A coherent picture for the band structure near the G point and the associated fundamental optical transitions in wurtzite (WZ) GaN, including the electron and hole effective masses and the binding energies of the free excitons associated with different valence bands, has been derived from time-resolved photoluminescence measurements and a theoretical calculation based on the local density approximation. We also determine the radiative recombination lifetimes of the free excitons and neutral impurity (donor and acceptor) bound excitons in WZ GaN and compare ratios of the radiative lifetimes with calculated values of the ratios obtained with existing theories of free and bound excitons.

Time-resolved photoluminescence has been employed to probe the free-excitonic transitions and their dynamic processes in GaN grown by molecular beam epitaxy (MBE). The exciton photoluminescence spectral line shape, quantum yield, and recombination lifetimes have been measured at different excitation intensities and temperatures, from which the binding energy of an exciton, the energy band gap, and the free-exciton radiative recombination lifetimes of GaN grown by MBE have been obtained. Our results have demonstrated the superior crystalline quality as well as ultrahigh purity of the investigated sample, implying a new major breakthrough in MBE growth technologies for GaN.

In this rapid communication, we present the new electromagnetic resonant cavity properties of the gold nanoshell. The gold nanoshell is regarded as a nanometre electromagnetic resonant cavity, and the cavity resonance displays tunability controlled by the radii of the dielectric core. The main mode of nanocavity resonant absorption is obtained, which shows a redshift during the reaction of the gold nanoshells. The classical electrodynamics theory is used to describe this gold-coated mini nanoshell cavity, and it shows agreement with experimental results.

The room temperature Hall factors in 6H-SiC have been measured under different magnetic fields. It is found that the Hall coefficient RH increases with magnetic field, implying a Hall factor of less than one at zero magnetic field. This behavior has been accounted for by a model based on electron mass anisotropy in 6H-SiC. The Hall factor results show that in 6H-SiC the mass along the c direction is a factor of 5 larger than that in the transverse direction and that the electron-phonon scattering time is aobut 1 ps at room temperature, which is consistent with a recent optically detected resonance spectroscopy measurement. Our results indicate that the Hall measurement can be utilized as an effective and simple method for studying scattering mechanisms as well as for determining anisotropic transport properties in SiC and that a correction of the Hall factor is necessary in order to determine the carrier concentration and mobility accurately.

Dynamics of the neutral-acceptor-bound exciton transition (the I1 line) in a Mg doped p-type GaN epitaxial layer grown by metalorganic chemical vapor deposition (MOCVD) have been studied by time-resolved photoluminescence emission spectroscopy. Two emission lines in the I1 transition region have been resolved in the time-resolved spectra, possibly due to the existence of two energy states of the Mg impurities after postgrowth thermal annealing. The recombination lifetimes of the acceptor-bound exciton transition have been measured under different conditions including temperature, excitation intensity, and emission energy. From these measurements, a value of about 450ps for the radiative recombination lifetime has been obtained, which is an important physical quantity for optoelectronic device applications based on GaN.