Tadatsugu Minami

Transparent conducting oxide semiconductors for transparent electrodes

The present status and prospects for further development of polycrystalline or amorphous transparent conducting oxide (TCO) semiconductors used for practical thin-film transparent electrode applications are presented in this paper. The important TCO semiconductors are impurity-doped ZnO, In2O3 and SnO2 as well as multicomponent oxides consisting of combinations of ZnO, In2O3 and SnO2, including some ternary compounds existing in their systems. Development of these and other TCO semiconductors is important because the expanding need for transparent electrodes for optoelectronic device applications is jeopardizing the availability of indium-tin-oxide (ITO), whose main constituent, indium, is a very expensive and scarce material. Al- and Ga-doped ZnO (AZO and GZO) semiconductors are promising as alternatives to ITO for thin-film transparent electrode applications. In particular, AZO thin films, with a low resistivity of the order of 10−5 Ω cm and source materials that are inexpensive and non-toxic, are the best candidates. However, further development of the deposition techniques, such as magnetron sputtering or vacuum arc plasma evaporation, as well as of the targets is required to enable the preparation of AZO and GZO films on large area substrates with a high deposition rate.

Transparent conducting p-type NiO thin films prepared by magnetron sputtering

Abstract Transparent and conductive thin films consisting of p -type nickel oxide (NiO) semiconductors were prepared by r.f. magnetron sputtering. A resistivity of 1.4 × 10 −1 ohms cm and a hole concentration of 1.3 × 10 19 cm −3 were obtained for non-intentionally doped NiO films prepared at a substrate temperature of 200°C in a pure oxygen sputtering gas. An average transmittance of about 40% in the visible range was obtained for a 110 nm thick NiO film. A semitransparent thin film pin diode consisting of p -NiO/i-NiO/i-ZnO/ n -ZnO layer having a voltage-current rectification characteristic and an average transmittance above 20% in the visible range was fabricated on a glass substrate.

Highly Conductive and Transparent Aluminum Doped Zinc Oxide Thin Films Prepared by RF Magnetron Sputtering

Highly conductive films of Al-doped ZnO have been prepared by rf magnetron sputtering of a ZnO target with Al2O3 dopant of 1–2 wt% in content added. Films with resistivity as low as 2×10-4 Ωcm and transmittance above 80% at the wavelength between 400 and 800 nm can be produced on low temperature substrate with a relatively high deposition rate. It is shown that a stable resistivity for use in various ambients at high temperature can be attained for the films. The characteristic features of Al-doped ZnO films are their high carrier concentration and low mobility in comparison with non-doped ZnO films.

Group III impurity doped zinc oxide thin films prepared by RF magnetron sputtering

The detailed study of electrical properties in group III impurity doped ZnO thin films prepared by rf magnetron sputtering is described. The resistivity is lowered by doping of B, Al, Ga and In into ZnO films. The characteristic features of ZnO films doped with group III elements except for B are their high carrier concentration and low mobility. Variation of the mobility with the impurity content is roughly governed by the ionized impurity scattering. It is shown that the doped ZnO films exhibit the resistivity dependence on film thickness below 300 nm.

Zinc‐oxide thin‐film ammonia gas sensors with high sensitivity and excellent selectivity

A sensor with a high sensitivity and an excellent selectivity for ammonia gas was prepared by using sputtered ZnO thin films. The sensor exhibited an increase of resistance for exposure to ammonia gas whereas it exhibited a decrease of resistance for exposure to many other gases such as inflammable and organic gases. The resistance change and the selectivity of the sensor were enhanced by doping group III metal impurities such as Al, In, and Ga. The lower limit of the detection for ammonia gas was about 1 ppm at a working temperature of 350 °C.

Highly transparent and conductive rare earth-doped ZnO thin films prepared by magnetron sputtering

Abstract Highly transparent and conductive thin films of ZnO doped with a rare-earth element, Sc or Y, have been prepared by d.c. magnetron sputtering using a powder target. The resistivity of the ZnO:Sc thin films was always lower than that of the ZnO:Y thin films; a resistivity in the order of 10 −4 Ω cm was obtained in these films. The resistivity of the ZnO:Sc thin films decreased as the Sc 2 O 3 content was increased up to about 2 wt.%; any further increase of the Sc 2 O 3 content caused the resistivity to increase. A resistivity of 3.1 A×10 −4 Ω cm was obtained in ZnO:Sc thin films prepared on a glass substrate at a temperature of 200°C with a Sc 2 O 3 content of 2 wt.%. An average transmittance of above 85% in the visible range was obtained for doped ZnO thin films. The electrical and optical properties, as well as the thermal stability of resistivity, of the ZnO:Sc thin films were comparable to those of ZnO:Al.

Electrical and optical properties of zinc oxide thin films prepared by rf magnetron sputtering for transparent electrode applications

Zinc oxide films were prepared on unheated glass substrates by rf magnetron sputtering under an applied external dc magnetic field in pure argon gas, and electrical and optical properties of the deposited films were investigated. Highly transparent films with resistivity as low as 10−4 Ω cm, which were weakly oriented perpendicular to the substrate surface(c‐axis orientation), could be produced with a relatively high deposition rate on the substrate suspended perpendicular to the target surface by controlling the sputtering gas pressure and the external dc magnetic field, without any postdeposition preparative treatment. The Hall mobility of the film with the highest conductivity was about 120 cm2/V sec, which was the highest yet reported for thin films on ZnO. The increase in the conductivity was related to the increase in Hall mobility which was caused by the decrease of carrier scattering from grain boundaries due to the grain growth resulting from the improvement of crystallization. The improvement of...

Transparent and conductive multicomponent oxide films prepared by magnetron sputtering

Transparent and conductive thin films of multicomponent oxides composed of binary and/or ternary compounds with varied chemical compositions have been prepared by rf or dc magnetron sputtering. Transparent conducting oxide (TCO) films were prepared in multicomponent oxides composed of ZnO, SnO2, Ga2O3, or MgO combined with In2O3 or ZnO combined with SnO2, containing ternary compounds such as Zn2In2O5, In4Sn3O12, GaInO3, MgIn2O4, and ZnSnO3. In addition, the TCO films were prepared in a multicomponent oxide composed of combinations of these ternary compounds. In particular, the Zn2In2O5 and In4Sn3O12 films exhibited a resistivity of 2×10−4 Ω cm, comparable to that of indium–tin–oxide films. TCO films could always be obtained in multicomponent oxides consisting of combinations of metal oxides as long as these oxides were TCO film materials or transparent conductors; the mixture ratio of the components could be varied over all possible compositions. The electrical, optical, and chemical properties, the band-...

Work function of transparent conducting multicomponent oxide thin films prepared by magnetron sputtering

Abstract The work function of transparent conducting multicomponent oxide (TCO) films is reported. TCO films consisting of binary oxides, such as In 2 O 3 , SnO 2 and ZnO, and ternary oxides, such as Zn 2 In 2 O 5 , In 4 Sn 3 O 12 , GaInO 3 , ZnSnO 3 and MgIn 2 O 4 , were prepared by magnetron sputtering. In addition, transparent conducting films consisting of multicomponent oxides composed of combinations of these binary or ternary oxides were also prepared by magnetron sputtering. The work function of these TCO films was measured by ultraviolet photoelectron spectroscopy operated in air. It was found that the work function as well as the electrical, optical and chemical properties of transparent conducting multicomponent oxide films could be controlled by varying the chemical composition.

High-Efficiency Oxide Solar Cells with ZnO/Cu2O Heterojunction Fabricated on Thermally Oxidized Cu2O Sheets

High conversion efficiencies were achieved in low cost n–p heterojunction oxide solar cells with an Al-doped ZnO (AZO)/non-doped ZnO (ZO)/Cu2O structure. This achievement was made possible by the formation of an n-ZO thin-film layer, prepared with an appropriate thickness by low damage deposition, on high quality Cu2O sheets produced by the thermal oxidization of copper sheets: n-ZO thin film optimal thickness ranges from 30 to 50 nm. Photovoltaic characteristics such as an open circuit voltage of 0.69 V, a fill factor of 0.55 and a conversion efficiency of 3.83% were attained under simulated AM1.5G solar illumination.