JP2012117093A - Transparent conductive film and substrate with transparent conductive film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ZnO-based transparent conductive film having a crystal structure which has practically sufficient electrical conductivity and transmittance, and a low refractive index, and can be deposited at a low temperature, and to provide a substrate with a transparent conductive film where the ZnO-based transparent conductive film is deposited on a film.SOLUTION: The transparent conductive film is composed of a zinc oxide containing at least one of aluminum and gallium in an amount within a range of 1-10 wt.% and is formed on a transparent substrate. The transparent conductive film is characterized in that transmittance in a visible light region is ≥85%, the refractive index at a wavelength of 550 nm is within a range of 1.75-1.80, and specific resistance is ≤8.0×10Ω cm.

Description

  The present invention relates to a transparent conductive film and a substrate with a transparent conductive film, and in particular, has high conductivity and high transmittance useful as a transparent electrode used in a display device such as a liquid crystal display element and a plasma light emitting element and a transparent electrode for a solar cell. The present invention relates to a transparent conductive film and a substrate with a transparent conductive film that have a low refractive index and can be formed in a low temperature range.

The transparent conductive film has both high transmittance and high conductivity in the visible light region. Transparent electrodes for display elements such as liquid crystal display elements, plasma light-emitting elements, EL (electroluminescence) elements, solar cells, TFTs (Thin film transistor) and other transparent electrodes of various light receiving elements.
Conventionally, as the transparent conductive film, tin oxide (SnO ₂ ) containing antimony or fluorine as a dopant deposited on a glass substrate, indium oxide (In ₂ O ₃ ) containing tin as a dopant, zinc oxide (ZnO), or the like In particular, indium oxide to which tin is added (hereinafter referred to as ITO) is widely used in various devices because a film having high conductivity, that is, low resistance can be easily obtained.

  However, in the case of an indium oxide-based material such as ITO, indium rare metal is expensive, and there are difficulties such as containing a toxic component that the indium element adversely affects the environment and the human body. There is a need for indium-based transparent conductive film materials. As a non-indium-based material, zinc oxide (ZnO) -based materials that are abundantly embedded as resources, are relatively inexpensive, and are friendly to the human body are attracting attention.

  By the way, the manufacturing method of the above transparent conductive films is divided roughly into physical vapor deposition methods (dry process), such as sputtering and vapor deposition, and methods by coating (wet process). Among the dry processes, the film formation method by sputtering is most widely adopted because it can reduce the size of the apparatus, simplify the film formation process, and reduce the cost associated with the recent technological progress. The same applies to the formation of the transparent conductive film. In sputtering, it is a general tendency that film formation at a high temperature of about 300 to 500 ° C. improves the crystallinity of the film and becomes dense, and also stabilizes the performance. In many cases, the substrate temperature is set to 300 ° C. or higher.

As the characteristics of the transparent conductive film, high transmittance and high conductivity (low specific resistance) in the visible light region are the most important indicators, and both of these are sufficient as practical levels for the ZnO-based transparent conductive film. Technological development has been promoted to achieve high performance. In particular, when considering the transparency of the entire substrate on which the transparent conductive film is formed, the refractive index of the film is the refractive index of the substrate (glass or film) in order to suppress reflection at the interface between the substrate and the transparent conductive film. It is advantageous to be close to. However, since the refractive index of these substrates is lower than the refractive index of a normal ZnO-based transparent conductive film, it has been a problem to make the ZnO-based transparent conductive film have a low refractive index.
On the other hand, in the display element etc., the movement which replaces a transparent substrate from glass to a resin film is remarkable from weight reduction, the ease of shape processing, and the ease of handling. However, when a film is used as the substrate, the film must be formed at a temperature that the substrate can withstand, so that the film formation at a temperature of about 200 ° C. or lower is required.

  In consideration of the above problems, a technique for forming a ZnO-based transparent conductive film that achieves both high permeability and high conductivity by sputtering at a relatively low temperature has been proposed (for example, Patent Document 1). , 2, 3).

Patent Document 1 realizes a ZnO film having a specific resistance of about 10 ⁻⁴ Ω · cm and a visible light transmittance of 85% or more at a film forming temperature of 150 ° C., but nothing is mentioned regarding the refractive index. Absent. Patent Document 1 discloses a method of applying a magnetic field perpendicular to the target surface and opposite to the magnetron magnetic field in addition to a normal magnetron cathode, but in order to realize this method, the film forming apparatus is changed. It is necessary and the manufacturing process becomes complicated.
Patent Document 2 discloses that a zinc oxide-based transparent conductive film laminate having a specific resistance of 3.5 × 10 ⁻⁴ Ω · cm can be obtained at a substrate temperature of 200 ° C. using a target obtained by adding Ga to Zn. Yes. However, the visible light transmittance of the zinc oxide-based transparent conductive film laminate is only described as 80% or more including the substrate, and detailed physical property values of the zinc oxide-based transparent conductive film laminate are unknown. . Further, as in Patent Document 1, no mention is made of the refractive index of the zinc oxide-based transparent conductive film laminate.
Patent Document 3 discloses a method for controlling the refractive index of a ZnO-based transparent conductive film, which is significantly lower than the refractive index of 1.61, which is usually 1.8 to 1.9. A low refractive index film is disclosed. However, the specific resistance and transmittance of the low refractive index film are not described.

JP 61-214306 A JP 2009-199986 A JP 2010-43334 A

As a practical standard of the transparent conductive film, a specific resistance of 10 ⁻⁴ Ω · cm or less and a transmittance of 85% or more in the visible light region are a guideline for high conductivity and high transmittance. And about the refractive index of a transparent conductive film, since it is common to use glass or a film for a board | substrate, 1.8 or less is preferable. As described above, transparent conductive films that satisfy practical standards in any one of conductivity, transmittance, and refractive index are disclosed in Patent Documents 1, 2, and 3, but in Patent Documents 1, 2, and 3, The disclosed transparent conductive film does not satisfy practical standards in terms of conductivity, transmittance, and refractive index. Furthermore, there has been a demand for a transparent conductive film that satisfies practical standards in terms of conductivity, transmittance, and refractive index, and can be formed at a low temperature. Therefore, since such a transparent conductive film has not been obtained, the elucidation of the crystal structure for the ZnO-based transparent conductive film to have the above characteristics has not been advanced.

  An object of the present invention is to solve the above-mentioned problems, have a practically sufficient high conductivity and high transmittance, and a ZnO-based transparent conductive film having a low refractive index close to the refractive index of a commonly used substrate. Is to provide. Another object of the present invention is to use a film forming condition at a low temperature of 230 ° C. or lower applicable to a film substrate without using a complicated film forming apparatus and method (within a normal sputtering technique). An object of the present invention is to provide a ZnO-based transparent conductive film having high conductivity and high transmittance sufficient for practical use and having a low refractive index close to the refractive index of a commonly used substrate.

The subject is a transparent conductive film made of zinc oxide containing at least one of aluminum or gallium in a range of 1 to 10 wt%, and formed on a transparent substrate, the transparent conductive film being in the visible light region. This is solved by having a transmittance of 85% or more, a refractive index in the range of 1.75 to 1.80 at a wavelength of 550 nm, and a specific resistance of 8.0 × 10 ⁴ Ω · cm or less.

The ZnO-based transparent conductive film of the present invention is obtained by a sputtering method using a target containing at least one of aluminum or gallium, and both conductivity and transmittance are equal to or higher than those of the ITO film. The specific resistance is 8.0 × 10 ⁻⁴ Ω · cm or less even in the case of the largest room temperature film formation, and is less than 6.0 × 10 ⁻⁴ Ω · cm in the film formation at 100 to 230 ° C. If a film forming temperature in this range is employed, the ZnO-based transparent conductive film according to the present invention has a film thickness of 600 nm (nm is equivalent to 10 Ω / □), which is required for the transparent conductive film for thin film solar cells. 0.001 μm), that is, a film thickness of 1 μm or less can satisfy the above requirement, which is advantageous for productivity. In addition, the transmittance is particularly high on the short wavelength side. Further, since the refractive index is low, there is little reflection at the interface with the glass or film substrate. As a result, the transmittance of the substrate with a transparent conductive film is improved, and it has excellent characteristics as a transparent electrode for a display device.

At this time, the lattice constant in the c-axis direction of the unit crystal lattice is in the range of 5.23 to 5.28 、, the lattice constant in the a-axis direction is in the range of 3.24 to 3.26 、, and the density is 5.55. It is preferable in the range of ˜5.65 g / cm ² .
In the ZnO-based transparent conductive film of the present invention, the lattice constant in the a-axis direction is the same as that of the conventional example, whereas the crystal growth in the c-axis direction proceeds and the lattice constant in the c-axis direction is significantly increased. . That is, a more clear columnar crystal structure with a higher column height is exhibited. Corresponding to the structure, the density is low, and the gap is increased in space. The refractive index of a substance is determined by the sum of atomic refractions of atoms or molecules (ions) constituting the substance, as shown by the Lorentz-Lorentz equation. Here, atomic refraction is represented by the product of the polarizability of each atom and the number of atoms per unit volume. The ZnO-based transparent conductive film according to the present invention has a low density and a small number of individual atoms per unit volume. As a result, a smaller refractive index is obtained, which contributes to high transmittance. In spite of the large number of voids, the lattice constant in the a-axis direction in which current normally flows as a transparent conductive film is equivalent to that of the conventional example, so that an extremely unique crystal structure in which the conductivity does not decrease is realized. The above characteristics are realized.

Further, it is more preferable that the carrier concentration is larger than 1.0 × 10 ²¹ / cm ³ and the band gap is in the range of 3.93 to 4.00 eV.
The band gap is larger than the band gap of the transparent conductive film obtained by the conventional technique, and the transmittance on the short wavelength side is improved.

Furthermore, it is preferable that the transparent substrate is formed at a temperature of 230 ° C. or lower during film formation by a sputtering method.
Thus, since a transparent conductive film can be formed at a low temperature, it can be applied to a film substrate. Further, it is also suitable for forming a film on a color filter of a liquid crystal display device or a semiconductor PN junction.

The subject is a substrate with a transparent conductive film on which the transparent conductive film according to claim 4 is formed,
The transparent substrate is solved by being a resin film.
Thus, since the transparent conductive film can be formed at a low temperature of 230 ° C. or lower, it can be formed even on a film-like substrate having a lower heat-resistant temperature than glass or the like. As a result, it can be set as the transparent conductive film provided with the transparent conductive film. Since a substrate with a transparent conductive film is flexible and lightweight, its application to portable display devices and the like is widened.

The ZnO-based transparent conductive film of the present invention has practically sufficient high conductivity and high transmittance as the film itself, and has a low refractive index, so that reflection at the substrate interface is reduced. A substrate with a transparent conductive film having a low electrical resistance can be obtained. Thus, since it is possible to create a transparent conductive film that satisfies practical standard values in terms of conductivity, transmittance, and refractive index, transparent electrodes for display devices such as liquid crystal display elements and plasma light emitting elements, or transparent for solar cells If used as electrodes, their performance can be significantly improved.
Furthermore, since it has practically sufficient high conductivity, high transmittance, low refractive index and can be formed at a low temperature, it is possible to form a ZnO-based transparent conductive film on a film having low heat resistance. Therefore, a transparent conductive film can be provided.

It is a graph which shows the specific resistance of the transparent conductive film of this invention. It is a graph which shows the spectral characteristic (transmittance) of the transparent conductive film of this invention. It is a graph which shows the refractive index of the transparent conductive film of this invention. It is a graph which shows the c-axis lattice constant of the transparent conductive film of this invention. It is a graph which shows the a-axis lattice constant of the transparent conductive film of this invention. It is a graph which shows the density of the transparent conductive film of this invention. It is a graph which shows the volume of the unit cell of the transparent conductive film of this invention. It is a graph which shows the carrier concentration of the transparent conductive film of this invention. It is a graph which shows the mobility of the transparent conductive film of this invention. It is a graph which shows the band gap of the transparent conductive film of this invention. It is a graph which shows the extinction coefficient in wavelength 380nm of the transparent conductive film of this invention. It is a graph which shows the extinction coefficient in wavelength 400nm of the transparent conductive film of this invention. It is the transmittance | permeability simulation at the time of using the transparent conductive film of this invention as the transparent electrode of a light-emitting body board | substrate.

  The transparent conductive film which concerns on embodiment of this invention is demonstrated based on drawing. The materials, arrangements, configurations, and the like described below do not limit the present invention and can be variously modified within the scope of the gist of the present invention.

  FIG. 1 to FIG. 12 show the behavior of the characteristics and crystal structure of the transparent conductive film according to the two embodiments of the present invention depending on the film formation temperature in comparison with the conventional example. 1 to 13 relate to a transparent conductive film according to an embodiment of the present invention. FIG. 1 is a graph showing specific resistance, FIG. 2 is a graph showing spectral characteristics (transmittance), and FIG. 3 is a refractive index. FIG. 4 is a graph showing the c-axis lattice constant, FIG. 5 is a graph showing the a-axis lattice constant, FIG. 6 is a graph showing the density, and FIG. 7 is a graph showing the volume of the unit cell. 8 is a graph showing the carrier concentration, FIG. 9 is a graph showing the mobility, FIG. 10 is a graph showing the band gap, FIG. 11 is a graph showing the extinction coefficient at a wavelength of 380 nm, and FIG. 12 is at a wavelength of 400 nm. FIG. 13 is a graph showing the extinction coefficient, and FIG. 13 is a transmittance simulation when the transparent conductive film of the present invention is used as a transparent electrode of a light emitter substrate.

[Embodiment 1]
The transparent conductive film of the present invention is formed on a glass substrate by a sputtering method. Details of the film forming conditions are as follows.
As the glass substrate, a product manufactured by Corning Corporation, product number 1737, dimensions 10 cm × 10 cm × thickness 1.1 mm was used. Prior to film formation, ultrasonic cleaning with pure water and pure water cleaning were performed, followed by drying.
As the sputtering target, a Ga-added ZnO (called GZO) target (AGG Ceramics) containing 5.7 wt% of gallium (Ga) in terms of Ga ₂ O _{3 in} ZnO is used, and is formed by a magnetron sputtering apparatus. Filmed. The content ratio of gallium is not limited to this, and it may be contained in the range of 1 to 10 wt% in the transparent conductive film. Alternatively, ZnO to which aluminum is added instead of gallium may be used. The film thickness was 150 nm.
Evacuation was performed by a turbo molecular pump until the degree of vacuum before film formation reached 7 × 10 ⁻⁶ Torr, and sputtering was performed by introducing a mixed gas of Ar and H ₂ into 1 mTorr. At this time, the mixing ratio of Ar and H ₂ was 95: 5.
Sputtering was performed with a power source in which a high frequency (RF) with a frequency of 13.65 MHz was superimposed on a direct current (DC) power source. The ratio of each power was 1 kW: 1 kW. The substrate temperature at the time of film formation was controlled by adjusting the sheath heater output, and film formation was performed under six conditions of 230 ° C. or lower, that is, room temperature (about 20 ° C.), 100 ° C., 150 ° C., 180 ° C., 200 ° C., 230 ° C. . The obtained sample was made into Examples (a) 1 to (a) 6 in order from the lowest substrate temperature during film formation.

[Embodiment 2]
Samples obtained under two conditions of substrate temperatures of 150 ° C. and 230 ° C. under the same conditions as in Embodiment 1 except that the sputtering gas was Ar were used as Examples (b) 1 and (b) 2.

[Comparative example]
As a comparative example related to the prior art, sputtering power source: DC, sputtering gas: Ar, and film formation under six conditions of room temperature (about 20 ° C.), 100 ° C., 150 ° C., 180 ° C., 200 ° C., 230 ° C. These samples were referred to as Comparative Examples 1-6.

  Table 1 shows a list of film forming conditions for the examples and comparative examples.

Examples (a) and (b) according to Embodiments 1 and 2 of the present invention and comparative examples according to the related art will be described in detail with reference to the drawings.
FIG. 1 shows the behavior of specific resistance with the film formation temperature (substrate temperature during film formation) as the horizontal axis and the specific resistance as the vertical axis. The specific resistance was measured with a resistivity meter.
In Examples (a) and (b) (Embodiments 1 and 2), although the specific resistance tends to increase on the low temperature side, there is a significant difference from Comparative Examples, and Examples (a) and (b) It was shown that the specific resistance is smaller than that of the comparative example. In Example (a), even at room temperature (20 ° C.) film formation, the 10 ⁻⁴ Ω · cm level equivalent to the ITO film is realized. More specifically, the specific resistance is 8.0 × 10 ⁻⁴ Ω · cm or less, and when assembled into a device such as a display device, the device can consume less power.
FIG. 2 shows a comparison of spectral characteristics on the glass substrates of Example (a) 3 and Comparative Example 3 formed at 150 ° C. In Example (a) 3, it can be seen that the transmittance is improved particularly on the short wavelength side. More specifically, the transmittance is 85% or less in the visible light region (430 nm to 850 nm). If there is a difference in transmittance in this region, the brightness of the display device is more noticeable than the difference in transmittance due to the relationship with human visibility.

  FIG. 3 shows the behavior of the refractive index at a wavelength of 550 nm determined by a spectroscopic ellipsometer depending on the film forming temperature for the transparent conductive films of Examples (a) and (b) and the comparative example. Although the refractive index tends to increase as the film forming temperature decreases, the tendency is small in Examples (a) and (b), whereas the refractive index increases to 1.9 in the comparative example of room temperature film formation. In Example (a), 1.8 or less is maintained, and more specifically, the range is from 1.75 to 1.80. The refractive index of the glass and film used for the transparent substrate is generally in the range of 1.5 to 1.6, but the transparent conductive film of Example (a) has a small difference from the refractive index of the substrate. Reflection at the interface is suppressed, and the transmittance as a substrate with a transparent conductive film can be improved.

4 and 5 are graphs showing the lattice constants of the c-axis and a-axis of the crystal obtained from the measurement results with an X-ray diffractometer (XRD), with the substrate temperature during film formation as the horizontal axis. FIG. In the ZnO-based transparent conductive film of the present invention, crystals grow remarkably in the c-axis direction, and the difference from the comparative example is large particularly in the low temperature region of the film formation temperature. On the other hand, the lattice constant in the a-axis direction is equivalent to the comparative example.
The density is shown in FIG. 6, and the volume of the unit cell is shown in the same graph in FIG. As a result of preferential crystal growth in the c-axis direction, the ZnO-based transparent conductive film of the present invention has a columnar structure with many gaps in the crystal, and the unit cell volume increases. As a result, the density is smaller than that of the comparative example. In general, in the case of a crystal having a large gap and a low density, the conductivity usually decreases, that is, the specific resistance increases. However, as described later, in the examples of the present invention, the specific resistance is a comparative example according to the prior art. Is lower than. That is, it can be said that it has a unique crystal structure in which the conductivity is improved while the structure has a little gap.
At this time, as shown in FIGS. 4 and 5, the lattice constant of the unit crystal lattice in the c-axis direction is 5.23 to 5.28 、, and the lattice constant in the a-axis direction is 3.24 to 3.26 Å. is there. And as FIG. 6 shows, a density is 5.55-5.65 g / cm < ³ >. The fact that the a-axis and c-axis lattice constants and densities are in the above ranges is considered to contribute to electrical and optical characteristics, as will be described later.

FIG. 8 shows a comparison of carrier concentration, and FIG. 9 shows a comparison of mobility. These were measured by the Hall effect measurement method. The carrier concentration in Example (a) of the present invention has a tendency to slightly decrease in the low temperature range, but the change due to the film forming temperature is not so large. On the other hand, in the comparative example, the carrier concentration is greatly reduced on the low temperature side. FIG. 9 shows that the mobility is slightly lower than that of the conventional example with respect to the embodiments (a) and (b) of the present invention. In detail, it is considered that the carrier concentration is higher than 1.0 × 10 ²¹ cm ⁻³ ), thereby realizing high conductivity (low specific resistance) superior to the comparative example according to the prior art.

  FIG. 10 shows the relationship between the substrate temperature during film formation and the band gap of the transparent conductive film calculated from the spectral transmittance. Also in the band gap, the difference between the examples (a) and (b) and the comparative example is particularly large in a low temperature range, and when the substrate temperature during film formation is 200 ° C. or lower, the band gap of the comparative example rapidly decreases. On the other hand, in Examples (a) and (b), it is almost constant. This difference corresponds to the fact that Example (a) 3 in FIG. 2 has less light absorption and higher transmittance on the short wavelength side. At this time, specifically, the band gap is 3.93 to 4.00 eV.

  11 and 12 are graphs showing extinction coefficients obtained by a spectroscopic ellipsometer at wavelengths of 380 nm and 400 nm, respectively. Although there is variation in the data of the comparative example, Examples (a) and (b) are significantly smaller values than the comparative example, and the difference is more remarkable at 380 nm. This difference in extinction coefficient also contributes to the high transmittance on the short wavelength side.

  FIG. 13 is a result of simulating the transmittance of Example (a) 3 and Comparative Example 3 on the assumption that an organic EL element is formed based on the spectral characteristics of FIG. In this simulation, a structure in which three layers of a glass substrate, a ZnO-based transparent conductive film, and an organic light emitting layer are stacked is used, and the refractive index of the organic light emitting layer is set to 1.7. In Example (a) 3, the transmittance is improved as an effect due to the low refractive index, and the average transmittance at 350 to 700 nm is about 2.6%, particularly 350 to 550 nm (in the visible region). In the short wavelength region), the average transmittance is improved by 15.5%. As a result, in the display device (organic EL element, liquid crystal element, etc.), the luminance of blue accompanying an increase in transmittance in the short wavelength region increases, and a bright and clear white display is possible as a whole.

Claims (5)

A transparent conductive film made of zinc oxide containing at least one of aluminum and gallium in a range of 1 to 10 wt%, formed on a transparent substrate,
The transparent conductive film
The transmittance in the visible light region is 85% or more,
The refractive index at a wavelength of 550 nm is in the range of 1.75 to 1.80,
A transparent conductive film having a specific resistance of 8.0 × 10 ⁻⁴ Ω · cm or less. The lattice constant in the c-axis direction of the unit crystal lattice is in the range of 5.23-5.28Å,
The lattice constant in the a-axis direction is in the range of 3.24 to 3.26 mm,
The transparent conductive film according to claim 1, wherein the density is in the range of 5.55 to 5.65 g / cm ³ . The carrier concentration is greater than 1.0 × 10 ²¹ / cm ³ ;
The transparent conductive film according to claim 2, wherein the band gap is in the range of 3.93 to 4.00 eV.   The transparent conductive film according to claim 1, wherein the transparent conductive film is formed by a sputtering method with a temperature of the transparent substrate during film formation being 230 ° C. or lower. A substrate with a transparent conductive film on which the transparent conductive film according to claim 4 is formed,
The substrate with a transparent conductive film, wherein the transparent substrate is a resin film.

Images