Optical gain properties of wurtzite GaN/Al0.2Ga0.8N quantum well lasers are theoretically analyzed using physical parameters from ab initio calculations for the first time. The valence band of wurtzite GaN exhibits strong non-paraboticity, and the hole density of states is significantly large in comparison with the conventional zincblende crystals. This valence band feature causes high transparency cartier density of 7.5×1018 cm-3 in the 50 Å thick GaN quantum well. This result predicts that the threshold current of wurtzite GaN/AlGaN quantum well laser is higher than the conventional lasers with zincblende crystals.

The reaction of silicon nitride films on silicon substrates in sub- and supercritical water at temperatures between 100 and 400 °C has been studied for the first time. The etching rates for the silicon nitride films were higher than those for silicon dioxide films under the same conditions. A novel selective etching method for silicon nitride films using subcritical water has been proposed. The etching mechanism for silicon nitride films in subcritical water was studied by means of infrared spectroscopy and ion chromatography. The etching selectivity at 200 °C and 10 MPa reached 70. An etching rate of 7.5 nm/min for the silicon nitride films was obtained under the same conditions. The selectivity value was higher than that of the conventional method using phosphoric acid, while the etching rate was comparable. Moreover, water is nontoxic; therefore, the proposed method is environmentally friendly. The proposed selective etching method for silicon nitride films using subcritical water is one of the promising candidates for future etching technologies.

An increase of 70% in the catastrophic optical damage (COD) level of AlGaInP visible laser diodes is achieved by sulfur treatment. From transmission electron microscope and energy dispersive microanalysis, we have confirmed that most of the oxide at the mirror facets is replaced by sulfur after this treatment. It is thought that oxide at the facets introduces surface states which cause the COD, and removal of the oxide by sulfur treatment results in the higher COD level.

Three-dimensional fluid simulations are performed in a horizontal reactor for GaN epitaxy. Attention is paid to the effect of gas flow velocity at the inlet and gas pressure. It is found that the gas flow rate rather than the velocity or the pressure is a key parameter which controls the spatial distribution of streamlines, temperature and gas-phase species. As the gas flow rate increases, the size of return-flow or flow-separation appearing near the gas entrance of the expansion region with a tapering angle increases. This causes velocity peaking near the reactor symmetry plane and complicated transport of gas-phase species along the streamlines of the return-flow. If an optimum gas flow rate which gives minimum return-flow and uniform macroscopic spatial distribution for flow pattern and gas-phase species can be determined, then it is desirable to change the gas flow velocity and the gas pressure on the condition that the gas flow rate is maintained.

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