Effect of hydrogen dilution on electronic properties of a-SiH x films deposited by low-frequency plasma
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A.S. Abramov A.F. Ioffe Physico-Technical Institute, St. Petersburg 194021, Russia
P. Rosales and A. Sibaja National Institute for Astrophysics, Optics and Electronics, Apdo. Postal 51y216, Puebla 7200, Mexico (Received 3 January 2003; accepted 27 May 2003)
The effect of hydrogen dilution during plasma deposition on hydrogen incorporation and the optical and electrical properties of a-SiHx films were studied. The films were grown in capacitive low-frequency (f ⳱ 10 and 110 kHz) discharge in SiH4 diluted with H2, varying the ratio RH of the gases H2/SiH4 from RH ⳱ 0 to 40. The optical absorption coefficient and optical bandgap were changed with RH. Si–H bonding, studied by infrared spectroscopy, depended on RH. Hydrogen concentration in the films estimated from infrared spectra was in the range 20–30%. We observed the significant effect of RH on the temperature dependence of conductivity (T) and on the subgap absorption spectra measured by the constant photocurrent method. The reduction of subgap absorption up to 1.5 order of magnitude was observed with increasing RH. I. INTRODUCTION
Amorphous silicon films and related materials are widely used in electronics and are commonly produced by the means of radio frequency (rf) (at a frequency f ⳱ 13.5 MHz) plasma deposition. Recently, very-highfrequency (VHF) discharge in the range 50–100 MHz has attracted much attention because the deposition rate is several times higher and the resulting films are of high electronic quality.1 High-efficiency solar cells ( ⳱ 12.4%) have been fabricated at high deposition rate.2 However, increasing the plasma frequency makes it difficult to uniformly load power into the discharge and, consequently, deposit uniform films over large areas by VHF plasma deposition.3 The highest deposition rate in plasma deposition Vd ⳱ 100 A/s has been achieved in the laboratory,4 but not enough electronic characterization data are available in the literature. Device-quality amorphous silicon films have, however, been deposited in low frequency (LF; f ⳱ 55 kHz) plasma at the relatively high deposition rate Vd ⳱ 30 Å/s.5 This type of discharge has been little studied for the deposition of silicon and related films. This discharge is of interest for the following reasons: ions impinging growing films possess higher energies, providing high deposition rates, and low frequency discharge is relatively simple to realize technically. Nevertheless, very little is known about the electronic characteristics of these films and their relation to the deposition conditions. 1918
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J. Mater. Res., Vol. 18, No. 8, Aug 2003 Downloaded: 26 Nov 2014
The goal of this work is to study the hydrogen dilution effect on both the microstructure and the electronic properties of silicon films deposited in LF at f ⳱ 10 kHz and 110 kHz capacitive discharge. II. EXPERIMENTAL
The samples of a–SiHx films were prepared in a plasma deposition system (AMP 3000, Applied Materials Inc.) using SiH4 diluted with H2. Gases of semiconductor purity were used in th
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