Electrical transport in Hydrogenated Nanocrystalline Silicon Thin Films

S.CONCARI, R.H.BUITRAGO

Advances in condensed matter and material research Vol 6

Nova Publisher

Año: 2009; p. 70 - 95

Different models for the electric charge flow in doped as well as in undoped hydrogenated nanocrystalline silicon (nc-Si:H) thin films have been proposed. However, there is no agreement about how the electrical transport occurs in these heterogeneous systems.  The electrical properties of nc-Si:H deposited using Very High-Frequency Plasma Enhanced Chemical Vapour Deposition (VH-PECVD), depend on the various deposition conditions, such as concentration of the dopant gases, ratio of hydrogen dilution,  deposition temperature and pressure, radio frequency power, etc. The nucleation and the growth characteristics of the films are modified by these deposition parameters, changing the structural properties and therefore the electrical ones. The effects of temperature (270 – 450 K) and applied electric field (0.1 – 2 x 104 V/cm) on the transport properties of intrinsic nc-Si:H thin films as well as p-type doped with Boron prepared by VH-PECVD are analysed. Variable Range Hopping (VRH) between defects near the Fermi level was established as a predominant electronic transport mechanism for intermediate applied fields. At very low applied electric fields, dark conductivity becomes field-dependent. The non-ohmic behaviour of the conductivity observed is analysed in terms of the hopping transport equations. Even when electronic transport is still not well understood, variable range hopping could highlight some mechanisms of current flow in non homogeneous semiconductor thin films. Other researchers have begun using the same model to fit experimental data of conductivity on many other mixed-phase thin films materials, even when no theory has been formulated to explain electrical transport in nanocrystalline thin films.