Polarization dependent Landau level crossing in a two-dimensional electron system in MgZnO/ZnO-heterostructure

We report electrical transport measurements in a tilted magnetic field on a high-mobility two-dimensional electron system confined at the MgZnO/ZnO heterointerface. The observation of multiple crossing events of spin-resolved Landau levels (LLs) enables the mapping of the sequence of electronic states. We further measure the renormalization of electron spin susceptibility at zero field and the susceptibility dependence on the electron spin polarization. The latter manifests the deviation from the Pauli spin susceptibility. As the result, the crossing of spin-resolved LLs shifts to smaller tilt angles and the first Landau level coincidence event is absent even when the magnetic field has only a perpendicular component to the 2DES plane.Comment: 5 pages, 4 figure

Nonlinear response of a MgZnO/ZnO heterostructure close to zero bias

We report on magnetotransport properties of a MgZnO/ZnO heterostructure subjected to weak direct currents. We find that in the regime of overlapping Landau levels, the differential resistivity acquires a quantum correction proportional to both the square of the current and the Dingle factor. The analysis shows that the correction to the differential resistivity is dominated by a current-induced modification of the electron distribution function and allows us to access both quantum and inelastic scattering rates.Comment: 4 pages, 3 figure

Temperature dependent magnetotransport around ν\nu= 1/2 in ZnO heterostructures

The sequence of prominent fractional quantum Hall states up to $\nu$=5/11 around $\nu$=1/2 in a high mobility two-dimensional electron system confined at oxide heterointerface (ZnO) is analyzed in terms of the composite fermion model. The temperature dependence of \Rxx oscillations around $\nu$=1/2 yields an estimation of the composite fermion effective mass, which increases linearly with the magnetic field. This mass is of similar value to an enhanced electron effective mass, which in itself arises from strong electron interaction. The energy gaps of fractional states and the temperature dependence of \Rxx at $\nu$=1/2 point to large residual interactions between composite fermions.Comment: 5 pages, 4 Figure

Electron scattering times in ZnO based polar heterostructures

The remarkable historic advances experienced in condensed matter physics have been enabled through the continued exploration and proliferation of increasingly richer and cleaner material systems. In this work, we report on the scattering times of charge carriers confined in state-of-the-art MgZnO/ZnO heterostructures displaying electron mobilities in excess of 10⁶ cm²/V s. Through an examination of low field quantum oscillations, we obtain the effective mass of charge carriers, along with the transport and quantum scattering times. These times compare favorably with high mobility AlGaAs/GaAs heterostructures, suggesting the quality of MgZnO/ZnO heterostructures now rivals that of traditional semiconductors

Precise calibration of Mg concentration in Mg_xZn_(1−x)O thin films grown on ZnO substrates

The growth techniques for Mg_xZn_(1−x)O thin films have advanced at a rapid pace in recent years, enabling the application of this material to a wide range of optical and electrical applications. In designing structures and optimizing device performances, it is crucial that the Mg content of the alloy be controllable and precisely determined. In this study, we have established laboratory-based methods to determine the Mg content of Mg_xZn_(1−x)O thin films grown on ZnO substrates, ranging from the solubility limit of x ∼ 0.4 to the dilute limit of x < 0.01. For the absolute determination of Mg content, Rutherford backscattering spectroscopy is used for the high Mg region above x = 0.14, while secondary ion mass spectroscopy is employed to quantify low Mg content. As a lab-based method to determine the Mg content, c-axis length is measured by x-ray diffraction and is well associated with Mg content. The interpolation enables the determination of Mg content to x = 0.023, where the peak from the ZnO substrate overlaps the Mg_xZn_(1−x)O peak in standard laboratory equipment, and thus limits quantitative determination. At dilute Mg contents below x = 0.023, the localized exciton peak energy of the Mg_xZn_(1−x)O films as measured by photoluminescence is found to show a linear Mg content dependence, which is well resolved from the free exciton peak of ZnO substrate down to x = 0.0043. Our results demonstrate that x-ray diffraction and photoluminescence in combination are appropriate methods to determine Mg content in a wide Mg range from x = 0.004 to 0.40 in a laboratory environment

Alloy disorder modulated electron transport at Mg_xZn_(1-x)O/ZnO heterointerface

High-mobility electron systems in two dimensions have been the platform for realizing many fascinating quantum phenomena at low temperatures. Continuous demand to improve the sample quality has necessitated the investigations of various disorders influencing the quantum transport. Here, we study the effect of short-ranged alloy disorder on the scattering of two-dimensional electron system in Mg_xZn_(1-x)O/ZnO. For this purpose, we employ a modified interface profile consisting of Mg_(0.01)Zn_(0.99)O/ZnO with a thin (2nm) Mg_xZn_(1-x)O interlayer with x ranging from 0.005 to 0.4. This interlayer design allows us to investigate scattering mechanisms at a nearly constant carrier density as the interlayer is found not to significantly affect the carrier density but enhance alloy disorder. While the transport scattering time (τ_(tr)) shows a strong correlation with x, the quantum scattering time (τ_q) remains insensitive to x. The large variation in the τ_(tr)/τ_q ratio (from 16.2 to 1.5 corresponding to x from 0.005 to 0.4) implies a change in the dominant scattering mechanism from long range towards short range with increasing x. The insensitivity of τq on x indicates the scattering rate is not dominated by the alloy disorder. This implies that other scattering mechanisms, likely unintentional background impurities or remote surface disorders, are dominant in limiting τ_q, and therefore providing a prospect for pursuing ever higher levels in the quality of the two-dimensional electron system in Mg_xZn_(1-x)O/ZnO system