Mg2Si film ~2.3 μm was synthesized by reactive deposition Mg on Si(111) at 340 °C in UHV. The photoresponse of backlighted Al/Si/Mg2Si Schottky structure represents the bell-shaped curve with the peak at 1045 nm and intensity 29 mA/W, 105 mA/W and 195 mA/W under the 0 V, 1 V and 5 V bias respectively with FWHM ~130 nm.