Uniformity and control of surface morphology during growth of GaN by molecular beam epitaxy

A “modulated growth” technique has been introduced to achieve large area uniformity and surface morphology control during rf plasma assisted molecular beam epitaxy of GaN. The technique consists of modulating the surface coverage of Ga through short-period shuttering of Ga, N, or Ga and N together (e.g., 50–100Å growth periods), to achieve alternating high and low surface coverages of Ga on the (0001) GaN surface. The periods of growth with high Ga flux provide saturation coverage of Ga (∼2.5 ML Ga plus Ga droplets) over the full wafer, while the subsequent growth with low Ga flux facilitates a time-averaged Ga flux which is just below the crossover for droplet formation at the growth temperature. The growth transients in the Ga droplet regime are necessary to maintain smooth, pit-free surface morphologies, and the subsequent growth with low Ga flux suppresses droplet buildup over time. The process is monitored in situ utilizing reflection high energy electron diffraction and line-of-sight quadrupole mass spectroscopy. Results are presented which demonstrate that this approach is an effective means to achieve uniform surfaces over both 1∕4 and 2in. wafer sizes during nonbonded growth. The effects of temperature nonuniformities were mitigated and a surface free of both pitting and droplets has been realized over large areas.