Microwave mobility in profiled poly-Si thin films deposited on glass by hot-wire CVD

Abstract

To study the mobility and lifetime of charge carriers in thin film polycrystalline silicon deposited by hot-wire chemical vapor deposition (HWCVD), time-resolved microwave conductivity (TRMC) measurements have been performed. With this technique it is possible to monitor the change in conductivity on pulsed laser excitation on a nanosecond time-scale, without contacting the layer with electrodes. Pulsed laser excitation has been performed at a laser wavelength of 320 nm. The samples studied are Poly1 (highly defective), Poly2 (device quality) and profiled layers of Poly1 and Poly2. The results for the Poly1 film show that lifetimes of the charge carriers generated on front and back side illumination are similar and below a nanosecond. For Poly2, the mobility in the substrate region (μ=0.17 cm 2/Vs) is more than one order of magnitude lower than in the top region (μ=3.8 cm 2/Vs). The introduction of a thin Poly1 layer, acting as a seed layer for the Poly2, results in a relatively small increase of the mobility in the substrate region compared to standard Poly2, while TRMC signals upon front illumination remain approximately constant.