Restricted epitaxial growth during thermal crystallization of nanocrystalline silicon: experiments and modeling

Hydrogenated nanocrystalline silicon (nc-Si:H) has attracted greater attention because of its improved transport properties with respect to hydrogenated amorphous silicon (a-Si:H) [1]. In addition, its deposition conditions are compatible with amorphous silicon technology which makes it possible to use both materials in the same device. In this sense, it has been proposed as a candidate for the circuits connecting amorphous thin film transistors (TFTs) in active matrix liquid crystal displays [2] and has been used as a part of a-Si:H photovoltaic solar cells and other devices [3]. Essentially, the high crystalline fraction (up to 90%) of nc-Si:H results in high carrier mobility and electrical conductivity. However, in contrast with polycrystalline silicon obtained by crystallization of a-Si:H [4], the conductivity of nc-Si:H cannot be described by a simple law of mixtures between the values for its amorphous and crystalline phases [5]. Deviations from this simple behaviour are explained by the important role of grain boundaries between crystallites and the amorphous phase [6]. They act as potential barriers to charge transport. Important changes in conductivity are observed when the grain boundaries are modified by thermal annealing [7, 8].