Characterization of electrical properties and defects in Er- and Yb-doped ZnO thin films grown by sol-gel spin coating

Abstract

ZnO thin films have been used in various applications such as optoelectronic devices, solar cell window layers, UV detectors and space applications. The devices based on ZnO should be able to operate in harsh radiation conditions and over a wide range of temperatures, so the information of radiation effects on devices performance is required. ZnO is a wide, direct band gap semiconductor, and this makes it a good candidate for optoelectronic devices and an ideal host lattice for doping by rare-earth elements such as Er and Yb. In this study, the effect of rare-earth (Er and Yb) doping and co-doping of ZnO thin films deposited using a sol-gel spin coating technique, as well as the temperature dependence of the electrical properties of the Schottky diodes based on these films were investigated. The electrical properties and defects in Er-doped, Yb-doped and (Er, Yb) co-doped ZnO thin films were studied. The effect of alpha particle irradiation on the electrical properties of the Schottky barrier diodes and the resulting defects in the undoped ZnO thin films were also studied. The SEM images revealed the grain shape of the undoped material changed from spherical shape to rod-shaped after doping by Er and Yb (both 3 at.%). The XRD spectrum exhibited peaks corresponding to the hexagonal wurtzite structure with random orientations in all samples. A sharp peak corresponding to the E2 (high) phonon mode in the Raman spectra confirmed the wurtzite ZnO structure along with other small peaks observed. The films exhibited good transmittance in the visible region and a sharp absorption peak in the UV region. The optical band gap of the films was found to increase as the Er concentration increased, and decreased after co-doping with (Er, Yb) at different concentrations. Photoluminescence spectroscopy of Er and Yb doped found that all samples exhibited strong UV emission and a broad green emission band, with the undoped sample showing weaker green emission. The electrical properties improved through doping with either Er or Yb as well as co-doping of both Er and Yb, and were also found to improve with an increase in the measurement temperature (Schottky barrier height increased and ideality factor decreased). A common defect seen in single crystal ZnO E4 was also detected in the ZnO thin films deposited using the sol-gel spin coating method. Alpha particle irradiation induced new defects in the undoped ZnO thin films, some new and some corresponding to known defects.