Surface effects of vapour-liquid-solid driven Bi surface droplets formed during molecular-beam-epitaxy of GaAsBi

Abstract

Herein we investigate a (001)-oriented GaAs1−xBix/GaAs structure possessing Bi surface droplets capable of catalysing the formation of nanostructures during Bi-rich growth, through the vapour-liquid-solid mechanism. Specifically, self-aligned “nanotracks” are found to exist trailing the Bi droplets on the sample surface. Through cross-sectional high-resolution transmission electron microscopy the nanotracks are revealed to in fact be elevated above surface by the formation of a subsurface planar nanowire, a structure initiated mid-way through the molecular-beam-epitaxy growth and embedded into the epilayer, via epitaxial overgrowth. Electron microscopy studies also yield the morphological, structural, and chemical properties of the nanostructures. Through a combination of Bi determination methods the compositional profile of the film is shown to be graded and inhomogeneous. Furthermore, the coherent and pure zincblende phase property of the film is detailed. Optical characterisation of features on the sample surface is carried out using polarised micro-Raman and micro-photoluminescence spectroscopies. The important light producing properties of the surface nanostructures are investigated through pump intensity-dependent micro-PL measurements, whereby relatively large local inhomogeneities are revealed to exist on the epitaxial surface for important optical parameters. We conclude that such surface effects must be considered when designing and fabricating optical devices based on GaAsBi alloys.

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

“…The presence of a "two-composition layer" (or sequences of layers with different Bi contents and a morphology similar to the one detected here) has been already observed in dilute bismide epilayers, 9,17,25,26 in most cases in connection with the existence of Bi droplets on the surface. 17,[25][26][27] The role of surface droplets during growth of dilute bismides and its impact on Bi incorporation inhomogeneities are currently under active investigation. 9,17,18,[25][26][27] Hence, a comprehensive and dedicated study of the effect of surface droplets on the observed morphology depicted in Figs.…”

Bismuth Quantum Dots in Annealed GaAsBi/AlAs Quantum Wells

“…3, green curve) at 269 and 290 cm −1 corresponds to the GaAs-like transverse optical (TO) and longitudinal optical (LO) phonon modes, respectively [16–18]. In the backscattering geometry, the TO band is symmetrically forbidden for the ideal GaAs crystal [17, 18], but Bi-induced crystalline structure disorder breaks the symmetry of GaAs crystalline lattice and activates TO mode. Two other broad Bi-induced vibrational modes visible near 227 and 181 cm −1 can be attributed to GaBi-like vibrational modes [18].…”

Assessing the Nature of the Distribution of Localised States in Bulk GaAsBi

“…In all instances a low substrate temperature of <400 °C is required, with a low As 2 :Ga flux ratio of around 0.5 21 favourable to achieving Bi incorporation in excess of 5%. The necessity for GaAsBi to be grown at low temperatures is mainly due to a large miscibility gap, which results in a poor solubility of Bi into the GaAs host lattice for all but a narrow range of growth temperatures 23 . This is attributed to the large atomic size difference between As and Bi atoms, leading to an increase in substitutional energy due to increased compressive strain 24 .…”