Microwave absorption caused by free carriers was investigated. A 9.35 GHz microwave interferometer was constructed. The transmissivity of 525-mm-thick silicon substrates decreased from 60.4 to 3.8% as the resistivity decreased from 1000 to 4 cm. This characteristic was explained well by a numerical analysis using the free carriers absorption theory. Microwave free carrier photo absorption caused by lightinduced carriers was also investigated for p-type silicon samples coated with 100 nm

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... y grown SiO 2 layers as well as SiO x layers deposited by the vacuum evaporation method. The effective minority carrier lifetime and recombination velocity were analyzed in the case of the photo induced carrier generation with 532 nm light illumination. The effective minority carrier lifetime was increased from 360 to 540 ms and the recombination velocity was decreased from 78 to 30 cm/s by 1:3 Â 10 6 Pa H 2 O vapor heat treatment at 260 C for 3 h for light illumination at 0.315 mW/cm 2 in the case of the thermally grown SiO 2 /Si because of the passivation of SiO 2 /Si interfaces. They were markedly increased from 30 to 380 ms and from 1300 to 60 cm/s, respectively, by the H 2 O vapor heat treatment in the case of the vacuumevaporated SiO x /Si. #