Lawrence Berkeley National Laboratory

Thin film heteroepitaxy of polar materials such as GaN grown by MOCVD, MBE or HVPE Molecular Beam Epitaxy (MBE), and grown on SiC or Al2O3 is frequently hampered by the formation of structural defects - mostly dislocations, nanotubes and pinholes. The nanotubes and pinholes are empty areas either extending along the growth direction (tubes) or forming v-shape defects on {101?1} planes. In most cases they are associated with dislocations, but in some cases these defects are formed in dislocation-free areas. In this presentation it will be shown that the screw dislocations present in HVPE samples are decorated by pinholes arranged on top of each other ("bamboo" structure), but these defects are not found in MBE samples grown on the top of HVPE samples. These might suggest either different core structure of a screw dislocation in HVPE and MBE grown material or suggest a different purity of these two materials. By applying a direct reconstruction of the phase and amplitude of the scattered electron wave from a focal series of high resolution images, the core structures of screw dislocations in both materials have been studied in order to see if the core structure in HVPE template layer and the MBE overlayer is different. This will be discussed in detail during the presentation.