In-situ Investigation of the Mechanical and Electrical Properties of Nanosized Silicon Powders

Abstract

ABSTRACTThe use of nanosized Silicon powders in nanoelectronics and photovoltaics enables new technologies and promises to reduce the production costs of devices like solar cells and printable electronics significantly. However, to understand their electrical behavior and mechanical properties, such systems must be examined carefully. In porous systems like powders, the macroscopic electrical properties result from transport mechanisms such as hopping and tunneling between particles as well as from structural properties such as the amount and shape of particle contacts. Theoretical approaches like the strongest stresses network or the brick layer model can only describe this complex relation in a simplyfied way and need to be accompanied by suitable experiments. Nanosized pure and doped Silicon powders, synthesized in a microwave supported plasma reactor, were characterized by determining in-situ the conductance, impedance, and the change of porosity while applying a uniaxial mechanical pressure ranging from 7.5 to 750MPa. The porosity change of the powder during electrical measurements was characterized by means of a laser interferometer to determine the mechanical properties of the powder more accurately. Conductance measurements as a function of the applied pressure show an exponential dependence for nanosized particles and a power law for microsized particles. Simple scaling considerations in respect of the particle size cannot explain this fundamentally different behavior. Therefore a more sophisticated model is needed. A time dependent change in conductance together with a decrease in porosity was observed while applying a constant pressure, suggesting friction limited compaction of the powder. For a constant external force, the comparison of different samples leads to a clear power law dependence between the conductance of pressed samples and their mean particle diameter. This size effect spans seven orders of magnitude of the conductance while the particle size changes by only a factor of ten, and it clearly exceeds any influence of the doping concentration and the variation of the sample mass. To separate the contributions of the particle cores, particle-particle, and particle-electrode contacts to the complex conductance and capacitance, impedance spectroscopy was performed. In agreement with the observed compaction of the powder, the spectra show a strong increase of the sample capacitance and conductance as a function of the applied pressure.