Cyclically Varying Hydrogen Dilution for the Growth of Very Thin and Doped Nanocrystalline Silicon Films by Hot-Wire CVD

Abstract

An important issue for the massive implementation of thin silicon technology in photovoltaic is the use of plastic substrates, which allow the use of roll-to-roll deposition systems and planar monolithic interconnection between the cells. However, the use of plastic substrates require a fully low temperature process; especially critical in the deposition of the thin amorphous (a-Si:H) or nanocrystalline (nc-Si:H) layers. Hot-Wire Chemical Vapour Deposition (HW-CVD) technique has demonstrated to be a good alternative to deposit quality thin films at low temperature. In this paper we focus our study on very thin (50 nm) n- and p-doped nc-Si:H films deposited at low substrate temperature around 100°C. We have observed that, in this low temperature deposition conditions, the promotion of an a Si:H incubation layer leads to a poor doping efficiency and poor electrical properties of the films. Hence, in addition to the optimization of the deposition conditions, we deposited doped layers by cyclically varying the hydrogen dilution (CVH) during deposition process. This CVH method promotes a layer-by-layer growth and inhibits the formation of the incubation layer. Several doped nc-Si:H layers have been deposited with and without this CVH method. The structural, electrical and optical properties of these films and advantage of CVH in improving the device quality of the thin doped layers are reported.