Reduction of threading dislocations in GaN grown on 'c' plane sapphire by MOVPE

The reduction of threading dislocations (TDs) in GaN is very important as TDs act as nonradiative recombination centres and generally reduce the luminescence efficiency of GaN-based optoelectronic devices, particularly UV emitting devices. In order to reduce the density of TDs, we have used an in-situ ELO technique by depositing a non-uniform SiNx mask (as revealed by STEM-EDX) using simultaneous flow of silane and ammonia gas, either on a sapphire substrate or on a GaN pseudo-substrate. The effect of several growth parameters, such as V/III ratio, duration of Si/N treatment, pressure and temperature, on the morphology, structural quality and reduction mechanisms of TDs in GaN have been investigated and the effects of the first two parameters are reported here. To start the epilayer growth with a low V/III ratio gives rise to predominantly 3D growth, forming islands with multiple inclined facets and a vanishing top (0001) facet. A high V/III ratio enhances the lateral growth, and islands with inclined {-2112} facets together with wide flat top (0001) facets are formed. The duration of the Si/N treatment and the low V/III ratio growth regime are optimised in order to reduce the TD density down to 8x10(7) cm(-2), as measured from plan-view TEM images. It is observed that TDs bend through 90 degrees at inclined side facets. The role of atomic ledges moving across inclined facets which are responsible for bending the line direction of the TDs is briefly discussed.