Abstract
The dilute bismide alloy GaAs
1-
x
Bi
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-
x
Bi
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.
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 GaAs
1−x
Bi
x
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.
•Auxiliary numerical tools are introduced to analyze quasi stationary states naturally formed in multislice simulations.•State-correlation functions reveal a generic exponential decay of transversely ...bound states’ excitation coefficients.•Transfer of intensity from bound states to emerging HOLZ rings is quantified.•Thermal scattering effects within the frozen-phonon model and point defects are shown to enhance the attenuation.
High-energy electrons that are used as a probe of specimens in transmission electron microscopy exhibit a complex and rich behavior due to multiple scattering. Among other things, understanding the dynamical effects is needed for a quantitative analysis of atomic-resolution images and spectroscopic data. In this study, state-correlation functions are computed within the multislice approach that allow to elucidate behaviors of transversely bound states in crystals. These states play an important role as a large fraction of current density can be coupled into them via focused electron probes. We show that bound states are generically unstable and decay monoexponentially with crystal depth. Their attenuation is accompanied by a resonant intensity transfer to Bessel-like wavefunctions that appear as Laue rings in the far-field diffraction patterns. Behaviors of bound states are also quantified when thermal effects are included, as well as point defects. This approach helps to bridge the Bloch wave and multisliced electron propagation pictures of dynamical scattering providing new insights into fundamental solutions of the wave equation, and may assist in developing quantitative STEM/TEM imaging techniques.
High‐resolution transmission electron microscopy (HRTEM) study of statistically large number of Bi quantum dots (QD) in annealed GaAsBi/AlAs multiple quantum well (MQW) structures is presented in ...this work. Superlattices containing 20 alternating periods of 10 nm thick GaAsBi quantum well and AlAs 20 nm thick barrier layers are grown on semi‐insulating GaAs at 330 °C temperature. Dispersed Bi quantum dots are formed within the bismide layers during a post‐growth annealing at 750 °C. Energy dispersive X‐ray (EDX) mapping and high‐angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM) analysis reveal that GaAsBi phase is altered during the annealing treatment forming a partially Bi‐depleted GaAsBi phase and semiconducting Bi nanoparticles. AlAs layers are found to act as barriers for Bi out‐diffusion, thus controlling the size of the QDs. The analysis of HRTEM micrographs confirms that Bi quantum dots consist of a rhombohedral Bi phase randomly distributed within zinc blende GaAsBi layers. Geometric phase analysis (GPA) also shows that GaAsBi layers are strained with respect to AlAs layers, as supported by the high‐resolution X‐ray diffraction (HRXRD) data. These findings demonstrate a highly controlled synthesis of Bi‐based quantum dots and pave the way for future applications in the field of infrared devices.
GaAsBi/AlAs multiple quantum well structure is investigated by high‐resolution transmission electron microscopy (HRTEM). This study reveals that after annealing treatment pure rh‐Bi phase Bi quantum dots form in GaAsBi layers. Sizes of quantum dots are found to be highly uniform, as determined by the layer thicknesses, and randomly crystallographically oriented within the matrix. TEM–geometric phase analysis (GPA) shows that GaAsBi layers are strained, as also supported by high‐resolution X‐ray diffraction (HRXRD).
CdTe is the second most-widely deployed photovoltaic (PV) material due to its high efficiency and low manufacturing costs. Currently, polycrystalline CdTe (poly-CdTe) has a record efficiency of ~22%, ...which is still well below the theoretical limit (~30%). Polycrystalline CdTe films that have incorporated Cl or Se show higher efficiency, possibly due to the segregation of these ions to the grain boundaries (GBs), where they may passivate the dangling bonds. However, the efficiency enhancement mechanisms of passivants in CdTe GBs, and the feasibility of employing alternative passivants, have not been well explored. Here, we present a systematic computational study of CdTe GBs with potential passivants, namely S, P, As, Se, and Sb on Te sites, and Na, Mg, Al, Sc, Cu, and Zn on Cd sites. Density functional theory (DFT) calculations were performed on GB dislocation core structures derived from scanning transmission electron microscopy (STEM) images of a model GB (bicrystal). We computed the segregation thermodynamics, electronic density of states, and charge variations near doped CdTe GBs. We find that segregation of impurities to GBs is thermodynamically favorable. For a Te-terminated core, Se, S, and P on Te sites effectively reduce midgap states. For both Cd- and Te- terminated dislocation cores, Sc and Al reduce midgap states when substituted for Cd atoms. The greatest improvement was achieved with co-doping, i.e. simultaneously substituting Te with Se and substituting Cd with Cu or Al. The elimination of midgap states is predicted to increase the photovoltaic efficiency of CdTe by reducing the recombination at grain boundaries.
•CdTe solar cell efficiency depends highly on grain boundary properties.•Large scale first principles computational study was performed on realistic grain boundary models.•For a Te-terminated core, Se, S, P, and As on Te sites effectively reduce midgap states and improve efficiency.•For both Cd- and Te- terminated dislocation cores, Sc and Al on Cd sites reduce the midgap states and improve efficiency.
Group III–V semiconductor multi-junction solar cells are widely used in concentrated-sun and space photovoltaic applications due to their unsurpassed power conversion efficiency and radiation ...hardness. To further increase the efficiency, new device architectures rely on better bandgap combinations over the mature GaInP/InGaAs/Ge technology, with Ge preferably replaced by a 1.0 eV subcell. Herein, we present a thin-film triple-junction solar cell AlGaAs/GaAs/GaAsBi with 1.0 eV dilute bismide. A compositionally step-graded InGaAs buffer layer is used to integrate high crystalline quality GaAsBi absorber. The solar cells, grown by molecular-beam epitaxy, achieve 19.1% efficiency at AM1.5G spectrum, 2.51 V open-circuit voltage, and 9.86 mA/cm
2
short-circuit current density. Device analysis identifies several routes to significantly improve the performance of the GaAsBi subcell and of the overall solar cell. This study is the first to report on multi-junctions incorporating GaAsBi and is an addition to the research on the use of bismuth-containing III–V alloys in photonic device applications.
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