SCANNING-DLTS

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Submitted on 1 Jan 1989

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SCANNING-DLTS

O. Breitenstein

To cite this version:

O. Breitenstein. SCANNING-DLTS. Journal de Physique Colloques, 1989, 50 (C6), pp.C6-101-C6-

110. �10.1051/jphyscol:1989609�. �jpa-00229640�

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REVUE DE PHYSIQUE

APPLIQUEE

Colloque C6, Suppl6ment au n06, Tome 24, Juin 1989

SCANNING-DLTS

0. BREITENSTEIN

Institute of Solid. State Physics and Electron Milcroscopy of the Academy of Sciences of the GDR, Weinberg 2, DDR-4050 Halle, D.R.G.

R'esumg

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^DLTS ^d bakayage (SDLTS) e s t une t e c h n i q u e c o u r a n t e de l a m i c r o s c o p i e g l e c t r o n i q u e

2

b a l a y a g e p o u r l a d g t e c t i o n d e s d i s t r i b u t i o n s l o c a l e s d e s c e n t r e s

a

p r o f o n d n i v e a u . Dans l e r a p p o r t l e s f o n d a t i o n s p h y s i c a l e s d e l a t e c h n i q u e SDLTS s o n t t r a i t e e s e t l e s n e c e s s i t g e s de l a i n s t r u m e n t a t i o n s o n t d i s c u t g e s . La p r a t i q u e d e mesure e s t d g c r i t e t i l l u s t r ' e 8i l t a i d e de q u e l q u e exemples e x p e r i m e n t a l s . F i n a l e m e n t l e s p o s s i b i l i t s s e t l i m i t a t i o n s d e SDLTS s o n t c r i t i q u e m e n t r e v u s .

A b s t r a c t

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S c a n n i n g Deep L e v e l T r a n s i e n t S p e c t r o s c o p y (SDLTS) i s a c u r r e n t SF&l t e c h n i q u e f o r t h e d e t e c t i o n of t h e l o c a l d i s t r i b u t i o n of deep l e v e l c e n t r e s i n s e m i c o n d u c t o r s . The c o n t r i b u t i o n d e a l s w i t h t h e p h y s i c a l f o u n d a t i o n s of t h e SDLTS t e c h n i q u e and i t d i s c u s s e s t h e demands on t h e i n s t r u m e n t a t i o n . The measurement p r a c t i c e i s d e s c r i b e d and i l l u s t r a t e d by s e v e r a l e x p e r i m e n t a l examples. F i n a l l y , t h e p o s s i b i l - i t i e s a n d l i m i t a t i o n s of SDLTS a r e c r i t i c a l l y reviewed.

1

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INTRODUCTION

Scanning-DLTS (SDLTS) r e p r e s e n t s t h e c o m b i n a t i o n of t h e DLTS d e t e c t i o n t e c h - n i q u e w i t h t h e l o c a l e x c i t a t i o n f a c i l i t y of a s c a n n i n g e l e c t r o n m i c r o s c o p e

(SEN). The f u n c t i o n of t h i s method i s t o image t h e s p a t i a l d i s t r i b u t i o n of n o n - r a d i a t i v e r e c o m b i n a t i o n c e n t r e s i n s e m i c o n d u c t o r s p a c e c h a r g e s t r u c t u r e s

( p n j u n c t i o n s , S c h o t t k y - d i o d e s ) . A s a r u l e , i t i s used i n c o n j u n c t i o n w i t h t h e EBIC imaging t e c h n i q u e t h a t e a s i l y a l l o w s t h e l o c a l i z a t i o n o f c r y s t a l d e f e c t s and o t h e r i n h o m o g e n e i t i e s i n t h e s e s t r u c t u r e s . U n l i k e EBIC, however, SDLTS r e p r e s e n t s a s p e c t r o s c o p i c t e c h n i q u e e n a b l i n g t h e i d e n t i f i c a t i o n of t h e d e f e c t s d i s p l a y e d w i t h r e s p e c t t o t h e i r t h e r m a l i o n i z a t i o n energy. IVIore- o v e r , SDLTS a l l o w s one t o image t h e d i s t r i b u t i o n o f c e n t r e s a c t i n g a s t r a p - p i n g c e n t r e s r a t h e r t h a n a s r e c o m b i n a t i o n c e n t r e s t h a t would n o t a f f e c t t h e E B I C c o n t r a s t . I n t h i s s e n s e , SDLTS and EBIC r e p r e s e n t complementary t e c h - n i q u e s w i t h t h e i r r e s u l t s s u p p l e m e n t i n g e a c h o t h e r .

The SDLTS t e c h n i q u e was f i r s t proposed by P e t r o f f and Lang / I / . T h e i r a r - rangement used t h e c u r r e n t d e t e c t i o n p r i n c i p l e ( s e e n e x t c h a p t e r ) on t h i n n e d specimens w i t h i n a STEN, t h u s a l l o w i n g t h e c o r r e l a t i o n of t h e r e s u l t s n o t o n l y w i t h t h e EBIC-image b u t a l s o w i t h TEN! i n v e s t i g a t i o n s . A f t e r s e v e r a l p r o m i s i n g r e s u l t s o b t a i n e d by t h i s g r o u p ( c f . e.g. /2/ and /3/) t h e method was f u r t h e r d e v e l o p e d t o work a l s o w i t h c a p a c i t a n c e d e t e c t i o n and t o be con-

t r o l l e d by a microcomputer s y s t e m l a r g e l y r a t i o n a l i z i n g t h e measurement p r a c t i c e and t h e image d i s p l a y ( B r e i t e n s t e i n and H e y d e n r e i c h , /4/ and / 5 / ) . TTevertheless, t h e number of groups vrorking w i t h SDLTS i s s t i l l l i m i t e d ( s e e

e.g. /6/ t o /9/). I t i s much t o be hoped t h a t t h e f u t u r e a v a i l a b i l i t y of a commercial s p e c i a l SDLTS equipment w i l l encourage a b r o a d e r a p p l i c a t i o n of t h i s p r o m i s i n g t e c h n i q u e .

I n t h e f o l l o v ~ i n g t h e p h y s i c a l f o u n d a t i o n of SDLTS w i l l be reviewed. Then, some demands on t h e i n s t r u m e n t a t i o n w i l l be d i s c u s s e d . I n c h a p t e r 4 t h e measurement p r a c t i c e i s d e s c r i b e d and i l l u s t r a t e d by i n t r o d u c i n g some measurement examples. F i n a l l y , a c r i t i c a l r e v i e w w i l l be g i v e n of t h e p o s s i b i l - i t i e s and l i m i t a t i o n s of t h e t e c h n i q u e .

2

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PHYSICAL POL2DATIONS

The b a s i c p h y s i c a l p r o c e s s e s u n d e r l y i n g t h e SDLTS t e c h n i q u e a r e d e s c r i b e d e.g. by Sah e t a l . / l o / and i n t h e o r i g i n a l DLTS p a p e r by Lang /11/. There-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989609

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f o r e , t h e y w i l l be reviewed h e r e a s b r i e f l y a s p o s s i b l e f o r d e s c r i b i n g t h e method and i t s e v a l u a t i o n , b u t s p e c i a l a t t e n t i o n w i l l be payed t o t h e phys- i c s of t h e e l e c t r o n beam e x c i t a t i o n i n comparison t o c o n v e n t i o n a l DLTS t r a p i i l l i n g u s i n g b i a s p u l s e s .

The b a s i c i d e a o f t h e DLTS p r o c e d u r e i s t o a p p l y p e r i o d i c a l l y t r a p f i l l i n g p u l s e s t o t r a p s l o c a t e d i n a r e v e r s e - b i a s e d s p a c e c h a r g e s t r u c t u r e (pn-junc- t i o n , S c h o t t k y - d i o d e ) . These t r a p s a r e i n t h e r m a l e q u i l i b r i u m i n t h e i o n i z e d s t a t e and g e t f i l l e d by t h e s e p u l s e s . Then, a f t e r each f i l l i n g t h e s y s t e m t e n d s t o r e a c h .the-ma1 e q u i l i b r i u m by t h e r m a l l y e m i t t i n g c a r r i e r s i n t o t h e c o r r e s p o n d i n g band, where t h e y a r e swept away by t h e e l e c t r i c f i e l d w i t h i n t h e s p a c e c h a r g e r e g i o n . The t i m e c o n s t a n t

re

of t h i s t h e r m a l e m i s s i o n , b e i n g t h e i n v e r s e of t h e s o - c a l l e d e m i s s i o n r a t e en; f o r e l e c t r o n s o r h o l e s , r e s p e c t i v e l y , i s governed by t h e sample t e m p e r a t u r e

8

and t h e t r a p p i n g e n e r g y

rn - - J -

"t

re

⁼ l / e n ; p = ----B--, exp Nc;vCn;

(g: d e g e n e r a c y f a c t o r of t h e l e v e l ; ^Nc.,: e f f e c t i v e d e n s i t y of s t a k e s i n t h e c o n d u c t i o n o r v a l e n c e band, r e s p e c t i v e l y ; cn;p: c a p t u r e c o e f f i c i e n t f o r e l e c t r o n s o r h o l e s , r e s p e c t i v e l y ; k: Boltzmann c o n s t a n t ) . The e m i s s i o n l e a d s t o a m e a s u r a b l e c u r r e n t t r a n s i e n t

J ( t ) = en;p VtNte exp ( - e t ) n ; P

(Vt: volume from where t h e e m i s s i o n c o n t r i b u t e s t o t h e s i g n a l ; N t : t r a p den- s i t y ; e: e l e m e n t a r y c h a r g e ; t: time a f t e r t h e e x c i t a t i o n p u l s e ) and a capa- c i t a n c e t r a n s i e n t

'tNte exp ( - e n ; p t )

" a t ) = 4(V+VD)

(V: a p p l i e d r e v e r s e b i a s ; VD: d i f f u s i o n v o l t a g e ) , which a r e t h e two a l t e r n a - t i v e p r i m a r y measurement s i g n a l s f o r current-DLTS and capacitance-DLTS, r e - s p e c t i v e l y . Under t h e p r e s u p p o s i t i o n of a homogeneous t r a p d i s t r i b u t i o n and t o t a l t r a p f i l l i n g i n t h e whole s p a c e c h a r g e r e g i o n , ( 3 ) can b e w r i t t e n i n t h e more p o p u l a r form

(ND-NA: n e t doping c o n c e n t r a t i o n ; Co: b a s i c sample c a p a c i t a n c e ) . F o r c u r r e n t - DLTS, t h e s i g n a l p o l a r i t y i s i n d e p e n d e n t of w h e t h e r e l e c t r o n s o r h o l e s a r e

e m i t t e d , t h e s i g n a l a m p l i t u d e i s p r o p o r t i o n a l t o t h e e m i s s i o n r a t e , a n d a l s o m i n o r i t y c a r r i e r s e m i t t e d o u t s i d e t h e s p a c e c h a r g e r e g i o n may c o n t r i b u t e t o t h e t r a n s i e n t measured i f t h e y a r e a b l e t o d r i f t t o t h e s p a c e c h a r g e r e g i o n . F o r capacitance-DLTS, on t h e o t h e r hand, m i n o r i t y c a r r i e r e m i s s i o n l e a d s t o p o s i t i v e t r a n s i e n t s a n d m a j o r i t y c a r r i e r e m i s s i o n t o n e g a t i v e o n e s , t h e a m p l i t u d e of t h e t r a n s i e n t i s i n d e p e n d e n t of t h e e m i s s i o h r a t e , and o n l y c a r - r i e r e m i s s i o n w i t h i n t h e s p a c e c h a r g e r e g i o n l e a d s t o a c a p a c i t a n c e t r a n - s i e n t . The a c t u a l DLTS s i g n a l i s formed from t h e measured t r a n s i e n t s i g n a l by a s i g n a l c o s r e l a t i o n . p r o c e d u r e , which i n t h e s i m p l e s t c a s e r e p r e s e n t s t h e s u b t r a c t i o n of t h e s i g n a l v a l u e s a t two t i m e s t l and t 2 a f t e r t h e end of t h e f i l l i n g p u l s e /11/. T h i s d i f f e r e n c e h a s a maximum i f t h e e m i s s i o n r a t e h a s a c e r t a i n v a l u e b e i n g

-

I n ( t 2 / t l )

e

-

n; P (5)

t2-tl

f o r capacitance-DLTS; f o r current-DLTS t h e maximum i s d e f i n e d by t h 6 s o l u - t i o n of t h e e q u a t i o n

1

-

^en._{. P}t ₁ ₌ _{exp (-e} ( t 2 - t , ) )

.

I - e t n ; P

n;P 2

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Hence, t h e s e l e c t i o n of t l and t 2 opens a s o - c a l l e d " r a t e windowr1, i n which t h e emission r a t e h a s t o f a l l f o r t h e DLTS-peak t o appear. Other p o s s i b i l - i t i e s of t h e DLTS-correlation a r e d i s c u s s e d e.g. by M i l l e r e t a l . /12/.

I n a standard-DLTS experiment, a c e r t a i n r a t e window i s s e l e c t e d and t h e sample temperature i s s l o w l y scanned t y p i c a l l y scanning t h e range between l i q u i d n i t r o g e n temperature and somewhat above room temperature. Then, ac- oording t o ( 1 ) t h e emission r a t e s of a l l t r a p s of t h e sample change and a DLTS peak s i g n a l a p p e a r s always a t a c e r t a i n temperature governed by ( I ) and

( 5 ) o r (61, r e s p e c t i v e 1

,

w i t h a peak h e i g h t governed by t h e r e s p e c t i v e t r a p d e n s i t y a c c o r d i n g t o ( 2 q t o ( 4 ) . Repeating t h i s procedure f o r d i f f e r e n t r a t e windows l e a d s t o a s h i f t of t h e temperature p o s i t i o n of t h e DLIS-peaks. The r e s u l t of t h i s procedure i s t h e measurement of t h e temperature dependence of t h e emission r a t e , a l l o w i n g t h e e s t i m a t i o n of t h e i o n i z a t i o n energy Et of t h e corresponding l e v e l s a c c o r d i n g t o ( 1 ) (ArrIienius p l o t ) . S p e c i a l t e c h n i q u e s u s i n g t h e v a r i a t i o n of t h e f i l l i n g p u l s e w i d t h and t h e f i l l i n g p u l s e h e i g h t o r i n t r o d u a i n g a second v c l e a r w - p u l s e a r e known t o measure t h e c a p t u r e c r o s s s e c t i o n s o r t h e depth d i s t r i b u t i o n of t h e l e v e l s /11/.

I n a Scanning-DLTS experiment t h e sample i s mounted t o a c r y o s t a t w i t h i n a SEM ( s e e Fig. I ) , t h e temperature i s f i x e d a t t h e temperature of a c e r t a i n DLTS-peak f o r t h e g i v e n r a t e window, t h e r e v e r s e b i a s i s permanently a p p l i e d t o t h e sample, and o n l y a s m a l l amount of t h e sample a r e a i s e x c i t e d p e r i - o d i c a l l y u s i n g t h e focused e l e c t r o n beam of t h e SEM b e i n g p u l s e d a c c o r d i n g t o t h e s p e c t r o m e t e r a c t i o n . Then, s o l e l y t h e l e v e l s w i t h i n t h i s e x c i t e d a r e a a r e f i l l e d and c o n t r i b u t e t o t h e s i g n a l , whereas t h e r e s t of t h e sample r e - w i n s p a s s i v e . The s i g n a l a t t h e s p e c t r o m e t e r o u t p u t t h u s r e f l e c t s t h e l o c a l d e n s i t y of t h e l e v e l s w i t h i n t h i s e x c i t e d a r e a . Scanning t h e e l e c t r o n beam o v e r t h e sample t h e n l e a d s

t.o

a l i n e s c a n o r , i f a two-dimensional image s c a n i s performed, t o a n image being connected w i t h t h e l o c a l d i s t r i b u t i o n of t h e v e r y l e v e l s e l e c t e d by t h e sample temperature and t h e r a t e window. Scanning t h e sample temperature and m a i n t a i n i n g a f i x e d beam p o s i t i o n , on t h e o t h e r hand, a l l o w one t o measure a DLTS spectrum b e i n g c h a r a c t e r i s t i c f o r t h i s v e r y region. We would propose t o c a l l t h e l a t t e r procedure Local-DLTS r a t h e r t h a n Scanning-DLTS, s i n c e no image i s produced here.

A d e c i s i v e d i f f e r e n c e between standard-DLTS and Scanning-DLTS a r i s e s from t h e d i f f e r e n t t r a p f i l l i n g c o n d i t i o n s f o r both methods, i n some c a s e s r e v e a l i n g d i f f e r e n t t r a p t y p e s f o r a standard-DLTS experiment and f o r an e l e c t r o n beam e x c i t e d SDLTS- o r Local-DLTS experiment, on t h e o t h e r hand. I n b o t h c a s e s t h e e x c i t a t i o n mechanism h a s t o be c o n s i d e r e d a c c o r d i n g a s a pn-junction o r a

P i g , 1

-

SDLTS p r i n c i p l e and s i m p l e s t arrangement u s i n g an x y - p l o t t e r f o r s i g n a l r e c o r d i n g .

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Schottky-diode i s t h e sample under i n v e s t i g a t i o n . I f i n a standard-DLTS experiment t h e r e v e r s e b i a s i s reduced t o z e r o o r o n l y p a r t l y reduced, o n l y m a j o r i t y c a r r i e r s w i l l be c a p t u r e d i n t o t h e corresponding l e v e l s ( m a j o r i t y c a r r i e r p u l s e ) . I f a pn j u n c t i o n i s forward-biased d u r i n g t h e p u l s e , b o t h e l e c t r o n s and h o l e s w i l l be c a p t u r e d ( i n j e c t i o n p u l s e ) . For a S c h o t t k y diode, however, a s a r u l e forward b i a s i n g does n o t i n j e c t m i n o r i t y c a r r i e r s , t h e o n l y d i f f e r e n c e a s t o a m a j o r i t y c a r r i e r p u l s e i s t h a t t h e l e v e l s a r e f i l l e d up t o t h e i n t e r f a c e l a y e r and a l s o i n t e r f a c e s t a t e s a r e f i l l e d and may con- t r i b u t e t o t h e DLTS-signal. Only f o r probably non-ideal Schottky b a r r i e r s some a u t h o r s have r e p o r t e d a l s o m i n o r i t y c a r r i e r t r a p f i l l i n g a f t e r " i n j e c - t i o n " p u l s e s t o appear.

Note t h a t u n l i k e Scanning-DLTS i n a 1 1 t h e s e standard-DLTS c a s e s t h e c a p t u r i n g occurs under z e r o f i e l d c o n d i t i o n s . Under e l e c t r o n beam e x c i t a t i o n , however, due t o t h e permanently a p p l i e d b i a s t h e c a p t u r e always t a k e s p l a c e w i t h i n a more o r l e s s - s t r o n g e l e c t r i c f i e l d , a t l e a s t a s l o n g a s l e v e l s w i t h i n t h e

s p a c e charge r e g i o n a r e considered. Pig. 2 summarizes t h e most important ex- c i t a t i o n geometries w i t h t h e i r b a s i c p r o p e r t i e s . Note t h a t , a s a r u l e , t h e c a p t u r e p r o c e s s i s c o n s i d e r e d t o o c c u r n o t due t o h o t c a r r i e r s w i t h i n t h e g e n e r a t i o n volume but r a t h e r t o t h e r m a l i z e d e x c e s s c a r r i e r s d r i f t i n g away from t h e g e n e r a t i o n volume t o and through t h e s p a c e charge r e g i o n . For a l l geometries, of c o u r s e , t h e two complementary t y p e s e x i s t . S i n c e t h e p e n e t r a - t i o n d e p t h of t h e e l e c t r o n beam s t r o n g l y v a r i e s w i t h t h e SEM a c c e l e r a t i o n v o l t a g e , t h i s p r o v i d e s a s p e c i a l degree of freedom e.g. t o c o n s i d e r a c e r - t a i n pn f u n c t i o n t o be e i t h e r deep ( f o r low a c c e l e r a t i o n v o l t a g e s ) o r shallow f o r h i g h a c c e l e r a t i o n v o l t a g e s ) . lmalogously, a l a r g e r e v e r s e b i a s and/or a low a c c e l e r a t i o n v o l t a g e p r e f e r e n t i a l l y e n a b l e s m a j o r i t y c a r r i e r t r a p s t o be d e t e c t e d i n S c h o t t k y d i o d e s , and v i c e v e r s a . Note t h a t f o r a Schottky-diode t h e g e n e r a t e d m i n o r i t y c a r r i e r s always have t o d r i f t through

t h e metal-semiconductor i n t e r f a c e . Thus, i f t h e diode i s no i d e a l one, hence i f a c e r t a i n oxide l a y e r o r o t h e r i n t e r f a c e - r e l a t e d s t a t e s i n t h e c r y s t a l e x i s t , t h e s e s t a t e s a r e always f i l l e d and c o n t r i b u t e t o t h e SDLTS-signal, g e n e r a l l y forming a broad and r e l a t i v e l y temperature-independent s i g n a l p a r t . Therefore, t h e demands on t h e q u a l i t y of a Schottky-diode f o r b e i n g used f o r t h e SDLTS-application a r e much h i g h e r than f o r b e i n g used i n standard-DLTS experiments

.

Apart from t h e d i f f e r e n t t y p e s of c a r r i e r s b e i n g i n t r o d u c e d i n t o t h e s p a c e charge r e g i o n a c c o r d i n g t o t h e d i f f e r e n t e x c i t a t i o n g e o m e t r i e s , t h e r e a r e two f a c t o r s t h a t might p r e v e n t t h e appearance of a c e r t a i n DLTS-signal i n an

predominmtly majority carrier flovl

Schottky diode

m a j o r i t y and minority c a r r i e r flow

Fig. 2

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SDLIS e x c i t a t i o n geometries.

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e l e c t r o n beam e x c i t e d experiment ( Scanning-DLTS o r Local-DLTS)

,

even i f i t had been d e t e c t e d i n a standard-DLTS experiment b e f o r e . The f i r s t one i s t h e e l e c t r i c f i e l d mentioned above. It i s p o s s i b l e t h a t a c e r t a i n c a p t u r e c r o s s s e c t i o n i s s t r o n g l y field-dependent s o t h a t no t r a p s a r e a b l e t o be f i l l e d w i t h i n t h e e l e c t r i c f i e l d . The p h y s i c a l mechanism might simply be t h a t with- i n t h e s p a c e charge r e g i o n s t h e c a r r i e r s a r e " h o t H , and even i f t h e y were c a p t u r e d t h e i r k i n e t i c energy would s u f f i c e t o cause a spontaneous reemis- s i o n process. A second f a c t o r o n l y h o l d s f o r l e v e l s i n t h e m a j o r i t y c a r r i e r gap h a l f having a l a r g e r c a p t u r e c r o s s s e c t i o n f o r m i n o r i t y c a r r i e r s . Note t h a t f o r a standard-DLTS m a j o r i t y c a r r i e r p u l s e a l s o t h e s e l e v e l s can be f i l l e d , s i n c e no m i n o r i t y c a r r i e r s a r e p r e s e n t . For Scanning-DLTS, however, t h e r e i s no s h a r p d e l i m i t a t i o n of t h e e x c i t a t i o n r e g i o n , hence a t l e a s t a s m a l l amount of m i n o r i t y c a r r i e r s a r e always p r e s e n t d u r i n g t h e e x c i t a t i o n pulse. These m i n o r i t y c a r r i e r s may recombine w i t h m a j o r i t y c a r r i e r s a l r e a d y t r a p p e d , a g a i n p r e v e n t i n g t h e appearance of t h e s e l e v e l s i n an SDLTS-experiment. For t h i s c a s e Woodham and Booker have proposed t h e SDDLTS method u s i n g t h e e l e c t r o n beam a s a " c l e a r t t - p u l s e /8/. The problem w i t h t h i s method i s j u s t t h a t t h e dynamic r a n g e of t h e DLTS-measurement h a s t o exceed t h e r a t i o of t h e sample a r e a t o t h e e x c i t e d a r e a .

U n f o r t u n a t e l y , one can o b v i o u s l y n o t be s u r e t o f i n d a g a i n a l e v e l i n an SDLTS-experiment t h a t p r e v i o u s l y had been c h a r a c t e r i z e d by standard-DLTS

.

P a r t i c u l a r l y f o r Schot tky-diodes

,

a s a r u l e , standard-DLTS r e v e a l s o n l y m a j o r i t y c a r r i e r c e n t r e s , whereas Scanning-DLTS p r e f e r e n t i a l l y d e p i c t s mino- r i t y c a r r i e r t r a p s . Note, however, t h a t many i m p o r t a n t l e v e l s a s t h e EL2- l e v e l i n n-GaAs o r t h e Au-acceptor i n n - s i l i c o n a r e a b l e t o be e x c i t e d i n SDLTS-experiments on S c h o t t k y b a r r i e r s , though t h e y a r e m a j o r i t y c a r r i e r t r a p s ( s e e n e x t s e c t i o n ) . Note a l s o t h a t t h e o p t i c a l l y e x c i t e d DLTS experiment (ODLTS) u s i n g a n LED, a Laser-diode o r a f l a s h lamp f o r e x c i t a t i o n , a s a r u l e , y i e l d s e x c i t a t i o n c o n d i t i o n s v e r y s i m i l a r t o t h o s e i n SDLTS o r Local-DLTS. It i s , t h e r e f o r e , a d v i s a b l e t o c h a r a c t e r i z e t h e samples b e f o r e , u s i n g t h i s t e c h n i q u e ( s e e c h a p t e r 4).

3

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IITSTRUMENTATION

A s Pig. 1 shows, t h e b a s i c i n s t r u m e n t s f o r c a r r y i n g o u t SDLTS a r e a SEM pro- v i d i n g a p u l s e d e l e c t r o n probe, a s r y o s t a t , and an a p p r o p r i a t e DLTS s p e c t r o - meter. Almost a l l SEX o r S T m a r e s u i t a b l e provided t h e y a r e equipped w i t h a beam b l a n k i n g u n i t and have enough s p a c e f o r a c r y o s t a t . The c r y o s t a t should be c o n s t r u c t e d t o move a s l i t t l e a s p o s s i b l e d u r i n g t h e t e m p e r a t u r e s c a n i n o r d e r t o have a w e l l - d e f i n e d p o s i t i o n f o r Local-DLTS experiments. S p e c i a l c a r e ( a n t i c o n t a m i n a t i o n f a c i l i t y , o i l - f r e e vacuum) has t o be t a k e n t o p r e v e n t sample contamination a t low temperatures. The r e s o l u t i o n of t h e SEM i s un- c r i t i c a l , s i n c e t h e s p a t i a l r e s o l u t i o n i s g e n e r a l l y governed by c a r r i e r s p r e a d i n g e f f e c t s , o r i t i s s e l e c t e d i n a well-defined manner ( s e e n e x t sec- t i o n ) . Also a L a s e r Scanning Microscope might be a p p r o p r i a t e f o r b e i n g used i n SDLTS i f i t i s equipped w i t h a n e l e c t r o n i c l i g h t chopper; t h e o n l y problem would be t h a t i t r e q u i r e s s e m i t r a n s p a r e n t c o n t a c t s o r unmet'allized pn- j u n c t i o n s t o be used, and t h a t t h e p e n e t r a t i o n depth i s f i x e d by t h e wave- l e n g t h .

Very s p e c i a l demands, however, have t o be made on t h e DLTS-spectrometer b e i n g used. The b a s i c d i f f i c u l t y i n SDLTS and Local-DLTS i s t h a t t h e DLTS- s i g n a l i s p r o p o r t i o n a l t o t h e e x c i t e d a r e a , hence t h e SDLTS-signal i s smal- l e r t h a n t h e standard-DLTS s i g n a l of t h e same sample by a f a c t o r sample a r e a / e x c i t e d a r e a , which t y p i c a l l y i s i n t h e o r d e r of 104. Hence, t h e s p e c t r o - meter should p r o v i d e a n extremely h i g h s e n s i t i v i t y , and n e v e r t h e l e s s a r e l a - t i v e l y l o n g i n t e g r a t i o n time has t o be s e l e c t e d i n o r d e r - t o g e t t h e s i g n a l o u t of t h e n o i s e . Meanwhile, h i g h l y s e n s i t i v e current-DLTS systems (Borsuk and Swanson / 1 3 / ) a s w e l l a s capacitance-DLTS systems have been i n t r o d u c e d

( M i s r a c h i e t a l . /14/; B r e i t e n s t e i n /15/). These h i g h l y s e n s i t i v e oapa- citance-DLIS systems a r e based on s p e c i a l resonance-tuned b r i d g e c i r c u i t s and o f f e r s e n s i t i v i t i e s below 10-5 pP, whioh f o r a s p a t i a l r e s o l u t i o n of a few microns corresponds t o a t r a p d e n s i t y below 1014 cm-3 o r l e s s t h a n 1000 d e t e c t e d atoms. The l o n g i n t e g r a t i o n time, however, l e a d i n g t o a measurement time i n t h e o r d e r of h a l f a n hour o r more, f o r a two-dimensional image c o m p l i c a t e s a n on-line image r e c o r d i n g a s a y-modulation r e p r e s e n t a t i o n o r v i a t h e SEU CRT-screen. T h e r e f o r e , t h e o n l y s o l u t i o n t o e a s e t h e measure-

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ment and image d i s p l a y i s t o u s e a microcomputer system f o r t h e SDLTS-measurement c o n t r o l and d i s p l a y . The computer s h o u l d a t l e a s t be a b l e t o s o a n t h e e l e c t r o n beam p o s i t i o n , t o d i g i t i z e t h e measured SDLTS-values and t o d i s p l a y t h e p r o p e r l y s c a l e d r e s u l t (y-modulation r e p r e s e n t a t i o n o r gray- p a t c h e s image) e i t h e r on t h e CRT-screen of t h e SEM o r on a g r a p h i e monitor o r a g r a p h i c p l o t t e r . I f t h e computer i s a d d i t i o n a l l y a b l e t o ~ o n t r o l t h e r a t e window, t h e p u l s e width and t h e sample b i a s , hence i f t h e DLTS s p e c t r o - meter i t s e l f can be c o n t r o l l e d by t h e computer, s e v e r a l a d d i t i o n a l measurement p o s s i b i l i t i e s a r i s e ( s e e n e x t s e c t i o n ) .

Another i m p o r t a n t demand on t h e SDLTS s p e c t r o m e t e r i s t h a t i t h a s t o a l l o w a s i m p l e s w i t c h i n g between SDLTS-measurement and EBIC-display of t h e sample, i n o r d e r t o c o r r e l a t e t h e SDLTS-image t o t h e p o s i t i o n s of t h e d e f e c t s w i t h i n t h e sample. F o r a current-DLTS system, which anyway i s equipped w i t h a sample c u r r e n t q m p l i f i e r , t h i s i s a t r i v i a l problem; a capacitance-DLTS system, however, has e s p e c i a l l y t o be equipped w i t h a n EBIC-amplifier. Then, t h e consequent s o l u t i o n i s t o d e s i g n t h e SDLTS-spectrometer a s a combined c u r r e n t - and capacitance-DLTS s p e c t r o m e t e r o f f e r i n g t h e d i f f e r e n t p o s s i b i l - i t i e s of b o t h t e c h n i q u e s t o be used a t choice.

4

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MEASUREMENT PRACTICE

A s a r u l e , f o u r d i f f e r e n t s t e p s may be a s s o c i a t e d w i t h a Scanning-DLTS i n - ves t i g a t i o n :

1. An e l e c t r o n beam e x c i t e d DLTS-measurement ( t e m p e r a t u r e s c a n ) of t h e whole sample a s t o i d e n t i f y t h e l e v e l s a p p e a r i n g under e l e c t r o n beam e x c i t a t i o n ( i d e n t i f i c a t i o n measurement).

2. The a c t u a l SDLTS measurement ( l i n e s c a n o r image s c a n ) . 3. The o p t i m i z a t i o n of t h e e x c i t a t i o n i n t e n s i t y .

4. The check of t h e p h y s i c a l s i g n i f i c a n c e of c e r t a i n SDLTS image s t r u c t u r e s . The i d e n t i f i c a t i o n measurement simply r e p r e s e n t s a Local-DLTS measurement u s i n g a s t r o n g l y defocused beam and a h i g h beam c u r r e n t t o e x c i t e almost t h e whole sample a r e a . Note t h a t t h e c o n d i t i o n s f o r c e r t a i n peaks t o a p p e a r can be optimized by v a r y i n g t h e a c c e l e r a t i o n v o l t a g e and/or t h e sample b i a s ac- c o r d i n g t o t h e d i s c u s s i o n i n c h a p t e r 2. I n o r d e r t o measure t h e thermal ac- t i v a t i o n energy of t h e l e v e l s a c c o r d i n g t o (I), t h e i d e n t i f i c a t i o n measurement h a s t o be r e p e a t e d f o r d i f f e r e n t r a t e windows; a s a r u l e t h i s i n v e s t i g a - t i o n can be c a r r i e d o u t a l s o o u t s i d e t h e SEM u s i n g o p t i c a l e x c i t a t i o n tech- n i q u e s (ODLTS)

.

The e x c i t e d a r e a of t h e SDLTS-experiment should be i n t h e o r d e r of t h e p i x e l a r e a of t h e image ( d i a m e t e r = d i s t a n c e between t h e measurement p o i n t s ) . I f t h e e x c i t e d a r e a i s l a r g e r t h e s p a t i a l r e s o l u t i o n drops, and i f i t i s smal- l e r t h e s i g n a l h e i g h t i s u n n e c e s s a r i l y reduced, and c e r t a i n i m p o r t a n t s i g n a l p a r t s w i l l probably n o t be d i s p l a y e d due t o t h e incomplete s c a n n i n g of t h e a r e a . The two f a c t o r s governing t h e e x c i t e d a r e a a r e t h e a r e a of beam i n c i - dence and t h e e x c i t a t i o n i n t e n s i t y ( t h e product of t h e beam c u r r e n t times t h e p u l s e w i d t h ) , governing t h e e f f e c t of t h e i n e v i t a b l e c a r r i e r s p r e a d i n g . I f t h e e x c i t a t i o n i n t e n s i t y i s t o o h i g h , a l s o l e v e l s w i t h i n a c e r t a i n r e g i o n o u t s i d e t h e a r e a of i n c i d e n c e a r e f i l l e d ( w o v e r e x p o s u r e e f f e c t v ) . The lower l i m i t of t h e s p a t i a l r e s o l u t i o n i s s e t by t h e d i a m e t e r of t h e e x c i t e d volume w i t h i n t h e c r y s t a l . The a r e a of i n c i d e n c e can be f i x e d i n a well-defined way e i t h e r by u s i n g a d e f i n e d defocus of t h e beam o r by performing a " p i x e l scanf1 a s i t was i n t r o d u c e d by Woodham and Booker /8/ f o r Local-DLTS, hence by q u i c k l y scanning t h e focused e l e c t r o n beam a c r o s s t h e d e s i r e d p i x e l a r e a a t l e a s t once d u r i n g each e x c i t a t i o n pulse. I n Scanning-DLTS t h i s p i x e l s c a n can be managed by computer-control t o a c h i e v e a complete non-overlapping subsequent scanning of t h e whole a r e a p i x e l by p i x e l . I f t h e a r e a of i n c i - dence i s w e l l - d e f i n e d , t h e problem of f i n d i n g o u t t h e optimum beam c u r r e n t and/or t h e optimum p u l s e width may be s o l v e d most r e l i a b l y by performing a computer-controlled p u l s e width s c a n ( s e e B r e i t e n s t e i n and Heydenreich /5/).

The optimum i s reached i f t h e s i g n a l dependence of t h e p u l s e width t e n d s t o become s u b l i n e a r ( b e g i n n i n g s a t u r a t i o n ) . A good g e n e r a l r u l e of thumb i s t o work w i t h a n e x c i t a t i o n i n t e n s i t y a s s m a l l a s p o s s i b l e t o have a s i g n a l - t o - n o i s e r a t i o s t i l l a c c e p t a b l e . Then, i f image d e t a i l s of one p i x e l i n s i z e a r e p r e s e n t i n t h e image, a n a d d i t i o n a l c a r r i e r s p r e a d i n g e f f e c t can be ex- cluded.

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The q u e s t i o n of t h e p h y s i c a l s i g n i f i c a n c e of t h e SDLTS s i g n a l h a s a l r e a d y been d i s c u s s e d elsewhere / 5 / . Vfhether some s t r u c t u r e i s indeed caused by t h e i n t e r e s t i n g p o i n t d e f e c t i n s t e a d of t h e s u p e r p o s i t i o n of s p e c t r a l l y d i f f e - r e n t s i g n a l p a r t s can most e a s i l y be checked by performing a Local-DLTS i n - v e s t i g a t i o n a t and b e s i d e s a n i n t e r e s t i n g d e t a i l i n t h e SDLTS-image. An a l t e r n a t i v e procedure c o n s i s t s i n a computer-controlled r a t e window s c a n

( i s o t h e r m a l DLTS, s e e Okushi and Tokamaru / 1 6 / ) , which can most e f f e c t i v e l y be combined w i t h a l i n e s c a n a c r o s s t h e i n t e r e s t i n g d e t a i l (combined r a t e window/line s c a n , s e e below). It should be mentioned h e r e t h a t t h e q u e s t i o n of t h e p h y s i c a l s i g n i f i c a n c e should a c t u a l l y a l s o imply t h e check of t h e homogeneity of t h e n e t doping c o n c e n t r a t i o n w i t h i n t h e r e g i o n of i n t e r e s t . Note t h a t , a t l e a s t a s l o n g a s t r a p emission i n s i d e t h e s p a c e charge r e g i o n

i s c o n s i d e r e d , l o c a l f l u c t u a t i o n s of t h e n e t doping c o n c e n t r a t i o n may a l s o l e a d t o SDLTS-contrasts, even i f t h e t r a p c o n c e n t r a t i o n does n o t vary. For S c h o t t k y diodes t h e r e a r e some methods (energy-dependent /17/ o r bias-dependent /18/ EBIC-measurements) t o measure t h e homogeneity of t h e n e t doping c o n c e n t r a t i o n . The second f a c t o r p r o b a b l y c a u s i n g an induced SDLTS-contrast i s a l o c a l l y v a r y i n g d i f f u s i o n l e n g t h b e i n g i n d i c a t e d by t h e EBIC-contrast i t s e l f . This l e a d s e i t h e r t o a l o c a l l y v a r y i n g e x c i t a t i o n i n t e n s i t y o r , f 6 r current-SDLTS where c a r r i e r emission o u t s i d e t h e s p a c e charge r e g i o n domi- n a t e s , t o v a r i a t i o n s of t h e c o n t r i b u t i n g c r y s t a l volume. F o r t h e l a t t e r c a s e P e t r o f f e t a l . have proposed a deconvolution procedure / 3 / . There a r e , how- e v e r , c r i t e r i a of p l a u s i b i l i t y i n d i c a t i n g t h e v a l i d i t y of SDLTS-results.

Thus, i f an a n t i c o r r e l a t i o n between t h e EBIC- und t h e SDLIS-image e x i s t s ( l a r g e SDLTS-signal i n r e g i o n s of low EBIC-signal) o r i f t h e SDLTS-contrast s t r o n g l y exceeds t h e EXiIC-contrast, t h e r e s u l t can be r e g a r d e d t o be q u a l i - t a t i v e l y v a l i d . Only i f t h e EBIC- and t h e SDLTS-images a r e l o o k i n g v e r y s i m i l a r , an EBIC-induced SDLTS-contrast has t o be assumed. Thus, q u a n t i t a t i v e SDLTS-statements being s c a l e d i n c o n c e n t r a t i o n u n i t s might r e q u i r e some ad- d i t i o n a l e f f o r t t o ensure t h a t f l u c t u a t i o n s of t h e t r a p d e n s i t y a r e indeed t h e o n l y f a c t o r governing t h e SDLTS-contrast, o r t o t a k e o t h e r f a c t o r s prop- e r l y i n t o account. According t o eqs. ( 2 ) and ( 3 ) , however, i t i s always jus- t i f i e d t o s c a l e a n SDLTS r e s u l t i n u n l t s of d e t e c t e d charges.

I n t h e f i r s t experimental example measurements were c a r r i e d out u s i n g a Schottky-diode on a b e v e l l e d s t r u c t u r e c o n s i s t i n g of a l a s e r - r e c r y s t a l l i z e d n-poly-Si l a y e r on an o x i d i z e d S i s u b s t r a t e (SOS-structure). Fig. 3 shows t h e standard-DLTS spectrum and t h e e l e c t r o n beam e x c i t e d DLTS-spectrum of t h e sample. I t i s obvious t h a t t h e n e g a t i v e ( e l e c t r o n t r a p ) peak a p p e a r i n g

Fig. 3

-

Standard-DLTS spectrum ( a ) and e l e c t r o n beam e x c i t e d DLTS spectrwn ( b ) of a L a s e r - r e c r y s t a l l i z e d SOS-structure (Au-Schottky diode).

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i n t h e standard-DLTS spectrum does n o t a p p e a r I n t h e SDLTS i d e n t i f i c a t i o n measurement. I n s t e a d , a p o s i t i v e ( h o l e t r a p ) peak a p p e a r s a t 267 K and an a d d i t i o n a l broad p o s i t i v e band probably belonging t o i n t e r f a c e s t a t e s appears.

The h o l e t r a p energy was measured t o be 5002 50 meV, t h e s u p e r p o s i t i o n w i t h t h e broad band p r e v e n t e d a more e x a c t d e t e r m i n a t i o n . Fig. 4 shows a n EBIC image of a c e r t a i n sample r e g i o n ( a ) and t h e c o r r e s p o n d i n g SDLTS-image measured a t 267 K ( b ) . The EBIC-image r e p r e s e n t s a number of l i n e d e f e c t s t h a t might be d i f f e r e n t t y p e s of g r a i n boundaries a n d / o r d i s l o c a t i o n s . The s t r o n g

dark l i n e s e p a r a t i n g t h e r e g i o n c o n t a i n i n g d e f e c t s from t h e d e f e c t - f r e e r e - g i o n ( s u b s t r a t e ) i s t h e S i 0 2 - l a y e r v i s i b l e i n t h i s b e v e l l e d s t r u c t u r e . The SDLTS-ima e c l e a r l y r e v e a l s t h a t o n l y p a r t of t h e d e f e c t s g i v e r i s e t o SDLTS- c o n t r a s t $ s e e arrows i n Fig. 4 a ) . Though t h e n a t u r e of t h e 500 meV l e v e l s i s n o t y e t c l e a r and t h e t y p e s of t h e c r y s t a l d e f e c t s have n o t y e t been i d e n t i - f i e d h e r e , t h e SDLTS measurement c l e a r l y i n d i c a t e s t h e e x i s t e n c e of d i f f e r e n t t y p e s of d e f e c t s i n t h i s sample, which cannot be p r e d i c t e d from t h e EBIC- image.

The second example shows r e s u l t s of t h e i n v e s t i g a t i o n of a Au-Schottky d i o d e on a n n-type LEC GaAs c r y s t a l . One of t h e most i n t e r e s t i n g l e v e l s i n t h i s ma- t e r i a l i s t h e EL2 l e v e l , a p p e a r i n g i n standard-DLTS experiments a s w e l l a s i n Scanning-DLTS a s a n e l e c t r o n t r a p l e v e l . Fig. 5 shows t h e EBIC-image ( a )

Fig. 4

-

E3IC-image ( a ) and SDLTS-image of a 500 meV h o l e t r a p l e v e l (b) of a b e v e l l e d r e g i o n of a L a s e r - r e c r y s t a l l i z e d SOS-structure.

Fig. 5

-

EBIC-image ( a ) and SDLTS-image of t h e EL2-level ( b ) of a n n-type LEC GaAs-sample (Au-Sohottky d i o d e ) .

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and t h e SDLTS-image of t h e EL2 l e v e l d i s t r i b u t i o n ( b ) . The s i g n of t h e SDLTS- s i g n a l h a s been chosen s o t h a t b r i g h t c o n t r a s t corresponds t o a l a r g e t r a n - s i e n t s i g n a l (which i s a c t u a l l y n e g a t i v e h e r e ) ; t h i s should be t h e g e n e r a l way t o p r e s e n t SDLTS r e s u l t s . It i s obvious t h a t t h e EL2 l e v e l i s e v e r y t h i n g but homogeneously d i s t r i b u t e d i n t h i s c r y s t a l . I n s t e a d , i t seems t o be con- c e n t r a t e d around groups of c r y s t a l d e f e c t s v i s i b l e i n t h e EJ3IC-image. An i n - homogeneous n e t doping c o n c e n t r a t i o n can be r u l e d out h e r e t o be r e s p o n s i b l e f o r t h i s s t r o n g SDLTS-contrast, s i n c e t h i s would l e a d t o a d i f f e r e n t EBIC- c o n t r a s t i n t h e corresponding r e g i o n s . Q u a l i t a t i v e l y s i m i l a r r e s u l t s have been found on s. i. GaAs c r y s t a l s u s i n g ir-microscopic t e c h n i q u e s a t low temperatures /9/. Note t h a t f o r t h e SDLTS-investigations of EL2 o n l y a h e a t i n g s t a g e i s n e c e s s a r y r a t h e r t h a n a c r y o s t a t .

A s t h e l a s t example of t h i s paper, Fig. 6 demonstrates t h e check of t h e phys- i c a l s i g n i f i c a n c e of a a e r t a i n SDLTS-structure. The sample was a Au-Schottky diode on a p l a s t i c a l l y deformed n-GaAs sample c o n t a i n i n g t h e EL2 l e v e l and t h e 400 meV *Haf1-hole t r a p l e v e l (Wosinski and B r e i t e n s t e i n /20/) a s t h e do- minant p o i n t d e f e c t l e v e l s . Moreover, t h e e l e c t r o n beam e x c i t e d DLTS-spectrum r e v e a l e d a broad h o l e t r a p - t y p e band t h a t i s superimposed w i t h t h e Ha- l e v e l s i g n a l and probably a l s o w i t h t h e EL2-signal, and t h a t most probably was caused by t h e def orma tion-induced d i s l o c a t i o n s themselves. The q u e s t i o n was, whether some l o c a l s i g n a l maximum of t h e SDLTS-signal was i n d e e d caused by a s t r o n g e r p o i n t d e f e c t s i g n a l r a t h e r t h a n by a s t r o n g e r d i s l o c a t i o n - i n - duced s i g n a l i n t h i s p o s i t i o n . Fig. 6 shows t h e SDLTS-line s c a n s f o r t h e Ha- and t h e EL2-signal w i t h t h e two maxima i n t h e 50 /urn-position. The s i g n of t h e EL2-signal i s i n v e r t e d s o t h a t b o t h s i g n a l s a r e p o s i t i v e . The c o r r e - sponding combined r a t e window/line s c a n s , where t h e beam i s scanned a l o n g t h e t r a c e of t h e SDLTS-line s c a n , and i n any p o s i t i o n a r a t e window s c a n i s performed, u n i u e l y enable t h e d i s t i n c t i o n between t h e corresponding p o i n t d e f e c t s i g n a l ?peak i n t h e r a t e window-dependence i n t h e corresponding p o s i - t i o n ) and d i f f e r e n t superimposed s p e c t r a l c o n t r i b u t i o n s . Note t h a t i n Fig. 6 t h e z e r o - l i n e s of t h e SDLTS-signal f o r b o t h r a t e window/line s c a n s a r e s h i f ted.

5

-

^SUMMARY

Scanning-DLTS a s a s p e c i a l SEMi t e c h n i q u e u s i n g t h e DLTS d e t e c t i o n p r i n c i p l e o f f e r s unique p o s s i b i l i t i e s of checking t h e homogeneity of t h e i n c o r p o r a t i o n of deep l e v e l d e f e c t s i n semiconductor s p a c e charge s t r u c t u r e s . E s p e c i a l l y

Fig. 6

-

SDLTS-line s c a n s f o r t h e Ha-level and t h e EL2-level i n p l a s t i c a l l y deformed n-GaAs /19/ and r a t e window/line s c a n s measured a t t h e temperatures of b o t h s i g n a p appearing.

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i t s s p e c t r o s c o p i c p o s s i b i l i t i e s i n c o m b i n a t i o n w i t h t h e E B I C t e c h n i q u e a l l o w t h e d e t a i l e d i n v e s t i g a t i o n of i n t e r a c t i o n s between p o i n t d e f e c t s and ex- t e n d e d c r y s t a l d e f e c t s , which i s of g r e a t b a s i c p h y s i c a l a s w e l l a s techno- l o g i c a l i m p o r t a n c e . I t s d e t e c t i o n l i m i t , which i n f a v o u r a b l e c a s e s i s below 1000 atoms p e r s c a n n i n g p o i n t a n d c o r r e s p o n d s t o a t r a p d e n s i t y of l e s s t h a n 1014 cm-3 f o r a few m i c r o n s of s p a t i a l r e s o l u t i o n , exceeds t h a t of most o t h e r m i c r o a n a l y t i c a l methods.

N e v e r t h e l e s s , t h e d e t e c t i o n s e n s i t i v i t y i s s t i l l a m a j o r f a c t o r l i m i t i n g t h e f i e l d of a p p l i c a t i o n o f SDLTS, s i n c e deep l e v e l s may be e l e c t r i c a l l y a c t i v e i n even l o w e r c o n c e n t r a t i o n s . Note a l s o t h a t n o t a l l t y p e s o f deep l e v e l s may be e x c i t e d b y t h e e l e c t r o n beam i n a g i v e n s e m i c o n d u c t o r s t r u c t u r e . Moreover, a d d i t i o n a l i n v e s t i g a t i o n s a r e n e c e s s a r y t o i n t e r p r e t a n SDLTS r e - s u l t q u a n t i t a t i v e l y , hence t o p r o v e t h a t i t r e a l l y r e f l e c t s a c o n c e n t r a t i o n d i s t r i b u t i o n o r t o t a k e o t h e r sample i n h o m o g e n e i t i e s p r o p e r l y i n t o a c c o u n t . I f , however, a c e r t a i n l e v e l t u r n s o u t t o b e e x c i t a b l e b y t h e e l e c t r o n beam, i t i s q u i t e e a s y t o check i f i t i s i n c o r p o r a t e d homogeneously a r o u n d c e r t a i n c r y s t a l d e f e c t s , o r n o t . Thus, q u a l i t a t i v e i n f o r m a t i o n ( c o n c e n t r a t i o n en- hancement o r r e d u c t i o n a r o u n d c e r t a i n d e f e c t s ) i s r e l a t i v e l y e a s y t o o b t a i n , and t h i s i s t h e d e c i s i v e new i n f o r m a t i o n j u s t i f y i n g t h e u s e of SDLTS. The e x p e r i m e n t a l expense of t h e method i s , of c o u r s e , r a t h e r h i g h , b u t i t c o u l d be r e m a r k a b l y r e d u c e d i f a commercial s p e c i a l SDLTS-equipment were a v a i l a b l e . Acknowledgement

The a u t h o r i s i n d e b t e d t o R. Dietmann, W. E b e r h a r d t , M. Op e l t and t h e mem- b e r s of t h e IFE microcomputer a p p l i c a t i o n g r o u p ( a l l H a l l e P f o r t h e i r sup- p o r t i n c o n s t r u c t i n g t h e SDLTS equipment, and t o B. T i l l a k ( ~ r a n k f u r t / ~ d e r ) a n d V. Chubarenko losco scow), t o C. F r i g e r i and R. F o r n a r i ( b o t h Parma), and t o T. Woosinski (warsaw) f o r p r o v i d i n g samples and e x p e r i m e n t a l a s s i s t a n c e i n t h e measurement examples 1, 2 a n d 3, r e s p e c t i v e l y . The s t i m u l a t i n g i n t e r e s t of P r o f . J. Heydenreich ( H a l l e ) i n t h i s work i s g r a t e f u l l y acknowledged.

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