Aluminum scandium gallium nitride film, method for manufacturing the same, and piezoelectric element

Abstract

To provide at least one of the following: an aluminum scandium gallium nitride film having a higher piezoelectric constant and a smoother surface compared to conventional AlScGaN films; a method for manufacturing the same; and a piezoelectric element. [Solution] An aluminum scandium gallium nitride film containing aluminum, scandium, gallium, and nitrogen, represented by the following compositional formula (A). (AlxScyGaz)N···(A) (In the above empirical formula (A), x is greater than 0, y is greater than 0, z is 0.09 or less, and x + y + z is 1.)

Description

[Technical Field]

[0001] This disclosure relates to aluminum scandium gallium nitride films, methods for producing the same, and piezoelectric elements. [Background technology]

[0002] Frequency filters equipped with piezoelectric films are used to pass electrical signals in a specific frequency band. These include SAW filters, which utilize surface acoustic waves (SAW) generated on the surface of the piezoelectric film, and Bulk Acoustic Wave (BAW) filters, which utilize the vibration of the bulk of the piezoelectric film. In recent years, for such frequency filters, there has been a demand for piezoelectric films with higher piezoelectric constants than conventional piezoelectric films in order to widen the bandwidth of the frequency band through which electrical signals can pass. Aluminum scandium gallium nitride (hereinafter also referred to as "AlScGaN") films are expected to be such piezoelectric films with high piezoelectric constants. Patent Document 1 discloses an AlScGaN film deposited by a ternary simultaneous reactive sputtering method using an Al target, an Sc target, and a GaN target. Patent Document 2 also discloses an AlScGaN film deposited by a sputtering method using an AlScGaN target. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] International Publication No. 2023 / 190869 [Patent Document 2] International Publication No. 2024 / 190444 [Overview of the project] [Problems that the invention aims to solve]

[0004] However, both the AlScGaN film described in Patent Document 1 and the AlScGaN film described in Patent Document 2 had insufficient piezoelectric constants, and there was room for improvement in terms of piezoelectric properties before they could be used as piezoelectric films. Furthermore, for piezoelectric films, high surface smoothness leads to a more uniform distribution of the electric field, enabling more efficient energy conversion and thus improving the performance of the piezoelectric film. For this reason, high surface smoothness is desirable for AlScGaN films. However, the AlScGaN film described in Patent Document 2 had room for improvement in terms of surface smoothness.

[0005] The present disclosure aims to provide at least one of the following: an AlScGaN film having a higher piezoelectric constant and a smoother surface compared to conventional AlScGaN films; a method for manufacturing the same; and a piezoelectric element. [Means for solving the problem]

[0006] The inventors of this disclosure focused on the composition of the AlScGaN film in order to increase the piezoelectric constant and improve the surface smoothness of the AlScGaN film compared to conventional AlScGaN films. As a result, the inventors of this disclosure found that by having a specific composition, an AlScGaN film with a higher piezoelectric constant and a smoother surface compared to conventional AlScGaN films can be realized. In other words, the content of the present invention is as described in the claims, and the gist of this disclosure is as follows. (1) An AlScGaN film containing aluminum, scandium, gallium, and nitrogen, represented by the following compositional formula (A). (AlxScyGaz)N···(A) (In the above compositional formula (A), x is greater than 0, y is greater than 0, z is 0.09 or less, and x + y + z is 1.) (2) The AlScGaN film described in (1) having a thickness of 600 nm or more. (3) The AlScGaN film described in (1) or (2), wherein the coefficient of variation of the piezoelectric constant defined by the following formula (B) is 0.15 or less. Coefficient of variation of piezoelectric constant = standard deviation of piezoelectric constant (pC / N) / piezoelectric constant (pC / N) ··· (B) (4) The AlScGaN film according to any one of (1) to (3), wherein in the compositional formula (A), y is 0.40 or more and 0.55 or less. (5) The AlScGaN film according to any one of (1) to (4), wherein in the compositional formula (A), z is 0.06 or more. (6) The AlScGaN film according to any one of (1) to (5), wherein a crystal phase of the AlScGaN film has a wurtzite structure. (7) The AlScGaN film according to any one of (1) to (6), which is a sputtered film. (8) A method for producing the aluminum scandium gallium nitride film according to any one of (1) to (7), A method for producing an AlScGaN film, comprising a film forming step of producing an AlScGaN film by sputtering an AlScGa alloy sputtering target, wherein in the film forming step, a gas pressure of a sputtering gas is set to 0.4 Pa or more. (9) A piezoelectric element comprising a piezoelectric film, A piezoelectric element, wherein the piezoelectric film comprises the aluminum scandium gallium nitride film according to any one of (1) to (7).

Effects of the Invention

[0007] According to the present disclosure, at least one of an AlScGaN film having a higher piezoelectric constant than conventional AlScGaN films and a smooth surface, a method for producing the same, and a piezoelectric element is provided.

Brief Description of Drawings

[0008] [Figure 1] FIG. 1 is a cross-sectional view illustrating one embodiment of the piezoelectric element of the present disclosure.

Mode for Carrying Out the Invention

[0009] Embodiments of the present disclosure will be described in detail by way of examples. However, the present disclosure is not limited to the following embodiments. In addition, the present embodiments shall include any combination of each configuration and parameter disclosed in the present specification, and the scope of any combination of upper and lower limits of values disclosed in the present specification shall also be included in the present embodiments.

[0010] <AlScGaN膜> First, an embodiment of the AlScGaN film of the present disclosure will be described. In the present embodiment, the AlScGaN film is a film mainly composed of AlScGaN. The AlScGaN film of the present embodiment contains aluminum, scandium, gallium and nitrogen, and is represented by the following composition formula (A). (AlxScyGaz)N···(A) In the above composition formula (A), x is greater than 0, y is greater than 0, z is 0.09 or less, and x+y+z=1. Compared with conventional AlScGaN films, this AlScGaN film has a higher piezoelectric constant and a smooth surface.

[0011] The crystal phase of the AlScGaN film preferably contains a hexagonal crystal, and more preferably consists of a hexagonal crystal. The crystal phase of the AlScGaN film preferably has a wurtzite structure. In this case, the piezoelectric constant of the AlScGaN film tends to be higher than that in the case where the crystal phase has a sodium chloride structure. The AlScGaN film may be either a sputtered film or a non-sputtered film, but is preferably a sputtered film. In this case, a uniform and high-density film can be obtained compared to the case where the AlScGaN film is a non-sputtered film.

[0012] (Composition) z is preferably 0.085 or less, more preferably 0.08 or less. Although z is greater than 0, it may be 0.03 or more or 0.06 or more. When z is 0.06 or more, the piezoelectric constant of the AlScGaN film is easily increased. The combination of the upper and lower limits of z may be any of the combinations of the upper and lower limits listed above, but examples include values ​​greater than 0 and less than or equal to 0.09, 0.03 to 0.085, or 0.06 to 0.08.

[0013] x may be 0.10 or greater, 0.20 or greater, 0.30 or greater, or 0.40 or greater. x may be 0.90 or less, 0.70 or less, 0.60 or less, or 0.50 or less. The combination of the upper and lower limits of x may be any of the combinations of the upper and lower limits listed above, but examples include 0.10 or more and 0.90 or less, 0.20 or more and 0.70 or less, 0.30 or more and 0.60 or less, or 0.40 or more and 0.50 or less.

[0014] y may be 0.20 or greater, 0.30 or greater, or 0.40 or greater, but 0.40 or greater is preferred. In this case, the piezoelectric constant of the AlScGaN film becomes higher. y may be 0.65 or less, 0.60 or less, or 0.55 or less, but it is preferably 0.55 or less. In this case, the piezoelectric constant of the AlScGaN film becomes higher. The combination of the upper and lower limits of y may be any of the combinations of the upper and lower limits listed above, but it is preferable that y be between 0.20 and 0.65, between 0.30 and 0.60, or between 0.40 and 0.55. In particular, it is preferable that y be between 0.40 and 0.55. In this case, the piezoelectric constant of the AlScGaN film becomes higher. In this embodiment, the composition can be determined by general scanning electron microscope-energy dispersive X-ray (hereinafter also referred to as "SEM-EDS") measurement. Specifically, a general EDS-equipped SEM (e.g., JSM-IT800, manufactured by JEOL Ltd.) can be used, and SEM observation can be performed under the following conditions. Using the software attached to the EDS-equipped SEM, the ratio (at.%) of the number of atoms of each element Al, Sc, Ga, and N to the total number of atoms of Al, Sc, Ga, and N can be determined, excluding the elements of the substrate and electrodes. Then, the ratios of the number of atoms of Al, Sc, and Ga, with the ratio of the number of atoms of N set to 1, can be determined as x, y, and z, respectively. (Observation conditions) Acceleration voltage: 15kV Observation magnification: 500x Observation field of view: 5 fields of view

[0015] In the above compositional formula (A), it is particularly preferable that z is 0.09 or less, y is 0.40 or more and 0.55 or less, and the crystalline phase has a wurtzite-type structure. In this case, the piezoelectric constant of the AlScGaN film becomes higher.

[0016] (thickness) The thickness of the AlScGaN film is not particularly limited, but it is preferably 600 nm or more. In this case, the surface smoothness of the AlScGaN film is further improved. The thickness of the AlScGaN film described above is more preferably 700 nm or more, 800 nm or more, 900 nm or more, or 1000 nm or more, as this tends to result in a higher surface smoothness. The thickness of the AlScGaN film may be 10,000 nm or less, 5,000 nm or less, 2,000 nm or less, 1,500 nm or less, or 1,200 nm or less. When the thickness of the AlScGaN film is 10,000 nm or less, the resonant frequency can be made higher when the AlScGaN film is used as a piezoelectric film in a frequency filter. The above combinations of upper and lower limits for the thickness of the AlScGaN film may be any combination of the above upper and lower limits, and examples include 600 nm to 10000 nm, 700 nm to 5000 nm, 800 nm to 2000 nm, 900 nm to 1500 nm, or 1000 nm to 1200 nm. In this embodiment, the thickness of the AlScGaN film can be determined by measuring the height difference between the substrate portion and the film portion using a general-purpose laser microscope (for example, VK-X250 / 260, manufactured by Keyence Corporation).

[0017] (Piezoelectric constant) The piezoelectric constant of the above AlScGaN film is not particularly limited, but a larger value is preferable. In this embodiment, the piezoelectric constant is the piezoelectric constant d33 The piezoelectric constant d 33 This parameter represents the amount of charge generated per unit stress when stress is applied in the thickness direction of an AlScGaN film. (D is the piezoelectric constant.) 33 For a sample prepared by forming a molybdenum (Mo) electrode with a diameter of 50 μm and a thickness of 75 ± 25 nm on an AlScGaN film using electron beam deposition at room temperature, a general d 33 This refers to the average value of the piezoelectric constant d measured at 30 measurement points at room temperature using a meter (e.g., "PM300," manufactured by PiezoTest, Ltd.). Here, the measurement points are the intersections (30 points) of line segments drawn to divide the AlScGaN film vertically into 6 sections and horizontally into 7 sections when viewed from its surface. A higher piezoelectric constant is preferable. Preferably, the piezoelectric constant is 10 pC / N or higher, more preferably 15 pC / N or higher, and particularly preferably 20 pC / N or higher. The piezoelectric constant may be 100 pC / N or less, 80 pC / N or less, or 60 pC / N or less. Piezoelectric constant d 33 The combination of the upper and lower limits may be any combination of the upper and lower limits listed above, but examples include 10 pC / N or more and 100 pC / N or less, 15 pC / N or more and 80 pC / N or less, or 20 pC / N or more and 60 pC / N or less.

[0018] (Coefficient of variation of piezoelectric constant) The coefficient of variation of the piezoelectric constant defined by equation (B) below is not particularly limited, but a smaller value is preferable. Coefficient of variation of piezoelectric constant = Standard deviation of piezoelectric constant (pC / N) / Piezoelectric constant (pC / N) ... (B) The standard deviation of the piezoelectric constant can be calculated from the individual piezoelectric constants d measured at 30 measurement points and their average value. The smaller the coefficient of variation of the piezoelectric constant, the more preferable it is. The coefficient of variation of the piezoelectric constant is preferably 0.15 or less, more preferably 0.13 or less, or 0.11 or less. When the coefficient of variation of the piezoelectric constant is 0.15 or less, the in-plane variation of the piezoelectric constant d is smaller than that when the coefficient of variation of the piezoelectric constant exceeds 0.15, so that the performance when the AlScGaN film is used as a piezoelectric film can be stabilized. The coefficient of variation of the piezoelectric constant is 0 or more, and may be 0.04 or more or 0.08 or more. The combination of the upper limit and lower limit of the coefficient of variation of the piezoelectric constant may be any combination of the above upper limits and lower limits, and examples thereof include 0 or more and 0.15 or less, 0.04 or more and 0.13 or less, and 0.08 or more and 0.11 or less.

[0019] (Surface Roughness) The surface roughness of the AlScGaN film is not particularly limited, but the smaller it is, the more preferable it is. The surface roughness refers to a value obtained as an arithmetic average value of surface roughness measured under the following conditions using a general scanning probe microscope (for example, SPM-9600, manufactured by Shimadzu Corporation). (Measurement Conditions) Measurement mode: Tapping mode AFM Scanning speed: 0.5 Hz Scanning range: 2 μm × 2 μm Number of pixels: 512 × 512 Measurement temperature: 25±5°C The surface roughness is preferably 5 nm or less, more preferably 4 nm or less. The surface roughness is 0 nm or more, and may be 2 nm or more or 3 nm or more. The combination of the upper limit and lower limit of the surface roughness may be any combination of the above upper limits and lower limits, and examples thereof include 0 nm or more and 5 nm or less, or 2 nm or more and 4 nm or less.

[0020] <Method for Producing AlScGaN Film> The AlScGaN film of this embodiment can be manufactured by any method as long as it satisfies the above characteristics. For example, the AlScGaN film of this embodiment is obtained by a manufacturing method that includes a film deposition step of manufacturing an AlScGaN film by sputtering an AlScGa alloy sputtering target, wherein the gas pressure of the sputtering gas in the film deposition step is 0.4 Pa or higher.

[0021] The sputtering target includes a bulk material. The sputtering target may further include a backing plate as a support that is bonded to the bulk body. The sputtering target may also include a bonding material between the bulk body and the backing plate. Various materials can be used for the bonding material, but indium is preferred in terms of suppressing thermal diffusion and thermal expansion during sputtering.

[0022] The bulk material contained in the AlScGa alloy sputtering target has AlScGa as its main component, and more specifically, at least one of single crystals and polycrystalline materials having AlScGa as its main component, and even more specifically, polycrystalline materials having AlScGa as its main component. In this specification, "main component" refers to a component whose content in the bulk material is 90% by mass or more.

[0023] The crystalline phase of AlScGa preferably includes a hexagonal structure, and more preferably consists of a hexagonal structure. The crystalline phase of AlScGa preferably has a wurtzite-type structure. In this case, the piezoelectric constant of the AlScGaN film tends to be higher compared to when the crystalline phase has a sodium chloride-type structure.

[0024] In AlScGa, the proportion of Al atoms to the total number of Al, Sc, and Ga atoms may be, for example, 15 at.% or more, 25 at.% or more, or 35 at.% or more. The percentage of Al may be 65 at.% or less, 55 at.% or less, or 45 at.% or less. The combination of the upper and lower limits for the percentage of Al may be any of the above combinations of upper and lower limits, and examples include 15% at. to 65% at.%, 25% at.% to 55% at.%, or 35% at.% to 45% at.%.

[0025] In AlScGa, the proportion of Sc to the total number of Al, Sc, and Ga atoms may be, for example, 25 at.% or more, 35 at.% or more, or 45 at.% or more, and is preferably 45 at.% or more. In this case, the piezoelectric constant of the AlScGaN film becomes higher. The proportion of Sc may be 75 at.% or less, 65 at.% or less, or 55 at.% or less, but 55 at.% or less is preferable. In this case, the piezoelectric constant of the AlScGaN film becomes higher. The combination of the upper and lower limits for the proportion of Sc may be any of the above combinations of the upper and lower limits, but the piezoelectric constant d of the AlScGaN film 33 From the viewpoint of increasing the concentration, it is preferable that the concentration be 25 at.% or more and 75 at.% or less, 35 at.% or more and 65 at.% or less, or 45 at.% or more and 55 at.% or less.

[0026] In AlScGa, the proportion of Ga in the total number of Al, Sc, and Ga atoms is preferably 20 at.% or less, more preferably 15 at.% or less, and particularly preferably 10 at.% or less. The proportion of Ga is greater than 0 at.%, but may be 5 at.% or more, or 8 at.% or more. The combination of the upper and lower limits for the percentage of Ga may be any of the above combinations, but examples include greater than 0 at.% and less than or equal to 20 at.%; 5 at.% to 15 at.%; or 8 at.% to 10 at.%. The method for producing the bulk material is arbitrary, but it can be obtained by melting a mixture of Al, Sc, and Ga raw materials to form a melted and disintegrated material. As a method for melting the mixture of Al, Sc, and Ga raw materials, at least one of the melting method and the arc melting method can be used, and the arc melting method can be cited as an example. The form of each raw material is not particularly limited and may be at least one of powder and flakes, or more specifically, powder.

[0027] The substrate used for film formation includes a support substrate. The support substrate is not particularly limited, but examples of support substrates include at least one of silicon substrates and glass substrates, such as silicon substrates, silicon carbide substrates, glass substrates containing alkali-free glass or quartz, gallium nitride substrates, substrates with a wurtzite-type crystalline structure, oxide crystal substrates such as sapphire and magnesia. Among these, silicon substrates are preferred as the support substrate.

[0028] The above substrate may further have an oxide layer on a support substrate, if necessary. Examples of oxides constituting the oxide layer include at least one of titania and silica. The oxide layer may be a single layer or a laminate of multiple layers.

[0029] As for the sputtering method, one or more methods can be appropriately selected from the group consisting of DC sputtering, RF sputtering, AC sputtering, DC magnetron sputtering, RF magnetron sputtering, ECR sputtering, pulsed laser deposition, and ion beam sputtering. Among these, at least one of the DC magnetron sputtering method and the RF magnetron sputtering method is preferred because it enables uniform and high-speed film deposition over a large area.

[0030] The gas pressure of the sputtering gas (hereinafter also simply referred to as "gas pressure") is 0.4 Pa or higher. By setting the gas pressure to 0.4 Pa or higher, the resulting AlScGaN film has a piezoelectric constant d compared to a conventional AlScGaN film. 33 The surface becomes smoother and the density increases. The gas pressure is preferably 0.5 Pa or higher, more preferably 0.6 Pa or higher, and particularly preferably 0.7 Pa or higher. The gas pressure may be 2.0 Pa or less, 1.5 Pa or less, 1.2 Pa or less, or 1.0 Pa or less. The combination of the upper limit and lower limit of the gas pressure may be any combination of the above upper and lower limits, and examples include 0.4 Pa or more and 2.0 Pa or less, 0.5 Pa or more and 1.5 Pa or less, 0.6 Pa or more and 1.2 Pa or less, or 0.7 Pa or more and 1.0 Pa or less.

[0031] Further, the temperature of the substrate during film formation (hereinafter also referred to as "film formation temperature") is preferably 700°C or lower, and more preferably 600°C or lower. Setting the film formation temperature to 700°C or lower tends to increase the piezoelectric constant of the AlScGaN film. The film formation temperature may be 150°C or higher, 200°C or higher, 300°C or higher, or 400°C or higher. Setting the temperature of the substrate during film formation to 150°C or higher tends to increase the piezoelectric constant of the AlScGaN film. Examples of the film formation temperature include 150°C or higher and 700°C or lower, 200°C or higher and 600°C or lower, or 300°C or higher and 600°C or lower.

[0032] The sputtering gas is not particularly limited as long as it is a gas type that causes sputtering by discharge. Examples of such a gas include inert gases, and further at least one of nitrogen and argon. These may be used alone or as a mixture of two types. The sputtering gas is preferably a mixed gas of nitrogen and argon. When the sputtering gas is a mixed gas of nitrogen and argon, the partial pressure of nitrogen is not particularly limited, and may be, for example, 10% or more, or 20% or more. The partial pressure of nitrogen may be 40% or less, or 30% or less. The combination of the upper limit and lower limit of the partial pressure of nitrogen may be any combination of the above upper and lower limits, and examples include 10% or more and 40% or less, or 20% or more and 30% or less.

[0033] In order to stably generate plasma, the discharge power density in sputtering is 1 W / cm 2 or more, 5 W / cm 2 or more, or 7 W / cm 2 or more, and also 25 W / cm 2Below, 20W / cm 2 Below or 15W / cm² 2 The following is acceptable: 1 W / cm² 2 More than 25W / cm 2 Below, 5W / cm 2 More than 20W / cm 2 Below or 7W / cm² 2 More than 15W / cm 2 The following are listed: The value of z in the above composition formula (A) can be adjusted by controlling the discharge power density during sputtering. For example, the value of z tends to decrease as the discharge power density during sputtering decreases.

[0034] The sputtering time (hereinafter also referred to as "deposition time") can be adjusted as appropriate according to the size of the substrate and the desired thickness of the AlScGaN film. The longer the deposition time, the thicker the AlScGaN film can be. For example, it can be 5 minutes or more, 10 minutes or more, or 1 hour or more, and also 24 hours or less, 18 hours or less, or 15 hours or less. Examples of deposition times include 5 minutes to 24 hours, 10 minutes to 18 hours, or 1 hour to 15 hours.

[0035] <Piezoelectric material> Next, the piezoelectric element of this embodiment will be described with reference to Figure 1. The piezoelectric element 10 shown in Figure 1 is equipped with a piezoelectric film 3, and the piezoelectric film 3 includes the AlScGaN film described above. The piezoelectric element 10 may further include a first electrode 2 and a second electrode 4 so as to sandwich the piezoelectric film 3. The piezoelectric element 10 may also further include a substrate 1. The substrate 1 can be provided, for example, so as to be laminated on the first electrode 2. With this piezoelectric element 10, since the piezoelectric film 3 includes an AlScGaN film with a smooth surface, when the piezoelectric element 10 has a film thinner than the piezoelectric film 3 (for example, an electrode film) on its surface, irregularities are less likely to occur on that film. Furthermore, the piezoelectric film 3 can effectively generate stress when a voltage is applied. Therefore, the performance of the piezoelectric element 10 can be improved.

[0036] The piezoelectric element 10 can be used in frequency filters such as SAW filters that utilize surface acoustic waves (SAW) generated on the surface of the piezoelectric film, and bulk acoustic wave (BAW) filters that utilize the bulk vibrations of the piezoelectric film, as well as in sensors, actuators, and the like. [Examples]

[0037] The present disclosure will be described below with reference to examples. However, the present disclosure is not limited to these examples.

[0038] <Fabrication of sputtering targets> Al, Sc, and Ga raw materials were prepared. Next, these raw materials were placed in a water-cooled copper mold with a diameter of 65 mm and a height of 12 mm, such that the atomic ratio of Al, Sc, and Ga was Al:Sc:Ga = 40:50:10. At this time, the Al, Sc, and Ga raw materials were placed from the bottom of the mold in order of increasing melting point (i.e., Sc, Al, Ga). Subsequently, the raw materials in the mold were melted in an arc melting furnace under an Ar atmosphere with a current of 100A for 5 minutes, and then cooled to room temperature. In this way, an ingot with an atomic ratio of Al, Sc, and Ga of Al:Sc:Ga = 40:50:10 was obtained by the arc melting method. The obtained ingot was ground into a disc-shaped ingot with a diameter of 2 inches (51 mm) and a thickness of 5 mm. The obtained ingot and an oxygen-free copper backing plate were joined together via indium solder, which served as the bonding layer (joining material). In this way, a disc-shaped sputtering target with a backing plate was fabricated.

[0039] (Example 1) Using the sputtering target obtained as described above, an AlScGaN film (sputtered film) was deposited on a silicon substrate by sputtering under the following deposition conditions (see Table 1). <Film deposition conditions> ·Equipment used: CMS-6400 combined sputtering apparatus Lower electrode layer: Molybdenum (thickness: 200 nm) Upper electrode layer: Molybdenum (thickness: 100 nm) ·Sputtering method: RF magnetron sputtering ·Type of sputtering gas: Ar+N₂ (nitrogen partial pressure: 25%) ·Gas pressure: 0.7 Pa ·Film formation time: 720 minutes

[0040] (Comparative Example 1) An AlScGaN film (sputtered film) was obtained in the same manner as in Example 1, except that the gas pressure was changed from 0.7 Pa to 0.3 Pa.

[0041] (Comparative Example 2) An AlScGaN film (sputtered film) was obtained in the same manner as in Example 1, except that the gas pressure was changed from 0.7 Pa to 0.2 Pa.

[0042] <Evaluation of AlScGaN Film> Composition of the AlScGaN film, thickness of the AlScGaN film, piezoelectric constant d 33 , piezoelectric constant d 33 standard deviation, piezoelectric constant d 33 coefficient of variation (C), and surface roughness serving as an indicator of surface smoothness were determined as follows. The results are shown in Table 2.

[0043] (1) Composition The composition was determined by scanning electron microscope-energy dispersive X-ray measurement. Specifically, using an EDS-equipped SEM (equipment name: JSM-IT800, manufactured by JEOL Ltd.), SEM observation was performed under the following conditions, and using the software attached to the EDS-equipped SEM, the percentage (atm%) of the number of atoms of each element of Al, Sc, Ga and N relative to the total number of atoms of Al, Sc, Ga and N was determined under the condition that elements of the substrate and the electrode were excluded. Then, when the percentage of the number of atoms of N is set to 1, the percentages of the number of atoms of Al, Sc and Ga were determined as x, y and z, respectively. (Observation conditions) Acceleration voltage: 15 kV Observation magnification: 500× Observation field of view: 5 fields of view

[0044] (2) Thickness of AlScGaN film The height difference between the substrate and the film was measured using a laser microscope (product name: VK-X250 / 260, manufactured by Keyence Corporation), and this height was defined as the thickness of the AlScGaN film.

[0045] (3) Piezoelectric constant d 33 Samples were prepared by forming molybdenum (Mo) electrodes with a diameter of 50 μm and a thickness of 75 ± 25 nm on the front and back surfaces of the AlScGaN films obtained in Example 1 and Comparative Example 1 as described above, using electron beam deposition at room temperature. For this sample, d 33 Using a meter (product name: "PM300", manufactured by Piezo Test, Ltd.), the piezoelectric constant d was measured at 30 measurement points at room temperature, and the arithmetic mean of these measurements was taken as the piezoelectric constant d. 33 The measurement points were determined as follows: Here, the measurement points were the intersections (30 points) of line segments drawn to divide the AlScGaN film vertically into 6 sections and horizontally into 7 sections when viewed from its surface. Furthermore, the AlScGaN film in Comparative Example 2 had poor film quality and the piezoelectric constant d was low in several respects. 33 Unable to measure it, the piezoelectric constant d was measured at 30 points, and the piezoelectric constant d was taken as the average value of those measurements. 33 Since it is not possible to calculate the piezoelectric constant at one point, the piezoelectric constant d 33 This was shown as follows. Piezoelectric constant d 33 A score of 10 pC / N or higher was considered acceptable.

[0046] (4) Piezoelectric constant d 33 Standard deviation For the AlScGaN films of Example 1 and Comparative Example 1, the piezoelectric constant d values ​​measured at 30 measurement points and the piezoelectric constant d obtained as described above were compared. 33 The standard deviation was calculated from the values. Note that for the AlScGaN film of Comparative Example 2, the piezoelectric constant d 33Since this is based on the piezoelectric constant at a single point and it was not possible to calculate the standard deviation, the standard deviation is indicated as "-" in Table 2.

[0047] (5) Piezoelectric constant d 33 coefficient of variation For the AlScGaN films of Example 1 and Comparative Example 1, the standard deviation calculated as described above and the piezoelectric constant d obtained as described above were used. 33 Based on the value of , the piezoelectric constant d is derived from the following equation (B). 33 The coefficient of variation (C) was calculated. Piezoelectric constant d 33 coefficient of variation = piezoelectric constant d 33 Standard deviation (pC / N) / piezoelectric constant d 33 (pC / N)···(B) Furthermore, for the AlScGaN film in Comparative Example 2, the standard deviation could not be calculated as described above, therefore the piezoelectric constant d 33 The coefficient of variation is indicated as "-" in Table 2.

[0048] (6) Surface roughness Surface roughness was determined as the arithmetic mean of surface roughness measured under the following conditions using a scanning probe microscope (product name: SPM-9600, manufactured by Shimadzu Corporation). A surface roughness of 5 nm or less was considered acceptable. (Measurement conditions) Measurement mode: Tapping mode AFM Scanning speed: 0.5Hz Scanning area: 2 μm × 2 μm Pixel count: 512 x 512 Measurement temperature: 25±5℃

[0049] [Table 1] [Table 2]

[0050] As shown in Table 2, the AlScGaN film of Example 1 had a high piezoelectric constant and sufficiently low surface roughness. In contrast, the AlScGaN film of Comparative Example 1 had a low piezoelectric constant and high surface roughness. The AlScGaN film of Comparative Example 2 had low surface roughness but a low piezoelectric constant. Furthermore, the AlScGaN film of Example 1 had a piezoelectric constant d 33 The coefficient of variation was 0.11, which was found to be smaller than that of Comparative Example 1.

[0051] Based on the above, it has been confirmed that the AlScGaN film of this disclosure has a higher piezoelectric constant and a smoother surface compared to conventional AlScGaN films. [Explanation of Symbols]

[0052] 1...Substrate, 2...First electrode, 3...Piezoelectric film (AlScGaN film), 4...Second electrode, 10...Piezoelectric element.

Claims

1. An aluminum scandium gallium nitride film containing aluminum, scandium, gallium, and nitrogen, represented by the following compositional formula (A). (AlxScyGaz)N...(A) (In the above compositional formula (A), x is greater than 0, y is greater than 0, z is 0.09 or less, and x + y + z is 1.)

2. The aluminum scandium gallium nitride film according to claim 1, having a thickness of 600 nm or more.

3. The aluminum scandium gallium nitride film according to claim 1 or 2, wherein the coefficient of variation of the piezoelectric constant defined by the following formula (B) is 0.15 or less. Coefficient of variation of piezoelectric constant = Standard deviation of piezoelectric constant (pC / N) / Piezoelectric constant (pC / N) ... (B)

4. The aluminum scandium gallium nitride film according to claim 1, wherein in the composition formula (A), y is 0.40 or more and 0.55 or less.

5. The aluminum scandium gallium nitride film according to claim 1, wherein z is 0.06 or greater in the composition formula (A).

6. The aluminum scandium gallium nitride film according to claim 4, wherein the crystalline phase of the aluminum scandium gallium nitride film has a wurtzite-type structure.

7. The aluminum scandium gallium nitride film according to claim 1, which is a sputtered film.

8. A method for producing an aluminum scandium gallium nitride film according to claim 1 or 2, The process includes a film deposition step to produce an aluminum scandium gallium nitride film by sputtering an AlScGa alloy sputtering target. In the aforementioned film formation process, the gas pressure of the sputtering gas is set to 0.4 Pa or higher. A method for manufacturing aluminum scandium gallium nitride films.

9. A piezoelectric element comprising a piezoelectric film, A piezoelectric element wherein the piezoelectric film includes the aluminum scandium gallium nitride film described in claim 1 or 2.

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