Observation of microwave-induced magnetoresistance oscillations in a ZnO/Mg$_x$Zn$_{1-x}$O heterojunction

Microwave-induced magnetoresistance and magnetoconductance oscillations in the two-dimensional electron system have been detected in a ZnO/Mg$_x$Zn$_{1-x}$O heterojunction. The structure has a high electron density of $n=6.5\times 10^{11}$ cm$^{-2}$ and a very low mobility of $10^{5}\,$cm$^2$ V$^{-1}$s$^{-1}$. This clearly indicates that the mobility of two-dimensional electrons is not a critical parameter for the observation of microwave-induced oscillations. The samples were both Hall bars and Corbino disks. At least four oscillations have been resolved. Their amplitude increases with the magnetic field and all oscillations with numbers $>1$ have the same phase of $0.25$. It has been shown that the microwave-induced photoelectromotive force is also a periodically oscillating function of the inverse magnetic field.