Optical Study of SiO 2 /nanocrystalline-Si Multilayers Using Ellipsometry
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Optical Study of SiO2/nanocrystalline-Si Multilayers Using Ellipsometry Kang-Joo Lee1, Tae-Dong Kang1, Hosun Lee 1,a, Seung Hui Hong1, Suk-Ho Choi1, Kyung Joong Kim2, and Dae Won Moon2 1
Department of Physics and Institute of Natural Sciences, Kyung Hee University, Suwon 449701, Korea 2 Nano Surface Group, Korea Research Institute of Standards and Science, P.O.Box 102, Yusong, Taejon 305-600, Korea ABSTRACT Using variable-angle spectroscopic ellipsometry, we measure the pseudo-dielectric functions of as-deposited and annealed SiO2/SiOx multilayers (MLs). The SiO2(2nm)/SiOx(2nm) MLs have been prepared under various deposition temperature by ion beam sputtering. The annealing at temperatures ≥ 1100°C leads to the formation of Si nanocrystals (nc-Si) in the SiOx layer of MLs. Transmission electron microscopy images clearly demonstrate the existence of nc-Si. We assume a Tauc-Lorentzian lineshape for the dielectric function of nc-Si, and use an effective medium approximation for SiO2/nc-Si MLs as a mixture of nc-Si and SiO2. We successfully estimate the dielectric function of nc-Si and its volume fraction. We find that the volume fraction of nc-Si decreases after annealing, with increasing x in as-deposited SiOx layer. This result is compared to expected nc-Si volume fraction, which was estimated from stoichiometry of SiOx. INTRODUCTION In order to realize light-emitting-diode using silicon-based materials, intensive investigations on nanocrystalline silicon (nc-Si) have been carried out. Due to the confinement of electron and hole carriers in nano-scale volumes, we expect enhanced luminescence efficiency due to increasing recombination rate of carriers as well as the visible luminescence arising from quantum confinement effect [1]. Notably, superlattices composed of alternating nc-Si and SiO2 layers have been given much attention due to a large volume fraction of nc-Si and the controllability of the size and density of nc-Si crystallites [2-3]. One promising method of nc-Si/SiO2 MLs relies on the growth of SiOx/SiO2 multilayers (MLs) and subsequent heat treatment [4]. We used ion beam sputtering deposition (IBSD) method to grow SiOx/SiO2 MLs. IBSD has the advantage of low operational pressure of the ion sources and the precise control of the ion beam parameters, compared to plasma-based rf sputtering techniques. Defect densities are relatively low because a neutralized ion beam is used for sputtering and a substrate is not immersed in the plasma [5,6]. In order to optimize nc-Si/SiO2 MLs for optoelectronic devices, we need various structural and optical characterization methods [2,3,5,7]. Using spectroscopic ellipsmetry, we can characterize the structural and optical properties at the same time. In detail, we can estimate the thickness of the nc-Si and SiO2 layers and the volume fraction of nc-Si, as well as the dielectric function of nc-Si. Recently, several researchers successfully adopted Tauc-Lorentz (TL) model for the dielectric function of nc-Si of the diameters of 1 to 3 nm [8,9]. a)
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