PULSED-LASER ATOM-PROBE STUDY OF Al-GaAs INTERFACES

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PULSED-LASER ATOM-PROBE STUDY OF Al-GaAs INTERFACES

O. Nishikawa, M. Yanagisawa, M. Nagai

To cite this version:

O. Nishikawa, M. Yanagisawa, M. Nagai. PULSED-LASER ATOM-PROBE STUDY OF Al-GaAs INTERFACES. Journal de Physique Colloques, 1986, 47 (C7), pp.C7-303-C7-308.

�10.1051/jphyscol:1986752�. �jpa-00225947�

PULSED-LASER ATOM-PROBE STUDY OF Al-GaAs INTERFACES

O. NISHIKAWA,

M.

YANAGISAWA and M. NAGAI

Department o f Materials Science and Engineering, The Graduate School at Nagatsuta, Tokyo Institute o f Technology,

4259 Nagatsuta, Midori-ku, Yokohama 227, Japan

Abstract

-

The interfaces of a thin Al layer and non-doped, Zn- doped and Si-doped GaAs were mass analyzed by the pulsed-laser atom-probe. The variation of depth profile of composition across the interfaces with temperatures indicates that the diffusibility of Al and Ga across the A:-GaAs interfaces appears to be high for non-doped and Zn-doped GaAs and low for Si-doped GaAs. The depth profiles also indicate that

A s

is not mobile for the present temperature range, below 3 5 0 K , and serves as a reference marker for diffusing Ga and Al.

1 - INTRODUCTION

The voltage-pulse atom-probe(A-P) mass analysis of Al-GaAs interfaces revealed the atomically abrupt Al-GaAs interface at low temperatures and clarified the formation process of an AlAs layer by the Al-Ga exchange reaction and the effect of dopants in GaAs on the interdif- fusion of Ga and AlIl]. However, it was noticed that the A-P tended to fail to detect most Ga atoms released by the exchange reaction and many Ga atoms of GaAs and Gap, resulting in erroneous compositions for

these compound semiconductors[2,3]. Another disadvantage of the

voltage-pulse A-P is poor transmittance of a high voltage pulse through a semiconductor with low conductivity. Application of a pulsed-laser beam has beenifound to be highly effective for promoting Si field evaporation[4] and stoichiometrically expected compositions of the compound semiconductors were obtained[5,6]. The present study is the extension of the voltage-pulse A-P study of clarifying the effect of dopants on the Al-Ga exchange reaction and the diffusibility of Al and Ga across the interfaces, utilizing the unique capability of the pulsed-laser A-P.

II

- EXPERIMENTAL

The high-mass-resolution A-Pr71 and a nitrogen laser with the pulse width of

0.8

ns were used. The resistivity of non-doped GaAs is 2 . 1 ~ 105 Q - c m and the carrier concentrations of Si-doped and Zn-doped GaAs

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986752

C7-304

JOURNAL

DE PHYSIQUE

a r e n = 2 x 1 0 1 8 cm-3 and p = 8 x 1 0 1 7 cm-3, r e s p e c t i v e l y . The s p e c i m e n t i p s w e r e p r e p a r e d . b y c u t t i n g 0 . 3 m m - t h i c k w a f e r s i n t o s q u a r e r o d s w i t h d i m e n s i o n s o f 0 . 3 ~ 0 . 3 x 1 0 m m 3 a n d b y e t c h i n g o n e e n d o f t h e r o d t o a s h a r p t i p [ l ] . The o r i e n t a t i o n o f a l 1 t i p s was [ 1 1 0 ] a n d a s m a l l a r e a o n t h e h e m i s p h e r i c a l a p e x o f t h e s p e c i m e n n e e d l e was a n a l y z e d . The s p e c i m e n t e m p e r a t u r e was m a i n t a i n e d b e l o w 50 K e x c e p t f o r t h e h e a t i n g p e r i o d a f t e r t h e A l d e p o s i t i o n .

I I I - RESULTS

Non-heated S i - d o p e d GaAs e x h i b i t e d a n a t o m i c a l l y a b r u p t Al-GaAs i n t e r - f a c e a s e x p e c t e d f r o m t h e r e s u l t o f t h e v o l t a g e - p u l s e A-P a n a l y s i s [ l ] , F i g . 1. The a b s c i s s a o f F i g . 1 i s t h e t o t a l number o f d e t e c t e d i o n s and t h e o r d i n a t e i s t h e number o f A l , Ga and A s i o n s p l o t t e d r e s p e c - t i v e l y i n t h e o r d e r o f d e t e c t i o n . F i g u r e 1 i n d i c a t e s t h e d e t e c t i o n o f a n e q u a l number o f Ga and A s a t o m s , g i v i n g t h e s t o i c h i o m e t r i c composi- t i o n o f GaAs. F o r Zn-doped GaAs, t h e i n t e r d i f f u s i o n o f A l a n d Ga a c r o s s t h e i n t e r f a c e f o r m s a few a t o m i c l a y e r t h i c k Al-Ga-As mixed l a y e r , F i g . 2 . F o r non-doped GaAs, A l a n d A s a r e c l e a r l y s e p a r a t e d a t t h e i n t e r f a c e b u t some G a a t o m s d i f f u s e i n t o t h e A l l a y e r a n d t h e G a c o n c e n t r a t i o n i n t h e s u b ç t r a t e GaAs i s l o w , F i g . 3 .

I n t e r d i f f u s i o n o f A l and Ga i s more p r o n o u n c e d by h e a t i n g t h e s p e c i m e n a t 270 K f o r 1 min. F i g u r e 4 shows t h e d e p t h p r o f i l e o f c o m p o s i t i o n a c r o s s t h e i n t e r f a c e o f A l and S i - d o p e d GaAs. The A l d i f f u s i o n a c r o s s t h e i n t e r f a c e i s e x p e c t e d £rom t h e r e s u l t o f t h e v o l t a g e - p u l s e A-P a n a l y s i s , b u t Ga a t o m s a r e a l s o d e t e c t e d £rom t h e Al-mixed l a y e r . However, t h e Ga c o n c e n t r a t i o n i n t h e mixed l a y e r a n d A l - f r e e GaAs i s l o w , s h o w i n g t h e Ga d e f i c i e n c y . For t h e i n t e r f a c e s w i t h non-doped a n d Zn-doped GaAs, A l d i f f u s i o n i s more a c t i v e a n d a l 1 A l a t o m s d i f f u s e i n GaAs, F i g s . 5 a n d 6 , and Ga d e f i c i e n c y was n o t i c e d f o r non-doped GaAs, F i g . 6 .

Even a t 3 5 0 K , s o m e A l a t o m s r e m a i n e d u n r e a c t e d , F i g . 7 . The d e p t h p r o f i l e o f Zn-doped GaAs h e a t e d a t 350 K i s s i m i l a r t o t h a t o b s e r v e d a t 270 K I F i g . 8 . Non-doped GaAs f o r m e d a n AlAs l a y e r a t t h i s low t e m p e r a t u r e b u t a s e r i o u s Ga d e f i c i e n c y was a l s o n o t i c e d , F i g . 9 .

F i g . 1

-

D e t e c t i o n s e q u e n c e o f f i e l d - e v a p o r a t e d i o n s

£rom t h e A l - d e p o s i t e d S i - d o p e d GaAs. The t i p t e m p e r a t u r e was k e p t b e l o w 50 K t h r o u g h o u t t h e e x p e r i m e n t . The v e r t i c a l l i n e i n d i c a t e s t h e a b r u p t Al-GaAs i n t e r f a c e .

Al1 figures show the same depth profile for Al: the number of AS atoms detected increases fairly linearly with the total number of detected atoms after the detection of the first As atom. This suggests that As is not mobile below 350

K

and serves as the reference marker for find- ing the position of diffusing Al and Ga atoms. Then the depth profile of Figs. 5 and

6

indicate that the Al diffusion into GaAs is more active for non-doped and Zn-doped GaAs than for Si-doped GaAs.

Unlike the result of the voltage-pulse A-P analysis, no Ga deficiency was noticed below 50

K.

However, the deficiency was noticed when the specimen was heated above 270

K

and the number of Al atoms detected exceeded 130, that is, the Al layer is thicker than 10 atomic layers, assuming that the analyzing area deepens one atomic layer by the detection 1 0 - 20 atoms. Since the stoichiometric composition is obtained at low temperatures, it is not appropriate to attribute the

Total number of atoms (Al + Ga + As)

ul

E

^40.

+ o

.,-

_O

n à5

_-

G

^{20- J ' i}

Z AsJJ -

J ^{. .}

- - . , - -

$-<

,g ,..:

Ga ^.,- ^.,-<- - - - . - - . - - - . . - . . . . . . , - - -

O0 20 40 60 80

Total number of atoms (Al + Ga + As)

F i g . 2 -

The cumulative number of Al, Ga and As atoms vs that of each element detected by mass analysis of Al/

Zn-doped GaAs .

The specimen temperature was kept below 50

K.

The vertical lines indicate the Al-Ga-As rnixed layer.

Fig. 3

-

Field- evaporation sequence of Al, Ga and As ions

£rom the

Al-

deposited, non- doped GaAs.

The specimen

temperature was

kept below

50 K

throughout the

experiment.

C7-306

JOURNAL

DE

PHYSIQUE

Ga d e f i c i e n c y t o t h e p r e f e r e n t i a l f i e l d e v a p o r a t i o n o f Ga b u t i t c a n b e e x p l a i n e d a s t h e r e s u l t o f G a d i f f u s i o n t h r o u g h t h e mixed and A l l a y e r s t o t h e s u r f a c e and f u r t h e r s u r f a c e d i f f u s i o n £rom t h e a n a l y z i n g a p e x a r e a t o t h e t i p s h a n k . However, t h e e f f e c t o f a t h i c k A l l a y e r o n t h e Ga d i f f u s i o n r e m a i n s a p u z z l e .

No Ga d e f i c i e n c y was n o t i c e d f o r t h e i n t e r f a c e o f A l a n d Zn-doped GaAs.

One p e c u l i a r f i n d i n g i s t h a t t h e number o f A l a t o m s d e t e c t e d £rom t h e i n t e r f a c e w i t h Zn-doped GaAs n e v e r e x c e e d e d 40 e x c e p t f o r t h e non- h e a t e d s p e c i m e n , F i g . 2. The amount o f A l d e p o s i t i o n i s c o n t r o l l e d by t h e t e m p e r a t u r e o f t h e A l e v a p o r a t o r a n d d e p o s i t i n g t i m e . T h i s method i s r a t h e r c r u d e a n d c a u s e s a c e r t a i n r a n g e o f f l u c t u a t i o n i n t h e t h i c k - n e s s o f A l l a y e r s b u t t h e o b s e r v e d t h i c k n e s s i s t h i n n e r t h a n t h e l i m i t o f t h e f l u c t u a t i o n . The low A l c o n c e n t r a t i o n s i n t h e Al-Ga-As mixed l a y e r , F i g s . 5 a n d 8 , may s u g g e s t t h a t A l d i f f u s e s d e e p i n t o GaAs a n d t h e A l c o n c e n t r a t i o n i n t h e mixed l a y e r i s n o t h i g h e n o u g h t o a c t i v a t e t h e Ga d i f f u s i o n . E x t e n s i v e s t u d y a t t h e t e m p e r a t u r e s b e t w e e n 50 K a n d 200 K is r e q u i r e d t o c l a r i f y t h e e f f e c t o f d o p e d Zn o n t h e A l

400 800

Total number of atoms (AI + Gc

I

+ As)

F i g . 4 - D e t e c t i o n s e q u e n c e o f f i e l d - e v a p o r a t e d i o n s

£rom t h e Al- d e p o s i t e d , S i - d o p e d GaAs h e a t e d a t 270 K f o r 1 min. The Al-Ga-As mixed l a y e r is

i n d i c a t e d by t h e v e r t i c a l l i n e s .

F i g . 5 - D e t e c t i o n s e q u e n c e o f f i e l d - e v a p o r a t e d i o n s

£rom t h e A l - d e p o s i t e d , Z n - doped GaAs. A l 1 A l a t o m s were d e t e c t e d a f t e r t h e d e t e c t i o n o f t h e f i r s t As i o n , i n d i c a t i n g t h a t a l 1 A l a t o m s c r o s s e d t h e Al- GaAs i n t e r f a c e .

V

-

CONCLUSION

The p r e s e n t s t u d y s u c c e s s f u l l y d e m o n s t r a t e d t h a t t h e p u l s e d - l a s e r A-P c a n m a s s a n a l y z e h i g h l y r e s i s t i v e s e m i c o n d u c t o r s a n d c l a r i f y t h e e f f e c t o f t h e d o p a n t f o r t h e i n t e r d i f f u s i o n o f Ga a n d A l a c r o s s t h e i n t e r f a c e s .

The d e p t h p r o f i l e o f c o m p o s i t i o n i n d i c a t e s t h a t A s r e m a i n s a t t h e o r i g i n a l p o s i t i o n a n d s e r v e s a s t h e r e f e r e n c e m a r k e r f o r t h e d i f f u s i n g A l a n d Ga. T h e n , S i - d o p e d GaAs i s f o u n d t o b e m o s t r e s i s t i v e f o r t h e A l a n d Ga d i f f u s i o n a c r o s s t h e i n t e r f a c e . The Ga d i f f u s i o n t h r o u g h t h e

i n t e r f a c e s o f A l a n d non-doped GaAs i s a c t i v e e n o u g h t o f o r m a n AlAs l a y e r e v e n a t 350 K . O b s e r v e d Ga d e f i c i e n c y c a n b e e x p l a i n e d a s t h e r e s u l t o f a c t i v e Ga d i f f u s i o n t h r o u g h t h e mixed a n d A l l a y e r a n d f u r t h e r s u r f a c e d i f f u s i o n . The p r e s e n t s t u d y s u g g e s t s t h a t t h e s m a l l

Total number of atoms (Al

+

Ga + As)

F i g . 6

-

F i e l d - e v a p o r a t i o n s e q u e n c e o f A l , Ga a n d A s i o n s f r o m t h e non- d o p e d GaAs h e a t e d a t 270 K f o r 1 min. The f i g u r e i n d i c a t e s t h a t a l 1 A l a t o m s c r o s s e d t h e i n t e r - f a c e a n d d i f f u s e d i n GaAs.

F i g . 7 - D e t e c t i o n s e q u e n c e o f f i e l d - e v a p o r a t e d i o n s f r o k t h e Al- d e p o s i t e d

,

S i - d o p e d GaAs h e a t e d a t 350 K f o r 1 min. Most A l a t o m s r e m a i n o u t - s i d e o f t h e i n t e r - f a c e .

Total

I

number of atoms (Al + Ga + As 1

C7-308 J O U R N A L D E PHYSIQUE

Fig. 9 - ^Detection

sequence of field- evaporated ions

£rom the Al- deposited, non- doped GaAs heated at 350 K for 1 min.

The AlAs layer is formed at this low temperature.

80.

E

O + O Y- O

àj

4 0 III

E

Z II

number of Al atoms detected £rom the Zn-doped GaAs is the result of promoted Al diffusion by doped Zn.

REFERENCES

150

Total number of atoms (AI + Ga + As)

d

2

,.

^-

: ,..- , -,- - , r 2 . . . ..y

,- i

. ,' - (.

, - m . f

. -

,F

-

7

.. L

.-' 2

~- - 2 ---r

. I I

, -f>

8 - <

. , ' -r , - . . . ., ,r

Ga.:.' -

*As

' #

[l] Nishikawa, O., Kaneda,

O.,

Shibata, M. and Nomura,

E.,

Phys. Rev.

Lett., 53 (1984) 1252.

[2] Sakurai,

T.,

Hashizume, T., Jimbo, A. and Sakata, T.,

J.

de Phys.

Cg (1984) 453

[3] z s h i k a w a , O., Kawada,

H.,

Nagai,

Y

and Nomura, E.,

J.

de Phys.

C9 (1984) 465.

[4] E o n g l T. T., Appl. Phys. Lett., (1984) 1149.

[5] Nishikawa, O., Nomura, E., Yanagisawa,

M.

and Nagai, M., Proc.

32nd Intern. Field Emission Symp. (1985).

[6] Cerezo, A., Grovenor,

C. R.

M. and Smith, G. E.

W.,

Proc. 32nd Intern. Field Emission Symp. (1985).

[7] Nishikawa, O. et al., Rev. Sci. Instrum. (1981) 810.

Fig. 8 - Detection sequence of field- evaporated ions

£rom the Al- deposited, Zn- doped GaAs heated at 350 K for 1 min.

The number of Al

atoms detected is

exceptionally

small.