Real-Time Spectroscopic Ellipsometry Studies of Zinc Oxide Deposition
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1074-I07-11
Real-Time Spectroscopic Ellipsometry Studies of Zinc Oxide Deposition Christian C. Fesenmaier1, Xiaonan S. Li2, Bobby To2, and Dean H. Levi2 1 Electrical Engineering, Stanford University, Stanford, CA, 94305 2 National Renewable Energy Laboratory, Golden, CO, 80401 ABSTRACT Zinc oxide is increasingly being studied as an eventual replacement for indium tin oxide as a transparent conducting oxide (TCO) for thin film solar cells. In order to better understand the growth process of ZnO, as well as provide for accurate control on the National Renewable Energy Laboratory’s TCO deposition system, undoped RF-magnetron sputtered ZnO films on silicon were analyzed in situ using real-time spectroscopic ellipsometry (RTSE). A large wavelength range (245-994 nm) was measured in order to derive information about optical properties both below and above the bandgap. Atomic force microscopy (AFM) measurements were also taken to confirm ellipsometry surface roughness results and provide additional insight into the surface morphology. In light of observed poor fit of typical two-layer optical models to in situ ellipsometry data, we propose a model film structure consisting of two layers: a bulk ZnO layer of graded density that increases in thickness as the growth proceeds and a growth zone at the top of the film of approximately constant thickness with a slightly larger bandgap, lower density, and decreased excitonic absorption. Unfortunately, this model is insufficient to explain the evolution of the film during the early growth period, corresponding to film thicknesses less than 100 nm. Nevertheless, the presence of a growth zone for films above a certain thickness should provide some insight into the growth process of RF-magnetron sputtered ZnO. INTRODUCTION Transparent conducting oxides have become increasingly promising materials for a multitude of optoelectronic devices, particularly next-generation thin film solar cells. Of the materials demonstrating optical transparency in the visible wavelength range and high electrical conductivity, indium-doped tin oxide (ITO) has been the most widely used. Indium supply concerns in addition to sensitivity to process temperatures and plasma treatments have spurred research into replacements for ITO. Foremost among them is doped zinc oxide, which is composed of abundant elements, easy to produce on a large scale, and compatible with a variety of processes and mechanical demands. Zinc oxide is a II-VI semiconductor with a wide bandgap (~3.3 eV) that makes it highly transparent over the visible range. Doping with several elements, including aluminum and gallium, allow for control of the charge concentration and thus conductivity of the film. While crystalline ZnO forms a wurzite structure, research into the use of ZnO as the top contact for solar cells has focused on the potential for inexpensive large-area coatings of non-crystalline material. RF-magnetron sputtering has proven to be a useful and scalable tool for depositing ZnO films for this purpose.
Sputter deposition of Zn
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