Growth and characterisation of GaN with reduced dislocation density

Two GaN MOVPE growth methods to reduce the threading dislocation (TD) density have been explored. The combined effects of (1) in situ SiNx masking of the sapphire substrate and (2) starting the epitaxial growth at low V-to-Ill ratio on the GaN film quality were studied by atomic force microscopy, transmission electron microscopy and high-resolution X-ray diffraction. It was found that the annealing condition of the low-temperature nucleation layer after in situ SiNx masking is critical in order to decrease the density of nucleation sites and hence increase the average grain size to about 5 mum. However, the coalescence of large grains with vertical side facets results in the formation of dense bundles of TDs at the grain boundaries combined with large numbers of basal-plane dislocation loops throughout the film. The formation of these dislocations can be prevented by starting the epilayer growth at low V-to-III ratio, resulting in the formation of grains with inclined side facets. The interaction of the TDs with the inclined side facets causes the dislocations to bend 90degrees as the grains grow in size and coalesce. GaN films with dislocation densities as low as 1 x 10(8) cm(-2), giving full-width at half-maximum values of 180 and 220 arcsec for respectively (002) and (302) omega scans, were achieved by the combination of in situ masking and low V-III ratio epilayer growth. Hall carrier mobility values in excess of 900 cm(2) V-1 s(-1) were deduced for Si-doped layers. (C) 2004 Elsevier Ltd. All rights reserved.