Optical Properties of Homoepitaxial GaN

For a long time fundamental studies of GaN were limited by the unsatisfactory quality of the samples. The major problem that hangered the development of technology was the lack of good-quality, lattice-matched substrates for MOCVD epitaxy. Since 1995, with the work of K. Pakula et al. [ 1 ], the growth of homoepitaxial GaN MOCVD layers has allowed substantial progress in the optical studies of this interesting material. GaN monocrystal plates, which were used as substrates, were grown from a dilute solution of atomic nitrogen in liquid gallium at a temperature of 1600 °C and a nitrogen pressure of about 15-20 kbar [ 2 ]. Crystals grown by this method form as platelets with their hexagonal c-axis [0001] perpendicular to the surface. The homoepitaxial layers grown on such substrates have the same orientation. The stress in such MOCVD layers is small (A/N) < 2 x 10 -4 ), as verified by X-ray measurements [ 3 ]. Luminescence and reflectivity results [ 4 ] reveal their good optical properties. The increasing quality of grown homoepitaxial GaN layers during the last 5 years, visualized in Figure 5.1 , allows the observation in luminescence spectra of discrete lines with a halfwidth of 0.1 meV. Optical studies of such samples provide new insight into the band energy structure and the nature of the impurities, which dominate the optical properties of this material. Some of the results obtained for homoepitaxial layers have been reviewed in Refs. [ 5-9 ].