Atsushi Tsukazaki

Quantum Hall Effect in Polar Oxide Heterostructures

We observed Shubnikov–de Haas oscillation and the quantum Hall effect in a high-mobility two-dimensional electron gas in polar ZnO/MgxZn1–xO heterostructures grown by laser molecular beam epitaxy. The electron density could be controlled in a range of 0.7 × 1012 to 3.7 × 1012 per square centimeter by tuning the magnesium content in the barriers and the growth polarity. From the temperature dependence of the oscillation amplitude, the effective mass of the two-dimensional electrons was derived as 0.32 ± 0.03 times the free electron mass. Demonstration of the quantum Hall effect in an oxide heterostructure presents the possibility of combining quantum Hall physics with the versatile functionality of metal oxides in complex heterostructures.

Blue Light-Emitting Diode Based on ZnO

A near-band-edge bluish electroluminescence (EL) band centered at around 440 nm was observed from ZnO p–i–n homojunction diodes through a semi-transparent electrode deposited on the p-type ZnO top layer. The EL peak energy coincided with the photoluminescence peak energy of an equivalent p-type ZnO layer, indicating that the electron injection from the n-type layer to the p-type layer dominates the current, giving rise to the radiative recombination in the p-type layer. The imbalance in charge injection is considered to originate from the lower majority carrier concentration in the p-type layer, which is one or two orders of magnitude lower than that in the n-type one. The current-voltage characteristics showed the presence of series resistance of several hundreds ohms, corresponding to the current spread resistance within the bottom n-type ZnO. The employment of conducting ZnO substrates may solve the latter problem.

Correlation between the photoluminescence lifetime and defect density in bulk and epitaxial ZnO

Influences of point defects on the nonradiative processes in ZnO were studied using steady-state and time-resolved photoluminescence (PL) spectroscopy making a connection with the results of positron annihilation measurement. Free excitonic PL intensity naturally increased with the increase in the nonradiative PL lifetime (τnr). Density or size of Zn vacancies (VZn) decreased and τnr increased with increasing growth temperature in heteroepitaxial films grown on a ScAlMgO4 substrate. Use of homoepitaxial substrate further decreased the VZn density. However, τnr was the shortest for the homoepitaxial film; i.e., no clear dependence was found between τnr and density / size of VZn or positron scattering centers. The results indicated that nonradiative recombination processes are not solely governed by single point defects, but by certain defect species introduced by the presence of VZn such as vacancy complexes.

Observation of the fractional quantum Hall effect in an oxide

The quantum Hall effect arises from the cyclotron motion of charge carriers in two-dimensional systems. However, the ground states related to the integer and fractional quantum Hall effect, respectively, are of entirely different origin. The former can be explained within a single-particle picture; the latter arises from electron correlation effects governed by Coulomb interaction. The prerequisite for the observation of these effects is extremely smooth interfaces of the thin film layers to which the charge carriers are confined. So far, experimental observations of such quantum transport phenomena have been limited to a few material systems based on silicon, III-V compounds and graphene. In ionic materials, the correlation between electrons is expected to be more pronounced than in the conventional heterostructures, owing to a large effective mass of charge carriers. Here we report the observation of the fractional quantum Hall effect in MgZnO/ZnO heterostructures grown by molecular-beam epitaxy, in which the electron mobility exceeds 180,000 cm(2) V(-1) s(-1). Fractional states such as ν = 4/3, 5/3 and 8/3 clearly emerge, and the appearance of the ν = 2/5 state is indicated. The present study represents a technological advance in oxide electronics that provides opportunities to explore strongly correlated phenomena in quantum transport of dilute carriers.

Pulsed laser deposition of thin films and superlattices based on ZnO

The pulsed laser deposition (PLD) technique has been applied for the epitaxial growth of ZnO for more than two decades. The emergence of high-temperature stability of the excitonic lasing was first demonstrated in a microcrystalline ZnO film grown by PLD leading to recent remarkable growth in this field. A number of attempts have been made to improve the crystallinity for realizing p-type materials in the quest for ZnO-based short wavelength light emitting devices (LEDs). In this paper, we describe practical advantages of PLD and currently accomplished intrinsic properties of ZnO films according to the abundant literature. We find that correlation between Hall mobility and lateral grain size captures the effect of grain boundaries for the films grown on sapphire substrates. On the other hand, advantages of the use of lattice-matched ScAlMgO4 substrate are evidenced by the lower residual electron density, higher mobility and sharper exciton peaks in the photoluminescence and absorption spectra. We also focus on the wide-band-gap ternary alloy, MgxZn1−xO, especially in terms of the composition dependence of its lattice parameters and band-gap in two different crystallographic phases, to discuss the stability of this metastable compound. The studies on the PLD growth of multilayer and superlattices are briefly reviewed. We finally present the current capability of electron and hole doping by incorporating Ga and N into films grown on (0001) ScAlMgO4 substrates. We conclude that the PLD technique and related technologies have now mature to meet the requirements for fabricating UV-LEDs.

Nitrogen doped MgxZn1−xO/ZnO single heterostructure ultraviolet light-emitting diodes on ZnO substrates

We have grown nitrogen-doped MgxZn1−xO:N films on Zn-polar ZnO single crystal substrates by molecular beam epitaxy. As N-sources, we employed NO-plasma or NH3 gas itself. As x increased, optimum growth temperature window for smooth film morphology shifted to higher temperatures, while maintaining high N-concentration (∼1×1019 cm−3). The heterosructures of MgxZn1−xO:N (0.1≤x≤0.4)/ZnO were fabricated into light emitting diodes of 500-μm-diameter. We observed ultraviolet near-band-edge emission (λ∼382 nm) with an output power of 0.1 μW for a NO-plasma-doped LED and 70 μW for a NH3-doped one at a bias current of 30 mA.

Trajectory of the anomalous Hall effect towards the quantized state in a ferromagnetic topological insulator

Quantized resistivity values for 2D electron systems don’t necessarily result from an external magnetic field as in the ‘normal’ quantum Hall effect; they can arise due to a materials intrinsic ferromagnetism too—the quantum anomalous Hall effect. Experiments with a ferromagnetic topological insulator now establish how the anomalous states can be mapped onto the normal states.

Gallium concentration dependence of room-temperature near-band-edge luminescence in n-type ZnO:Ga

We investigated the optical properties of epitaxial n-type ZnO films grown on lattice-matched ScAlMgO4 substrates. As the Ga doping concentration increased up to 6×1020cm−3, the absorption edge showed a systematic blueshift, consistent with the Burstein–Moss effect. A bright near-band-edge photoluminescence (PL) could be observed even at room temperature, the intensity of which increased monotonically as the doping concentration was increased except for the highest doping level. It indicates that nonradiative transitions dominate at a low doping density. Both a Stokes shift and broadening in the PL band are monotonically increasing functions of donor concentration, which was explained in terms of potential fluctuations caused by the random distribution of donor impurities.

Insulator-to-metal transition in ZnO by electric double layer gating

The authors report high-density carrier accumulation and a gate-induced insulator-to-metal transition in ZnO single-crystalline thin-film field effect transistors by adopting electric double layers as gate dielectrics. Hall effect measurements showed that a sheet carrier density of 4.2×1013cm−2 was achieved. The highest sheet conductance at room temperature was ∼1mS, which was sufficient to maintain the metallic state down to 10K. These results strongly suggest the versatility of electric double layer gating for various materials.

Electron transport in ZnO thin films

Epitaxial n-type ZnO films grown by a laser molecular-beam epitaxy method were investigated by the temperature-dependent Hall-effect technique. The 300K carrier concentration and mobility were about ns∼1016cm−3 and 440cm2∕Vs, respectively. Transport characteristics are calculated by solving the Boltzmann transport equation using a variational method. Mobility limit of 430cm2∕Vs was calculated at 300K. The temperature dependence of the mobility for an undoped film is calculated and agrees favorably well with experimental data if physical parameters are chosen so as to approach those. In the experimental “mobility versus concentration” curve, unusual phenomenon was observed, i.e., mobilities at ns∼5×1018cm−3 are significantly smaller than those at higher densities above ∼1020cm−3. It is qualitatively explained in terms of electron-plasmon interaction.