t r i c h n e s s of i n f o r m a t i o n , a s was e v i d e n t from Reimer' s review t a l k . The d i f f e r e n t time-resolved spectroscopy experiments a r e j u s t now coming t o t h e f o r e . The experiments by Vardeny, Tauc and Fang on induced a b s o r p t i o n i n amorphous SiH, S e , As2Se3, u s i n g 2 eV r a d i a - t i o n and o b s e r v a t i o n s o v e r t h e time domain of 0.5 t o 150 p s , showed d i s t i n c t d i f f e r - e n c e s among t h e t h r e e m a t e r i a l s . Apparently t h e r m a l i z a t i o n t o a band edge i s over i n a t l e a s t 2 p s , and t h e t h e r m a l i z a t i o n time (and t r a p p i n g t i m e ) i s s h o r t e s t i n a-Si.
Photoluminescence i n t h e time domain of nanoseconds t o microseconds h a s become an e s t a b l i s h e d t e c h n i q u e , . a n d t h e s p e c t r a d i s p l a y e f f e c t s of c a r r i e r r e l a x a t i o n through band t a i l s t a t e s a s w e l l a s e f f e c t s of Coulomb i n t e r a c t i o n s . Of some i n t e r e s t was C o l l i n s ' demonstration (which agreed with t h e d e d u c t i o n s of Vardeny e t a l . ) t h a t t h e i n i t i a l e l e c t r o n - h o l e s e p a r a t i o n was independent of t h e e x c e s s energy of t h e e x c i - t i n g photon o v e r band gap energy. C o l l i n s a l s o p r e s e n t e d evidence t h a t it might be p o s s i b l e , from t h e time decay o f PL u s i n g d i f f e r e n t photon e n e r g i e s of e x c i t a t i o n , t o d i f f e r e n t i a t e between i n i t i a l e x c i t a t i o n i n t o extended s t a t e s and i n t o l o c a l i z e d s t a t e s . Since t h e e n e r g y a t which t h i s o c c u r s c o r r e s p o n d s t o a r e g i o n where t h e a b s o r p t i o n spectrum i s f e a t u r e l e s s , t h i s might j u s t be an i n d i c a t i o n of t h e e l u s i v e
" m o b i l i t y gap".
Auston r e p o r t e d some p r e l i m i n a r y , b u t very i n t e r e s t i n g , d a t a on induced photo- c o n d u c t i v i t y i n t h e p s domain. He found t h a t t h e room t e m p e r a t u r e "microscopic"
m o b i l i t y was about t h e same f o r e v a p o r a t e d , CVD and glow-discharge S i of very d i f - f e r e n t GDOS. He a l s o found an a c t i v a t e d m o b i l i t y ( a c t i v a t i o n energy 58 meV) i n t h e glow d i s c h a r g e sample, although he s t r e s s e d t h a t t h i s c o u l d t u r n o u t t o be an a r t i - f a c t . I f it i s n ' t , t h e n what i s going on h e r e ? P o l a r o n i c hopping? Trapping and r e - l e a s e i n t o shallow t r a p s , on a s c a l e of 10 p s ?
These measurements seem bound t o give u s more i n f o r m a t i o n i n t h e immediate f u t u r e . T h e i r i n t e r p r e t a t i o n w i l l be l i n k e d t o t h a t of experiments on d r i f t m o b i l i t y v e r s u s T, and on dispersive/nondispersive t r a n s p o r t . May we a l s o t h i n k of spin- resonance experiments i n t h i s time domain?
Now l e t me make a few comments on t h e very important t o p i c of t h e d e t e r m i n a t i o n o f t h e gap d e n s i t y o f s t a t e s . DLTS and space charge l i m i t e d c u r r e n t measurements may now be added t o t h e o t h e r s l i s t e d on t h e s i x t h viewgraph. My p e r s o n a l conclusion i s t h a t no one method h a s y e t been e s t a b l i s h e d a s c l e a r l y s u p e r i o r . But, i n t o o few c a s e s I f i n d , a r e e x p e r i m e n t e r s a t t e m p t i n g t o a p p l y two o r more methods t o t h e same sample i n o r d e r t o o b t a i n comparative r e s u l t s . T h i s should be done. I t a l s o seems t o me t o be e s s e n t i a l t h a t experimenters r e p o r t i n d e t a i l how t h e y p r e p a r e t h i n f i l m samples f o r measurement, s i n c e many r e s u l t s depend i n t i m a t e l y on t h e s t r u c t u r e and chemistry o f i n t e r f a c e s .
F i e l d E f f e c t . - I t i s probably c o r r e c t t h a t t h e problem o f a n a l y s i s of t h e f i e l d e f f e c t t o o b t a i n from t h e observed induced c u r r e n t
a
h a s been s o l v e d . The ques- t i o n l e f t i s t h e i n t e r p r e t a t i o n of how much of t h e deduced DOS i s a t t r i b u t a b l e t o t h e bulk and how much t o s u r f a c e and i n t e r f a c e . The f i e l d e f f e c t c u r r e n t flows i n a channel l e s s t h a n 100 wide. The l a b o r a t o r i e s r e p o r t i n g f i e l d e f f e c t r e s u l t s , m a r i l y now on glow-discharge a-SiH, t e l l us of s t a t e d e n s i t i e s of 1017 eV-Y:;
midgap. I t i s my guess t h a t t h e s e a r e c o r r e c t t o , s a y , a f a c t o r o f 2 o r 3. However, I know t h a t samples with s i m i l a r DOS, prepared b o t h by s p u t t e r i n g and glow d i s c h a r g e
(5% of s o l a r c e l l m a t e r i a l ) , o f t e n d o n ' t show f i e l d e f f e c t a t a l l , because t h e i n t e r f a c e h a s been damaged. Thus, t h i s i s h a r d l y a t e c h n i q u e s u i t a b l e f o r r o u t i n e c h a r a c t e r i z a t i o n . We had a l s o a whole s e s s i o n on Staebler-Wronski e f f e c t s which em- p h a s i z e d t h e d i f f i c u l t i e s of assignment of t h e DOS between bulk and s u r f a c e . C ( V ) and C(d) t e c h n i q u e s a r e l e s s s u s c e p t i b l e t o t h e problems of t h e s u r f a c e , although t h e y i n v o l v e o t h e r d e b a t a b l e assumptions.
A t t h i s conference we h e a r d a l o t about DLTS which should avoid i n t e r f a c e prob- lems, and i n f a c t i s supposed t o be a b l e t o d i f f e r e n t i a t e between s u r f a c e and bulk e f k c t s . We h e a r d o f DOS a s low a s 1014 cm-3 measured on samples which had been doped, which i n c r e a s e s t h e DOS. I t should be mentioned though t h a t Exxon h a s r e p o r t e d
c o n t a i n s a t l e a s t 1020 cm-3 i m p u r i t i e s ? Is valence s a t i s f a c t i o n t h i s c l o s e t o a r e a l i t y ? F r a n k l y , I j u s t d o n ' t know, j u s t a s I d o n ' t know where any flaws i n t h e experiments may l i e .
F i n a l l y , i n t h i s s e c t i o n , l e t - m e draw your s p e c i a l a t t e n t i o n t o t h e experiments on m i c r o c r y s t a l s of t h e o r d e r of 100 s i z e . I have a l r e a d y r e f e r r e d t o Veprek's c o n t r i b u t i o n . I found e s p e c i a l l y i n t e r e s t i n g P r o f e s s o r S p e a r ' s examination of a num- b e r of p r o p e r t i e s a s a f u n c t i o n of m i c r o c r y s t a l l i t e s i z e ; t h a t group i n c l u d e d t h e demonstration t h a t t h e Hall e f f e c t changes s i g n f o r a c r y s t a l l i t e s i z e n e a r 20
W,
a l l o t h e r m a t e r i a l p r o p e r t i e s b e i n g assumed unchanged. P r o f e s s o r Spear r e p o r t e d t h a t t h e m i c r o c r y s t a l l i n i t y d i d n o t appear t o a f f e c t t h e microscopic m o b i l i t y which r e - mained a t a very low v a l u e . I f i n d t h i s s t r a n g e , more e s p e c i a l l y s i n c e my own i n t u i - t i o n i s t h a t even i n t h e amorphous m a t e r i a l e s t i m a t e s o f t h e microscopic m o b i l i t y have been low by about an o r d e r of magnitude. M o b i l i t i e s of 50 cm2/v-S would n o t s u r p r i s e me.
The low temperature ( f o r c e r t a i n c o n d i t i o n s of p r e p a r a t i o n ) a t which micro- c r y s t a l s appear s u g g e s t s t h a t a l l of us examine whether we have i n t h e p a s t r e p o r t e d measurements on m a t e r i a l of some degree of m i c r o c r y s t a l l i n i t y . The whole s u b j e c t s u g g e s t s t h e u t i l i t y of measurement of many o t h e r p r o p e r t i e s v e r s u s g r a i n s i z e and subsequent e x t r a p o l a t i o n back t o g a i n new i n f o r m a t i o n about t h e p r o p e r t i e s of small a s s e m b l i e s , perhaps s o small t h a t t h e d i s t i n c t i o n s between c r y s t a l l i n e and amorphous a r e q u i t e i n d i s t i n c t .
Permit me, a s I c l o s e , a few words of p r a i s e and c r i t i c i s m f o r t h e r e p o r t p r e - s e n t a t i o n s themselves. There were many s p l e n d i d i n v e s t i g a t i o n s r e p o r t e d h e r e . Those s p e a k e r s with a few w e l l - c o n s t r u c t e d l e g i b l e s l i d e s , who had e v i d e n t l y thought through t h e b e s t way t o t r a n s f e r i n f o r m a t i o n t o an i n t e r n a t i o n a l a u d i e n c e , d e s e r v e s p e c i a l t h a n k s and p r a i s e
-
and f u t u r e emulation. I n some c a s e s I should have thought many fewer s l i d e s t o be p r e f e r a b l e , w i t h fewer i n c i d e n t a l o r p e r i p h e r a l com- ments, and slow p r e s e n t a t i o n s I n E n g l i s h t o an audience f o r whom t h a t i s n o t t h e n a t i v e tongue. B e t t e r t o make t h r e e good p o i n t s i n f i f t e e n minutes t h a n f i f t e e n p o i n t s e v e r y t h r e e minutes.I n more g e n e r a l t e r m s , though, t h i s h a s been a r i c h program, worthy of i t s p r e d e c e s s o r s . I know very w e l l what it must have c o s t t h e o r g a n i z e r s i n time and energy, and I am s u r e I speak f o r everyone i n o f f e r i n g t h a n k s and c o n g r a t u l a t i o n s f o r what h a s been arranged h e r e i n Grenoble.
NEXTCONFERENCEANNOUNCEMENT
The 10th International Conference on Amorphous and Liquid Semiconductors will b e held at Tokyo during 22nd
