JP2025005832A - Substrate with deep ultraviolet light-transmitting transparent electrode and method for manufacturing substrate with deep ultraviolet light-transmitting transparent electrode - Google Patents

Abstract

To provide a base material with a deep ultraviolet transmission type transparent electrode having high permeability to deep ultraviolet, and a method for manufacturing the base material with the deep ultraviolet transmission type transparent electrode.SOLUTION: A base material with a deep ultraviolet transmission type transparent electrode includes a transparent base material and a transparent oxide layer formed on the transparent base material. The transparent base material is any base material selected from the group consisting of a sapphire base material, an aluminum nitride base material and a base material constituted of a mixed crystal of aluminium nitride and gallium nitride. The transparent oxide layer consists of metal oxide having SnO2 as a main component and including a Ta element. An amount of the Ta element of the metal oxide is 0.8 atom% or more and 7.0 atom% or less to the total of an Sn element and the Ta element of the metal oxide.SELECTED DRAWING: Figure 1

Description

The present invention relates to a substrate with a deep ultraviolet light-transmitting transparent electrode and a method for manufacturing a substrate with a deep ultraviolet light-transmitting transparent electrode.

Transparent electrodes having transparent conductive compositions are used in display devices such as touch panels and displays, light control devices such as smart windows, light emitting devices such as LEDs, light receiving devices such as solar cells, etc. As the above devices become more widespread, there is a demand for the development of transparent conductive compositions with properties suitable for each application. For example, there is a demand for the development of transparent conductive compositions for ultraviolet light to improve the efficiency of ultraviolet light devices.

Non-Patent Document 1 discloses that a thin film composed of a transparent conductive composition in which tin dioxide is doped with tantalum is formed on an r-plane sapphire substrate by a metal organic chemical vapor deposition (MOCVD) method. The transparent conductive composition disclosed in Non-Patent Document 1 shows a result that the ratio of Ta element to Sn element is 0 to 8 atomic %, and when the ratio of Ta element to Sn element is 0 to 8 atomic %, the maximum carrier density is 3.0 to 4.0 × 10 ²⁰ cm ^-3 . Non-Patent Document 1 discloses that the transparent conductive composition shows high transmittance to ultraviolet light (UV-B) with a wavelength of 280 to 320 nm.

Non-Patent Document 2 discloses that a thin film composed of a transparent conductive composition in which tin dioxide is doped with antimony (Sb) is formed on an r-plane sapphire substrate by molecular beam epitaxy. Non-Patent Document 2 discloses that the transparent conductive composition exhibits high transmittance to ultraviolet light with wavelengths of 300 nm to 400 nm.

Non-Patent Document 3 discloses that a thin film containing (111)-oriented indium tin oxide (ITO) as a transparent conductive composition is formed on a c-plane sapphire substrate by MOCVD. Non-Patent Document 3 discloses that the transparent conductive composition has high transmittance, with average transmittances of 94% and 74% for ultraviolet rays with wavelengths of 315 to 400 nm (UV-A) and ultraviolet rays with wavelengths of 280 to 315 nm (UV-B).

“UV-vis transparent conducting Ta-doped SnO2 epitaxial films grown by metal-organic chemical vapor deposition”, He et al., Mater. Res. Bull. (2019), doi: 10.1016/j.materresbull.2019.05.013 “Conductivity and transparency limits of Sb-doped SnO2 grown by molecular beam epitaxy”, Martinez-Gazoni et al., Phys. Rev. B (2018), doi:10.1103/PhysRevB.98.155308 “Highly ultraviolet transparent textured indium tin oxide thin films and the application in light emitting diodes”, Chen et al., Appl. Phys. Lett. (2017), doi:10.1063/1.4986452

However, the transparent conductive compositions disclosed in Non-Patent Documents 1 to 3 had low transmittance to deep ultraviolet light (UV-C) with a wavelength of 280 nm or less. Deep ultraviolet light is expected to have effects such as sterilization, water purification, and air purification, and in recent years, there has been a demand for improving the external quantum efficiency η _ext of deep ultraviolet light-emitting devices. Accordingly, there is a demand for a substrate with a transparent electrode that is highly transmittable to deep ultraviolet light in order to improve the light extraction efficiency η _lee of deep ultraviolet light.

The present invention was made in consideration of the above circumstances, and aims to provide a substrate with a deep ultraviolet light-transmitting transparent electrode that has high transparency to deep ultraviolet light, and a method for manufacturing a substrate with a deep ultraviolet light-transmitting transparent electrode.

The present invention provides the following means to solve the above problems.

[1] A substrate with a deep ultraviolet light transmitting transparent electrode according to one aspect of the present invention includes a transparent substrate and a transparent oxide layer formed on the transparent substrate, the transparent substrate being any substrate selected from the group consisting of a sapphire substrate, an aluminum nitride substrate, and a substrate composed of a mixed crystal of aluminum nitride and gallium nitride, the transparent oxide layer being made of a metal oxide containing _SnO2 as a main component and Ta element, and the amount of Ta element in the metal oxide is 0.8 atomic % or more and 7.0 atomic % or less with respect to the sum of Sn element and Ta element in the metal oxide.

[2] In the substrate with a deep ultraviolet light transmitting transparent electrode of [1] above, in the transparent oxide layer, the metal oxide may contain 1.0 atomic % or more and 5.0 atomic % or less of Ta element relative to Sn element.

[3] In the substrate with a deep ultraviolet light transmitting transparent electrode according to the above [1] or [2], the transparent oxide layer may have a carrier density of 4.5×10 ²⁰ cm ⁻³ or more.

[4] In the substrate with a deep ultraviolet light transmitting transparent electrode described above in [1] to [3], the transparent oxide layer may be an epitaxial layer of the metal oxide.

[5] A method for producing a substrate with a deep ultraviolet light transmitting transparent electrode according to one embodiment of the present invention includes a preparation step of preparing a substrate selected from the group consisting of a sapphire substrate, an aluminum nitride substrate, and a substrate composed of a mixed crystal of aluminum nitride and gallium nitride as a transparent substrate, and a film formation step of placing a target in which the amount of Ta element relative to the sum of the amounts of Sn element and Ta element is 0.8 atomic % to 7.0 atomic % facing the transparent substrate, and performing physical vapor deposition to form a transparent oxide layer composed of a metal oxide on the transparent substrate.

[6] In the method for producing a substrate with a deep ultraviolet light transmitting transparent electrode according to the above [5], the target may be mainly composed of _SnO2 , and the amount of the Ta element may be 0.8 atomic % or more and 7.0 atomic % or less.

[7] In the method for manufacturing a substrate with a deep ultraviolet light transmitting transparent electrode as described above in [5] or [6], the target may have an amount of Ta element of 1.0 atomic % or more and 5.0 atomic % or less.

[8] In any one of the above methods for producing a substrate with a deep ultraviolet light transmitting transparent electrode [5] to [7], the film forming step may be performed by a pulsed laser deposition method.

The present invention provides a deep ultraviolet light-transmitting transparent electrode and a method for manufacturing a deep ultraviolet light-transmitting transparent electrode that has a transparent conductive composition with a small amount of constituent elements and has high transparency to deep ultraviolet light.

FIG. 1 is a cross-sectional view showing an example of the configuration of a substrate with a deep ultraviolet light transmitting transparent electrode according to one embodiment of the present invention. FIG. 2 is a diagram for explaining a method for producing a substrate with a deep ultraviolet light transmitting transparent electrode according to one embodiment of the present invention, showing a state in which a film formation process is performed by a PLD method. FIG. 3(a) is a diagram showing the band structure of _SnO2 , and FIG. 3(b) is a diagram showing the band structure of the transparent conductive composition according to this embodiment. 1 shows X-ray diffraction patterns of substrates with deep ultraviolet light transmitting transparent electrodes of Examples 1-1 to 1-5 and substrates with electrodes of Comparative Examples 1 to 3. FIG. 5(a) is a graph showing the resistivity of the deep-ultraviolet light-transmitting transparent electrode-equipped substrates of Examples 1-1 to 1-5 and the electrode-equipped substrates of Comparative Examples 1 to 3; FIG. 5(b) is a graph showing the carrier density of the deep-ultraviolet light-transmitting transparent electrode-equipped substrates of Examples 1-1 to 1-5 and the electrode-equipped substrates of Comparative Examples 1 to 3; and FIG. 5(c) is a graph showing the electron mobility of the deep-ultraviolet light-transmitting transparent electrode-equipped substrates of Examples 1-1 to 1-5 and the electrode-equipped substrates of Comparative Examples 1 to 3. 1 is a graph showing the internal transmittance spectra of the deep ultraviolet light transmitting transparent electrode-equipped substrates of Examples 1-1 to 1-5 and the electrode-equipped substrates of Comparative Examples 1 to 3. 1 is a graph showing the transmittance, reflectance, and internal transmittance of the deep ultraviolet light transmitting transparent electrode-attached substrates of Examples 1 to 4 and Example 2. FIG. 8(a) is a graph showing the sheet resistance and internal transmittance of the electrode-attached substrates of Examples 1-1 to 1-5, Example 2, Example 3, and Comparative Examples 1 to 6 at a wavelength of 280 nm, and FIG. 8(b) is a graph showing the sheet resistance and internal transmittance of the electrode-attached substrates of Examples 1-1 to 1-5 and Comparative Examples 1 to 6 at a wavelength of 260 nm.

Below, an example of an embodiment of the present invention will be described in detail with reference to the drawings. Note that the drawings used in the following description may show enlarged characteristic parts for the sake of convenience in order to make the features of the present invention easier to understand. For this reason, the dimensional ratios of each component may differ from the actual ones.

[Substrate with ultraviolet light-transmitting transparent electrode]
Fig. 1 is a cross-sectional view showing an example of the configuration of a deep ultraviolet light transmitting substrate with a transparent electrode according to one embodiment of the present invention. The substrate with a transparent electrode 10 shown in Fig. 1 is an optical laminate including a transparent substrate 1 and a transparent oxide layer 2 formed on the transparent substrate 1. The substrate with a transparent electrode 10 preferably includes the transparent substrate 1 and the transparent oxide layer 2.

(Transparent substrate)
The transparent substrate 1 is a substrate that serves as the base of the substrate 10 with a transparent electrode. The transparent substrate 1 is any substrate selected from the group consisting of a sapphire substrate, an aluminum nitride substrate, and a substrate composed of a mixed crystal of aluminum nitride and gallium nitride. When the substrate with an ultraviolet light-transmitting transparent electrode according to this embodiment is used as an LED, the transparent substrate 1 contains an aluminum nitride substrate or a mixed crystal of aluminum nitride and gallium nitride as a main component, and the transparent oxide layer 2 is formed on the transparent substrate 1 that is a part of the element. As the transparent substrate, any of a-plane, c-plane, r-plane, and m-plane may be used. The transparent substrate 1 is a substrate that contacts the transparent oxide layer 2, and another substrate having high transparency to deep ultraviolet light may be provided on the surface of the transparent substrate 1 opposite to the transparent oxide layer 2. For example, when the transparent substrate 1 is an aluminum nitride substrate, the aluminum nitride substrate may be formed on a sapphire substrate. For example, the aluminum nitride substrate may be formed on an a-plane sapphire substrate or a c-plane sapphire substrate. Similarly, when the transparent substrate 1 is a sapphire substrate, the sapphire substrate may be formed on an aluminum nitride substrate. For example, a c-plane sapphire substrate may be formed on a c-plane aluminum nitride substrate.

Since the transparent substrate 1 has a c-plane orientation, the transparent oxide layer 2 grown by physical vapor deposition on the transparent substrate 1 inherits the orientation of the transparent substrate 1 and grows in the c-plane.

The transparent substrate 1 is made of the above-mentioned substrate and therefore exhibits high transmittance to deep ultraviolet light. The transparent substrate 1 is not limited to a flat substrate, but may be a substrate having a curved shape, or may be a so-called flexible substrate having high flexibility.

The thickness of the transparent substrate 1 can be set arbitrarily, but is, for example, 40 μm or more and 1000 μm or less. The transparent substrate 1 is preferably a substrate that can be wound up and unwound onto a roll. If the transparent substrate 1 is a substrate that can be wound up and unwound onto a roll and the thickness of the transparent substrate 1 is within the above range, the substrate 10 with transparent electrodes can be manufactured by a roll-to-roll method, and high productivity can be achieved.

(Transparent oxide layer)
The transparent oxide layer 2 is made of a metal oxide mainly composed of _SnO2 and containing Ta element, and the amount of Ta element in the metal oxide is 0.8 atomic % or more and 7.0 atomic % or less with respect to the sum of Sn element and Ta element in the metal oxide. The metal oxide constituting the transparent oxide layer 2 is represented by the general formula (1).
Sn _100-x Ta _x O ₂ ...(1)
(In the formula, x satisfies 0.8≦x≦7.0.)

In the above metal oxide, the Ta element relative to the sum of the Sn element and the Ta element is preferably 1.0 atomic % or more and 5.0 atomic % or less, more preferably 1.5 atomic % or more and 4.0 atomic % or less, and even more preferably 2.0 atomic % or more and 3.0 atomic % or less. That is, in the above formula (1), x satisfies 0.8≦x≦7.0, preferably satisfies 1.0≦x≦5.0, more preferably satisfies 1.5≦x≦4.0, and even more preferably satisfies 2.0≦x≦3.0.

As described above, the transparent oxide layer 2 is formed by inheriting the orientation of the transparent substrate 1. The transparent oxide layer 2 is an epitaxial layer of the above-mentioned metal oxide. The transparent oxide layer 2 is configured, for example, to have one type of orientation in the perpendicular direction to the surface, and one to three types of orientation in the in-plane direction depending on the crystals that make up the substrate.

The resistivity of the transparent oxide layer 2 is, for example, 1.0×10 ⁻⁴ Ω·cm or more and 5.0×10 ⁻⁴ Ω·cm or less, preferably 4.0×10 ⁻⁴ Ω·cm or less, and more preferably 3.0×10 ⁻⁴ Ω·cm or less.

The carrier density of the transparent oxide layer 2 is, for example, 2.8×10 ²⁰ cm ^-3 or more, preferably 4.5×10 ²⁰ cm ^-3 or more, more preferably 5.1×10 ²⁰ cm ^-3 or more, and even more preferably 6.0×10 ²⁰ cm ^-3 or more. The transparent oxide layer 2 provided in the substrate with transparent electrode 10 according to this embodiment is manufactured by the manufacturing method described below, so that the amount of carrier density relative to the amount of doped Ta element is high, and the doped Ta element is prevented from becoming inactive. Therefore, the transparent oxide layer 2 exhibits high electrical conductivity.

The mobility of the transparent oxide layer 2 is, for example, 15 cm ² V ^-1 s ^-1 or more, preferably 25 cm ² V ^-1 s ^-1 or more, more preferably 40 cm ² V ^-1 s ^-1 or more, and further preferably 60 cm ² V ^-1 s ^-1 or more.

The transmittance of deep ultraviolet light with a wavelength of 280 nm through the transparent oxide layer 2 is 30% or more, preferably 35% or more, more preferably 40% or more, even more preferably 50% or more, and particularly preferably 60% or more.

The thickness of the transparent oxide layer 2 is, for example, 10 nm or more and 1000 nm or less, and preferably 50 nm or more and 300 nm or less.

The figure of merit (FoM) of the substrate 10 with transparent electrodes is expressed by formula (2). In formula (2), ρ represents the resistivity of the transparent oxide layer 2, and α represents the absorption coefficient of the substrate 10 with transparent electrodes. The resistivity ρ is calculated by formula (2-1), and the absorption coefficient α is calculated by formula (2-2). In formulas (2-1) and (2-2), R represents the electrical resistance of the transparent oxide layer 2, t represents the film thickness, w represents the width, l represents the electrode spacing, and T represents the light transmittance.
FoM=(ρ・α) ^-1 ...(2)
ρ=R×t×w/l...(2-1)
α=-(log _e T)/t...(2-2)

The FoM of the substrate 10 with transparent electrodes for deep ultraviolet light with a wavelength of 280 nm is, for example, 0.05 or more, and preferably 0.1 or more.

[Method for producing substrate with deep ultraviolet light transmitting transparent electrode]
Hereinafter, a method for manufacturing a substrate with a deep ultraviolet light transmitting transparent electrode according to the above embodiment will be described. The method for manufacturing a substrate with a transparent electrode according to one embodiment of the present invention includes a preparation step of preparing any substrate selected from the group consisting of a sapphire substrate, an aluminum nitride substrate, and a substrate made of a mixed crystal of aluminum nitride and gallium nitride as a transparent substrate, and a film formation step of arranging a target in which the amount of Ta element relative to the sum of the amounts of Sn element and Ta element is 0.8 atomic % to 7.0 atomic % facing the transparent substrate, and performing physical vapor deposition to form a transparent oxide layer made of a metal oxide on the transparent substrate.

(Preparation process)
First, a c-plane sapphire substrate or a c-plane aluminum nitride substrate is prepared. The c-plane sapphire substrate or the c-plane aluminum nitride substrate may be a substrate made of Al ₂ O ₃ or AlN, which is c-plane oriented. Alternatively, an AlN film may be formed on an Al ₂ O ₃ substrate with a c-plane or a-plane orientation, and a c-plane aluminum nitride substrate may be formed on the c-plane sapphire substrate. Similarly, an Al ₂ O ₃ film may be formed on an AlN substrate with a c-plane orientation, and a c-plane sapphire substrate may be formed on the c-plane aluminum nitride substrate.

(Film forming process)
Next, a target containing Ta element of 0.8 atomic % or more and 7.0 atomic % or less with respect to the sum of Sn element and Ta element is placed facing the transparent substrate, and a physical vapor deposition (PVD) method is performed to form a transparent oxide layer composed of a metal oxide on the transparent substrate. The film formation process can be performed by a physical vapor deposition method such as a sputtering method, a pulse laser deposition method, or a vacuum evaporation method, and is preferably performed by a pulse laser deposition method.

Figure 2 is a diagram for explaining a method for manufacturing a substrate with a deep ultraviolet light transmitting transparent electrode according to one embodiment of the present invention, and shows how a film formation process is performed by the PLD method. The PLD method is performed, for example, by using a pulsed laser deposition apparatus (PLD apparatus) 50. The pulsed laser deposition apparatus 50 includes, for example, a sample mounting stage 20 and a target mounting stage 21 inside a vacuum chamber 40.

A transparent substrate 1 is placed on the sample setting stage 20. The transparent substrate 1 is arranged so that the surface on which the transparent oxide layer is laminated faces the target T described later. The target T is placed on the target setting stage 21. The target T is a substrate weighed so that the amount of Ta element relative to the sum of Sn element and Ta element is 0.8 atomic % or more and 7.0 atomic % or less. The amount of Ta element relative to the sum of Sn element and Ta element in the target is preferably 1.0 atomic % or more and 5.0 atomic % or less, more preferably 1.5 atomic % or more and 4.0 atomic % or less, and even more preferably 2.0 atomic % or more and 3.0 atomic % or less. The target is, for example, a substrate in which a predetermined amount of Ta element is doped into _SnO2 .

The pulsed laser deposition apparatus 50 is connected to, for example, an exhaust device 31 that reduces the pressure inside the vacuum chamber 40 and a supply device 32 that supplies gas into the vacuum chamber 40. The exhaust device 31 is, for example, a vacuum pump. In the PLD method, for example, the exhaust device 31 evacuates the vacuum chamber 40. Next, a predetermined gas is supplied from the supply device 32 so that the vacuum chamber 40 has a predetermined atmosphere, and the exhaust device 31 adjusts the vacuum chamber 40 to a predetermined pressure. The vacuum chamber 40 is adjusted to, for example, oxygen radicals or oxidizing gas such as ozone or N ₂ O. The inside of the vacuum chamber 40 is adjusted to, for example, an oxygen partial pressure of 0.1 mTorr to 1 Torr, and preferably adjusted to 1 mTorr to 50 mTorr from the viewpoint of the film formation rate.

In the pulsed laser deposition device 50, a pulsed laser L focused by a focusing lens (not shown) is irradiated onto the surface of the target T. This knocks out or evaporates the constituent particles of the target T, generating a plume. The constituent particles of the target T contained in the plume are deposited on the transparent substrate 1, forming a transparent oxide layer 2 on the surface of the transparent substrate 1.

The film formation process is performed, for example, while heating the transparent substrate 1. The heating temperature of the transparent substrate 1 is, for example, 200°C or more and 900°C or less, preferably 300°C or more and 700°C or less, and more preferably 450°C or more and 650°C or less. The transparent substrate 1 may be heated by an infrared laser (IR laser) (not shown), or may be heated by resistance heating or the like.

The target T may be in the form of a pellet or a powder.

In FIG. 1, an example in which only one target T is provided is shown, but multiple targets T may be provided. When multiple targets T are provided, the amount of Ta element relative to the amount of Sn element and Ta element in the multiple targets may be adjusted to be 0.8 atomic % or more and 7.0 atomic % or less, preferably adjusted to be 1.0 atomic % or more and 5.0 atomic % or less, more preferably adjusted to be 1.5 atomic % or more and 4.0 atomic % or less, and even more preferably adjusted to be 2.0 atomic % or more and 3.0 atomic % or less.

According to the above embodiment, a transparent oxide layer 2 is formed that includes a transparent conductive composition with a small amount of constituent elements. The transparent conductive composition according to the above embodiment is formed by doping the base material _SnO2 with a high concentration of Ta to introduce carrier electrons, and the Burstein-Moss shift associated with the increase in carrier electron concentration improves the transparency to deep ultraviolet light. Since the transparent conductive composition according to the above embodiment is formed so as to have a high carrier electron concentration, a significantly large Burstein-Moss shift is realized, and the transparency to deep ultraviolet light is high.

FIG. 3(a) is a diagram showing a band structure of SnO ₂ , and FIG. 3(b) is a diagram showing a band structure of the transparent conductive composition according to this embodiment. The Burstein-Moss shift will be described with reference to FIG. 3(a) and FIG. 3(b). As shown in FIG. 3(a), SnO ₂ has an energy gap of E _g in the forbidden band between the valence band VB and the conduction band CB. On the other hand, as shown in FIG. 3(b), when Ta is doped into SnO ₂ to increase the carrier density, carrier electrons occupy the bottom of the conduction band, and the apparent band gap (optical gap) becomes large.

The deep ultraviolet light-transmitting substrate with transparent electrodes according to the above embodiment is suitable for display devices, dimming devices, and light-emitting devices. Specifically, it can be used as a substrate with transparent electrodes for deep ultraviolet light suitable for LEDs that emit ultraviolet light, particularly LEDs that emit not only UV-A and UV-B but also UV-C (deep ultraviolet light).

The following describes examples of the present invention. The present invention is not limited to the following examples.

[Example 1]
First, in a preparation step, a c-plane aluminum nitride substrate, which is a transparent substrate, was prepared by cleaning with an organic solvent. The prepared c-plane aluminum nitride substrate had a flat shape and a thickness of about 0.5 mm.

Next, in the film formation process, a transparent oxide layer was formed on the c-plane aluminum nitride substrate by the PLD method. The target was prepared by the following procedure: a pellet containing 1 atomic % Ta element relative to the sum of Sn element and Ta element.

Powders of Ta ₂ O ₅ and SnO ₂ were weighed and mixed so that the Ta content was 1 atomic % relative to the sum of the Sn and Ta elements, and the mixture was molded and then heated and sintered in the air to prepare a target.

This target was placed on a target mounting stage in the vacuum chamber of the PLD device. A c-plane sapphire substrate was placed on the sample mounting stage so that it faced the target. The sample mounting stage was configured so that the substrate placed on it could be heated by an infrared laser.

Using the above PLD device, the inside of the vacuum chamber was adjusted so that oxygen radicals were being supplied, and the oxygen partial pressure was adjusted to 5 mTorr. The transparent substrate was heated to 500°C using an infrared laser, and a KrF excimer laser (pulse width approximately 20 ns) was irradiated onto the target surface from the laser inlet to form a transparent oxide layer on the transparent substrate, producing a substrate with a deep ultraviolet light-transmitting transparent electrode.

Hereinafter, the substrate with a deep ultraviolet light transmitting transparent electrode formed with a transparent conductive composition containing 1 atomic % of Ta element relative to the sum of Sn element and Ta element, produced by the above procedure, will also be referred to as Example 1-1.

(Example 1-2)
A substrate with a deep ultraviolet light transmitting transparent electrode was produced in the same manner as in Example 1-1, except that a target in which the amount of Ta element relative to the sum of Sn element and Ta element was 1.5 atomic % was used.

(Examples 1 to 3)
A substrate with a deep ultraviolet light transmitting transparent electrode was produced in the same manner as in Example 1-1, except that a target in which the amount of Ta element relative to the sum of Sn element and Ta element was 2 atomic % was used.

(Examples 1 to 4)
A substrate with a deep ultraviolet light transmitting transparent electrode was produced in the same manner as in Example 1-1, except that a target in which the amount of Ta element relative to the sum of Sn element and Ta element was 3 atomic % was used.

(Examples 1 to 5)
A substrate with a deep ultraviolet light transmitting transparent electrode was produced in the same manner as in Example 1-1, except that a target in which the amount of Ta element relative to the sum of Sn element and Ta element was 5 atomic % was used.

(Comparative Example 1)
A substrate with an electrode was produced in the same manner as in Example 1-1, except that a target in which the amount of Ta element relative to the sum of Sn element and Ta element was 0.3 atomic % was used.

(Comparative Example 2)
A substrate with an electrode was produced in the same manner as in Example 1-1, except that a target in which the amount of Ta element relative to the sum of Sn element and Ta element was 0.6 atomic % was used.

(Comparative Example 3)
A substrate with an electrode was produced in the same manner as in Example 1-1, except that a target in which the amount of Ta element relative to the sum of Sn element and Ta element was 10 atomic % was used.

[Analysis of constituent phases]
The constituent phases were analyzed by X-ray diffraction (XRD) for the substrates with deep ultraviolet light transmitting transparent electrodes of Examples 1-1 to 1-5 and the substrates with electrodes of Comparative Examples 1 to 3. The analysis of the constituent phases by X-ray diffraction was performed under the following conditions.
・X-ray source: Cu Kα ray (output: 40kV, current: 40mA)
Scanning range: 2θ = 20° to 90°
Step time: 0.2 s/step
Scan speed: 6°/min
Step width: 0.02°
・Detector: Semiconductor array detector

Figure 4 shows the X-ray diffraction patterns of the substrates with deep ultraviolet light transmitting transparent electrodes of Examples 1-1 to 1-5 and the substrates with electrodes of Comparative Examples 1 to 3. In Figure 4, the locations marked with the symbol * are peaks based on the constituent phases of the substrate.

In all samples, in addition to the peaks due to the constituent phases of the substrate, the peaks identified were the SnO ₂ (200) peak and the SnO ₂ (400) peak, confirming that a layer based on SnO ₂ was epitaxially grown on the substrate.

[Measurement of resistivity, carrier density, and mobility]
The resistivity, carrier density, and mobility of the deep ultraviolet light transmitting transparent electrode-attached substrates of Examples 1-1 to 1-5 and the electrode-attached substrates of Comparative Examples 1 to 3 were measured by the following procedures.

(Resistivity)
The resistivity was measured by measuring the electrical resistance by a four-terminal method for the substrates with deep ultraviolet light transmitting transparent electrodes of Examples 1-1 to 1-5 and the substrates with electrodes of Comparative Examples 1 to 3. Fig. 5(a) is a graph showing the resistivity of the substrates with deep ultraviolet light transmitting transparent electrodes of Examples 1-1 to 1-5 and the substrates with electrodes of Comparative Examples 1 to 3.

In FIG. 5(a), the horizontal axis represents the amount (atomic %) of Ta relative to the sum of Sn and Ta, and the vertical axis represents resistivity (Ω·cm). In FIG. 5(a), the results of Comparative Example 1, Comparative Example 2, Example 1-1, Example 1-2, Example 1-3, Example 1-4, Example 1-5, and Comparative Example 3 are plotted in order of the amount (atomic %) of Ta relative to the sum of Sn and Ta. From the results shown in FIG. 5(a), it was confirmed that the deep ultraviolet light transmitting transparent electrode-attached substrates of Examples 1-1 to 1-5 exhibited excellent conductivity of 0.0004 (Ω·cm) or less at room temperature, and Examples 1-1 to 1-4 exhibited particularly excellent conductivity of 0.0003 (Ω·cm) or less.

The sheet resistance of the transparent oxide layer was calculated by dividing the resistivity by the thickness of the transparent oxide layer. The thickness of the transparent oxide layer was measured using a stylus step gauge.

(Carrier Density)
The carrier density was measured by performing Hall effect measurement and four-terminal electrical resistance measurement on the substrates with deep ultraviolet light transmitting transparent electrodes of Examples 1-1 to 1-5 and the substrates with electrodes of Comparative Examples 1 to 3. Fig. 5(b) is a graph showing the carrier density of the substrates with deep ultraviolet light transmitting transparent electrodes of Examples 1-1 to 1-5 and the substrates with electrodes of Comparative Examples 1 to 3.

The horizontal axis in FIG. 5(b) is the same as the horizontal axis in FIG. 5(a), and the vertical axis in FIG. 5(b) represents carrier density (cm ⁻³ ). In FIG. 5(b), an ideal straight line in the case where one electron is emitted per doped Ta atom is shown by a dashed line. The dashed line is based on the number density of Ta atoms calculated from the lattice volume of the SnO ₂ crystal and the Ta doping amount. As shown in FIG. 5(b), it was confirmed that Example 1-1, Example 1-2, and Example 1-3 are on the dashed line and have ideal carrier density. In particular, Example 1-2 to Example 1-5 showed a carrier density of 4.0×10 ²⁰ (cm ⁻³ ) or more, and Example 1-3 to Example 1-5 showed a carrier density of 5.0×10 ²⁰ (cm ⁻³ ) or more. In Examples 1-4, 1-5, and Comparative Example 3, the carrier density is below the value of the dashed line, and it is considered that the carrier density is saturated at about 6.5×10 ²⁰ (cm ^-3 ). The carrier densities (×10 ²⁰ cm ^-3 ) of Examples 1-1, 1-2, 1-3, 1-4, and 1-5 were 2.9, 4.2, 5.1, 6.1, and 6.2, respectively. The carrier densities (×10 ²⁰ cm ^-3 ) of Comparative Examples 1, 2, and 3 were 1.0, 1.7, and 4.0.

(Mobility)
The mobility was measured by Hall effect measurement and four-terminal electrical resistance measurement for the deep ultraviolet light transmitting transparent electrode-attached substrates of Examples 1-1 to 1-5 and the electrode-attached substrates of Comparative Examples 1 to 3. FIG. 5(c) is a graph showing the electron mobility of the deep ultraviolet light transmitting transparent electrode-attached substrates of Examples 1-1 to 1-5 and the electrode-attached substrates of Comparative Examples 1 to 3. The vertical axis in FIG. 5(c) represents the mobility (cm ² V ^-1 s ^-1 ). As shown in FIG. 5(c), Examples 1-1 to 1-5 exhibited mobilities of 20 (cm ² V ^-1 s ^-1 ) or more, Examples 1-1 to 1-4 exhibited mobilities of 40 (cm ² V ^-1 s ^-1 ) or more, and Example 1-3 exhibited a mobility of 50 (cm ² V ^-1 s ^-1 ) or more.

[Internal transmittance measurement]
The spectra of transmittance T% and reflectance R% in the lamination direction were measured using a spectrophotometer (manufactured by JASCO Corporation, model number: V-670) for the deep ultraviolet light transmitting transparent electrode-attached substrates of Examples 1-1 to 1-5 and the electrode-attached substrates of Comparative Examples 1 to 3. Based on the transmittance T and reflectance spectra, the spectrum of the internal transmittance τ% of the electrode-attached substrate was calculated by calculating {T/(100-R)}×100.

Figure 6 is a graph showing the internal transmittance spectra of the deep ultraviolet light-transmitting transparent electrode-attached substrates of Examples 1-1 to 1-5 and the electrode-attached substrates of Comparative Examples 1 to 3. As shown in Figure 6, it was confirmed that Examples 1-1 to 1-5 exhibit high transmittance even in the deep ultraviolet region with a wavelength of 280 nm or less. For example, for deep ultraviolet light with a wavelength of 280 nm, the internal transmittance of Examples 1-1 to 1-5 was 35% or more, the internal transmittance of Examples 1-2 to 1-5 was 40% or more, and the internal transmittance of Examples 1-4 and 1-5 was 60% or more.

Example 2
A substrate with a deep ultraviolet light transmitting transparent electrode was produced in the same manner as in Example 1-4, except that the transparent substrate was heated to a temperature of 600° C. That is, in Example 2, the amount of Ta element relative to the sum of Sn element and Ta element in the target was 3 atomic %.

Example 3
Except for changing the transparent substrate to one made of Al ₂ O ₃ , a substrate with a deep ultraviolet light transmitting transparent electrode was produced in the same manner as in Example 2. That is, in Example 3, c-plane sapphire was used as the transparent substrate, the amount of Ta element relative to the sum of Sn element and Ta element in the target was 3 atomic %, and the temperature at which the transparent substrate was heated during the film formation process was 600°C.

(Comparative Example 4)
Comparative Example 4 is experimental data on the substrate with electrodes disclosed in Non-Patent Document 1. That is, Comparative Example 4 is experimental data on the substrate with electrodes produced by forming a (101)-oriented Ta-doped _SnO2 thin _film on an α- _Al2O3 (012)-oriented substrate by MOCVD.

As Comparative Example 4, FIG. 8 shows cases where the amount of Ta element relative to the sum of Sn element and Ta element in the source gas is 4%, 6%, and 8%.

Comparative Example 4-1 corresponds to experimental data in which the amount of Ta element relative to the sum of Sn element and Ta element is 4%.
Comparative Example 4-2 corresponds to experimental data in which the amount of Ta element relative to the sum of Sn element and Ta element is 6%.
Comparative Example 4-3 corresponds to experimental data in which the amount of Ta element relative to the sum of Sn element and Ta element is 8%.

(Comparative Example 5)
Comparative Example 5 is data on the substrate with _electrodes disclosed in Non-Patent Document 2. That is, Comparative Example 5 is experimental data on an r-plane _Al2O3 substrate on which a (101)-oriented Sb-doped _SnO2 thin film is formed to prepare a substrate with electrodes. In Comparative Example 5, the amount of Sb element relative to the sum of Sn element and Sb element is adjusted to be 0.014.

(Comparative Example 6)
Comparative Example 6 is experimental data on the substrate with electrodes disclosed in Non-Patent Document _3. That is, Comparative Example 6 is experimental data on the substrate with electrodes produced by forming a (111)-oriented Sn-doped _In2O3 thin film, a so-called ITO thin film, on a c-plane _Al2O3 substrate by _MOCVD .

[Measurement of internal transmittance]
The internal transmittance of the substrates with transparent electrodes of Examples 2 and 3 was measured in the same manner as in Example 1.

[Measurement of sheet resistance]
In the same manner as in Example 1, the resistivity and thickness of the transparent oxide layer in the substrate with a transparent electrode of each of Examples 2 and 3 were measured, and the sheet resistance of the substrate with a transparent electrode of each of Examples 2 and 3 was calculated.

Figure 8(a) is a graph showing the sheet resistance and internal transmittance of the electrode-attached substrates of Examples 1-1 to 1-5 and Comparative Examples 1 to 6 at a wavelength of 280 nm, and Figure 8(b) is a graph showing the sheet resistance and internal transmittance of the electrode-attached substrates of Examples 1-1 to 1-5 and Comparative Examples 1 to 6 at a wavelength of 260 nm. In Figures 8(a) and 8(b), the internal transmittance of the electrode-attached substrates of Comparative Examples 4 to 6 and the sheet resistance of the oxide layer laminated on the substrate of the electrode-attached substrate are plotted as described in non-patent literature.

8(a) and 8(b), the figure of merit (FoM) of the substrate with a transparent electrode is shown by a dashed line. The figure of merit (FoM) is expressed by the following formula (2).
FoM=(ρ・α) ^-1 ...(2)
In formula (2), ρ represents the resistivity of the transparent oxide layer, and α represents the absorption coefficient of the substrate with a transparent electrode. The absorption coefficient α is calculated from the internal transmittance and film thickness t by the formula τ = exp(-αt). It can be said that transparent electrodes with similar figures of merit (FoM) have similar performance.

As shown in FIG. 8(a), the deep ultraviolet light transmitting transparent electrode-attached substrates of Examples 1-1 to 1-5, Example 2 and Example 3 have the same sheet resistance as the electrode-attached substrate of Comparative Example 4 in which a Ta-doped SnO ₂ thin film is formed on an Al ₂ O ₃ substrate by MOCVD, and it was confirmed that they exhibit significantly superior transmittance to deep ultraviolet light with wavelengths of 280 nm and 260 nm. Therefore, the deep ultraviolet light transmitting transparent electrode-attached substrates of Examples 1-1 to 1-5 have a superior figure of merit (FoM) compared to the electrode-attached substrates of Comparative Example 4-1 to Comparative Example 4-3. In addition, the FoM of all the measurement results at a wavelength of 280 nm was 0.5 or less.

On the other hand, the deep ultraviolet light transmitting transparent electrode-attached substrates of Examples 1-1 to 1-5 have lower sheet resistance and exhibit superior conductivity compared to the electrode-attached substrates of Comparative Examples 5 and 6. It was also confirmed that the deep ultraviolet light transmitting transparent electrode-attached substrates of Examples 1-1 to 1-5 exhibit significantly superior transmittance to deep ultraviolet light with wavelengths of 280 nm and 260 nm compared to the electrode-attached substrates of Comparative Examples 5 and 6. Therefore, the deep ultraviolet light transmitting transparent electrode-attached substrates of Examples 1-1 to 1-5 have superior figure of merit (FoM) compared to the electrode-attached substrates of Comparative Examples 5 and 6.

1: transparent substrate, 2: transparent oxide layer, 10: substrate with transparent electrode,
20: sample installation stage, 21: target installation stage, 31: exhaust device, 32: supply device,
40: vacuum chamber, 50: pulsed laser deposition apparatus,
T: target, L: pulse laser

Claims (8)

A transparent substrate and a transparent oxide layer formed on the transparent substrate,
The transparent substrate is any one selected from the group consisting of a sapphire substrate, an aluminum nitride substrate, and a substrate made of a mixed crystal of aluminum nitride and gallium nitride;
The transparent oxide layer is made of a metal oxide containing _SnO2 as a main component and Ta element;
The substrate with a deep ultraviolet light transmitting transparent electrode, wherein the amount of Ta element in the metal oxide is 0.8 atomic % or more and 7.0 atomic % or less based on the sum of Sn element and Ta element in the metal oxide. The substrate with a deep ultraviolet light transmitting transparent electrode according to claim 1, wherein the metal oxide in the transparent oxide layer contains 1.0 atomic % or more and 5.0 atomic % or less of Ta element relative to Sn element. 3. The substrate with a deep ultraviolet light transmitting transparent electrode according to claim 2, wherein the transparent oxide layer has a carrier density of 4.5×10 ²⁰ cm ⁻³ or more. The substrate with a deep ultraviolet light transmitting transparent electrode according to claim 1 or 2, wherein the transparent oxide layer is an epitaxial layer of the metal oxide. A preparation step of preparing a transparent substrate selected from the group consisting of a sapphire substrate, an aluminum nitride substrate, and a substrate made of a mixed crystal of aluminum nitride and gallium nitride;
and a film formation step of: disposing a target, in which the amount of Ta element is 0.8 atomic % or more and 7.0 atomic % or less relative to the sum of the amounts of Sn element and Ta element, facing the transparent substrate, and performing a physical vapor deposition method to form a transparent oxide layer composed of a metal oxide on the transparent substrate. 6. The method for producing a substrate with a deep ultraviolet light transmitting transparent electrode according to claim 5, wherein the target is mainly composed of _SnO2 and has an amount of Ta element of 0.8 atomic % or more and 7.0 atomic % or less. The method for producing a substrate with a deep ultraviolet light transmitting transparent electrode according to claim 5 or 6, wherein the amount of the Ta element in the target is 1.0 atomic % or more and 5.0 atomic % or less. The method for producing a substrate with a deep ultraviolet light transmitting transparent electrode according to claim 5 or 6, wherein the film forming process is performed by a pulsed laser deposition method.

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