Influence of Active Nitrogen Species on the Nitridation Rate of Sapphire
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, M.R. Millecchia , C.D. Stinespring
,
and T.H. Myers
Department of Physics, Department of Chemical Engineering, West Virginia University, Morgantown, WV 26506; [email protected] Cite this article as: MRS Internet J. Nitride Semicond. Res. 4S1, G3.10 (1999) ABSTRACT The operating regimes of two rf-plasma sources, an Oxford CARS-25 and an EPI Unibulb, have been extensively characterized. By changing the exit aperture configuration and using an electrostatic deflector, the Oxford source could produce either primarily atomic nitrogen, atomic nitrogen mixed with low energy ions, or a large flux of higher energy ions (>65 eV) as the active species in a background of neutral molecular nitrogen. The EPI source produced a significant flux of metastable molecular nitrogen as the active species with a smaller atomic nitrogen component. Nitridation of sapphire using each source under the various operating conditions indicate that the reactivity was different for each type of active nitrogen. Boron contamination originating from the pyrolytic boron nitride plasma cell liner was observed. INTRODUCTION GaN has been grown by many techniques, each with its own form of active nitrogen species. Molecular beam epitaxy (MBE), using either electron cyclotron radiation (ECR) or radio frequency (rf) plasma sources, represents a particularly complex case since these sources produce ions of various energies, atoms, and potentially, nitrogen molecules in a metastable state. The reactivity of the individual species may play a crucial role in growth kinetics, particularly in the formation of point defects. One measure of the reactivity of a given species is indicated by its efficiency at the nitridation of sapphire, a common step in the nucleation process of GaN. The different reactivity of the various species is evident from the wide range of nitridation times observed [1,2,3,4]. We report the results of a study determining the active species from two rf-plasma sources under different operating conditions, and assess the relative reactivity of the various nitrogen species based on nitridation rates for sapphire. RF-PLASMA SOURCE CHARACTERIZATION Two rf-plasma sources were used to produce various species of nitrogen for the nitridation of sapphire. These sources were an Oxford Applied Research (Oxfordshire, England) CARS-25 source and an EPI Vacuum Products (St. Paul, MN) Unibulb source. The Oxford source featured a removable pyrolytic boron nitride (PBN) aperture plate allowing use of different hole configurations. We investigated plates with 1, 9, 37, and 255 holes. As the number of holes increased, the size of the holes was decreased proportionately in order to maintain the overall conductance of the plate. The EPI source contained the standard Unibulb configuration with a 400-hole aperture resulting in an approximately 50% increase in conductance. The sources were mounted in a UHV chamber in direct line of sight to an Extrel quadrupole mass spectrometer for characterization. The mass spectrometer's repeller grid was biased separately to
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