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 che mical vapor deposition (MOCVD)grown GaN(0001)/AlN/6H-SiC substrates was pro bed using NH3-seeded supersonic molecular beams. NH3 was seeded in H-2 and He and antiseeded in N-2 and Ar in order to obtain incident kinetic energie s of 0.08-1.8 eV. Nozzle temperatures of 35-600 degreesC were used to adjus t the NH3 internal energy. Intense NH3 beams (fluxes > 2x10(15)cm(-2)s(-1) at the substrate) are produced for low seeding percentages (<5%) in the lig hter 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 mo lecular 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 (alpha (0)(NH3)) values from GaN growth k inetics data. An alpha (0)(NH3) value of 0.14 at 750 degreesC was determine d 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 molecu les (0.03 eV) from a leak valve indicate an alpha (0)(NH3) of 0.29. Growth rate measurements using NH3 beams with kinetic energies of 0.08-1.8 eV conf irmed that alpha (0)(NH3) generally de-creases with increasing incident kin etic energy, leading us to conclude that NH3 chemisorption on GaN(0001) is unactivated and occurs via a precursor-mediated mechanism. Internal energy enhancement of NH3 chemisorption via a precursor-mediated channel is propos ed to explain the effects of nozzle temperature on GaN growth kinetics. The effects of NH3 incident kinetic energy on film morphology are indirect. Ro ugh, highly faceted films are observed under Galimited growth conditions. T he surface morphology of films grown under NH3-limited conditions changes f rom rough to smooth as the effective V/III ratio is decreased. (C) 2001 Els evier Science B.V. All rights reserved.