Liquid-phase epitaxial growth of GaSb-related compounds on sulphide treated (100) GaSb substrates

T.V. Cvova I.A. Andreev E.V. Kunitsyna M.P. Mikhailova V. P. U I i n Y. P. Ya kovl ev

-7 Abstract: The authors have performed theoretical and experimental investigations of the chemical interaction between mono- and disulphide water sodium solutions and (100) surfaces of GaSb. It is shown that the pre-epitaxial sulphide treatment of the GaSb substrate improves morphology and interface abruptness of the GaSbl(3aInAsSb heterostructures grown by liquid phase epitaxy (LPE) .

I

1 Introduction

Gallium antimonide and its solid solutions are widely used in optoelectronic devices for the ecologically important spectral range 2 - 5 p [1, 21. However, these materials are characterised by a high density of surface states and the Fermi-level pinning, as well as high chemical activity which causes quick formation of sur- face oxides. One of the main problems of heterostruc- tures is the relatively high levels of the reverse dark current and the high leakage current via surface states. Consequently, considerable advances in processing technology, including surface passivation techniques are required for further development of GaSb-based heterojunction devices.

Wet treatment in a sulphide based solution (H,S, Na2S, (NH&S) has been used, mainly for GaAs and related compounds [3-81. Until now surface treatments of GaSb have not been given sufficient attention. So the aim of this work was to understand the passivation process for GaSb and to use our knowledge to improve the interface quality.

We investigate sulphide chemical passivation of GaSb including: etching of GaSb in sulphide solution, chemical modelling of these passivated surfaces, and pre-epitaxy sulphide treatment of the GaSb substrate. We will show that the sulphide treatment of (100) GaSb substrates performed before epitaxial growth can 0 IEE, 1998 IEE Proceedings online no. 19982307 The authors are wlth A.F. Ioffe Physical-Technical Institute RAS, Politekhnicheskaya, 26; 194021 St. Petersburg, Russia

provide a better crystalline quality (of the grown epilay- ers and an abrupt interface.

2 Sulphide treatment

We carried out surface etching experiments on two (100)n-GaSb substrates: doped Te (n = 1.8 x 10'7cm-3) and undoped (100) GaSb (p = (1.0--1.4) x 1017cm-3).

The sulphide treatments were performed using 0.6M and 2.4M Na2S solutions of pH 13.6 and 13.9, respec- tively, and also 1 M Na2S2 solutions. Part of the initial surface of the semiconductor wafers was covered by a SiOz protecting layer. The material etching rate was determined using etching depth crystal exposure dependence. After exposure in the solution the crystals were washed with deionised water and then were dried in a nitrogen flow. The Si02 film was then dissolved and the etching depth was determined by profilometer.

3000 -

-

2000 - 4 U

0 0 5 10 15 20 25

t. min Fig. 1 aqueous solutions with molarity O.6M and 2.4.44 0.6M: 1-iz-GaSb (open circles) 3-p-GaSb (open triangles) 2.4 M: 2-n-GaSb (solid circles) 4-j-GaSb (solid rhombus) All bymbols corrcspond to experimental data

Etching depth OJ Gas6 against duration of treatment in NazS

Fig. I presents the results obtained when n- and p- GaSb were treated in the 0.6M and 2.4M Na2S

303

solutions. It was found that the etching process is nonuniform in both solutions. The etch rate decreases with time. The etch rates for n-GaSb in both solutions are approximately the same in the initial stage of the treatment. However, the etching process ends very quickly in the saturated (2.4M) solution. It sh'ould be noted that an etching effect is observed for p-type GaSb only under illumination by an incandescent lamp. At the same time, the dynamics of etching of p - GaSb are highly nonuniform in the 0.6M and 2.4M solutions. On the other hand, the etching of GaSb was not detected after poly-sulphide (Na,S,) treatment. Our results on Na2S passivation seem to indicate that two successive passivation stages occur on the surface during sulphide treatment. First, for short times the passivating layer is formed which, as observed by others [9], reduces the surface recombination velocity and realises an electronic passivation, but does not protect against etching. Then, after stopping the etching process, we found the second passivation stage, when a stable oxide-free protective sulphide layer was formed.

We have shown that the mechanism of the chemical passivation includes a competition between the process of etching and the steady formation of the passivating monolayer of S ad-atoms in the bridge-site position between Sb-atoms on the (100) surface. Formmation of the passivating overlayer, and hence the termination of the etching process occurs faster in the Na2S2 solution because of the higher adsorption activity of S-, anions. As a result of the sulphide treatment the seniicclnductor surface obtained a microrelief, formed by the (100) Sb- S(S,)-terraces fragments divided by monomolecular steps.

3 substrates

LPE growth on sulphide passivated GaSb

As the GaSb surface exhibits very quick oxidisation at atmosphere, sulphide treatments performed before epi- taxial growth could provide a better quality of grown epitaxial layers. It is known that the presence of sul- phur in the melt under the epitaxial process ha.s a sub- stantial influence on the growth process and on the electrical and optical properties of the grown layers [lo,

Basing on our results on the etching of GaSb in sul- phide solutions [12], we chose a Na,S water solution to treat the substrate before LPE growing of Gao,781no,22. As0.18Sb0.82 quaternary layers. Note that the necessary

111.

Ga0.78~n0.22As0.18Sb0.82

0.28 r 1

duration of passivation for epitaxial growth should be the same as that observed for etching saturation when a stable oxide-free protective sulphide layer is formed (see Fig. 1). For treatment we used monosulphide Na2S solutions in a range of concentrations from 0.6 to 2.4M. We also used polysulphide solutions Na,S,. The temperatures of the solutions were of the order of 60°C in all cases. As a reference case we did not use pre-epi- taxial sulphide treatment of the substrate. In the second and third cases we used pre-epitaxy nionosulphide and polysulphide treatments, respectively. The substrate annealing was performed under H, flow before epitax- ial procedure. The anneal temperature was equal 620°C for all experiments, but annealing durations changed from 15 min to 100 min. Gao7xInoz2AsolsSbo82 layers were grown at a temperature of 600°C.

4 Results and discussion

We have studied influence of the sulphide treatment on the interface quality. TEM and X-ray diffraction meas- urements show indeed that InGaAsSbiGaSb heter- ostructures grown on sulphide treated (100) GaSb substrates possess an improved morphology and exhibit an abrupt interface. All the samples showed a small mismatch between the lattice periods of the layers and the substrates ( A d a = (4-5) x We measured X-ray diffraction rocking curve GaSbiGaInAsSb heter- ostructures grown on sulphide-treated and nontreated substrates. We found that for both types of samples annealed at 620°C for 60 min and more before the LPE growth, the shapes of the X-ray diffraction rocking curves were the same (Fig. 2a). In contrast, X-ray dif- fraction rocking curves of the passivated samples annealed in times of less than 30 min exhibit oscilla- tions (see Figs. 3a and 4a). Fig. 4u shows that the number of oscillations are higher for the case of polysulphide solutions. An appearance of the oscilla- tions in X-ray diffraction rocking curves indicates the improvement of heterojunction abruptness.

From TEM micrographs (see Figs. 2b, 3h and 4b) one can clearly see that sulphide passivation without long annealing considerably improves the morphology and interface abruptness. To summarise the results of the TEM observations, a strong abrupt interface was obtained for the case of polysulphide treatment (see Fig. 4h). This result supports the conclusion stated above.

We suggest, based on our observations, that a continuous chemisorbed sulphur layer formed as a

I 1

0.04

-400 -200 0 200 arcsec

E +- grown layer

- interface

- substrate

H 25nm

a b

Fig. 2 annealed at T = 620 "C f o r I00 nzin bejbre the LPE procedure a X-ray diffraction rocking curves

X-ray diJfaction rocking curves and cross-sectional TEM image of n-GaSb/p-GuInAsSb structures grown on GuSb substrate treated by HCl: H,O,

h TEM image

IEE Proc.-Oploelectron , Vol. 145, No. 5, October 1598 304

, Ga0.76~n0,22As0.18Sb0.82

0.25

U)

3 c ’ 0.15-

A i?

0.05

U) 020 c

A L21 , ,A , ,

0 04

-320 -1 60 0 160 arcsec H 25nm

-

#grown layer

interface

substrate

GaSb

-

I I -

a b Fi .3 an! I M NUJ, water solution, unnealed ut T = 620”Cjbr 1.5 min bejore the LPEpioceilure

X-ray d$fraction rocking curves and cross-secrioiial TEM image of n-GaSb&GizlnArSb structures grown on GaSb suhstriite treated by HCI: H,O

a X-ray diffraction rocking curves h TEM image

H 25 nm 160 0 160 arcsec

a b Fig. 4 and I M Na&, waler solution, annealed at T = 620°C for 1.5 nim hefijrc. the LPE procedure a X-ray diffraction rocking curves

X-ruy dQf7wction rocking curves and cross-sectional TEA4 image of n-GaSb/p-GahAsSb slructures grown on GuSb ,substrate treated by HCl: H,O

b TEM image

result of passivation protects the surface against further etching by growth solution (melt) at the contact with substrates. We think that the absence of oscillation in the X-ray diffraction rocking curves after long annealing is due to thermal desorption of the sulphur layer.

Finally, we can interpret our results in terms of the proposed chemical model (see Fig. 5). As mentioned earlier, under sulphide treatment the (100) GaSb sur- face obtained a microrelief of Sb-S(SJ-terraces. Fur- ther modification of the surface chemistry under LPE growth conditions consists of a dissociation of Sb-S bonds and a formation of Ga-S surface bonds. The two-dimensional Ga-S phase preserves the substrate etching by the melt, but does not prevent the epitaxial growth. At the same time the plural elemental steps on the substrate act as growth crystallisation centres. This mechanism provides an homogeneity of teinsions in the growing layer and a decrease of dislocation density of the interface.

physisorbed layer chemosorbed layer crystall surface layer+

GaSb

b Fi,g. 5 ajter annealing ut T > 600°C n Before b Artcr

Chemical state of lhe sulphide treuted (100) surface hefbre iind

5 Conclusion

We have studied the etching behaviour of GaSb in water sulphide solutions. Using these results we were

IEE Pro( -0ptoalecfion Vol 145, No 5 October 1998

able to understand the chemistry of sulphide passiva- tion of this semiconductor. This knowledge was then used for the next practical application. We have dem- onstrated that the pre-epitaxial sulphide treatment of (100) GaSb substrates allows us to obtain an abrupt heterojunction with improved morphology of the inter- face.

6 Acknowledgments

We wish to thank N.N. Faleev ancl A.A. Sitnikova for X-ray diffraction and TEM measurements and helpful discussions.

References

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