Doping Dependence of Chlorine Incorporation in SiCl 4 -based Microcrystalline Silicon Films
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Doping Dependence of Chlorine Incorporation in SiCl4-based Microcrystalline Silicon Films Wolfhard Beyer, Reinhard Carius and Uwe Zastrow Institut für Photovoltaik, Forschungszentrum Jülich, D-52425 Jülich, Germany ABSTRACT For SiCl4-based microcrystalline silicon films the doping dependence of chlorine and hydrogen incorporation was studied. The results reveal a Fermi level dependence with a maximum chlorine (and hydrogen) incorporation for a Fermi level somewhat above midgap. As an explanation, a Fermi level dependence of the chlorine release rate during film growth is considered, similar as valid for hydrogen diffusion and desorption. INTRODUCTION Microcrystalline silicon (µc-Si) is of interest for application in thin film solar cells and other thin film devices. For its preparation, usually plasma enhanced chemical vapor deposition (PECVD) is applied with silane and hydrogen mixtures as precursor gases. The replacement of silane by the less dangerous gas SiCl4 (silicon tetrachloride) was recently explored and rather high conductivities of up to 300 (Ωcm)-1 were achieved for boron-doped material [1]. Such boron-doped films were found to have rather low chlorine and hydrogen concentrations of 2-3 at.% at a substrate temperature TS =250°C. On the other hand, phosphorus-doped and undoped µc-Si films showed (at the same TS) much higher chlorine and hydrogen content with chlorine concentrations ranging from 5 to 10 at.% [1]. Thus, the variation of rather small boron and phosphorus concentrations seems to control the incorporation of much larger amounts of Cl and H. Aiming to clarify the nature of these doping effects, we prepared and studied a series of SiCl4based compensated microcrystalline silicon films doped with both boron and phosphorus. EXPERIMENTAL The microcrystalline silicon films were prepared by plasma deposition (PECVD) employing the same regime as applied previously [1], namely a high dilution of SiCl4 in hydrogen, a rather high pressure of 4 mbar and a high rf (13.56 MHz) power of typically 60 W (~1W/cm2). A hydrogen flow of 100 to 200 sccm and a flow of SiCl4 of 1 to 8 sccm were used to obtain silicon fractions in the gas phase of SF = 1 to 5 %. For doping, flows of diborane (B2H6) and phosphine (PH3) were added. Typical substrate temperature was TS = 250°C. As substrates, crystalline silicon and quartz were used. For structural characterization, Raman measurements were employed (using the 488 nm line of an argon laser).The chemical composition was analyzed by secondary ion mass spectrometry (SIMS) and infrared (IR) absorption measurements. For electrical characterization, dark conductivity (using coplanar geometry) as well as thermoelectric power were used. RESULTS AND DISCUSSION Results of singly doped SiCl4-based microcrystalline Si films are shown in Fig. 1 for fixed silicon (SiCl4) gas phase fraction of SF = 3 % [1]. Plotted in Fig. 1a is the doping dependence of
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Figure 1. Room temperature conductivity σ and crystalline fraction X (Fig. 1a) and concentration c (Fig. 1b) of bond
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