High-nitrogen-pressure growth of GaN single crystals: Doping and physical properties

Growth of GaN under high-pressure high-temperature conditions allows one to obtain large-size high-quality GaN single crystals. These crystals have high concentration of free electrons, most likely due to a high concentration of O impurity replacing nitrogen in the N sublattice. The incorporation of oxygen impurity during high-pressure growth of GaN single crystals was investigated using quantum mechanical density functional theory calculations. It was shown that the adsorption of oxygen in liquid group III metals (Al, Ga and In) leads to dissociation of the O molecule. The dissociation process proceeds without energy barrier. The transition of oxygen from the adsorbed position into the interior of the Al has been also investigated. The results of calculations indicate that the direct transition energy barrier is about 3 eV. This indicates that the dissolution of oxygen into liquid group III metals proceeds via Brownian motion of O-containing clusters. This also explains the difference between the solid and liquid surfaces: the solid surfaces undergo passivation by oxygen, whereas in the liquid metal the oxygen is dissolved. The doping of Mg during growth leads to a change of the electric properties of GaN crystals-they become highly resistive. Mg doping changes the morphology of the plate-like GaN crystals. The physical properties of GaN:Mg crystals will be reviewed and compared with undoped GaN crystals. Beryllium doping is considered as an alternative route to obtaining p-type GaN. The doping with Be during growth increases the resistivity of the Be-doped GaN. However, the optical properties of Be-doped GaN crystals are different. These properties will be compared with Mg-doped and undoped GaN crystals.