COMBINED ELECTRON MICROSCOPICAL AND DLTS (ESP, DSLTS) INVESTIGATIONS IN SEMICONDUCTORS

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COMBINED ELECTRON MICROSCOPICAL AND DLTS (ESP, DSLTS) INVESTIGATIONS IN

SEMICONDUCTORS

O. Breitenstein, J. Heydenreich

To cite this version:

O. Breitenstein, J. Heydenreich. COMBINED ELECTRON MICROSCOPICAL AND DLTS (ESP, DSLTS) INVESTIGATIONS IN SEMICONDUCTORS. Journal de Physique Colloques, 1983, 44 (C4), pp.C4-207-C4-215. �10.1051/jphyscol:1983425�. �jpa-00223044�

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COMBINED ELECTRON MICROSCOPICAL AND DLTS ( E S P . DSLTS) INVESTIGATIONS IN SEMICONDUCTORS

0. Breitenstein and J. Heydenreich

Akademie der Wissenschaften der D.D.R., Inst-itut fur Festkorperyhys-ik und Elektronennrikroskopie, D.D.R.-402V Halle/Saale, Weinberg 2, V.R.G.

Résumé - Un système DLTS a* balayage, mesurant la capacité, est introduit. Il est caractérisé par une grande sensibilité de détection. Des exemples de mesures (silicium, composés

III -V)dans lesquelles DLTS et SEM/EBIC sont combinées, démontrent la fonction et les possibilités du système.

Abstract - A capacitively measuring Scanning DLTS system is introduced having a high detection sensitivity. Measuring examples achieved by combined application of SDLTS and SEM/

EBIC on silicon and III-V compound samples demonstrate the possibilities of the method and the performance of the system.

By combined application of transmission electron microscopy (TEM) or of scanning transmission electron microscopy (STEM) and scanning electron microscopy (SEM) in the electron-beam induced current

(EBIC) mode (or also in the cathodoluminescence mode) according to the pioneering investigations of Ravi and Varker /1, 2/ the electrical activity of crystallographically well characterized dislocations in semiconductors has been investigated by different research groups, especially by Ourmazd and Booker /3/» by Petroff et al. A / , by Fathy, Sparrow and Valdre- /5/» and also by the Halle group /6/.

In the majority of cases the results of the investigations point to the fact that dislocations decorated by impurities or by impurity agglomerates (e.g. precipitates), resp. /7-9/ have a special electrical activity, e.g. a high recombination efficiency for minority carriers, as could be shown by interpreting electron-beam induced currents.

Thus, the detection of the electrical activity of point defects necessary for many fields of semiconductor research is especially important for solving problems of the kind mentioned above. Provided that there are deep energy levels the Deep Level Transient Spectro- scopy (DLTS), originally proposed by Lang /10/, is applicable advan- tageouslyj besides the detection of the energy levels of the defects also their relative concentrations can be estimated down to very low concentrations. Moreover, the capture coefficients to electrons and holes can be measured which are of interest concerning the estima- tion of the recombination activity of a centre. As is well-known the DLTS method is based on the measurement of capacity changes (or the generated currents) in a space charge structure (pn junction or Schottky barrier) after an excitation pulse as a function of the temperature. The aspired statements can be gained from the analysis of the temperature-dependence of the signal. Isolated point defects show a relatively sharp signal maximum at a well-defined-temperature, which depends on the activation energy of the level. For ex-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983425

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C4-208 JOURNAL DE PHYSIQUE

tended d e f e c t s l i k e d i s l o c a t i o n s t h e method i s I n p r i n c i p l e a l s o a p p l i c a b l e t a s a r u l e t h e temperature dependence of the s i g n a l i s , however, much s m a l l e r than f o r i s o l a t e d p o i n t d e f e c t s , and there- f o r e a q u a n t i t a t i v e interpretation o f the r e s u l t s i s r a t h e r compli- cated.

A s an e s p e c i a l l y advantageous mode of Deep I e v e l T r a n s i e n t Spectro- scopy t h e s p a t i a l l y r e s o l v i n g method of Scanning Deep h v e l Tran- s i e n t Spectroscopy (SDI%PS) according t o P e t r o f f and Lang /?I/ can be used. By e x c i t i n g t h e l e v e l s by a scanning e l e c t r o n probe ( a t a chosen temperature) an SDmS image i s obtained, which shows the l o c a l change of the d e n s i t y of e l e c t r o n i c l e v e l s a t a c e r t a i n energy i n t h e band gap given by t h e e x i s t i n g d e f e c t s , and the tem- p e r a t u r e dependence o f the image c o n t r a s t a g a i n f a c i l i t a t e s

s t a t e m e n t s on t h e nature of the i n t e r e s t i n g d e f e c t s .

The paper s h a l l be d i v i d e d i n t o t h e f o l l o w i n g two s e c t i o n s :

I ) D e s c r i p t i o n of a computer-controlled SDLTS system having a h i g h d e t e c t i o n s e n s i t i v i t y

2 ) Combined a p p l i c a t i o n of SEM (EBIC) and SDIlllS f o r t h e c h a r a c t e r i - z a t i o n of the e l e c t r i c a l a c f i v i t y of i n d i v i d u a l extended c r y s t a l d e f e c t s and t h e i r c o r r e l a t i o n t o p o i n t d e f e c t s

1 ) The main problem i n t h e pra-ctical r e a l i z a t i o n of the SDLTS tech-

-

nique i s t o r e a c h the h i g h e s t p o s s i b l e d e t e c t i o n s e n s i t i v i t y . This requirement i s a n a t u r a l consequence of the f a c t t h a t here only t h e s m a l l sample a r e a under t h e focused e l e c t r o n beam c o n t r i b u t e s t o t h e DLTS s i g n a l , whereas t h e r e s t of t h e sample remains p a s s i v e , There- f o r e t h e SDL!L'S s i g n a l h e i g h t depends on t h e s p a t i a l r e s o l u t i o n t h a t t y p i c a l l y i s a t b e s t i n t h e micron range, s i m i l a r a s i n t h e EBIC imaging mode, b u t can be worsened by e l e c t r o n beam defocusing r e s u l t i n g i n a l a r g e r SDLTS s i g n a l height. Moreover, t h e signal-to- noise r a t i o depends on t h e s i g n a l i n t e g r a t i o n time f o r e a c h meas- u r i n g p o i n t governing t h e t o t a l measure time s o t h a t t h e SDI%PS measuring procedure, always f o r c e s a compromise between t h e d e s i r e d s p a t i a l r e s o l u t i o n and the signal-to-noise r a t i o on t h e one hand and a j u s t i f i a b l e measure time on the o t h e r hand, t h a t i s l i m i t e d by p r a c t i c a l c o n s i d e r a t i o n s and by t h e s t a b i l i t y of the system

components,

I n t h e o r i g i n a l SDLTS-system introduced by P e t r o f f and h n g /11/

current-DLTS was used, f o r i t should have a h i g h e r d e t e c t i o n s e n s i - t i v i t y than the more f a m i l i a r capacitance-DUTS. It h a s been shown, however, by s e v e r a l a u t h o r s t h a t i t should a l s o be p o s s i b l e t o r e a c h h i g h e s t s e n s i t i v i t y using a capacitance based system ( s e e e-g.

Borsuk and Swanson /12/ o r Misrachi e t a l . /13/), which would have t h e advantage t h a t one can d i s t i n g u i s h between e l e c t r o n and hole t r a p s and t h a t t h e s e n s i t i v i t y i s almost independent of t h e emis- s i o n r a t e . Therefore, t h e SDVPS system t o be d e s c r i b e d was based on a s p e c i a l l y developed h i g h l y s e n s i t i v e capacitance meter t h a t has been d e s c r i b e d elsewhere ^/74/,Using t h i s capacitance meter i n a c o r r e l a t o r DLTS system under optimum cond'tionsa noise l e v e l corresponding t o a capacitance change of 1 O~ pl? h a s been reached using a n i n t e g r a t i o n time gf only -100 ma t h a t belongs t o a d e t e c t i o n l i m i t of s t i l l s e v e r a l 10- of the n e t doping c o n c e n t r a t i o n f o r a s p a t i a l r e s o l u t i o n of 4/um a s w i l l be demonstrated i n Fig. 3 i n t h e f o l l o w i n g paragraph.

I n Fig. 7 t h e f u n c t i o n a l b l o c k diagram of t h e v e r y l a t e s t v e r s i o n of o u r SDL'I'S system i s shown using a c e n t r a l i z e d microcom u t e r control.. It i s based on t h e scanning e l e c t r o n microscope

PmM)

^type

BS-300 from Tesla (Czechoslovakia) b e i n g completed by s e v e r a l a d d i t i o n a l assemblies necessary f o r SDIIPS a p p l i c a t i o n . These a r e

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I1 I I I1 I1 II II II I 1 1 1 1

1280 I P R G ~ D A T I PT I D ~ S P ~ Z D A C ~ D L T S ~ Z D A ~ A D C

IMUX I

D L T S

p l o t t e r ryostot

f a c i l i t y , a l i q u i d c i t r c - gen e v a p o r a t i o n c r y o s t a t t h a t w i l l be d e s c r i b e d elsewhere /15/, and a l i q u i d n i t r o g e n c r y o t r a p t o reduce contamination a t low sample tempera- t u r e s . The a c t u a l DU2S s p e c t r o m e t e r c o n s i s t s of t h r e e p a r t s : %he capac- i t a n c e meter w i t h t h e p r e a m p l i f i e r l o c a t e d w i t h i n t h e microscope, an analogue s i g n a l cor- r e l a t o r s i m i l a r t o t h a t of M i l l e r e'c a l . /76/

w i t h t h e e x c e p t i ~ n t h a t Fig. 1 Block diagram of a computer- here t h e c o r r e l a t i o n

c o n t r o l l e d SDLTS system f u n c t i o n i s d e l i v e r e d by t h e computer, a n d an analogue l i n e a r i n t e g r a t o r t o average t h e r e s u l t over a c e r t a i n i n t e g r a t i o n period. Not t i l l then t h e r e s u l t i s d i g i t i z e d by t h e ADC of t h e microcomputer, i s s t o r e d a f t e r w a r d s , and t h e beam posi- t i o n on t h e sample i s changed. T h i s kind of computer c o n t r o l I s somewhat unusual and can be c a l l e d "hybrid conceptw f o r i t u s e s b o t h analogue and d i g i t a l s i g n a l p r o c e s s i n g componentsb Its b a s i c i d e a s w i l l be i l l u s t r a t e d now i n comparison t o h i t h e r t o e x c i s t k n g computerized s t a n d a r d DLTS systems.

S e v e r a l computer-controlled DIlPS systems a r e known from the l i t e r a - t u r e ( s e e e.g. K. Asada e t a l . /'l7/, K.E. Wang / ? 8 / , R e &f'ew'evre / l g / ) . They a r e a l l designed t o have t h e capacitance meter a s t h e o n l y analogue device on the DLTS signal. path. The c a p a c i t a n c e v a l u e s a r e d i g i t i z e d p o i n t by p o i n t a t c e r t a i n t i m e s a f t e r t h e f i l l i n g p u l s e and a r e then processed on ,the computer, s t o r e d aad f i n a l l y d i s p l a y e d i n an a p p r o p r i a t e manner. I n comparison t o a

c o n v e n t i o n a l DLTS system c o n t r o l t h e s e concepts y i e l d a h i g h e r measuring comfort, an e s s e n t i a l time r e d u c t i o n because a l l meas- urements can be c a r r i e d out on one temperaturs scan, a s w e l l a s t h e p o s s i b i l i t y t o a p p l y new modes of measarement, f o r example t h e i s o t h e r m a l D I E S method r e p o r t e d e.g. by H. Okushi e t a l , 1'20/~ Buf concerning SDLTS a l l t h e s e concepts have one disadvantage: because of t h e p o i n t by p o i n t capacifance measurement p a r t i c u l a r l y a t low emission r a t e windows most of the measure t i a e p a s s e s away witfaou'r;

g i v i n g a n y p h y s i c a l information. Note t h a t t h e d u r a t i o n 92 t h e a c t u a l d i g i t i z i n g e v e n t i s t y p i c a l l y much s m a l l e r t h a n t h e time i n t e r v a l between two measurement e v e n t s . That means w i t h noisy s i g n a l s t h e measurement i s not optimized w i t h r e s p e c t t o t h e detec- t i o n s e n s i t i v i t y , which i s , however, t h e d e c i s i v e f a c t o r for SBLTLSW Moreover, a problem may a i s from measuring a t h i g h e r emissioo r a t e windows ( s a y some 0 s Then t h e computer i s a l r e a d y q u i t e busy w i t h t h e D ~ r o u t i n e o r g a n i z a t i o n and t h e d a t a y i e l d i s v e r y s h i g h , s o t h a t an inexpensive microprocessor system having medium c a l c u l a t i o n speed can a l r e a d y be overburdened*

On t h e c o n t r a r y , i n the system d e s c r i b e d t h e a c t u a l DLTS s i g n a l i s formed by applying analogue t e c h n i q u e s , and t h e computer only has t h e f o l l o w i n g t a s a s : a ) t o d e l i v e r r e p e t i t i v e l y t h e e x p o n e n t i a l c o r r e l a t i o n f u n c t i o n and an e x c i t a t i o n p u l s e t r i g g e r s i g n a l using a s p e c i a l DLW hardware, b ) t o d i g i t i z e and t o s t o r e t h e measured

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C4-210 JOURNAL DE PHYSIQUE

v a l u e s and t o c o n t r o l t h e e l e c t r o n beam scan over the sample, c ) t o c o n t r o l the sample temperature using a temperature c o n t r o l u n i t , and f i n a l l y d ) t o d i s p l a y t h e SDLTS image e i t h e r on an x-y r e c o r d e r o r on t h e S E M s c r e e n i n an a p p r o p r i a t e manner. The DT/PS hardware i s t h e key component of the whole concept t o r e a c h a high working speed without overburdening t h e computer because i t i s c o n t r o l l e d d i r e c t l y by t h e 10 Mc system c l o c k and h a s i t s own c o n t r o l l o g i c t o organize

t h e a c t u a l DLIPS r o u t i n e independent of the computer. The concepts of i t s s t r u c t u r e a s w e l l a s of system software w i l l be described elsewilere /I 5 / ,

Summarizing the p o t e n t i a l advantages of t h i s hybrid SDLTS-system i t should be pointed out t h a t i t uses p r a c t i c a l l y t h e whole meas- u r e time f o r c a p t u r i n g t h e t r a n s i e n t s i g n a l because t h e computer i s a b l e t o c a r r y out independently necessary o p e r a t i o n s d u r i n g t h e measure time. Therefore t h e systertl should be regarded a s optimum w i t h r e s p e c t t o a h i g h e s t p o s s i b l e signal-to-noise-ratio w i t h i n a given izeasure time. Moreover, the f a s t DUCS hardware a l l o w s t o

c a r r y o u t c e r t a i n o p e r a t i o n s a t a h i g h e r speed than t h e computer would e v e r reach, and i t allows a r e l a t i v e l y simple microprocessor system t o be used, It should a l s o be mentioned, however, t h a t t h e s e advantageous claim a h i g h e r expense f o r developing the s p e c i a l hardware and t h e analogue components.

2 ) The a p p l i c a t i o n of scanning DLTS r e l a t e d t o the d e t e c t i o n of Beep l e v e l c e n t r e s i n well-defined microregions of the specimen comprising t h e e l e c t r i c a l c h a r a c t e r i z a t i o n of i n d i v i d u a l c r y s t a l d e f e c t s i s e s p e c i a l l y advantegeous i f used i n combination w i t h o t h e r e l e c t r o n microscopical imaging techniques. I n the f o l l o w i n g examples of t h e combined a p p l i c a t i o n of t h e s e methods a r e given. It should be noted t h a t these SDLQS r e s u l t s have not y e t been obtained by t h e computerized SDI%PS system b u t by the f o r e g o i n g one u s i n g t h e same p r i n c i p l e b u t on l i n e r e c o r d i n g and analogue system c o n t r o l technique s ,

Fig. 2 i s connected w i t h t h e i n v e s t i g a t i o n of a c r o s s s e c t i o n through the pn j u n c t i o n r e g i o n of a GaP LED. I n the SEM image a ) , which i s a s u p e r p o s i t i o n of a secondary e l e c t r o n and an EBIC micrograph, the p+n $unction i t s e l f i s v i s i b l e a s a b r i g h t l i n e , The decreased i n t e n s i t y a t t h e s i t e i n d i c a t e d by an arrow p o i n t s t o an i n c r e a s e d recombination e f f i c i e n c y p o s s i b l y induced by t h e presence of c r y s t a l d e f e c t s . The d a r k l i n e r e g i o n t o t h e r i g h t of t h e pn j u n c t i o n , imaged by secondary e l e c t r o n s , i s r e l a t e d t o t h e gold bonding c o n t a c t i n t h e p+-region of t h e LED. I n a s t a n d a r d DIlPS

examination the sample showed a hole t r a p w i t h an a c t i v a t i o n energy

P i g b ² Cross s e c t i o n of a po juncfion i n GaP

a ) S E M image ( s u p e r p o s i t i o n of SE and EBIC micrograph) b-d) SDPS Y-modulation r e p r e s e n t a t i o n s a t 315 K ( b ) , a t ²⁷⁶K on DUPS peak 500 meV hole t r a p ( c ) , a t 215 K ( d )

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t h r e e s e r i e s of SDIES scans c r o s s i n g t h e pn

,

^lOprn

,

j u n c t i o n and taken a t t h r e e d i f f e r e n t sample

temperatures: b ) below the 500 meV l e v e l peak temperature, c ) a t peak temperature, 11~"~F21200at0ms and d ) a t a h i g h e r temperature. Since t h e

2 -,-\ ^:^-- .J\*-, - pn junction goes i n t o the depth of t h e image

3s--e--+-.

^{a n}^SDDTSs i g n a l can of course be expected -->.

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-1 -k ^>^.^-^.^. ----x" = ,-

-- :>-,.;' k.L_ z 5,- o n l y n e a r the l i n e where t h e pn junction

; - - c r o s s e s the sample surface. F i r s t , one almost

z-?zz-=

-

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homogeneously d i s t r i b u t e d s i g n a l p a r t i s

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visibLe showing o n l y a s l i g h t t e m p e ~ a t u r e

---

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, dependence t h a t might o r i g i n a t e from sur-

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f a c e s t a t e s . But, moreover, o n l y i n t h e po-

FxF2=%$- *

- 2 - - - s i t i o n of t h e EBIC-mln&mum and only a t the

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-7%- - temperature corresponding t o the 500 meV

: - ~ E + Y ; ~

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---, 4 Ak l e v e l a s h a r p S D m S s i g n a l maximum o c c m s t h a t can be i n t e r p r e t e d a s a s t r o n g accumu- Fig. 3 l a t i o n of t h e c e n t r e s r e s p o n s i b l e f o r t h e

500 meV DIES s i g n a l . The inhomogeneous in- representation c o r p o r a t i o n of t h i s l e v e l i s influenced most o f a pn j u n c t i o n probably by the presence of c r y s t a l d e f e c t s , (cross section) in whose n a t u r e has not yet been i d e n t i f i e d * Gap on 500 meV

DIES-pe a k tempera- Fig. 3 shows again t h e SDLTS image of t h e t u r e same sample r e g i o n a t a b e t t e r r e s o l u t i o n . It

i s v i s i b l e t h a t t h e ^TOOmeV SDblPS s i g n a l a c t u a l l y c o n s i s t s of d i f f e r e n t p a r t s . Please note t h e s e n s i t i v i t y s c a l e and t h e s p a t i a l r e s o l u t i o n of only a few microns.

The i n v e s t i g a t i o n o f t h e a c t i v i t y of i n d i v i d u a l d i s l o c a f i o n s may be demonstrated by Fig. 4. The f i g u r e shows t h e comparison of two Y-modulation images of a copper-doped p - s i l i c o n c r y s t a l having a n 81-Schottky diode, a ) f o r EBIC and b ) f o r DIES imaging mode. The two d i s t i n c t l o c a l minima i n t h e EBIC micrograph corresponding t o t h e d e f e c t s a r e a l s o r e p r e s e n t e d i n the SDTG image, The SDLTS mapping h a s been taken a t the temperature of a 500 meV copper-

r e l a t e d donor peak

Fig. 4

SEMLEBIC ( a ) and SDIII1S ( b )

Y-modula t i o n r e p r e s e n t a t i o n of d e f e c t s i n p-Si (copper doped) c o n t a i n i n g a

Schottky b a r r i e r region

(temperature:

3g8 T j

r a t e window: q O s- and

-

under c e r t a i n con- d i t i o n s (e ^.$: homogeneous doping)

-

t h l s s i g n a l r e f l e c t s t h e concentra- t i o n of t h i s l e v e l around t h e d e f e c t . While i n t h e l a r g e r surrounding of t h e d e f e c t s a h i g h e r con- c e n t r a t i o n of e x c i t e d l e v e l s i s r e v e a l e d , the r e g i o n n e a r the d i s l o - c a t i o n shows a diminished c o n c e n t r a t i o n of e x c i t e d l e v e l s . Here the g e t - t e r i n g a c t i o n of t h e d i s l o c a t i o n s has prob- a b l y i n a c f i v a t e d copper atoms e l e c t r i c a l l y w i t h r e s p e c t t o the DUPS process; t h u s a h i n t a t t h e s t a t e of d e c o r a t i o n of these d i s l o c a t i o n s i s give n

.

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C4-212 JOURNAL DE PHYSIQUE

Microplasmae i n semiconductor device s t r u c t u r e s , e .g. pre-breakdown s i t e s i n pn- o r Schottky j u n c t i o n s can a l s o be i n v e s t i g a t e d by t h e SDLRS technique. According t o Fig. 5 a s an example IOOR cm p-8i

c r y s t a l s , deformed plas- t i c a l l y a t 1000°C and a f t e r w a r d s prepared w i t h an A 1 ~ c h o t t k y - c o n t a c t were s t u d i e d . The s t a n d a r d DIES spectrum ( a ) shows a broad DLTS-band, which might be r e l a t e d t o the presence of extended c r y s t a l d e f e c t s , e.g. d i s l o c a t i o n s . Some peaks i n t h e spectrum p r e s e n t a t lower tempera- t u r e s a r e probably r e l a t e d

b

t o t h e e x i s t e n c e of p o i n t d e f e c t s . A t the temperature marked b s t h e arrow t h e SDI/I'S s i g n a l should be

./ETzai& -

^-- ^..

---

^-=.. r e l a t e d mainly t o extended

:E*<r-%g.&!=- < ^=:522:-

Lq2.z?* c - . =. --- .g c r y s t a l d e f e c t s , e.g. d i s -

. -

&.,>; - -

,&~5~:z~w~-~:-~~L l o c a t i o n s . Scanning e l e c - t r o n microscopic ZBIC i n -

.-zFk-:-, .,. .s&-- ^:^.-^.^:

.=-. . -=

G=+~;+.:F~~--- .-.- --:.- v e s t i g a t i o n s , c a r r i e d out

ezcF:

^:^-^-^<⁼^..^:^. ^{.. .-}

^-?;=

a t lower temperatures

, ~Z:L-~.->-~~~.

. .7:=.7~7-==-=

.. - - (about 200 K) enabled

l o c a l microplasmae t o be

, - a m i d e t e c t e d , which obviously

C

d

o r i g i n a t e d from t h e a c t i o n Fig. 5 SDE'S Y-modulation r e p r e s e n t a - of t h e e l e c t r o n beam and

t i o n s of d e f e c t s i n a p-Si vanished a f t e r t h e specimen c r y s t a l ( p l a s t i c a l l y deformed had been annealed up t o a t 'I00O0C) c o n t a i n i n g a ¹OO°C. Analogously i n the Schottky b a r r i e r v e r y p o s i t i o n s o f t h e a ) DLTS spectrum, b , c , d ) SDLHS micropLasmae t h e SDI%CS micrographs taken a t 240 K i n v e s t i g a t i o n s y i e l d l o c a l ( s e e arrow) showing t h e f o r - SDLTS s i g n a l v a r i a t i o n s , mation of d e f e c t s (micro- which

-

a s can be seen i n plasmae) by r e p e a t e d e l e c t r o n t h e r e p r e s e n t a t i o n s ,

impact b ) , c ) , d )

-

enhanced w i t h

t h e time of i n v e s t i g a t i o n i n c r e a s i n g . Obviously t h e formation of microplasmae i s r e l a t e d t o t h e a c t i v a t i o n of e l e c t r i c a l l y a c t i v e c e n t r e s under t h e a c t i o n of t h e e l e c t ~ o n beam i n a low temperature range. Since t h e m a t e r i a l was piasbsically deformed and the microplasmae were found t o a r i s e r e p e k i t i v e l y i n the same p o s i t i o n s an i n f l u e n c e of g e n e r a t e d d i s -

l o c a t i o n s can be taken i n t o consideration. Obviously under t h e a c t i o n of t h e e l e c t r o n beam a t lower temgeratures the e l e c t r o n i c s t r u c t u r e of c e r t a i n d e f e c t s i s changed l e a d i n g t o t h e observed nicroplasmae and a t the same time being d e t e c t a b l e by SDIIPS. Such combined SDLTS and EBIC i n v e s t i g a t i o n s can be expected t o h e l p i n e l u c i d a t i n g t h e nature of such microplasmae and t h e i r p o s s i b l e connection w i t h c r y s t a l d e f e c t s i n f u t u r e .

I n the l b s f example SDLTS r e s u l t s should be introduced o f t h e main e l e c t r o n t r a p i n GaAs a t Ec-0,8 eV, commonly l a b e l l e d a s the ^EL² l e v e l , It became a n o t h e r sub j e e t of i n t e r e s t a f t e r some r e c e n t experimental r e s u l t s had y i e l d e d new arguments on i t s p h y s i c a l nature. So i t was found t o i n c r e a s e i n concentrqtion a f t e r p l a s t i c deformation of the m a t e r i a l s i m i l a r l y t o t h e AS& paramagnetic c e n t r e , s u g g e s t i n g t h a t t h e EL 2 l e v e l i s t h i s v e r y a n t i s i t e

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F i g i e l s k i / 2 2 / t o a l a r g e e x t e n t these a n t i s i t e d e f e c t s can be generated during t h e g l i d e motion of d i s l o c a t i o n s . I n t h e i n v e s t i g a t i o n s GaAs c r y s t a l s were used t h a t had been deformed a t 4 0 0 ' ~ by u n a x i a l p l a s t i c compression.

Then S c h o t t k y - b a r r i e r s on (711) o r i e n t e d s l i c e s were prepared by g o l d evaporation:

ohmic c o n t a c t s were made by a l l o y i n g I n (1gAg)-dots.

Pig. 6 D i s l o c a t i o n s i n (111b Fig. 6 a ) shows a t y p i c a l d e f e c t o r i e n t e d GaAs deformed s t r u c t u r e of such a type of a by u n a x i a l p l a s t i c com- deformed c r y s t a l i n a H'VEM

p r e s s i o n a t 4000C, image a f t e r a p p r o p r i a t e t h i n n i n g a ) HVEM micrograph, of t h e specimen. A s expected the b ) SEMBBIC micrograph major d e f e c t s were s t r a i g h t dLs-

l o c a t i o n l i n e s o r i e n t e d pre- f e r e n t i a l l y i n ( ~ 1 0 ) d i r e c t i o n . The a n a l y s i s of t h e s e d i s l o c a t i o n s predominantly y i e l d e d 60° dislocations w i t h a Burgers v e c t o r of a./2<170). An EBIC-micrograph of t h e same sample before t h i n n i n g i s shown i n Fig. 6b. The s t r a i g h t c o n t r a s t l i n e s seem t o belong t o the observed d i s l o c a t i o n s l y i n g p a r a l l e l and having a c e r t a i n recombi- n a t i o n a c t i v i t y .

Fig. 7 Defects i n - G ~ A S (111 orien- t e d ) deformed bg u n a x i a l p l a s t i c com- p r e s s i o n a t 400 C , a ) SEBd/EBIC survey image, b ) SDDTS Y-modulation repre- s e n t a t i o n of the EL 2 s i g n a l

of a n o t h e r sample from t h e same c r y s t a l . The s h a r p b l a c k s t r a i g h t l i n e s a r e no c r y s t a l d e f e c t s b u t r a t h e r s u r f a c e r e l a t e d p o l i s h i n g t r a c e s . Besides, again t h e more o r l e s s p a r a l l e l de- f e c t s a r e v i s i b l e showing an o r i e n t a t i o n n e a r l y d i a g o n a l t o t h e image. Here t h e d i s l o c a t i o n d e n s i t y i s not homogenoust a r e l a - t i v e l y b r i g h t s t r i p e w i t h a lower d e f e c t d e n s i t y i s v i s i b l e whereas i n the upper c e n t r a l p a r t and i n t h e lower region t o t h e r i g h t t h e d i s l o c a t i o n den- s i t y i s much higher. S l i d e no. To shows an SDIDS Y-modulation r e p r e s e n t a t i o n i n p e r s p e c t i v e of the same r e g i o n w i t h t h e experimen- t a l parameters s e l e c t e d t o image t h e EL 2 concentra- t i o n (T = 930C r a t e

window = 3 0 s"). Note t h a t t h e DLTS s i g n a l goes i n negative d i r e c t i o n h e r e , so

"valleystt i n the SDI%PS image a r e r e g i o n s of h i g h EL 2 c o n c e n t r a t i o n and

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C4-214 JOURNAL DE PHYSIQUE

" h i l l s 1 * a r e r e g i o n s of lower c o n c e n t r a t i o n . The SDLTS-image corre- l a t e s v e r y w e l l w i t h t h e l o c a l d i s l o c a t i o n d e n s i t y a s can be seen i n t h e EBIC image. E s p e c i a l l y the n e a r l y d i a g o n a l l y l y i n g b r i g h t s t r i p e belongs t o an analogous r e g i o n of a low EL 2 s i g n a l ( s e e arrow). A q u a n t i t a t i v e a n a l y s i s o f t h e SDLTS s i g n a l y i e l d e d EL 2 c o n c e n t r a t i o n v a r i a t i o n s by a f a c t o r o f t h r e e t o f o u r , c o n t r a r y t o t h e much lower EBIC-contrast t h a t was b e s t i n the o r d e r of 20 %.

The l o c a l correspondence between t h e EL 2 s i g n a l and t h e d i s l o - c a t i o n d e n s i t y i s e s t a b l i s h e d up t o the SDYPS-re'solution of 3-5/um.

It should be noted t h a t a unique c o r r e l a t i o n has been a s p i r e d t o between the EL 2 s i g n a l and s i n g l e d a r k EBIC-signal p o i n t s , b u t could not y e t be achieved. It cannot be excluded t h a t the reason f o r t h i s i s due t o the s t i l l i n s u f f i c i e n t SDL!l'S d e t e c t i o n s e n s i t i v - i t y . G e n e r a l l y , however, t h e s p a t i a l c o r r e l a t i o n s between the EL 2 s i g n a l and t h e d i s l o c a t i o n d e n s i t y are w e l l e s t a b l i s h e d , and t h e r e s u l t s a r e compatible with, t h e EL 2 g e n e r a t i o n model of Fi- g i e l s k i /22/.

Summary

DLTS, e s p e c i a l l y t h e Scanning-DIIPS techniques should be shown t o be a p p l i e d very u s e f u l l y i n comparison t o e l e c t r o n microscopical

technique, e s p e c i a l l y t o TEM and SEM/EBIC i n v e s t i g a t i o n s . The a p p l i - c a t i o n o f t h e s e d i f f e r e n t methods t o t h e same sample can y i e l d new i n f o r m a t i o n on the n a t u r e o r t h e e l e c t r i c a l a c t i v i t y of extended d e f e c t s and p o i n t d e f e c t s , r e s p e c t i v e l y , which should be demon- s t r a t e d by s e v e r a l examples. Though such combined i n v e s t i g a t i o n s a r e s t i l l a t t h e i r i n i t i a l s t a g e , one may assume t h e i r a p p l i c a t i o n t o e l u c i d a t e problems long d i s c u s s e d , f o r example concerning t h e mechanism of i m p u r i t y d e c o r a t i o n of extended d e f e c t s . The r e l a - t i v e l y h i g h e x p e r i m e n t a l expense of t h e method can be c o n s i d e r a b l y reduced w i t h o u t compromises concerning the d e t e c t i o n s e n s i t i v i t y u s i n g t h e d e s c r i b e d computer c o n t r o l system.

Acknowledgments

The a u t h o r s would l i k e t o thank D r . R i c h t e r ( H a l l e ) and D r .

Wosinski (Warsaw) f o r t h e i r e x p e r i m e n t a l c o o p e r a t i o n , D r . Oelgart and D r . Lemke ( b o t h B e r l i n ) f o r p r o v i d i n g samples, and Th. Nerst- heimer, A. P i p p e l , ill1. Taege and J.M. Langaer ( a l l H a l l e ) f o r t h e i r s u p p o r t i n c o n s t r u c t l n g t h e computerized SDLTS-system.

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