Different zones of nominally undoped AlN bulk single crystals were investigated using optical absorption and cathodoluminescence. Incorporation of impurities and formation of intrinsic defects during growth strongly depend on the facet which forms the zone. Following first principles calculations and earlier observations on AlN epilayers, the end-points of the observed optical transitions were attributed to ionization levels of oxygen, VAl2-/3-, and (VAl-ON)-/2-. According to the strength of the respective optical transitions in different zones, it was found that oxygen contamination increased in the order Al-polar (00•1), rhombohedral {10•2} and prismatic {10•0}, N-polar (00•¯1). It was concluded that the low UV transmittance typically observed in bulk AlN was caused by the formation of VAl and (VAl-ON). These deep levels formed in the presence of oxygen, which was the major contaminant in bulk AlN.

Point Defect Content and Optical Transitions in Bulk Aluminum Nitride Crystals. M.Bickermann, B.M.Epelbaum, O.Filip, P.Heimann, S.Nagata, A.Winnacker: Physica Status Solidi B, 2009, 246[6], 1181-3