In various electronic devices, conductive oxides are widely used as conductive materials for electrodes, wiring and the like. Recently, in the field of electronic devices, a device is required to be microminiaturized as small as possible, due to a demand of higher integration and multilayered wiring.
In this context, when a crystalline oxide is used as a conductive oxide, a limitation is indicated for the microminiaturization of the device. That is, it is known that conductivity becomes non-continuous as the size of an electrode or a wiring composed of a crystalline material approaches a crystal size. Therefore, an electrode and the like are required to have a size at least 3 times larger than that of a crystal size. Since a crystal size of a common crystalline conductive oxide is 50 to 100 nm, an electrode and the like having a size of less than 150 to 300 nm can not be produced using a crystalline conductive oxide.
On the other hand, since there are no such limitations for an amorphous conductive oxide, an electrode and the like having a more microminiature size can be formed.
As amorphous conductive oxides, for example, IZO (indium-zinc composite oxide), IGZO (indium-gallium-zinc composite oxide) and the like are known. Conventionally, films composed of these amorphous conductive oxides have been formed by gas phase methods, such as the sputtering method, the laser ablation method and the vapor deposition method, for example. However, a gas phase method requires massive and expensive equipment, and the productivity of films is also low, resulting in cost-ineffective film manufacturing.
In recent years, a technique in which an amorphous conductive oxide film is formed by a less expensive liquid phase process has been reported (C. K. Chen, et al., Journal of Display Technology, Vol. 5, No. 12, pp 509-514 (2009)). This technique involves a formation of an IZO film by applying a solution composition containing indium chloride, zinc chloride and acetonitrile as a precursor of an oxide on a substrate, and heating it. However, this technique has not yet been subjected to practical use because a film obtained by the technique do not have sufficient conductivity since its volume resistivity is greater than 1 Ω·cm. For example, in order to use the film in a gate electrode for a thin layer transistor, its volume resistivity is required to be 0.1 Ω·cm or less. In addition, the low thermal stability of amorphous IZO and IGZO is a problem. These materials can maintain an amorphous state up to 500° C. at the highest and undergo crystallization at 500 to 600° C. Therefore, they are not applicable for electronic devices which require a processing temperature of 500° C. or higher.
In view of these, a method is demanded for forming a conductive oxide film having high conductivity and a stable amorphous state by an inexpensive liquid phase process.
Recently, the present inventors completed a technique in which an amorphous conductive strontium-ruthenium composite oxide film by a liquid phase process, and filed a patent application thereof (Japanese Patent Application No. 2010-98200). The technique is an excellent technique in which an amorphous conductive oxide film having high conductivity can be formed by a liquid phase process comprising heating at a relatively low temperature of below 450° C. However, exposure of a film obtained by the technique to temperature of 450° C. or higher causes crystallization into a perovskite structure. Therefore, the stability of the amorphous structure at high temperature is not sufficient, if considering every manufacturing step of all electronic devices.
Meanwhile, a semiconductor device such as a diode and a transistor shows its functions through a junction between semiconductors showing different types of conductivity such as a pn junction and a pin junction. For many years, such a semiconductor has been manufactured using a metalloid element such as silicon and germanium. These materials are less than satisfactory as an industrial semiconductor material because their manufacturing cost is high, and they are susceptible to decomposition at high temperature of 600° C. or higher.
In this context, for example, an oxide semiconductor such as an In—Ga—Zn—O-based semiconductor is a potential material having various attractive properties including that it can also be prepared at a low temperature by a simple method such as the coating method, and that surrounding atmosphere at the time of preparation is not particularly required to be controlled, and in addition, that a thin film obtained shows optical transparency.
However, there is no other choice but to use, at least in part, conventional materials to manufacture a practical semiconductor device because most of known oxide semiconductors are an n-type semiconductor, leaving the above problems not completely solved.
Only a few oxide semiconductors showing p-type conductivity have been reported. For example, Applied Physics 97, 072111 (2010) and Applied Physics 93, 032113 (2008) described crystalline SnO showing p-type conductivity. However, the method of preparing it is very complicated. Briefly, according to the above literature in 2010, an amorphous SnO film is deposited on a substrate by radiofrequency magnetron sputtering, and a SiO2 cap layer is further formed on the amorphous SnO film by sputtering. Subsequently, two step annealing at different surrounding atmospheres and temperatures was performed to obtain a crystalline SnO thin film showing p-type conductivity. Such complicated manufacturing steps lack industrial applicability. In addition to this, a crystalline SnO obtained has insufficient p-type semiconductivity.