FORMATION AND PROPERTIES OF MBE GROWN AlSb-GaSb (100) INTERFACES

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FORMATION AND PROPERTIES OF MBE GROWN AlSb-GaSb (100) INTERFACES

C. Raisin, H. Tegmousse, L. Lassabatère

To cite this version:

C. Raisin, H. Tegmousse, L. Lassabatère. FORMATION AND PROPERTIES OF MBE GROWN AlSb-GaSb (100) INTERFACES. Journal de Physique Colloques, 1987, 48 (C5), pp.C5-77-C5-80.

�10.1051/jphyscol:1987512�. �jpa-00226703�

JOURNAL DE PHYSIQUE

Colloque C 5 , supplement au n 0 l l , Tome 48, novembre 1987

FORMATION AND PROPERTIES OF MBE GROWN AlSb-GaSb (100) INTERFACES

C. RAISIN, H. TEGMOUSSE and L. LASSABATERE

Universitt5 des Sciences et des Techniques du Zanguedoc, Laboratoire d l E t u d e s des Surfaces, Interfaces et Composants, CNRS-UA 04-0787, Place Eugene Bataillon, F-34060 Montpellier Cedex, France

Ce t r a v a i l p o r t e sur 1161aboration p a r j e t s mol6culaires des interfaces GaSb-AlSb e t AlSb-GaSb, e t sur l ' e t u d e pas h pas p a r d i f f r a c t i o n d'electrons rapides, par e f f e t Auger, par mesures du t r a v a i l de sortie de l a surface de depart e t de l ' i n t e r f a c e en formation. On precise ainsi l a qualite 6lectronique des surfaces, interfaces, et, m e t en evidence des diffusions d ' A l pour l ' i n t e r f a c e GaSb/AlSb e t pas de diffusion notable de Ga pour AlSb/GaSb.

I n this paper, we present results, we have obtained on GaSb-A1Sb o r AlSb-GaSb i n t e r f a c e s grown b y M BE. We f i r s t focuss on t h e surface properties obtained b y AES, R H E E D , work function topographies. Then, we describe t h e f o r m a t i o n o f t h e i n t e r f a c e and t h e evolution o f i t s properties during growth. We d e t a i l AES and work f u n c t i o n mezsuroments and shcw A1 migration a t t h e GaSb/AlSb i n t e r f a c e and no noticeable Ga diffusion a t t h e AlSb/GaSb interface.

I - INTRODUCTION.

For a few years, research on GaSb and alloys have been increasing continuously.

Type I ( GaSb-GaAlSb) o r t y p e I1 (GaSb - InAs) superlattices present interesting proper- t i e s resulting p a r t i c u l a r l y f r o m s t r a i n a t t h e i n t e r f a c e and f r o m r e l e v a n t effects.

However, t h i s strain i s n o t t o o high and these interfaces are good enough t o be used i n electronic devices and i n quantum w e l l structure. Photodetector and lasers integra- t i n g these interfaces and o p t i c a l f i b e r s achieved with these materials are promising.

New developments can be expected i n t h e n e x t future.

MBE i s now intensively used, t o grown these materials (1-5).

I n this paper, we present results r e l e v a n t t o t h e elaboration's conditions, t o t h e physico-chemistry and t o t h e electronic properties o f GaSb-A1Sb interfaces we have g r o w n by t h i s technique.

11 - 6 R O W T H APPARATUS A N D CONDITIONS

The samples were grown i n a MBE system which has been previously described This system which was home made, i s ^CO mposed of t h r e e chambers.

-One evaporation chamber w i t h 7 Riber 110 PBN/cells, w i t h a R HEED, apparatus and a mass spectro meter.

-One chamber f o r Auger and work function studies.

-One chamber t o keep i n stock several samples. _{* .}

The vacuum i n t h e t h r e e chamber i s i n t h e 10-'' Ton. range. The samples can be transfered f r o m one chamber t o another without breaking t h e vacuum. Under these conditions, we are able t o characterize a l l t h e phases o f t h e i n t e r f a c e formation.

Furthermore, b y using a Kelvin probe which i s moved i n f r o n t o f t h e surface of t h e substrate o r i n f r o n t o f t h e layer, we deduce i n f o r m a t i o n on t h e u n i f o r m i t y of t h e layer, on t h e work f u n c t i o n 9 and on t h e photovoltage SPY. This l a s t technique i s v e r y useful because it does n o t a f f e c t t h e properties of t h e i n t e r f a c e during measure-

ments. I t s sensitivity is high and variations as small as 1.5 meV can be detected.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987512

JOURNAL DE PHYSIQUE

El- S U R F A C E P R E P A R A T I O N 1 - Substrate surface

The samples 100 oriented, obtained f r o m MCP wafer (350 p) were f i r s t mechanical- l y polished and etched i n d i f f e r e n t chemical solutions. Studies of the surface composi- t i o n having shown t h a t the best results (small concentration of C, 0) were obtained with Br-Methanol, we have used this solution.

The samples were then inserted into the chamber and heated a t 300°C. The sur- faces prepared i n this way show reproducible properties. The carbon concentration is low but they are covered by a thin oxyde layer. In order t o eliminate it, substrates were annealed a t 580°C. Under these conditions, Ga203, Sb205, were craked and desorbed. But a t this temperature, Ga and Sb do not evaporate congruently and the surface becomes Sb poor. In order, t o avoid this depletion i n Sb, we annealed the sample under a Sb f l u x (10-' Torr).

The surfaces prepared i n this way were clean and ordered, and depending on the flux presented the following reconstruction : (1x3) f o r Ga rich surfaces, (2x3) f o r Sb r i c h surfaces.

Similar experiments with an As flux lead t o the conclusion t h a t the surface recon- struction i s determined by the nature o f the vapor flux. The reconstruction was cha- racteristic o f GaAs f o r an As flux, (on GaSb as well as on GaAs) and of GaSb f o r Sb f l u x on GaAs or GaSb.

Besides AES, we performed EELS and work function measurements. EELS showed the typical peaks corresponding t o valence-band-conduction-band transitions, t o Sb (4d), Ga (3d) core level excitation. Work function ( $ ) and surface photovoltage (SPV) topographies obtained by moving a metallic probe i n f r o n t o f the sample and measuring the contact potential difference and the surface photovoltage are presented i n Fig.

1. For GaSb, SPV # 0 and $ variation along the surface are smaller than 100 meV.

The surface barrier if present, is small ( i t is not the case f o r GaAs substrate) and the variations i n 9 probably result from variations o f the electronic affinity.

2

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Starting from the previous substrates, we have grown GaSb or AlSb layer (growth r a t e 1.5. AO/s f o r GaSb , 0.5 - 5 A0 / s f o r AlSb - ^T substrate # + 580°C, vacuum

# 10- Tow. These surface are clean and ordered but the work function and surface 9 photovoltage topographies obtained i m mediatly a f t e r the growth, show noticeable varia- tions : the surfaces are not uniform (Fig. 1). Their electronic properties, very sensitive t o small modifications, vary from point t o point..

IU

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We have grown the following interface : AlSb/GaSb substrates, AlSb/GaSb M BE grown/GaSb substrate, G aSb/AlSb M BE grow n/GaSb.

In the case o f the AlSb/GaSb substrate, the interface was disturbed by the sub- strate morphology.

Therefore, we have grown AlSb o r GaSb layers on a MCP substrate covered by a l p m MBE grown GaSb buffer. Some results we have got by AES, w o r ~ function measu- rements are given i n Fig. 2. When growing AlSb on GaSb, Ga peaks decrease and A1 peaks increase but 50 A" o f AlSb are enough t o attenuate Ga peaks. Furthermore, the r a t i o o f Ga (55 eV) peaks before and during AlSb growth, fastly decrases. Ga atoms

may diffuse i n t o AlSb but this diffusion is n o t significant.

I n the case o f the growth of GaSb over AlSb A1 peaks and A1 (68 eV) / A1 (1395 eV) r a t i o s progressively decrease when increasing the top layer thickness but A1 is s t i l l detected f o r 150 A" o f GaSb. Simultaneously the r a t i o A1 (68 eV/Sb (26 eV) goes through a maximum for 1-2 A" o f GaSb and this illustrate an A1 transfer between the last layer of AlSb and the f i r s t layer of GaSb.

Therefore, A1 atoms diffuse i n t o GaSb and this diffusion is significative and result i n noticeable modification. Indeed measurements of work function variations shown i n Fig.2 emphasize high modifications i n 9 during the first monolayer growth. first increases subsequently reaches a maxi mu m value f o r half a monolayer, then decreases and f i n a l l y becomes equal t o GaSb work function

D I S C U S S I O N S

Diffusion o f A1 i n t o GaSb cannot be explained by taking into account the heat of formation o f GaSb and AlSb ; the heat o f formation of AlSb is higher than t h a t of GaSb and the situation is n o t favorable t o an A1 exchange from AlSb t o GaSb.

However, the values o f the heat o f formation found i n the literature do not exactly correspond t o the conditions of these experiments and t o interface reaction.

Therefore f r o m a thermodynamical point o f view, may be this exchange cannot be excluded. Furthermore, we can also envisage the formation of Ga clusters i n top o f AlSb. The energy needed t o promote the exchange could be supplied by the formation o f these clusters as it is the case f o r metal deposits on I I I - V compounds (7). However, i n view o f thermodynamical data, the hypothesis of the formation of small clusters o f A1 (on top o f AlSb) supplying A1 atoms f o r the diffusion i n t o GaSb seems more probable.

The work function variations can be associated with this exchange. The last layer o f AlSb lose A1 atoms and the f i r s t layer o f GaSb seems Sb poor. As A1 and Sb vacancies induce, respectively donor and acceptor states (8), 9 variations can be explained by an electron transfer from the last layer o f AlSb t o the first layer of GaSb and b y a resulting increase i n electronic a f f i n i t y 7 . When the thickness of the layer increases, varies i n order t o get the value characteristic of the thick layer.

The 50-70 Ao correspond t o the depth o f t h e interface.

IV - C O N C L U S I O N

Surfaces prepared obtain by M B E present electronic properties (affinity, barrier) much more uniform than other surfaces (cleaved or chemically cleaned). The growth o f AlSb/GaSb and GaSb/AlSb brings t o the fore the diffusion of A1 which cannot be simply explained by considering the heats o f formation. Possibly this diffusion is assisted by strains i n the layer (9).

V - B I B L I O G R A P H Y

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B3 535 /1985/

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3. Tsang W.T., Olson N.A.

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4. Hirayama Y., Ohmori Y., Okamoto H. - J. vac Sci. Techn. 23 - 488 /1984/

5. Gotoh H., Sasamoto K., Kuroda S., Kimata M. - Phys. Stat. Sol. (a) 75

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/1983/

6. C . Raisin, B. Saguintaah, H. Tegmousse, B. Girault, C. Alibert - Ann. Telecom.

41-50 /1986/

7 Williams R.H., i n Physics and chemistry of 111-V Compounds- Carl. W . Wilmsen Ed. Plenum Press - New York /1985/

8. Spicer W .E., Lindau L, Skeath P., Su C.Y, Chye P.-J. Vac sci. Technol. 17 1019 - 1027 - /1980/

9. Zur A., MC G i l l T.C. - J. Vac. Sci. Technol. B 3 (4) 1055

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C5-80 JOURNAL DE PHYSIQUE

Work function and surface photovol- t a g e topographies

1.a n t y p e GaSb etched surface

1.b n t y p e GaSb annealed a t 580°

u n d e r Sb flux : surface clean and o r d e r e d

1.c M B E grown p t y p e GaSb 1.d. M B E grown p t y p e AlSb

1.c' M B E grown p type GaSb a f t e r 24 h-un- d e r u l t r a high vacuum.

1.d' M BE grown p t-vpe AlSb a f t z r 24 h.

under ultra high vacuum

Auger s p e c t r a during t h e growth of GaSb on AlSb

e = thickness of GaSb film (Ao)

2

Work function variations during t h e growth o f GaSb on AlSb

A i s r e l a t i v e t o etched GaSb s u r f a c e s and B t o clean and ordered GaSb s u r f a c e s

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