ZnO based oxide semiconductors have attracted much attention for light emitting devices and photo detectors in short visible wavelength due to its unique properties such as a wide direct bandgap of 3.3 eV and a large exciton binding energy of 60 meV. p-type Mg_xZn_1-xO (0 ≤ x ≤ 0.2) has been required for realizing a light emitting device in ZnO based heterostructure or as an active layer in UV photodetectors. We present here a demonstration of a photodetection in Mg_xZn_1-xO Schottky photodetectors with surface treatment on Mg_xZn_1-xO film and a systematic analysis with the Mg content of the formation and concentration of deep levels found throughout the lower half of the bandgap of Mg_xZn_1-xO by deep level optical spectroscopy (DLOS).