Suppression of Plasma Damage on SnO 2 by Means of a Different Surface Chemistry Using Dichlorosilane
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Suppression of Plasma Damage on SnO2 by Means of a Different Surface Chemistry Using Dichlorosilane T. Nakashima, M. Kondo, Y. Toyoshima and A. Matsuda Thin film Silicon Solar Cells Super Laboratory, Electrotechnical Laboratory, Ibaraki, Japan ABSTRACT We report on that plasma damage on SnO2 can be suppressed by using surface termination by chlorine. It was found that the darkening of SnO2 is decreased and a wider gap p-a-Si material is obtained by using SiH2Cl2 especially at the higher reaction pressure and at the lower substrate temperature. The suppression of darkening of SnO2 and wide optical gap is correlated to chlorine contents in the film. It is demonstrated that SiH2Cl2 is also beneficial for boron-doped material, indicating a suitable material for a window layer of solar cells. INTRODUCTION The fabrication process of a p-i-n solar cell with a superstrate structure includes the deposition of a p-layer on a transmittance conductive oxide (TCO) coated substrate such as SnO2. In plasma enhanced chemical vapor deposition (PECVD) which is widely employed for solar cell fabrication with a source gas mixture of SiH4/H2, SnO2 is damaged by plasma due to reduction by atomic hydrogen, resulting in the darkening of SnO2 due to the appearance of metallic tin (Sn). This darkening causes a decrease of short circuit current (Isc) of solar cells particularly at higher substrate temperatures, and therefore the process temperature of the solar cells is limited. Very recently, it was found that surface coverage during the growth of thin film silicon using dichlorosilane/hydrogen source gas is not hydrogen but chlorine[1]. This result suggests that chlorine termination leads to a reduction of SnO2 darkening. In this paper, we report the effect of the different surface chemistry of chlorine on the reduction of SnO2. EXPERIMENT Samples were prepared using capacitively coupled RF PECVD from a gas mixture of H2 and SiH2Cl2 (or SiH4). The typical deposition conditions are shown in Table I. The deposition temperature is measured at near the substrate. The samples for measuring optical properties were deposited on Corning #7059 glass and SnO2 coated substrate (ASAHI-U) at the same time. The optical gap (Eopt) and film thickness was determined from transmittance (T) and reflectance (R) spectra. The optical gap was determined from Tauc plot. To evaluate the degree of the darkening of SnO2, normalized transmittance is used with T&R spectra of samples deposited on ASAHI-U substrate.
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Table I. the typical fabrication conditions Deposition temperature 180-250 °C Reaction pressure 30-300 mtorr Background pressure 10-7 torr RF power density 16 mW/cm2 Gas flow rate SiH2Cl2 (or SiH4) 10 sccm 50 sccm H2 The normalized transmittance, Tnorm, is obtained by a ratio of the integrated transmittance for SnO2 with and without a-Si:H layers (figure 1). To suppress the optical interference effect, transmittance is obtained by T/(1-R). The integrated transmittance is used in the region between 1000nm and 2500nm, because the presence of metallic
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