Theoretical and experimental investigation of point defects in cubic boron nitride
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Theoretical and experimental investigation of point defects in cubic boron nitride Nicholas L. McDougall1, Jim G. Partridge1, Desmond W. M. Lau2, Philipp Reineck2, Brant C. Gibson2, Takeshi Ohshima3 and Dougal G. McCulloch1 1
Physics, School of Science, RMIT University, GPO Box 2476, Melbourne, Victoria, 3001, Australia. 2 ARC Centre of Excellence for Nanoscale BioPhotonics, RMIT University, GPO Box 2476, Melbourne, Victoria, 3001, Australia. 3 Japan Atomic Energy Research Institute, 1233 Watanuki, Takasaki, Gunma 370-1292, Japan. ABSTRACT Cubic boron nitride (cBN) is a synthetic wide band gap material that has attracted attention due to its high thermal conductivity, optical transparency and optical emission. In this work, defects in cBN have been investigated using experimental and theoretical X-ray absorption near edge structure (XANES). Vacancy and O substitutional defects were considered, with O substituted at the N site (ON) to be the most energetically favorable. All defects produce unique signatures in either the B or N K-edges and can thus be identified using XANES. The calculations coupled with electron-irradiation / annealing experiments strongly suggest that ON is the dominant defect in irradiated cBN and remains after annealing. This defect is a likely source of optical emission in cBN. INTRODUCTION Cubic boron nitride (cBN) is a synthetic wide band gap material with similar structure to diamond. It has attracted attention due to its high thermal conductivity, optical transparency and may be doped n- or p-type [1-3]. Like diamond, sub band gap fluorescence occurs in cBN and produces spectral features that have been assigned to point defects [4]. However, identification of a particular type of point defect often requires pairing of experiments with theoretical calculations. Electron irradiation is routinely used to study effects of intentionally induced defects in materials [5, 6], and has proven successful in increasing the concentration of defectrelated fluorescent centers in diamond [7, 8]. Electron irradiation has also been used to help study defects and their influence on photoluminescence in single crystal and micro-crystalline cBN [9, 10]. Following irradiation, three additional zero-phonon line peaks in the emission spectra have been observed: RC1 (546 nm), RC2 (577 nm) and RC3 (623nm). These spectral features have been tentatively assigned to neutral or charged N vacancies [10-12], or vacancy complexes involving C or O [9]. It is clear alternative methods are required to identify defects in cBN. X-ray absorption near edge structure (XANES) probes the unoccupied density of states and is therefore sensitive to changes in local bonding caused by defects [13]. In this work, we compare experimental XANES of as-received, electron irradiated and irradiated-annealed nanocrystalline cBN powders. Interpreting features in XANES can be problematic without supporting theory. Hence, we compare these experimental results with ab initio calculations of XANES from vacancies and substitution defects.
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