Gamma-ray induced photo emission from ZnO single crystal wafer: Comparison with GaN

Abstract Gamma-ray induced photo emission from ZnO is investigated for as-grown single crystal wafers at room temperature. The ZnO crystals are irradiated with gamma-ray of 1.17 and 1.33 MeV from a cobalt-60 source. The gamma-ray induced photo emission is analyzed by a charge coupled device equipped spectrometer using the optical fiber cable placed in front of ZnO. In the gamma-ray induced photo emission from as-grown ZnO wafers, the green luminescence (GL) is observed with a peak at around 580 nm in the spectrum around 420 nm–900 nm. The main GL peak is attributed to the singly ionized oxygen vacancy in the as-grown crystal. The emission intensity increases almost in proportion to the wafer thickness when the thickness of the single crystal wafer increases from 0.5 mm to 1.5 mm, suggesting that the gamma-ray induced photo emission is generated in all parts of the crystal due to the strong penetration of gamma-rays. The emission intensity is about 1/60 of gamma-ray induced yellow luminescence from GaN single crystal wafers observed in our previous study [Appl. Phys. Lett. 118 (2021) 032106]. It is suggested that a part of the electrons in the singly ionized oxygen vacancies in ZnO are excited to the conduction band by the Compton electrons generated by the gamma-ray irradiation and oxygen vacancies become the doubly ionized states, reducing their contribution to GL.