The effectiveness and reliability of estimating the dislocation density in GaN thin films and bulk crystals by defect selective etching in eutectic KOH/NaOH had already been successfully demonstrated. The results of applying this technique to bulk AlN crystals were reported. Etching produced hexagonal pits on the Al-polar (00▪1) plane, while hexagonal hillocks formed on the N face. On the basis of synchrotron white-beam X-ray topography, it was deduced that the etch pits for Al polarity formed mainly at dislocations. The optimum etching temperature for Al-polarity was 350 to 380C. This was typically 50 to 100C higher than that for N polarity; thus indicating the higher stability of Al-polarity. For Al-polarity AlN single crystals grown onto Si-face 6H-SiC (00▪1) substrates, the dislocation density was about 107/cm2. For self-seeded crystals, the dislocation densities for Al-polarity and the hillock densities for N-polarity were both of the order of 103/cm2. As far as the dislocation density was concerned, self-seeded crystals were of better quality than crystals grown onto Si-face 6H-SiC (00▪1) substrates.

Defect-Selective Etching of Bulk AlN Single Crystals in Molten KOH/NaOH Eutectic Alloy. D.Zhuang, J.H.Edgar, B.Strojek, J.Chaudhuri, Z.Rek: Journal of Crystal Growth, 2004, 262[1-4], 89-94