Comparative investigation of high resolution transmission electron microscopy and Fourier transform infrared spectroscopy for GaN films on sapphire substrate

A comparative study on GaN/sapphire has been performed by transmission electron microscopy (TEM) and infrared reflectance (IR). TEM observations reveal that both the undoped and Si doped GaN epilayers have large density of threading dislocations. Dislocations in the undoped GaN tend to form open core structure, while dislocation lines in the Si-doped GaN are very sharp and the strain contrast is much more confined. It is believed that Si-doping causes the increase in surface energy, resulting in the change of dislocation configurations from open core structures in undoped GaN to much more confined dislocation lines. Frank dislocation loops are also found lined up at a depth of about 110±10 nm from the interface. High resolution TEM study also reveals that the GaN buffer layer grown at low temperature has transformed into its thermodynamically stable wurtzite structure during the high temperature post-buffer GaN epilayer growth process. The comparative infrared reflectance shows the corresponding behaviour. The interference fringes of the Si doped sample, compared with the undoped one, shows a contrast damping and reflectance reduction behavior, suggesting the presence of a transition/defect layer near the interface.