Gas-source molecular beam epitaxy (GS-MBE), utilizing Ga and NH3, and reactive-ion MBE (RIMBE), incorporating both thermal NH3 and low-energy NHx+ ions, were used to grow single crystal GaN(0001) layers on Al2O3(0001) at temperatures T-s between 700 and 850 degrees C with deposition rates of 0.2-0.5 mu m h(-1). The RIMBE experiments were carried out with incident NHx+/Ga flux ratios J(NHx+)/J(Ga) = 1.9-3.2 and NHx+ acceleration energies E(NHx+) = 45-90 eV. Plan-view and cross-sectional transmission electron microscopy analyses showed that the primary defects in the GS-MBE films were threading dislocations having either pure edge or mixed edge/screw characteristics with Burgers vectors ($) over bar b = 1/3[2 ($) over bar 1 ($) over bar 10], basal-plane stacking faults with displacement vectors ($) over bar R = 1/6[02 ($) over bar 23], and prismatic stacking faults with ($) over bar R = 1/2[($) over bar 1101]. In the case of RIMBE films, no stacking faults or residual ion-induced defects were observed with E(NHx+) = 45 eV and T-s greater than or equal to 800 degrees C. However, increasing E(NHx+) to greater than or equal to 60 eV at T-s = 800 degrees C gave rise to the formation of residual ion-induced point-defect clusters observable by transmission electron microscopy (TEM). Increasing T-s to 850 degrees C with E(NHx+) greater than or equal to 60 eV resulted in the ion-induced defects aggregating to form interstitial basal and prismatic dislocation loops, whose number densities depended upon the ion flux, with Burgers vectors 1/2[0001] and 1/3[2 ($) over bar 1 ($) over bar 10], respectively. Unlike previously published results for RIMBE growth with N-2(+), GaN growth kinetics with NHx+ were not found to be a strong function of either ion-to-thermal flux ratios or ion acceleration energies.