Microcrystalline Silicon Thin-Films Grown by Plasma Enhanced Chemical Vapour Deposition - Growth Mechanisms and Grain Size Control

Besides plasma reactions, Plasma Enhanced Chemical Vapor Deposition of silicon thin films at low temperatures involves surface and subsurface reactions of the impinging radicals and ions. Dilution of the silane feedstock in hydrogen or high dissociation of pure silane results in a large flux of atomic hydrogen towards the substrate, which can induce a transition from amorphous to macrocrystalline silicon growth. In this paper we review previous results based on the layer-by-layer technique, which demonstrate that the ratio of atomic hydrogen with respect to silicon radicals is the main parameter governing the nature of the films and allows for the growth of fully crystallized thin layers on various substrates. These studies highlight the importance of subsurface reactions on microcrystalline silicon formation. We show that the driving force for the formation of stable nuclei is the achievement of a highly porous and hydrogen-rich layer. The plasma is also a source of ions which have varying effects on the film properties, depending on the energy and identity (H⁺ and SiH_x⁺) of the impinging ions. We discuss the role of ion energy on the different stages of the growth and show that this can be used to control the grain size from a few nanometers up to few tens of nanometers. In particular, we highlight the role of ion energy on the different stages of the growth. Finally, we will present results concerning the use of silicon tetrafluoride as a feedstock, which allow us to achieve polycrystalline silicon thin films (∼ 100 nm thick) even at a substrate temperature of 200 °C.