Contactless measurements of microwave-induced resistance oscillations in a ZnO/MgZnO heterojunction

Abstract

The high-frequency conductivity of the two dimensional electron system formed at a ZnO/MgZnO heterojunction was investigated in the 4--18 GHz frequency range at high Landau level fillings and in the regime of both integer quantum Hall effect. The conductivity was probed by measuring the transmission of the broadband coplanar waveguide deposited on the sample surface. At low magnetic fields in case the sample was additionally subjected to the exciting radiation of the 60--140 GHz frequencies, the high-frequency conductivity exhibited well developed microwave-induced oscillations. Remarkably, these oscillations were detected in the whole range of probe frequencies studied---up to 18 GHz---in contrast to GaAs/AlGaAs heterostructures, where microwave-induced oscillations were reported to smear out at probe frequencies around 1 GHz. Furthermore, for each exciting frequency the phase and the period of the microwave-induced oscillations revealed no dependence on the probe frequency and coincided with the period and phase of the microwave-induced resistance oscillations studied independently by dc transport measurements in the same sample. The characteristic probe frequency at which the oscillations start to degrade corresponds to the characteristic scattering time in the region between quantum scattering rates deduced from the cyclotron resonance linewidth and that extracted from the magnetic field dependence of the microwave-induced resistance oscillations.