Theory of Electron Energy Loss Spectroscopy and its Application to Threading Edge Dislocations in GaN
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Theory of Electron Energy Loss Spectroscopy and its Application to Threading Edge Dislocations in GaN C. J. Fall,1 R. Jones,1, ∗ P. R. Briddon,2 A. T. Blumenau,3 T. Frauenheim,3 and M. I. Heggie4 1 School
of Physics, University of Exeter, Exeter EX4 4QL, United Kingdom of Physics, University of Newcastle upon Tyne, Newcastle NE1 7RU, United Kingdom 3 Theoretische Physik, Universit¨ at Paderborn, D-33098 Paderborn, Germany 4 CPES, University of Sussex, Falmer, Brighton BN1 9QJ, United Kingdom 2 Department
ABSTRACT
The electronic structure of dislocations in GaN is controversial. Several experimental techniques such as carrier mobility studies and cathodoluminescence experiments have indicated that dislocations are charged while theoretical studies point to intrinsic states and/or point defect accumulation along the core as a source of electrical activity. Electron Energy Loss Spectroscopy (EELS) studies have the ability to probe the electronic structure of extended defects. Here we report first principles calculations of the EELS spectrum applied to edge dislocations in GaN. It is found that the electrostatic potential at N atoms in the vicinity of the dislocation varies by the order of a volt and casts doubt on any simple interpretation of core loss spectroscopy. On the other hand, low loss spectroscopy leads directly to detailed information about any gap states. The low loss spectrum obtained by the theory is in good agreement with recent experimental work and indicates that threading dislocations in p-type GaN possess acceptor levels in the upper half of the gap. INTRODUCTION
The atomic and electronic structures of dislocations in GaN is a topic of current high interest and is still controversial [1]. Nano-pipes associated with open-core threading screw dislocations have been imaged by transmission electron microscopy (TEM) [2, 3], but full-core structures have also been seen for edge, screw, and mixed dislocations using Z-contrast imaging [4]. The electrical and structural properties of screw dislocations have been found to depend sensitively on the growth conditions [5]. Full-core structures are expected to lead to gap states, which, if filled, would lead to dislocation line charging. Scanning capacitance spectroscopy results have suggested that GaN threading dislocations are negatively charged [6], as have electron holography measurements [7]. A recent ballistic electron emission microscopy study has however concluded that threading dislocations do not have a fixed negative charge, and are, if anything, positively charged at the surface [8]. Initial theoretical investigations of GaN screw and edge dislocations suggested that dislocation core reconstructions lead to a lack of deep gap states [9]. Subsequent calculations, however, have predicted deep levels associated with GaN edge dislocations, leading in particular to charge accumulation at dislocations [10, 11]. Furthermore, a recent theoretical study of GaN edge dislocations has shown evidence for empty gap-states associated with full-core GaN edge disloc
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