Direct imaging of reverse-bias leakage through pure screw dislocations in GaN films grown by molecular beam epitaxy on GaN templates

Pure screw dislocations are found to be the origin of excess reverse bias leakage in GaN films grown by molecular beam epitaxy on GaN templates. A scanning current-voltage microscope (SIVM) was used to map the spatial locations of leakage current on high quality GaN films under reverse bias. Two samples with similar total dislocation density (~$10^9 cm^{-2}$) but with pure screw dislocation density differing by an order of magnitude were compared. We found that the density of reverse bias leakage spots correlates well with pure screw dislocation density, not with mixed dislocation density. Thus, pure screw dislocations have a far more detrimental impact on gate leakage than edge or mixed dislocations.