The electrical and luminescent properties of the GaN epitaxial films grown on AlN-coated sapphire by reactive molecular beam epitaxy have been studied. The GaN films on AlN epitaxial films have larger Hall mobilities and show more intense cathodoluminescence peaks at a wavelength of 360 nm than those of the GaN films grown directly on sapphire, which suggests that the crystal qualities of GaN films are improved by use of AlN-coated sapphire as substrates. The lattice matching and small difference of the thermal expansion coefficients between GaN and AlN are considered to result in the improvements.

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A large variation in wavelength from the ultraviolet to longer than 2 µm could be achieved in the GaN-rich side of the GaN1-x As x alloy due to the large bowing of bandgap energy. Layers of GaN1-x As x are grown on (0001) sapphire substrates by electron cyclotron resonance molecular beam epitaxy (ECR-MBE) using an ion-removed ECR radical cell after the growth of GaN buffer layers. During the growth of GaN1-x As x layers, a streaky reflection high-energy electron diffraction (RHEED) pattern was observed. The excitonic photoluminescence (PL) peak from the GaN-rich side of the GaN1-x As x layer shows a large red shift as the As content changes. When an As content of up to x=0.009 is attained, a bandgap bowing parameter of 19.6 eV is experimentally obtained. Such a large value of the bowing parameter is promising for applications to optical devices operating over wide range of wavelength.

GaN layers were grown on C-, A-, R-, and M-plane sapphire substrates by the electron cyclotron resonance-molecular beam epitaxy technique. We addressed a combined utilization of Raman spectroscopy, photoluminescence ͑PL͒ and reflectance measurements to investigate the optical properties of these high-quality GaN layers. First order optical phonons of A 1 , E 1 , and E 2 symmetries were observed in the Raman spectra and the peaks are indicative of the wurtzite crystal structure. All three intrinsic exciton transitions arising from A, B, and C interband transitions were observed in reflectance measurements. The PL spectra were dominated by A and B free exciton transitions and the recombination of an exciton bound to a neutral donor. The experimental data clearly revealed a thickness-dependent change of the biaxial strain in the GaN layers grown on ͑0001͒ C-plane sapphire. The residual strain induced in these layers was found to have a strong influence in determining the energies of the excitonic transitions. Resonant Raman scattering measurements were performed by temperature tuning of fundamental gap in 1.0 m GaN on C-plane sapphire. The influence of epitaxial strain in free exciton properties of GaN layers grown on various orientations of sapphire has been discussed based on the PL and reflectance results. The exciton binding energies were estimated in the GaN layers grown on C-, A-, and M-plane sapphire substrates. Polarized Raman measurements were performed on GaN layers grown on various orientations of sapphire and we observed quasipolar modes of both E 1 and A 1 symmetries. An additional broad photoluminescence band centered around 2.74 eV was observed in the GaN layers grown on R-and M-plane sapphire substrates. The defect induced Raman scattering in resonance with this band shows strong Raman scattering peaks resulting from the transition between energy levels of donor species or defect states.