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Metal-ferroelectric-insulator-semiconductor (MFIS) structure has been prepared using SrBi2Ta2O9 thin film whose surface was modified by irradiation of nitrogen or oxygen radical. By this modifying, leakage current is reduced and C-V characteristic shows clear memory window and sharp slopes that correspond to good interface layer. characterization of x-ray photoelectron spectroscopy and ultraviolet photoyield spectroscopy have exhibited that the SBT thin films with nitrogen treatment have the higher threshold energy than that without the irradiation. Memory window of the MFIS structures are in the range of 1-2V when the gate voltage is varied from 3-6V. Retention time of ON and OFF states is 1 week in the structure irradiated with oxygen radical and 12 days in that with nitrogen radical although that without the irradiation is only 3 hours.