Effect of GaN Surface Treatment on the Morphological and Optoelectronic Response of Violet Light Emitting Diodes
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Effect of GaN Surface Treatment on the Morphological and Optoelectronic Response of Violet Light Emitting Diodes Muhammad Jamil, James R. Grandusky and Fatemeh Shahedipour-Sandvik College of Nanoscale Science and Engineering, University at Albany-State University of New York, 255 Fuller Rd, Albany, NY 12203, U.S.A. ABSTRACT We report on the study of the effect of various surface chemical treatment processes of nGaN template layers used for subsequent growth of light emitting diode (LED) structures. The treatment procedure included cleaning in organic solvents, organic solvents followed by 5 minutes of HCl, organic solvents and 5 minutes of HCl followed by 2 minutes and finally 10 minutes of HF treatment. Chemical, optical and electrical properties of the surfaces of GaN and InGaN-based LED structures were systematically investigated by x-ray photoemission spectroscopy (XPS), auger electron spectroscopy (AES), atomic force microscopy (AFM), photoluminescence (PL) and electroluminescence (EL) spectroscopy. GaN layers that were grown on the samples treated with HCl and HF showed dramatically different surfaces having high density of 3D structures with high roughness. As measured by AFM, growth of the LED structure on top of the GaN layer continued the 3D-growth mode. LED structures grown on the HCl and HF treated GaN template layers showed minimal to no PL and EL emission and failed after a short period. We suggest a qualitative model of the growth that could potentially explain the underlying phenomena leading to such pronounced changes in the optoelectronic properties and surface conditions of the LED structures due to the treatment of the initial template layers. INTRODUCTION The potential of III-Nitride based materials for the solid state lighting market, high power, high frequency devices, and chemical and biological sensors have made them so attractive that in less than ten years we have witnessed commercial availability of devices with high performance despite the high level of defects they possess. In order to better control the performance of these devices, there is a great need to understand the interaction of GaN layers with its environment. A clean surface is necessary for growth and subsequent processing of III-Nitride materials including good contact formation [1-3]. Treatment of GaN surfaces with HF and HCl solutions followed by in-situ desorption were found to have lowest coverage of oxygen and carbon contaminants with complete thermal desorption of all contaminants achieved at temperatures ~900°C in vacuum [4]. King and coworkers [5] also reported that ex-situ treatment of the GaN surface with HCl and HF based solutions produced surfaces with the lowest levels of oxygen but significant concentrations of Cl and F remained. However, the issue that has not received much attention in the past is how these residues behave in a typical MOCVD growth chamber in the presence of NH3 and hydrogen. The effect that these residues have on the homoepitaxy of GaN layers as well as device structures is of great
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