4:00 PM - 6:00 PM
[14p-PB8-7] Investigation of photoresponsive defect states in electron-irradiated ZnO
by positron annihilation lifetime spectroscopy and electron spin resonance
Keywords:ZnO, semiconductor, LED
It is commonly recognized that wide-bandgap compound semiconductor zinc oxide (ZnO) has potentially better properties compared with widely developed Si and GaN. However, the nature of native defects, in particular, persistent photoconductivity (PPC), has not been sufficiently clarified yet. Previous theoretical and experimental studies suggested that transient energy levels related to charge states of the anion vacancy (Vo) cause PPC. In this study, we use positron annihilation lifetime spectroscopy to investigate defects relating to PPC.
A single crystal ZnO synthesized by a hydrothermal growth method was irradiated to 6×1018 e−/cm2 with an 8 MeV electron beam from a KURNS-LINAC for introducing point defects. After that, the sample was measured by electron spin resonance (ESR) and positron annihilation lifetime spectroscopy at 77 K under light illumination with blue and red light emitting diodes (LED). We found that the average positron lifetime during the red-LED illumination were lower by about 10 ps than that during the blue-LED illumination. Calculated positron lifetimes for VZn, VZn-H and VO have been reported to be 207 ps, 179 ps and 159 ps, respectively. In principle, positron annihilation in the positively ionized vacancies cannot be observed. The excitation of VO during the blue LED illumination increased the number of positively ionized Vo and then the annihilation in VZn or VZn-H is considered to be dominant. The excited VO returned to the ground state by illumination with the red LED, and some of the positively charged VO were neutralized, leading to the decrease in the average positron lifetime. These results provide new insight into our understanding into the mechanism of PPC caused by anion vacancy at ZnO.
A single crystal ZnO synthesized by a hydrothermal growth method was irradiated to 6×1018 e−/cm2 with an 8 MeV electron beam from a KURNS-LINAC for introducing point defects. After that, the sample was measured by electron spin resonance (ESR) and positron annihilation lifetime spectroscopy at 77 K under light illumination with blue and red light emitting diodes (LED). We found that the average positron lifetime during the red-LED illumination were lower by about 10 ps than that during the blue-LED illumination. Calculated positron lifetimes for VZn, VZn-H and VO have been reported to be 207 ps, 179 ps and 159 ps, respectively. In principle, positron annihilation in the positively ionized vacancies cannot be observed. The excitation of VO during the blue LED illumination increased the number of positively ionized Vo and then the annihilation in VZn or VZn-H is considered to be dominant. The excited VO returned to the ground state by illumination with the red LED, and some of the positively charged VO were neutralized, leading to the decrease in the average positron lifetime. These results provide new insight into our understanding into the mechanism of PPC caused by anion vacancy at ZnO.