NONCONTACTING MEASUREMENTS OF PHOTOCARRIER LIFETIMES IN BULK-FILM ANDPOLYCRYSTALLINE THIN-FILM SI PHOTOCONDUCTIVE DEVICES BY PHOTOTHERMAL RADIOMETRY

Laser-induced and frequency-scanned infrared photothermal radiometry w as applied to a crystalline-Si photoconductive device, and to polysili con thin-film photoconductors deposited on oxidized Si substrates by a n LPCVD method. A detailed theoretical model for the radiometric signa l was developed and used to measure the free photoexcited carrier plas ma recombination lifetime, electronic diffusivity and surface recombin ation 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 devic e, but was limited by the high resistivity of the LPCVD thin-film devi ces, In the case of the bulk-Si device. the results of photothermal ra diometry were compared with, and corroborated by, frequency-scanned ph otocurrent measurements. More sophisticated analysis was shown to be r equired for the interpretation of the polysilicon photoconductor frequ ency-responses, perhaps involving the fractal nature of carrier transp ort in these grain-structured devices. (C) 1996 American Institute of Physics.