Modern Physics Letters B

Recent advances in silicon technology have pushed the silicon properties to its theoretical limits. Therefore, wide band gap semiconductors, such as silicon carbide (SiC) and gallium nitride (GaN) have been considered as a replacement for silicon. The discovery of these wide band gap semiconductors have given the new generation power devices a magnificent prospect of surviving under high temperature and hostile environments. The primary focuses of this review are the properties of GaN, the alternative substrates that can be used to deposit GaN and the substitution of SiO₂ gate dielectric with high dielectric constant (k) film. The future perspectives of AlGaN/GaN heterostructures are also discussed, providing that these structures are able to further enhance the performance of high power devices.

A method of calculation of donor impurity states in a quantum well is developed. The used techniques have made it possible to find the binding energy both of ground and excited impurity states attached to each QW subband. The positions of the resonant states in 2D continuum are determined as poles of corresponding wave functions. As a result of such an approach the identification of resonant states in 2D continuum is avoided, introducing special criterions. The calculated dependences of binding energies versus impurity position are presented for various widths of Si/Si_1-xGe_x quantum wells.

Under the non-resonant condition, we investigated the optical bistability (OB) in a three-level quantum well system inside a unidirectional ring cavity. We find that the incoherent pumping fields and the quantum interference induced by incoherent pumping fields can affect the optical bistability dramatically, which can be used to manipulate efficiently the threshold intensity and the hysteresis loop. These results are achieved by applying the two incoherent pumping fields, so they are very different from the conventional schemes that coherent driving fields are used to control the optical bistability. Thus, it may provide some new possibilities for technological applications in optoelectronics and solid-state quantum information science.

The optical properties and plasmon resonances coupling of ordered gold nanoshell arrays are investigated theoretically by means of finite-difference time-domain (FDTD) theory. We showed that the thickness, size and inter-shell distance of the nanoshells can tune the optical transmission of the system and the highly geometry-dependent plasmon response can be seen as an interaction between the essentially fixed-frequency plasmon response of a nanosphere and that of a nanocavity for the nanoshells. We also revealed the two different resonance modes by analyzing the spatial distributions of electric field component E_z. We proposed that the peaks of the lower energy mainly originate from the sphere plasmons coupling and the peaks of the higher energy are mainly attributed to the coupling between the sphere plasmons and the cavity plasmons.

The electronic structures of nitrogen (N)-doped TiO₂ anatase (101) surfaces have been investigated by density functional theory (DFT) plane-wave pseudopotential method with general gradient approximation (GGA) + U (Hubbard coefficient) method being adopted to describe the exchange-correlation effects. Both substitutional and interstitial N doping have been considered in this paper as well as the case of surface N adsorption. The results demonstrated that there is no obvious band gap narrowing observed by introducing N impurities except several N 2p states lying in the gap, whereas the introduction of oxygen vacancy was confirmed playing an important role on band gap narrowing. The results accord well with some experimental results.

By taking into account the interface scattering effect in a d-wave superconductor (S)/insulator layer (I)/d-wave superconductor (S) junction, the temperature dependence of the critical current and the current-phase relation are studied theoretically. It is found that both the barrier scattering and the roughness scattering at the interface always suppress the Andreev reflection and the current-phase relation is almost sinusoidal in the junction. The Josephson current strongly depends on the crystalline axis orientation of the d-wave superconductor in the junction. Some different phenomena appear depending on whether the crystal orientations of the superconductors on the two sides are the same or not, and this is mainly presented in the influence of the zero-energy states formation at the interface on the critical current which changes with temperature and phase.

The thermal activation problem of a bistable system driven by correlated noises with time delay is investigated by means of numerical simulations. The simulation results indicate: (1) For the case of the bistable system with linear delay, the phenomenon of noise enhanced stability (NES) is enforced by increasing delay time τ as the multiplicative noise intensity D is smaller, but is weakened as D is larger. (2) For the case of the bistable system with cubic delay, the NES becomes faintness as τ increases. (3) For the case of the bistable system with global delay, the NES is still restrained by increasing τ with smaller D, and in some circumstances, the activation rate as a function of τ exists a peak structure, which demonstrating the emergence of resonant activation.