Gamma-ray irradiation effect on ZnO bulk single crystal: Origin of low resistivity

Abstract

We report the origin of low resistivity in ZnO bulk single crystals with thicknesses of 500 μm caused by gamma-ray irradiation. The crystals are irradiated at room temperature with gamma-rays of 1.17–1.33 MeV from a cobalt-60 source. The gamma-ray dose is 170 kGy. The resistivity at room temperature decreases from 4.1 × 104 Ω cm to 3.1 × 102 Ω cm by the irradiation. After the irradiation, slight increase in green luminescence at around 530 nm is observed, suggesting that zinc vacancy (VZn), oxygen vacancy (VO) and oxygen interstitial (Oi) are induced by Compton electrons emitted by the gamma-ray irradiation because this PL peak is a superposition consisting of the emissions relating to the VZn (∼490 nm), VO (∼530 nm), and Oi (∼580 nm). The PL intensity of VO is eight times larger than that of VZn. The existence of VO+ (g value is 1.996) is also observed by the electron paramagnetic resonance. This signal disappears by illuminating red LED with a wavelength of 654 nm, indicating VO+ to VO2+ transition. The existence of VZn would suggest the formation of zinc interstitial (Zni). In analogy with the low resistivity after Al-implanted ZnO, the origin of the low resistivity in gamma-ray irradiated ZnO is attributed to the Zni located at ∼30 meV below the conduction band.