High-nitrogen-pressure growth of GaN single crystals: doping and physicalproperties

Abstract

Growth of GaN under high-pressure high-temperature conditions allows one to obtainlarge-size high-quality GaN single crystals. These crystals have highconcentration of free electrons, most likely due to a high concentration of Oimpurity replacing nitrogen in the N sublattice. The incorporation of oxygen impurityduring high-pressure growth of GaN single crystals was investigated using quantum mechanical density functional theorycalculations. It was shown that the adsorption of oxygen in liquid groupIII metals (Al, Ga and In) leads to dissociation of the O2 molecule. Thedissociation process proceeds without energy barrier.The transition of oxygen from the adsorbed position into the interior of the Al has beenalso investigated. The results of calculations indicate that the directtransition energy barrier is about 3 eV. This indicates that the dissolution ofoxygen into liquid group III metals proceeds via Brownian motion of O-containing clusters.This also explains the difference between the solid and liquidsurfaces: the solid surfaces undergo passivation by oxygen, whereas in the liquid metalthe oxygen is dissolved.The doping of Mg during growth leads to a change of the electric properties of GaNcrystals - they become highly resistive. Mg doping changes the morphology of theplate-like GaN crystals. The physical properties of GaN:Mg crystals will bereviewed and compared with undoped GaN crystals.Beryllium doping is considered as an alternative route to obtaining p-type GaN. Thedoping with Be during growth increases the resistivity of the Be-doped GaN.However,the optical properties of Be-doped GaN crystals are different. These propertieswill be compared with Mg-doped and undoped GaN crystals.