MOVPE growth of AlN layers on bulk AlN substrates with low threading dislocation density (< 10(5) cm(-2)) can result in enhanced defect formation. Chemo-mechanical polishing (CMP) of bulk AlN using colloidal silica sol can produce locally disturbed surface with SiOx residuals. These surface disturbances lead to generation of threading dislocations (similar to 10(8) cm(-2)) in homoepitaxially grown AlN layers. These dislocations show a tendency to form nano- and even micropipes, which may be associated with oxygen accumulation along the dislocation lines. As result, the subsequently grown AlGaN-based layer structures exhibit a high number of v-pits and micropipes. Inductively coupled plasma etching of AlN substrate surface prior to MOVPE growth results in clean AlN surfaces and improves the AlGaN layer quality.