We report on the one-dimensional (1D) heteroepitaxial growth of In(x)Ga(1-x)As (x = 0.2-1) nanowires (NWs) on silicon (Si) substrates over almost the entire composition range using metalorganic chemical vapor deposition (MOCVD) without catalysts or masks. The epitaxial growth takes place spontaneously producing uniform, nontapered, high aspect ratio NW arrays with a density exceeding 1 × 10(8)/cm(2). NW diameter (∼30-250 nm) is inversely proportional to the lattice mismatch between In(x)Ga(1-x)As and Si (∼4-11%), and can be further tuned by MOCVD growth condition. Remarkably, no dislocations have been found in all composition In(x)Ga(1-x)As NWs, even though massive stacking faults and twin planes are present. Indium rich NWs show more zinc-blende and Ga-rich NWs exhibit dominantly wurtzite polytype, as confirmed by scanning transmission electron microscopy (STEM) and photoluminescence spectra. Solar cells fabricated using an n-type In(0.3)Ga(0.7)As NW array on a p-type Si(111) substrate with a ∼ 2.2% area coverage, operates at an open circuit voltage, V(oc), and a short circuit current density, J(sc), of 0.37 V and 12.9 mA/cm(2), respectively. This work represents the first systematic report on direct 1D heteroepitaxy of ternary In(x)Ga(1-x)As NWs on silicon substrate in a wide composition/bandgap range that can be used for wafer-scale monolithic heterogeneous integration for high performance photovoltaics.

Periodic high aspect ratio GaAs nanopillars with widths in the range of 500-1000 nm are produced by metal-assisted chemical etching (MacEtch) using n-type (100) GaAs substrates and Au catalyst films patterned with soft lithography. Depending on the etchant concentration and etching temperature, GaAs nanowires with either vertical or undulating sidewalls are formed with an etch rate of 1-2 μm/min. The realization of high aspect ratio III-V nanostructure arrays by wet etching can potentially transform the fabrication of a variety of optoelectronic device structures including distributed Bragg reflector (DBR) and distributed feedback (DFB) semiconductor lasers, where the surface grating is currently fabricated by dry etching.

We report the fabrication of degenerately doped silicon (Si) nanowires of different aspect ratios using a simple, low-cost and effective technique that involves metal-assisted chemical etching (MacEtch) combined with soft lithography or thermal dewetting metal patterning. We demonstrate sub-micron diameter Si nanowire arrays with aspect ratios as high as 180:1, and present the challenges in producing solid nanowires using MacEtch as the doping level increases in both p- and n-type Si. We report a systematic reduction in the porosity of these nanowires by adjusting the etching solution composition and temperature. We found that the porosity decreases from top to bottom along the axial direction and increases with etching time. With a MacEtch solution that has a high [HF]:[H(2)O(2)] ratio and low temperature, it is possible to form completely solid nanowires with aspect ratios of less than approximately 10:1. However, further etching to produce longer wires renders the top portion of the nanowires porous.

The equations for threshold-current density Jth, differential quantum efficiency ηd, and maximum wallplug efficiency ηwp,max for quantum-cascade lasers (QCLs) are modified for electron leakage and backfilling. A thermal-excitation model of “hot” injected electrons from the upper laser state to upper active-region states is used to calculate leakage currents. The calculated characteristic temperature T0 for Jth is found to agree well with experiment for both conventional and deep-well (DW) QCLs. For conventional QCLs ηwp,max is found to be strongly temperature dependent; explaining experimental data. At 300 K for optimized DW-QCLs, front-facet, continuous-wave ηwp,max values >20% are projected.

One-dimensional crystal growth allows the epitaxial integration of compound semiconductors on silicon (Si), as the large lattice-mismatch strain arising from heterointerfaces can be laterally relieved. Here, we report the direct heteroepitaxial growth of a mixed anion ternary InAsyP1-y nanowire array across an entire 2 in. Si wafer with unprecedented spatial, structural, and special uniformity across the entire 2 in. wafer and dramatic improvements in aspect ratio (>100) and area density (>5 × 10(8)/cm(2)). Heterojunction solar cells consisting of n-type InAsyP1-y (y = 0.75) and p-type Si achieve a conversion efficiency of 3.6% under air mass 1.5 illumination. This work demonstrates the potential for large-scale production of these nanowires for heterogeneous integration of optoelectronic devices.

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Phase transition and coexistence of 2H (trigonal prismatic structure) and 1T′ (distorted octahedral structure) phases occur easily in molybdenum ditelluride (MoTe 2 ) when compared with other 2D MX 2 type (M = Mo, W and X = S, Se) transition metal dichalcogenides (TMDs) because of small discrepancies in the cohesive energy. [1][2][3][4] Phase-engineered 2D TMDs, particularly MoTe 2 films including 2H, 1T′, and 1T phases, are very attractive candidates for numerous electronic applications, such as ambipolar field-effect transistors (FETs), environmental sensors, superconductors, spintronics, and valley optoelectronics. [5][6][7][8] Atomically thin-layer 2H MoTe 2 possesses a narrow bandgap energy of 1 eV in comparison to the bandgap energy (1.89 eV) of monolayer MoS 2 and is a potential candidate for various optoelectronic device applications, such as solar cells and photodetectors. [3,8,9] From the electronic device application point of view, the 2H and the 1T phases, i.e., semiconducting and semimetal MoTe 2 are applicable as a 2D materials beyond molybdenum disulfide such as molybdenum ditelluride (MoTe 2 ) have attracted increasing attention because of their distinctive properties, such as phase-engineered, relatively narrow direct bandgap of 1.0-1.1 eV and superior carrier transport. However, a wafer-scale synthesis process is required for achieving practical applications in next-generation electronic devices using MoTe 2 thin films. Herein, the direct growth of atomically thin 1T′, 1T′-2H mixed, and 2H phases MoTe 2 films on a 4 in. SiO 2 /Si wafer with high spatial uniformity (≈96%) via metal-organic vapor phase deposition is reported. Furthermore, the wafer-scale phase engineering of few-layer MoTe 2 film is investigated by controlling the H 2 molar flow rate. While the use of a low H 2 molar flow rate results in 1T′ and 1T′-2H mixed phase MoTe 2 films, 2H phase MoTe 2 films are obtained at a high H 2 molar flow rate. Field-effect transistors fabricated with the prepared 2H and 1T′ phases MoTe 2 channels reveal p-type semiconductor and semimetal properties, respectively. This