Kinetics and gas-surface dynamics of GaN homoepitaxial growth using NH3-seeded supersonic molecular beams

The kinetics of homoepitaxial growth of GaN thin films on metal-organic chemical vapor deposition (MOCVD)-grown GaN(0 0 0 1)/AlN/6H-SiC substrates was probed using NH3-seeded supersonic molecular beams. NH3 was seeded in H2 and He and antiseeded in N2 and Ar in order to obtain incident kinetic energies of 0.08–1.8 eV. Nozzle temperatures of 35–600 °C were used to adjust the NH3 internal energy. Intense NH3 beams (fluxes >2×1015 cm−2 s−1 at the substrate) are produced for low seeding percentages (<5%) in the lighter carrier gases, because the heavier species (NH3) is focused along the centerline of the beam. The NH3 flux is proportional to the ratio of its molecular weight to the average molecular weight of the binary gas mixture. A steady-state Langmuir–Hinshelwood kinetics model was used to extract zero-coverage NH3 sticking coefficient (αNH30) values from GaN growth kinetics data. An αNH30 value of 0.14 at 750 °C was determined using seeded supersonic beams of NH3 in He with incident kinetic energies of 0.4–0.5 eV. In comparison, GaN growth rates using low-energy NH3 molecules (0.03 eV) from a leak valve indicate an αNH30 of 0.29. Growth rate measurements using NH3 beams with kinetic energies of 0.08–1.8 eV confirmed that αNH30 generally decreases with increasing incident kinetic energy, leading us to conclude that NH3 chemisorption on GaN(0 0 0 1) is unactivated and occurs via a precursor-mediated mechanism. Internal energy enhancement of NH3 chemisorption via a precursor-mediated channel is proposed to explain the effects of nozzle temperature on GaN growth kinetics. The effects of NH3 incident kinetic energy on film morphology are indirect. Rough, highly faceted films are observed under Ga-limited growth conditions. The surface morphology of films grown under NH3-limited conditions changes from rough to smooth as the effective V/III ratio is decreased.