JPH0657410A - Selective light-transmissive zno thin film - Google Patents

Abstract

PURPOSE:To form a ZnO thin film excellent in optical characteristics by sputtering a target consisting essentially of ZnO under specified conditions. CONSTITUTION:A target consisting essentially of ZnO is sputtered at >=2Pa gas pressure and >=100 deg.C substrate temp. to form a ZnO thin film with the light transmittance depending on the light incident angle. In this case, the film thickness is not specified but ordinarily controlled to 2000Angstrom to 5mum, especially to 5000Angstrom to 2mum. A ZnO thin film excellent in optical characteristics, especially in the light scattering effect, is produced in this way.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin film containing ZnO as a main component (hereinafter referred to as "ZnO-based thin film") and a method for forming the thin film. More specifically, Zn whose light transmittance depends on a light incident angle.
The present invention relates to an O-based thin film and a method for forming the same.

The thin film of the present invention can be used as, for example, a light-collecting electrode of a solar cell or a window coating material for light control.

[0003]

2. Description of the Related Art In recent years, there has been an increasing demand for transparent conductive films as transparent electrodes for display devices such as solar cells and liquid crystals.
As such a material, ITO (tin-doped indium oxide), tin oxide, zinc oxide and the like are well known.

Further, these materials are properly used in appropriate places for their respective uses. For example, ITO is mainly used in the field where low resistance is required (for example, transparent electrode of liquid crystal), and in the field where optical characteristics such as light scattering effect are required (for example, collector electrode of solar cell). Mainly tin oxide is used.

In particular, it is well known that, in a thin film requiring optical characteristics, a tin oxide thin film having a textured structure can provide a light scattering confinement effect and improve the photoelectric conversion efficiency of a solar cell. Further, recently, in order to supplement the stability of tin oxide, it has been studied to coat a zinc oxide film on the surface of the tin oxide film. However, ITO and tin oxide have low plasma resistance and have a problem in stability.

Therefore, recently, ZnO-based thin films, which are excellent in chemical stability, particularly in plasma resistance, have attracted attention.

The ZnO-based thin film is characterized by being inexpensive, excellent in transparency, and having the same conductivity as ITO when dopant is added, and having no problems of reduction by plasma, deterioration, and impurity diffusion. To be

However, none of the conventional ZnO-based thin films are satisfactory in terms of optical properties such as light scattering (light confinement) effect, and tin oxide whose performance is still insufficient is used in solar cells and the like. Is the current situation.

[0009]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and an object thereof is to provide a ZnO-based thin film excellent in optical characteristics, particularly light scattering effect.

[0010]

The inventors of the present invention have conducted extensive studies to solve the above problems, and as a result, found a ZnO-based thin film having optical characteristics in which the transmittance depends on the light incident angle. Has been completed.

That is, the present invention provides a thin film containing ZnO as a main component, in which the grain units constituting the film are filled with forming a void on the surface of the film, and the light transmittance of the film depends on the light incident angle, and a method for forming the thin film. Is. Hereinafter, the present invention will be described in detail.

First, the thin film containing ZnO as a main component of the present invention includes a thin film made of ZnO alone, a thin film containing a dopant for adding conductivity to ZnO, and the like.

Examples of such dopants include elements having a valence of at least positive trivalence, such as I
IIA group, IIIB group, IVA group, IVB group, VA group,
VB group, VIA group, VIB group, VIIA group, VIII
One or more elements selected from group, lanthanoid, or actinoid series elements can be shown, and when these elements are used as dopants, the content is usually 0.1 to 20 relative to zinc. 0.0 atom%, especially 0.5 to
By setting the range to be 5.0 atomic%, ZnO having low resistance can be obtained.
It is preferable because a system thin film can be obtained.

Next, the ZnO-based thin film of the present invention is one in which the grain units forming the film are filled with voids formed on the film surface.

The crystal structure of the surface of the ZnO-based thin film of the present invention is shown in FIG. 1, and the crystal structure of the surface of the conventional ZnO-based thin film is shown in FIG. The ZnO-based thin film of the present invention shown in FIG.
It can be seen that the grain units forming the film are not densely packed like the conventional ZnO-based thin film shown in (3), but are filled by forming voids on the film surface.

The grain unit constituting the above-mentioned film is a column-shaped or granular particle agglomerate, and the size thereof is not particularly limited, but is usually about 0.1 to 10 μm, and is formed on the film surface. The size of the voids is approximately 0.01 to 10 μm
Is in the range.

The column-shaped or granular group of agglomerates may be in an amorphous state, but it is preferably a polycrystal with advanced crystallization. Furthermore, it is acceptable for these polycrystals to form irregularities on the film surface.

The thickness of the film of the present invention is not particularly limited, but a thickness of generally 2000 angstroms to 5 microns, particularly 5000 angstroms to 2 microns is preferable.

The ZnO-based thin film of the present invention has the above structure, and the light transmittance of the film depends on the light incident angle.
The degree of the dependency depends on the film to be obtained and the wavelength of the incident light. For example, the light transmittance of the vertically incident light is 4 degrees.
It is possible to obtain a dependency of 5% or more on the light transmittance of incident light of 5 degrees, and further, a dependency of 10% or more on the optical transmittance.

By the way, the reason why the light transmittance of the ZnO-based thin film of the present invention depends on the light incident angle is that the particle unit forming the film filled with voids on the film surface is further secondary aggregated. It is considered that this is due to the fact that particles are formed, a space such as a grain boundary is formed for each aggregated particle, and a wavy curved surface with a cycle of about 1 to 100 μm is formed on the film surface.

The ZnO-based thin film is a semiconductor,
Since the light having a wavelength of less than nm does not transmit and the light having an infrared wavelength may be reflected due to its conductivity, the above dependency is generally 400 to 2000 nm, particularly 500 to 1000 nm.
It tends to be exhibited in the wavelength range of.

Therefore, since the ZnO-based thin film of the present invention has selective translucency in which the light transmittance depends on the light incident angle as described above, it is transparent when the film is directly viewed, and when viewed at an angle. , Looks like milky.

Next, a method of forming the ZnO-based thin film of the present invention will be described.

The ZnO thin film of the present invention can be formed by, for example, a sputtering method. The sputtering method is not particularly limited, and DC sputtering, RF sputtering, and high rate sputtering in which a magnetron is introduced can be applied.

As the combination of the target and the sputtering gas to be used, a combination of the oxide sintered body target and an inert gas such as pure argon, a combination of the metal target and a mixed gas of argon and oxygen is used. Etc.,
Among these, it is preferable to employ a combination using an oxide sintered body target because the control of sputtering conditions is easy.

The method of forming the ZnO-based thin film of the present invention by the sputtering method using the oxide sintered body will be described below.

In the method of the present invention, the sputtering gas pressure and the substrate temperature during sputtering are important factors. The sputtering gas pressure is 2 Pa or higher, and the substrate temperature at that time is 100 ° C. or higher. If the gas pressure is less than 2 Pa,
The transparency of the obtained ZnO-based thin film is improved as a whole, while when the substrate temperature is lower than 100 ° C., the light transmittance of the obtained thin film at an oblique incident angle is improved. The rate becomes independent of the incident angle of light.

Although the upper limit of each condition is not particularly limited, if the gas pressure exceeds 20 Pa, it may be difficult to maintain a stable sputter discharge, and sputtering with a substrate temperature exceeding 600 ° C. is seldom. Not realistic, gas pressure is 3 Pa to 20 Pa, substrate temperature is 100
It is preferable to carry out the sputtering at a temperature of 600 to 600 ° C., particularly preferably 200 to 300 ° C.

[0029]

EXAMPLES The present invention will be described below based on examples.
The invention is in no way limited to the examples.

Example 1 A zinc oxide sintered body containing 2% by weight of aluminum oxide (sintering density of 5.2 g / cm ³ ) was used as a target.
A ZnO-based thin film containing Al as a dopant was formed by DC magnetron sputtering. The sputtering was carried out in a pure argon atmosphere at a sputtering gas pressure of 8 Pa and a glass substrate (# 705 manufactured by Corning Incorporated) was used as a substrate.
9) was used and the substrate was placed in a position parallel to the target. The substrate temperature during sputtering is 250
℃ was made.

The crystal structure of the surface of the obtained thin film is shown in FIG. 1. The units constituting this film were filled with minute voids.

The resistivity of the obtained thin film is 8 × 10 ^−4.
Ω · cm, the transmittance of vertically incident light at a visible light wavelength of 500 to 700 nm is 45 for obliquely incident light of 45 degrees.
It was a thin film having a high transmittance of at least%. The light transmission characteristics of this thin film are shown in FIG.

Example 2 A ZnO thin film was formed in the same manner as in Example 1 except that the substrate was placed at a position perpendicular to the target and sputtering was performed.

The specific resistance of the obtained thin film is 8 × 10 ⁻⁴ Ω ·
It was cm, and it was a thin film similar to the thin film obtained in Example 1 and having different transmittances at light incident angles of 90 ° and 45 °. Example 3 A ZnO-based thin film was formed in the same manner as in Example 1 except that the substrate temperature was 300 ° C. and the film thickness was increased.

The obtained thin film has a film thickness of from 5000 angstroms to 2 by raising the substrate temperature to 300.degree.
In the wide range of the thickness of the micron, a dense film having a dense inside and filling the surface of the film with a granular unit characteristic of the present invention was obtained. The substrate temperature is 300 ℃
In the case, the crystallization of the film proceeded, and the surface became polycrystallized, and the unevenness became remarkable. The specific resistance was 4 × 10 ⁻⁴ Ω · cm over the entire surface of the substrate. In addition, crystallization progressed from the thin film obtained in Example 1, and as a result, irregularities on the film surface due to the growth of grain units were observed. The light transmittance of the film was a thin film having different transmittances at the light incident angles of 90 ° and 45 ° as in Example 1.

Example 4 A ZnO thin film was formed in the same manner as in Example 3 except that the substrate temperature was 400 ° C.

The obtained thin film had strong adhesion as in Example 3, and the specific resistance was 5 × 10 ⁻⁴ Ω · cm. Further, the crystallization was further increased as compared with the thin film obtained in Example 3, and the grain units were grown accordingly. The light transmittance of the film was a thin film having different transmittances at the light incident angles of 90 ° and 45 ° as in Example 1.

Example 5 A ZnO thin film was formed in the same manner as in Example 3 except that the substrate temperature was 500 ° C.

The obtained thin film had strong adhesion as in Example 4, and the specific resistance was 6 × 10 ⁻⁴ Ω · cm. Further, crystallization was further increased as compared with the thin film obtained in Example 4, and grain units were also grown accordingly. The light transmittance of the film was a thin film having different transmittances at the light incident angles of 90 ° and 45 ° as in Example 1.

Comparative Example 1 A ZnO thin film was formed in the same manner as in Example 1 except that the substrate temperature during sputtering was room temperature.

The obtained thin film has a specific resistance of 4 × 10 ⁻⁴ Ω.
Cm, excellent transparency, no substantial difference in light transmittance (excluding interference effect) between vertically incident light and obliquely incident light of 45 degrees at a wavelength of 500 to 700 nm was not observed. The light transmittance of the obtained thin film is shown in FIG.

Further, the light transmission characteristics of the substrate are shown in FIG. 5, and no substantial difference in light transmittance between the vertically incident light and the 45 ° obliquely incident light was observed in the substrate.

Comparative Example 2 ZnO was prepared in the same manner as in Example 1 except that the substrate temperature during sputtering was room temperature and the gas pressure was 1 Pa.
A system thin film was formed.

The obtained thin film has a specific resistance of 4 × 10 ⁻⁴ Ω.
Cm, more excellent in transparency than Comparative Example 1, 500 to
No substantial difference in light transmittance (excluding interference effect) was observed between vertically incident light and obliquely incident light of 45 degrees at a wavelength of 700 nm.

Comparative Example 3 A ZnO thin film was formed in the same manner as in Example 1 except that the substrate temperature during sputtering was 300 ° C. and the gas pressure was 1 Pa.

The obtained thin film has a specific resistance of 4 × 10 ⁻⁴ Ω.
-Cm, excellent in transparency as in Comparative Example 2, 500 to 70
No substantial difference in light transmittance (excluding interference effect) was observed between vertically incident light and 45 ° obliquely incident light at a wavelength of 0 nm.

[0047]

As described above, the ZnO-based thin film of the present invention is required as a thin film having a light scattering effect and a dimming function,
It shows the light incident angle dependency of the light transmittance, and is expected to have a light scattering effect and a light control function. Therefore, the ZnO-based thin film of the present invention can provide an excellent function when used as a light-collecting electrode of a solar cell or a window coating material.

[Brief description of drawings]

FIG. 1 is an electron micrograph showing a crystal structure of a surface of a ZnO-based thin film obtained in Example 1.

FIG. 2 is an electron micrograph showing a crystal structure of a surface of a conventional ZnO-based thin film.

FIG. 3 is a diagram showing the normal incident light and the light transmittance at an incident angle of 45 degrees of the ZnO-based thin film obtained in Example 1.

FIG. 4 is a diagram showing the normal incident light and the light transmittance at an incident angle of 45 degrees of the ZnO-based thin film obtained in Comparative Example 1.

FIG. 5 is a diagram showing a vertically incident light and a light transmittance at an incident angle of 45 degrees of a glass substrate.

Claims (2)

[Claims] 1. Particles constituting a film are filled in the film surface by forming voids, and the light transmittance of the film depends on the incident angle of light Z.
A thin film composed mainly of nO. 2. Sputtering is carried out using a target containing ZnO as a main component, at a sputtering gas pressure of 2 Pa or higher and a substrate temperature of 100 ° C. or higher.
A method of forming a thin film containing as a main component.