Tomoteru Fukumura

An oxide-diluted magnetic semiconductor: Mn-doped ZnO

Epitaxial thin films of an oxide-diluted magnetic semiconductor, Mn-doped ZnO, were fabricated by pulsed-laser deposition technique. Solubility of Mn into ZnO exceeds thermal equilibrium limit as a result of nonequilibrium film growth process. As Mn content is increased, the lattice constants of both a and c axes of wurtzite Zn1−xMnxO films (x<0.35) increase and the band gap expands although considerable in-gap absorption develops. Itinerant electrons over 1019 cm−3 can be doped into the Zn1−xMnxO films by Al doping, in contrast to low carrier density in the other II–VI diluted magnetic semiconductors. The temperature dependence of the resistivity is almost metallic and considerable magnetoresistance is observed at low temperatures.

High throughput fabrication of transition-metal-doped epitaxial ZnO thin films: A series of oxide-diluted magnetic semiconductors and their properties

Combinatorial laser molecular-beam epitaxy method was employed to fabricate epitaxial ZnO thin films doped with all the 3d transition metal (TM) ions in a high throughput fashion. The solubility behavior of TM ions was discussed from the viewpoints of the ionic radius and valence state. The magneto-optical responses coincident with absorption spectra were observed for Mn- and Co-doped samples. Cathodoluminescence spectra were studied for Cr-, Mn-, Fe-, and Co-doped samples, among which Cr-doped ZnO showed two sharp peaks at 2.97 eV and 3.71 eV, respectively, at the expense of the exciton emission peak of pure ZnO at 3.25 eV. Different magnetoresistance behavior was observed for the samples codoped with n-type carriers. Ferromagnetism was not observed for Cr- to Cu-doped samples down to 3 K.

Magnetic properties of Mn-doped ZnO

We report on the magnetic properties of an oxide-diluted magnetic semiconductor (DMS), Zn0.64Mn0.36O. The temperature dependence of the magnetization shows a spin-glass behavior with the large magnitude of the Curie–Weiss temperature, corresponding to a stronger antiferromagnetic exchange coupling than other II–VI DMSs. The small effective Mn moment (x∼0.02) under high field also represents a strong antiferromagnetic exchange coupling in this compound.

Electrically induced ferromagnetism at room temperature in cobalt-doped titanium dioxide.

The magnetic properties of a magnetic insulator can be controlled by an electric field at room temperature. The electric field effect in ferromagnetic semiconductors enables switching of the magnetization, which is a key technology for spintronic applications. We demonstrated electric field–induced ferromagnetism at room temperature in a magnetic oxide semiconductor, (Ti,Co)O2, by means of electric double-layer gating with high-density electron accumulation (>1014 per square centimeter). By applying a gate voltage of a few volts, a low-carrier paramagnetic state was transformed into a high-carrier ferromagnetic state, thereby revealing the considerable role of electron carriers in high-temperature ferromagnetism and demonstrating a route to room-temperature semiconductor spintronics.

High mobility thin film transistors with transparent ZnO channels

We have fabricated high performance ZnO thin film transistors (TFTs) using CaHfOx buffer layer between ZnO channel and amorphous silicon?nitride gate insulator. The TFT structure, dimensions, and materials set are identical to those of the commercial amorphous silicon (a-Si) TFTs in active matrix liquid crystal display, except for the channel and buffer layers replacing a-Si. The field effect mobility can be as high as 7 cm2?V-1?s-1 for devices with maximum process temperature of 300?C. The process temperature can be reduced to 150?C without much degrading the performance, showing the possibility of the use of polymer substrate.

Modeling and simulation of polycrystalline ZnO thin-film transistors

Thin-film transistors (TFTs) made of transparent channel semiconductors such as ZnO are of great technological importance because their insensitivity to visible light makes device structures simple. In fact, there have been several demonstrations of ZnO TFTs achieving reasonably good field effect mobilities of 1–10 cm2/V s, but the overall performance of ZnO TFTs has not been satisfactory, probably due to the presence of dense grain boundaries. We modeled grain boundaries in ZnO TFTs and performed simulation of a ZnO TFT by using a two-dimensional device simulator in order to determine the grain boundary effects on device performance. Polycrystalline ZnO TFT modeling was started by considering a single grain boundary in the middle of the TFT channel, formulated with a Gaussian defect distribution localized in the grain boundary. A double Schottky barrier was formed in the grain boundary, and its barrier height was analyzed as a function of defect density and gate bias. The simulation was extended to TFTs ...

Magneto-optical properties of ZnO-based diluted magnetic semiconductors

Magnetic circular dichroism (MCD) spectra of Zn1−xTMxO (TM=Sc, Ti, V, Cr, Mn, Co, Ni, and Cu) films were measured to clarify their magneto-optical properties. Noticeable magneto-optical effect was not observed for Zn1−xScxO, Zn1−xTixO, Zn1−xVxO, and Zn1−xCrxO. On the contrary, Zn1−xMnxO, Zn1−xCoxO, Zn1−xNixO, and Zn1−xCuxO showed pronounced negative MCD peaks near 3.4 eV which correspond to the band gap of the host ZnO semiconductor. These results show that ZnO alloyed with Mn, Co, Ni, and Cu is a diluted magnetic semiconductor with strong exchange interaction between sp-band carriers and localized d electrons.

Exploration of oxide-based diluted magnetic semiconductors toward transparent spintronics

A review is given for the recent progress of research in the field of oxide-based diluted magnetic semiconductor (DMS), which was triggered by combinatorial discovery of transparent ferromagnet. The possible advantages of oxide semiconductor as a host of DMS are described in comparison with conventional compound semiconductors. Limits and problems for identifying novel ferromagnetic DMS are described in view of recent reports in this field. Several characterization techniques are proposed in order to eliminate unidentified ferromagnetism of oxide-based DMS unidentified ferromagnetic oxide (UFO). Perspectives and possible devices are also given.

Rutile-type oxide-diluted magnetic semiconductor: Mn-doped SnO2

Epitaxial films of an oxide-diluted magnetic semiconductor with rutile structure, Mn-doped SnO2, have been fabricated by pulsed-laser deposition. As the Mn content increases, systematic changes in lattice constants and in-gap absorption are observed. Magnetization measurements show almost paramagnetic behavior. The injection of n-type carrier over 1020 cm−3 is achieved by Sb doping. A Sn0.95Mn0.05O2:Sb film shows giant positive magnetoresistance as large as 60% at 5 K.

Ferromagnetism in Co-Doped TiO2 Rutile Thin Films Grown by Laser Molecular Beam Epitaxy

Epitaxial TiO2 rutile films were fabricated on α-Al2O3 (1012) substrate in the layer-by-layer fashion by laser molecular beam epitaxy. Ablation with a Co-doped TiO2 target produced single phase of rutile film with the concentration of Co between 0 and 5%. Some ferromagnetic domain structures were observed in CoxTi1-xO2 rutile films by a scanning superconducting quantum interference device microscope at 3 to 90 K. The magnetic hysteresis could also be observed even at room temperature with a magnetic moment of ~ 1 µB/Co atom.