We have employed the plane-wave pseudopotential method to study point defect complexes in GaN and AlN. The results reveal that defect complexes consisting of dominant donors bound to cation vacancies are likely to be formed in both materials. The position of the electronic levels in the band gap due to these defect complexes is shown to correlate well with the experimentally commonly observed broadband luminescence both in GaN and in AlN. The origin of the large bandwidth of the luminescence spectrum is discussed.

Abstract-Nonlinear effects in single-crystal silicon microresonators are analyzed with the focus on mechanical nonlinearities. The bulk acoustic wave (BAW) resonators are shown to have orders-of-magnitude higher energy storage capability than flexural beam resonators. The bifurcation point for the silicon BAW resonators is measured and the maximum vibration amplitude is shown to approach the intrinsic material limit. The importance of nonlinearities in setting the limit for vibration energy storage is demonstrated in oscillator applications. The phase noise calculated for silicon microresonator-based oscillators is compared to the conventional macroscopic quartz-based oscillators, and it is shown that the higher energy density attainable with the silicon resonators can partially compensate for the small microresonator size. Scaling law for microresonator phase noise is developed.[1246]

We have studied oxygen point defects with the plane-wave pseudopotential method in GaAs, GaN, and AlN. The calculations demonstrate a qualitatively different behavior of oxygen impurities in these materials. O As in GaAs acts as a deep center with an off-center displacement and negative-U behavior, in agreement with the experimental data. O N in GaN is found to be a shallow donor with a low formation energy, and is suggested to act as a partial source for the unintentional n-type conductivity commonly observed in GaN. O in AlN is also found to easily substitute for N, which is consistent with the experimentally observed large oxygen concentrations in AlN. However, O N in AlN is shown to be a deep center due to the wide band gap, in contrast with O N in GaN. Our calculations thus predict that isolated oxygen acts as a DX-type center in Al x Ga 1Ϫx N alloys. Results for other oxygen point defect configurations and for the dominant native defects are also presented.

A method for sintering nanoparticles by applying voltage is presented. This electrical sintering method is demonstrated using silver nanoparticle structures ink-jet-printed onto temperature-sensitive photopaper. The conductivity of the printed nanoparticle layer increases by more than five orders of magnitude during the sintering process, with the final conductivity reaching 3.7 × 10(7) S m(-1) at best. Due to a strong positive feedback induced by the voltage boundary condition, the process is very rapid-the major transition occurs within 2 µs. The best obtained conductivity is two orders of magnitude better than for the equivalent structures oven-sintered at the maximum tolerable temperature of the substrate. Additional key advantages of the method include the feasibility for patterning, systematic control of the final conductivity and in situ process monitoring. The method offers a generic tool for electrical functionalization of nanoparticle structures.

We use molecular dynamics simulations and ab initio calculations to study the structures and formation probabilities of isolated surface defects produced by ion irradiation of (1000) graphite. We improve the conventionally used Tersoff potential [J. Tersoff, Phys. Rev. Lett. 61, 2879 (1988)] to realistically describe interlayer forces in graphite and high-energy processes in carbon. We identify three defect structures which correspond to experimentally observed hillocks on graphite surfaces, and examine their formation at different implantation energies. [S0031-9007(96)00757-0]