Yasuhiro Igasaki

The effects of deposition rate on the structural and electrical properties of ZnO:Al films deposited on (112̄0) oriented sapphire substrates

Aluminum doped zinc oxide (ZnO:Al) films were deposited on (1120) oriented sapphire substrates heated to 200 °C with a radio‐frequency (rf) power ranging from 25 to 170 W for a deposition rate in the range 0.7–27.4 nm min−1 by rf‐magnetron sputtering from a ZnO target mixed with Al2O3 of 2 wt. %. All of the films deposited were (0001) oriented single‐crystalline films with an internal stress. The stress was increased and degraded the crystallinity of the epitaxial film as a deposition rate was increased, and thus the Hall mobility and the resistivity of the film were, respectively, decreased and increased. However, the resistivities obtained were in the range about 1.4–3.0×10−4 Ω cm, the values comparable to those for indium tin oxide film presently used as a transparent electrode.

Substrate temperature dependence of electrical properties of ZnO:Al epitaxial films on sapphire (12̄10)

ZnO:Al films were deposited on (1210) oriented sapphire substrates heated up to 400 °C by rf magnetron sputtering from a ZnO target mixed with Al2O3 of 2 wt%. Films deposited on a substrate heated to a temperature in the range 50–350 °C were (0001) oriented single crystals but those grown at 400 °C consisted of crystallites with the (0001) and (1101) orientation. The former films had relatively smooth surfaces whereas the latter exhibited very rough surfaces. Electrical properties such as resistivity, carrier concentration, and the Hall mobility were measured as a function of substrate temperature. The carrier concentration decreased as the substrate temperature was increased up to 300 °C, although the Al content remained unchanged in this temperature range. From these measurements, it was found that the native donors were important as a source of carriers, even in ZnO:Al films. However, it was found that the Hall mobilities for films with a thickness of more than 200 nm experienced minor changes over a...

Spectroscopic ellipsometry of epitaxial ZnO layer on sapphire substrate

Optical properties of epitaxial ZnO layers have been studied in the spectral region from 1.5 to 5.4 eV using four-zone null spectroscopic ellipsometry. An existing model dielectric function based on excitonic structure near direct band gap has been improved by including a high-energy absorption term. Surface layer, corresponding to the surface roughness, was found to be essential to fit the spectroellipsometric data obtained. Two kinds of samples have been studied: ZnO layers prepared on (0001) and (1120)-oriented sapphire substrates. The surfaces of the first ones were found to be more rough.

Argon gas pressure dependence of the properties of transparent conducting ZnO:Al films deposited on glass substrates

Aluminium doped zinc oxide (ZnO:Al) films were deposited on amorphous substrates heated up to 200°C with a radio frequency (rf) power of 100 W by rf magnetron sputtering from a ZnO target mixed with Al2O3 of 2 wt.%. Argon gas pressure during deposition was in the range 0.08–2.7 Pa. As argon gas pressure was increased, the deposition rate and the grain size were decreased and the surface roughness was increased. Furthermore, the carrier concentration and the Hall mobility were decreased and thus the electrical resistivity was increased. However, the optical transmittance of about 90% was maintained over the argon pressure range. The resistivity of the film deposited at argon gas pressure of 0.13 Pa was about 2.5×10−4 Ω cm, a value comparable to that for indium tin oxide film presently used as a transparent electrode.

The effects of substrate bias on the structural and electrical properties of TiN films prepared by reactive r.f. sputtering

TiN films of the f.c.c. phase were grown by reactive r.f. sputtering on negatively biased substrates. The effects of the bias voltages on electrical properties such as the resistivity ϱ, the differential change in resistivity with respect to temperature dϱ/dT, the Hall coefficients and the superconductivity were studied in terms of the structural features of the films prepared. At sufficient substrate bias voltage (≈ 100 V) it was found that films with stable electrical properties could be produced.

The effects of zinc diffusion on the electrical and optical properties of ZnO:Al films prepared by r.f. reactive sputtering

Abstract ZnO:Al transparent conducting films with very low resistivity were easily prepared by the following method: ZnO:Al/Zn/ZnO:Al three-layered films were deposited by r.f. reactive sputtering from a composite target consisting of zinc disc and aluminium thin wires and then zinc atoms were diffused by heat treatment in vacuum. In the case that after deposition of a ZnO:Al layer for 5 min, zinc was deposited for 3 s followed by 25 min of deposition of a ZnO:Al layer, we could obtain ZnO:Al films with a resistivity of about 4 × 10 -4 Ω cm and transparency above 90% by annealing at 350°C for 2 h. We found that the diffusion of zinc atoms was conducted through grain boundaries and caused an increase in carrier concentration and Hall mobility and hence a decrease in resistivity. Furthermore, it was also found that diffused zinc atoms were very effective in suppressing an increase in resistivity in air.

Structure and Electrical Properties of Titanium Nitride Films

Metallic titanium, in nitrogen atmosphere with specified pressures (pN) ranging from 10-6 to 10-4 Torr, was evaporated on glass substrates heated at temperatures (Ts) between 300 to 500°C. X-ray analysis revealed that the films were composed of α-titanium, distorted titanium phase, amorphous phase, and TiN. The distorted titanium phase was found to extend to a high concentration of nitrogen (x=0.5). In particular, the specimens evaporated at pN=2.0×10-5 Torr and Ts=500°C had maximum resistivity of 270 µΩ-cm and a very small negative value of the temperature coefficient of resistance (0 to -20 ppm/°C) for an extended range of temperatures (-190 to 200°C). The excess resistivity of the distorted titanium phase can be mainly interpreted in terms of two kinds of carrier scattering, one due to nitrogen atoms randomly distributed at vacant interstitials, another attributable to grain boundaries involved.

Doping effects on optical properties of epitaxial ZnO layers determined by spectroscopic ellipsometry

Optical properties of Al- and Ga-doped ZnO layers have been studied in the spectral range from 1.5 to 5.4 eV using a four-zone null spectroscopic ellipsometer and in the spectral range from 0.5 to 6.5 eV using near-normal incidence reflectivity measurements. The layers were prepared by RF magnetron sputtering onto (1 1 2 0) oriented single-crystal sapphire substrates. Al- and Ga-doping gives rise to a shift of the fundamental absorption edge from 3.4 to 3.7 eV. The model dielectric function (MDF) based on an excitonic structure derived by Tanguy [Phys. Rev. B 60 (1999) 10660] was completed by the Sellmeier and Drude terms. The Drude term describes a free-electron contribution originating from presence of the dopant. Spectroscopic ellipsometry and reflectometry are very sensitive to a surface roughness. The surface roughness was modeled by a surface layer of the Bruggeman effective medium and by diffraction theory.

Epitaxial growth of aluminum-doped zinc oxide films on (11–.0) oriented sapphire substrates

ZnO:AI films were deposited on (11–0) oriented sapphire substrates heated up to 400°C with an RF power ranging from 25 to 170 W by RF magnetron sputtering from a ZnO target mixed with AI 2 O 3 of 2 wt%. Films deposited on substrates heated to a temperature in the range 50-350°C were (0001) oriented single crystals but those grown at 400°C consisted of crystallites with the (0001) and (101) orientation. The epitaxial relationships between ZnO:AI films and the substrates were determined by using the reflective electron diffraction patterns from the films and the back-reflection Laue patterns from the substrates. From these measurements, it was found that there were two types of epitaxial relationships between the (0001) ZnO:AI films and the (1120) sapphire substrates. One was [1010]ZnO:AI ll[00111]sapphire and the other [2110]ZnO:AI ‖[0001]sapphire.

The preparation of highly oriented InSe films by electrodeposition

Abstract InSe films were deposited from a mixture of indium chloride and selenium oxide in aqueous solution by electrodeposition on titanium-coated glass substrates and were crystallized by annealing. The effects of the parameters during deposition, such as current density, deposition potential versus saturated calomel electrode, acidity and concentration of source materials, and annealing conditions were studied. The crystal structure, surface morphology, and the composition of the films were studied by X-ray diffraction using the Cu K α line, scanning electron microscopy, and wavelength dispersive X-ray spectroscopy, respectively. As a result, we found that highly (001)-oriented InSe films could be prepared by electrodeposition of InSe films with indium contents of about 50 at% followed by annealing in an Ar atmosphere at 400°C for 30 min.