Kinetics and gas-surface dynamics of GaN homoepitaxial growth using NH 3-seeded supersonic molecular beams

Abstract

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 NH 3-seeded supersonic molecular beams. NH 3 was seeded in H 2 and He and antiseeded in N 2 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 NH 3 internal energy. Intense NH 3 beams (fluxes >2×10 15 cm −2 s −1 at the substrate) are produced for low seeding percentages (<5%) in the lighter carrier gases, because the heavier species (NH 3) is focused along the centerline of the beam. The NH 3 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 NH 3 sticking coefficient ( α NH 3 0) values from GaN growth kinetics data. An α NH 3 0 value of 0.14 at 750 °C was determined using seeded supersonic beams of NH 3 in He with incident kinetic energies of 0.4–0.5 eV. In comparison, GaN growth rates using low-energy NH 3 molecules (0.03 eV) from a leak valve indicate an α NH 3 0 of 0.29. Growth rate measurements using NH 3 beams with kinetic energies of 0.08–1.8 eV confirmed that α NH 3 0 generally decreases with increasing incident kinetic energy, leading us to conclude that NH 3 chemisorption on GaN(0 0 0 1) is unactivated and occurs via a precursor-mediated mechanism. Internal energy enhancement of NH 3 chemisorption via a precursor-mediated channel is proposed to explain the effects of nozzle temperature on GaN growth kinetics. The effects of NH 3 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 NH 3-limited conditions changes from rough to smooth as the effective V/III ratio is decreased.