Koki Yano

High-Mobility Thin-Film Transistors with Polycrystalline In–Ga–O Channel Fabricated by DC Magnetron Sputtering

Oxide thin-film transistors (TFTs) were fabricated using a polycrystalline In–Ga–O (IGO) thin film as the n-channel active layer by direct current magnetron sputtering. The 50-nm-thick IGO TFT showed a field-effect mobility of 39.1 cm2 V-1 s-1, a threshold voltage of 1.4 V, and a subthreshold gate voltage swing of 0.12 V/decade. The polycrystalline IGO thin film showed the cubic bixbyite structure of In2O3 without an obvious preferred orientation. The average grain size of polycrystalline IGO was approximately 10 µm. The high mobility of IGO TFT is related to the In2O3 crystalline phase and large grain size of the IGO film.

Ambipolar field-effect transistor based on organic-inorganic hybrid structure

The authors developed an ambipolar field-effect transistor (FET) based on an organic-inorganic hybrid structure that consisted of an indium zinc oxide and pentacene double layer fabricated on a SiO2∕n++-Si substrate. Although the FETs based on an indium zinc oxide or pentacene single layer only showed unipolar FET characteristics, the hybrid FET showed definite ambipolar FET characteristics. The authors obtained a highly saturated field-effect hole and electron mobilities of 0.14 and 13.8cm2∕Vs. Furthermore, the authors demonstrated electroluminescence from hybrid FETs using tetracene as an emitting layer. The authors’ success shows that the hybridization of organic and inorganic materials opens up a new field in electronics.

Thermal analysis of amorphous oxide thin-film transistor degraded by combination of joule heating and hot carrier effect

Stability is the most crucial issue in the fabrication of oxide thin-film transistors (TFTs) for next-generation displays. We have investigated the thermal distribution of an InSnZnO TFT under various gate and drain voltages by using an infrared imaging system. An asymmetrical thermal distribution was observed at a local drain region in a TFT depending on bias stress. These phenomena were decelerated or accelerated with stress time. We discussed the degradation mechanism by analyzing the electrical properties and thermal distribution. We concluded that the degradation phenomena are caused by a combination of Joule heating and the hot carrier effect.

High-Performance Thin Film Transistor with Amorphous In2O3–SnO2–ZnO Channel Layer

We have developed a high-mobility and high-processability oxide semiconductor using amorphous In2O3–SnO2–ZnO (a-ITZO) as the channel material. An a-ITZO thin-film transistor (TFT) was fabricated by a back-channel-etch process. Its field effect mobility was more than 20 cm2 V-1 s-1 and its subthreshold swing was 0.4 V s-1, which makes it a promising candidate for next-generation TFTs.

Structure and Internal Stress of Tin-Doped Indium Oxide and Indium–Zinc Oxide Films Deposited by DC Magnetron Sputtering

Representative transparent conductive oxide films, such as tin-doped indium oxide (ITO) and indium–zinc oxide (IZO) films, were deposited by dc magnetron sputtering using corresponding oxide targets under various total gas pressures (Ptot) ranging from 0.3 to 3.0 Pa. The ITO films deposited at a Ptot lower than 0.7 Pa were polycrystalline and were found to have a large compressive stress of about 1.5 ×109 Pa, whereas the ITO films deposited at 1.5–3.0 Pa were amorphous and had a low tensile stress. In contrast, all the IZO films deposited at a Ptot range of 0.3–3.0 Pa showed an entirely amorphous structure, where the compressive stress in the IZO films deposited at a Ptot lower than 1.5 Pa was lower than that in the ITO films. Such compressive stress was considered to be generated by the atomic peening effect of high-energy neutrals (Ar0) recoiled from the target or high-energy negative ions (O-) accelerated in the cathode sheath toward the film surface.

Relationship between variable range hopping transport and carrier density of amorphous In2O3–10 wt. % ZnO thin films

The electrical transport characteristics in amorphous Zn doped In2O3 films have been investigated in the range from 2 × 1017 cm−3 to 6 × 1020 cm−3 of the carrier concentration Ne. For films with Ne > 3 × 1020 cm−3, it is found that the Hall mobility μH is limited by ionized impurity scattering. However, for films with Ne   0 to insulating behavior with dρ/dT < 0 near Ne≈1 × 1020 cm−3 with decreasing Ne. The transport mechanism of carriers in the high-resistivity region is discussed by considering a model based on the Ioffe-Regel criterion. For the film with highest resistivity with Ne ≈ (5 − 6) × 1017 cm−3 among the present films, the ρ(T) show a change from Mott variable-range hopping (ρ ∝ exp T−1/4) to ρ ∝ expT−1/2 at approximately 10 K with decreasing temperature.

Superconductivity in transparent zinc-doped In2O3 films having low carrier density

Abstract Thin polycrystalline zinc-doped indium oxide (In2O3–ZnO) films were prepared by post-annealing amorphous films with various weight concentrations x of ZnO in the range 0x 0.06. We have studied the dependences of the resistivity ρ and Hall coefficient on temperature T and magnetic field H in the range 0.5T 300 K, H6 Tfor 350 nm films annealed in air. Films with 0x0.03 show the superconducting resistive transition. The transition temperature Tc is below 3.3 K and the carrier density n is about 1025–1026 m−3. The annealed In2O3–ZnO films were examined by transmission electron microscopy and x-ray diffraction analysis revealing that the crystallinity of the films depends on the annealing time. We studied the upper critical magnetic field Hc2 (T) for the film with x = 0.01. From the slope of dHc2 /dT, we obtain the coherence length ξ (0) ≈ 10 nm at T = 0 K and a coefficient of electronic heat capacity that is small compared with those of other oxide materials.

Amorphous indium-tin-zinc oxide films deposited by magnetron sputtering with various reactive gases: Spatial distribution of thin film transistor performance

This work presents the spatial distribution of electrical characteristics of amorphous indium-tin-zinc oxide film (a-ITZO), and how they depend on the magnetron sputtering conditions using O2, H2O, and N2O as the reactive gases. Experimental results show that the electrical properties of the N2O incorporated a-ITZO film has a weak dependence on the deposition location, which cannot be explained by the bombardment effect of high energy particles, and may be attributed to the difference in the spatial distribution of both the amount and the activity of the reactive gas reaching the substrate surface. The measurement for the performance of a-ITZO thin film transistor (TFT) also suggests that the electrical performance and device uniformity of a-ITZO TFTs can be improved significantly by the N2O introduction into the deposition process, where the field mobility reach to 30.8 cm2 V–1 s–1, which is approximately two times higher than that of the amorphous indium-gallium-zinc oxide TFT.

Transport properties and microstructures of polycrystalline In2O3–ZnO thin films

We prepared polycrystalline In2O3–ZnO films by post annealing the amorphous films (1.0 wt % ZnO) at 200 °C with various annealing times ta 0≤ta≤20 h. We have measured the electric resistivity and Hall mobility and also observed film structures by not only the x-ray diffraction but also scanning transmission electron microscopy (STEM) with electron energy-loss spectroscopy (EELS). We have found the following: (1) Hall mobility takes the maximum with respect to the carrier density and the annealed films clearly show the superconductivity of which transition temperature increases with increase in ta. (2) The data on EELS spectra mapping of indium plasmon indicate that droplets of the pure indium phase exist on grain boundaries and near the interface between the film and the glass substrate. However, it seems that these droplets do not form an electrical conducting path but contribute to the scattering centers for carrier electrons, from the dispersed distribution of these droplets in STEM-EELS spectra mappin...

Electron-phonon scattering in amorphous In2O3–ZnO films

For amorphous transparent conductive oxide In2O3–ZnO films over a wide range of resistivities ρ, the temperature dependences of ρ and Hall coefficient RH have been measured in the temperature range of 2.0–300 K. The low-resistivity films show a metallic characteristic (dρ/dT>0), although high-resistivity films show an insulating behavior (dρ/dT<0). Even in metallic films, however, the resistivity slightly increases with decreasing temperature below 20 K because of the term ρquanta(T) due to quantum effects. Through a careful analysis, we have found that the ρ(T) of metallic films changes in the form of ρ(T)−ρ0−ρquanta∝ρ0T2 at temperatures below ∼100 K. This temperature dependence can be explained by the interference term ρel-imp-ph between the impurity scattering and the electron-phonon scattering. At temperatures of 20–300 K, it has been found that ρ(T) agrees well with the sum of the Gruneisen–Bloch term ρel-ph(T)=βel-phF(T,Θd) and the term ρel-imp-ph(T)=Bel-imp-phG(T,Θd). From analyses, with the coeffi...