Undoped GaN-based metal-oxide-semiconductor high-electron-mobility-transistors (MOS-HEMTs) with atomic-layer-deposited Al2O3 gate dielectrics are fabricated with gate lengths from 1 μm up to 40 μm. With a two-dimensional numerical simulator, we report simulation results of the GaN-based MOS-HEMTs using field-dependent drift velocity model. A developed model, taking into account polarization-induced charges and defect-induced traps at all of the interfaces and process-related trap levels of bulk traps measured from experiments, is built. The simulated output characteristics are in good agreement with reported experimental data. The effect of the high field at the drain-side gate edge and bulk trap density of GaN on the output performance is discussed in detail for the device optimization. AlGaN/GaN/AlN quantum-well (QW) MOS-HEMTs have been proposed and demonstrated based on numerical simulations. The simulation results also link the current collapse with electrons spreading into the bulk, and confirm that a better electron localization can dramatically reduce the current collapse for the QW-MOS-HEMTs. Due to the large band edge discontinuity and effective quantum confinement of the AlGaN/GaN/AlN quantum well, the parasitic conduction in the bulk is completely eliminated.

Electrical spin injection into and spin extraction from a wide-bandgap semiconductor SiC at room temperature were demonstrated via Schottky junctions. The spin relaxation time of SiC could reach 300 ps, overwhelming that of Si with similar carrier density due to the smaller atomic number. We also found that there existed two channels in SiC/CoFeB Schottky junctions for spin relaxation, one from bulk SiC and the other from interfacial defect states within the barrier whose spin relaxation times were about 1 ns. The bias condition controlled transport channels via bulk or defect states within the barrier and then affected the effective spin relaxation process. Realization of spin injection into SiC shed light on spintronics of wide-bandgap semiconductors such as spin-resolved blue light emitting diodes and high power/temperature spintronics.

Several nanotechnology applications are based on the promising scheme of highly anisotropic magnetic nanomaterials. Using this idea, we investigated the structure, magnetic properties, and interfacial exchange anisotropy effects of the Ni/CrO and Fe/CrO core-shell nanowires (NWs) geometry. A template-based strategy was developed to synthesize Ni (Fe)-CrO core-shell NWs, which combines a wet-chemical route and electrodeposition within the nanopores of the membranes. Structural determination in correlation with magnetic testing shows that the crystalline CrO-nanoshells (NSs) cause an enhanced exchange bias, providing an extra source of anisotropy that leads to their magnetic stability. This core-shell NWs geometry, with enhanced anisotropy, should, therefore, motivate further study related to the applicability of anisotropic nanostructures. Our design opens a new pathway to obtain optimized heterostructured nanomaterials exhibiting tunable magnetic properties.

We propose new topological insulators in hexagonal wurtzite-type binary compounds based on the first principles calculations. It is found that two compounds AgI and AuI are three-dimensional topological insulators with a naturally opened band-gap at Fermi level. From band inversion mechanism point view, this new family of topological insulators is similar with HgTe, which has s (Γ 6 ) -p (Γ 8 ) band inversion. Our results strongly support that the spin-orbit coupling is not an essential factor to the band inversion mechanism; on the contrary, it is mainly responsible to the formation of a global band gap for the studied topological insulators. We further theoretically explore the feasibility of tuning the topological order of the studied compounds with two types of strains. The results show that the uniaxial strain can contribute extremely drastic impacts to the band inversion behavior, which provide an effective approach to induce topological phase transition.

By using first-principles calculations, we have systematically investigated the phase stability, magnetism and electron-filling behavior of vanadium-based inverse Heusler compounds. Our calculation results indicate that, due to the complex hybridization of the d orbitals for the vanadium atom, the electronic structures of the vanadium-based inverse Heusler compounds show two opened gaps (one locates in the spin-up channel and the other in the spin-down channel) near the Fermi level, originating from different bonding states. Based on the unique electronic structures, we proposed a generalized electron-filling rule, which can qualitatively explain the unusual change of the molecular spin magnetic moment as a function of the total number of valence electrons observed in the vanadium-based inverse Heusler compounds. Moreover, most of the vanadium-based inverse Heusler compounds have a negative formation energy, which indicates that they are promising to be synthesized experimentally.

In this paper, by first principle calculations, we investigate systematically the band topology of a new half-Heusler family with composition of I(A)-III(A)-IV(A). The results clearly show that many of the I-III-IV half-Heusler compounds are in fact promising to be topological insulator candidates. The characteristic feature of these new topological insulators is the naturally strong band inversion strength (up to -2eV) without containing heavy elements. Moreover, we found that both the band inversion strength and the topological insulating gap can be tailored through strain engineering, and therefore would be grown epitaxially in the form of films, and useful in spintronics and other applications.

Based on first-principles calculations, we investigate the influence of tetrahedral covalent-hybridization between main-group and transition-metal atoms on the topological band structures of binary HgTe and ternary half-Heusler compounds, respectively. Results show that, for the binary HgTe, when its zinc-blend structure is artificially changed to rock-salt one, the tetrahedral covalent-hybridization will be removed and correspondingly the topologically insulating band character lost. While for the ternary half-Heusler system, the strength of covalent-hybridization can be tuned by varying both chemical compositions and atomic arrangements, and the competition between tetrahedral and octahedral covalent-hybridization has been discussed in details. As a result, we found that a proper strength of tetrahedral covalent-hybridization is probably in favor to realizing the topologically insulating state with band inversion occurring at the Γ point of the Brillouin zone.

FeMn nanowires have been synthesized by employing DC electro-deposition technique provided with constant stirring during the growth. The use of anodic aluminum oxide (AAO) templates made it possible to get well aligned nanowires with average diameter around 100 nm. Magnetic field annealing with field strength of 1 T applied at angle 0° and 90° to nanowires axis at different annealing temperatures has been employed to study the variation in structural and magnetic properties of nanowires. XRD analysis shows poor crystallinity of as-synthesized arrays but cubic structure with (110) preferred orientation has been resulted after the annealing process. Furthermore, vibrating sample magnetometer (VSM) has been employed to study the saturation magnetization (Ms), squareness ratio (SQ=Mr/Ms) and coercivity (Hc) of the as-synthesized and annealed samples. The as deposited and annealed NWs arrays show the coherent rotation for magnetization reversal process.