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Fabrication of n-channel polycrystalline silicon thin-film transistors (poly-Si TFTs) at a low temperature is reported. 13.56 MHz-oxygen plasma at a 100 W, 130 Pa at 250°C for 5 min, and heat treatment at 260°C with 1.3×106-Pa-H2O vapor for 3 h were applied to reduction of the density of defect states in 25-nm-thick silicon films crystallized by irradiation of a 30 ns-pulsed XeCl excimer laser. Defect reduction was numerically analyzed. Those treatments resulted in a high carrier mobility of 830 cm2/Vs and a low threshold voltage of 1.5 V at a laser crystallization energy density of 285 mJ/cm2.