We have systematically studied the strain dependence of the free-exciton resonance energies in wurtzite GaN by photoreflectance measurements using well-characterized samples. The experimental data have been analyzed using the appropriate Hamiltonian for the valence bands in wurtzite GaN and determined the values of the crystal field splitting, the spin–orbit splitting, the shear deformation potential constants, and the energy gap in the unstrained crystal. Discussions are given on the strain dependence of the energy gaps, of the effective masses, and of the binding energies for the free-exciton ground states as well as on the valence-band parameters.

The polarization of photoluminescence (PL) was investigated on (11̄00) GaN grown by metalorganic vapor phase epitaxy. We found that the PL intensity and wavelength have polarization dependence parallel and perpendicular to the c axis. We quantitatively analyzed the dependence and found that, since the crystal field of wurtzite GaN along the c axis is strong enough to fix the |z〉 axis of p functions at the c axis, the difference in symmetry between three valence bands appears as the polarization anisotropy in radiative emission, even in bulk GaN.

Exciton resonance energies of hexagonal (h-) GaN(0001) epilayers were determined by a combination of high-resolution modulated photoreflectance methods. The results were analyzed thoretically using the Luttinger-Kohn type Hamiltonian for the valence bands under the in-plain biaxial stress, and we obtained the shear deformation potential constants and energy gap in unstrained crystal. Occurrence of the anticrossing of B and C valence bands in tensile biaxially strained h-GaN was suggested.

We grew GaN crystals by metalorganic vapor phase epitaxy on (111) and (100) MgAl2O4 substrates. We obtained a single-crystal GaN layer with a specular surface on the (111) substrate. The full width of half-maximum of the x-ray rocking curve for a 3.6 μm thick GaN layer was 310 s, comparable to the reported values for GaN on Al2O3 substrates. In the room-temperature photoluminescence, a band-edge emission at around 360 nm was dominant. A smooth cleaved (11̄00) facet of the GaN epitaxial layer was obtained, assisted by the inclined (100) cleavage of the (111) MgAl2O4 substrate. We intend this cleaved facet, which is normal to the surface, to be used as a cavity mirror in a laser diode.

InGaN multiple quantum well (MQW) laser diodes were fabricated on (0001)Si oriented 6H–SiC substrate using low-pressure metal organic vapor phase epitaxy (LP-MOVPE). The laser oscillation was observed above the threshold current of 800 mA at a peak wavelength of 414.3 nm under pulsed current injection at room-temperature. The pulse duration was 300 ns and the repetition frequency was 1 kHz. The threshold current density and differential efficiency were estimated to be 16 kA/cm2 and 0.03 W/A, respectively. The full width at half maximum (FWHM) of the lasing emission lines was between 0.03 nm and 0.21 nm. Streak-shaped far field patterns were clearly observable. The lifetime of the laser diode was more than 5 hours.