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

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.

Figure 1

Typical dc voltage measured at the Shottky diode detector output vs the magnetic field. The probe frequency was fixed at 7 GHz and the temperature was equal to 1.5 K. The positions of several Shubnikov–de Haas minima are indicated. In the inset the experimental setup is schematically illustrated.

Figure 2

(a) Typical dependence of the transmitted power on the magnetic field in the presence of exciting radiation (red line) and without any exciting radiation (blue line). The frequencies of the probe and exciting radiation were fixed at 7 and 140 GHz; temperature was 1.5 K. (b) The variation of the transmitted power induced by the exciting radiation measured with the aid of lock-in technique under the same conditions as in panel (a). The positions of the first several oscillations are indicated. (c) The dependence of the inverse magnetic field positions of the oscillations on $i=n-1/4$ for minima and $i=n+1/4$ for maxima. The solid line represents the fit with the equation $i=2\pi \text{F}{m}^{*}/eB$.

Figure 3

Magnetic field dispersion of the microwave-induced oscillations of the transmitted power through the CPW line. The correspondent oscillations number is denoted near each data set. The lines represent fits according to $\mathrm{F}=(n+1/4)eB/2\pi m^{*}$ for the position of $n\text{th}$ maxima and $\mathrm{F}=(n-1/4)eB/2\pi m^{*}$ for the $n\text{th}$ minima. The fits were performed with a single free parameter—effective electron mass $m^{*}=0.33m_0$. The probe frequency was fixed at $f=7\mathrm{GHz}$.

Figure 4

On the left panel the variation of the transmitted power induced by the exciting radiation of 75.9 GHz frequency is presented for a set of probe frequencies. The traces are offset for clarity. On the right panel the transmitted power is demonstrated in the regime of high magnetic field where the Shubnikov–de Haas oscillations are fully developed. The power value is normalized by the amount of the transmitted power at the exact filling factor of $\nu =5$. The traces are offset for clarity.