Joseph Falson

Magnesium Doping Controlled Density and Mobility of Two-Dimensional Electron Gas in MgxZn1-xO/ZnO Heterostructures

The magnesium content in MgxZn1-xO/ZnO heterostructures grown by molecular beam epitaxy enables the careful control of the carrier density of the two-dimensional electron system down to 5.6×1010 cm-2 while retaining a mobility of 200,000 cm2 V-1 s-1 when pursuing magnesium concentrations as low as x = 0.0038. By selecting an optimum magnesium content (x~0.01), the mobility is enhanced to over 700,000 cm2 V-1 s-1 due to reduced impurity levels associated with the use of pure distilled ozone and avoiding interface roughness scattering. This control technique allows access to the coherent transport region with strong electron–electron interaction.

Temperature-dependent magnetotransport around ν=1/2 in ZnO heterostructures.

The sequence of prominent fractional quantum Hall states up to ν=5/11 around ν=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 R(xx) oscillations around ν=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 R(xx) at ν=1/2 point to large residual interactions between composite fermions.

MgZnO/ZnO heterostructures with electron mobility exceeding 1 × 10 6 cm 2 /Vs

The inherently complex chemical and crystallographic nature of oxide materials has suppressed the purities achievable in laboratory environments, obscuring the rich physical degrees of freedom these systems host. In this manuscript we provide a systematic approach to defect identification and management in oxide molecular beam epitaxy grown MgZnO/ZnO heterostructures which host two-dimensional electron systems. We achieve samples displaying electron mobilities in excess of 1 × 106 cm2/Vs. This data set for the MgZnO/ZnO system firmly establishes that the crystalline quality has become comparable to traditional semiconductor materials.

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.

Optical probing of MgZnO/ZnO heterointerface confinement potential energy levels

Low-temperature photoluminescence and reflectance measurements were employed to study the optical transitions present in two-dimensional electron systems confined at MgxZn1–xO/ZnO heterojunctions. Transitions involving A- and B-holes and electrons from the two lowest subbands formed within the confinement potential are detected. In the studied density range of 2.0–6.5 × 1011 cm−2, the inter-subband splitting is measured and the first excited electron subband is shown to be empty of electrons.

Calibration and control of in-plane Mg doping distribution in MgxZn1−xO/ZnO heterostructures grown by molecular beam epitaxy

The in-plane Mg doping distribution in molecular beam epitaxy grown MgxZn1−xO/ZnO heterostructures is mapped by low-temperature photoluminescence measurements in an effort to evaluate and control the resultant inhomogeneity formed during the growth process. In an unrotated sample, the independent configuration effects of the O3 and Mg source cells are clearly demonstrated in a composition spread due to flux gradients, while this inhomogeneity is suppressed by sample rotation during the growth. The present mapping results provide an important means for investigating improved doping regimes with the aim of enhancing the quality of quantum transport observable at the MgxZn1−xO/ZnO heterointerface.

Composite fermion liquid to Wigner solid transition in the lowest Landau level of zinc oxide

Interactions between the constituents of a condensed matter system can drive it through a plethora of different phases due to many-body effects. A prominent platform for it is a dilute two-dimensional electron system in a magnetic field, which evolves intricately through various gaseous, liquid and solid phases governed by Coulomb interaction. Here we report on the experimental observation of a phase transition between the composite fermion liquid and adjacent magnetic field induced phase with a character of Wigner solid. The experiments are performed in the lowest Landau level of a MgZnO/ZnO two-dimensional electron system with attributes of both a liquid and a solid. An in-plane magnetic field component applied on top of the perpendicular magnetic field extends the Wigner-like phase further into the composite fermion liquid phase region. Our observations indicate the direct competition between a composite fermion liquid and a Wigner solid formed either by electrons or composite fermions.In two-dimensional electron systems, strong Coulomb interactions lead to the formation of new phases. Here the authors observe a transition between two of these correlated phases, a composite fermion liquid and Wigner solid, in a zinc oxide heterostructure.