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

Exploring the Implementation of GaAsBi Alloys as Strain-Reducing Layers in InAs/GaAs Quantum Dots

This paper investigates the effect of GaAsBi strain reduction layers (SRLs) on InAs QDs with different Bi fluxes to achieve nanostructures with improved temperature stability. The SRLs are grown at a lower temperature (370 °C) than the usual capping temperature for InAs QDs (510 °C). The study finds that GaAs capping at low temperatures reduces QD decomposition and leads to larger pyramidal dots but also increases the threading dislocation (TD) density. When adding Bi to the capping layer, a significant reduction in TD density is observed, but unexpected structural changes also occur. Increasing the Bi flux does not increase the Bi content but rather the layer thickness. The maximum Bi content for all layers is 2.4%. A higher Bi flux causes earlier Bi incorporation, along with the formation of an additional InGaAs layer above the GaAsBi layer due to In segregation from QD erosion. Additionally, the implementation of GaAsBi SRLs results in smaller dots due to enhanced QD decomposition, which is contrary to the expected function of an SRL. No droplets were detected on the surface of any sample, but we did observe regions of horizontal nanowires within the epilayers for the Bi-rich samples, indicating nanoparticle formation.

Tunable bandgap and isotropic light absorption from bismuth-containing GaAs core-shell and multi-shell nanowires

Semiconductor core-shell nanowires based on the GaAs substrate are the building blocks of many photonic, photovoltaic and electronic devices, thanks to their associated direct bandgap and highly tunable optoelectronic properties. The selection of a suitable material system is crucial for custom designed nanowires tailored for optimised device performance. Bismuth-containing GaAs materials are an imminent class of semiconductors which not only enable an exquisite control over the alloy strain and electronic structure but also offer the possibility to suppress internal loss mechanisms in photonic devices. Whilst the experimental efforts to incorporate GaBixAs1-x alloys in the nanowire active region are still at an early stage, the theoretical understanding of the optoelectronic properties of such nanowires is only rudimentary. This work elucidates and quantifies the role of nanowire physical attributes such as its geometry parameters and bismuth incorporation in designing light absorption wavelength and polarisation response. Based on the multi-million atom tight-binding simulations of the GaBixAs1-x/GaAs core-shell and GaAs/GaBixAs1-x/GaAs multi-shell nanowires, our results predict a large tuning of the absorption wavelength, ranging from 0.9 μm to 1.6 μm, which can be controlled by engineering either Bi composition or nanowire diameter. The analysis of their strain profiles indicates a tensile character leading to significant light-hole mixing in the valence band states. This offers a possibility to achieve polarisation-insensitive light interaction, which is desirable for several photonic devices involving amplification and modulation of light. Furthermore, at low Bi compositions, the carrier confinement is quasi type-II, which further broadens the suitability of these nanowires for a myriad of applications requiring large carrier separations. The presented results provide a systematic and comprehensive understanding of the GaBixAs1-x nanowire properties and highlight new possibilities for future technologies in photonics, quantum optics and solar energy harvesting.

Effects of Bi incorporation on recombination processes in wurtzite GaBiAs nanowires

The effects of Bi incorporation on the recombination process in wurtzite (WZ) GaBiAs nanowires are studied by employing micro-photoluminescence (μ-PL) and time-resolved PL spectroscopies. It is shown that at low temperatures (T < 75 K) Bi-induced localization effects cause trapping of excitons within band-tail states, which prolongs their lifetime and suppresses surface nonradiative recombination (SNR). With increasing temperature, the trapped excitons become delocalized and their lifetime rapidly shortens due to facilitated SNR. Furthermore, Bi incorporation in the GaBiAs NW is found to have a minor influence on the surface states responsible for SNR.

Atomic-Resolution EDX, HAADF, and EELS Study of GaAs1-xBix Alloys

The distribution of alloyed atoms in semiconductors often deviates from a random distribution which can have significant effects on the properties of the materials. In this study, scanning transmission electron microscopy techniques are employed to analyze the distribution of Bi in several distinctly MBE grown GaAs1-xBix alloys. Statistical quantification of atomic-resolution HAADF images, as well as numerical simulations, are employed to interpret the contrast from Bi-containing columns at atomically abrupt (001) GaAs-GaAsBi interface and the onset of CuPt-type ordering. Using monochromated EELS mapping, bulk plasmon energy red-shifts are examined in a sample exhibiting phase-separated domains. This suggests a simple method to investigate local GaAsBi unit-cell volume expansions and to complement standard X-ray-based lattice-strain measurements. Also, a single-variant CuPt-ordered GaAsBi sample grown on an offcut substrate is characterized with atomic scale compositional EDX mappings, and the order parameter is estimated. Finally, a GaAsBi alloy with a vertical Bi composition modulation is synthesized using a low substrate rotation rate. Atomically, resolved EDX and HAADF imaging shows that the usual CuPt-type ordering is further modulated along the [001] growth axis with a period of three lattice constants. These distinct GaAsBi samples exemplify the variety of Bi distributions that can be achieved in this alloy, shedding light on the incorporation mechanisms of Bi atoms and ways to further develop Bi-containing III-V semiconductors.

GaAs1-xBix growth on Ge: anti-phase domains, ordering, and exciton localization

The dilute bismide alloy GaAs_1-xBi_x has drawn significant attention from researchers interested in its fundamental properties and the potential for infrared optoelectronics applications. To extend the study of bismides, molecular-beam heteroepitaxy of nominally 1.0 eV bandgap bismide on Ge substrates is comprehensively investigated. Analysis of atomic-resolution anti-phase domain (APD) images in the direct-epitaxy revealed a high-density of Ga vacancies and a reduced Bi content at their boundaries. This likely played a key role in the preferential dissolution of Bi atoms from the APD interiors and Bi spiking in Ge during thermal annealing. Introduction of GaAs buffer on offcut Ge largely suppressed the formation of APDs, producing high-quality bismide with single-variant CuPt_B-type ordered domains as large as 200 nm. Atomic-resolution X-ray imaging showed that 2-dimensional Bi-rich (111) planes contain up to x = 9% Bi. The anomalously early onset of localization found in the temperature-dependent photoluminescence suggests enhanced interactions among Bi states, as compared to non-ordered samples. Growth of large-domain single-variant ordered GaAs_1-xBi_x films provides new prospects for detailed analysis of the structural modulation effects and may allow to further tailor properties of this alloy for optoelectronic applications.

Controlling Bi-Provoked Nanostructure Formation in GaAs/GaAsBi Core-Shell Nanowires

We control the formation of Bi-induced nanostructures on the growth of GaAs/GaAsBi core-shell nanowires (NWs). Bi serves as not only a constituent but also a surfactant and nanowire growth catalyst. Thus, we paved a way to achieve unexplored III-V nanostructures employing the characteristic supersaturation of catalyst droplets, structural modifications induced by strain, and incorporation into the host GaAs matrix correlated with crystalline defects and orientations. When Ga is deficient during growth, Bi accumulates on the vertex of core GaAs NWs and serves as a nanowire growth catalyst for the branched structures to azimuthal <112>. We find a strong correlation between Bi accumulation and stacking faults. Furthermore, Bi is preferentially incorporated on the GaAs (112)B surface, leading to spatially selective Bi incorporation into a confined area that has a Bi concentration of over 7%. The obtained GaAs/GaAsBi/GaAs heterostructure with an interface defined by the crystalline twin defects in a zinc-blende structure can be potentially applied to a quantum confined structure. Our finding provides a rational design concept for the creation of GaAsBi based nanostructures and the control of Bi incorporation beyond the fundamental limit.

Variation of the photoluminescence spectrum of InAs/GaAs heterostructures grown by ion-beam deposition

This work reports on an experimental investigation of the influence of vertical stacking of quantum dots, the thickness of GaAs potential barriers, and their isovalent doping with bismuth on the photoluminescence properties of InAs/GaAs heterostructures. The experimental samples were grown by ion-beam deposition. We showed that using three vertically stacked layers of InAs quantum dots separated by thin GaAs barrier layers was accompanied by a red-shift of the photoluminescence peak of InAs/GaAs heterostructures. An increase in the thickness of the GaAs barrier layers was accompanied by a blue shift of the photoluminescence peak. The effect of isovalent Bi doping of the GaAs barrier layers on the structural and optical properties of the InAs/GaAs heterostructures was investigated. It was found that the Bi content up to 4.96 atom % in GaAs decreases the density of InAs quantum dots from 1.53 × 1010 to 0.93 × 1010 cm-2. In addition, the average lateral size of the InAs quantum dots increased from 14 to 20 nm, due to an increase in the surface diffusion of In. It is shown that isovalent doping of GaAs potential barriers by bismuth was accompanied by a red-shift of the photoluminescence peak of InAs quantum dots of 121 meV.

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

A comprehensive assessment of the nature of the distribution of sub band-gap energy states in bulk GaAsBi is presented using power and temperature dependent photoluminescence spectroscopy. The observation of a characteristic red-blue-red shift in the peak luminescence energy indicates the presence of short-range alloy disorder in the material. A decrease in the carrier localisation energy demonstrates the strong excitation power dependence of localised state behaviour and is attributed to the filling of energy states furthest from the valence band edge. Analysis of the photoluminescence lineshape at low temperature presents strong evidence for a Gaussian distribution of localised states that extends from the valence band edge. Furthermore, a rate model is employed to understand the non-uniform thermal quenching of the photoluminescence and indicates the presence of two Gaussian-like distributions making up the density of localised states. These components are attributed to the presence of microscopic fluctuations in Bi content, due to short-range alloy disorder across the GaAsBi layer, and the formation of Bi related point defects, resulting from low temperature growth.

Unidirectional lateral nanowire formation during the epitaxial growth of GaAsBi on vicinal substrates

We report on enhanced control of the growth of lateral GaAs nanowires (NWs) embedded in epitaxial (100) GaAsBi thin films enabled by the use of vicinal substrates and the growth-condition dependent role of Bi as a surfactant. Enhanced step-flow growth is achieved through the use of vicinal substrates and yields unidirectional nanowire growth. The addition of Bi during GaAsBi growth enhances Ga adatom diffusion anisotropy and modifies incorporation rates at steps in comparison to GaAs growth yielding lower density but longer NWs. The NWs grown on vicinal substrates grew unidirectionally towards the misorientation direction when Bi was present. The III/V flux ratio significantly impacts the size, shape and density of the resulting NWs. These results suggest that utilizing growth conditions which enhance step-flow growth enable enhanced control of lateral nanostructures.

Bismuth Quantum Dots in Annealed GaAsBi/AlAs Quantum Wells

Formation of bismuth nanocrystals in GaAsBi layers grown by molecular beam epitaxy at 330 °C substrate temperature and post-growth annealed at 750 °C is reported. Superlattices containing alternating 10 nm-thick GaAsBi and AlAs layers were grown on semi-insulating GaAs substrate. AlAs layers have served as diffusion barriers for Bi atoms, and the size of the nanoclusters which nucleated after sample annealing was correlating with the thickness of the bismide layers. Energy-dispersive spectroscopy and Raman scattering measurements have evidenced that the nanoparticles predominantly constituted from Bi atoms. Strong photoluminescence signal with photon wavelengths ranging from 1.3 to 1.7 μm was observed after annealing; its amplitude was scaling-up with the increased number of the GaAsBi layers. The observed photoluminescence band can be due to emission from Bi nanocrystals. The carried out theoretical estimates support the assumption. They show that due to the quantum size effect, the Bi nanoparticles experience a transition to the direct-bandgap semiconducting state.

More features, more tools, moreCrysTBox

A new release of theCrysTBoxsoftware is introduced. The original toolbox allows for an automated analysis of transmission electron microscope (TEM) images and for crystallographic visualization. The existing tools, which are capable of highly precise analyses of high-resolution TEM images, as well as spot, disc and ring diffraction patterns, are extended to include a tool for automatically measuring TEM sample thickness using convergent beam electron diffraction in a two-beam approximation. An implementation of geometric phase analysis is newly available, employing one of the existing tools to identify parameters and indices of crystallographic planes depicted in the input image and allowing easier and more accurate analysis. The crystallographic visualization capabilities are extended as well. Along with the simulated diffraction pattern and atomic structure, a stereographic projection and inverse pole figure tool is newly offered. A new tool able to visualize the atomic structure of two different phases and their interface is also introduced.