Drastic changes in electronic properties of a ZnO-SiO2-Au device by ultraviolet illumination

Abstract

Oxidative gases are chemisorbed on the semiconductor surface by capturing electrons of a semiconductor and then are desorbed by releasing electrons of chemisorption states [1]. The releasing of electrons can be attained by thermionic emission, field emission, and hole injection. The thermionic process is predominant as the temperature is raised. In general, chemisorptive states of oxidative gases are located at a few electronvolts below the bot tom of the conduction band of an n-type semiconductor, e.g. 0.8 to 1.0 eV for O2 on the ZnO surface [2], so that a high temperature is necessary to excite electrons from the chemisorptive states into the conduction band. On the other hand, the field-assisted desorption of oxygen was recently confirmed in an c~-F%O3-Au diode, as reported in a previous paper [3]. Desorption by hole injection was found in the photodesorption of oxygen from ZnO powder [4]. However, the desorption by hole injection has not been qualitatively studied, therefore the purpose of this work was to elucidate this process, and it is applied to develop a gas sensor with high sensitivity even at room temperature. For this purpose, a Z n O SiO2-Au device is fabricated to generate effectively holes at the semiconductor surface by band gap illumination, and the adsorption and desorption of oxygen is intensively investigated by considering the change in the leakage current of the device. ZnO film was deposited on a glass substrate (Corning 7059) using an r.f. magnetron sputtering apparatus. The sputtering was performed at 50W r.f. power under 1 Pa of an 95% Ar + 5% 02 atmosphere for 135 min. SiO2 was subsequently deposited on the sputtered ZnO surface. Sputtering conditions were the same as those for the ZnO sputtering. Gold was deposited on the SiO2 surface by vacuum evaporation. A high-pressure 500 W mercury lamp was used as a light source for ultraviolet illumination. Photocurrent spectra were measured using a 500 W xenon lamp, a monochromator , a light chopper (80 Hz) and a lock-in amplifier. Impedance measurements of the Z n O SiO2-Au device were carried out for the assignment of the resistance change before and after ultraviolet illumination. Two peaks of (0 0 2) and (0 0 4) reflections were seen in X-ray diffraction patterns of the sputtered ZnO film, except for a negligible weak peak of(1 0 3) reflection. Thus, the sputtered ZnO film had a c-axis