Different zones of nominally undoped AlN bulk single crystals were investigated

using optical absorption and cathodoluminescence. The incorporation of impurities

and the formation of intrinsic defects during growth depended strongly upon the

facet which formed the zone. Following first-principles calculations and earlier

observations of AlN epilayers, the endpoints of the observed optical transitions

were assigned to ionization levels of oxygen, VAl

2-/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 of bulk AlN.

Point Defect Content and Optical Transitions in Bulk Aluminium Nitride Crystals.

M.Bickermann, B.M.Epelbaum, O.Filip, P.Heimann, S.Nagata, A.Winnacker:

Physica Status Solidi B, 2009, 246[6], 1181-3