Molecular beam epitaxy of GaN(0001) utilizing NH3 and/or NH+x ions: Growth kinetics and defect structure

Abstract

Gas-source molecular beam epitaxy (GS-MBE), utilizing Ga and NH3, and reactive-ion MBE (RIMBE), incorporating both thermal NH3 and low-energy NH+x ions, were used to grow single crystal GaN(0001) layers on Al2O3(0001) at temperatures Ts between 700 and 850 °C with deposition rates of 0.2–0.5 μm h−1. The RIMBE experiments were carried out with incident NH+x/Ga flux ratios JNH+x/JGa=1.9–3.2 and NH+x acceleration energies ENH+x=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 b̄=1/3〈21̄1̄0〉, basal-plane stacking faults with displacement vectors R̄=1/6〈022̄3〉, and prismatic stacking faults with R̄=1/2〈1̄101〉. In the case of RIMBE films, no stacking faults or residual ion-induced defects were observed with ENH+x=45 eV and Ts≥800 °C. However, increasing ENH+x to ≥60 eV at Ts=800 °C gave rise to the formation of residual ion-induced point-defect clusters observable by transmission electron microscopy (TEM). Increasing Ts to 850 °C with ENH+x≥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〈21̄1̄0〉, respectively. Unlike previously published results for RIMBE growth with N+2, GaN growth kinetics with NH+x were not found to be a strong function of either ion-to-thermal flux ratios or ion acceleration energies.