Mg-doped GaN samples prepared by reactive molecular beam epitaxy have been investigated in an attempt to gain insight into the impurity incorporation and the origin of auto doping in otherwise undoped GaN films. The Hall and secondary ion mass spectroscopy data were utilized for the analysis of possible background impurities such as Si, O, and H in an effort to ascertain whether the background electron concentration is of impurity origin or native defect origin. The data appear to support the N vacancy as a possible cause of auto-n-type doping seen in undoped GaN. The effect of the ammonia flow rate on the incorporation of Mg atoms in GaN films and on the behavior of H were studied for layers grown onc-plane sapphire as well as 6H–SiC. Increased incorporation of Mg with larger ammonia flow rates is attributed to Ga vacancies and accompanying site selection. Moreover,p-GaNfilms grown under high ammonia flux are reported with a hole concentration, mobility, and resistivity of about8×1017 cm−3,26 cm2/V s,and 0.3 &OHgr; cm, respectively. ©1997 American Institute of Physics.