Tetsuhiro Tanabe

Fabrication and photoresponse of a pn-heterojunction diode composed of transparent oxide semiconductors, p-NiO and n-ZnO

Transparent trilayered oxide films of ZnO/NiO/indium tin oxide were heteroepitaxially grown on a YSZ (111) substrate by pulsed-laser deposition combined with a solid-phase-epitaxy technique, and were processed to fabricate a p-NiO/n-ZnO diode. The diode exhibited a clear rectifying I–V characteristic with an ideality factor of ∼2 and a forward threshold voltage of ∼1 V. Although the photoresponsivity was fairly weak at the zero-bias voltage, it was enhanced up to ∼0.3 A W−1 through the application of a reverse bias of −6 V under the irradiation of 360 nm light, a value comparable to that of commercial devices.

Uniaxial locked epitaxy of ZnO on the a face of sapphire

High-quality, c-oriented ZnO epitaxial films have been grown on the a surface using molecular-beam epitaxy. The use of a-oriented sapphire eliminates rotational domains and related structural defects which have limited the use of ZnO in electronic applications. The ZnO epitaxial layers are uniquely oriented with the ZnO/sapphire orientational relationship [0001]‖[1120] and 〈1120〉‖[0001]. This unique orientation is a consequence of the anisotropy of the a-sapphire surface in conjunction with a strong correlation along a single direction leading to the term uniaxial locked epitaxy. High-resolution x-ray diffraction measurements show an increase in x-ray lateral coherence length from several tens of nanometers to >0.7 μm for growth of c-oriented ZnO on the a surface as opposed to the c surface of sapphire.

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.

Growth of Undoped ZnO Films with Improved Electrical Properties by Radical Source Molecular Beam Epitaxy

High-quality undoped ZnO epitaxial films with mobilities as high as 120 cm2V-1s-1 and carrier concentrations as low as 7.6 ×1016 cm-3 have been grown on (1120) a-sapphire substrates using low-temperature buffer layers, a slow substrate cooling process and a modified oxygen radical cell. Pole figure measurements reveal that a-plane sapphire substrates are effective for the elimination of 30° rotation domains, which usually appear in the case of ZnO growth on c-sapphire. The low-temperature buffer layers allow high-temperature growth, because initial ZnO growth does not occur with high initial growth temperature. The use of slow substrate cooling prevents the deterioration of the electrical properties of the ZnO films. Use of quartz insulators in the oxygen radical cell eliminates aluminum contamination, which is a serious problem when using conventional alumina insulators.

Spatial composition fluctuations in blue-luminescent ZnCdO semiconductor films grown by molecular beam epitaxy

Abstract The optical and structural properties of textured ZnCdO films that exibit a newly found distinct blue luminescence have been investigated. Microscopic compositional fluctuations of Cd, associated with hexagonal grain structures, were observed. Models for the blue-luminescence process are proposed.

High Electron Mobility Exceeding 104 cm2 V-1 s-1 in MgxZn1-xO/ZnO Single Heterostructures Grown by Molecular Beam Epitaxy

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.

Effects of oxygen plasma condition on MBE growth of ZnO

We report growth condition optimizations of ZnO films by plasma-assisted molecular beam epitaxy. Effects of oxygen plasma conditions on film quality were evaluated by photoluminescence, X-ray characterization and scanning electron microscopy. From the growth experiments on c-plane sapphire and bulk ZnO (+ c, - c) substrates, it was suggested that the surface morphology is strongly related with the growth direction of ZnO parallel to its c-axis, and was found to be controllable by optimization of oxygen plasma conditions.

Plasma-assisted Molecular Beam Epitaxy of High Optical Quality MgZnO Films on Zn-polar ZnO Substrates

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.

Blue Photoluminescence from ZnCdO Films Grown by Molecular Beam Epitaxy

Blue luminescence from textured ZnCdO films grown by molecular beam epitaxy was observed. The properties of ZnCdO films grown at various Zn/Cd partial pressures were studied by several techniques including X-ray diffraction, fluorescence (FL) microscopy and Auger electron spectroscopy. With increasing CdO content, the properties of ZnCdO films changed sharply at specific Zn/Cd partial pressures. Some of the films grown at approximately the threshold pressures showed blue luminescence at room temperature, with large Stokes shift leaving the absorption edge in the UV region. By FL microscopy, it was observed that the blue luminescent regions were nonuniformly distributed, suggesting composition fluctuation.

Direct correlation between the internal quantum efficiency and photoluminescence lifetime in undoped ZnO epilayers grown on Zn-polar ZnO substrates by plasma-assisted molecular beam epitaxy

The equivalent internal quantum efficiency (ηinteq) at 300K of the near-band-edge excitonic photoluminescence (PL) peak in ZnO epilayers grown by plasma-assisted molecular beam epitaxy on Zn-polar ZnO substrates was directly correlated with the PL lifetime (τPL) for the first time. This relation seems to be universal for O-polar ZnO films grown by other methods. Present homoepitaxial ZnO epilayers grown above 800°C exhibited atomically flat surfaces, and the best full-width-at-half-maximum value of (0002) ZnO x-ray diffraction ω-rocking curves was 17.6arcsec. The high-temperature growth also led to a long τPL of 1.2ns at 300K. As a result, a record high ηinteq value (9.6%) was eventually obtained under an excitation density of 5W∕cm2 (He–Cd, 325.0nm). The homoepitaxial Zn-polar ZnO films grown by molecular beam epitaxy are coming to be used for p-n junction devices.