Structural and Electronic Properties of SiCl 4 -based Microcrystalline Silicon Films
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Structural and Electronic Properties of SiCl4-based Microcrystalline Silicon Films Wolfhard Beyer, Reinhard Carius, Michael Lejeune and Uwe Zastrow Institut für Photovoltaik, Forschungszentrum Jülich, D-52425 Jülich, Germany ABSTRACT Structural and electronic properties of SiCl4-based microcrystalline silicon films were studied. A rather dense (non-porous) material structure is obtained near the transition to amorphous material, in particular at substrate temperatures of 250°C and above. Boron doping results in very high conductivity values while for phosphorus doping only lower values are reached. This latter effect is attributed to a different microstructure with lower crystalline fraction, higher hydrogen and chlorine content and increased porosity in highly phosphorusdoped material. INTRODUCTION Microcrystalline silicon (µc-Si:H) is of interest for application in thin film solar cells and other thin film devices. For its deposition, silane (SiH4) is commonly used as a precursor gas. In order to explore the replacement of this dangerous substance by the less dangerous and less expensive silicon tetrachloride (SiCl4), we investigated deposition and properties of SiCl4-based micro-crystalline silicon films [1,2]. Here, we focus on structural and electronic properties of the material as a function of deposition conditions. EXPERIMENTAL The films were prepared using plasma deposition at conditions leading for silane-based material to high deposition rates and high solar cell efficiencies [3], namely a high dilution of the silicon containing gas in hydrogen, a high pressure of 4 mbar (measured by a baratron vacuum gauge) and a high rf (13.56 MHz) power of typically 60 W (1 W/cm2). The flow of hydrogen was kept constant at 100 sccm while the flow of SiCl4 was varied. 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) as well as effusion measurements of hydrogen and implanted helium (employing a heating rate of 20 K/min). The chemical composition of the material was analyzed by infrared (IR) absorption measurements. For electrical characterization, dark conductivity (using coplanar geometry) as well as thermoelectric power were employed. RESULTS AND DISCUSSION As shown in Fig.1 for undoped material, the growth rate increases as the silicon fraction in the process gas (SiCl4 - H2 mixture) rises. At a silicon fraction (SF) of about 3 %, the growth rate exceeds 2 Å/s. Also shown in Fig. 1 is the crystalline fraction in the material, determined from the relative intensities of the Raman peaks near 480, 500 and 520 cm-1, with the latter two attributed to crystalline material [4]. The crystalline fraction is found to decrease smoothly
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Figure 1. Deposition rate and crystalline fraction of undoped Si:Cl:H films (TS= 250°C) as a function of silicon fraction in the gas phase.
Figure 2
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