Electrical properties of Silicon nanodots/Si rich carbide systems for photovoltaic application

Silicon nano-crystals (Si NCs) embedded in a dielectric matrix are studied in view of their application as tunable band-gap absorbers in all-silicon multi-junction photovoltaic cells. Although high conductivity and high mobility are requirements for the matrix in order to guarantee an efficient carrier collection, the ideal properties of the nc-Si are more related to the ability of absorbing photons beyond a given energy, and emitting photogenerated carriers into the surrounding matrix. The carrier collection efficiency in this composite system is supposed to depend on the barrier height of the surrounding matrix. In the present case we are dealing with Si NCs embedded in a SiC matrix, which guarantees a lower barrier to the Si NCs with respect to SiO2 matrix and better transport properties. SiC/SiC:Si multilayers produced by Plasma Enhanced Chemical Vapor Deposition (PECVD) have been annealed to obtain Si NCs formation. Various experimental conditions (Si NCs size, doping level, crystallinity) have been considered. The structural parameters were determined by Reflectance and Transmittance spectroscopy analyses. From this the calculated thickness of multilayers is in the range of 131-178 nm. Macroscopical as well as microscopical electrical characterizations were carried out in order to understand the conduction mechanisms. Conductivity measurements as a function of temperature showed that the conductivity ranged from 10-6-10-3S/cm and the activation energy range is 0.19-0.4 eV. Atomic Force Microscopy (AFM) was used to characterize the topography of the sample surface. Roughness values are of the order of 0.6 nm while phase contrast dynamical mode AFM mapping allowed us to obtain an estimated average value of the nano-crystal diameter around 10 nm. Conductive-AFM was performed to identify the conductive path at the nano-crystals level. The Si NCs showed enhanced conductivity with respect to the surrounding matrix. The measurements were correlated with optically determined crystallized fraction and with thermally activated transport properties. The combination of electrical with optical measurements, associated to AFM observation at the nanoscale, gives a fundamental insight in the silicon nano-dots conduction properties.