Point defect content and optical transitions in bulk aluminum nitride crystals

Abstract

Different zones of nominally undoped AlN bulk single crystals are 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, we assign the endpoints of the observed optical transitions to ionization levels of oxygen, VAl2–/3–, and (VAl–ON)–/2–. According to the strength of the respective optical transitions in different zones, we find that oxygen contamination increases in the order Al‐polar (0001), rhombohedral {10$ \bar 1 $2} and prismatic {10$ \bar 1 $0}, N‐polar (000$ \bar 1 $). We conclude that the low UV transmittance typically observed in bulk AlN is caused by the formation of VAl and (VAl–ON). These deep levels form in the presence of oxygen, which is the major contaminant in bulk AlN. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

UV transparent single‐crystalline bulk AlN substrates

“…Thus, they cannot be used as deep UV transparent substrate. Note that similar spectra of bulk AlN have been recently reported by different research groups [4][5][6][7]. In contrast, crystals grown on bulk AlN substrates with reduced oxygen contamination show spectra with significantly reduced UV optical absorption as seen in Fig.…”

“…According to our recently established point defect model for bulk AlN [4], the main transitions and their in- [6,12]). The 2.78 eV band is assumed to be caused by a transition from V Al to O N and should thus depend on the concentrations of both defects.…”

“…However, even in nominally pure AlN substrates significant optical absorption at photon energies below the band-gap of AlN of approx. 6.1 eV is generally observed [4][5][6][7]. The resulting low transparency in the UV wavelength range is clearly inacceptable for emitters in which the light is coupled out through the substrate.…”

Growth of AlN bulk crystals on SiC seeds: Chemical analysis and crystal properties

“…On the other hand, Schulz et al [54] link the CL intensity at 3-4 eV -which they attribute to the presence of aluminum vacancies V Al 3-in accordance to previous reports [51,53,[60][61][62] -to a steep increase in optical absorption above 4 eV (i.e., deep-UV opacity). Our group found a similar coincidence in 'nominally undoped' AlN, where both the CL bands in the 3-4 eV range and the optical absorption in the 4-5 eV range strongly decrease as a function of source purification [63].…”

“…An absorption band at 2.8 eV is regularly observed in nominally pure AlN crystals [50,51]. There it has been attributed to nitrogen vacancies V N [48,52], to a transition from the valence band to negatively changed aluminum vacancies V Al 3- [53], or to a transition from the V Al 3-/2-level to a donor level in the upper half of the band-gap [51].…”