Spontaneous formation of nanostructures by surface spinodal decomposition in GaAs1−xBix epilayers

Abstract

We report on the spontaneous formation of lateral composition modulations (LCMs) in Ga(As,Bi) epilayers grown by low-temperature (<300 C) molecular beam epitaxy (MBE) on GaAs(001). Both cross-section and plan-view transmission electron microscopy techniques are used to investigate the nature of the LCMs, consisting of Bi-rich cylinder-like nanostructures lying along the [001] growth direction. The observed LCMs are the consequence of a two-dimensional phase separation process occurring at the surface of the growing epilayers, and their columnar nature is consistent with a surface-directed spinodal decomposition process. Although LCMs are thermodynamically driven, we show how they can be kinetically controlled, in particular, through the As/Ga flux ratio and the substrate temperature. This is a result of LCMs developing from surface atomic diffusion processes, since the atomic dimer configurations on the surface alter adatom diffusivity. The significant role of the surface reconstructions is also discussed. V C 2015 AIP Publishing LLC.

Microstructure and interface analysis of emerging Ga(Sb,Bi) epilayers and Ga(Sb,Bi)/GaSb quantum wells for optoelectronic applications

“…It has been reported that the amplitude of lateral CMs in Ga(As,Bi) is about D[Bi]/[Bi] 30%. 7,8,23 In Ga(As,Bi) with an average Bi content of [Bi] avg ¼ 5%, the presence of a 30% modulation induces a contrast of about 16%, which is readily detectable (we estimate that 3-3.6% contrast is the lower limit of detection that we achieve using g 002 DFTEM 28 ). Assuming that the amplitude of CMs in Ga(Sb,Bi) is as large as that in Ga(As,Bi), a 30% modulation in Ga(Sb,Bi) with [Bi] avg ¼ 14% (or in Ga(Sb,Bi) with [Bi] avg ¼ 11.5% Bi, as a representative composition for the QW samples) would induce a contrast of about 5.7% (5.1%), which is significantly smaller than in Ga(As,Bi) but still detectable.…”

“…4 Among Bi-containing III-V semiconductors, Ga(As,Bi) is the most investigated material, and recent works report morphological instabilities, clustering, and composition modulations (CMs) depending on the specific growth conditions. [5][6][7][8][9] On the contrary, GaSb compounds alloyed with Bi have been far less studied (the first reports on the alloy date back to 2012), 10,11 and questions such as alloy stability, segregation, or solubility limits are still unexplored. In spite of the recent demonstration of laser emission, material developments are still required for the further use of Ga(Sb,Bi) in practical devices.…”

“…Unfortunately, in the present situation, there is a lack of suitable references (e.g., the not-yet synthesized endpoint GaBi compound 1 ), thus limiting EDS quantitative capabilities and awarding g 002 DFTEM a potential suitability for chemical quantification in this material system. Furthermore, g 002 DFTEM imaging is a very valuable technique for the investigation of compositional inhomogeneities in Ga(As,Bi) 7,8,22 since it allows the detection of small variations (around 0.5%) in the Bi content and, in particular, of composition fluctuations which cannot be detected using conventional XRD techniques. 23 Theoretical estimations of the g 002 diffracted intensity in Ga(Sb,Bi) predict, however, that the intensity contrast to GaSb is extremely low, with I 002-GaSbBi /I 002-GaSb 1.19 for Ga(Sb,Bi) with 14% Bi, which is in marked difference to Ga(As,Bi) where I 002-GaAsBi /I 002-GaAs 4.7 for Ga(As,Bi) with 14% Bi, as observed in Fig.…”

Impact of Rotation Rate on Bismuth Saturation in GaAsBi Grown by Molecular Beam Epitaxy

“…However, more recent work has shown that good bismuth incorporation can be achieved under wider ranges of V/III ratios when growing with arsenic tetramers (As 4 ). 12 GaAsBi has a tendency to phase separate over a wide range of bismuth fractions (x = 0.02 to 0.13), resulting in films demonstrating strain-influenced lateral compositional modulations, 13 coarse-scale vertical phase separation brought on by droplet formation, 14 or CuPt B -type atomic ordering. 15,16 Phase separation or spinodal decomposition can yield materials with inconsistent optical and electrical properties, 17 as well as introduce defects and trap states that degrade performance.…”

A comparative study of epitaxial InGaAsBi/InP structures using Rutherford backscattering spectrometry, X-ray diffraction and photoluminescence techniques

“…For example, anti-sites, where the group III or V sites have an element from the other site, as has also been seen in GaAs(Bi) [32,33], may increase the observed lattice mismatch. We speculate that it is also possible that in InGaAsBi, bismuth gives rise to inhomogeneities leading to the formation of clusters or other localised structures in the layer similar to that observed in GaAsBi [20,34,35,36]. Additionally, observations of surface roughness from the SEM images ( Fig.…”