Improvement of the Photoconductive Characteristics of ZnO Single Crystals by Annealing

Abstract

The photoconductive characteristics of undoped ZnO single crystals grown by the hydrothermal method were improved by annealing in oxygen gas. The photocurrent in the visible light region decreased with annealing and became about 1/105 and about 1/104 of the value of the as-grown single crystal with annealing at 500°C and 600°C, respectively. The peak value of the photocurrent also decreased with annealing. The decay time of the photocurrent of annealed samples at 600°C was less than 1/104 of the value of the as-grown sample. Photosensitivity, visible rejection ratio and photoresponsivity of the sample annealed at 600°C were 4 × 104 (under illumination of 10 μW/cm2 at a wavelength of 360 nm), 3 × 103 and 2 × 103 A/W, respectively. These values are comparable to the values for a sample using a nitrogen-doped ZnO (ZnO:N) single crystal. Annealing of undoped ZnO single crystals at a relatively low temperature (500°C or 600°C) is effective for obtaining ZnO single crystals for an ultraviolet (UV) sensor with high performance. It is thought that the defects originating in oxygen vacancies of undoped ZnO single crystals were decreased by annealing in oxygen gas.