Influence of Hydrogen Dilution on Properties of Silicon Films Prepared by D.C. Saddle-Field Glow-Discharge: Observation
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Influence of Hydrogen Dilution on Properties of Silicon Films Prepared by D.C. Saddle-Field Glow-Discharge: Observation of Microcrystallinity T. Allen1, I. Milostnaya, D. Yeghikyan, K. Leong, 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 In the D.C. saddle field glow discharge deposition the transition from amorphous to microcrystalline silicon thin films occurs when the silane concentration in the gas phase drops below 10%. We report here the results of Raman spectroscopy, SEM, TEM, and HRTEM studies of the film morphology. We estimate the average crystallite size to be in the range of 5 to 7 nm and the crystalline volume fraction of 25 to 35%. INTRODUCTION Amorphous Si is widely used for large area photovoltaic and microelectronic applications [1]. The use of microcrystalline Si is expected to improve stability against light-induced degradation and provide more efficient doping over that offered by amorphous silicon. Recently, we reported on the growth of mixed phase amorphous-microcrystalline silicon using the D.C. saddle field glow discharge deposition method [2]. The films were grown using hydrogen dilution of silane during the deposition. We were able to identify the growth conditions and the types of substrates that promote microcrystallinity. In this work we present the structural properties of saddle field glow discharge deposited microcrystalline Si films as a function of hydrogen dilution. The films were studied using Raman spectroscopy, SEM, TEM and highresolution TEM. EXPERIMENTAL The deposition process is described in [2]. The samples studied in this paper were grown on pyrex substrates at a temperature of 250°C, a chamber pressure of 200 mTorr, and an anode current of Ian= 30 mA. The source gas was a mixture of silane and hydrogen. The silane concentration in percent, C=[SiH4]/([SiH4]+[H2]), ranged from 2.4% to 100%. For most films the flow rate was 10 sccm, but one deposition with a flow rate of 5 sccm was also performed. The thickness of the films was measured with a profilometer (Tencor) and by SEM (Hitachi model S4500). The film thickness ranged from 0.2 to 0.5 microns and was uniform within ±10%. SEM was also used to study the film morphology. Raman spectra were collected at room temperature in the near back-scattering geometry using the 514.32 nm line of an Ar+ laser at a power output of 140 mW. To ensure that the sample surface was not thermally damaged, the laser beam was attenuated using an absorbing filter. A visual check of the surface was carried out before and after each measurement using an optical microscope. The crystalline Si Raman signal at 520 cm-1 was used to reference the spectra. In order to prepare samples for TEM measurements, films were lifted from glass substrates using hydrofluoric acid, and then mounted on TEM copper grids. TEM
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