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.

We have grown nitrogen-doped Mg x Zn 1−x O : N films on Zn-polar ZnO single crystal substrates by molecular beam epitaxy. As N-sources, we employed NO-plasma or NH 3 gas itself. As x increased, optimum growth temperature window for smooth film morphology shifted to higher temperatures, while maintaining high N-concentration ͑ϳ1 ϫ 10 19 cm −3 ͒. The heterosructures of Mg x Zn 1−x O:N ͑0.1Յ x Յ 0.4͒ / ZnO were fabricated into light emitting diodes of 500-m-diameter. We observed ultraviolet near-band-edge emission ͑ ϳ 382 nm͒ with an output power of 0.1 W for a NO-plasma-doped LED and 70 W for a NH 3-doped one at a bias current of 30 mA.

We report on a high performance visible-blind Schottky ultraviolet photodiode composed of a ZnO ͑0001͒ bulk single crystal and a transparent conducting polymer, poly͑3,4-ethylenedioxythiophene͒ poly͑styrenesulfonate͒, fabricated with a simple spin-coating process at room temperature in air. The quantum efficiency as high as unity in ultraviolet region and a visible rejection ratio of about 10 3 were achieved in the spectral response of the photodiode under zero-bias condition. The normalized detectivity of the photodiode was evaluated to be 3.6ϫ 10 14 cm Hz 1/2 / W at 370 nm.

Nominally undoped MgxZn1-xO/ZnO (x = 0.05 and 0.08) single heterostructures were prepared on Zn-polar ZnO substrates by using plasma assisted molecular beam epitaxy (MBE). The samples showed a metallic conductivity below 50 K and a mobility exceeding 104 cm2 V-1 s-1 at 0.5 K. We observed quantum Hall effect accompanying Shubnikov–de Haas oscillations, in which zero-resistance states were clearly seen above 5 T. Rotation experiments in magnetic field suggest strong two-dimensional carrier confinement at low temperatures. The results indicate that the MBE grown films have much higher quality than the previously reported samples grown by pulsed laser deposition.

The excellent structural and optical properties of pseudomorphic MgxZn1-xO films (0≤x≤0.39) are reported in this work. The MgxZn1-xO films were grown on Zn-polar ZnO substrates by plasma-assisted molecular beam epitaxy. Those MgxZn1-xO films for which x≤0.18 exhibited atomically flat surfaces, and the typical full-width-at-half-maximum (FWHM) value of the (0002) X-ray diffraction ω-rocking curves for these films was 35 arcsec. The FWHM values were less than 100 meV for the near-band-edge photoluminescence (PL) at 300 K. We observed PL lifetimes of the order of ns, and the longest fast-decay component reached 3.5 ns for the Mg0.12Zn0.88O alloy.