Effects of Si-doping on the Microstructure of AlGaN/GaN Multiple-quantum-well
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Effects of Si-doping on the Microstructure of AlGaN/GaN Multiple-quantum-well R. Liu and F. A. Ponce Department of Physics and Astronomy, Arizona State University, Tempe, AZ 85287 S-L. Sahonta and D. Cherns H. H. Wills Physics Laboratory, University of Bristol, Bristol BS8 1TL, UK H. Amano and I. Akasaki Department of Materials Science and Engineering, Meijo University, Nagoya 468, Japan ABSTRACT The effects of silicon-doping on the microstructure of Al0.07Ga0.93N/GaN multiplequantum-well (MQW) have been studied by TEM. Significant changes of surface morphology and dislocation core structures have been observed due to Si-doping in the Al0.07Ga0.93N barriers. Threading dislocations create surface pits in the MQW as a result of Si doping. With an increasing doping level, the pits change the shape from small faceted pyramid to large cone. The formation mechanism of the surface pits has been discussed from both dynamics and kinetics points of view. We have also observed nanopipes constrict to form closed core screw dislocations in the MQW due to Si-doping. INTRODUCTION Applications of UV-LEDs using GaN active regions have been obstructed by the weak luminescence efficiency [1]. In comparison to InGaN quantum well (QW), GaN QW appears to suffer more from the non-radiative recombination nature of threading dislocations and the quantum-confined Stark effect caused by the internal field [1]. This is simply because GaN QW is free of alloy fluctuation, which is believed to be essential for a high radiative recombination rate in InGaN QW [1]. For further improvement of the luminescence efficiency, Si doping in barrier layers is promising, because the doping design is expected to improve radiative efficiency by screening the internal field that separates the electron-hole wave function [2]. Si doping in InGaN barrier layers in InGaN/InGaN MQW has been found to improve the optical gain of the laser diodes [3]. A study of the optical properties of Si-doped AlGaN/GaN MQW has been reported earlier [2]. In the present work, the doping effect on the microstructure is studied by TEM. EXPERIMENTAL The studied samples are a set of three Al0.07Ga0.93N/GaN MQW structures grown on GaN by metal-organic vapor phase epitaxy. Detailed growth parameters were reported earlier [4]. The MQW consists of 5 periods of AlGaN/GaN structure. The difference between the samples is the Si doping level in the AlGaN barriers. The first sample is nominally undoped. And the other two have very high doping levels, which are reflected by their high carrier concentrations: about 4x1019 cm-3 and 9x1019 cm-3 respectively. In comparison, the background carrier concentration for the nominally undoped one is about 1x1016 cm-3. The microstructure of these samples has been characterized by TEM in
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cross-section and plan-view. The microscopes used in this study are JEOL 4000FX operated at 400kV and Philip CM200 FEG operated at 200kV. The foil specimens were prepared by mechanical polishing followed by Ar+ ion milling. RESULTS AND DISCUSSION Surface pit
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