Surface photovoltage response of zinc oxide microrods on prismatic planes: effect of UV, temperature and oxygen ambience

In this study, we report the time evaluation of work function measurements on ZnO microrods in Kelvin probe method under photoexcitation, called Surface Photovoltage (SPV). The SPV measurements are one of the most important techniques for characterizing both surface and bulk of the semiconductors. The ZnO microrods synthesized by hydrothermal method are highly crystalline with typical length of 50–100 µm and hexagonal cross section of about 2 µm. Upon drop-casting, most of the rods lie flat on the surface making the basal (002) plane vertical and the prismatic (101) and (100) plane on top. Thus, the SPV measurements are performed on these prismatic planes which are non-polar in nature. The SPV response of microrods has been studied for UV laser of photon energy 3.45 eV which is higher than that of the observed bandgap of 3.3 eV of ZnO microrods. The response is studied in varying oxygen ambience and temperatures. The average work function of the material at room temperature (25 °C) and ambient atmosphere is found to be 4.52 ± 0.3 eV. The work function was found to decrease with increasing temperature and upon photoexcitation. The photovoltage recovery time depicts that the presence of oxygen in atmosphere greatly accelerates the photocurrent recovery through trapping of photo-generated charge carriers by the surface states. The minority carrier diffusion length thus estimated from variable energy photoexcitation is found to be 160 nm.