Conduction Mechanisms in Hydrogenated Nanocrystalline Silicon

Hydrogenated nanocrystalline silicon (nc-Si:H) is an attractive material for photovoltaic applications, nevertheless some of its physical properties have been investigated only in recent times. In particular, the investigation of the transport mechanisms has up to now led to controversial results. This is mainly due to the complexity of nc-Si:H, as several phases and many defects and impurities coexist. The doping process further increases the complexity of the system as dopant atoms can segregate at nanocrystals (ncs) or at the boundaries between different phases. An extended study of the conduction mechanisms at microscopic level of nc-Si:H thin films is here reported. The films have been deposited by Low Energy Plasma Enhanced Chemical Vapor Deposition, at deposition temperatures from 200 to 400°C and SiH4 dilution ratios from 1% to 50%, which resulted in crystal fractions ranging from 25 to 75%. p-type and n-type doped layers were obtained by using B2H6 and PH3 gases, respectively. Sub-micron resolution current maps have been obtained by conductive atomic force microscopy. In the undoped samples all the maps presented a clear evidence of enhanced conduction in the ncs, while the disordered tissue surrounding them was mostly non-conductive. The conduction, furthermore, occurs mainly at the ncs independently of the crystalline fraction of the films. Doped films show a quite different behavior: nanocrystals are still more conductive than the surrounding tissue, but their localization in the map is different from that of intrinsic films. These results have been compared with macroscopic conductivity measurements. A unified model to interpret both microscopic and macroscopic results is advanced.