Investigation of defect states in light-irradiated single-crystal ZnO by low-temperature positron annihilation lifetime spectroscopy

Abstract

Positron annihilation lifetime spectroscopy of single crystalline ZnO during light illumination has been performed in conjunction with electron spin resonance (ESR) to investigate defects related to persistent photoconductivity. The intensity of the ESR signal changes depending on light wavelengths and the apparent positron lifetime during red-light illumination is approximately 6 ps lower on average than that during blue-light illumination. The results suggest that the excitation of VO during blue-light illumination increases positively ionized VO, while the relaxation of VO during red-light illumination decreases positively ionized VO, leading to a decrease in the apparent positron lifetime.

Transient positron annihilation spectroscopy under light irradiation at liquid nitrogen temperature: evaluation of defect states in single-crystal ZnO

“…In our previous study, it was reported that under red light irradiation at liquid nitrogen (LN 2 ) temperature, the apparent positron lifetime of single-crystal ZnO was, on average, approximately 6 ps lower compared to that under blue light irradiation. 3) This results indicate that the presence of positively charged oxygen-vacancy (V O ) increases during blue light irradiation (excitation) and decreases during red light irradiation (relaxation), leading to a decrease in the apparent positron lifetime. These observed behaviors of vacancies are closely related to the phenomenon of persistent photoconductivity (PPC) in ZnO.…”

“…Previous studies have indicated that red light irradiation reduces the apparent positron lifetime, suggesting an increase in the number of neutral oxygen vacancies. 3) If positrons are captured by V O , the number of positrons annihilated with Zn electrons would increase, resulting in a spectrum approaching the Zn spectrum (i.e. an increase around 20 × 10 −3 m 0 c in Fig.…”