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GaN nanorod films have been grown on Si(001) substrates with native silicon oxides by radio-frequency plasma-enhanced molecular beam epitaxy. GaN nanorod films are made up of single-crystalline nanorods with a so-called (0001) fiber-like texture. Each nanorod is elongated along c axis in perpendicular to the substrate surface and has no preferential axis in film plane. Excellent electron field emission characteristics were observed for the fabricated GaN nanorod films with a field emission threshold as low as 1.25V∕μm at a current density of 0.1μA∕cm2 and a field emission current density as high as 2.5mA∕cm2 at an applied field of 2.5V∕μm. These excellent characteristics are attributed to the geometrical configuration of nanorods and their good crystalline quality as well as the low electron affinity of GaN.

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.

Polycrystalline GaN layers were grown on W, Mo, Ta, and Nb metal substrates by gas-source molecular-beam epitaxy using an ion-removal, electron-cyclotron-resonance radical cell. X-ray diffraction rocking curves showed preferential GaN(0002) or GaN(10–11) orientations. The grain sizes ranged from 100 to 800 nm. Strong photoluminescence (PL) emission without yellow luminescence was observed from these polycrystalline GaN layers. At 77 K, PL peaks at 3.46 and 3.26 eV were observed, and their temperature dependence fit a simple relation based on the number of phonons. The higher-energy peak probably was due to the free excitonic transition in hexagonal GaN.

GaN-based magnetic semiconductors are expected to exhibit ferromagnetism even at high temperatures. We have studied the magnetic and optical properties of transition-metal- and rare-earth-doped GaN. GaCrN showed ferromagnetic characteristics at 7–400 K. Clear hysteresis and clear saturation were observed in the magnetization versus magnetic field (M–H) curves at all measuring temperatures. We observed photoluminescence (PL) emission from GaCrN layers. GaGdN also showed ferromagnetic characteristics at 7–400 K. Sharp PL emission was observed for GaGdN. X-ray diffraction and EXAFS measurements showed no phase separation, and that the Cr and Gd atoms substitute Ga sites. Applications to novel devices controlling charges (electrons and holes), spins and photons are expected.

GaN layers are grown on (0001) sapphire substrate by electron cyclotron resonance molecular beam epitaxy (ECR-MBE) using an ECR plasma cell with ion removal magnets on the cell top for the nitrogen source. The efficiency of the ion removal magnets in this ECR plasma cell is 99%. High-quality GaN layers are obtained. In particular, (2×2) and (4×4) RHEED (reflection high-energy electron diffraction) patterns are observed during GaN growth and during cooling after growth, respectively, indicating a flat and smooth surface of GaN. These results show the superiority of the ion-removed ECR plasma cell.