Mikio Shimada

Materials, Devices, and Circuits of Transparent Amorphous-Oxide Semiconductor

This paper presents the following recent investigations of transparent amorphous-oxide semiconductors (TAOS) from materials to devices and circuits. 1) Composition of metals in TAOS are widely explored with the aim of seeking semiconductors suitable for the channel layers of thin-film transistors (TFTs) composing backplanes for flat-panel displays. It is found in combinatorial approaches to the materials exploration that indium-based ternary TAOS (In-X-O) and their TFTs show the properties and the performance as good as those of the most popular material of amorphous In-Ga-Zn-O (alpha-IGZO) when X = Zn or Ge. 2) Defects and impurities in TAOS are investigated by theoretical approaches. The first-principle calculation of the electron states reveals that charge-neutral oxygen vacancy or interstitial forms the density of states around mid-gap level and does not generate carriers directly, while hydrogen impurity raises the Fermi level beyond the conduction-band minimum and acts as a donor in TAOS. 3) Device structures of TAOS-TFTs are also investigated extensively for better performance and stability. It is found in channel-etch type TFTs with bottom-gate inverse-stagger structures that the TFT characteristics and stability are significantly improved by chemically removing the back-channel layer in a wet-etching process. Coplanar homojunction (CH) structure is proposed as a novel device structure where conductive alpha-IGZO regions work as the source and drain electrodes to the channel region of semiconductor alpha-IGZO. The CH TFTs show excellent characteristics and stability, with low series resistance without any difficulty in making good electrical contact between metals and TAOS. 4) Circuits using TAOS-TFTs are demonstrated. A ring oscillator composed of fifteen-stage inverters with a buffer circuit operates as designed by circuit simulation with a TFT model for hydrogenated amorphous Si TFTs. Pixel circuits composed of three TFTs and one transparent capacitor successfully drive organic light-emission diode cells without a planarization layer on a 2-in diagonal panel having 176 times144 times 3 pixels.

Enhanced Piezoelectric Constant of (1-x)BiFeO3?xBiCoO3 Thin Films Grown on LaAlO3 Substrate

Thin films of a rhombohedral-tetragonal solid solution (1-x)BiFeO3–xBiCoO3 with x = 0–0.30 were prepared by chemical solution deposition on LaAlO3(001) and (La0.5,Sr0.5)CoO3(001)/LaAlO3(001) substrates. A BiCoO3-type tetragonal structure with a large c/a of 1.23 was grown adjacent to the substrate whereas a BiFeO3-type rhombohedral one was grown on the tetragonal phase. Co substitution in the rhombohedral phase led to enhancement of piezoelectric constant and d33 reached 100 pm/V, almost double that of BiFeO3.

Microscopic structure and electrical transport property of sputter-deposited amorphous indium-gallium-zinc oxide semiconductor films

We report on microscopic structures and electrical and optical properties of sputter-deposited amorphous indium-gallium-zinc oxide (a-IGZO) films. From electron microscopy observations and an x-ray small angle scattering analysis, it has been confirmed that the sputtered a-IGZO films consist of a columnar structure. However, krypton gas adsorption measurement revealed that boundaries of the columnar grains are not open-pores. The conductivity of the sputter-deposited a-IGZO films shows a change as large as seven orders of magnitude depending on post-annealing atmosphere; it is increased by N2-annealing and decreased by O2-annealing reversibly, at a temperature as low as 300°C. This large variation in conductivity is attributed to thermionic emission of carrier electrons through potential barriers at the grain boundaries, because temperature dependences of the carrier density and the Hall mobility exhibit thermal activation behaviours. The optical band-gap energy of the a-IGZO films changes between before and after annealing, but is independent of the annealing atmosphere, in contrast to the noticeable dependence of conductivity described above. For exploring other possibilities of a-IGZO, we formed multilayer films with an artificial periodic lattice structure consisting of amorphous InO, GaO, and ZnO layers, as an imitation of the layer-structured InGaZnO4 homologous phase. The hall mobility of the multilayer films was almost constant for thicknesses of the constituent layer between 1 and 6 A, suggesting rather small contribution of lateral two-dimensional conduction It increased with increasing the thickness in the range from 6 to 15 A, perhaps owing to an enhancement of two-dimensional conduction in InO layers.

Structural Transformation of Hexagonal (0001)BaTiO3 Ceramics to Tetragonal (111)BaTiO3 Ceramics

A ceramic slurry that contains a 6H-type Ba(Ti0.95Mn0.05)O3 powder was casted into a plaster mold under 10 T magnetic field to form a green compact of (0001)-oriented Ba(Ti0.95Mn0.05)O3. After sintering the green compact at 1300 °C in air, it was confirmed that the (0001)-oriented 6H-type perovskite structure transformed to a (111)-oriented 3C-type perovskite structure. The structural transformation was again examined using hexagonal BaTiO3 prepared by reducing pseudo-cubic BaTiO3 powder in H2 atmosphere. In this case, the preferred (0001) orientation was not confirmed for the green compacts. After sintering the green compacts at 1300 °C in air, mixed crystal orientations of (100)/(001) and (111) were observed for the resultant tetragonal BaTiO3 ceramics. This (100)/(001) orientation was suppressed by annealing the hexagonal BaTiO3 powder at 1000 °C before slip-casting, leading to highly (111)-oriented ceramics. It was found that the green compacts of (0001)-oriented hexagonal BaTiO3 can transform into (111)-oriented tetragonal BaTiO3 ceramics, maintaining the macroscopic crystal orientations due to a similar atomic stacking along [0001] of 6H-type BaTiO3 and [111] of 3C-type BaTiO3.

Relaxor Characteristics of BaTiO3-Bi(Mg1/2Ti1/2)O3 Ceramics

Relaxor characteristics of (1-x)BaTiO3-xBi(Mg1/2Ti1/2)O3 (x=0.1–0.7) ceramics were investigated. Microstructural observation showed second phases and no domain structure for the sample with x=0.6. Deviation from the Curie-Weiss behavior was found in temperature dependence of the inverse permittivity for all the samples. The stronger dielectric dispersion was found for x=0.6 and 0.7 and they were described by the Vogel-Fulcher relationship. The temperature dependence of the remanent polarization and coercive field indicated the freezing temperature was 100~150°C for x=0.6. The strong dielectric dispersion of x=0.6 is believed to be induced by the structural disorder due to the second phases.

Microstructure and Piezoelectric Properties of BaTiO3-Bi(Mg1/2Ti1/2)O3-BiFeO3 Ceramics

Barium titanate (BaTiO3, BT)-bismuth magnesium titanate (Bi (Mg1/2Ti1/2)O3, BMT)-bismuth ferrite (BiFeO3, BF) solid solution ceramics were prepared using a conventional solidstate synthesis, and their piezoelectric properties and microstructure were investigated. Strain electric field curves of the 0.3BT-0.1BMT-0.6BF ceramics with a single perovskite phase were ferroelectric butterfly-like curves. A strain maximum / electric field maximum (Smax/Emax) was 330 pm/V. Transmission electron microscopy revealed ferroelectric-like domain structure in the 0.3BT-0.1BMT-0.6BF ceramics.

Piezoelectric enhancement of relaxor-based lead-free piezoelectric ceramics by nanodomain engineering

0.3BaTiO₃-0.1Bi(Mg_1/2Ti_1/2)O₃-0.6BiFeO₃ ceramics were either doped with vanadium or sintered in calcined powder with the same composition. Compared to an undoped ceramic sintered without the calcined powder, both ceramics showed reduced leakage current densities (lower than 1 × 10^-7 A/cm²) and absence of dielectric relaxation behaviors observed in frequency-and temperature-dependent dielectric measurements. The Curie temperatures of both samples were higher than 460 °C. The maximum field-induced strain over the applied field, S_max/E_max, of 366 pm/V of the undoped ceramic sintered without the calcined powder increased to 455 and 799 pm/V for the V-doped sample and the sample sintered with the calcined powder, respectively. The increase was related to a reduced concentration of bismuth vacancy - oxygen vacancy defect dipoles.

Rhombohedral-tetragonal structural change and enhanced piezoelectric constant in (1-x)BiFeO 3 -xBiCoO 3 thin films

Thin films of rhombohedral-tetragonal solid solution (1-x)BiFeO3-xBiCoO3 (BFCO) with x = 0-0.30 were prepared by chemical solution deposition on SrTiO3(001) and LaAlO3(001) substrates. Thin films grown on SrTiO3 substrate shows a gradual second order change from a rhombohedral phase to a tetragonal phase with c/a = 1.04. The piezoelectric constant d33 initially increased with increasing Co content, reached 80 pm/V at x = 0.05, and gradually decreased at x above 0.05. Films on LaAlO3(001) having smaller lattice parameter than SrTiO3 shows a distinct change from the rhombohedral phase to the tetragonal phase with a large c/a = 1.23. d33 of films on LAO reached 100 pm/V at x = 0.10, almost double that of BiFeO3. Co substitution in the rhombohedral phase enhances the piezoelectric response.