Structural properties of AlN crystals grown by physical vapor transport

Structural properties of different AlN material grown by physical vapor transport (freestanding single crystals vs. bulky material) are compared. In polycrystalline samples, void formation leading to dark and opaque crystallites and grain boundaries is restricted to high growth temperatures and to grains grown with N-polarity. Single-crystalline freestanding AlN exhibits a zonal structure as growth takes place on multiple faces simultaneously. As a conclusion, optical transmission of different grains varies regardless of void formation: Grains grown with N-polarity appear darker and more opaque than grains grown in other directions. Presumably, impurity incorporation and/or intrinsic defect formation depends on the crystallographic orientation of the facet where growth occurs. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.