Non-contacting measurements of photocarrier lifetimes in bulk- and polycrystalline thin-film Si photoconductive devices by photothermal radiometry

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Abstract
Laser-induced and frequency-scanned infrared photothermal radiometry was applied to a crystalline-Si photoconductive device, and to polysilicon thin-film photoconductors deposited on oxidized Si substrates by an LPCVD method. A detailed theoretical model for the radiometric signal was developed and used to measure the free photoexcited carrier plasma recombination lifetime, electronic diffusivity and surface recombination velocity of these devices, with the simultaneous measurement of the bulk thermal diffusivity. A trade-off between detectivity/gain and frequency-response bandwidth was found via the lifetime dependence on the wafer background temperature rise induced by Joule-heating due to the applied bias. This effect was most serious with the bulk-Si device, but was limited by the high resistivity of the LPCVD thin-film devices. In the case of the bulk-Si device, the results of photothermal radiometry were compared with, and corroborated by, frequency-scanned photocurrent measurements. More sophisticated analysis was shown to be required for the interpretation of the polysilicon photoconductor frequency-responses, perhaps involving the fractal nature of carrier transport in these grain-structured devices.