Extended Defects in GaN: a Theoretical Study
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Abstract We present density-functional theory studies for a variety of surfaces and extended defects in GaN. According to previous theoretical studies1 {101O} type surfaces are electrically inactive. They play an important role in GaN since similar configurations occur at open-core screw dislocations and nanopipes as well as at the core of threading edge dislocations. Domain boundaries are found to consist of four-fold coordinated atoms and are also found to be electrically inactive. Thus, except for full-core screw dislocations which possess heavily strained bonds all investigated extended defects do not induce deep states into the band-gap. However, electrically active impurities in particular gallium vacancies and oxygen related defect complexes are found to be trapped at the stress field of the extended defects. 1. Introduction GaN has recently been the subject of considerable interest due to its optoelectronic properties. In particular the wide band gap (3.4 eV for wurtzite GaN) makes blue light applications feasible. Defect-induced electronic states in the band gap can significantly alter the optical performance. This fact becomes extremely important in laser devices, where parasitic components in the emission spectrum are highly undesirable. Moreover, point defects could be trapped in the stress field of extended defects giving rise to charge accumulated in the vicinity. The resulting electrostatic field leads to electron scattering which will severely affect the electron mobility (see Look and Sizelove2 for a recent model). Therefore, there is considerable interest in understanding the microstructure of extended defects in GaN and their interaction with point defects. In this paper we present the geometries, energetics and electrical properties of extended defects in GaN using an ab initio local density-functional (LDF) cluster method, AIMPRO, and a self-consistent charge density-functional tight-binding method SCC-DFTB. The latter can be used in large supercells and enables the formation energy of the defects to be found. Details of the methods and their application to GaN and oxygen related defect complexes in GaN have been given previously 3'- and will not be repeated here.
G 3.29 Mat. Res. Soc. Symp. Proc. Vol. 537 ©1999 Materials Research Society
2. Threading Dislocations in GaN A. Threading Screw Dislocations We consider first a screw dislocation with a full core'. Full core screw dislocations have 6 recently been observed by Xin et al. using the high resolution Z-contrast imaging technique . The presence of atoms so close to the dislocation axis leads to severely strained bond lengths distorted by as much as 0.4 A. Consequently it is not surprising that such dislocations possess deep gap states ranging from E, + 0.9 to E, + 1.6 eV and shallow states around E, - 0.2 eV. An analysis of these gap states revealed that the states above E, are localised on N core atoms, whereas those below E. are localised on both Ga and N core atoms. The strong distortion of the bonds of the core atoms leads to a high li
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