By taking advantage of interfacial misfit arrays and carefully engineering parameters during molecular beam epitaxy, we report the growth of GaSb-on-Si(001) virtual substrates whose structural properties rival the current state-of-the-art, while maintaining a total epitaxial thickness of <= 3 mu m. To demonstrate the value of these virtual substrates, we have integrated them with nBn photodetectors composed of InAs/InAsSb-strained layer superlattices. The resulting devices exhibit low noise and background limited infrared photodetection at temperatures as high as 160 K, with sufficient quality for use in thermal imaging. This work advances ongoing Si photonics efforts, particularly for low cost, large-area focal plane arrays with high operating temperatures.

We report a comparative analysis between InGaAs and GaAsSb-based top-down cylindrical nanowire arrays fabricated from p-i-n heterostructures. Optical constants measured via spectroscopic ellipsometry were incorporated into finite-difference time-domain simulations to optimize array geometries. Simulations indicated that GaAsSb nanowire arrays with a diameter of 314 nm and a period of 904 nm achieved a peak absorption of similar to 98% at 1550 nm, while InGaAs nanowire arrays with a 326 nm diameter and 1090 nm period reached similar to 76%. Measurements of fabricated arrays showed absorption of similar to 89% at 1628 nm (GaAsSb) and enhanced sub-bandgap absorption at 1766 nm (InGaAs) at room temperature. These results were confirmed through modeling of the as-fabricated nanowire diameters 330 nm and 411 nm, demonstrating excellent agreement between theory and experiment. In both material platforms, nano-structuring improved the absorption relative to planar films, demonstrating near-unity absorption suitable for practical single-photon detection.

Strain due to lattice mismatch is frequently used during molecular beam epitaxy of III-V semiconductor materials to both modify the electronic band structure and to drive nanostructure self-assembly. We harness these effects to synthesize tensile-strained quantum dots whose light emission is dramatically red-shifted relative to their bulk band gap energy. We demonstrate tunable light emission from tensile-strained InGaAs/GaSb quantum dots, which represents a path towards future optoelectronic devices operating in the mid-wave infrared

Epitaxial combination of transition metal nitrides and group III-nitrides holds significant potential for novel device architectures, given their wide array of properties and similar lattice constants. However, the mixture of hexagonal and cubic crystals limits structural quality and has stymied development. This work will discuss the molecular beam epitaxy synthesis of metastable cubic GaN on 3C-SiC templates and its integration with cubic ZrN and NbN superconductors in single and multilayer heterostructures. The fully cubic nature of GaN and the epitaxial nature of all layers are confirmed via in situ and ex situ techniques. The electrical transport properties of transition metal nitrides on cubic GaN (001) are compared to those grown directly on 3C-SiC (001) and c-plane hexagonal GaN templates. The determination of a similar growth window for cubic wide-bandgap and superconducting metal nitrides creates a platform for new epitaxial device architectures and potential applications in metamaterials, quantum information science, and condensed matter physics.

Purpose Historically, graduate education’s goal was to prepare academics; now most science, technology, engineering and/or mathematics (STEM) graduate students (GSs) go on to nonacademic careers. STEM GSs must be equipped for success regardless of career aspirations, which can be done by strengthening GSs’ professional identities. This study aims to explore an interdisciplinary partnership designed to strengthen STEM GS professional identity. Design/methodology/approach The STEM Partnership Project (SPP), asked STEM GSs to serve as disciplinary experts and teach STEM content to elementary teacher candidates (TCs) so the TCs could design and teach an elementary science lesson. GSs also enrolled in a one-credit course to support SPP participation and activities. Over five semesters, the authors collected data from 28 STEM GSs across different disciplines and degree programs in the form of course assignments, surveys and interviews. Findings The SPP supported the development of a professional identity by having GSs serve as and feel like experts; increasing GSs’ sense of belonging in their field; increasing GSs’ self-confidence that they could (learn to) teach a wide variety of audiences; and raising GSs’ awareness of their ability to serve others via their field. Originality/value The SPP’s outcomes were consistent across STEM disciplines, did not require GSs to take on large amounts of coursework, nor did it cost much beyond materials for the various lessons. Furthermore, the key components that strengthened GSs’ professional identities could be adapted for different contexts and institutions.

The quantum Bragg mirror detector (QBMD) is an innovative class of photodetectors designed to explore optical transitions from the ground state to bound states or to leaky electronic states in the continuum. In this study, we explore an asymmetric QBMD based on GaAs/AlGaAs with a leaky electronic state in the continuum. The use of GaAs/AlGaAs provides unique flexibility in potential barrier manipulation, which is unattainable in InGaAs/InAlAs heterostructures lattice matched to InP. By tailoring the layers compositions and thicknesses, the heterostructure has been engineered to explore a transition at 111 meV (l = 11.2 µm) from the ground state to a leaky electronic state in the continuum. A judicious choice of the GaAs/AlGaAs physical parameters was carried out and the calculated photovoltaic photocurrent and absorption exhibit a strong correspondence.

We present recent experiments for tensile-strained Ge QDs on InAlAs(111)A where we observe that growth proceeds via the Stranski–Krastanov (SK) growth mode at lower temperatures and transitions to the Volmer-Weber (VW) growth mode at higher temperatures. We discuss atomistic kinetic Monte Carlo simulations that show how the SK to VW growth mode transition is affected by model parameters. We find that systems are more likely to transition from a SK to a VW mode when the bond energy between substrate atoms is weaker than the bond energy between atoms of the deposited material. We also show how entropic effects due to intermixing can stabilize a layer-by-layer growth mode in certain parameter regimes.

We demonstrate that when highly lattice-mismatched GaSb layers are grown on GaAs(111)A substrates, the strain can be relieved by a self-assembled array of interfacial misfit (IMF) dislocations. This 2D array consists of periodically spaced, pure 60 degrees dislocations that lie in the plane of the GaSb/GaAs(111)A interface. The efficient strain relief provided by the IMF means that the GaSb exhibits good material quality, with threading dislocation densities in the 2-3 x 10(8) cm(-2) range. Other through-film defects associated with twinned GaSb(111) regions have densities between 0.2 and 2 x 10(8) cm(-2). The ability to grow GaSb on GaAs substrates with a (111) orientation creates research opportunities for the integration of dissimilar materials.

We demonstrate the ability to tailor self-assembled growth of In0.5Ga0.5As quantum dots (QDs) on GaSb(111)A surfaces by molecular beam epitaxy. Spontaneous formation via the Volmer-Weber growth mode produces QDs with excellent structural and optical quality. By harnessing tensile strain to reduce their band gap energy, these QDs are characterized by light emission that extends into the midwave infrared wavelength range of 3.2-3.9 μm (0.318-0.388 eV). As we increase QD size, we can tune the band alignment from type-III to type-II, where light emission occurs due to interband recombination between quantum confined electrons in the InGaAs QDs and holes in the GaSb barriers. Of particular interest is an unusual blue-shift in emission wavelength with increasing QD size, which we attribute to the incorporation of Sb into the InGaAs QDs from the GaSb barriers. By expanding this approach to produce tensile-strained QDs from other narrow band gap semiconductors, we anticipate the development of a range of highly tunable mid-infrared light sources.

In this work we present a far-infrared asymmetric quantum Bragg mirror detector (QBMD) based on GaAs/AlGaAs. The heterostructure is designed to explore a transition at 111 meV (11,2 μm) between the ground state and a leaky electronic state in the continuum. The calculated photovoltaic photocurrent shows a good agreement with the expected absorption.

Intercalation-type metal oxides are promising negative electrode materials for safe rechargeable lithium-ion batteries due to the reduced risk of Li plating at low voltages. Nevertheless, their lower energy and power density along with cycling instability remain bottlenecks for their implementation, especially for fast-charging applications. Here, we report a nanostructured rock-salt Nb 2 O 5 electrode formed through an amorphous-to-crystalline transformation during repeated electrochemical cycling with Li + . This electrode can reversibly cycle three lithiums per Nb 2 O 5 , corresponding to a capacity of 269 mAh g −1 at 20 mA g −1 , and retains a capacity of 191 mAh g −1 at a high rate of 1 A g −1 . It exhibits superb cycling stability with a capacity of 225 mAh g −1 at 200 mA g −1 for 400 cycles, and a Coulombic efficiency of 99.93%. We attribute the enhanced performance to the cubic rock-salt framework, which promotes low-energy migration paths. Our work suggests that inducing crystallization of amorphous nanomaterials through electrochemical cycling is a promising avenue for creating unconventional high-performance metal oxide electrode materials.

In the Stranski–Krastanov growth mode for heteroepitaxial systems, layer-by-layer growth is followed by the formation and growth of three-dimensional (3D) islands. In this paper, we use a kinetic Monte Carlo method to simulate this growth mode behavior. We present a detailed and systematic investigation into the effects of key model parameters including strain, growth temperature, and deposition rate on this phenomenon. We show that increasing the strain lowers the apparent critical thickness that is defined by the onset of 3D island formation. Similarly, increasing the growth temperature lowers the apparent critical thickness, until intermixing, and the resulting relevance of entropic contributions, become more significant. We also report the impact on Stranski–Krastanov growth of more model-specific parameters, such as bond strengths between constituent atoms of the system, and surface energy anisotropies.

Controlling dimensionality and strain in actinide heterostructures will provide unrivaled opportunities for exploring novel quantum phenomena. We discuss the promises, challenges, and synthesis routes for these actinide-bearing heterostructures with complex electron correlations for functional and energy materials.

Actinide-based compounds exhibit unique physics due to the presence of 5f electrons, and serve in many cases as important technological materials. Targeted thin film synthesis of actinide materials has been successful in generating high-purity specimens in which to study individual physical phenomena. These films have enabled the study of the unique electron configuration, strong mass renormalization, and nuclear decay in actinide metals and compounds. The growth of these films, as well as their thermophysical, magnetic, and topological properties, have been studied in a range of chemistries, albeit far fewer than most classes of thin film systems. This relative scarcity is the result of limited source material availability and safety constraints associated with the handling of radioactive materials. Here, we review recent work on the synthesis and characterization of actinide-based thin films in detail, describing both synthesis methods and modeling techniques for these materials. We review reports on pyrometallurgical, solution-based, and vapor deposition methods. We highlight the current state-of-the-art in order to construct a path forward to higher quality actinide thin films and heterostructure devices.

We present a novel semiconductor single-photon source based on tensile-strained (111)-oriented GaAs/InAlAs quantum dots (QDs) exhibiting ultrasmall exciton fine-structure splitting (FSS) of <= 8 mu eV. Using low-temperature micro-photoluminescence spectroscopy, we identify the biexciton-exciton radiative cascade from individual QDs, which, combined with small FSS, indicates these self-assembled GaAs(111) QDs are excellent candidates for polarization-entangled photon-pair generation.

Quantum dots that store large tensile strains represent an emerging research area. We combine experiments and computational modeling to investigate the self-assembly of Ge and GaAs tensile-strained quantum dots (TSQDs) on In0.52Al0.48As-(111)A Comparing these two nominally similar material systems highlights how differences in adatom kinetics leads to distinct features of Ge and GaAs TSQD self-assembly. The energy barrier to diffusion of Ge adatoms is higher than that for Ga adatoms, while forming a stable island requires six Ge atoms and four Ga atoms. Unusually, these critical cluster sizes do not increase as we raise the substrate temperature. Radial distribution scaling shows that both Ge and GaAs TSQDs preferentially nucleate at a particular distance from their neighbors. This deeper understanding of the physics of Ge(111) and GaAs(111)A TSQD self-assembly will enable researchers to more effectively tailor these nanostructures to specific optoelectronic applications.

Data management plan for NSF Education and Human Resources (EHR) Innovations in Graduate Education (IGE) Program Proposal

Epitaxial heterostructures of narrow-gap IV-VI and III-V semiconductors offer a platform for new electronics and mid-infrared photonics. Stark dissimilarities in the bonding and the crystal structure between the rocksalt IV–VIs and the zincblende III–Vs, however, mandate the development of nucleation and growth protocols to reliably prepare high-quality heterostructures. In this work, we demonstrate a route to single crystal (111)-oriented PbSe epitaxial films on nearly lattice-matched InAs (111)A templates. Without this technique, the high-energy heterovalent interface readily produces two populations of PbSe grains that are rotated 180° in-plane with respect to each other, separated by rotational twin boundaries. We find that a high-temperature surface treatment with the PbSe flux extinguishes one of these interfacial stackings, resulting in single-crystalline films with interfaces that are mediated by a monolayer of distorted PbSe. While very thin PbSe-on-InAs films do not emit light, hinting toward a type-III band alignment, we see strong room temperature photoluminescence from a 1.5 μm thick film with a minority carrier lifetime of 20 ns at low-excitation conditions and bimolecular recombination at high excitation conditions, respectively, even with threading dislocation densities exceeding 108 cm−2. We also note near-complete strain relaxation in these films despite large thermal expansion mismatch to the substrate, with dislocations gliding to relieve strain even at cryogenic temperatures. These results bring to light the exceptional properties of IV-VI semiconductors and the new IV-VI/III-V interfaces for a range of applications in optoelectronics.

Two infrared photodetectors based on submonolayer quantum dots, having a different InAs coverage of 35% and 50%, were grown, processed and tested. The detector with the larger coverage yielded a specific detectivity of 1.13×1011 cm Hz1/2 W-1 at 12K, which is among the highest values reported in the literature for that kind of device.