Mosaic structure and its influence on carrier mobility in undoped hexagonal GaN thin film

The different kinds of mosaic structure in a series of hexagonal GaN thin films, prepared by low pressure metal-organic chemical vapor deposition, were studied by high resolution X-ray diffraction analysis and transmission electron microscopy (TEM). The rocking curves of (0002), (101̄2), and (202̄1) planes were measured, and some calculation was carried out to distinguish two different mosaic structures: the in-plane twist and the out-of-plane tilt. The different threading dislocations were deduced from the corresponding mosaic structures. It is found that the carrier mobility is much more sensitive to the substrate normal tilt than to the in-plane twist of the grain. Dislocations with different Burgers vectors are suggested to exert different influence on carrier mobility of the GaN film. Most of the dislocations in the film are in-plane twists, with a lesser amount of out-of-plane tilts, which strongly influences the mobility. The result may help explain the fact that the density of the dislocations in the GaN thin films is very high, but the films still show a high efficiency.