Patent Application
20170106406
This application is based upon and claims the benefit of priority from Japanese patent application No. 2015-204489, filed on Oct. 16, 2015, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a method for manufacturing zinc oxide films and to, in particular, a method for manufacturing zinc oxide films using the Liquid Phase Deposition (LPD) method.
2. Description of Related Art
Recently, use of zinc oxide (ZnO), which material is low in cost, as a replacement for tin-doped indium oxide (ITO) containing rare elements as a material for transparent electrodes has been examined. ZnO exhibits n-type conductivity because of oxygen deficiency or interstitial zinc. It is known that the conductivity of ZnO can be improved by doping an element of group IV such as Al, Ga, or the like as a dopant. The conductive ZnO films with resistivity of the order of 10−4 Ω·cm have been prepared so far by using sputtering and ion plating.
One of film deposition methods for conductive ZnO films is a vapor phase method such as sputtering and ion plating. However, the manufacturing cost of the vapor phase method such as sputtering can be excessively high because such a method includes a process that requires a large vacuum device. Further, such a method is not suitable for increasing areas of films and mass production of films.
Japanese Unexamined Patent Application Publication No. 2012-144384 discloses a technique related to a method for manufacturing zinc oxide films using the LPD method. The technique disclosed in Japanese Unexamined Patent Application Publication No. 2012-144384 teaches a method for manufacturing conductive zinc oxide films, which includes: depositing a zinc oxide film on a substrate by the LPD method in the presence of organic acid; and irradiating the substrate with UV light to remove the organic acid from the zinc oxide film. Such a technique can provide a method for manufacturing conductive ZnO films that achieves an increase in areas of the films and mass production of the films at a low cost and a reduced environmental impact.
In the technique disclosed in Japanese Unexamined Patent Application Publication No. 2012-144384, two source solutions, which are a zinc nitrate aqueous solution and an ammonia aqueous solution containing citric acid, are separately prepared, the source solutions are separately transferred to a substrate, and the source solutions are mixed on the substrate (on a reaction field), so that a zinc oxide film is formed on the substrate.
However, the present inventors have found a problem in such a configuration that separately transfers two source solutions to a substrate in that two transfer paths for the source solutions need to be prepared, thereby complicating a device configuration of a manufacturing device.
In light of the above circumstances, an object of the present invention is to provide a method for manufacturing zinc oxide films that can simplify a device configuration of a manufacturing device.
In an exemplary aspect of the present invention, a method for manufacturing zinc oxide films includes steps of: mixing zinc salt, aqueous ammonia, and organic acid to prepare a source solution containing a zinc ammine complex; depositing a zinc oxide film on a substrate using the source solution by a liquid phase deposition method; and irradiating the deposited zinc oxide film with UV light to remove the organic acid from the deposited zinc oxide film.
According to the present invention, it is possible to provide a method for manufacturing zinc oxide films that can simplify a device configuration of a manufacturing device.
The above and other objects, features and advantages of the present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Hereinafter, an exemplary embodiment of the present invention will be described with reference to the drawings.
A method for manufacturing zinc oxide films according to this exemplary embodiment uses Liquid Phase Deposition (LPD) method. The LPD method is a method for depositing the crystals of metal oxide from an aqueous solution. By the LPD method, a crystalline thin film of metal oxide can be formed on a surface of a substrate by causing a synthesis reaction of the metal oxide in the aqueous solution and then depositing the crystals of the metal oxide directly on the substrate. According to the LPD method, the crystalline thin film of the metal oxide can be formed under a condition of a relatively low temperature of not more than 100 Celsius degrees, and thus the heat treatment process for the crystallization such as the sol-gel method will become unnecessary.
Firstly, zinc salt, aqueous ammonia, and organic acid are mixed to prepare a source solution containing a zinc ammine complex. As the zinc salt according to this exemplary embodiment, zinc nitrate [Zn(NO3)2], zinc sulfate [ZnSO4], zinc chloride [ZnCl2], or hydrate of these zinc salts (e.g., Zn(NO3)2.nH2O (n is natural number)) may be used. For example, when zinc nitrate [Zn(NO3)2] is used as the zinc salt, zinc nitrate in the mixed solution is ionized into divalent ions as shown in the following equation (1).
[Equation 1]
Zn(NO3)2+aq→Zn2++2NO3− (1)
Further, the aqueous ammonia is equilibrated as shown in the following equation (2).
[Equation 2]
NH3+H2O→NH4++OH− (2)
The divalent zinc ions (Zn2+) in the equation (1) and hydroxide ions (OH−) in the equation (2) are reacted as shown in the following equation (3), and zinc hydroxide (Zn(OH)2) is formed.
[Equation 3]
Zn2++2OH−→Zn(OH)2↓ (3)
The zinc hydroxide (Zn(OH)2) is reacted with aqueous ammonia as shown in the following equation (4), and a zinc ammine complex (tetraammine zinc ion complex) is generated.
[Equation 4]
Zn(OH)2+4NH3→[Zn(NH3)4]2++2OH− (4)
In the invention according to this exemplary embodiment, organic acid is mixed in the process for preparing the source solution. The organic acid is a substance for controlling an orientation of ZnO crystals. As for growth rates of crystallographic planes of ZnO in the aqueous solution, in most cases, a growth rate of a plane (0001) of ZnO crystal in an aqueous solution is the highest among other planes of ZnO. In such a state, the needle-like crystals showing anisotropic growth in the c-axis direction are randomly jumbled up on the substrate to form the needle point holder, and thus a uniform film structure may not be obtained. However, a dense crystalline film can be obtained by adding a substance that is absorbed on the plane (0001) of ZnO to the deposition reaction system so as to prevent the anisotropic growth in the c-axis direction. Organic acid containing a carboxylic group (COOH) can be used as a substance that is absorbed on the plane (0001) of ZnO.
In this exemplary embodiment, such organic acid may include, for example, citric acid (e.g., [C6H5O7]3− or salt thereof) and another acid such as maleic acid or dimelcaptosuccinic acid as the dicarboxylic acid and phenol phtalein or eosin-Y as the monocarboxylic acid, or sodium salt, ammonium salt, or amine salt thereof.
For example, in this exemplary embodiment, Zn(NO3)2.6H2O may be used as the zinc salt, and C6H5O7Na3 may be used as the organic acid. In this case, C6H5O7Na3 may be added to 5 mM Zn(NO3)2.6H2O so that a concentration of C6H5O7Na3 will become 10 mM or less. In other words, a concentration ratio of Zn(NO3)2.6H2O to C6H5O7Na3 (i.e., [Zn(NO3)2.6H2O]:[C6H5O7Na3]) may be adjusted within a range of 5:1 to 5:10. The reason for adding C6H5O7Na3 to become a concentration of 10 mM or less is that C6H5O7Na3 with a concentration greater than 10 mM will inhibit a zinc oxide film from being formed on the substrate.
More preferably, 1 to 4 mM of C6H5O7Na3 may be added to 5 mM Zn(NO3)2.6H2O. In other words, a concentration ratio of Zn(NO3)2.6H2O to C6H5O7Na3 (i.e., [Zn(NO3)2.6H2O]:[C6H5O7Na3]) may be adjusted within a range of 5:1 to 5:4.
A concentration of the ammonia is adjusted to become 140 mM or greater for, for example, 5 mM Zn(NO3)2.6H2O. In other words, the concentration of the ammonia is adjusted in such a way that a concentration ratio of ammonia to the Zn(NO3)2.6H2O will become 28 (i.e., 28/1) or higher. As an ammine complex is not formed with a low ammonia concentration, a zinc oxide film will not be formed as well. It is thus necessary to adjust the concentration of the ammonia to be greater than or equal to a predetermined concentration. When the concentration of the ammonia is low, zinc hydroxide particles are generated, which cause a nozzle of a manufacturing device to be clogged.
Next, a zinc oxide film is deposited on the substrate by the LPD method using the source solution prepared in the step S1. To be more specific, the substrate that is in contact with the source solution prepared in the step S1 is heated so that a temperature thereof will become an appropriate film deposition temperature (50 to 100 Celsius degrees). Then, a supersaturated state of the deposition reaction system is changed by the heat, and ZnO is directly deposited on the surface of the substrate as shown in the equation (5).
[Equation 5]
[Zn(NH3)4]2++2OH−→ZnO↓+4NH3+H2O (5)
In this exemplary embodiment, as a crystallization reaction shown in the above equation (5) progresses in the presence of the organic acid, anisotropic growth in the c-axis direction of ZnO crystals is prevented, and thus a continuous dense ZnO crystalline film is formed on the surface of the substrate.
As has been described above, as the method for manufacturing zinc oxide films according to this exemplary embodiment using the LPD method does not require the heat treatment process, it can be applied not only to glass substrates and silicon substrates but also to plastic substrates.
Note that the step S2 (the process for depositing a zinc oxide film on a substrate) in the method for manufacturing zinc oxide films according to this exemplary embodiment can be performed by the spin spray method.
The source container 14 holds a source solution 15 produced in the step S1. The source solution 15 held in the source container 14 is transferred through the pipe 13. The pipe 13 is connected to the nozzle 12. The source solution 15 transferred through the pipe 13 is uniformly sprayed from the nozzle 12 onto the substrate 20. In this manner, a zinc oxide film can be deposited on the substrate 20 using the spin spray device 1.
Note that the manufacturing device shown in
Step S3: Process for Irradiating Zinc Oxide Film with UV Light
Next, the zinc oxide film deposited on the substrate is irradiated with UV light in order to remove the organic acid from the zinc oxide film deposited in the step S2. In the process for depositing the zinc oxide film on the substrate (step S2), the organic acid is incorporated into the film when a zinc oxide crystalline film is formed on the substrate. This consequently leads to a defect derived from the organic acid in the crystalline film, and the defect generates carriers. As the generated carriers are trapped in the organic acid, the zinc oxide crystalline film exhibits almost no conductivity when the step S2 is completed.
In the step S3, the organic acid incorporated into the crystalline film is removed using a photocatalytic activity of zinc oxide. That is, the organic acid in the film is photolyzed by irradiating the zinc oxide crystalline film with UV light. As a result, the carriers trapped in the organic acid are discharged inside the crystalline film, and then preferable conductivity is provided to the zinc oxide crystalline film. Note that the UV light to be used preferably has an appropriate wavelength taken a transmittance and the like into consideration so that the UV light reaches throughout the crystalline film in order to photolyze all the organic acid.
In the technique disclosed in Japanese Unexamined Patent Application Publication No. 2012-144384, two source solutions, which are a zinc nitrate aqueous solution and an ammonia aqueous solution containing citric acid, are separately prepared, the source solutions are separately transferred to a substrate, the source solutions are mixed on the substrate (on a reaction field), and then a zinc oxide film is formed. However, there is a problem in such a configuration that separately transfers two source solutions to a substrate in that two transfer paths for the source solutions need to be prepared, thereby complicating a device configuration of a manufacturing device.
Thus, in the method for manufacturing zinc oxide films according to this exemplary embodiment, in the process for preparing a source solution (step S1), zinc salt, aqueous ammonia, and organic acid are mixed to prepare a source solution containing a zinc ammine complex. Then, the source solution is transferred to the substrate to deposit a zinc oxide film (step S2). As there is only one source solution in the method for manufacturing zinc oxide films according to this exemplary embodiment, only one transfer path for the source solution is needed, thereby simplifying the device configuration of the manufacturing device.
Further, in the method for manufacturing zinc oxide films according to this exemplary embodiment, one source solution is transferred to the substrate to deposit a zinc oxide film on the substrate. It is therefore possible to form a zinc oxide film more uniformly than when two source solutions, which are a zinc nitrate aqueous solution and an ammonia aqueous solution containing citric acid, are separately transferred in order to form a zinc oxide film (see Japanese Unexamined Patent Application Publication No. 2012-144384).
As an example of characteristics of the zinc oxide film formed by the method for manufacturing zinc oxide films according to this exemplary embodiment, which has been explained above, a resistivity is within a range of 1.3×10−2 to 9.1×10−1 (Ω·cm), preferably 1.31×10−2 to 9.19×10−1 (Ω·cm), a transmittance at a film thickness 400 nm is within a range of 89 to 92(%), and the crystal structure is (002) oriented. To be more specific, an intensity ratio of X-ray diffraction (XRD) is (100):(002):(101)=0:9.5:1. That is, the (100) peak is not confirmed, there are mostly (002) peaks indicating the c-axis orientation, and a few (101) peaks. In other words, a zinc oxide film preferentially oriented with the c-axis is obtained.
In this way, by the method for manufacturing zinc oxide films according to this exemplary embodiment, it is possible to form a uniform zinc oxide film and achieve a zinc oxide film with a high transmittance.
Hereinafter, an example of the present invention will be described.
Next, zinc salt, aqueous ammonia, and organic acid were mixed to prepare a source solution containing a zinc ammine complex (step S13). Zinc nitrate hexahydrate [Zn(NO3)2.6H2O] was used as the zinc salt, and trisodium citrate [C6H5O7Na3] was used as the organic acid. More specifically, the source solution was adjusted so that a concentration of zinc nitrate hexahydrate [Zn(NO3)2.6H2O] was 5 mM, a concentration of aqueous ammonia (NH3) was 450 mM, and a concentration of trisodium citrate [C6H5O7Na3] was 2 mM.
Next, a zinc oxide film was formed on the substrate using the spin spray device 1 shown in
After that, the zinc oxide film produced in the step S14 is irradiated with UV light in order to remove the organic acid from the zinc oxide film. More specifically, the zinc oxide film was irradiated with UV light with a broad wavelength within a range of 300 to 400 nm using a black light for 24 hours.
In a comparative example, a zinc oxide film produced by the method disclosed in the related art (Japanese Unexamined Patent Application Publication No. 2012-144384), which uses two source solutions, was prepared. Samples of deposited zinc oxide films not irradiated with UV light were also prepared.
A result of an evaluation on the zinc oxide films produced as above will be described below.
X-ray diffraction measurements have been performed on these zinc oxide films. As a result, both of the zinc oxide films manufactured by the present invention (one source solution) and the related art (two source solutions) were crystallized. In a comparison between the zinc oxide films manufactured when the present invention (one source solution) was used and when the related art (two source solutions) was used, the (100) peak was the maximum when the related art (two source solutions) was used, whereas the (002) peak was the maximum when the present invention (one source solution) was used. As a result of examinations by the present inventors, it was found that the (100) peak was the maximum also in the present invention (one source solution) by adjusting the amount of citric acid to be added.
From the invention thus described, it will be obvious that the embodiments of the invention may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended for inclusion within the scope of the following claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2015-204489 | Oct 2015 | JP | national |
