Novel Approaches to Nitride Film Growth: Seeded Supersonic Molecular Beam Methods

Abstract

Seeded supersonic molecular beams can provide high-intensity sources of precursor molecules with tunable translational energies for III-nitride film growth; however, non-equilibrium (kinetic) phenomena associated with supersonic free jet expansions can affect the translational energy, intensity, and energy resolution of the resulting molecular beams. Time-of-flight (TOF) methods are used herein to measure the translational energies, flux intensities, and parallel speed ratios of species in seeded supersonic molecular beams generated from binary gas mixtures containing He, N₂, Ar, and Kr. Aerodynamic focusing of heavy molecules along the centerline of the free jet was found to result in mass separation (i.e., heavy species enrichment). The enrichment data scale to a common dimensionless parameter, the Stokes number at the nozzle throat. An empirical correlation is provided that allows enrichment ratios to be calculated for binary mixtures using only the species mass ratio, the Schmidt number, and the nozzle Reynolds number (calculated using the nozzle stagnation conditions and the physical properties of the carrier gas). The enrichment correlation was found to predict with reasonable accuracy the relative flux intensities of triethylgallium (TEG)/He and TEG/N₂ supersonic molecular beams. Optical emission spectroscopy (OES) and a novel TOF-appearance potential mass spectrometry (APMS) technique were used to characterize the active nitrogen species generated by a radio-frequency discharge supersonic jet (RFD-SSJ) source. The results indicate that the RFD-SSJ source yields supersonic beams containing primarily ground-state (₄S) atomic nitrogen. Homoepitaxial GaN(0001) films were grown by supersonic jet epitaxy (SJE) and characterized by reflection high-energy electron diffraction (RHEED), scanning electron microscopy (SEM) and atomic force microscopy (AFM). Topological scaling analysis applied to the AFM data indicated that surface diffusion is the dominant surface transport mechanism in the growth of GaN films by SJE.