Toshiaki Aiba

High-mobility thin-film transistor with amorphous InGaZnO4 channel fabricated by room temperature rf-magnetron sputtering

Thin-film transistors (TFTs) were fabricated using amorphous indium gallium zinc oxide (a-IGZO) channels by rf-magnetron sputtering at room temperature. The conductivity of the a-IGZO films was controlled from ∼10−3to10−6Scm−1 by varying the mixing ratio of sputtering gases, O2∕(O2+Ar), from ∼3.1% to 3.7%. The top-gate-type TFTs operated in n-type enhancement mode with a field-effect mobility of 12cm2V−1s−1, an on-off current ratio of ∼108, and a subthreshold gate voltage swing of 0.2Vdecade−1. It is demonstrated that a-IGZO is an appropriate semiconductor material to produce high-mobility TFTs at low temperatures applicable to flexible substrates by a production-compatible means.

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.

Characterization of nanoporous Si thin films obtained by Al–Si phase separation

Nanoporous silicon films of ultrahigh pore densities and large surface areas have been fabricated by sputtering an Al–Si target and subsequent removal of the deposited film’s Al regions by etching in a sulfuric acid solution. The resulting nanoporous films are mainly composed of amorphous silicon and have cylindrical pores with an average pore density exceeding 1016pores∕m2. These nanoporous films can be crystallized by thermal annealing in a H2 atmosphere to improve their electrical properties. The electrical properties of the crystallized nanoporous films, which behave as p-type semiconductors, are very similar to those of electrochemically etched porous Si.

A New Superconductor with Tc=47 K in the Bi-Sr-Cu-O System (Nominal Composition=Bi4Sr8+xCu5-xOy)

A new superconductive bismuth cuprate, with a nominal composition of Bi4Sr8+xCu5-xOy, was discovered using resistivity and magnetization measurements. The onset transition temperatures of as-synthesized and He-annealed samples are 42 K and 47 K, respectively. This superconductive phase is very unstable at the synthesis temperature, and thus, changes into Bi4Sr8Cu5O19+α and Bi2Sr3Cu2Oy with prolonged calcination. Because of this instability, a single phase cannot be obtained. X-ray diffraction measurements indicate the existence of a new phase with a large-value peak of d=19.8 A.

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.