Theoretical Study of the Electronic Structure of Threading Edge Dislocations in GaN

Abstract

Although silicon is the material mainly used for power devices, we are now approaching the performance limits of power devices based on silicon. Moreover, it is difficult to improve their performance due to the intrinsic physical properties of silicon. Gallium nitride (GaN), on the other hand, is attracting a lot of attention as an alternative material for future power devices. The bandgap of GaN is three times as wide as that of silicon, and GaN has some superior features, such as its high thermal conductively, large dielectric breakdown field, and high saturation velocity for the electrons. Therefore GaN is expected to become a key material in the next generation of power devices, enabling higher voltages and higher speed operation than current devices. Moreover, the use of GaN will allow miniaturization of the devices and higher temperature operation. During the crystal growth of GaN, however, a high density of threading dislocations appears due to the lattice mismatch with the sapphire (Al2O3), silicon carbide (SiC). Moreover, it has been reported that threading dislocations degrade the performance of GaN-based electronic devices fabricated even on GaN substrates. There have been some experimental studies in which screw or mixed dislocations have been reported as being the primary sources for the leakage current in GaN-based devices [1], whereas threading edge dislocations were found not to contribute to the leakage current [2]. Therefore, we need to fabricate GaN-based power devices to examine the electronic behavior at threading dislocations in GaN and to clarify the origin of the leakage current. As for theoretical studies, the electronic structure of threading dislocations in GaN is not fully understood, although Akiyama et al. reported the electronic structures of threading dislocations by determining the atomistic structures by empirical inter-atomic potential calculations [3]. The purpose of this theoretical study is to discover whether threading edge dislocations contribute to the leakage current or not. To do this, we used first principles calculations based on density functional theory (DFT) to examine the electronic structure at threading edge dislocations with Burghers vectors of 1/3[11-20]. 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). Figure 1