ATOM-PROBE STUDY OF Al-GaAs INTERFACES

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ATOM-PROBE STUDY OF Al-GaAs INTERFACES

O. Nishikawa, O. Kaneda, M. Shibata, E. Nomura

To cite this version:

O. Nishikawa, O. Kaneda, M. Shibata, E. Nomura. ATOM-PROBE STUDY OF Al-GaAs INTER- FACES. Journal de Physique Colloques, 1984, 45 (C9), pp.C9-459-C9-464. �10.1051/jphyscol:1984976�.

�jpa-00224465�

ATOM-PROBE STUDY

OF

Al-GaAs INTERFACES

O . Nishikawa, O . Kaneda, M . Shibata and E . Nomura

Department of Materials Science and Engineering, The Graduate School a t Nagatsuta, Tokyo I n s t i t u t e of Technoloa~, 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan

Résumé

-

La réaction d'échange Al-Ga à l'interface Al-GaAs a été étudiée en déposant une couche de Al sur des pointes de GaAs dopées au Zn et au Si dans la chambre de la sonde à atomes.

Tandis que le GaAs dopé au Si montre un interface Al-GaAs atomi- quement abrupt à 200 K I le GaAs dopé au Zn forme une couche compos6e Al-As. La température de seuil plus basse pour la réac- tion d'échange Al-Ga pourrait être attribuée à une réaction facilitée par Zn. Lorsque la température augmente, la concentra- tion en As dans la couche mélangée augmente et l'épaisseur de la couche de Al n'ayant pas réagi diminue. A 900 IZ, la couche composée avec la stoechiométrie de AlAs et un interface atomi- quement abrupt avec le substrat GaAs se forme.

Abstract - The Al-Ga exchange reaction at the Al-GaAs interface was studied by depositing an Al layer on Zn-doped and Si-doped GaAs tips in the atom-probe chamber. While Si-doped GaAs exhibited an atomically abrupt Al-GaAs interface at 200 K, Zn- doped GaAs formed an Al-As mixed layer. The observed lower threshold temperature for the Al-Ga exchange reaction could be attributed to the promoted reaction by Zn. As temperature rose, the As concentration in the mixed layer increased and the thickness of an unreacted Al layer decreased. At 900 K the mixed layer with the stoichiometry of AlAs and an atomically abrupt interface with the substrate GaAs was forme<.

1 - INTRODUCTION

It is known that a heterojunction such as AlAs-GaAs is fairly stable and requires a high temperature(-900°C) to cause appreciable Al-Ga interdiffusion. However, Laidig et al. found that Zn diffused into an AlAs-GaAs superlattice at 500-600°C promotes the interdiffusion[l].

We realized that the atom-probe make it possible to analyze the compositional variation at the interfaces in an atomic dimension and the effect of dopants on the interdiffusion would be clarifiedby mass analyzing interfacial composition after heating the junction at various temperatures. In this study an Al layer was deposited on Zn- doped and Si-doped GaAs and then the temperature effect on the inter- diffusion was examined by varying the specimen temperature £rom below 150 K to above 950 K.

II

-

EXPERIMENTAL

The high mass resolution atom-prober21 was used. Wafers of 0.3 mm-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984976

C9-460 JOURNAL DE PHYSIQUE

thick Zn-doped and Si-doped GaAs ( p = 2 x 10' cm-3 and n = 2 x 10' cm-3, respectively) were cut into a square rod of 0.3mmx 0 . 3 m m x 1 0 m m and one end of the rod was chemically etched in a solution of H2S0, : H202

= 2 : 1 at 60°C until a needle shaped specimen with a tip radius of less than 1000fi was obtained. The other end was inserted into a Pt tube which was spot welded on a Mo loop. The specimen can be cooled down to 20 K by a cryogenic refrigerator and heated by resistively heating the loop. The temperature was measured by a chromel-alumel thermocouple which was inserted into the Pt tube and clamped together with the specimen. The number of atoms detected from one atomic layer was approximately 10 to 20.

III

-

RESULTS AND DISCUSSION

Al deposited on a Zn-doped [Olll-oriented GaAs and the secceeding mass analysis was proceeded at 150 K. In order to visialize the variation of the chemical composition with depth, detected ions were plotted by taking the abscissa as the total number of detected ions and the ordinate as the numbers of Al, Ga, and As ions in the order of detec- tion, Ffg. 1. The first 16 atoms are Al and then 6 As atoms detected, indicating that the Al coverage on GaAs is approximately a monolayer.

Note that the Al-GaAs interface indicated by the vertical line in the figure is clear and no mixed layer, which could be noticed by the disordered detection of Al, Ga and As, is seen.

The slope of the plotted lines represents the local concentration of each element. Thus the slopes of Ga and As should be parallel for GaAs. However, the slope of As is steeper than that of Ga, indicating a deficiency of a large number of Ga atoms. The cause of the errone- ous composition of GaAs is not known at present[3].

A mixed layer of Al, As and Ga was found when the temperature w$s raised to 200 K I Fig. 2. After the detection of 8 Al atoms, atoms and one Ga atom were detected with 8 Al atoms. The

50

I I I 1 1 1 1 I I

Al on GaAs

As

V>

- Detected a t 150K

k Background pressure

:

t

6 x IO-8Pa -

...

Ga ...

....

...

...

... ^Al

.

. . .

#-

O

50

100

Total number of ions

(

AI+ A s t G a 1

Fig. 1 - The cumulative number of Al, Ga and As atoms versus the number of each element detected by mass analyzing Al deposited Zn- doped [Olll-oriented GaAs at 150 K.

contrary the Si-doped [001]-oriented tip does not form a mixed layer at 200 K , Fig. 3. In this case also, the atoms detected immediately after the last Al atom were As atoms. The formation of the mixed

60

-

^{Al on Ga As}

Kept at 200 K

Background pressure : 6 X 10-'pa

O 40

-

*i

O

-

9 8

O 20 4 0 60 80 100 120

Total number of atoms ( LU + G a + As i

Fig. 2 - The cumulative number of Al, Ga and As atoms versus the number of each element detected by mass analyzing Al deposited Zn- doped [Olll-oriented GaAs at 200 K.

Ai on GaAs As

20

-

^{Kept at 200 K}

+ O Background pressure : 8 x 1 O-' Pa

O w- O L a2

O

f i o - z

/. i

0 ; 1 1 I 1

-

O 10 20 30 40 50

Total number of otoms ( AI+ Ga

+

As

Fig. 3

-

The cumulative number of Al, Ga and As atoms versus the number of each element detected by mass analyzing A l deposited Si- doped [OOlI-oriented GaAs at 200 K.

C9-462

JOURNAL DE PHYSIQUE

layer at 200 K for the Zn-doped GaAs could be attributed to the promoted diffusion of Ga and Al by Zn. However, no Zn atoms were detected because of the low concentration and a small number of detected atoms.

Al on Ga As

Heated at 4 0 0 K for 100 sec

Ë

Background pressure

:

6 X 1

o - ~

Pa

O 5 0

-

+ O

<C

-

O L al

n

-

^-

&

,

3 /.

Z

-

^-

, '

-

0 ;

,

^I I I I L 1 1

-

O 5 0 100 150

Total nurnber o f atorns ( AI t Ga t A s )

Fig. 4 - The total number of Al, Ga and As atoms detected versus the number of each element detected from Al deposited Si-doped [001]- oriented GaAs. The specimen was heated at 400 K for 100 sec after Al

deposition and then mass analyzed at 200 K.

Total number of atoms f AI+Ga+As)

Fig. 5 - The total number of atoms versus the number of each element detected from Al deposited Zn-doped [Olll-oriented GaAs. The specimen was heated at 500 K for 20 sec and analyzed at 200 K.

A s and 6 Ga a t o m s a r e mixed f o r m i n g a n i n t e r f a c i a l monoatomic l a y e r b e t w e e n t h e u n r e a c t e d A l l a y e r a n d t h e s u b s t r a t e GaAs, F i g . 4. A g a i n A s a t o m s w e r e d e t e c t e d r i g h t a f t e r t h e d e t e c t i o n o f t h e l a s t A l atom.

T h i s may s u g g e s t t h a t A l r e p l a c e s t h e Ga a t o m s o f t h e f i r s t l a y e r o f GaAs a t t h e i n t e r f a c e a n d d i r e c t l y i n t e r a c t w i t h A s o f t h a t l a y e r and t h a t t h e r e p l a c e m e n t o f Ga a t o m s o f t h e 2nd l a y e r [ 4 ] i s u n l i k e l y . A t h i c k mixed l a y e r f o r m e d a t 500 K w i t h a l o w A s c o n c e n t r a t i o n c o n t a i n e d n o Ga atom. A g a i n t h e i n t e r f a c e o f t h e mixed l a y e r a n d t h e s u b s t r a t e was a t o m i c a l l y a b r u p t , F i g . 5. When t h e S i - d o p e d [ 0 0 1 ] - o r i e n t e d s p e c i m e n was h e a t e d a t 900 K , a l 1 A l a t o m s w e r e bound w i t h A s a t o m s f o r m i n g t h e mixed l a y e r w i t h t h e s t o i c h i o m e t r y o f A l A s w h i c h a l s o e x h i b i t e d a n a b r u p t i n t e r f a c e w i t h t h e s u b s t r a t e , F i g . 6 .

F i g . 6

-

The t o t a l number o f a t o m s v e r s u s t h e numbers o f A l , Ga a n d A s a t o m s d e t e c t e d f r o m A l d e p o s i t e d S i - d o p e d f 0 0 1 1 - o r i e n t e d GaAs.

The s p e c i m e n was h e a t e d a t 900 K f o r 70 s e c and mass a n a l y z e d a t 200 K .

The a b r u p t i n t e r f a c e c o u l d b e a r e s u l t o f t h e s t r o n g Al-As b i n d i n g . The A l a t o m s , w h i c h a s s u m e t h e Ga s i t e s by t h e Al-Ga e x c h a n g e r e a c t i o n , a r e s o s t a b l y bound w i t h A s a t o m s t h a t t h e y d o n ' t d i f f u s e f u r t h e r . Thus t h e A l - A s mixed l a y e r i s grown by unbound f r e e A l a t o m s w h i c h c a n d i f f u s e f u r t h e r i n t o GaAs. They a r e s t a b i l i z e d by t h e e x c h a n g e r e a c t i o n a s s o o n a s t h e y e n c o u n t e r u n r e a c t e d Ga a t o m s . A f t e r h e a t i n g a b o v e 500 K , Ga a t o m s w e r e d e t e c t e d n e i t h e r o n t h e s u r f a c e n o r i n t h e mixed l a y e r . T h i s rnay i n d i c a t e t h a t t h e - d i f f u s i v i t y o f Ga i n t h e mixed l a y e r i s s i g n i f i c a n t l y h i g h e r t h a n t h a t - o f A l a b o v e 500 K a n d t h e Ga a t o m s r e l e a s e d by t h e Al-Ga

e x c h a n g e r e a c t i o n d i f f u s e t h r o u g h t h e mixed l a y e r . Then t h e Ga a t o m s come t o t h e s u r f a c e and m i g r a t e f u r t h e r t o t h e t i p s h a n k .

JOURNAL DE PHYSIQUE

IV

-

CONCLUSION

The present study successfully demonstrated that the atom-probe has the unique capability of clarifying the interfacial composition in an atomic dimension. The following conclusions were obtained:

1. Zn doped in GaAs promotes the Al-Ga exchange reaction at the Al- GaAs interface.

2. The As concentration in the Al-As mixed layer increased with temperature and the layer with the stoichiometric composition of AlAs was formed at 900 K.

3. The Al-GaAs and AlAs-GaAs interfaces are atomically abrupt.

REFERENCES

[l] Laidig, W. D., Holinyak, Jr., N., Camras, M. D., Hees, K., Coleman, J. J., Dakus, P. D. and Bardeen, J., Appl. Phys. Lett.

2

(1981) 776.

[2] Nishikawa, O., Kurihara, K., Nachi, M., Konishi, M. and Wada, M., Rev. Sci. Instrum. 52 (1981) 810.

131 Nishikawa, O., ~ a w a d a , H., Nagai, Y. and Nomura, E., Proc. 3lSt Intern. Field Emission Symposium (Paris, 1984).

[41 Kahn, A., Carelli, J., Kanani, D., Duke, C. B., Paton, A. and Brillson, L., J. Vac. Sci. Technol.

2

(1981) 331.