In order to investigate the electronic states of threading edge dislocations in GaN, it was necessary to prepare an initial atomic structure for the dislocations. Our initial atomic structure, which has a supercell containing about 200 atoms, is shown in Fig. 1. Fig. 1(a) and Fig. 1(b) show this model in the [0001] and [1-210] directions, respectively. The model has a periodic boundary condition in the [0001] direction, parallel to the dislocation. The surface of the model shown Fig. 1(a) is terminated by fictitious hydrogen atoms [4] and the model is surrounded by a vacuum region. The shape of the model is like a square pillar. First, the model structure was optimized. It is known that several types of core configurations are proposed for 1/3[11-20] threading dislocations in GaN. In this study we focused on an 8-atom ring core configuration, a symmetric 4-atom ring core configuration, and 5/7-atom ring core configurations. The electronic structures for these relaxed atomic models were calculated.
Structural optimization and electronic state calculations were performed using VASP (Vienna Ab-initio Simulation Package) [5], which is a first principles calculation code based on DFT.
The electronic densities of states of several core configurations are shown in Fig. 2. Fig. 2(a), Fig. 2(b), Fig. 2(c) and Fig. 2(d) show the electronic densities of states for bulk wurtzite GaN, an 8-atoms ring core, a 5/7-atoms ring core, and a symmetric 4-atoms ring core, respectively. In Fig. 2, the Fermi level is shown by the black lines at 0 eV, and the bandgaps are shown by the shaded blue or green parts. It is known that undoped GaN generally has n-type semiconductor characteristics. Therefore, focusing on the blue parts above the Fermi level, the bandgap of every core configuration is similar to that of bulk GaN. In other words, no defect levels can be seen near the bottom of the conduction band. These results demonstrate that threading edge dislocations in gallium nitride do not contribute to leakage current in GaN-based devices.
Moreover, we also discuss the atomic and electronic structures using larger scale systems.
Acknowledgement
This work was conducted as part of a project entrusted with “research and development for next generation of power devices that contributes to the realization of an energy saving society” (MEXT; Ministry of Education, Culture, Sports, Science and Technology).