Deposition Techniques of Thin-Film Silicon

Thin-film silicon, either amorphous or nanocrystalline, is predominantly prepared by chemical vapor deposition techniques, though there are exceptions, such as lift-off tech­niques by which a thin film of crystalline silicon is ripped off from a wafer [2-4].

Advanced Silicon Materials for Photovoltaic Applications, First Edition. Sergio Pizzini. © 2012 John Wiley & Sons, Ltd. Published 2012 by John Wiley & Sons, Ltd.

It is common practice in this field that high temperature (>1000°C) deposition from suitable silicon precursors generates polycrystalline silicon deposits, as is the case of polycrystalline silicon deposited in Siemens-type reactors (see Chapter 2). Here, the grain size is of the order of micrometers. Deposition at low temperatures (<300°C) invariably generates amorphous or nanocrystalline silicon, or mixtures of both. Depending on the application, the device quality will be different, and the primary goal of a thin-film deposition process is to get the proper device quality, both in term of structure and electrical and optical properties.

The most appropriate technique to produce device-quality films is chemical vapor deposition (CVD), which includes a large varieties of techniques, arranged in descend­ing order of ion energies; direct current (DC) plasma CVD (high), standard radio­frequency plasma-enhanced CVD (St RF PECVD) (standard), very high frequency (VHF) PECVD (moderate), expanding thermal plasma (ETP) CVD, low-energy (LE) PECVD, microwave (MW) PECVD (low), hot-wire (HW) CVD (no ions). Of the above tech­niques, PECVD is currently used as direct deposition processes in mainly capacitive coupled configurations or employing electrodes consisting of parallel-plate electrodes. However, the inductively coupled plasma configuration, which was in fact the technique that produced the first amorphous silicon (a-Si) layer [5] has also been receiving atten­tion because of its potential advantage of increasing the deposition rate. The DC plasma CVD, which was once used by Solarex, USA to make a-Si and a-SiGe films and solar cells, is no longer considered as a viable technique, though it is still used in combination with other deposition techniques, such as ETP and LEPECVD, as described below.