Growth and properties of single crystalline GaN substrates and homoepitaxial layers

It is shown that high quality crystals of III-V nitrides can be grown at high gas pressure (up to 20 kbar) and high temperature (up to 2000 K). From the analysis of the thermodynamic properties of AlN, GaN and InN, which are briefly summarized in the paper. it follows that the best conditions for crystal growth at available pressures and temperature conditions can be achieved for GaN. The crystallization of AlN is less efficient due to the relatively low solubility of AlN in liquid Al. The possibility for the growth of InN crystals is strongly limited, since this compound loses its stability at T>600 °C, even at 2 GPa N2 pressure. Recently, high quality, transparent and colorless, 5 mm GaN platelets have been grown from solution in liquid gallium at N2 pressure up to 20 kbar. The mechanisms of nucleation and growth of GaN crystals are discussed on the basis of the experimental results. Structural, electrical and optical properties of these crystals are reported in the paper. The crystals have been used as substrates for the homoepitaxial growth of GaN by MOCVD. Both n- and p-type layers have been obtained by doping with Si and Mg. At low temperature (4.2 K) very strong and narrow PL lines at 3.4666 eV (FWHM = 1.0 meV) assigned to an exciton bound to a neutral acceptor and at 3.4711 eV (FWHM = 1.3 meV) and 3.4719 eV (FWHM = 1.3 meV) due to excitons bound to two neutral donors have been found for undoped GaN layers.