Screw Dislocations in MBE GaN Layers Grown on Top of HVPE Layers: Are They Different?
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Screw Dislocations in MBE GaN Layers Grown on Top of HVPE Layers: Are They Different? Z. Liliental-Weber, D. Zakharov, J. Jasinski, and J. Washburn, M.A. O’Keefe and H. Morkoca Lawrence Berkeley National Laboratory, Berkeley, CA 94720 m/s 62/203 a Virginia Commonwealth University, Richmond, VA ABSTRACT Transmission Electron Microscopy was applied to study HVPE template and MBE overlayers in plan-view and cross-section. It was observed that screw dislocations in the HVPE layers are decorated by small voids arranged along the screw axis. However, no voids were observed along screw dislocations in MBE overlayers grown with excess Ga, despite the fact that Ga droplets were observed on the layer surface as well as imbedded in the layer. By applying a direct reconstruction of the phase and amplitude of the scattered electron wave from a focal series of high-resolution images, the core structures of screw dislocations in both materials have been studied and show that all screw dislocations have filled cores. Dislocation cores in MBE samples grown Ga-rich and N-rich show no substantial differences and no stoichiometric change compared to the matrix. However, in HVPE materials, single atomic columns show substantial differences in intensities and indicate the possibility of Ga presence. These Ga-rich cores might be responsible for the attraction impurities forming voids in their close vicinity. INTRODUCTION Epitaxial growth of (0001) GaN on Al2O3 leads to high concentrations (typically 109-1011 cm-2) of threading edge, screw and mixed dislocations which propagate vertically from the GaN/Al2O3 interface to the GaN surface [1-2]. Despite this high density of dislocations, a high emitting efficiency has been achieved for optical devices such as light-emitting diodes (LEDs) and laser diodes (LDs) [3]. This behavior may be contrasted with that of GaAs-based LDs where a value of dislocation density of about 104 cm-2 is usually sufficient to prevent laser action [4]. The most common explanation for this phenomenon is that the threading dislocations in GaN do not have electronic states in the band gap. However, this is still a controversial issue, and there is no agreement on this subject between different scientific groups [5-8]. Density functional theoretical calculations [8] and early experimental work [4] suggested that threading dislocations in GaN are not non-radiative recombination centers. However, the increase in optical properties that is achieved [3] by reducing the threading dislocation density by use of the epitaxial lateral overgrowth method [9] provides circumstantial evidence that dislocations do have some deleterious effect. Atomic force microscopy (AFM) combined with cathodoluminescence (CL) [10], and plan-view transmission electron microscopy (TEM) combined with CL [11], clearly show threading dislocations to be related to dark spots in band edge emission CL images. This effect is consistent with theoretical calculations suggesting that dislocations can be charged depending on doping and growth conditions
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