Dislocation Mobilities in GaN from Molecular Dynamics Simulations
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Dislocation Mobilities in GaN from Molecular Dynamics Simulations N. Scott Weingarten1 1 U.S. Army Research Laboratory, Building 4600, Aberdeen Proving Ground, MD 21005, U.S.A. ABSTRACT The results of molecular dynamics (MD) simulations of dislocation glide in GaN using a Tersoff potential are presented. The simulation methodology involves applying a constant shear stress to a single crystal system containing an individual dislocation, with multiple slip systems considered. Upon reaching a steady state, the dislocation velocity as a function of applied stress and temperature are determined. Edge dislocations with a-type Burgers vectors in the basal, prismatic and pyramidal planes have been analyzed over the temperature range of 300-1300K. The results from simulations of c-type edge dislocations at 1300 K are also presented. INTRODUCTION Gallium nitride (GaN) is a semiconductor with band gap properties that make it an excellent material for use in optoelectronic devices, such as light-emitting diodes (LEDs) and high electron mobility transistors (HEMT)1. Growing GaN on a non-native substrate results in a large number of threading dislocations, reducing the performance capability2. Experimental and theoretical efforts have been aimed at reducing the number of dislocations3,4,5 since methods available for cubic crystals do not work for GaN6. The use of GaN substrates significantly reduces dislocation densities7, and dislocation-free nanomembranes of GaN have been exfoliated for possible subsequent use as substrates8, although this research is in its early stages. Furthermore, dislocations may be necessary for building devices that include, for example, an AlxGa1-xN layer on top of the GaN9. Threading dislocations in GaN therefore remain of interest to the community10. Dislocation dynamics (DD) simulations are a mesoscale modeling technique in which a system is modeled through dislocation movements and interactions in the material. These simulations can be useful in understanding how dislocation motion can be manipulated to reduce the number of dislocations in GaN11. Dislocation mobility functions are required to run such simulations, information that is typically gleaned from classical molecular dynamics (MD) simulations. In this paper, we present results of dislocation glide in GaN via MD simulations. The velocity of individual a-type edge dislocations is determined as a function of temperature and stress for three slip systems: basal, prismatic, and pyramidal. Edge dislocations with a c-type Burgers vector are also considered, including a discussion of the core structure of these dislocations. THEORY The methodology for achieving dislocation glide in classical MD simulations has been described in more detail elsewhere12. A crystal with a single dislocation is generated such that the Burgers vector points in the x-direction, and the normal to the slip plane is in the z-direction. An additional force is applied to atoms in a small region near the top and bottom (in the z-
direction) of the system; the force is parall
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