1. Field of the Invention
The present invention relates to an oxide sintered body which mainly consists of gallium, indium, and oxygen, and a transparent base material comprising an oxide film obtained using the oxide sintered body and its oxide film.
In particular, it relates to a transparent base material comprising an oxide sintered body having a low content of an indium oxide phase, and an oxide film with high light transmittance at a near-ultraviolet region wherein a film is formed by using the oxide sintered body as a sputtering target.
2. Description of the Related Art
Since a transparent conductive oxide film is excellent in electrical conductivity and light transmittance in a visible region, it has been used as a transparent electrode of various devices.
As a practical thing, tin oxide (SnO2) which contains antimony and fluorine as a dopant, zinc oxide (ZnO) which contains aluminium and gallium as a dopant, indium oxide (In2O3) which contains Sn as a dopant etc., have been known.
Especially, among them, the indium oxide film which contains Sn as dopant is called ITO (Indium-Tin-Oxide) film, and it has been extensively used since the transparent conductive oxide film having low resistance can be obtained easily.
As a method of forming a transparent conductive oxide film, the sputtering method, the evaporation method, the ion plating method, and the chemical solution coating method have been used widely.
Among such methods, a sputtering method is an effective method, when using material with a low vapor pressure, or when precise film thickness control is needed.
In the sputtering method, generally, argon gas is used under gas pressure of about 10 Pa or less, a substrate is used as an anode, and a sputtering target which is a raw material of the transparent conductive oxide film to be formed as a cathode and voltage is supplied to them.
Between the electrodes to which voltage is applied, glow discharge occurs, and then argon plasma occurs, and argon ions in plasma collide with the sputtering target of the cathode.
Particles which are flipped one after another off by this collision are deposited one by one on the substrate, and a thin film is formed.
The sputtering method is classified according to generating method of argon plasma. A method using plasma generated by high frequency power is called as RF sputtering method, and a method using plasma generated by direct current power is called as direct current sputtering method.
Especially, the direct current sputtering method is an optimal film forming method since it has such features that there are less heat damages to a substrate, high-speed film forming is possible, power supply equipment is cheap, and operation is simple and so on.
Generally, the direct current sputtering method is used for formation of ITO film.
The ITO film formed at room temperature shows low specific resistance of 5×10−4 Ω·cm.
The ITO film is good also about the light transmittance of a visible region, and has the light transmittance of an average of 80% or more.
Moreover, it is excellent at chemical and thermal stability.
The luminescent material and luminescence device which have a function of near-ultraviolet light luminescence (for example, wavelength of 300 nm˜400 nm) (for example, LED, laser, organic or inorganic EL) have been widely used and these development have been made briskly (with respect to a near-ultraviolet LED, refer to Applied physics, volume 68 (1999), No. 2, pp. 152-155, and SEI Technical Review, September, 2004 (No. 165), and pp. 75˜78) Applied physics, the 68th volume (1999), No. 2, pp. 152˜155 and the SEI technical review, the September, 2004 (No. 165), and pp. 75˜78)
A transparent electrode is indispensable to these electron devices also.
In a conventional luminescence device in which importance is given to visible light with wavelength of 400 nm˜800 nm, the ITO film and the transparent conductive oxide film of ZnO and the like or SnO2 and the like have been used as a transparent electrode.
These conventional transparent conductive oxide films had characteristics such that an average transmittance of a visible light with wavelength of 400 nm˜800 nm is excellent, but to a near ultraviolet light with wavelength that was short wave less than 400 nm, transmittance was not sufficient since an absorption occurs at the wavelength of 400 nm.
The following proposals have been made for a transparent conductive oxide film applied to a luminescent material or a luminescence device (for example, LED, laser, organic or inorganic EL) which has a luminescence function of the near-ultraviolet light (for example, wavelength of 300 nm-400 nm).
In Japanese published unexamined patent application Toku Kai Hei 7-182924, it has been proposed that gallium indium oxide (GaInO3) which is doped by a little amount of different valent dopants like a quadrivalent ion.
It is disclosed that since a crystal film of this oxide is excellent at transparency and has low refractive index of about 1.6, refractive-index consistency with a glass substrate is improved, and furthermore, electrical conductivity comparable as that of a broad prohibition area semiconductor which has been currently used can be realized. However, as for the crystal film disclosed there, absorption of a near-ultraviolet light occurs, and it is difficult to use it industrially without improvement since film forming at a high temperature, that is, a substrate temperature of 250° C.˜500° C. is required.
In Published Unexamined Patent Application Toku Kai 2002-093243, an ultraviolet transparent conductive oxide film has been proposed, and it has been disclosed that the ultraviolet transparent conductive oxide film is characterized in that it consists of Ga2O3 crystal, and in the range of the wavelength of 240 nm˜800 nm, or wavelength (240 nm˜400 nm), it is transparent, and has electrical conductivity owing to an oxygen defect or a dopant element, and manufacturing is carried out by using one of methods of pulsed laser deposition method, sputtering method, CVD method, and MBE method under such condition that one element or more elements of Sn, Ge, Si, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W is used as a dopant, and a substrate temperature is set at 600° C.˜1500° C., and oxygen partial pressure is set at 0˜1 Pa, have been shown.
In order to acquire electrical conductivity, it is necessary to form a film of Ga2O3 crystal film shown in the above, at a substrate temperature of 600° C.˜1500° C.
Since this temperature range is too high, industrial use is very difficult.
Recently, inventors of the present invention have found, as disclosed in the patent application No. 2005-252788, a new transparent conductive thin laminated film, which has not only a high transmittance in a visible region, and a low surface resistance (6Ω/□˜500Ω/□), but also has a high light transmittance in a visible light short wavelength region with wavelength of 380 nm˜400 nm and also in a near-ultraviolet light region (300 nm˜380 nm) of short wavelength.
Namely, the inventors have found out that the above-mentioned subject can be solved, by having paid attention to a transparent conductive film having a lamination structure in which a surface of a metal thin film is covered by a transparent thin film of oxide, in a transparent conductive film wherein the transparent thin film of oxide is thin film of an amorphous oxide which mainly consists of gallium, indium, and oxygen, or the transparent thin film of the amorphous oxide which mainly consists of gallium and oxygen, and the gallium contained in the transparent thin film of oxide is contained at a rate 35 at. % or more, and less than 100 at. % to all metal atoms.