A study of dislocations in AlN and GaN films grown on sapphire substrates

The dislocations in epitaxial AlN film directly grown on (0 0 0 1) sapphire have been investigated by transmission electron microscope and X-ray diffraction (XRD), including direct comparison with conventional GaN film grown on sapphire substrates. Unlike GaN, the threading dislocations (TDs) of AlN are almost entirely of pure-edge character, and the screw dislocation density is much less than that of GaN film, leading to a very narrow full-width-at-half-maximum of ∼70 arcsec in (0 0 0 2) XRD of AlN film. The origin of screw TDs in GaN is found to be primarily related to the highly faulted layer at the interface region, which is introduced by using a low temperature (LT) nucleation layer. The high temperature AlN growth without any LT initial layer leads to the dramatic reduction of screw dislocations in AlN. Simultaneously, the AlN growth is dominated by two-dimensional step-flow mode while the island growth that occurs for GaN film on sapphire is suppressed. Moreover, we obtained a very low dark current density of 1.1×10−9 A/cm2 at a 5 V bias on AlN metal-semiconductor-metal diodes, which indicates a lower defect density and good material quality of our AlN thin film.