The effects of proton irradiation energy on dc characteristics of AlGaN/GaN metal-oxide semiconductor high electron mobility transistors (MOSHEMTs) using Al2O3 as the gate dielectric were studied. Al2O3/AlGaN/GaN MOSHEMTs were irradiated with a fixed proton dose of 5 × 1015 cm−2 at different energies of 5, 10, or 15 MeV. More degradation of the device dc characteristics was observed for lower irradiation energy due to the larger amount of nonionizing energy loss in the active region of the MOSHEMTs under these conditions. The reductions in saturation current were 95.3%, 68.3%, and 59.8% and reductions in maximum transconductance were 88%, 54.4%, and 40.7% after 5, 10, and 15 MeV proton irradiation, respectively. Both forward and reverse gate leakage current were reduced more than one order of magnitude after irradiation. The carrier removal rates for the irradiation energies employed in this study were in the range of 127–289 cm−1. These are similar to the values reported for conventional metal-gate high-electron mobility transistors under the same conditions and show that the gate dielectric does not affect the response to proton irradiation for these energies.
Tin selenide thin films have been grown by molecular beam epitaxy on GaAs (111)B substrates at a growth temperature of 150°C, and a microstructural study has been carried out, primarily using the technique of transmission electron microscopy. The Se:Sn flux ratio during growth was systematically varied and found to have a strong impact on the resultant crystal structure and quality. Low flux ratios (Se:Sn = 3:1) led to defective films consisting primarily of SnSe, whereas high flux ratios (Se:Sn > 10:1) gave higher quality, single-phase SnSe 2. The structure of the monoselenide films was found to be consistent with the Space Group Pnma with the epitaxial growth relationship of [011] SnSe //[ ] GaAs , while the diselenide films were consistent with the Space Group , and had the epitaxial growth relationship [ ] SnSe2 //[ ] GaAs .
Lattice-matched heterovalent II-VI/III-V semiconductor structures, such as quantum wells and double heterostructures consisting of ZnSe/GaAs and ZnTe/GaSb, are grown using single and dual-chamber molecular beam epitaxy systems by utilizing migration-enhanced epitaxy and a substrate temperature ramp method. Specific elemental overpressures are utilized after each epilayer growth to control the surface termination and to prevent defective III-VI compounds from forming at the heterovalent interfaces. Characterization using x-ray diffraction and transmission electron microscopy confirms sharp interfaces and coherent bonding between the heterovalent materials. Photoluminescence measurements show optical transitions from the heterovalent double heterostructures and quantum wells, as well as evidence for midgap defect states in the III-V layers. The III-V layers have a very low density of structural defects, but some stacking faults are observed in the II-VI layers.
Rock-salt lead chalcogenides such as PbTe are of much current interest for the physics study of quantum materials as a topological insulator and practical applications for infrared photodetectors. Heterocrystalline (rock-salt on zincblende) and heterovalent PbTe/CdTe/InSb heterostructures are grown on (001) InSb substrates using a single-chamber molecular beam epitaxy system. Elemental Pb and Te sources are used to independently vary the flux conditions at the heterocrystalline interface in nearly lattice-matched PbTe/InSb and PbTe/CdTe heterostructures. A streaky (1 × 1) surface reconstruction is observed during the growth of thicker PbTe layers on both InSb and CdTe, signifying smooth layer-by-layer growth. The thickness required for smooth PbTe growth on nearly lattice-matched zincblende materials can be minimized with the proper choice of growth conditions, particularly at the heterocrystalline interface. Characterization with x-ray diffraction indicates good crystalline quality, and observations by transmission electron microscopy reveal sharp interfaces between the PbTe and CdTe films.
We describe the structural evolution of dilute magnetic (Sn,Mn)Se films grown by molecular beam epitaxy on GaAs (111) substrates, as revealed by transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. When the Mn concentration is increased, the lattice of the ternary (Sn,Mn)Se films evolves quasi-coherently from a SnSe2 two-dimensional (2D) crystal structure into a more complex quasi-2D lattice rearrangement, ultimately transforming into the magnetically concentrated antiferromagnetic MnSe 3D rock-salt structure as Mn approaches 50 at. % of this material. These structural transformations are expected to underlie the evolution of magnetic properties of this ternary system reported earlier in the literature.
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