Keiichiro Masuko

Influence of antiferromagnetic exchange interaction on magnetic properties of ZnMnO thin films grown pseudomorphically on ZnO (0001¯) single-crystal substrates

This study investigated the influence of antiferromagnetic exchange interaction on magnetic properties of ZnMnO thin films grown on ZnO (0001¯) single-crystal substrates. Using pulsed laser deposition, ZnMnO films with Mn concentrations as high as 14.1at.% were prepared. Investigations revealed that the lattice constant along the c axis of the ZnMnO films increases linearly with increasing Mn concentration. The lattice constant along the a axis is independent of the Mn concentration and is equal to that of the ZnO substrate, suggesting pseudomorphic growth of ZnMnO films. An atomic force microscopy image of the ZnMnO film grown at 640°C shows a step-and-terrace structure. An interference fringe around the ZnMnO 0004 symmetric diffraction image is clearly visible in the 2θ-ω x-ray diffraction profile, suggesting that the samples have a smooth surface and an abrupt interface. The intrinsic magnetic properties of these pseudomorphically grown ZnMnO films with smooth surfaces were evaluated. At temperatures g...

Electrical Characteristics of Controlled-Polarization-Type Ferroelectric-Gate Field-Effect Transistor

We propose a novel ferroelectric-gate field-effect transistor (FET) with a polar semiconductor channel, which is called a controlled-polarization-type ferroelectric-gate FET. The essential factor for this device is the stabilization of the spontaneous polarization of the ferroelectric film by the spontaneous polarization of a polar semiconductor, such as ZnO or GaN. We demonstrate two types of controlled-polarization-type ferroelectric-gate thin film transistors (TFTs), which are the ZnO/YMnO3/Pt bottom-gate-type structure and Pt/PZT/ZnO top-gate-type structure. Both structures show n-channel-type transistor operation with the memory effect originating from ferroelectricity. From the electrical characteristics of ferroelectric-gate TFTs, the effects of ferroelectricity, gate leakage current, space charge, and charge injection on the device operation are discussed.

Photoluminescence properties peculiar to the Mn-related transition in a lightly alloyed ZnMnO thin film grown by pulsed laser deposition

We have investigated photoluminescence properties of ZnO and Zn0.95Mn0.05O thin films at 10K grown on a (0001¯) ZnO crystal substrate by pulsed laser deposition. The structural characterization with x-ray diffraction and atomic force microscopy demonstrates the pseudomorphic growth of the Zn0.95Mn0.05O thin film and the atomically smooth surface. It has been found that a photoluminescence band originating from the d-d transition of Mn2+ in the Zn0.95Mn0.05O thin film appears in the energy region of deep-level transitions in a ZnO crystal: The photoluminescence-decay time is in the order of sub-milliseconds. The photoluminescence-excitation spectrum of the Mn-related transition exhibits a peaky structure with a broad profile at the energy lower than the A-exciton energy by ∼100meV. This indicates that the light incorporation of Mn to ZnO leads to a negative energy shift of the band-gap energy. The broad profile of the band-edge transition observed in the photoluminescence-excitation spectrum suggests that ...

Effect of Mn doping on the electric and dielectric properties of ZnO epitaxial films

The Mn doping effects on the electric and dielectric properties of ZnO epitaxial films were studied. To optimize the stoichiometry of undoped ZnO film prepared by pulsed laser deposition, we evaluated the intensity ratio of the optical emission from excited oxygen atoms (O*) and excited zinc atoms (Zn*) generated by ablating a ZnO ceramic target in plume. The leakage current of undoped ZnO film decreased with an increase of the ratio I(O777*)∕I(Zn335*). Mn doping of ZnO was also effective in decreasing the leakage current. The ZnO:4at.% Mn had a very small leakage current, eight orders of magnitude less than that of undoped ZnO. With a Mn concentration above 4at.%, on the other hand, the leakage current was increased. The activation energy in ZnO:Mn obtained from the temperature dependence of the ac conductivity revealed that oxygen vacancy was responsible for the large current at the Mn concentration below 2at.%. For the sample with the Mn concentration of 2–3at.%, dominant conduction in ZnO:Mn epitaxial...

Effects of Postannealing on Orientation and Crystallinity of P-Type Transparent Conducting CuScO2 Thin Films

Highly c-axis-oriented CuScO2 thin films were successfully fabricated from polycrystalline CuScO2 thin films by pulsed laser deposition and followed by postannealing treatment. The oxygen pressure effects during postannealing on the surface morphology, and crystallographic and optical properties of the films were investigated. The growth orientation, crystallinity along both out-of-plane and in-plane directions, and surface morphology of the films were significantly improved by postannealing at an optimal oxygen pressure. Using a film postannealed under optimal annealing conditions, the optical and electrical properties of the film were measured. The optical average transmittance of the film was greater than 70% in the visible/near-infrared regions, and the energy gap for calculated direct allowed transition was 3.7 eV. The Hall coefficient measured using the van der Pauw electrode configuration was 1.9×10+1 cm3 C-1, indicating p-type conduction. The resistivity, carrier concentration, and Hall mobility of the film at room temperature were 9.9×10+1 Ω cm, 3.2×10+17 cm-3, and 2.0×10-1 cm2 V-1 s-1, respectively.

Spin-dependent transport in a ZnMnO∕ZnO heterostructure

We report a spin-dependent transport property in modulation-doped ZnMnO:Al∕ZnMnO∕ZnO heterostructures. Using ZnO (0001¯) single crystal substrates, the heterostructures with atomically smooth surface and interface were prepared. For the sample with the Mn concentration of 12at.%, the electron confinement at the ZnMnO∕ZnO interface was recognized, indicating that ZnMnO layer acted as a barrier layer. In this paper, we investigated if an s-d exchange interaction exists between s electron conducting in the ZnO channel layer and Mn d spin in the ZnMnO barrier layer at the interface. To eliminate the Lorentz force and weak localization effects from the magnetoresistance (MR), the MR was measured under the magnetic field parallel to the sample surface. For a ZnMnO∕ZnO heterostructure with the sheet carrier concentration of 5.46×1012cm−2, at 1.85K, the parallel-field MR revealed that a positive MR was dominant below 0.8T, while a negative MR was recognized above 0.8T. The positive MR was well fitted to a Brillou...

Fabrication and Magneto-Transport Properties of Zn0.88-xMgxMn0.12O/ZnO Heterostructures Grown on ZnO Single-Crystal Substrates

The transport properties of Zn0.88-xMgxMn0.12O/ZnO modulation-doped heterostructures (x≤0.15) were investigated. The heterostructures were fabricated on ZnO single-crystal substrates by a pulsed laser deposition system. Atomic force microscope observation and X-ray diffraction analysis suggested that Zn0.88-xMgxMn0.12O layers have atomically flat surface and excellent crystallinity. The results of Hall measurement for Zn0.88-xMgxMn0.12O/ZnO modulation-doped heterostructure with x=0.075 revealed that the carrier concentration and the electron mobility were 5.1×1012cm-2 and 800 cm2/Vs at 10 K, respectively, suggesting that the carrier confinement effect exits at the heterointerface between Zn0.88-xMgxMn0.12O barrier layer and ZnO channel layer. In the magnetoresistance (MR) measurement at 1.85 K, a positive MR behavior was observed below 0.5 T, while a negative MR behavior was recognized above 0.5 T. The slope of the positive MR decreased with increasing the temperature and was well fitted to the Brillouin function with S=5/2. The electrical and magneto-transport properties were very similar to those of Zn0.88Mn0.12O/ZnO heterostructures without doping Mg.