Wurtzite-type epilayers with Ga- and N-polarity were grown by reactive molecular beam epitaxy, using a plasma-activated N source, onto c-plane sapphire. The polarities were confirmed by convergent-beam electron diffraction. High-resolution X-ray diffraction, atomic force microscopy and transmission electron microscopy were used to characterize structural defects in the films. The various topographic features of Ga- and N-polarity samples, and their appearance after wet etching, were correlated with measured X-ray rocking curve peak widths for both symmetrical [00•2] and asymmetrical [10•4] diffraction. In the case of Ga-polarity samples, the [00•2] diffraction was narrower than the [10•4] diffraction. In N-polarity samples, the [00•2] peaks were broader than [10•4]. The half-widths of [10•4] peaks for both polarity types were in the range of 5 to 7arcmin. This indicated the existence of a high density of pure edge threading dislocations, as well as perhaps other defects, lying parallel to the c-axis. The 1 to 2arcmin [00•2] linewidths of Ga-polarity samples suggested the presence of a low density of screw dislocations, corresponding to transmission electron microscopic observations in which the screw dislocation density was less than 107/cm2. In N-polarity samples, the [00•2] diffraction peak was typically wider than 5arcmin; thus suggesting either a higher density of edge dislocations and inversion domains in N-polarity samples, or columnar structural features in atomic force microscopic images, where the effective coherence length for X-ray diffraction was sharply reduced.

A Comparative Study of MBE-Grown GaN Films Having Predominantly Ga- or N-Polarity. F.Yun, D.Huang, M.A.Reshchikov, T.King, A.A.Baski, C.W.Litton, J.Jasinski, Z.Liliental-Weber, P.Visconti, H.Morkoç: Physica Status Solidi B, 2001, 228[2], 543-7