GaSb layers with low defect density deposited on (001) GaAs substrate in two-dimensional growth mode using molecular beam epitaxy

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Highlights

  • Suppression of spiral growth of GaSb deposited on GaAs substrate using IMF array.

  • Flowing terraces and lack of mounds on GaSb surface.

  • GaSb on GaAs with atomically smooth surface and low EPD of 8 × 106cm−2.

Abstract

We report on the growth of fully relaxed and smooth GaSb layers with reduced density of threading dislocations, deposited on GaAs substrate. We prove that three parameters have to be controlled in order to obtain applicable GaSb buffers with atomically smooth surface: interfacial misfit (IMF), the etch pit density (EPD) and the growth mode.

The GaSb/GaAs interfacial misfit array and reduced EPD ≤1.0 × 107 cm−2 were easily obtained using As-flux reduction for 3 min and Sb-soaking surface for 10 s before the GaSb growth initiation. The successive growth of GaSb layer proceeded under the technological conditions described by the wide range of the following parameters: rG ∈ (1.5 ÷ 1.9) Å/s, TG ∈ (400 ÷ 520)°C, V/III ∈ (2.3 ÷ 3.5). Unfortunately, a spiral or 3D growth modes were observed for this material resulting in the surface roughness of 1.1 ÷ 3.0 nm. Two-dimensional growth mode (layer by layer) can only be achieved under the strictly defined conditions. In our case, the best quality 1-μm-thick GaSb buffer layer with atomically smooth surface was obtained for the following set of parameters: rG = 1.5 Å/s, TG = 530 °C, V/III = 2.9. The layer was characterized by the strain relaxation over 99.6%, 90° dislocations array with the average distance of 5.56 nm, EPD ∼8.0 × 106 cm−2 and 2D undulated terraces on the surface with roughness of about 1 ML. No mounds were observed. We belive that only thin and smooth GaSb layer with reduced EPD may be applied as the buffer layer in complex device heterostructures. Otherwise, it may cause the device parameters deterioration.

Introduction

Lattice-mismatched GaSb/GaAs heterostructures are of great interest for optoelectronic devices such as infrared lasers, detectors and transistors [[1], [2], [3]]. However, with the presence of mismatch, the layer growth is limited by the critical thickness, beyond which the material relieves the strain energy through the misfit dislocations propagating across the entire structure [4,5]. Threading dislocations worsen the device performance so there are many attempts to mitigate their detrimental effect [[6], [7], [8]]. One such growth technique involves 90° interfacial misfit (IMF) dislocations [9]. In GaSb/GaAs system these dislocations can be easily generated through the island formation at the beginning of GaSb growth [10]. The dislocations form periodic array and propagate solely laterally in the substrate-layer interface region resulting in almost completely relaxed material [11]. Many authors reported on the IMF arrays based on the results obtained mainly by the high resolution transmission electron microscopy (HRTEM) and high resolution X-ray diffractometry (HRXRD) [[12], [13], [14]]. The HRTEM images show the dislocation array periodicity of 5.6 nm. The HRXRD curves measured in 2θ–ω configuration as well as ω scans prove the full relaxation of high crystal quality material (the full width at half maximum (FWHM) of rocking curve is less than 250 arcsec for 1.0 μm-GaSb layer). There is however scarce information available about the estimation of dislocation density. KOH and three-component chemical solution of H2O:H2O2:HCl are used to reveal the etch pits, of which densities vary between 7 × 105 cm−2 [9] and 1 × 107 cm−2 [13]. The issue of the growth mode is usually omitted even though the surface roughness is one of the key parameters of buffer layers [[15], [16], [17]].

In this paper we demonstrate that the IMF array verified by both the misfit periodicity of about 5.6 nm and the etch pits density ∼107 cm−2 might be useless for device applications due to the rough morphology. The rougher the buffer surface the poorer the crystal quality of complex heterostructures grown on it. We prove that the ability of IMF array to be applied for the growth of complex mismatched heterostructures should be confirmed by three parameters of GaSb material on GaAs substrate: the full relaxation (higher than 98%) by periodic array of 90° misfit dislocations, 2D growth mode and the reduced EPD (∼1.0 × 107cm−2). Otherwise, the IMF can cause a deterioration of the device heterostructure quality [2].

Section snippets

Experiment

The aim of the experiment described in this paper is to prove the thesis that the three aforementioned parameters of GaSb material have to be taken into account simultaneously during the optimization of strain relaxation by IMF array, especially for the device applications.

The GaSb layers on (100) GaAs substrates were grown using molecular beam epitaxy in 32P Riber machine. The growth procedure was described in detail in the earlier works [18]. Briefly, after typical oxide desorption the GaAs

Crystal quality

All analysed samples are almost fully relaxed. The relaxation of R ≥ 99.6% was determined based on 004 symmetric and -2-24 asymmetric RSMs. The GaSb layers are characterized by the Δqz/Δqx ≥ 0.59 indicating that the relaxation process occurred by the generation of periodically arranged 90° misfit dislocations. The obtained results are listed in Table 2.

In Fig. 1 the symmetric 004 RSM used to determine the Δqz/Δqx ratio and the rocking curve measured for 1#H548 sample are presented. The latter

Conclusion

We reported on the applicable nature of IMF array of 90° dislocations. The strain relaxation by generation of periodically arranged edge dislocations is easily achievable. The wide range of epitaxial conditions allows for the growth of high quality GaSb material on GaAs substrates with reduced density of threading dislocations of n × 106 cm−2 and periodically placed 90° dislocations at the interface. However, only one sample has reached Δqz/Δqx = 0.60 and EPD = 8.0 × 106 cm−2 simultaneously

Acknowledgements

This work was partially supported by the National Science Centre (NCN), project No. 2013/11/B/ST7/04341 and by the National Centre for Research and Development (NCBR), project No. TECHMATSTRATEG1/347751/5/NCBR/2017. The authors would like to thank prof. K. Regiński for his support and assistance with these projects.

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