Formation of BN and AlBN During Nitridation of Sapphire Using RF Plasma Sources
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ABSTRACT Evidence is presented that nitrogen plasma sources utilizing a pyrolytic boron nitride liner may be a significant source of B contamination during growth and processing. Auger electron spectroscopy analysis performed during nitridation of sapphire indicate the resulting layers contain a significant amount of BN. The formation of Al1-xBxN would explain the observation of a lattice constant several percent smaller than AlN as measured by reflection high-energy electron diffraction. The presence of cubic inclusions in layers grown on such a surface may be related to the segregation of BN during the nitridation into its cubic phase.
INTRODUCTION Nitridation of sapphire is an important step for many approaches to growing Group III-nitrides. As such, there have been many studies of the nitridation of sapphire by molecular beam epitaxy (MBE) using ammonia, [1,2] an electron cyclotron resonance (ECR) plasma source, [3] or an rf-plasma source. [4,5,6] The chemical evolution of the nitridation layer has been studied by x-ray photoelectron spectroscopy (XPS) [4], indicating an initially increasing nitrogen signal which ultimately saturates. A wide range of times have been reported necessary in order to obtain a completely nitrided surface, from ten to twenty minutes for an ECR source to several hours with an RF source. This range of nitridation times can be adequately explained by the different reactivity of the various active nitrogen species. [5] After nitridation, some authors find a completely relaxed surface with a final lattice constant equal to that of AlN, [1,3] while others find a lattice constant consistently smaller than that of AlN. [6] The formation of an AlO1-xNx alloy has been invoked to explain the smaller lattice constant. However, AlO1-xNx formation is unlikely based on thermodynamic considerations, and is contradicted by an in-situ transmission electron microscopy study of nitridation. [7] Another consideration is the common observation of the nucleation of cubic grains at the epilayer-sapphire interface. Widmann et al. [6] have related this nucleation of cubic grains to nitridation conditions. In this paper, we present results which indicate that B contamination from an rf-plasma source may explain the latter two phenomena.
EXPERIMENTAL DETAILS Two rf-plasma sources are used in our lab used to produce various active species of nitrogen for the nitridation of sapphire and growth of GaN. These sources are an Oxford Applied Research (Oxfordshire, England) CARS-25 source and an EPI Vacuum Products
F99W3.33
(St. Paul, MN) Unibulb source. The Oxford source features a removable pyrolytic boron nitride (PBN) liner and aperture plate. The EPI source contains the standard PBN Unibulb configuration with a 400-hole aperture Nitridation experiments were performed in two separate systems. For chemical analysis of the evolution of the nitridation layer, nitridation was performed at 400 oC in a chamber connected to an Auger electron spectroscopy (AES) analysis system consisting of a Phi 545 Scanning AES Micro
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