UV transparent single-crystalline bulk AlN substrates

Bulk aluminum nitride (AlN) is a very promising substrate material for UV optoelectronics, and its UV transparency is of high interest for UV devices designed to emit through the substrate. We report on 500 μm thick bulk AlN substrates with plain UV transmittance exceeding 50% (i.e., absorption coefficients below 14 cm–1) for wavelengths from 220 nm to 380 nm in the main wafer area. Comparing the spectra of different AlN bulk samples, four major below band‐gap absorption bands are identified and interpreted based on a point defect model for AlN crystals. We conclude that a further reduction of oxygen contamination of the source material led to a significant decrease of UV absorption in both colorless and yellowish areas of those substrates. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)