Simulations of Defect-Interface Interactions in GaN
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INTRODUCTION Vacancy and interstitial native defects are known to have a major influence on the electrical and optical properties of GaN [1]. For example, the N vacancy acts as single donor, the Ga vacancy as an acceptor and the interstitials act as amphoteric defects. The formation energies of native defects in bulk GaN have been calculated using first principles methods and their values are quite well established [2,3]. However, these formation energies will change in the neighbourhood of extended planar defects such as stacking mismatch boundaries and inversion domain boundaries. These boundaries are commonly observed in epitaxially grown GaN but so far the interaction between such interfaces and native point defects in GaN has not been investigated and it remains unclear whether the formation of interstitials and vacancies near the boundaries is encouraged or discouraged. The calculation of defect-interface interactions requires computational cells containing several hundred atoms so as to avoid unwanted intercellular interactions when periodic boundary conditions are applied. This makes a first principles approach difficult and therefore we have used, in the first instance, a classical methodology which employs interatomic pair potentials that have been fitted to reproduce various bulk properties of GaN. This classical model is used to calculate the binding energy of Ga and N vacancies and interstitials to three commonly observed interfaces: the (10 1 0) stacking mismatch boundary (SMB), the (10 1 0) inversion domain boundary (IDB) and the (11 2 0) IDB. The atomic structures of these boundaries have been determined from transmission electron microscope observations [4-6] and are shown in figure 1. One particular point of interest has been the observation of two atomic structures for the (10 1 0) IDB [7]. The first boundary structure involves an inversion of the atomic species across the boundary. For the second boundary structure, referred to as IDB*, there is an additional translation of c/2 along the [0001]. First principles density functional calculations carried out by
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Northrup at al [8] have shown that this second structure, which contains no dangling bonds, has the lower formation energy and we therefore focus on this structure in the present work. Analysis of the electronic structure also revealed that IDB* does not introduce any interface states in the forbidden gap but that the (10 1 0) SMB introduces an occupied state 1.1 eV above the valence band maximum.
Figure 1. Observed boundary structures of (a) the (10 1 0) IDB*, (b) the (11 2 0) IDB and (c) the (10 1 0) SMB. Interstitial defects are placed in positions 1-4 and vacancy defects are placed in positions a-d. Dashed line indicates the location of the boundary.
METHODOLOGY
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The computational methodology and pair potential scheme are well established (for a review see Harding [9]) and therefore only a summary is given here. The long range electrostatic energy is evaluated through the Ewald summation [10]. The interaction bet
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