Conduction mechanisms in doped and undoped nc-Si for PV

Hydrogenated nanocrystalline silicon (nc-Si:H) is a interesting material for photovoltaic applications, mainly due to its higher stability against Staebler-Wronski effects with respect to amorphous silicon. Notwithstanding its promising properties, its physical characteristics have been only recently investigated, for example the investigation of the transport mechanisms has led to controversial results. This is mainly due to the complexity of this material, as several phases coexist: crystalline nano-grains, nanocrystalline columns, hydrogenated amorphous Si (a-Si:H), defects and voids. The doping process further increases the complexity of the system as dopant atoms can segregate at nanocrystals or at the boundaries between different phases. Recently [1], the investigation into the electrical transport mechanisms and routes has revealed the importance of percolation conduction through a network of connected disordered tissue encapsulating the columns in fully-crystalline undoped μc-Si:H, while different results [2] are in favor of an hopping assisted conduction mechanism. The present contribution deals with an extended study of the conduction mechanisms at both macroscopic and microscopic levels of hydrogenated nanocrystalline Si thin films grown with different conditions. The films have been deposited by Low Energy Plasma Enhanced Chemical Vapor Deposition (LEPECVD) using SiH4 and H2 precursor gases, the deposition temperatures ranged from 200 to 400°C, the SiH4 dilution ratios from 1% to 50%. The films were deposited on several substrates: crystalline Si, glass, glass covered with ITO or ZnO. The crystal fraction, as measured by Raman spectroscopy, ranged from 25 to 75% [1]. p-doped layers were obtained by varying the using B2H6 dilution rates. Both doped and undoped films were exhamined. Sub-micron resolution current maps have been obtained by conductive atomic force microscopy (C-AFM) applying a fixed voltage to a conductive tip. All the maps obtained present a clear evidence of enhanced conduction in the nanocrystals, while the disordered tissue surrounding the nano-grains (constituted mainly by amorphous region) is mostly nonconductive (fig.1). This result was obtained for all the undoped films examined whatever the substrate or the crystalline fraction.