P- and N-Type Microcrystalline SiC Fabricated by rf Plasma CVD with Ethane Gas
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P- and N-Type Microcrystalline SiC Fabricated by rf Plasma CVD with Ethane Gas T. Toyama, Y. Nakano, T. Kosuge, A. Asano, H. Okamoto Department of Systems Innovation, Graduate School of Engineering Science, Osaka University Toyonaka, Osaka 560-8531, Japan ABSTRACT We have been investigated p- and n-type microcrystalline Si1-xCx (µc-SiC) films fabricated by a conventional rf (13.56 MHz) plasma CVD method with a use of a new carbon source of C2H6 gas at a low substrate temperature on a glass substrate. The Si crystallites incorporated in µc-SiC films retain with a carbon content up to 9 at.%. Both of p- and n-type µc-SiC films show relative high dark conductivities of on the order of 10-3 S/cm with optical energy gaps, E04, of ~2 eV. In infrared spectra, any pronounced features due to C-Hn vibration mode are not found in C2H6-based µc-SiC films, which is different from the case of CH4-based µc-SiC films. INTRODUCTION Highly conductive p- and n-type microcrystalline silicon-carbon alloys (µc-SiC) consisting of Si crystallites embedded in amorphous Si1-xCx (a-SiC) are the key materials for optoelectronic devices due to their wide optical energy gap, and have been already applied to solar cells [1,2] as well as thin-film light-emitting diodes [3,4] in R&D level. In these experiments, the µc-SiC films were prepared by electron cyclotron resonance (ECR) plasma chemical vapor deposition (CVD) with a mixture of SiH4 and CH4 gas sources highly diluted by H2 gas [1–4]. Quite high dark conductivity of >10-1 S/cm and wide optical gap of >2.1 eV have been simultaneously achieved with a low substrate temperature of ~250°C. However, µc-SiC deposited by ECR plasma CVD has some disadvantages, e.g., with respect to deposition on transparent conductive oxide (TCO) layers, excess hydrogen radicals as well as significantly slow deposition rate of ≤~0.01 nm/s damage the TCO layer. On the other hand, some trials have been performed for fabrication of µc-SiC films by conventional rf (13.56 MHz) plasma CVD with a mixture of SiH4 and CH4 gases [5–8]. The highly conductive p- and n-type µc-SiC films with optical energy gaps up to ~2.1 eV have been achieved by the rf plasma CVD with a high H2 dilution, however, the crystalline volume fraction of deposited SiC films made by rf plasma CVD with CH4 gas as a carbon source markedly decreases with an increase in carbon content of the SiC film even if the carbon content is slightly over 3 at.% [8]. Quite recently, C2H6 has been proposed as a new carbon gas source for fabricating µc-SiC with hot-wire (catalytic) CVD [9]. Here we will demonstrate the results on p- and n-type µc-SiC fabricated by rf plasma CVD with the new carbon source of C2H6 gas. The Si crystallites are still incorporated in the deposited µc-SiC with a carbon content of 8 at.% showing high conductivities of on the order of 10-3 Scm-1 as well as an optical energy gap, E04, of ~2.0 eV. Additionally, the deposition rate of the µc-SiC remains ~0.1 nm/s.
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