Thickness-Dependant Electrical Characteristics of Nitrogen-Doped Polycrystalline 3C-SiC Thin Films Deposited by LPCVD
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Thickness-Dependant Electrical Characteristics of Nitrogen-Doped Polycrystalline 3C-SiC Thin Films Deposited by LPCVD Man I Lei1, Te-Hao Lee1 and Mehran Mehregany2 1 Department of Materials Science and Engineering, 2 Department of Electrical Engineering and Computer Science, Case Western Reserve University, Cleveland, OH 44106, U.S.A. ABSTRACT The effect of film thickness on the electrical resistivity of heavily-nitrogen-doped polycrystalline SiC (poly-SiC) thin films is investigated. The resistivity of poly-SiC thin films decreases by a factor of ~7 for thickness increasing from 100 nm-thick to 1.78 ȝm-thick; the resistivity begins to stabilize as thickness approaches 1 ȝm. The increased resistivity for the thinner films is shown to be related to the grain boundary effect. Secondary ion mass spectrometry indicates a nitrogen concentration of 9×1020 atoms/cm3 in the films. However, Hall measurements reveal that only 45% of the dopants are electrically active in the 100 nm-thick film. The percentage of active dopants rises to 80% when film thickness increases to 680 nm. From the studies of surface roughness and microstructure, it is seen that small grains are formed at the initial stage of deposition, which then grow into larger columnar grains as film thickness increases. The presence of a large density of grain boundaries and limited grain growth for the very thin films contribute to increased electrical resistivity from increased trapped mobile carriers and reduced carrier mobility. The free carrier trapping phenomenon can further be observed in the temperature-dependence of resistance measurement. INTRODUCTION Silicon carbide (SiC) is a high temperature semiconductor with excellent electrical, mechanical and chemical properties. It is therefore a promising material for realization of microand nano-electro-mechanical systems (i.e., MEMS and NEMS) that demand performance at high temperatures and in harsh environments [1, 2]. Polycrystalline 3C-SiC (poly-SiC) thin films are employed as key structural layers and temperature sensitive thermistors in MEMS devices [2, 3]. These films are also used to realize high-frequency nano-beam resonators and all-mechanical NEMS switches [4, 5]. These applications require high-quality poly-SiC films with controlled mechanical and electrical properties. Previous studies have focused on controlling the residual stress and doping concentration of poly-SiC films by manipulating gas flow, deposition pressure and temperature [6-8]. However, the effect of film thickness on electrical resistivity has not been studied. According to studies of polysilicon, the resistivity of the film is thickness-dependant, which is different than the constant resistivity measured from single-crystalline silicon films [9]. The effect of grain size plays an important role in resistivity of thin polysilicon films due to free carrier trapping and dopant segregation at the grain boundaries [9]. In this work, the effect of film thickness on the electrical resistivity of nitrogen-doped poly-SiC thin
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