Microcrystalline silicon -: Relation between transport and microstructure

Intrinsic microcrystalline silicon (Ac-Si:H) samples were prepared close to the border between microcrystalline and amorphous growth (at 4.5% dilution of SiH4 in H-2 by 13.56 MHz PECVD) in order to obtain a thick transition layer (d greater than or similar to0.6 mum) which allowed a detailed study of relation of transport and microstructure. Small grains (15-30 nm) and their aggregates (columns) with sizes 100-500 nm were observed by combined AFM with a conductive cantilever in simultaneously measured local topography and current map. With increasing muc-Si:H layer thickness the DC dark conductivity parallel to the substrate a, smoothly increases but three distinct regions are seen in its activation energy E-a and prefactor sigma (0). Within the transition layer from amorphous to fully microcrystalline growth an increasing number of small grains leads to decrease of activation energy. When the transition to microcrystalline growth is completed at d similar to0.6 mum, both E-a and sigma (0) drop sharply and are limited by column boundaries. Further growth with corresponding propagation of the columns keeps E-a constant but sigma (0) increases. Anisotropy of the columnar structure is responsible for the difference between transport properties parallel (\ \) and perpendicular ((perpendicular to)) to the substrate. Transport anisotropy was observed both as a difference in the diffusion lengths L((perpendicular to)) observed by surface photovoltage and L((perpendicular to)) observed by steady state photocarrier grating techniques and by comparing the sigma (d)(\ \) measured with coplanar electrodes parallel to the substrate with the true sigma (d)((perpendicular to)) obtained from AC impedance analysis.