Deposition Techniques and Processes Involved in the Growth of Amorphous and Microcrystalline Silicon Thin Films

Hydrogenated amorphous and microcrystalline silicon deposition has been a subject of research over the last four decades, supported by its increasing number of applications. Many deposition techniques involving physical (sputtering) or chemical (plasma enhanced chemical vapour deposition) processes have been studied. The choice of the deposition technique may help to favour some type of film precursor, in particular SiH₃ which is often considered as the most suitable to obtain device grade material. However, taking as a general case the growth of μc-Si:H films, we show that the growth process and film properties are mainly controlled by the surface and subsurface reactions. In particular, thanks to in-situ ellipsometry measurements, we demonstrate that there is a growth zone close to the film surface, where cross-linking reactions leading to bulk-like formation take place. In fact, the crystallization front may be located a few tens of nanometers below the surface exposed to the plasma, thus suggesting that the film properties are governed neither by the film precursor, nor by the deposition technique. Finally, we address the issue of the substrate dependence of the growth process, which is fundamental in the case of heterojunction solar cells.