Spatially resolved photoconductive properties of profiled polycrystalline silicon thin films

To study the mobility and lifetime of charge carriers in thin film polycrystalline silicon deposited by hot-wire chemical vapor deposition, time-resolved microwave conductivity measurements have been performed. Using this technique the change in conductivity in the polycrystalline silicon films after pulsed excitation can be monitored on a nanosecond time scale, without the use of electrodes. Due to the different penetration depths of the laser wavelengths used (320, 500, and 690 nm) combined with illumination from different sides, the photoconductivity in different regions within the sample can be measured. Four different samples of polycrystalline silicon deposited on Corning glass have been studied: Poly1 (highly defective), Poly2 (device quality) and profiled layers of Poly1 and Poly2. For front or back illumination, the conductivity transients for the Poly1 film are very similar and show that lifetimes of the charge carriers generated are less than 1 ns. For the Poly2 film the mobility in the interfacial substrate region (μ=0.17 cm2/V s) is more than 1 order of magnitude lower than in the top region (μ=3.8 cm2/V s). The formation of a thin Poly1 film on the surface of the Corning substrate, acting as a seed layer for the Poly2 layer, followed by the deposition of the Poly2 layer, results in only a relatively small increase in the mobility in the region close to the substrate as compared to the bare Poly2 layer, while the mobility in the top region remains approximately constant.