FIM AND ATOM-PROBE STUDIES OF Ni-GaAs INTERFACE

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FIM AND ATOM-PROBE STUDIES OF Ni-GaAs INTERFACE

M. Wada, H. Kita, O. Nishikawa

To cite this version:

M. Wada, H. Kita, O. Nishikawa. FIM AND ATOM-PROBE STUDIES OF Ni-GaAs INTERFACE.

Journal de Physique Colloques, 1984, 45 (C9), pp.C9-471-C9-476. �10.1051/jphyscol:1984978�. �jpa-

00224467�

FIM AND ATOM-PROBE STUDIES OF Ni-GaAs INTERFACE

M. vada, H. Kita and 0. Nishikawa

Department of Materiazs Science and Engineering, The Graduate SchooZ a t Nagatsuta, Tokyo I n s t i t u t e of TechnoZogy, 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan

Résumé - Une fine couche de Ni a été déposée sur un substrat dtAsGa et la formation d'un interface Ni/GaAs a été examinée en microscopie ionique de champ et par sonde à atomes.

Abstract - A thin layer of Ni was deposited on a GaAs substrate and the formation of Ni/GaAs interface layer was examined by the field ion microscope and the atom-probe.

Metal-semiconductor interface is a subject of considerable 5nterest and the structure and properties of the interfaces havebeenextensive- ly studied /1/. Ogawa /2/ has shown that Ni is highly reactive with GaAs and' forms compounds with Ga and As. In his study. chemical com- positions of the alloy layer formed by heating below 500°C were ex- amined by thé AES and the) structure of the layer was identified by the electron diiffraction (RHEED and TED) patterns. It has been demon-

formed on th& substrate was examined by field ion image and the compos~itiondl variation of the formed layer was analyzed by the AP.

1 1

Square rods 'of GaAs of a size 0.4 x 0.4 x 0.7 mm3 were sharpened in a solution of H2S04(l) and H202(1) and used for FIM and AP studies. The field ion image was observed with - ⁵ x 10-3 Pa of H2 gas at a tip temperature of -20 K. Ni was deposited on the field-evaporated GaAs tip surface while keeping the specimen temperature near 20 K. The backgro nd pressure during the Ni deposition was normally - 1 0 - ~ ~ a .

h

After 't e deposition, the specimen was heated at 500°C or at 600°C.

The dpec$men was again cooled down to near 20 K and the FIM image qf the heatkd specimen was observed. The compositional variation of /e Ni/GaAs interface was examined by the time-of-flight AP /6/. The tip temherature was about 200K which was considered to be an optimum tempeirature for the AP1analysis of the present GaAs specimens.

Fig. l(a) is an image of the [O011 -0riented GaAs substrate. Fig. l(b) is 4 e limage of as-deposited Ni layer on the substrate. Bright spots are]dnomly distributed and no ring pattern is neen. apd this is very dif $rent fr m the hydrogen image of a Ni crystal. When thisapparent- ly don-oryst2lline layer was field evaporated. a substrate image quite similaq ~to tAat in Fig. l(a) appeared and there was no indication of a presence O-f an interface layer between the Ni layer and the substrate.

Fig. l(c) is the image obtained after the Ni-deposited tip was heated for 3 s at 600°C. By this heating, a new phase seems to have appeared in the central region of Fig. l(c). The boundaries between this phase

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984978

C9-472 JOURNAL DE PHYSIQUE

F i g . 1 - (a);FIM image of t h e [001]- o r i e n t e d GaAs s u b s t r a t e a t 3.6 kV. The

(111) and ( ï I 1 > p l a n e s a r e e q u v a l e n t . The o t h e r s e t of (111) and (111) p l a n e s a r e n o t recognized, i n d i c a t i n g d i f f e r e n t evap- o r a t i o n behavior of t h e (111) and (711) t y p e s of p l a n e s . ( b ) ; A f t e r N i d e p o s i t i o n a t t i p temperature n e a r 20 K . No i n d i c a - t i o n o f c r y s t a l l i n e s t r u c t u r e i s seen.

Imaged a t 4.5 kV. (c);The Ni-deposited t i p was heated f o r 3 s a t 600°C. Ring p a t t e r n s due t o low-index p l a n e s a r e seen on t h e s i r e s corresponding t o t h e (OOl),

( I l l ) , (011) and (101) s u b s t r a t e p l a n e s . Imaged a t 4 . 5 kV.

and t h e s u b s t r a t e a r e c l e a r . The r e g i o n 1s s t a b l e under a h i g h f i e l d and t h e image becomes b l u r r e d a t a h i g h e r v o l t a g e , i n d i c a t i n g a pro- t r u s i o n of t h i s r e g i o n from t h e o r i g i n a l GaAs s u r f a c e . F i g . l ( c ) c l e a r l y shows t h e p r e s e n c e o f r i n g p a t t e r n s o r i g i n a t i n g from low-index p l a n e s . M i l l e r i n d i c e s i n t h e f i g u r e i s t h o s e f o r t h e GaAs s u b s t r a t e . I t i s n o t i c e d t h a t t h e low-index p l a n e s o f t h i s r e g i o n c o i n c i d e w i t h t h o s e o f t h e s u b s t r a t e ; l a r g e r i n g s due t o low-index p l a n e s o f t h e new phase a r e s e e n on t h e (001) s u b s t r a t e p l a n e and a l s o t h e r i n g s a r e n o t i c e d on p o s i t i o n s c o r r e s p o n d i n g t o t h e (011) and (101) s u b s t r a t e p l a n e s , a l t h o u g h t h e s e s u b s t r a t e p l a n e s t h e m s e l v e s do n o t show t h e r i n g p a t t e r n s a s shown i n F i g . l ( a ) . An a t o m i c p l a n e i s a l s o recog- n i z e d on t h e (111) s u b s t r a t e p l a n e . F i g . 1 seems t o i n d i c a t e t h a t t h e Ni-contained r e g i o n formed by h e a t i n g a t 6 0 0 ° C i s c o h e r e n t w i t h t h e s u b s t r a t e . The c o i n c i d e n c e of t h e s i t e s o f low-index p l a n e s o f t h i s r e g i o n w i t h t h e ( O O l ) , (101) and (111) GaAs s u b s t r a t e i n F i g . l ( c ) s u g g e s t s t h a t t h e s t r u c t u r e of t h e r e g i o n i s a l s o c u b i c , p o s s i b l y w i t h t h e same l a t t i c e c o n s t a n t a s t h a t of t h e s u b s t r a t e .

A t lower t e m p e r a t u r e s , i t was d i f f i c u l t t o o b s e r v e s t a b l e and l a r g e

( c ) ; N i - d e p o s i t e d t i p was h e a t e d f o r 600 s a t 500°C. Imaged a t 7 . 1 kV.

Ni-contained r e g i o n s . F i g . 2 i s t h e r e s u l t o b t a i n e d a f t e r 500°C h e a t - i n g . F i g . 2 ( a ) i s t h e [ O l l l - o r i e n t e d s u b s t r a t e image and 2 ( b ) i s t h e imageof t h e N i - d e p o s i t e d t i p h e a t e d f o r 30 s a t 500°C. Small i s l a n d s

o f a b o u t 2 t o 10 nm i n d i a m e t e r a r e o b s e r v e d . S i n c e t h e y a r e n o t s t a b l e a s t h o s e l a r g e r o n e s , c o m p o s i t i o n a l a n a l y s i s o f t h e s e s m a l l i s l a n d s by t h e AP was n o t p o s s i b l e . When t h e amount of N i d e p o s i t i o n was s u f f i c i e n t l y l a r g e and t h e h e a t i n g p e r i o d was i n c r e a s e d , l a r g e

i s l a n d s s u c h a s t h a t shown i n F i g . 2 ( c ) was sometimes o b s e r v e d . The r i n g p a t t e r n on t h e image i s n o t c l e a r .

Compositions of t h e Ni-contained r e g i o n s formed by h e a t i n g a t 500°C

and 600°C a r e a n a l y z e d by t h e AP. F i g . 3 i s t h e r e s u l t f o r a specimen

h e a t e d a t 500°C f o r 60 S . The a b s c i s s a i s a c u m u l a t i v e number of t h e

d e t e c t e d A s and Ga atoms and t h e o r d i n a t e i s a c u m u l a t i v e number of

t h e d e t e c t e d N i atoms. When one N i atom i s d e t e c t e d , t h e l i n e i s

drawn upwards by a u n i t l e n g t h and f o r a d e t e c t i o n o f an A s o r a Ga

atom, t h e l i n e i s t o t h e r i g h t . I n t h e f i g u r e , d e t e c t i o n of atoms

s t a r t s £romL t h e o r i g i n and p r o c e e d s t o a , b , ..., and t o g beyond

which t h e GaAs s u b s t r a t e a p p e a r s . From t h e o r i g i n t o t h e d e p t h a t f ,

C9-474 JOURNAL DE PHYSIQUE

F i g . 3 - Atom-probe a n a l y s i s o f Ni/GaAs i n t e r f a c e h e a t e d f o r 60 s a t 500°C. From t h e o r i g i n t o f , o n l y two A s atoms a r e d e t e c t e d , one i n b t o c and t h e o t h e r i n d t o e . Beyond f , Ga and A s a r e mixed .

Ni on GaAs Heated at 500°C for 60s.

20k,, ^{O 0 20 40 60}

⁸⁰ (1

Number of A s t G a atoms

t h e d e t e c t e d atoms a r e m o s t l y N i and some Ga atoms, and t h e r e a r e o n l y two A s atoms d e t e c t e d , one between b and c and one between d and e . From f t o g , Ga and A s a r e mixed i n N i b a s e . The f i g u r e i n d i c a t e s t h a t by t h i s h e a t i n g Ga atoms have d i f f u s e d o u t £rom t h e s u b s t r a t e i n t o t h e N i l a y e r . There i s a n i n d i c a t i o n o f a Ga r i c h r e g i o n i n t h e outmost s u r f a c e l a y e r , £rom t h e o r i g i n t o a , i n which no A s i s d e t e c t - e d . One i n t e r e s t i n g o b s e r v a t i o n i s t h a t i n t h e N i r i c h l a y e r between p o s i t i o n s a and f , Ga atoms a r e n o t randomly d i s t r i b u t e d i n t h e N i m a t r i x . I n t h e f i g u r e , r e g i o n s b t o c and d t o e which c o n t a i n Ga a r e surrounded by N i l a y e r s . The Ga-Ni phase diagram shows a p r e s e n c e o f NijGa (Cu3Au t y p e ) s u p e r l a t t i c e i n t h e N i b a s e / 7 / . Although t h e number o f d e t e c t e d atoms i n t h e s e r e g i o n s a r e s m a l l and a c c u r a t e com- p o s i t i o n a l d e t e r m i n a t i o n i s n o t p o s s i b l e , it i s l i k e l y t h a t t h e s e r e g i o n s c o r r e s p o n d t o t h e Ni3Ga s u p e r l a t t i c e phase. The Ga composi- t i o n i n t h e s e r e g i o n s i s r e a s o n a b l y c l o s e t o t h a t o f Ni3Ga.

Compositional p r o f i l e o f N i - d e p o s i t e d GaAs h e a t e d a t 600°C f o r 60 s i s

shown i n F i g . 4 . I t shows t h a t t h e r e a r e two d i s t i n c t r e g i o n s , 1 and

I I , o f d i f f e r e n t c o m p o s i t i o n s i n t h e Ni-contained phase. Region I I I

i s t h e GaAs s u b s t r a t e . Region 1 c o n s i s t s mostly o f N i and A s , and

o n l y two Ga atoms o u t o f 8 0 t o t a l atoms a r e d e t e c t e d i n t h i s r e g i o n .

Nurnbers o f d e t e c t e d N i and A s atoms a r e 55 and 23, r e s p e c t i v e l y . A

known b u l k compound o f N i and A s w i t h c o m p o s i t i o n s s i m i l a r t o t h i s i s

Ni5As2 /7/. Compositions i n r e g i o n II i s a p p a r e n t l y d i f f e r e n t £rom

t h a t i n r e g i o n 1. Although t h e t o t a l nurnber o f d e t e c t e d atoms i n II

i s 40 and r a t h e r s m a l l , t h e a v e r a g e s l o p e i n t h i s r e g i o n seems t o

l n d i c a t e a p r e s e n c e o f N ~ ( A S , G ~ ) ~ l a y e r . The r a t i o o f t h e nurnber of

A s atoms t o t h a t o f Ga i s a b o u t 2 . The i n t e r f a c e between r e g i o n II

and t h e s u b s t r a t e i s a b r u p t and t h e r e i s no s i g n o f a t r a n s i t i o n a l

r e g i o n . The t h i c k n e s s of r e g i o n I I l a y e r i s r o u g h l y e s t i m a t e d t o b e

2 p r 3 a t o m i c l a y e r s , B e c a u s e t h e number o f atoms d e t e c t e d £rom one

60- 600°C. I n r e g i o n 1, d e t e c t e d

+ - atoms a r e m o s t l y N i and A s ,

and o n l y 2 Ga atoms a r e i n Ni on GaAs t h i s r e g i o n . I n r e g i o n I I , Heated at GO& - t h e r a t i o o f nurnber o f A s for 60 s atoms t o t h a t of Ga atoms i s

Q

P a b o u t 2.

-

z

atomic l a y e r i s i n t h e r a n g e from 1 5 t o 20 by t h e p r e s e n t atom-probe / 6 / , and t h e t o t a l d e t e c t e d atoms i n t h i s r e g i o n i s 40. T h i c k n e s s o f r e g i o n 1 would b e 4 o r 5 atomic l a y e r s . I t was i n d i c a t e d e a r l i e r by F i g . l ( c ) t h a t t h e s e r e g i o n s formed by 600°C h e a t i n g were c o h e r e n t w i t h t h e s u b s t r a t e and had a c u b i c s t r u c t u r e . However, t h e b u l k s t r u c - t u r e o f N i 5 A s 2 i s hexagonal and t h a t o f N i A s 2 , i n c a s e it i s t h e same a s t h a t o f Ni(As,Ga)2, i s orthorhombic /7/. T h e r e f o r e t h e s t r u c t u r e of t h e s e s e v e r a l a t o m i c l a y e r s seems t o b e d i f f e r e n t from t h e b u l k s t r u c t u r e .

From F i g . 4 , it a p p e a r s a s i f many A s atoms o f t h e s u b s t r a t e have d i f f u s e d i n t o N i l a y e r and formed r e g i o n s 1 and I I . However, t h e r e i s no i n d i c a t i o n o f a n A s d e p l e t e d zone i n t h e s u b s t r a t e n e a r t h e i n t e r f a c e between r e g i o n s II and I I I . Furthermore, it h a s been

i n d i c a t e d by F i g . 3 t h a t t h e i n t e r f a c e r e a c t i o n o f N i and ~ a ~ s s u b s t r a - t e a t 500°C i s preceeded by t h e o u t d i f f u s i o n of Ga f r o m - t h e s u b s t r a t e i n t o t h e N i o v e r l a y e r . I t seems n a t u r a l , t h e r e f o r e , t o c o n s i d e r t h a t Ga o u t d i f f u s i o n h a s a l s o t a k e n p l a c e d u r i n g h e a t i n g a t 600°C. F i g . 4 , however, d o e s n o t show any p r e s e n c e o f o u t d i f f u s e d Ga atoms n e a r t h e s u r f a c e . One p o s s i b l e e x p l a n a t i o n would b e t h a t by t h e 600°C h e a t i n g , Ga atoms r e a c h f a s t e r t o t h e s u r f a c e and t h e y d i f f u s e a c r o s s t h e s u r - f a c e t o a r e a s o f l a r g e r a d i u s o f c u r v a t u r e o r f u r t h e r t o t h e shank o f t h e specimen. S i n c e t h e r e g i o n a n a l y z e d by t h e A P i s l i m i t e d t o a q u i t e s m a l l a r e a on t h e t i p apex, t h e d i f f u s e d s u r f a c e Ga atoms o u t o f t h e apex a r e a can n o t b e d e t e c t e d . The predominant Ga o u t d i f f u s i o n t h r o u g h t h e m e t a l l a y e r t o t h e s u r f a c e h a s been r e p o r t e d f o r Au-GaAs /8,9,10/, Pt-GaAs /Il/ and Al-GaAs/l2/ systems. The absence o f a mix- ed l a y e r o f N i and GaAs and t h e p r e s e n c e o f t h e Ni-contained r e g i o n I I i n which Ga c o n c e n t r a t i o n i s a b o u t one h a l f o f A s c o n c e n t r a t i o n , seem t o i n d i c a t e t h a t N i d i f f u s i o n towards t h e s u b s t r a t e i s l i m i f e d o n l y t o a low Ga c o n c e n t r a t i o n r e g i o n from which Ga atoms have d i f f u s e d o u t . Thus t h e r e a c t i o n o f Ni/GaAs i n t e r f a c e a t 600°C i n v o l v e s 1) a r a p i d o u t d i f f u s i o n o f Ga atoms t h r o u g h N i l a y e r , 2) N i d i f f u s i o n i n t o t h e Ga d e p l e t e d zone and 3 ) a f o r m a t i o n o f a s t a b l e Ni5As2 l a y e r .

Ogawa /2/ d e p o s i t e d a 110 nm t h i c k N i l a y e r and o b s e r v e d t h e f o r m a t i o n o f an NiAs-rich r e g i o n n e a r t h e GaAs s u b s t r a t e and a 8-NiGa-rich

r e g i o n n e a r t h e s u r f a c e by h e a t i n g a t 500°C f o r 5 min. The r e s u l t i s

q u i t e d i f f e r e n t £rom t h e p r e s e n t one i n which t h e t h i c k n e s s of t h e

Cg-476 JOURNAL DE PHYSIQUE

deposited Ni film is less than 10 atomic layers. This may be partly due to the rapid outdiffusion of Ga atoms through a thin Ni layer to the surface in the present experiment.

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