Reduction of Threading Dislocation Density in AlGaN by Indium Incorporation
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Reduction of Threading Dislocation Density in AlGaN by Indium Incorporation H. Kang, Z.C. Feng and I. Ferguson* School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA 30332 * [email protected] S.P. Guo and M. Pophristic EMCORE Corporation, 145 Belmont Drive, Somerset, NJ 08873, U.S.A ABSTRACT The addition of indium, even to small concentrations, to AlGaN has resulted in improved optical and doping properties for these materials. This paper is the first report of improved structural properties for indium containing AlGaN layers. A systematic series of the AlGaN layers with nominal concentration of 20% aluminum were grown by metal-organic chemical vapor deposition with traces amounts of indium incorporated into the layers (up to 0.15% indium). X-ray diffraction analysis of the layers was completed using Williamson Hall plots and reciprocal space mapping to investigate any change in the columnar structure of the initial AlGaN layers. It was found that the threading dislocation densities and lateral coherence length showed a systematic variation with indium incorporation. The threading dislocation density is lowered as indium composition increased with a corresponding increase in lateral coherence length. This indicates that even the incorporation of trace amounts of indium improves the structural properties of these epilayers. INTRODUCTION AlGaN layers grown by Metalorganic Chemical Vapor Deposition (MOCVD) with high aluminum composition are forming the basis of new ultraviolet (UV) optoelectronic devices. These devices have many applications including threat warning and biological agent detection. However, these devices are typically grown on sapphire and this results in a high threading dislocation (TD) density which degrades their quantum efficiency [1-3]. Therefore it is crucial to reduce the TD density in these AlGaN epilayers in order to develop UV sources with high quantum efficiency. Recently, InAlGaN has attracted much attention since it is possible to separately selecting bandgap energy and lattice mismatch when optimizing device structures [49]. However, there have also been reports that even the incorporation of small concentrations of indium in AlGaN results in improved optical and doping properties. The focus of this work is to complete a systematic study of the structural properties of AlGaN with increasing indium concentration. X-Ray Diffraction (XRD) measurements were used to investigate the densities of screw (Nscrew) and edge (Nedge) dislocations in these layers. Two techniques, viz., WilliamsonHall (W-H) plot and Reciprocal Space Mapping (RSM), were employed to distinguish and determine these dislocations in the epilayers. These techniques do not give the absolute measure of dislocation densities that transmission electron microscopy (TEM) but do provide a
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systematic method of rapidly comparing layers. The W-H plot shows a systematic decrease in dislocation density and corresponding increase in lateral coherence length for the AlGaN epilayer
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