A method of surface passivation of Cd1-xZnxTe (CZT) x-ray and gamma ray detectors has been established by using microwave-assisted atomic oxygen bombardment. Detector performance is significantly enhanced due to the reduction of surface leakage current. CZT samples were exposed to an atomic oxygen environment at the University of Alabama in Huntsville's Thermal Atomic Oxygen Facility. This system generates neutral atomic oxygen species with kinetic energies of 0.1-0.2 eV. The surface chemical composition and its morphology modification due to atomic oxygen exposure were studied by x-ray photoelectron spectroscopy and atomic force microscopy and the results were correlated with current-voltage measurements and with room temperature spectral responses to Ba-133 and Am-241 radiation. A reduction of leakage current by about a factor of 2 is reported, together with significant improvement in the gamma-ray line resolution.