AN ATOM-PROBE STUDY OF III-V COMPOUND SEMICONDUCTORS

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AN ATOM-PROBE STUDY OF III-V COMPOUND SEMICONDUCTORS

T. Sakurai, T. Hashizume, A. Jimbo, T. Sakata

To cite this version:

T. Sakurai, T. Hashizume, A. Jimbo, T. Sakata. AN ATOM-PROBE STUDY OF III-V COM- POUND SEMICONDUCTORS. Journal de Physique Colloques, 1984, 45 (C9), pp.C9-453-C9-458.

�10.1051/jphyscol:1984975�. �jpa-00224464�

AN

The I n s t i t u t e f o r SoZid S t a t e Physics, The U n i v e r s i t y o f Tokyo, Roppongi Minato-ku, Tokyo 106, Japan

Résumé - A l'aide d'une sonde atomique focalisée à temps de vol, nous avons analysé des pointes de GaAs préparées à partir de "wafers" cornercialement disponibles. Il a été démontré que (1) la composition dépend fortement des conditions expérimentales : pression partielle d'hydrogène, plan emetteur.

(2) Sous un bon vide (5 x 10-1 I~orr), la concentration de Ga (Ga/Ga +As) n'est que de 43% dans le plan (II 1) et (3) sous une pression de 3 x 1 0 - ~ ~ o r r d1H2 une concentration idéale peut être obtenue en utilisant un pulse de rapport 0,13. Nous pensons que la formation d'amas libres due à l'évapora- tion préférentielle de As est responsable de l'appauvrissement en Ga observé.

L'interface GaAs-Ti a également été étudiée. Nous trouvons que (1) Ti diffuse partiellement dans le substrat même à basse température (50 K ) . (2) A 1150 K et au dessus, Ti paraît former avec GaAs un composé intermétallique dont la composition est Ga :As :Ti = 1 :1 :1 possédant une interface fine avec le substrat.

Abstract - GaAs tips prepared £rom commercially available wafers were ana- lyzed using Our focusing-type time-of-flight atom-probe. It was found that (1) The composition depends strongly on the experimental conditions, such as H2 partial pressure and netplane. (2) Even in good vacuum ( 5 x 10-''~orr) Ga concentration (Ga/Ga +As) is only 43% at the (111) plane and (3) Ideal concentration can be obtained using pulse ratio of 0.13 in the presence of 3 x l ~ - ~ ~ o r r Hz. We believe that vacancy cluster formation due to the pre- ferential evaporation of As is responsible to the observed Ga depletion.

GaAs-Ti interface was also studied. Our findings are (1) Ti partially diffu- ses into the substrate even at cryogenic temperature 50 K. and (2) at and above 1130 K Ti appears to form an intermetallic compound with GaAs, whose composition is Gg :As : Ti = 1 :1 :l with a sharp interface with the under- lying bulk.

Imaging semiconductor surfaces by FIM is more difficult compared with metal surfaces.

This is even more so in the case of atom-probe study, where field evaporation must be performed in a controlled fashion. Field evaporation of semiconductors can be very different from that of metal surfaces primarily because of two reasons: (1) Electric field does not terminate right at the surface. Thus atoms at the second or third layer can in principle be field evaporated as well as the atoms at the first layer. and (2) The tecrahedral structure of semiconductors may result in higher anisotropy in bonding at the surface. Because of those features the semiconductor surface atoms tend to field evaporate sporadically in a cluster form (l)-possessing a large energy deficit. In the case of III-V compound semiconductors this diffi- culty of achieving orderly evaporation is further compounded by preferential evapo- ration of one of the elements.

Nevertherless, we have since 1976 been working on semiconductor atom-probe projects because of @yat interest in microanalysis of semiconductor surfaces and its metal interfaces. Here we report our progress on ToF atom-probe study of GaAs and its Ti interfaEes. Even though orderly field evaporation is difficult to achieve it is possible to do by introducing H2 gas in the system and raising the emitter temperature as is shown in Fig. 1. This figure shows progressive layer-by-layer Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984975

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e v a p o r a t i o n i n a s l i g h t l y d i f f e r e n t way compared w i t h t h e c a s e of m e t a l e m i t t e r s . The f i r s t l a y e r Ga atoms and t h e second l a y e r A s atoms a r e d e t e c t e d a l m o s t simul- t a n e o u s l y . F i e l d e v a p o r a t i o n of d o u b l e - l a y e r , p o i n t e d o u t f i r s t by Melmed s e v e r a l y e a r s ago, i s t a k i n g p l a c e due t o t h e f a c t t h a t t h e second l a y e r atom i s bonded t o PP t h e s u b s t r a t e by o n l y one bond i n s t e a d of t h r e e bonds l i k e t h e o u t e r m o s t l a y e r and t h u s i t c a n f i e l d e v a p o r a t e a t t h e f i e l d much lower t h a n t h a t f o r t h e f i e l d evapora- t i o n o f t h e f i r s t l a y e r atomL3) Because of t h i s , s p o r a d i c a l e v a p o r a t i o n c a n t a k e p l a c e even i n s i d e t h e n e t p l a n e , n o t o n l y a t t h e k i n k s i t e s , p o s s i b l y forming vacancy c l u s t e r s i n t h e p r o c e s s o f e v a p o r a t i o n . F i g . 1 a l s o shows t h a t l e s s Ga atoms a r e d e t e c t e d t h a n A s atoms a t t h e (111) p l a n e where Ga s i t s a t t h e s u r f a c e and t h u s one may e x p e c t more G a atoms t o b e d e t e c t e d .

We have found t h a t t h e composition we o b t a i n e d depends s t r o n g l y on t h e e x p e r i m e n t a l c o n d i t i o n s . Two most i m p o r t a n t p a r a m e t e r s a f f e c t i n g i t seem t o b e t h e p a r t i a l p r e s s u r e of hydrogen and p u l s e r a t i o (V IVtip). Our r e s u l t s a r e sumrnarized i n F i g . 2, where t h e d e t e c t e d G a c o n c e n t r a t i o n !s p l o t t e d a a i n s t p u l s e r a t i o f o r v a r i o u s e x p e r i m e n t a l c o n d i t i o n s . For i n s t a n c e , when 3 x10 T o r r Hz i s p r e s e n t , Ga c o n c e n t r a -

-9

t i o n a t t h e (111) p l a n e v a r i e s widelyfrom 70% a t p u l s e r a t i o of 0 . 1 t o 35% a t 0.25.

The e f f e c t o f H2 c a n b e s e e n i n t h e (111) A s t o p s u r f a c e b u t a t a l e s s e r d e g r e e . Not o n l y t h e composition b u t a l s o t h e mass h i s t o g r a m depend s o much on Hz p r e s s u r e . F i g . 3 compares t h e mass h i s t o g r a m s ( a ) and (b) i n t h e c a s e of f i e l d e v a p o r a t i o n a t GaAs(ll1) p l a n e u s i n p u l s e r a t i o o f 0.08 w i t h and w i t h o u t hydrogen. I n vacuum

(H21ess t h a n 5 x 10-81Torr) A s i s d e t e c t e d mainly a s doubly and s i n g l y charged a t o m i c i o n s and some i n dimer As2+ and t r i m e r

AS^+

AS^+,

These changes i n t h e h i s t o g r a m due t o hydrogen c a n b e e x p l a i n e d a s f o l l o w s : (1) Hz r e d u c e s t h e e v a p o r a t i o n f i e l d g r e a t l y (sometime a s much a s by h a l f ) and t h u s t h e f o r m a t i o n of doubly-charged i o n s , which r e q u i r e s h i g h e r f i e l d , becomes m f a v o r a b l e . and ( 2 ) Hydrogen promotes o r d e r l y one-by-one e v a p o r a t i o n from t h e k i n k s i t e , t h u s e f f e c t i v e l y e l i m i n a t i n g t h e p o s s i b i l i t y of forming c l u s t e r i o n s . O r d e r l y eva- p o r a t i o n , i n p r i n c i p l e , s h o u l d l e a d t o h a v i n g t r u e composition i n atom-probe ana- l i s i s , namely Ga:As = 1:l. T h i s , however, i s n o t n e c e s s a r i l y a c h i e v e d b e c a u s e of p r e f e r e n t k a l e v a p o r a t i o n of As i n t h e form o f AS+ upon r e a c t i n g w i t h hydrogen.

T h e r e f o r e i t i s n e c e s s a r y t o f i n d o p t i m a l c o n d i t i o n s t o a c h i e v e t r u e c o m p o s i t i o n by v a r y i n g p u l s e r a t i o and H2 p r e s s u r e . F i $ . 2 s u g g e s t s t h a t t h e p u l s e r a t i o o f 0.13 s h o u l d b e used i n t h e p r e s e n c e o f 3 x 10- T o r r Hz. Another i m p o r t a n t and u s e f u l c o n c l u s i o n found i n F i g . 2 is t h a t when f i e l d e v a p o r a t i o n i s c a r r i e d o u t i n vacnum, Ga c o n c e n t r a t i o n may n o t b e t h e e x p e c t e d 50% b u t a t l e a s t i t i s c o n s t a n t , approxi- m a t e l y 43%, i n d e p e n d e n t of t h e p u l s e r a t i o . T h i s makes Our a c h i e v i n g g o a l q u i t e s i m p l e and e a s i e r s i n c e a s i m p l e c o r r e c t i o n i s a l 1 needed t o c o n v e r t t h e d a t a i n t o t r u e c o m p o s i t i o n s .

I n o r d e r t o u n d e r s t a n d t h i s seemingly i n t r i g u i n g s i t u a t i o n more c l e a r l y , we have s t u d i e d semiconductor f i e l d e v a p o r a t i o n u s i n g b o t h focusing-type and s t r a i g h t - t y p e mode s i m u l t a n e o u s l y , which i s p o s s i b l e w i t h Our new ToF atom-probe. The s t r a i g h t - t y p e mode s h o u l d i n p r i n c i p l e d e t e c t a l 1 t h e i o n s r e g a r d l e s s of t h e i r energy d e f i c i t s w h i l e t h e f o c u s i n g -mode a c c e p t s o n l y t h e i o n s whose e n e r g y d e f i c i t s a r e w i t h i n 4%

o f t h e nominal k i n e t i c e r ~ e r ~ y . ( ~ ) The comparison i s g i v e n i n Fig. 4. A l 1 t h e ex- p e r i m e n t a l c o n d i t i o n s a r e same f o r b o t h e x p e r i m e n t s e x c e p t s l i g h t change i n a p p l i e d v o l t a g e i n o r d e r t o keep e v a p o r a t i o n r a t e c o n s t a n t . A s e x p e c t e d t h e s t r a i g h t - t y p e mode(Fig. 4a) shows t h a t b o t h Ga and As i o n s have c o n s i d e r a b l e amounts o f energy d e f i c i t s . I n t h e c a s e of AS+ i o n s t h e energy d e f i c i t (dE/E) r e a c h e s a s much a s 20%

w h i l e

AS^+

AS^+

A s u r p r i s i n g f a c t i n t h i s experiment i s t h e f o c u s i n g mode d i d produce t h e composi- t i p n c l o s e r t o t h e i d e a l Ga/As = 1 r a t h e r t h a n t h e s t r a i g h t - t y p e ToF a n a l y s i s , shown

e v a p o r a t e s i m u l t a n e o u s l y . S i n c e t h e e v a p o r a t i o n f i e l d o f Ga i s s l i g h t l y h i g h e r i n Our e x p e r i m e n t a l c o n d i t i o n t h a n A s , some of Ga atoms a r e l e f t i n t h e g a s phase with- o u t b e i n g i o n i z e d upon t h e d i s e c t i o n from t h e s u r f a c e due t o vacancy c l u s t e r forma- t i o n . The h i g h e r t h e e v a p o r a t i o n f i e l d , t h e more d e p l e t i o n of G a , which i s indeed observed. T h i s s l i g h t d i f f e r e n c e i n e v a p o r a t i o n f i e l d and t h u s e v a p o r a t i o n p r o z e s s c a n b e observed i f o n e watches t h e d . c . e v a p o r a t i o n c l o s e l y a t t h e (111) and (111) p l a n e s .

A l 1 t h e s e o b s e r v a t i o n s p o i n t o u t t h a t t h e atom-probe s t u d y o f s e m i c o n d u c t o r s i s a r a t h e r d i f f i c u l t t a s k , y e t i t i s p o s s i b l e t o c a r r y o u t s u c c e s s f u l l y , w i t h extreme c a r e .

One a r e a of g r e a t i n t e r e s t f o r atom-probe a p p l i c a t i o n s i s t h e c o m p o s i t i o n a l a n a l y s i s of semiconductor-metal i n t e r f a c e s . With r a p i d advancement i n semiconductor techno- l o g y , p r e c i s e c h a r a c t e r i z a t i o n o f t h e i n t e r f a c e s on a n a t o m i c s c a l e i s becoming a l - a o s t a n e c e s s i t y , p a r t i c u l a r l y i n t h e f i e l d o f m i c r o f a b r i c a t i o n . Here we r e p o r t a p r e l i m i n a r y s t u d y o f GaAs-Ti i n t e r f a c e .

A f t e r o b t a i n i n g a n a t o m i c a l l y c l e a n GaAs s u r f a c e , T i was d e p o s i t e d on i t u s i n g a c o n v e n t i o n a l e v a p o r a t o r ( T i wrapped on a W c o i l ) . The c r y o g e n i c r e f r i g e r a t i o n u n i t was k e p t o n f o r c o o l i n g a t i p d u r i n g t h e d e p o s i t i o n . However t h e t r u e t i p tempera-

t u r e i s n o t known s i n c e t h e e v a p o r a t o r was o n l y 15mm away from t h e t i p and i n e v i t a - b l y h e a t e d t h e t i p t o some d e g r e e . Atom-probe a n a l y s i s of t h i s t i p s u r f a c e was c a r r i e d o u t a t 45 K t o r e v e a l t h a t a p p r o x i m a t e l y 50% o f T i d i f f u s e d i n t o GaAs sub- s t r a t e . T h e i r c o m p o s i t i o n at t h e mixed l a y e r v a r i e s from o n e experiment t o a n o t h e r s u g g e s t i n g t h a t a l t h o u g h i n t e r d i f f u s i o n t a k e s p l a c e a t t h i s t e m p e r a t u r e , no i n t e r - m e t a l l i c compounds were formed. Annealing o f t h e t i p a t e l e v a t e d t e m p e r a t u r e s have produced no change u n t i l a f t e r 1130 K f o r t e n s e c . At t h i s a n n e a l i n g t e m p e r a t u r e , we found t h a t T i d i f f u s e s i n t o t h e b u l k t o form a uniform mixed l a y e r wher t h e com- p o s i t i o n was found t o b e Ga: A s : T i = 1: 1: 1. Also t h e i n t e r f a c e between t h e mixed l a y e r and t h e b u l k was r a t h e r s h a r p a s i s s e e n i n F i g . 6 . No f u r t h e r changes were n o t i c e d above t h i s a n n e a l i n g t e m p e r a t u r e . It i s w o r t h n o t i n g t h a t w i t h T i p r e s e n t on t h e s u r f a c e , Ga and As a r e d e t e c t e d a t a p p r o x i m a t e l y same r a t e . T h i s work i s c u r r e n t l y i n p r o g r e s s and more d e t a i l e d r e s u l t s w i l l b e p r e s e n t e d .

+Permanent Address: C o l l e g e of A r t and S c i e n c e , Osaka P r e f e c t u r e U n i v e r s i t y , Osaka, Japan.

R e f e r e n c e s

(1) T. S a k u r a i , R. J. C u l b e r t s o n , A. J. Melmed, S u r f . S c i . 78, L221 (1978).

(2) T. S a k u r a i , E. W. M u e l l e r , R. J. C u l b e r t s o n , A. J. Melmed, Phys.Rev. L e t t s .

g ,

578 (1977).

( 3 ) A. J. Melmed, A b s t r a c t o f t h e 2 1 s t F i e l d Emission Symposium, U n i v e r s i t y Park,PA USA (1974).

(4) T. S a k u r a i , T. Hashizume, A. Jimbo, Appl. Phys. L e t t s .

44,

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300 r ~ " ~T "

/ G a + fl s

a G»Rs ( 1 1 1 ) f

£ VE = 1 1 . 0 - 1 1 . 1KV f~ F l g. ! . L a y e r - b y - l a y e r e v a - y PU.LSE RflT10 = 0 . 2 1 f" p o r a t i o n a t t h e (111) p l a n e j - R00M TEMP J y a ° 8 GaAs shows t h a t b o t h Ga o /~ r' and As a r e e v a p o r a t i n g s i - w f y m u l t a n e o u s l y . A l s o n o t e g I ^J that l e s s nuraber of Ga a r e

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F i g . 2 . D e t e c t e d Ga ^ 8 0 * ( | | T ) < I O " ' ° T o r r c o n c e n t r a t i o n ( G a / G à +

As) a s a f u n c t i o n of ^ Q O ( | | | ) 3 X I 0 ~7T o r r H2

p u i s e r a t i o under ^ KO \ _ v a r i o u s c o n d i t i o n s . .—• ^t. ^ ^ \

( I I I ) 3 X I 0 ~ ' T o r r H,

In vacuum Ga c o n c e n t - ^Z —~ ^ ^ ^ C ^ "~

r a t i o n i s a p p r o x i m a t e - ad 2 ^ \ ^ ^ o

l y 43%, c o n s t a n t i n - £ 4 0 ^ \ A O S Ë T "° o d é p e n d e n t of p u i s e ^ »=—^u~o A

r a t i o . -z.

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24 0 ^ , . H25 X I 0 - " T o r r a

•^ Fig. 3(a). Mass histogram J=j=j 2 0 2 °f field evaporation of 2: . 2+ the (111) plane in vacuum.

g AS3 n 1 . d 1 1 J

0 50 100 150 MASS T0 CHARGE RATIO (m/n)

M A S S 10 CHARGE R A T I O ( n i / n )

W

L

n LO

6

-

k

Oi.

crn W

=11 I O

2

( a ) A s2+ ' ^{GaAs( l I I )}

Probed a t 54K : 1 . 2 ~ 1 0 - ~ T o r r

i V, = 8.80-9.49KV,V, = 1.88KV S t r a ight type

Channelplate a s a d e t e c t o r PULSE K A I lO=Ll. 0 8 ROOM TEMP.

H, 3x10.' J o r r

H+

O 0 1 O0 150

Channeltron a s a d e t e c t o r c d

G 250

W +

W 0 200

F i g . 3 ( b ) . Mass h i s t o g r a m of f i e l d e v a p o r a t i o n o f t h e (111) p l a n e i n t h e p r e - s e n c e of 3 x 1 0 - ~ T o r r Hz.

24, FaAs( l l l ) ( b )

*'

Probed a t 54K : 1 . 2 x 1 0 - ~ ~ o r r VE = 8.26-8.70KV,Vp = 1.76KV Focus ing type

F i g . 4 . Comparison of s t r a i g h t - t y p e ( a ) and f o c u s i n g - t y p e ( b ) ToF atom-probe a n a l y s i s i n t h e c a s e of G a A s ( l l 1 ) s u r f a c e a t 54 K i n 1 . 2 x l 0 - ' ~ o r r Hz. A l a r g e e n e r g y d e f i c i t i s e v i d e n t i n t h e c a s e of s t r a i g h t - t y p e r e s u l t .

MASS 10 CHARGE R A T I O ( m / n )

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F i g . 5. Unexpectedly f o c u s - i n g - t y p e mode (b) y i e l d s b e t t e r Ga c o n c e n t r a t i o n t h a n s t r a i g h t - t y p e mode.

A s i g n i f i c a n t p o r t i o n of Ga atoms a r e d e s o r b i n g from t h e s u r f a c e w i t h o u t b e i n g i o n i z e d and r e s u l t i n g i n Ga d e p l e t i o n i n t h e

(111) p l a n e . GaAs(l I I )

O

PI

i-

4 54K. 1.2x10-' Torr cn . ^{'1, =} 8.80-9. A9KV

4 V? = 1.88KV

\ .

S t r a i g h t type a . 03 Channelplate

L L O

aL W

E

I L

0

t l

O NUMBE? OF Ga 8 As ATOMS

6000

f

D e p t h prof le of TL/GaAs( l l l ) Fig. 6 . T i d i f f u s e s i n t o

GaAs s u b s t r a t e forming an An'ea1ed at / G a

800

f-4 =z

i- O

< cn

\ a a .

LL O

cz W m . z 3 Z

O

i n t e r m e t a l l i c cornPound, whose c o m p o s i t i o n a p p e a r s u t o b e Ga:As:Ti= 1:l:l.

>

-

1 GaAs( l I 1 )

54K. l.2xl0" Torr VE = 8.26-8.70KV Vp = 1 .76KV F O C U S L R ~ t y p e Canneltron

I

1

O NUMBER OF T O T A L ATOMS 600 ¹

O NUMBER OF Ga g As ATGHS 1200

1

DIU IIU N Y 1 - 1-