We have successfully grown Be-doped p-type GaN on an Al2O3(0001) substrate by RF-MBE by conserving the surface polarity during growth. It is found that the surface polarity of undoped GaN grown on Al2O3(0001) substrate is N-terminated, but the polarity changes from N to Ga during growth when Be-doping is performed. This polarity change results in extremely high resistivity of the grown layer and the defect level centered at approximately 2.0 eV appears prominently. By using the AlN buffer layer, GaN layer can be grown under the Ga-terminated condition from the beginning. By growing Be-doped GaN on the Ga-terminated GaN, we have successfully grown Be-doped p-type GaN without inducing polarity change. In addition, the optical property of the sample is dramatically improved. We have confirmed that the Be acceptor level is shallower than that of the Mg by approximately 100 meV.
