Relation between Electronic Properties and Density of Crystalline Agglomerates in Microcrystalline Silicon

A series of silicon thin films was made by very high frequency plasma enhanced chemical vapour deposition (VHF PECVD) at substrate temperatures below 100 °C at different hydrogen to silane dilution ratios. The electronic properties of these layers were studied as a function of the surface crystalline fraction as determined accurately from a combination of microscope images at different length scales (gathered by using different types of microscopes). The results show that the electrical conductivity increases monotonously as a function of crystalline surface coverage and no discontinuity is observed at the percolation threshold. An increase in conductivity of four orders of magnitude for layers with a high crystalline content is observed after annealing at temperatures up to 170 °C. Combined with the information that oxygen is incorporated at Si-H surface bond sites, this suggests that doping of the intergrain boundaries by oxygen might be dominantly responsible for the electronic properties of mixed phase silicon.