Nobuto Oka

Thermal transport properties of polycrystalline tin-doped indium oxide films

Thermal diffusivity of polycrystalline tin-doped indium oxide (ITO) films with a thickness of 200 nm has been characterized quantitatively by subnanosecond laser pulse irradiation and thermoreflectance measurement. ITO films sandwiched by molybdenum (Mo) films were prepared on a fused silica substrate by dc magnetron sputtering using an oxide ceramic ITO target (90 wt % In2O3 and 10 wt % SnO2). The resistivity and carrier density of the ITO films ranged from 2.9×10−4 to 3.2×10−3 Ω cm and from 1.9×1020 to 1.2×1021 cm−3, respectively. The thermal diffusivity of the ITO films was (1.5–2.2)×10−6 m2/s, depending on the electrical conductivity. The thermal conductivity carried by free electrons was estimated using the Wiedemann–Franz law. The phonon contribution to the heat transfer in ITO films with various resistivities was found to be almost constant (λph=3.95 W/m K), which was about twice that for amorphous indium zinc oxide films.

Experimental observation on the Fermi level shift in polycrystalline Al-doped ZnO films

The shift of the Fermi level in polycrystalline aluminum doped zinc oxide (AZO) films was studied by investigating the carrier density dependence of the optical band gap and work function. The optical band gap showed a positive linear relationship with the two-thirds power of carrier density ne2/3. The work function ranged from 4.56 to 4.73 eV and showed a negative linear relationship with ne2/3. These two phenomena are well explained on the basis of Burstein-Moss effect by considering the nonparabolic nature of the conduction band, indicating that the shift of Fermi level exhibits a nonparabolic nature of the conduction band for the polycrystalline AZO film. The variation of work function with the carrier density reveals that the shift of the surface Fermi level can be tailored by the carrier density in the polycrystalline AZO films. The controllability between the work function and the carrier density in polycrystalline AZO films offers great potential advantages in the development of optoelectronic dev...

Effects of Energetic Ion Bombardment on Structural and Electrical Properties of Al-Doped ZnO Films Deposited by RF-Superimposed DC Magnetron Sputtering

The bombardment of various types of energetic ions during rf-superimposed dc magnetron sputter deposition was investigated in detail and their effects on the structural and electrical properties of Al-doped ZnO (AZO) films were analyzed. Aside from the expected energetic negative oxygen ions (i.e., O- and O2-), various other negative ions (i.e., AlO-, AlO2-, AlO3-, ZnO-, and ZnO2-) with a high energy were clearly observed. Such negative ions were found to be generated on the target surface and accelerated towards the substrate by the full cathode voltage. Furthermore, we found that the energy of these negative ions decreased with decreasing plasma impedance by superimposing rf power on dc sputtering. The resistivity of the AZO films deposited using the rf-superimposed dc sputtering was much lower than that of the films deposited using conventional dc sputtering. Such a decrease in resistivity should be attributed to reducing the damage of AZO films by suppressing the bombardment energies of various types of energetic negative ions impinging on a growing film surface.

Electrical and optical properties of Nb-doped TiO2 films deposited by dc magnetron sputtering using slightly reduced Nb-doped TiO2−x ceramic targets

Nb-doped anatase TiO2 films were deposited on unheated glass by dc magnetron sputtering using slightly reduced Nb-doped TiO2−x targets (Nb concentration: 3.7 and 9.5 at. %) with various hydrogen or oxygen flow ratios. After postannealing in a vacuum (6×10−4 Pa) at 500 °C for 1 h, both films were crystallized into the polycrystalline anatase TiO2 structure. The resistivity decreased from 1.6×10−3 to 6.3×10−4 Ω cm with increasing Nb concentration from 2.8 to 8.0 at. %, where the carrier density increased from 5.4×1020 to 2.0×1021 cm−3 and the Hall mobility was almost constant at 5–7 cm2 V−1 s−1. The films exhibited a high transparency of over 60%–80% in the visible region.

In situ analyses on negative ions in the sputtering process to deposit Al-doped ZnO films

The origin of high energy negative ions during deposition of aluminum doped zinc oxide (AZO) films by dc magnetron sputtering of an AZO (Al2O3: 2.0 wt %) target was investigated by in situ analyses using the quadrupole mass spectrometer combined with the electrostatic energy analyzer. High energy negative oxygen (O−) ions which possessed the kinetic energy corresponding to the cathode sheath voltage were detected. The maximum flux of the O− ions was clearly observed at the location opposite to the erosion track area on the target. The flux of the O− ions changed hardly with increasing O2 flow ratio [O2/(Ar+O2)] from 0% to 5%. The kinetic energy of the O− ions decreased with decreasing cathode sheath voltage from 403 to 337 V due to the enhancement of the vertical maximum magnetic field strength at the cathode surface from 0.025 to 0.100 T. The AZO films deposited with the lower O− bombardment energy showed the higher crystallinity and improved the electrical conductivity.

Sputter deposition of Al-doped ZnO films with various incident angles

Al-doped ZnO (AZO) films were sputter deposited on glass substrates heated at 200 °C under incident angles of sputtered particles at 0° (incidence normal to substrate), 20°, 40°, 60°, and 80°. In the case of normal incidence, x-ray diffraction pole figures show a strong [001] preferred orientation normal to the film surface. In contrast, in the case wherein the incident angles were higher than 60°, the [001] orientation inclined by 25°–35° toward the direction of sputtered particles. Transmission electron microscopy revealed that the tilt angle of the [001] orientation increased with increasing angle of the incident sputtered particles, whereas the columnar structure did not show any sign of inclination with respect to the substrate plane.

Carrier Density Dependence of Optical Band Gap and Work Function in Sn-Doped In2O3 Films

Carrier density dependence of the optical band gap and work function were investigated in undoped and Sn-doped In2O3 films. The optical band gap and the work function of the films showed clear positive and negative linear relationships, respectively, with two-thirds power of the carrier density. Optical band gaps increased from 3.8 to 4.3 eV when the carrier density increased from 8.8×1019 to 8.2×1020 cm-3, whereas work functions decreased from 5.5 to 4.8 eV. These variations could be attributed to the shift in the Fermi energy with varying carrier density in highly degenerated semiconductors.

Thermal Conductivity of Amorphous Indium–Gallium–Zinc Oxide Thin Films

We investigated the thermal conductivity of 200-nm-thick amorphous indium–gallium–zinc-oxide (a-IGZO) films. Films with a chemical composition of In:Ga:Zn= 1:1:0.6 were prepared by dc magnetron sputtering using an IGZO ceramic target and an Ar–O2 sputtering gas. The carrier density of the films was systematically controlled from 1014 to >1019 cm-3 by varying the O2 flow ratio. Their Hall mobility was slightly higher than 10 cm2V-1s-1. Those films were sandwiched between 100-nm-thick Mo layers; their thermal diffusivity, measured by a pulsed light heating thermoreflectance technique, was ~5.4×10-7 m2s-1 and was almost independent of the carrier density. The average thermal conductivity was 1.4 Wm-1K-1.

Electronic State of Amorphous Indium Gallium Zinc Oxide Films Deposited by DC Magnetron Sputtering with Water Vapor Introduction

Amorphous indium gallium zinc oxide (a-IGZO) films were fabricated using dc magnetron sputtering with water vapor (H2O or D2O) introduction. To determine the incorporation pattern of water, films were characterized by secondary ion mass, Fourier transform infrared, Raman, and hard X-ray photoelectron spectroscopies. Chemically bound hydroxyl groups were observed, and more hydroxyl bonds existed nearer to the interface between the substrate and film than in the film. Furthermore, for a-IGZO films, subgap densities of states near valence band maxima increased with the H2O partial pressure during deposition, which can be attributed to defect-level generations due to H and O.

Thermophysical and electrical properties of Al-doped ZnO films

Thermal diffusivity of Al-doped ZnO (AZO) films with a thickness of 200 nm was quantitatively analyzed using a “rear heating/front detection type” nanosecond thermoreflectance system. AZO monolayer and Mo/AZO/Mo three-layered films were prepared on synthesized silica substrates by DC magnetron sputtering using high density ceramic ZnO–Al2O3 (Al2O3: 2.5 wt. %) and Mo metal targets. The thermal diffusivity and electrical resistivity of the deposited AZO films ranged 1.8 × 10−6 –2.4 × 10−6 m2 s−1 and 2.3 × 10−3–5.9 × 10−4 Ω cm, respectively. The thermal conductivity corresponding to the thermal diffusivity was one order of magnitude smaller than that of sintered AZO ceramics prepared from ZnO and Al2O3 powders. However, it was found to be larger than that of In2O3-based transparent conductive oxide (TCO) films with approximately the same electrical conductivity, thus implying that AZO can be considered an excellent material for diathermanous TCO circuits.