Influence of doping and Oxide Layer on the Conductivity of Silicon Nanoparticles
S. Hartner, A. Gupta, H. Wiggers
University of Duisburg-Essen, DE
Keywords: functionalization, conductivity, silicon, doping, imepdance spectroscopy
Abstract:We synthesized undoped and phosphorous-doped silicon nanoparticles in a microwave plasma reactor via pyrolyis of silane (SiH4). The phosphorous doping was achieved by adding phosphine with the precursor gas. The as-prepared, soft-agglomerated powders consist of spherical and highly crystalline nanoparticles in the size regime of about 50 nm [Fig 1a]. Etching of silicon nanoparticles with hydrofluoric acid (HF) resolves the agglomeration of the silicon nanoparticles and removes the oxide layer from their surface [Fig 1b]. Subsequently, we functionalized the silicon nanoparticles by covalent bonding of organic molecules to the particle surface. The electrical properties of as-prepared as well as functionalized nanoparticles were investigated by use of impedance spectroscopy. Therefore the silicon powders were compacted into pellets with a diameter of 5mm and an average thickness of 0.09mm. To prove the stability of the coating, the measurements were performed in dry, synthetic air as well as in hydrogen atmosphere at different temperatures ranging from 320K to 670K. As expected for semiconducting materials, the spectra show typical semicircles with an increase in conductivity with rising temperature (Fig.2). While the resistivity of as-prepared, surface-oxidized silicon particles was very high and increased with time at elevated temperatures, we observed no influence of the atmosphere on the resistivity of both, undoped as well as phosphorous-doped functionalized powders. From the low-frequency part of the impedance spectra (DC-conductivity), the activation energy for the undoped silicon was calculated to be 360meV while for the phosphorous-doped silicon an activation energy of 160meV was found suggesting phosphorous-states in the band-gap of the silicon particles. A similar effect was found by EPR measurements performed on undoped phosphorous-doped silicon nanoparticles . Surprisingly, the resistivity of undoped as well as doped particles did not differ significantly. This result is found to be consistent with the measurements on thin films of silicon nanoparticles  and supports the model of defect compensation discussed in .