Amorphous-Microcrystalline Silicon Films Obtained Using Hydrogen Dilution in a DC Saddle-Field Glow-Discharge
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Amorphous-Microcrystalline Silicon Films Obtained Using Hydrogen Dilution in a DC Saddle-Field Glow-Discharge T. Allen1, I. Milostnaya, D. Yeghikyan, F. Gaspari, N.P. Kherani, T. Kosteski, S. Zukotynski Department of Electrical and Computer Engineering, University of Toronto, Toronto, Ontario, CANADA, M5S 1A4 1 Department of Physics, Geology and Astronomy, University of Tennessee at Chattanooga, Chattanooga, TN 37403 U.S.A. ABSTRACT Amorphous-microcrystalline Si has been grown with hydrogen dilution using the DC saddle-field glow-discharge deposition technique. The five-electrode saddle-field system allows for independent control of the discharge parameters and of the substrate bias. The film structure was studied using Raman spectroscopy and SEM. We find that the structure of the films depends mainly on hydrogen dilution, substrate bias, electrical conductivity of the substrate, and chamber pressure. The deposition conditions, which promote growth of microcrystals, have been identified. It was found that the local electric field at the substrate surface plays a key role in obtaining microcrystallinity. INTRODUCTION Microcrystalline Si is an important material for photovoltaic applications. Crystallinity is expected to improve stability against light degradation and provide for more efficient doping than in amorphous Si [1]. Hydrogen dilution of silane during film deposition is known to promote growth of microcrystals in the amorphous matrix. The presence of microcrystals can be detected using Raman spectroscopy, by observing a characteristic peak at 510-517 cm-1 [2]. Films grown with hydrogen diluted silane have been studied extensively using plasmaenhanced chemical vapor deposition (PECVD) at RF frequencies (13.56 MHz) [3], very high frequency (VHF, 70 MHz) [4] and hot-wire techniques [5]. There are relatively few studies that deal with hydrogen dilution using DC deposition methods [6,7]. Platz and Wagner [7] have reported results of a study that compares the influence of the excitation frequency (DC, RF and VHF) and hydrogen dilution on film properties. We report here on the observation of microcrystallinity in films deposited using a mixture of silane and hydrogen in a DC saddle-field glow-discharge system [8]. EXPERIMENTAL DETAILS The DC saddle-field discharge is formed using an electrode configuration that consists of a semitransparent (wire mesh) anode sandwiched between two parallel semitransparent cathodes. Substrate holders (heated stainless steel plates) are positioned outside the cathodes, one on each side. A schematic diagram of the system can be found in reference [9]. The five-electrode configuration allows for independent control of discharge parameters and of substrate bias. The substrate holder can be grounded (grounded regime), left electrically isolated which leads to a positive bias, due to charging effects (floated regime), or externally biased (biased regime). Prior to deposition, a base pressure of 10-8 Torr was achieved. During deposition a constant deposition pressure (p), gas flow
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