DEFECTS IN CADMIUM TELLURIDE AS STUDIED BY SEM/EBIC

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DEFECTS IN CADMIUM TELLURIDE AS STUDIED BY SEM/EBIC

B. Sieber, M. Dupuy

To cite this version:

B. Sieber, M. Dupuy. DEFECTS IN CADMIUM TELLURIDE AS STUDIED BY SEM/EBIC.

Journal de Physique Colloques, 1983, 44 (C4), pp.C4-297-C4-304. �10.1051/jphyscol:1983435�. �jpa-

00223054�

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JOURNAL DE PHYSIQUE

Colloque C4, supplément au n°9, Tome 44, septembre 1983 page C4-297

DEFECTS IN CADMIUM TELLURIDE AS STUDIED BY SEM/EBIC

B. S i e b e r and M. Dupuy*

L.P.M. - C.N.R.S., 1, -place A. Briand, 92190 Meudon, France

*L.E.T.I./C.R.M.J> B.P. 85 X, 38041 Grenoble, France RESUME

Nous avons u t i l i s é l a méthode du courant i n d u i t dans l e microscope à balayage (MEB/EBIC) pour c a r a c t é r i s e r les défauts électriquement a c t i f s dans l e t e l l u r u r e de cadmium non dopé r e c u i t , de type n. Pour cela nous avons f a i t , sur une diode de Schottky Au/CdTe, des mesures Q u a n t i t a t i v e s de contraste e t de r é s o l u t i o n en f o n c - t i o n de l a tension d ' a c c é l é r a t i o n sur les défauts observés. Cela permet de connaitre- l e u r profondeur, e t d ' a v o i r des informations sur l e u r s t r u c t u r e .

ABSTRACT

The irduced current method, performed in a scanning electron microscope (SEM/EBIC) , has been used to characterize the electrically active defects in undoped n type, annealed cadmium telluride. For that purpose, quantitative measurements, on a Au/

CdTe Schottky diode, have been made of the contrast and resolution behaviour a s a function of the beam voltage. This allows to assess the depth of the defects, and to get information about their structure.

I. INTRODUCTION

The EBIC technique (electron beam induced conductivity) performed on bulk specimens in a scanning electron microscope (SEM), is a non destructive tool to investigate the spatial distribution of the electrical activity of defects in semiconductors. Both pn junctions and Schottky barriers have been used to make quantitative studies of the EBIC contrast of defects (point defects, clusters, dislocations). These studies have been performed mainly on silicon [1 - 13] for which theoretical models have been developped [14 - 17] .

The contrast theory in bulk specimens was first elaborated by DON0LAT0 [14, 16] who was essentially interested in "point-like" defects and dislocations perpendicular to the surface ("line-shaped" defects). Let us recall that the defect contrast C is equal to (Ig-I„)/In, IR being the background EBIC current, and I„ that at the defect. In a specimen geometry where i) the depletion region is negligible

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983435

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C4-298 JOURNAL DE PHYSIQUE

(compared t o t h e e l e c t r o n range Ro) iil the generation volume i s tanqent t o the surface, Donolato found t h a t , when decreasing the a c c e l e r a t i n g v o l t a q e Eo, t h e c o n t r a s t o f t h e d i s l o c a t i o n was increasing, and t h a t t h e c o n t r a s t o f " p o i n t - l i k e "

d e f e c t was going through a maximum depending on the e l e c t r o n range -Rn- t o t h e d e f e c t depth r a t i o . The image r e s o l u t i o n (w = h a l f w i d t h a t h a l f h e i g h t ) o f b o t h d e f e c t s was i n c r e a s i n q w i t h Eo, and was m a i n l y Rp deoendent.

The aim o f the present work i s t o f i n d o u t i f Donolato's model can be used i n CdTe c r y s t a l s , i n o r d e r t o assess t h e s t r u c t u r e o f defects. Two assumptions o f the model are n o t f u l l y v a l i d i n our case : i ) the d e p l e t i o n r e g i o n i s n o t n e g l i g i b l e . i i ) t h e generation volume i s n o t tangent t o t h e surface. l~fe have undertaken, i n a SEM, q u a n t i t a t i v e c o n t r a s t experiments as a f u n c t i o n o f t h e a c c e l e r a t i n q voltaqe, on various types o f defects.

11. EXPERIMENTAL

We have i n v e s t i g a t e d a m o n o c r y s t a l l i n e b u l k specimen o f n t y p e CdTe, which was elaborated by t h e THM technique [181. The specimen (3x3x0,5 mm 3 ), was n o t doped;

t o achieve n type, i t has been annealed under s a t u r a t e d cadmium pressure, f o r 6 H, a t 700°C, then quenched i n water.

The Schottky b a r r i e r ( 1 mm i n diameter) was made by evaporation, i n u l t r a - vacuum, of a t h i n f i l m o f g o l d ( 50 nm t h i c k ) a f t e r mechanical and chemical ( i n bromine- methanol ) p o l i s h o f t h e surface. An indium ohmic c o n t a c t was r e a l i z e d on t h e back side.

P r i o r t o observation i n t h e SEW, t h e c u r r e n t - v o l t a g e and capacitance-voltage c h a r a c t e r i s t i c s have been recorded; they gave an i d e a l i t y f a c t o r equal t o 1.2 a leakage c u r r e n t o f about 161° A f o r a reverse b i a s o f 5 V, and a doping l e v e l o f 1 . 5 x (which corresponds t o a depleted r e g i o n o f 0.9

m).

The i n t e r a c t i o n electron-beam-crystal i s described i n the usual aporoximations.

The depth dose f u n c t i o n as w e l l as t h e e l e c t r o n range Rp have been c a l c u l a t e d , as a f u n c t i o n o f Eo, from t h e formulae o f Kanaya and Okayama [191. The maxima o f the generation r a t e s are l o c a t e d a t about 0.2 Rp.

The c o n t r a s t and r e s o l u t i o n measurements were performed by r e c o r d i n q on a x

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t c h a r t recorder, the v a r i a t i o n o f t h e EBIC c u r r e n t w h i l e scanning t h e e l e c t r o n beam on s i n g l e l i n e through the d e f e c t ( t i m e = 60 sec; m a g n i f i c a t i o n = 4 x 10 3 ) . The measurements a r e absolute s i n c e t h e c u r r e n t i s d i r e c t l y taken o f f from t h e specimen.

IJe have used a c c e l e r a t i n g voltaqes Eo ranqinq from 30 down t o 10 k e V w i t h 5 keV steps.

Low i n j e c t i o n c o n d i t i o n s were respected through out.No b i a s was a p n l i e d .

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111. RESULTS AND DISCUSSION

Fiaure 1 shows t h e t y p i c a l c o n f i g u r a t i o n o f e l e c t r i c a l a c t i v e d e f e c t s i n t h e diode studied5observed a t 25, 20, 15 and 10 keV. Dark spots are present a t a d e n s i t y o f about

l o 6

cm-2.

From image a n a l y s i s and c o n t r a s t measurements, i t r e s u l t s t h a t the defects can be c l a s s i f i e d i n f o u r categories :

1) Dark spots which are i n c o n t r a s t a t 30 and 25 keV, and o u t o f c o n t r a s t a t e i t h e r 20 keV o r 15 keV o r 10 keV. They are r e f e r r e d (see f i g 1 ) as X1, X2 and X3 resoec- t i v e l y . The curves c = f ( E o ) and w = f (Eo) are p l o t t e d on f i g u r e s 2 and 3 f o r X2 and X3 r e s p e c t i v e l y . According t o Donolato's model, the c o n t r a s t maximum f o r p o i n t l i k e d e f e c t s i s obtained when t h e d e f e c t i s a t 0.8 Rp from the surface. This value i s probably too l a r g e s i n c e t h e depth dose f u n c t i o n i s a maximum a t 0.2 Rp i n s t e a d o f the uniform f u n c t i o n down t o Rp used by Donolato. IJe assume t h a t t h e c o r r e c t value must be between 0.2 and 0.8 Rp and deduce t h a t the p o s i t i o n o f X2 and X3 i s about 2 from the surface, X2 being deeper than X3 ( f i q 2 and 3 ) . The r e s o l u t i o n o f X3 i s q u i t e steady from 30 t o 20 keV, and then increases very r a p i d l y w i t h Eo.

This i s n o t the case o f X2 ( f i g u r e 2 ) . Defects l i k e X I , X2 and X3 have been found t o be t h e most numerous ones i n t h e observed area. Other s i m i l a r dark spots o f which an example c a l l e d X4, i s shown on f i g u r e 4, have a maximum c o n t r a s t a t Eo = 25 keV, corresponding t o a depth o f about 1

m.

llhen Eo decreases from 15 keV t o 10 keV, t h e beam induced generation i s e n t i r e l y w i t h i n t h e d e p l e t i o n r e a i o n and t h e c o n t r a s t increases a l i t t l e , w h i l e t h e r e s o l u t i o n improves by a f a c t o r o f 2 ( f i g 4).

2) Dark snots which a r e v i s i b l e from 30 down t o 10 key. Some o f them (one o f them i s named y on f i q 1 ) have a c o n t r a s t o f n o i n t 1 i ke defect w i t h a minimum a t 15 kev may bedue t o t h e i n f l u e n c e o f t h e deoleted zone. These d e f e c t s a r e deeper than those o f the f i r s t category. These two kinds o f p o i n t l i k e d e f e c t s m i a h t be due t o t e l l u - r i u m p r e c i p i t a t e s since CdTe i s arown i n l i o u i d Te. Transmission i n f r a - r e d micros- copy has shown no evidence o f such p r e c i n i t a t e s l a r g e r than 1

w,

which a r e observed when specimens are n o t quenched a f t e r t h e annealing treatment. The dark spots observed i n

EBIC

are n o t due t o such l a r g e p r e c i p i t a t e s .

3) Dark spots f o r which t h e c o n t r a s t increases when decreasina Eo have a l s o been observed (Z1 and Z2 on f i g 1 and 5). They could be 1 ine-shaped d e f e c t s ( d i s l o c a - t i o n s ) perpendicular t o t h e surface.

Some o f these d e f e c t s show a " d o t and h a l o " c o n t r a s t ( f i g 4 and 5 ) . S i m i l a r c o n t r a s t has been observed i n s i l i c o n f o r edge d i s l o c a t i o n s oaral l e l t o t h e surface [ 4 1

.

The

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JOURNAL DE PHYSIQUE

h a l o i s u s u a l l y a t t r i b u t e d t o an e l e c t r i c a l l y a c t i v e i m p u r i t y denuded zone around t h e defects. Such f l u c t u a t i o n s i n t h e EBIC c u r r e n t have probably the same o r i g i n , even i n t h e absence o f v i s i b l e d e f e c t s ( f i g 1 and 4).

4 ) A few l i n e shaped defects which l o o k l i k e d i s l o c a t i o n s i n c l i n e d t o the surface have been observed ( f i g 1 and 5 : A 1 and A2). The c o n t r a s t o f A 1 i s a maximum a t Eo = 30 keV ( c = 4,5 %) w h i l e t h a t o f A2 i s a maximum a t 20 keV ( c = 3 % ) .

CONCLUSIONS

I t has been shown t h a t q u a n t i t a t i v e measurements o f the EBIC c o n t r a s t as as f u n c t i o n o f the beam v o l t a g e can assess t h e s t r u c t u r e and the depth o f the defects, l o c a t e d a t a few microns below the surface. This i n d i c a t e s t h a t Donolato's model i s a p p l i - cable t o the case o f CdTe. A c r y s t a l l o g r a p h i c c h a r a c t e r i z a t i o n o f the d e f e c t s i s i n progress. I t w i l l a l l o w t o b e t t e r c o r r e l a t e EBIC images and s t r u c t u r a l defects.

Acknowledgments :

The authors a r e g r a t e f u l t o R. T r i b o u l e t f o r p r o v i d i n g CdTe c r y s t a l s , f o r h i s h e l p and f o r valuable discussions, and t o D. I m h o f f and J. Deschamps f o r t h e i r t e c h n i c a l he1 p .

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F i g u r e 1 : EBIC image o f t h e s t u d i e d area a t v a r i o u s a c c e l e r a t i n g voltages a) 25 keV

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b ) 20 keV

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c) 15 keV

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d) 10 keV.

The s e l e c t e d d e f e c t s are a l s o shown.

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C4-302 JOURNAL DE PHYSIQUE

F i g u r e 2 : V a r i a t i o n o f c o n t r a s t ( a ) and F i a u r e 3 : V a r i a t i o n o f c o n t r a s t ( a ) and r e s o l u t i o n ( b ) as a f u n c t i o n r e s o l u t i o n ( b ) , as a f u n c t i o n o f t h e beam voltage, o f o f the beam voltage, o f

d e f e c t X 2 . d e f e c t X '3.

F i g u r e 4a : E B I C image a t 25 keV and 1 5 keV. X4 e x h i b i t s d o t and halo c o n t r a s t a t 15 keV. The curved arrow shows a s p o t appearing a t 20 keV and e x h i b i t i n g a l s o t h e d o t and h a l o c o n t r a s t .

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F i g u r e 4b : c o n t r a s t behaviour o f X 4.

--

F i g u r e 4c : r e s o l u t i o n behaviour o f X 4.

I5 kev

Figure 5a : E B I C images o f d e f e c t s Z1, 22, A1 and A2 a t 25 keV and 15 keV. 22 e x h i b i t s d o t and h a l o c o n t r a s t a t 15 keV. A1 i s v i s - i b l e a t 25 keV b u t n o t a t 15 keV. A2 has a

c o n t r a s t which increases from 25 keV t o 5 10 15 !%!k!?$

15 keV.

F i g u r e 5b. : c o n t r a s t behaviour o f d e f e c t Z2. 0

u

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C4-304 JOURNAL DE PHYSIQUE

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