Dilute magnetic semiconductors and wide gap oxide semiconductors are appealing materials for magnetooptical devices. From a combinatorial screening approach looking at the solid solubility of transition metals in titanium dioxides and of their magnetic properties, we report on the observation of transparent ferromagnetism in cobalt-doped anatase thin films with the concentration of cobalt between 0 and 8%. Magnetic microscopy images reveal a magnetic domain structure in the films, indicating the existence of ferromagnetic long-range ordering. The materials remain ferromagnetic above room temperature with a magnetic moment of 0.32 Bohr magnetons per cobalt atom. The film is conductive and exhibits a positive magnetoresistance of 60% at 2 kelvin.

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 ${\mathrm{Zn}}_{\text{1−x}}{\mathrm{Mn}}_x\mathrm{O}$ films $\text{(x<0.35)}$ increase and the band gap expands although considerable in-gap absorption develops. Itinerant electrons over ${10}^{19} {\mathrm{cm}}^{\mathrm{-3}}$ can be doped into the ${\mathrm{Zn}}_{\text{1−x}}{\mathrm{Mn}}_x\mathrm{O}$ 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.

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.

We report on the magnetic properties of an oxide-diluted magnetic semiconductor (DMS), ${\mathrm{Zn}}_{\mathrm{0.64}}{\mathrm{Mn}}_{\mathrm{0.36}}\mathrm{O}.$ 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 $\text{(x̄∼0.02)}$ under high field also represents a strong antiferromagnetic exchange coupling in this compound.

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.

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 with many grain boundaries to quantitatively analyze the potential profiles that developed along the channel. One of the main differences between a polycrystalline ZnO TFT and a polycrystalline Si TFT is that the much smaller nanoscaled grains in a polycrystalline ZnO TFT induces a strong overlap of the double Schottky barriers with a higher activation energy in the crystallite and a lower barrier potential in the grain boundary at subthreshold or off-state region of its transfer characteristics. Through the simulation, we were able to estimate the density of total trap states localized in the grain boundaries for polycrystalline ZnO TFT by determining the apparent mobility and grain size in the device.