Conduction mechanisms in doped and undoped nc-Si for PV

Hydrogenated nanocrystalline silicon (nc-Si:H) is an attractive 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 in recent times investigated and many questions are still open. As an example, the investigation of the transport mechanisms has up to now 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, grain-boundaries 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 of the electrical transport mechanisms and routes has revealed the major importance of percolation conduction through the 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 in 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% [3]. p-doped layers were obtained by varying the using B2H6 dilution rates. Both doped and undoped films were examined. Sub-micron resolution current maps have been obtained by conductive atomic force microscopy (C-AFM) applying a fixed voltage to a conductive tip. In the undoped samples 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 is. The conduction, furthermore, occurs mainly at the Si nanocrystallites independently of the crystalline fraction of the films examined, while the spatial distribution and the concentration of conductive nano-grains significantly changes as a function of the film deposition parameters [3]. Doped films show a quite different behavior: nanocrystals are still more conductive than the surrounding tissue, but the most conductive nanocrystals are mainly located within the “valley” of the structure, i.e. at the boundaries between the columns (fig.2). For undoped material, conductivity measurements as a function of temperature revealed a Meyer Neldel Rule (MNR) for materials grown at a dilution factor between 1 and 6% and temperature range from 240 to 340 K. It is, thus, likely that these materials have a similar microstructure and that the conduction involves the amorphous phase, our MNR parameters being close to the one found for amorphous silicon. However, since the activation energy values is low and the conductivity relatively high, it is also possible that the conduction occurs through the crystalline phase as well, in agreement with C-AFM results. At higher dilution factors we observe a decrease in conductivity up to typical values for amorphous which seems more sensitive to the silane flow than to the dilution. We observe a strong enhancement of conductivity while increasing the doping ratio up to a threshold from which the conductivity decreases. Most probably, after saturating the substitutional sites, additional boron atoms will be incorporated on interstitial site in whic...