Domains of molecular beam epitaxial growth of Ga(In)AsBi on GaAs and InP substrates

doi:10.1016/j.jcrysgro.2015.11.021

Journal of Crystal Growth

Volume 436, 15 February 2016, Pages 56-61

https://doi.org/10.1016/j.jcrysgro.2015.11.021 Get rights and content

Highlights

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Bismuth containing III–V semiconductor alloys are promising for optoelectronic applications.
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The challenges of bismide growth are the formation of defects and spinoidal decomposition.
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Growth of bismides can be allocated to a variety of growth domains.

Abstract

We investigate the molecular beam epitaxial growth of GaAsBi and GaInAsBi layers on GaAs and InP-substrates as the materials are intended to serve as an active region in optoelectronic devices. The layers were grown at substrate temperatures between 250–400 °C and for all layers the growth rate was kept at a value of 1 ML/s. We show that bismuth incorporation into Ga(In)As is independent of the applied arsenic (As₄) overpressure and can be allocated to different growth domains depending solely on the parameters bismuth flux and substrate temperature, respectively. The maximum bismuth content that could be incorporated was as high as 20.0% in GaAs.

Introduction

Utilization of bismuth as an alloying element in III-V semiconductor growth has recently attracted increasing interest [1], [2], [3]. This is mainly due to the large bandgap reduction, $Δ E_{g}$ , induced by the incorporation of bismuth into a host material, which could lead to a considerable extension of the wavelength regime of GaAs and InP-based optoelectronic devices. Photoluminescence measurements on ${GaAs}_{1 - x} {Bi}_{x}$ layers grown on GaAs have shown a bandgap reduction as high as 88 meV/%Bi [4], [5]. For ${GaInAs}_{1 - x} {Bi}_{x}$ the reported value determined by spectrophotometry is 54 meV/%Bi [6]. Those values agree well with the theoretical predictions based on the valence band anticrossing (VBAC) model [7] and make the materials highly interesting for utillization as an optically active region emitting in the near and mid-infrared spectral range [1], [2], [3]. In order to achieve longer wavelengths, larger bismuth fractions are necessary.

In this work we investigate the growth parameter dependence of bismuth incorporation into GaAs and into GaInAs grown on InP substrates. We show that bismuth incorporation can be allocated to a variety of growth domains depending solely on the applied bismuth flux and the substrate temperature. In addition, we report about the difficulties encountered in bismide growth: the formation of defects and the occurence of spinoidal decomposition.

Section snippets

Experimental details

The GaAsBi and GaInAsBi layers investigated in this work were grown in a Varian GEN II MBE on semi-insulating (100)-GaAs and InP substrates, respectively. For all layers, the growth rate was set to 1 ML/s and the growth temperatures range from 250–400 °C. The values were adjusted by a thermocouple-based controller and the temperatures have been obtained by a temperature calibration procedure, where thermocouple-based temperatures have been correlated to literature values of transitions of surface

Growth conditions

It was been reported, that bismuth atoms show a tendency to surface segregate at typical MBE growth temperatures ( $> 400 ° C$ ) [12]. The atoms form a surface layer that enhances crystal quality and reduces surface roughness, but they are not incorporated themselves. Therefore, bismuth is denoted as a surfactant. Since the incorporation coefficient at high growth temperatures is close to zero, it requires very low growth temperatures $400 ° C$ to incorporate bismuth in significant amounts [1], [13], [14]

Conclusion

In this work growth studies of GaAsBi and GaInAsBi layers on GaAs- and InP-substrates were presented. All observations that were made can be summarized in the growth domain diagram shown in Fig. 6, which relates bismuth incorporation to growth temperature and bismuth evaporation flux. Bismuth incorporation requires low growth temperatures ( $< 410 ° C$ ). Below 330 °C, bismuth incorporation into GaAs, as well as GaInAs, depends linearly on the applied bismuth flux and is growth temperature independent.

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The spinodal decomposition temperatures of InBixAsyP1−x−y with x > 0.1 are slightly higher than those of GaBixAsyP1−x−y because of the smaller coherency strain energy of InBixAsyP1−x−y. The alloys with x < 0.25 are outside the region of the unstable states with respect to phase separation at the conventional growth temperatures of bismide alloys [1–3]. Thus, Ga(In)BixAsyP1−x−y in the wide Bi range are materials suitable for device applications.
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A systematic series of GaAsBi pin diodes was grown by MBE using different growth temperatures and Bi fluxes, to study the effect on the structural and opto-electronic properties of GaAsBi. The Bi contents of the diodes show both growth temperature and Bi flux dependences. The diodes grown at higher temperatures show evidence of long range inhomogeneity from X-ray diffraction (XRD) measurements, whereas samples of comparable Bi content grown at lower temperatures appear to have well defined, uniform GaAsBi regions. However, the high temperature grown diodes exhibit more intense photoluminescence (PL) and lower dark currents. The results suggest that growth temperature related defects have a greater influence on the dark current than bismuth related defects, and therefore GaAsBi devices should be grown at the highest temperature possible for the desired Bi content.
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The quality of the epitaxial GaAsBi layers appears to be sufficient for the fabrication of minority carrier devices. Gu et al. [55] produced an InGaAsBi detector lattice matched to the InP substrate by gas source MBE with a Bi content of 3.2%. The room temperature cut-off wavelength was 2.1 µm with low dark currents.
III/V semiconductor alloys have been extensively studied because of their usefulness for electronic and photonic devices. Nevertheless, the search for new alloys for specific applications continues. Often, thermodynamic factors restrict the compositional range accessible by epitaxial growth processes, particularly when the size difference between atoms mixing on a particular sublattice is large. This causes solid phase immiscibility, leading to important effects on the epitaxial growth, the resultant alloy properties, and, consequently, device performance. Stringent thermodynamic limits exist for a number of alloys being considered for advanced LED, laser, and solar cell applications where the atomic sizes are very dissimilar, such as GaInN, GaAsN and GaAsBi. This paper will review the basic thermodynamics of the epitaxial growth processes and mixing in semiconductor alloys, as well as the causes and consequences of the resultant complex microstructures.
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Domains of molecular beam epitaxial growth of Ga(In)AsBi on GaAs and InP substrates

Highlights

Abstract

Introduction

Section snippets

Experimental details

Growth conditions

Conclusion

J. Cryst. Growth.

J. Cryst. Growth

Surf. Sci.

Surf. Sci.

J. Cryst. Growth

J. Cryst. Growth

Appl. Phys. Lett.

Jpn. J. Appl. Phys.

Appl. Phys. lett

Appl. Phys. Lett.

Appl. Phys. Lett.

Appl. Phys. Lett.

Phys. Rev. B.

Phys. Rev. B.