The origin of nanopipes and micropipes in non-cubic GaN and SiC

Micro/nanopipes are linear defects along the c-axis of hexagonal polytypes of SiC and GaN that are currently the focus of much attention. It has been shown that these defects can be very detrimental to the electronic properties of devices manufactured from, at least, 6H-SiC. In this paper, the origin of these defects is discussed in terms of Frank's theory [1] that dislocations will have a hollow core when their Burgers vector is large. Two fundamental issues about such dislocations are addressed: their formation along the c-axis of the crystal, and their stability despite their large Burgers vectors [2]. The proposed model is based on the mosaic structure of sublimation-grown 6H- or 4H-SiC, and VPE-grown 2H-GaN on sapphire substrates. The presence of unit c-axis screw dislocations is attributed to the accommodation of low-angle twist boundaries in the mosaic structure. The formation of superscrew dislocations with large Burgers vector, which empty their cores to reduce the excessive strain energy there, is shown to be the result of 3c screw dislocations in the axis of triple junctions which "getter" the neighboring unit dislocations and simultaneously increase their diameter. The predictions of the model are compared with available data in the literature, and suggestions are made for the decrease of nano/micropipe density.