CATHODOLUMINESCENCE STUDIES OF DISLOCATIONS IN SEMICONDUCTORS

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CATHODOLUMINESCENCE STUDIES OF DISLOCATIONS IN SEMICONDUCTORS

M. Dupuy

To cite this version:

M. Dupuy. CATHODOLUMINESCENCE STUDIES OF DISLOCATIONS IN SEMICONDUCTORS.

Journal de Physique Colloques, 1983, 44 (C4), pp.C4-277-C4-287. �10.1051/jphyscol:1983433�. �jpa-

00223052�

JOURNAL DE PHYSIQUE

Colloque C4, supplément au n°9, Tome 44, septembre 1983 page C4-277

CATHODOLUMINESCENCE STUDIES OF DISLOCATIONS I N SEMICONDUCTORS

M. Dupuy

Laboratoire d'Electronique et de Technologie de l'Informatique, 38000 Grenoble, France

Résumé : Les divers accessoires du microscope à balayage qui permettent de réaliser des analyses en cathodoluminescence de semiconducteurs sont décrits.

De brèves notions de spectroscopie sont rappelées. Les contrastes de disloca- tions sont classés en "usuels" (point noir, point noir et halo brillant) et

"non usuels" (absence de contraste, contraste monochromatique brillant). Des exemples d'application récents sont donnés.

Abstract: The cathodoluminescence accessories of the scanning electron micros- cope f o r semiconductor examinations are described. Brief notions of specto- scopy are recalled. The dislocation contrasts are classified as "usual" (black dot, dot and halo) and "unusual" (no contrast, white contrast on monochromatic image). Examples of recent applications are given.

I - INTRODUCTION

Cathodoluminesce (CL) is an analytical tool of the scanning electron microscope (SEM) and of the scanning transmission electron microscope (STEM). The minority carrier lifetime, energy gap and other fundamental properties of semiconductors can be determined by use of this method with a spatial resolution of the order of 1 ym. In addition, crystal defects (precipitates, grain boundaries, dislocations) can be detected in the CL-mode. Now, it has been well established that in semiconductors, defects such as dislocations are harmful to the functioning of electronic and optical devices. Therefore, it can be easily understood why CL, which is non destructive, is such an attractive method of characterization. It has been widely developed in the last few years. In this review, some information concerning the different instrumental systems used for CL examination will be given, with the emphasis on the possibilities and performances of these systems. Then, the main recombination mechanisms will be presented. From the general theory of CL, an attempt will be made to draw some quantitative conclusions which will be useful in the last part, devoted to the CL studies of dislocations in semiconductors.

II - EXPERIMENTAL TECHNIQUE

The experimental technique has already been reviewed by Pfefferkorn et al [l] and Holt [2]. Only the main points of interest will be resumed below.

The first problem is detecting the light emitted by the specimen under electron bombardment. In figure 1, the energy band gaps Eg of the main covalent, III-V and II- VI semiconductors, and their corresponding wavelengths are presented together.

Indirect band gap materials are indexed with the subscript (1) For a few ternary compounds the band gap energy has been plotted as a function of the composition. The range of spectral response of detectors is indicated. Photomultipl iers,, with high gains and low noise amplification characteristics are best suited for wavelenghts up to 1.2 vim. Beyond this limit, photovoltatc detectors must be used with all the inherent drawbacks to these detectors (noise, low amplification). However, the study of the luminescent emission at these wavelengths are of the greatest interest. They correspond to either band to band recombination for narrow-gap semiconductors (IR

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983433

JOURNAL DE PHYSIQUE

InSb Ga As

1

0

I

0 1 1.5 2 2.5 Eg(eV)

A

^{p m )}

< :

:

10 5 2 1.6 1.2 1 0.8 0,7

0.6 0.5

IR

< -

^Visible

Photomultipliers PbS t

-

^InSb

-

^PbSe

I 1 Extrinsic Ge

Fig. 1 : Semiconductors band gaps and d e t e c t o r ranges.

d e t e c t o r a p p l i c a t i o n ) o r t o t h e recombination corresponding t o a f r a c t i o n o f theband gap when deep l e v e l s a r e i n v o l v e d . For i n s t a n c e very few CL s t u d i e s have been

presented on Cd Hg Te [3].

I n o r d e r t o achieve t h e h i g e s t p o s s i b l e s e n s i t i v i t y 1 ig h t c o l l e c t i o n may be improved by e i t h e r the f i t t i n g o f an e l l i p s o C d a l m i r r o r which focuses t h e l i g h t on t h e detectors o r by working i n t h e transmission c o n f i g u r a t i o n (TCL) [4]. I n t h e l a s t experimental set-up, t h e CL generated a t t h e s u r f a c e i s t r a n s m i t t e d through t h e sample (% 300 um i n thickness) and detected by a d e t e c t o r s i t u a t e d j u s t below t h e specimen. High c o l l e c t i o n e f f i c i e n c y i s achieved by the small d i s t a n c e between t h e CL source and t h e d e t e c t o r .

Generally, g r a t i n g monochromators have been used f o r s p e c t r a l CL measurements and f o r monochromatic imaging [5], sometimes a t wavelengths up t o 3 urn (63 .The wavelength r e s o l u t i o n i s o f t h e o r d e r o f 1 nm. The spectrum can a l s o be recorded simultaneously f o r a l l wavelengths (analogous t o EDS i n X-ray a n a l y s i s ) t h r o u h t h e use o f an o p t i c a l multichannel analyser (OM*) described b L l h n e r t e t a1 f 7 ] . I n s t e a d o f a monochromator, an i n t e r f e r o m e t e r can be used

[d.

The F o u r i e r t r a n s f o r m o f the i n t e r ferogram g i v e s t h e o p t i c a l spectrum. T h i s method g r e a t l y improves t h e d e t e c t i o n s e n s i t i v i t y e s p e c i a l l y i n the I R r e g i o n where t h e d e t e c t o r noise i s t h e main l i m i t a t i o n .

The beam b l a n k i n g technique used i n c o n j u n c t i o n w i t h a l o c k - i n a m p l t f i e r tmproves t h e s i g n a l t o n o i s e r a t i o and t h e image c o n t r a s t [ 9 ] . Tn a d d t t i o n i t p e r m i t s t h e m i n o r i t y c a r r i e r l i f e t i m e measurements. Depending upon t h e d i f f e r e n t systems adopted,

one can distinguish the sequential (sampling) technique [lo] [ll] [12], where the l i g h t i s detected i n a narrow time interval t, t

+

~t (variable t i s the time a f t e r the el-ectron beam i s switched o f f ) , and the simultaneous r e g i s t r a t i o n of the CL relaxa- t l o n time : the s i n g l e photon technique [8] applicable t o materials w i t h low CL, e f f i c i e n c i e s , and the s t r e a k camera technique [13] [14] p a r t i c u l a r l y useful f o r low lifetimes measurements (time resolution 100 p s ) .

Many i n s t a l l a t i o n s are provided w i t h a l i q u i d helium stage so the specimen can be cooled down t o 10°K f2] [8] [9] [ll] [14]. Many advantages a r e expected from CL studies a t low temperature : the l i n e s are usually brighter and sharper and therefore more s i g n i f i c a n t .

Finally, i t is worthwhile noticing t h a t some TEM-STEMS are equipped w i t h spectral detection and liquid he1 ium stage [15] [16]

.

Some dedicated STEMS a r e a1 so provided w i t h CL detection [14] which allows CL imaging w i t h a s p a t i a l resolution i n theorder of 100 nm with thin specimens.

111

-

SPECTROSCOPY

The CL emission i s usually devided i n t o i n t r i n s i c and e x t r i n s i c e f f e c t s . I n t r i n s i c e f f e c t s a r e related t o the semiconductor i t s e l f and include band t o bandrecombina- tion andfree exciton recombination. These t r a n s i t i o n s a r e not e f f i c i e n t i n i n d i r e c t gap semiconductors as they require the contribution of a phonon t o conservemomentum.

For band t o band recombination, the wavelength i s such t h a t hv = Eg and f o r f r e e exciton recombination (EX) we have

hv = Eg-Eex

Eex i s the exciton binding energy. In a hydrogenic model, Eex can be expressed a s

s i s the d i e l e c t r i c constant, m: and m: the electron and hole e f f e c t i v e mass respectively.

Extrinsic e f f e c t s a r e r e l a t i v e to the defects, present i n the semiconductor which produce localized s t a t e s i n the forbidden band gap. They include band-level recombi- nations, p a i r t r a n s i t i o n s and bound exciton recombinations. In the f i r s t case, the photon energy of an electron-acceptor level (eAO) or a donor level-hole (Doh) recombination is given by

hv = Eg

-

Ea (Ed)

where Ea (Ed) i s the acceptor (donor) binding energy, t h e thermal energy of f r e e particules being neglected.

Pair t r a n s i t i o n (DOAO) between an electron trapped on a donor and a hole on an acceptor gives a photon of energy :

9

h = Eg

-

(Ed

+

Ea)

-

e'

1

_{1181 1191 [201}

r i s the donor-acceptor distance. The l a s t term represents the coulombien energy.

In the case of a bound exciton which i s generally localized by neutral species, the photon energy is

hv = Eg

-

^Eex

-

^Eb

-

^(E,,

-

^{E n s )}

Eb i s the exciton-impurity binding energy.

For instance, i n the case of a bound exciton on a neutral acceptor (AOX), the corresponding recombination l i n e s a r e f i r s t l y A (y i s the chemical symbol of the acceptor) which i s the most intense,where the a

1

ceptor i s i n i t s fundamental s t a t e

(Els) and secondly AX (n

*

1 ) where a p a r t of the recombination energy i s given to

C4-280 JOURNAL DE PHYSIQUE

t h e h o l e , l e a v i n g t h e a c c e p t o r i n an e x c i t e d s t a t e (EQs, two h o l e t r a n s i t i o n ) . A1 1 t h e s e 1 in e s can have phonon r e p l i c a s . The i n t e n s i t y o f t h e n t h phonon r e p l i c a I n i s g i v e n by

I n = I.

-

N n !

I. i s t h e z e r o t h phonon l i n e i n t e n s i t y and N t h e number o f e m i t t e d p h o n o n s . N r e f l e c t s t h e c o u p l i n g o f t h e d e f e c t w i t h t h e l a t t i c e .

Recombination can o c c u r a t p o i n t d e f e c t s such as v a c a n c i e s , i n t e r s t i t i a l s a n d / o r i m p u r i t y complexes. Sometimes, t h i s i n t e r p r e t a t i o n can be m i s l e a d i n g : i n t h e c a s e o f ZnTe, two a c c e p t o r l e v e l s w e r e a t t r i b u t e d t o t h e Zn v a c a n c y . L a t e r on,by use o f CL and PL t e c h n i q u e s i t was demonstrated t h a t t h e s e l e v e l s w e r e i n f a c t r e l a t e d t o t h e presence o f i m p u r i t i e s such as Cu and L i 1211 [ 2 2 ] .

Not a l l t r a n s i t i o n s a r e r a d i a t i v e ; non r a d i a t i v e r e c o m b i n a t i o n can o c c u r by t h e A u g e r mechanism w h i c h t r a n s f e r s t h e r e c o m b i n a t i o n energy t o an e l e c t r o n o r a h o l e . As t h e l e v e l becomes deeper i n t h e band gap t h e phonon c o u p l i n g i n c r e a s e s . M u l t i p h o n o n r e c o m b i n a t i o n becomes more and more e f f i c i e n t and t h e r e f o r e l e s s and l e s s e f f e c t i v e f o r r a d i a t i v e recombinations [23] [ 2 4 ] . An example o f a deep l e v e l r a d i a t i v e t r a n s i - t i o n observed by photo1 uminescence i s g i v e n by Mircea-Roussel and Makram-Ebeid [25]

c o n c e r n i n g t h e mid-gap EL2 l e v e l i n Ga As.

The band t o band peak energy dependence on t e m p e r a t u r e must be t h e same as f o r t h e band gap, t h a t i s , f o r most semiconductors i t must i n c r e a s e w i t h d e c r e a s i n g t e m p e r a t u r e . C o n v e r s e l y , t h i s e f f e c t can be used t o measure t h e t e m p e r a t u r e o f t h e specimen under t h e impact o f t h e beam. Gatos e t a1 1261 used t h i s e f f e c t t o measure t h e t e m p e r a t u r e i n c r e a s e when t h e beam c u r r e n t was v a r i e d f r o m 350 t o 1150 nA on an InP specimen. An energy s h i f t o f 1 2 meV i n t h e band t o band l i n e was n o t i c e d , c o r r e s p o n d i n g t o a t e m p e r a t u r e i n c r e a s e o f a p p r o x i m a t e l y 40°C. I n t h e same way, Davidson e t a1 [27] w e r e a b l e t o measure l o c a l l y t h e w o r k i n g t e m p e r a t u r e o f a Gunn d i o d e .

As l o n g as t h e i n j e c t i o n l e v e l i s k e p t l o w , t h e peak h e i g h t s e x h i b i t a 1 in e a r r e l a t i o n - s h i p w i t h t h e beam c u r r e n t .

The most s p e c t a c u l a r t e m p e r a t u r e e f f e c t i s t o i n d u c e l a r g e v a r i a t i o n s i n t h e r e l a t i v e peak h e i g h t and band broadening as t h e t e m p e r a t u r e i n c r e a s e s . Only a t low t e m p e r a t u r e can an a c c u r a t e assessement o f t h e o r i g i n s o f t h e l e v e l s i n v o l v e d i n t h e recombina- t i o n s be a t t e m p t e d . R e c e n t l y , Chamonal e t a l [28] showed t h a t PL (4°K) and CL (15°K) s p e c t r a o f Cd Te doped by l i t h i u m d i f f u s i o n a r e comparable. Due t o i t s h i g h e r s p a t i a l r e s o l u t i o n , t h e CL t e c h n i q u e was a b l e t o demonstrate t h a t a p a r t o f L i d i f f u s i o n o c c u r r e d by a g r a i n boundary mechanism.

The e f f e c t o f t h e dopant c o n c e n t r a t i o n o n CL e f f i c i e n c y can be v e r y i m p o r t a n t . Cusano [29] measured t h e CL band edge e f f i c i e n c y i n Ga As as a f u n c t i o n o f t h e d o p i n

c o n c e n t r a t i o n . I n t h e case o f Te d o p i n g , t h e CL e f f i c i e n c y r i s e s by more t h a n

3

o r d e r s o f magnitude between 1016 and 1018 donors cm-3 and t h e n decreases by two o r d e r s o f magnitude t o 1 0 ~ 9 . c m - 3 . The maximum i s reached a t a c o n c e n t r a t i o n where t h e semi- conductor i s degenerate and b e f o r e p r e c i p i t a t i o n o c c u r s . S i m i l a r e f f e c t s were sh wn i n Ga Sb : T e [ 3 0 ] . W i t t r y was a b l e t o c o r r e l a t e t h e CL e f f i c i e n c y w i t h t h e l o c a l v a r i a t i o n o f Te c o n c e n t r a t i o n i n Ga As by e l e c t r o n probe m i c r o - a n a l y s i s (EPMA) [ 3 1 ] . When t h e CL s i g n a l v a r i a t i o n was v e r y i m p o r t a n t , t h e c o r r e s p o n d i n g EPMA s i g n a l v a r i a t i o n was a t t h e s e n s i t i v i t y l i m i t o f t h e method.

I n t h e case o f t e r n a r y a l l o y s , t h e energy p o s i t i o n o f t h e band t o band peak g i v e s an approximate v a l u e o f t h e c o n c e n t r a t i o n . I n some cases, t h e v a r i a t i o n o f t h e i n t e n s i t y i s more s e n s i t i v e . I n Gax A l l - , As, a v a r i a t i o n i n x induces a v a r i a t i o n i n CL i n t e n s i t y as Ga As i s a d i r e c t band gap semiconductor (CL e f f i c i e n t ) and A1 As i s an i n d i r e c t band gap semiconductor. T h i s e f f e c t has been used by L e v i n and Ladany [32]

t o r e v e a l c o m p o s i t i o n i n h o m o g e n e i t i e s i n a Ga A1 As f i l m d e p o s i t e d on Ga As.

I V

-

DISLOCATION STUDIES 1 I n t r o d u c t i o n

CL d i s l o c a t i o n c o n t r a s t w i l l be a r b i t r a r i l y devided i n t o "usual" and "unusual"

constrasts."Usual c o n t r a s t " r e f e r s t o the CL c o n t r a s t which i s most f r e q u e n t l y encountered i n semiconductors : the p o i n t o f emergence o f a d i s l o c a t i o n almost per- p e n d i c u l a r t o t h e surface appears e i t h e r as a "black d o t " o f a few micron i n diameter o r as a " d o t and h a l o " which c o n s i s t s of a b l a c k d o t surrounded by a r e g i o n (4 20um) o f CL i n t e n s i t y g r e a t e r than t h e b u l k one. "Unusual" c o n t r a s t r e f e r s t o d i s l o c a t i o n s e x h i b i t i n g e i t h e r a w h i t e c o n t r a s t o r no c o n t r a s t a t a l l .

Most o f t h e time, the d i s l o c a t i o n s observed i n t h e CL-mode have n o t beencharacteriz- ed by o t h e r methods. Nevertheless, c o r r e l a t i o n s have been made w i t h chemical e t c h i n g [33] [34] [35], X-ray topography [36] and w i t h TEM (see below).

Some a p p l i c a t i o n s o f t h e CL d i s l o c a t i o n s o b s e r v a t i o n w i l l begiven, i n c l u d i n g p l a s t i c deformation, i n t e r f a c e defects; o p t i c a l devices degradation and s e m i - i n s u l a t i n g Ga AS.

CL and EBIC s t u d i e s o f d i s l o c a t i o n s i n semiconductors havebeen reviewed by Booker [37]. Below some aspects o f t h i s paper w i l l be resumed and some new r e s u l t s presented.

2 "Usual c o n t r a s t s "

a)

Black-dot-sontrast

The CL i n t e n s i t y IB generated i n a b u l k m a t e r i a l by a r a d i a t i v e recombination mechanism can be expressed as :

Gib

I B = 7 . P L r (1)

G i s t h e g e n e r a t i o n f a c t o r o f e l e c t r o n - h o l e p a i r s p e r i n c i d e n t e l e c t r o n , ib t h e beam c u r r e n t and Q. t h e i n t e r n a l quantum e f f i c i e n c y f o r r a d i a t i v e

recombination.^^

^{i s the}

r a t i o o f t h e r a d i a t i v e recombination r a t e t o t h e recombination r a t e by a l l mecanisms ( r a d i a t i v e o r n o t ) . Therefore

G i b I B =

e

-I_

r (2)

T, i s t h e r a d i a t i v e recombination l i f e t i m e and T t h e e f f e c t i v e l i f e t i m e f o r r a d i a t i v e and non r a d i a t i v e recombination. I f T n r i s the n o n - r a d i a t i v e l i f e t i m e , then

I n a d i s l o c a t i o n , i f we assume t h a t a new n o n - r a d i a t i v e mechanism operates w i t h a l i f e t i m e TD, then the CL i n t e n s i t y generated a t t h e d i s l o c a t i o n w i l l be :

G i b ' 1

whi t h I D = -

-

_'r ^{( 4 )}

'B

can be expressed i n terms o f l i f e t i m e s . By s u b s t i t u t i n g ( 2 ) and (4) i n t o (6) we have:

C can a l s o be expressed as a functfon o f t h e measured l i f e t i m e s f n t h e b u l k (T) and a t t h e d i s l o c a t i o n ( 7 ' ) :

C

= 1

- j1

( 8 ) w i t h T T ( b l a c k c o n t r a s t )

Formulae (7) and (8) must be considered as very approximate since many f a c t o r s have been neglected. P a r t i c u l a r l y , i t has been assumed t h a t t h e recombination centres are n o t saturated(thereby assuming l i n e a r r e l a t i o n s h i p between i n j e c t i o n and CL genera*

C4-282 JOURNAL DE PHYSIQUE

t i o n ) a n d a l s o t h a t t h e l i f e t i m e s do n o t depend on t h e i n j e c t i o n d e n s i t y . Furthermore, t h e s u r f a c e recombination has n o t been taken i n t o account. The m i n o r i t y c a r r i e r l i f e t i m e approach f o r d i s l o c a t i o n c o n t r a s t has been reviewed by H o l t and D a t t a [38].

Black d o t c o n t r a s t has been e x t e n s i v e l y s t u d i e d b Davidson e t a1 r l 2 ] [27] [39] [40][41]

i n n type Gap ( n i t r o g e n doping ranging from 2.1018 up t o 5.1019 cm-3). They found t h a t t h e grown-in d i s l o c a t i o n s d i d n o t e x h i b i t t h e same c o n t r a s t b u t ranged from l O t o 40 %. The same f i g u r e was obtained i n Gap vapour phase S dopedepi-layers. It was n o t i c e d t h a t t h e c o n t r a s t increased w i t h sulphurdoping and was a minimum f o r d i s l o - c a t i o n s i n t r o d u c e d by p l a s t i c deformation.

L i f e t i m e measurements have been c a r r i e d o u t a t s i n g l e d i s l o c a t i o n s and i n b u l k . I n a l l cases, m i n o r i t y c a r r i e r l i f e t i m e i s t h e s m a l l e s t a t t h e d i s l o c a t i o n andincreases w i t h d i s t a n c e from t h e d i s l o c a t i o n reaching t h e b u l k value a t approximately one d i f f u s i o n length. Boulou an; S c h i l l e r [globtained s i m i l a r r e s u l t s f o r Gap. They gave t h e values T = 150 ns and T = 76 ns f o r a c o n t r a s t o f 45 % which i s i n agreement w i t h formula (8). However, Steckenborn e t a1 1421 i n Ga As ( n t y p e

-

1016 cm-3) found a l i f e t i m e increase a t t h e d i s l o c a t i o n s (+ 29 %1 although t h e c o n t r a s t was b l a c k (C = 76 % ) . T h i s c o n t r a d i c t o r y r e s u l t has n o t been s a t i s f a c t o r i l y explained.

The c o n t r a s t i s u s u a l l y i n s e n s i t i v e t o t h e wavelength a t which i t has been measured:

t h e shape o f t h e spectrum i s unchanged when approaching t h e d i s l o c a t i o n a p a r t from a s l i g h t v a r i a t i o n which was r e p o r t e d by Davidson and D i m i t r i a d i s [41] a t 720 nm i n Gap. I n InP too, no emission l i n e s p e c i f i c t o t h e presence o f d i s l o c a t i o n s was found by Bohm and F i s h e r [43] by s p a c i a l l y r e s o l v e d photoluminesce.

Rasul and Davidson 1391 n o t i c e d t h a t i n t h e case o f low n i t r o g e n doped Gap samples, t h e d i s l o c a t i o n c o n t r a s t remained t h e same as t h e temperature was v a r i e d . I n t h i s specimen, T, was much g r e a t e r than Tnr and r d and t h e r e f o r e t h e c o n t r a s t c o u l d be expressed as :

C = T n r 'Inr ^{+ T~}

The d i s l o c a t i o n c o n t r a s t o n l y depends on t h e r e l a t i v e n o n - r a d i a t i v e l i f e t i m e s a t t h e d i s l o c a t i o n and i n t h e b u l k . I f C i s constant, i t means t h a t t h e temperature

dependence o f Tnr and ^{T r} are t h e same. Although a s p e c i f i c non r a d i a t i v e mechanism a t d i s l o c a t i o n s cannot be r u l e d out, t h i s s t r o n g l y suggests t h a t t h e non r a d i a t i v e mechanism a t t h e d i s l o c a t i o n and i n t h e b u l k i s t h e same. T h i s experiment supports a non r a d i a t i v e mechanism a t p o i n t d e f e c t s which i s enhanced i n t h e v i c i n i t y o f t h e d i s l o c a t i o n .

Titmarsh and Booker 1441 c a r r i e d o u t a study o f t h e d i s l o c a t i o n c o n t r a s t s as a func- t i o n o f t h e i r Burgers vector. By TEM, seventeen d i s l o c a t i o n s were characterized.

They i n c l u d e d screw,edge, 30, 45 and 60" d i s l o c a t i o n s . Each d i s l o c a t i o n gave a s i m i l a r b l a c k d o t c o n t r a s t o f t h e same s i z e and i n t e n s i t y i n CL image.No c o r r e l a t i o n was found between t h e CL image and t h e d i s l o c a t i o n nature.

I n s i l i c o n , no s i n g l e d i s l o c a t i o n c o n t r a s t s have been r e p o r t e d . Only g r a i n boundaries have been v i z u a l i z e d 1451. They e x h i b i t a dark c o n t r a s t .

61

_Do:-a~d-halo-so~trast

T h i s s o r t o f c o n t r a s t u s u a l l y appears i n h i g h l y doped semiconductors : a t t h e l i f e t i m e c o n t r a s t an e f f e c t due t o v a r i a t i o n o f dopant c o n c e n t r a t i o n around t h e d i s l o c a t i o n w i l l be superimposed upon i t . I n t h e i r experiment on Ga As : Te ( n = 4.5 1018 cm-3), B a l k and a1 [46] c o r r e l a t e d a s h i f t i n peak emission w i t h Se c o n c e n t r a t i o n and t h e r e f o r e were a b l e t o show t h a t t h e Se c o n c e n t r a t i o n decreased when approaching t h e d i s l o c a t i o n . Presumably, Se p r e c i p i t a t i o n o c c u r r i n g a t t h e d i s - l o c a t i o n was r e s p o n s i b l e f o r t h i s denuded r e g i o n . CL increase (halo c o n t r a s t ) was t h e r e f o r e c o r r e l a t e d t o t h e dopant d i s t r i b u t i o n around t h e d i s l o c a t i o n . S i m i l a r observations were made by Boulou and S c h i l l e r (91.

Chu and a1 47 made a d e t a i l e d study i n Ga As : Te w i t h doping concentrations rang-

I ]

i n g from 10 7 u t o 5.1018 cm-3. Black dot, d o t and h a l o and halo c o n t r a s t s were i n v e s t i g a t e d . H~-TEM observations were c a r r i e d o u t i n o r d e r t o e s t a b l i s h a

c o r r e l a t i o n between t h e c o n t r a s t observed and t h e n a t u r e o f t h e c r y s t a l d e f e c t present.

The i n t e r e s t i n g p o i n t i s t h a t , i n many cases, t h e c o n t r a s t s were n o t associated w i t h a s i n g l e d i s l o c a t i o n b u t w i t h many d i s l o c a t i o n s and f o r p r e c i p i t a t i o n d i s l o c a t i o n loops. T h i s study c l e a r l y shows t h a t c a r e must be taken b e f o r e i n t e r p r e t a t i o n o f CL c o n t r a s t s . P a r a l l e l o r simultaneous TEM observations a r e h i g h l y d e s i r a 6 l e .

3 "Unusual c o n t r a s t s "

The f i r s t "unusual c o n t r a s t " w i l l be i l l u s t r a t e d by t h e work o f P e t r o f f and a1 [481 [49] c a r r i e d o u t i n a STEM a l l o w i n g d i r e c t comparison between TEM and CL images. On a (001) Ga As s u b s t r a t e a h e t e r o - e p i t a x i a l s t r u c t u r e Ga A1 ASP 1 Ga As/Ga A1 As was grown by l i q u i d phase e p i t a x y (LPE). A f t e r removing t h e Ga As s u b s t r a t e themismatch d i s l o c a t i o n s contained i n t h e t h r e e l a y e r specimen (1.5 vm i n t h i c k n e s s ) were observ- ed. They c o n s i s t e d o f a cross-hatched network o f m a i n l y 60" d i s l o c a t . o n s ( o r i e n t a t i o n s

11101 and [lT0[ and a few Lomer d i s l o c a t i o n s (b = 1 / 2 [110'] 1 = [110'] o r b = 1/2 [110] 1 = [ll~] ) . The 60" d i s l o c a t i o n s e x h i b i t e d a "usual" dark c o n t r a s t t y p i c a l o f enhanced non r a d i a t i v e recombinations. The two s e t s have n o t t h e same d i s l o c a t i o n density. I t has been demonstrated t h a t t h e s e t which c o n t a i n s t h e g r e a t e s t amount of d i s l o c a t i o n s ( s e t 1) was generated b e f o r e t h e s e t 2. The d i s l o c a t i o n s of s e t 2 are t h e r e f o r e more mobile and c o n t a i n more k i n k s than those o f s e t 1. However, i t was found t h a t t h e d i s l o c a t i o n c o n t r a s t f o r s e t 2 was l o w e r than f o r s e t 1. Therefore, i t can be s a i d t h a t k i n k s do n o t seem t o p l a y an i m p o r t a n t r o l e i n t h e assessment o f t h e non r a d i a t i v e e f f i c i e n c y associated w i t h a d i s l o c a t i o n .

The "unusual c o n t r a s t " r e f e r s t o Lomer d i s l o c a t i o n s which do n o t e x h i b i t any CL c o n t r a s t . 60" d i s l o c a t i o n s a r e known t o be d i s s o c i a t e d whereas Lomer d i s l o c a t i o n s are presumably undissociated and have no dangling bords. Therefore, i t i s thought t h a t t h e p e c u l i a r core s t r u c t u r e o f Lomer d i s l o c a t i o n s should p l a y a r o l e i n t h e i r n e u t r a l behaviour toward CL.

The second t y p e o f "unusual c o n t r a s t " r e f e r s t o a d i s l o c a t i o n w i t h CL enhancement a t a p a r t i c u l a r wavelength. From EL, some evidence o f d i s l o c a t i o n emission a t 0.5 eV i n p l a s t i c a l l y deformed germanium was i v e n by Ivanov [50] and B a r t h and a1 [51

c a r r i e d o u t a t 4'K, Drozdov and a1 7521 showed t h a t recombination r a d i a t i o n

were associated w i t h t h e presence o f d i s l o c a t i o n s i n s i l i c o n . Two s t r o n g l i n e s (0.812 and 0.875 eV) and two weak l i n e s (0.934 and 1.00 eV) were revealed.

P e t r o f f [53 c a r r i e d o u t CL observations on a molecular beam e p i t a x y (MBE) double h e t e r o s t r u c

l

u r e (Ga A1 As/Ga As/Ga A1 As) specimen. A t E = 1.512 eV (DOX l i n e ) , t h e d i s l o c a t i o n s e x h i b i t a "usual" dark c o n t r a s t . However, a t E = 1.504 eV which corres- pond t o an as y e t u n i d e n t i f i e d bound e x c i t o n c h a r a c t e r i s t i c o f MBE material, o n l y t h e d i s l o c a t i o n s which had o r i g i n a t e d i n t h e Ga As s u b s t r a t e changed t o g f v e a w h i t e c o n t r a s t . Therefore, i t was so thought t h a t t h e unknown c e n t r e ( i m p u r i t y o r p o i n t d e f e c t ) c l u s t e r s around t h e d i s l o c a t i o n s thus g i v i n g r i s e t o t h e w h i t e c o n t r a s t observed.

CL s t u d i e s o f d i s l o c a t i o n s i n n a t u r a l diamond were c a r r i e d o u t by Lang e t a1 [54'][55]

by use o f t h e CL topographic technique : th e specimen i s f l o o d e d w i t h a s t a t i o n a r y e l e c t r o n beam and t h e image recorded on f i l m by a simple o p t i c a l system. This exper- imental set-up a l l o w s a n a l y s i s o f t h e p o l a r i z a t i o n o f t h e l i g h t e m i t t e d by t h e specimen. The d i s l o c a t i o n s were found t o generate a b l u e emission. I n a d d i t i o n t h e d i s l o c a t i o n s , e s p e c i a l l y those contained i n s l i p l i n e s , e m i t t e d a p o l a r i z e d l i n e w i t h t h e E v e c t o r p a r a l l e l t o t h e s l i p plane. Pennycook e t a1 [56] obtained h i g h r e s o l u - t i o n CL d i s l o c a t i o n images i n a STEM. No c o r r e l a t i o n was found between t h e CL e f f i - ciency and t h e d i s l o c a t i o n Burgers v e c t o r . Moreover some d i s l o c a t i o n s d i d n o t e m i t any l i g h t andwereout o f c o n t r a s t i n CL imaging. However, i t was r e p o r t e d by Smmida and Lang [57] t h a t h i g h energy e l e c t r o n s quench t h e l i g h t emission and c o u l d e x p l a i n t h e bad c o r r e l a t i o n observed by Pennycook e t a l .

V

-

APPLICATIONS

1 P l a s t i c deformation

C4-284 JOURNAL DE PHYSIQUE

Davidson e t a1 [58] c a r r i e d o u t p l a s t i c deformation on a Gap specimen at700°C using a f o u r - p o i n t bending apparatus. A cross-hatched p a t t e r n was revealed on a (110) face by CL examination. The angle between t h e two s e t s o f l i n e s was 70" and t h i s was i n - t e r p r e t e d as the t r a c e o f the {Ill) s l i p planes. I n d i v i d u a l d i s l o c a t i o n s w i t h b l a c k d o t c o n t r a s t c o u l d be seen a t t h e ends o f t h e sample where t h e deformation was moderate.

Esquivel e t a1 [59] [60 i n t r o d u c e d e i t h e r a o r

p

d i s l o c a t i o n s i n n-Ga As bent crys-

1

t a l s . No s i g n i f i c a n t c anges r e l a t e d t o t h e type o f d i s l o c a t i o n were n o t i c e d by d i s l o c a t i o n c o n t r a s t , peak p o s i t i o n o r h a l f peak w i d t h o f the.CL emission l i n e . Continuous CL examination o f d i s l o c a t i o n motion have been c a r r i e d o u t by Maeda e t a1 [61] by use o f a deformation apparatus i n s t a l l e d i n the SEM chamber. Cd Te and CdS were deformed a t room temperature and 380°K r e s p e c t i v e l y . The p o i n t s o f emergence o f the d i s l o c a t i o n s e x h i b i t e d a dark s p o t c o n t r a s t . D i s l o c a t i o n sources and growth behaviour o f s l i p bands can be observed.

I n a r e c e n t paper, Maeda e t a1 [62] were a b l e t o observe t h e g l i d e motion o f a and 6 d i s l o c a t i o n s i n Ga As a t d i f f e r e n t temperatures, and t o study the e f f e c t o f t h e electron-beam i r r a d i a t i o n on t h e d i s l o c a t i o n v e l o c i t y . Above a c r i t i c a l temperature T (550 and 650°K f o r a and 6 d i s l o c a t i o n s r e s p e c t i v e l y ) , t h e temperature dependence

OF

t h e v e l o c i t y f o l l o w e d an Arrhenius formula w i t h an a c t i v a t i o n energy o f 1 eV f o r a

d i s l o c a t i o n s and 1.7 eV f o r 6 d i s l o c a t i o n s , and corresponded t o v e l o c i t y measurements r e a l i s e d i n darkness. Below T

,

t h e a c t i v a t i o n energy changed t o 0.3 eV and 1.1 eV f o r a and 6 d i s l o c a t i o n s . A t ~ O O O K , t h e m o b i l i t y o f a d i s l o c a t i o n s was found t o increase by a f a c t o r o f 10 compared t o t h e m o b i l i t y measured w i t h o u t i r r a d i a t i o n . These r e s u l t s were i n t e r p r e t e d i n terms o f t h e recombination enhanced d e f e c t motion (RDEM) mechanism. The r e d u c t i o n i n a c t i v a t i o n energy (0.7 eV and 1.1 eV f o r a and 6 d i s l o c a t i o n s r e s p e c t i v e l y ) , was r e l a t e d t o t h e energy r e l e a s e d by n o n r a d i a t i v e recom- b i n a t i o n o f excess c a r r i e r s a t d i s l o c a t i o n s .

2 I n t e r f a c e d e f e c t s

I n a d d i t i o n t o t h e experiments o f P e t r o f f 1481 1491 1531 a l r e a d y described, many authors have used t h e CL technique as a means o f c h a r a c t e r i z i n g t h e d e n s i t y o f m i s f i t d i s l o - cations. Kasano and Hasoki [63] i n v e s t i g a t e d Ga Asl-, Px/Ga As graded l a y e r s by CL where x was v a r i e d from 0 t o 0.4. From t h e s h i f t o f t h e band t o band peak wavelength, they deduced the compositional p r o f i l e (and thus t h e compositional g r a d i e n t ) and from the i n t e n s i t y v a r i a t i o n s they deduced a non r a d i a t i v e c e n t r e c o n c e n t r a t i o n ( s d i s l o - c a t i o n d e n s i t y ) . These two q u a n t i t i e s can be compared as a f u n c t i o n o f t h e shape of the g r a d i e n t f o r d i f f e r e n t specimens. S c h i l l e r [64] has c a r r i e d o u t t h e same k i n d o f assessment o f Gal,, In, As/Ga As graded l a y e r s .

Umanskii e t a1 1651 determined t h e c r i t i c a l value f o r

9

(a : l a t t i c e parameter) i n the Gal-,In, As/Gao.tj A10.5 As system f o r which the deformation changes from e l a s t i c t o i n e l a s t i c . They found a value o f 0.08 %. "Rake l i n e d e f e c t s " i n t h e a c t i v e l a y e r o f Ga A1 As double h e t e r o s t r u c t u r e l a s e r s have been v i z u a l i s e d by Gaw and Reynolds i n the transmission c o n f i g u r a t i o n [66].

3 LED and l a s e r degradation

TCL s t u d i e s o f t h e degradation o f l i g h t - e m i t t i n g devices were undertaken by Chin e t a1 who have presented a review on t h e s u b j e c t 1671. The degradation o f homojunction graded band-gap Ga A1 As : S i was s t u d i e d i n the transmission c o n f i g u r a t i o n r68j.

Dark l i n e d e f e c t s (DLDs) are observed t o o r i g i n a t e a t t h e s u b s t r a t e back surface and t o extend towards t h e j u n c t i o n . Only a t t h i s stage were t h e DLDs observed i n t h e electroluminescence image.

I n agreement w i t h previous work, i t was confirmed by CL t h a t , i n Ga A1 As double h e t e r o s t r u c t u r e LEDs, DLDs o r i g i n a t e a t p r e - e x i s t i n g d i s l o c a t i o n s

.

S i m f l a r obser- v a t i o n s were made by L e b a i l l y e t a1 [70] on Ga A1 As ectroluminescent diodes. Therefore, Ct can be used as a n o n d e s t r u c t i v e technique t o s e l e c t m a t e r i a l s t h a t w i l l produce devices w i t h l o n g l i f e t i m e s .

4 Semi-insulating Ga As

The c e l l s t r u c t u r e o f SI-Ga As i n g o t s was r e v e a l e d by Chin e t a1 [71]. The d i s l o c a - t i o n observed i n t h e dark s p o t contrast, d e l i n e a t e d l a r g e regions (> 10Pum) free from d i s l o c a t i o n s . The 1 uminescence e f f i c i e n c y i m e d i a t l y around t h e d i s l o c a t i o n s

(0 *

5 um) was found t o be low, t o i n c r e a s e over a 50 um wide r e i o n and then t o decrease a g a i n i n t h e d i s l o c a t i o n f r e e region. Kamei ji m a e t a1 T721 were a b l e t o c o r r e l a t e t h e CL c o n t r a s t o f t h e c e l l s t r u c t u r e w i t h i m p u r i t y segregation ( S i ,O,Cr) a t t h e d i s l o c a t i o n s by use o f t h e SIMS technique.

V I

-

CONCLUSION

The CL technique has undergone a r a p i d development i n these l a s t few years. S o p h i s t i - cated experimental set-up have been r e a l i z e d , a l l o w i n g c o n t r a s t e v a l u a t i o n on mono- chromatic images, accurate l i f e t i m e measurements and examination a t low temperatures.

F u r t h e r improvements a r e r e q u i r e d i n t h e l o n g wavelength area (> 1.2 um) t o enable s t u d i e s o f narrow-gap semiconductors and t r a n s i t i o n s i nvol v i ng mid-gap 1 eve1 s.

I n most cases t h e presence o f i m p u r i t i e s and/or p o i n t d e f e c t s associated w i t h d i s l o - c a t i o n s can be suspected to. e x p l a i n t h e c o n t r a s t behaviour o f d i s l o c a t i o n s .There i s s t i l l a need f o r fundamental experiments on "clean" d i s l o c a t i o n s c o r r e l a t e d w i t h a c a r e f u l c h a r a c t e r i z a t i o n i n c l u d i n g Burgers v e c t o r determination and study o f TEM core s t r u c - t u r e (weak- beam, h i g h r e s o l u t i o n )

.

F i n a l l y , CL has proved t o be a valuable non-destructive method i n i n v e s t i g a t i n g t h e d i s l o c a t i o n d i s t r i b u t i o n and i m p u r i t y inhomogenei t i e s i n semiconductors. However, t h e t h e o r i t i c a l 1 if e t i m e approach f o r d i s l o c a t i o n c o n t r a s t i s n o t s a t i s f a c t o r y i n many instances as t o o many p o i n t s a r e neglected, and a comprehensive CL theory i s s t i l l required.

REFERENCES

[I] PFEFFERKORN (6. ), BROKER (W. ) and HASTENRATH (M.

1,

SEM 1980,vol. I (AMF O'Hare, SEM I n c . ) , 251.

[2] HOLT(D.B. ), Microscopy o f Semiconducting M a t e r i a l s , I n s t . Phys. Conf. Ser.n060, 1981, 165.

[3] IVANOV-OMSK11 (V.I.), MALTSEVA (V.A.), BRITOV (A.D.) and SIVACHENKO (S.D.), Phys.

- S t a t . Sol. (a), 1 9 7 8 , s 77.

141 CHIN (A.K.), TEMKIN, (H.) and ROEDEL (R.J.), Appl. Phys. Lett., 1979, % 476.

[51 DAVIDSON (S.M.) and RASUL (A.), J. Phys. E : S c i . Instrum., 1976, 9, 43.

[61 BEAUVINEAU ( J . ) and SEMO (J.), Rev. Sci. Instrum., 1982, 53, 1573.

[7] LOHNERT (K. )

,

HASTENRATH (M. ) and KUBALEK (E. ), SEM 1979, v o l .I (AMF O'Hare, SEM I n c . ) , 229.

[8] DAVIDSON (S.M. ), CUMBERBATCH (T.J. ), HUANG (E. ) and MYHAJLENKO (S. ), Microscopy of Semiconducting M a t e r i a l s , I n s t . Phys. Conf. Ser. No 60, 1981, 191.

[91 BOULOU (M.) and SCHILLER (C.), J. Microsc. Spectrosc. Electron., 1981,

L

^39.

[lo]

BALK (L.J.), KUBALEK (E.) and MENZEL (E.), SEM 1975, v o l .I (AMF O'Hare, SEM Inc.), 447.

[ll] STECKENBORN (A. )

,

MUNZEL (H. ) and BIMBERG (D. )

,

Microscopy o f Semiconducting M a t e r i a l s , I n s t . Phys. Conf. Ser. No 60, 1981, 185.

t121 RASUL (A.) and DAVIDSON (S.M.), SEM 1977, v o l . 1 (AMF OIHare, SEM Inc.), 233.

[13] HASTENRATH (M.), BALK (L.J .), LOHNERT (K. ) and KUBALEK (E.), J. Microscopy, 1980,

118,

303.

[14] LOHNERT (K. )

,

HASTENRATH (M.

1,

BALK (L. J. ) and KUBALEK (E. )

,

Microscopy o f Semi- conducting M a t e r i a l s , I n s t . Phys. Ser. No 60, 1981, 179.

[15] PETROFF (P.M.), LANG (D.V.), STRUDEL

(J.L.)

and LOGAN (R.A.), SEM 1978, vol.1 (AMF O'Hare, SEM Inc.), 325.

[16] ROBERTS (S.H.) and STEEDS (J.W.), J. o f Cryst. Growth, 1982,

3,

^312.

[I71 PENNYCOOK (S.J.) and HOWIE (A.), P h i l . Mag. A, 1980,

3,

^809.

[I81 DEAN (P.J.), Progress i n S o l i d S t a t e Chemistry, Pergamon Press, 1973, 8, 1.

[19] HOPFIELD (J.J.), THOMAS (D.G.) and GESHENZON (M.), Phy. Rev. L e t t e r s , 1963, 162.

C4-286 JOURNAL DE PHYSIQUE

[20] MUNNIX (S.) and KARTHEUSER (E.), Phys. Rev. B, 1982, 26, 6776.

t211 BENSAHEL (D.), DUPUY (M.) and PFISTER (J.C.

1,

Phys. S t a t . Sol. (a), 1979, 55,211.

1221 BENSAHEL (D.), MAGNEA (N.) and DUPUY (M.), S o l i d S t a t e Corn., 1979, 30, 467.

C231 HENRY (C.H. ) and LANG (D.V.), Phys. Rev. B, 1977, 15, 989.

E241 TOYOZAWA (Y.), S o l i d S t a t e Elect., 1978, Zl, 1313.

[253 MIRCEA-ROUSSEL (A.) and MAKRAM-EBEID (S.), Appl. Phys. Lett., 1981, 38, 1007.

C261 GATOS (C.H.), VAUGHAN (J.J.), LAGOWSKI (J.) and GATOS (H.C.), J . Appl. Phys., 1981, 1464.

[271 DAVIDSON (S.M.) and RASUL (A.), SEM 1977, v o l .I (AMF OIHare, SEM Inc.), 225.

C281 CHAMONAL (J.P.), MOLVA (E.), DUPUY (M.) ACCOMD (R.) and PAUTRAT (J.L.), To be published i n Phys. S t a t . Sol. ( b ) .

C291 CUSANO (D.A. ), S o l i d S t a t e Comm., 1964,

2,

353.

C301 CHIN (A.K.) and BONNER (W.A.), Appl. Phys. Lett., 1982,

40.

^248.

1311 WITTRY (D.B.), Appl. Phys. L e t t . , 1966, -8 142.

C321 LEVIN (E.R.) and LADANY ( I . ) , J. Appl. Phys., 1978,49, 3025.

(331 CHIN (A.K.), J. Electrochem. Soc., 1982, 13, 369

041 CHIN (A.K.) and MAHAJAN (S), Appl. Phys. Lett., 1979, 35, 784.

L351 NAKAGAWA (K.), MAEDA (K) and TAKEUCHI (S.), Appl. Phys. L e t t . , 1979,

34.

574.

1361 KIFLAWI (I.) and LANG (A.R.), P h i l . Mag., 1976,

33.

^697.

[371 BOOKER ( G . R. ) Microscopy o f Semiconducting M a t e r i a l s , I n s t . Phys. Conf. Ser. No 60, 1981, 203.

[381 HOLT (D.B.) and DATTA (S.), SEM 1980, v o l . 1 (AMF OIHare, SEM Inc.), 259.

1391 RASUL (A.) and DAVIDSON (S .M. ), Microscopy o f Semiconducting M a t e r i a l s , I n s t . Phys. Conf. Ser. N060, 1981, 306.

[40] DIMITRIADIS (C.A.), HUANG (E. ) and DAVIDSON (S.M.), S o l i d S t a t e E l e c t . 1978,

21.

1419.

[41] DAVIDSON (S.M.) and DIMITRIADIS (C.A.), J. o f Microscopy, 1980,

118,

275 [ 4 a STECKENBORN (A.), MUNZEL (H. ) and BIMBERG (D.), J . o f Luminescence, 1981, 2=,

351.

[431 BOHM (K.) and FISCHER (B), J. Appl

.

Phys., 1979,

50,

5453.

C441 TITCHMARSH (J.M.), BOOKER (G.R.), HARDING (W.) and WIGHT (D.R.), J. o f Mat.

Science, 1977,12, 341.

[451 CUMBERBATCH (T. J. )

,

DAVIDSON (S.M. ) and MYHAJLENKO (S. )

,

Microscopy o f Semi- conducting M a t e r i a l s , I n s t . Phys. Conf. Ser. N060, 1981, 197.

[461 BALK (L.J.), KUBALEK (E.) and MENZEL (E.), SEM 1976, v o l .I (AMF OtHare, SEM Inc.), 257

[471 CHU (Y .M.), DARBY (D.B.) and BOOKER (G.R.), Microscopy o f Semiconducting Mate- r i a l s , I n s t . Phys. Conf. Ser. N060, 1981, 331.

[481 PETROFF (P.M.), LOGAN (R.A.) and SAVAGE (A.), Phys. Rev. L e t t e r s , 1980, 4& 287.

C491 PETROFF (P.M.), LOGAN (R.A.) and SAVAGE (A.), J. o f Microscopy, 1980,

118.

255.

C501 IVANON (Y.L.), Sov. Phys.- S o l i d State, 1965,

1,

629.

(511 BARTH (W.), BETTINI (M.) and OSTERTAG (U.), Phys. S t a t . Sol. (a), 1970,

5

K177.

[52] DROZDOV N.A. ), PATRIN (A.A. ) and TKACHEV (V. D. )

,

JETP L e t t . , 1976,

23.

598.

c531 PETROFF [P.M. ), Defects i n Semiconductors, North Holland, 1981, 457.

[541 KIFLAWI ( I . ) and LANG (A.R.), P h i l . Mag., 1974, 219.

[551 SUMIDA (N.) and LANG (A.R.), P h i l . Mag. A, 1981,

B ,

1277.

L561 PENNYCOOK (S. J.), BROWN (L.M.) and CRAVEN (A.J.), P h i l . Mag. A, 1980,41. 589.

1571 SUMIDA (N.) and LANG (A.R. ), Microscopy o f Semiconducting M a t e r i a l s , I n s t . Phys.

Conf. Ser. N060, 1981, 319.

[581 DAVIDSON S.M.)

,

IQBAL (M.Z. ) and NORTHROP (D.C. ), Phys. Stat,Sol

.

(al,l975,29,571.

L591 ESQUIVEL [A.L.), L I N (W.N.) and WITTRY (D.B.

1,

Appl. Phys. L e t t . , 1913 22 414.

C601 ESQUIVEL (A.L.), SEN

(S.)

and L I N (W.N.), J. Appl

.

Phys., 1976, 47,2588: -' [611 MAEDA (K.), NAKAGAWA (K.)

,

TAKEUCHI (S.) and SAKAMOTO (K. )J. o f Mat. Science,

1981,

16.

927.

[ 6 4 # MAEDA (K.), SAT0 (M.), KUBO (A.) and TAKEUCHI (S.), J. Appl. Phys., 1983,%,161.

1631 KASANO (H.), and HOSOKI (S.), J. Appl

.

Phys., 1975,

46,

^394.

[641 SCHILLER (C.), Ga As and Related Compounds, I n s t . Phys., Conf. Ser. N033a, 1977, 25

1651 UMANSKII (V.E.), KONNIKOV (S.G.)

,

GARBUZOV (D.Z. ), TULASHVILI (E.V .), ARSENT'EV (I.N.) and DEINEKINA (I.V.) Sov. Phys. Semicond., 1982,

16,

955.

[661 GAW (C.A.) and REYNOLDS (C.L.), Electron. L e t t . , 1981,

17,

285.

[67] CHIN (A.K.), JEMKIN (H.) and MAHAJAN (S.;, The B e l l System Techn. Journal, 1981,

a

^2187.

[68] CHIN (A,K.), KING (W.C.), LEONARD (T.J.), ROEDEL (R.J.), ZIPFEL (C.L.), KERAMIDAS (V.G.) and ERMANIS (F.), J. o f Electrochem. Soc., 1981, 128, 661.

[69] CHIN (A.K. )

,

KERAMIDAS (V.G.)

,

JOHNSTON J r . (W .D. )

,

MAHAJAN (S. ) and ROCCASECCA (D.D.), J, Appl

.

Phys., 1980,

51,

978.

[70] LEBAILLY (J.), DUPUY (M.), BOISROBERT (C.) and FRESSY (G.), IEEE Trans. Electron Devices, 1981,

w,

^390.

[71] CHIN (A,K,), YON NEIDA (A.R.) and CARUSO (R.), J. Electrochem. Soc., 1982,

129,

2386.

[72) KAMEJIMA (T.), SMIMURA (F.), MATSUMOTO (Y .), WATANABE (H.) and MATSUI (J .), Jap.

J. Apple, Phys,, 1982, 2l, L721.