Hiroshi Yanagi

P-type electrical conduction in transparent thin films of CuAlO2

Optically transparent oxides tend to be electrical insulators, by virtue of their large electronic bandgap (⩾3.1 eV). The most notable exceptions are doped versions of the oxides In2O3, SnO2 and ZnO—all n-type (electron) conductors—which are widely used as the transparent electrodes in flat-panel displays. On the other hand, no transparent oxide exhibiting high p-type (hole) conductivity is known to exist, whereas such materials could open the way to a range of novel applications. For example, a combination of the two types of transparent conductor in the form of a pn junction could lead to a ‘functional’ window that transmits visible light yet generates electricity in response to the absorption of ultraviolet photons. Here we describe a strategy for identifying oxide materials that should combine p-type conductivity with good optical transparency. We illustrate the potential of this approach by reporting the properties of thin films of CuAlO2, a transparent oxide having room-temperature p-type conductivity up to 1 S cm−1. Although the conductivity of our candidate material is significantly lower than that observed for the best n-type conducting oxides, it is sufficient for some applications, and demonstrates that the development of transparent p-type conductors is not an insurmountable goal.

p-channel thin-film transistor using p-type oxide semiconductor, SnO

This paper reports that among known p-type oxide semiconductors, tin monoxide (SnO) has a high hole mobility and produces good p-type oxide thin-film transistors (TFTs). Device-quality SnO films were grown epitaxially on (001) yttria-stabilized zirconia substrates at 575°C by pulsed laser deposition. These exhibited a Hall mobility of 2.4cm2V−1s−1 at room temperature. Top-gated TFTs, using epitaxial SnO channels, exhibited field-effect mobilities of 1.3cm2V−1s−1, on/off current ratios of ∼102, and threshold voltages of 4.8V.

Electronic structure and optoelectronic properties of transparent p-type conducting CuAlO2

Electrical and optical properties of CuAlO2, a p-type conducting transparent oxide, were examined for the thin films prepared by the pulsed laser deposition technique. The indirect and direct allowed optical band gaps were evaluated to be ∼1.8 and ∼3.5 eV, respectively. The conductivity at 300 K was ∼3×10−1 S cm−1 and its temperature dependence is of the thermal-activation type (activation energy ≈0.2 eV) at temperatures >220 K but is of the variable-range hopping type (log σ∝T−1/4) at <220 K. It was inferred that an admixed state of Cu 3d and O 2p primarily constitutes the upper valence band, which controls transport of positive holes, from a combined information on ultraviolet photoemission spectrum with x-ray photoemission spectrum. An energy band calculation by full-potential linearized augmented plane wave method substantiated the experimental findings. The present results gave a solid basis for our working hypothesis [Nature (London) 389, 939 (1997)] for chemical design of p-type conducting transpar...

SrCu2O2: A p-type conductive oxide with wide band gap

SrCu2O2 thin films were prepared on SiO2 glass substrates by pulsed laser deposition. The film deposited in O2 atmosphere of 7×10−4 Pa at 573 K showed high optical transmission in visible and near-infrared regions. Potassium was doped at Sr site for substitutional doping. The optical band gap of the K-doped film was estimated to be ∼3.3 eV. The dc electrical conductivity of the K-doped film at 300 K was 4.8×10−2 S cm−1 and the activation energy was 0.10 eV. Positive sign of Seebeck and Hall coefficients demonstrated the p-type conduction of the film. Hole concentration and mobility at 300 K were 6.1×1017 cm−3 and 0.46 cm2 V−1 s−1, respectively.

Epitaxial growth of transparent p-type conducting CuGaO2 thin films on sapphire (001) substrates by pulsed laser deposition

Transparent p-type conducting CuGaO2 thin films were prepared on α-Al2O3 (001) single-crystal substrates by pulsed laser deposition. The films were grown epitaxially on the substrates in an as-deposited state. X-ray pole figure analysis revealed that the films were composed of two types of epitaxial grains, both with c axes oriented perpendicular to the surface and a axes rotated 60° with respect to each other around the c axis. Observation of the CuGaO2 thin films by atomic force microscopy and high-resolution transmission electron microscopy substantiated this conclusion. The films have high optical transparency (∼80%) in the visible region, and the energy gap of CuGaO2 for direct allowed transition was estimated to be 3.6 eV. p-type conductivity was confirmed by Seebeck and Hall measurements. The electrical conductivity, carrier (positive hole) density, and Hall mobility of the films at room temperature were 6.3×10−2 S cm−1, 1.7×1018 cm−3, and 0.23 cm2 V−1 s−1, respectively.

Bipolarity in electrical conduction of transparent oxide semiconductor CuInO2 with delafossite structure

A transparent oxide semiconductor with delafossite structure, CuInO2, was found to exhibit both p-type and n-type conduction by doping of an appropriate impurity and tuning of proper film-deposition conditions. Thin films of Ca-doped or Sn-doped CuInO2 (optical band gap=∼3.9 eV) were prepared on α-Al2O3(001) single crystal substrates by pulsed laser deposition method. The films were deposited at 723 K in O2 atmosphere of 1.0 Pa for the Ca-doped films or 1.5 Pa for the Sn-doped films. The positive sign of the Seebeck coefficient demonstrated p-type conduction in the Ca-doped films, while the Seebeck coefficient of the Sn-doped films was negative indicating n-type conductivity. The electrical conductivities of Ca-doped and Sn-doped CuInO2 thin films were 2.8×10−3 S cm−1 and 3.8×10−3 S cm−1, respectively, at 300 K.

Subgap states in transparent amorphous oxide semiconductor, In–Ga–Zn–O, observed by bulk sensitive x-ray photoelectron spectroscopy

We investigated the electronic states in amorphous In–Ga–Zn–O films with high carrier concentrations by optical absorption and hard x-ray photoelectron spectroscopy (HX-PES). Films having different Hall mobilities were prepared and their annealing effects were examined. All HX-PES spectra showed Fermi edge structures and extra subgap densities of states (DOSs). Tail-like structures observed in the optical spectra originate from subgap DOSs (⪢1020cm−3) near valence band maximas (VBMs). Subgap DOSs near VBMs provide a reason why In–Ga–Zn–O thin film transistors show hard saturation in off states and are difficult to operate in an inversion p-channel mode.

Fabrication of transparent p–n heterojunction thin film diodes based entirely on oxide semiconductors

All oxide-based, transparent polycrystalline p–n heterojunctions on a glass substrate were fabricated. The structure of the diode was n+-ZnO electrode/n-ZnO/p-SrCu2O2/In2−xSnxO3 electrode on the substrate. The contact between the n- and p-type semiconducting oxides was found to be rectifying. The ratio of forward current to the reverse current exceeded 80 within the range of applied voltages of −1.5 to +1.5 V and the estimated diode factor (n value) was 1.62. The diode structure was fabricated on a glass plate with the total thickness of 1.3 μm and possessed an optical transmission of 70%–80% in the visible region.

Epitaxial growth of high mobility Cu2O thin films and application to p-channel thin film transistor

Cu2O epitaxial films were grown for high mobility p-channel oxide thin-film transistors (TFTs). The use of a (110) MgO surface and fine tuning of a growth condition produced single phase epitaxial films with hole Hall mobilities ∼90 cm2 V−1 s−1 comparable to those of single crystals (∼100 cm2 V−1 s−1). TFTs using the epitaxial film channels exhibited p-channel operation although the field-effect mobilities and the on-to-off current ratio were not yet satisfactory (∼0.26 cm2 V−1 s−1 and ∼6, respectively).

Superconductivity Induced by Co-Doping in Quaternary Fluoroarsenide CaFeAsF

A new quaternary fluoroarsenide CaFeAsF with the tetragonal ZrCuSiAs-type structure composed of alternate stacking of (FeAs)delta- and (CaF)delta+ layers was synthesized. CaFeAsF is a poor metal and shows the anomaly at approximately 120 K in temperature dependence of electrical conductivity. The electron doping by the partial replacement of the iron with cobalt suppresses the anomaly and induces the bulk superconductivity (optimal Tc = 22 K for CaFe0.9Co0.1AsF), analogous to recently discovered FeAs-based superconductors. The present results suggest that CaFeAsF is a promising candidate as a parent compound for high Tc superconductors.