Epitaxial thin film and method for producing same

Abstract

In order to form, on the surface of an LN single crystal, an LT single crystal, or the like, an epitaxial thin film that does not have rotating twin crystals and that exhibits characteristics comparable to those of an ideal perfect crystal, this epitaxial thin film obtained by adding, to lithium niobate or lithium tantalate serving as a base material, at least one alkali metal element of sodium, potassium, rubidium, cesium, or francium is epitaxially grown on the surface of a substrate by a high-frequency sputtering method.

Description

Epitaxial thin film and method for manufacturing the same

[0001] The present invention relates to an epitaxial thin film and a method for producing the same.

[0002] Lithium niobate (LiNbO), an oxide single crystal ３ , hereinafter also referred to as LN), and lithium tantalate (LiTaO ３ LN and LT are materials that have excellent piezoelectric and electro-optical properties. In recent years, techniques for forming thin films of these materials while maintaining their single-crystal properties have attracted attention from the perspective of improving the performance of communication and optical control devices that use these materials.

[0003] When forming single crystal thin films of LN or LT by sputtering, which is a type of vapor phase growth, appropriate optimization of process conditions is required to reproduce the perfect crystal structure and stoichiometric composition on the substrate.

[0004] Incidentally, the crystal structure of single crystals of LN and LT has the usual space group R3c structure (LiNbO ３ It is known that the space group R3-structure (also called ilmenite structure) can theoretically be formed as a crystal structure. ３ The difference between the ilmenite structure and the corundum structure is the stacking arrangement of cations along the polar axis in the crystal. Both structures share the corundum structure as the basic framework of the crystal structure, and can coexist within the crystal. Therefore, the ilmenite structure is LiNbO ３ This can manifest itself in the crystal as stacking faults (also called non-stoichiometric defects when caused by deviations from the stoichiometric composition) or superstructures.

[0005] LiNbO ３ However, in Non-Patent Document 1, reflection peaks with large interplanar spacing that do not appear in the space group R3c are observed in the X-ray diffraction scattering angle 2θ ranges of 11° to 22° and 25.3° to 31° for a lithium niobate crystal sample into which ilmenite-type stacking faults have been intentionally introduced.

[0006] In Non-Patent Document 2, potassium oxide (chemical formula: K) is used in the production of LN single crystals.２ A method for obtaining stoichiometric lithium niobate single crystals has been reported using a top-seeded solution growth method using a flux containing potassium. It is known that potassium does not act as an impurity (dopant) and is not mixed into the crystal.

[0007] Non-Patent Document 3 describes the importance of adding various dopants to the crystal in order to control the crystal growth and physical properties in the production of LN single crystals, but does not report the addition of potassium (K), rubidium (Rb), or cesium (Cs), which are alkali elements of the same group as lithium.

[0008] Patent Document 1 discloses a lithium niobate epitaxial thin film (ilmenite structure film) formed by a sputtering method. However, the crystal structure of the film is different from that of the ilmenite structure and LiNbO ３ No distinction is made between the structure and the film, and the film is not an epitaxial film of a single structure but is an epitaxial film containing rotational twins.

[0009] Patent No. 5862364

[0010] AV Kadetova et al., Materialia 28 (2023) 101770.K. Polgar, et. al., J. Cryst. Growth 177 (1997) 211-216.O. Sanchez-Dena, et.al., Crystals 2020, 10(11), 990.

[0011] In recent years, remarkable progress has been made in thinning LN and LT single crystals, and from the viewpoints of production cost, large area, and flexibility of the production process, a method of forming a thin film based on a bottom-up technique by film deposition is expected.

[0012] When forming a thin film of LN single crystal by sputtering, the constituent elements are supplied to the substrate surface in an atomically decomposed state during the elementary process, which tends to result in the formation of a lithium niobate thin film with impaired crystalline integrity, such as a thin film with rotational twins. As a result, such a thin film will have physical properties, such as piezoelectric properties, that are different from those expected from an ideal perfect crystal, and there is a risk that the performance of communication and optical control devices using such a thin film will not be fully achieved.

[0013] The problem to be solved by the present invention is to form an epitaxial thin film on the surface of an LN single crystal or an LT single crystal, which does not have rotational twins and exhibits properties that are comparable to those of an ideal perfect crystal.

[0014] The present invention, which solves the above problems, provides a method for producing an epitaxial thin film by adding one alkali metal element, sodium, potassium, rubidium, cesium, or francium, to a lithium niobate base material and depositing the resulting epitaxial thin film on the surface of a substrate by radio-frequency sputtering (hereinafter referred to as the first invention). The present invention, which solves the above problems, also provides a method for producing an epitaxial thin film by adding one alkali metal element, sodium, potassium, rubidium, cesium, or francium, to a lithium tantalate base material and depositing the resulting epitaxial thin film on the surface of a substrate by radio-frequency sputtering (hereinafter referred to as the second invention).

[0015] In the above inventions, the surface of the substrate may have a surface of lithium niobate single crystal or sapphire single crystal in the first invention, and may have a surface of lithium tantalate single crystal or sapphire single crystal in the second invention. Here, in the first invention, the entire substrate may be a substrate of lithium niobate single crystal or sapphire single crystal, or only the surface may be a substrate of lithium niobate single crystal or sapphire single crystal. Furthermore, in the second invention, the entire substrate may be a substrate of lithium tantalate single crystal or sapphire single crystal, or only the surface may be a substrate of lithium tantalate single crystal or sapphire single crystal.

[0016] In the first aspect of the present invention, a target containing lithium, niobium, oxygen, and any one of the alkali metal elements may be sputtered, or alternatively, a first target containing lithium, niobium, and oxygen and a second target containing any one of the alkali metal elements may be simultaneously sputtered. Also, in the second aspect of the present invention, a target containing lithium, tantalum, oxygen, and any one of the alkali metal elements may be sputtered, or alternatively, a first target containing lithium, tantalum, and oxygen and a second target containing any one of the alkali metal elements may be simultaneously sputtered.

[0017] In the above invention, the target containing lithium, niobium / tantalum, oxygen, and any one of the alkali metal elements has a composition formula Li ｘ A （１－ｘ） Nb / TaO ｙ It is more preferable that 0.920≦x≦0.995, where niobium and tantalum are alternatives, and A is any one of the alkali metal elements sodium, potassium, rubidium, cesium, and francium.

[0018] In the above invention, the temperature of the substrate may be set to less than 450° C., and epitaxial growth may be performed by the radio frequency sputtering method.

[0019] The above problems can also be solved by an epitaxial thin film manufacturing apparatus that uses the epitaxial thin film manufacturing method according to the above invention.

[0020] Further, the present invention for solving the above-mentioned problems is an epitaxial thin film formed on the surface of a lithium niobate single crystal or a sapphire single crystal, the epitaxial thin film being formed by adding one alkali metal element selected from sodium, potassium, rubidium, cesium, and francium to lithium niobate as a base material.

[0021] Further, in order to solve the above-mentioned problems, the present invention provides an epitaxial thin film formed on the surface of a lithium tantalate single crystal or a sapphire single crystal, the epitaxial thin film being formed by adding one alkali metal element selected from sodium, potassium, rubidium, cesium, and francium to lithium tantalate as a base material.

[0022] According to the present invention, an epitaxial thin film that does not have rotational twins and exhibits properties comparable to those of an ideal perfect crystal thin film can be formed on the surface of an LN single crystal or an LT single crystal.

[0023] Fig. 1 is a longitudinal sectional view showing one embodiment of an epitaxial thin film manufacturing apparatus of the present invention; Fig. 2 is a longitudinal sectional view showing another embodiment of an epitaxial thin film manufacturing apparatus of the present invention; Fig. 3 is a sectional view showing one embodiment of an epitaxial thin film of the present invention; Fig. 4 is a diagram showing X-ray diffraction results of a lithium niobate epitaxial thin film according to a comparative example of the present invention and an epitaxial thin film doped with a different alkali metal element according to an example; Fig. 5 is a pole figure showing the result of XRD pole measurement (2θ=23.715) of a lithium niobate epitaxial thin film according to a comparative example of the present invention and an epitaxial thin film doped with a different alkali metal element according to an example; Fig. 6 is a pole figure showing the result of XRD pole measurement (2θ=19.200) of a 128Y-cut lithium niobate substrate according to a comparative example of the present invention, a lithium niobate epitaxial thin film according to a comparative example, and an epitaxial thin film doped with a different alkali metal element according to an example. FIG. 1 is a diagram showing the results of observation by a piezoelectric response microscope (PFM) of a 128Y-cut lithium niobate substrate according to a comparative example of the present invention, an epitaxial thin film of lithium niobate according to a comparative example, and an epitaxial thin film doped with a different alkali metal element according to an example.

[0024] 1 is a longitudinal cross-sectional view showing one embodiment of an epitaxial thin film manufacturing apparatus 1 according to the present invention. The epitaxial thin film manufacturing apparatus 1 of this embodiment is a sputtering apparatus that uses a radio-frequency sputtering method to epitaxially grow a thin film on a surface of a substrate 2 having a surface of an LN single crystal, an LT single crystal, or a sapphire single crystal, and deposits the thin film by epitaxial growth on the surface of the substrate 2 when the surface of the substrate 2 is an LN single crystal or a sapphire single crystal, using lithium niobate as a base material and doping the base material with one of the alkali metal elements sodium, potassium, rubidium, cesium, and francium, or deposits the thin film by epitaxial growth on the surface of the substrate 2 when the surface of the substrate is an LT single crystal or a sapphire single crystal, using lithium tantalate as a base material and doping the base material with one of the alkali metal elements sodium, potassium, rubidium, cesium, and francium, and deposits the thin film by epitaxial growth on the surface of the substrate 2 when the surface of the substrate is an LT single crystal or a sapphire single crystal.

[0025] In the epitaxial thin film manufacturing apparatus 1 of this embodiment, when the surface of the substrate 2 is an LN single crystal or a sapphire single crystal, the target is a powder sintered body of lithium niobate doped with a different alkali metal element, in which lithium niobate is used as a base material and one of the alkali metal elements, sodium, potassium, rubidium, cesium, or francium, is added to the base material. Hereinafter, this target is also referred to as a different alkali metal element-doped LN powder sintered body. Furthermore, when the surface of the substrate 2 is an LT single crystal or a sapphire single crystal, the target is a powder sintered body of lithium tantalate doped with a different alkali metal element, in which lithium tantalate is used as a base material and one of the alkali metal elements, sodium, potassium, rubidium, cesium, or francium, is added to the base material. Hereinafter, this target is also referred to as a different alkali metal element-doped LT powder sintered body.

[0026] Then, high frequency power is applied to the target 121 made of the sintered LN powder or sintered LT powder with different alkali metal elements to form plasma, and positive ions accelerated by a negative DC bias generated on the surface of the target 121 are made to collide with the target 121. As a result, particles sputtered from the target surface are used as material for the epitaxial thin film. The substrate 2 is made of LiNbO ３Single crystal, LiTaO ３ Single crystal or sapphire single crystal (α-Al ２ O ３ ) surface, and LiNbO ３ Single crystal, LiTaO ３ The single crystal may be subjected to a reduction reaction treatment.

[0027] As shown in FIG. 1 , the epitaxial thin film manufacturing apparatus 1 of this embodiment includes a chamber 11 that can be set to a predetermined vacuum level, a sputtering electrode 12 provided in the chamber 11, and a susceptor 13 also provided in the chamber 11.

[0028] The chamber 11 according to this embodiment includes a film formation chamber 111, which faces a target 121 attached to a sputtering electrode 12 and a substrate 2 mounted on a susceptor 13. The film formation chamber 111 can be set to a predetermined vacuum level by an exhaust device 113 provided at an exhaust port 112 of the chamber 11.

[0029] The sputtering electrode 12 of this embodiment supplies high-frequency power to a target 121 attached to its tip. The frequency of this high-frequency power is not limited, but may include a frequency of 13.56 MHz. Furthermore, when the substrate surface is an LN single crystal or a sapphire single crystal, a target of an LN powder sintered compact doped with a different alkali metal element is used as the target 121. When the substrate surface is an LT single crystal or a sapphire single crystal, a target of an LT powder sintered compact doped with a different alkali metal element is used as the target 121.

[0030] The composition formula of the target of the LN powder sintered body containing different alkali metal elements is Li ｘ A （１－ｘ） NbO ｙ In the formula, it is more preferable that 0.920≦x≦0.995, and the composition formula of the target of the sintered compact of the LT powder containing different alkali metal elements is Li ｘ A （１－ｘ） Ta O ｙIn this case, it is more preferable that 0.920≦x≦0.995. That is, it is more preferable that the ratio of the different alkali metal element added is 0.5 atomic % to 8 atomic % with respect to lithium. In addition, the target of the different alkali metal element added LN powder sintered body or the different alkali metal element added LT powder sintered body contains lithium oxide Li ２ O may be contained to a certain extent.

[0031] The susceptor 13 of this embodiment is made of a conductive material, holds the substrate 2, and can be rotated at a predetermined rotation speed by a drive device 132 connected to a rotation shaft 131. The susceptor 13 may be provided with a counterbore (a recess corresponding to the outer shape of the substrate 2) so that deposition processing can be performed on multiple substrates 2 in one processing. The material constituting the susceptor 13 is not particularly limited, but examples include inconel, silicon carbide, graphite, and silicon. The conductivity of the susceptor 13 does not necessarily need to be exhibited at room temperature, and may be exhibited when the substrate 2 is heated to a temperature below 450°C.

[0032] A heater 133 is provided below the susceptor 13 to heat the substrate 2 to a predetermined temperature. The heating method of the heater 133 is not limited, but examples include a resistance heating method using graphite or silicon carbide as a resistor, and an optical heating method using a halogen lamp. Depositing an epitaxial thin film by high-frequency sputtering while maintaining the substrate 2 at a temperature below 450° C. promotes epitaxial growth of the deposited thin film while preventing reoxidation of the reduced LN single crystal or LT single crystal.

[0033] The rotation shaft 131 of the susceptor 13 is maintained in an electrically floating state. By electrically floating the rotation shaft 131, adjustments can be made to alleviate the impact of positive ions. By depositing a thin film while rotating the substrate 2 in an electrically floating state, it is possible to deposit a thin film uniformly over the entire surface of the substrate 2 while satisfying the requirement to alleviate the impact of negative ions.

[0034] The epitaxial thin film manufacturing apparatus 1 of this embodiment includes a radio frequency matching box 134 that is connected to the rotating shaft 131 of the susceptor 13 and is capable of adjusting the reactance of the electrically floating rotating shaft 131. The reactance here corresponds to the capacitance and inductance within the radio frequency matching box 134. The adjustment by the radio frequency matching box 134 is performed by diagnosing the state of electrical coupling of the substrate 2 with the radio frequency plasma by measuring the DC voltage generated within the radio frequency matching box 134 during deposition.

[0035] Here, when the DC voltage reaches its negative maximum value, it indicates that the capacitive substrate sheath and capacitance are in a series resonance state, and positive ions are attracted to the substrate 2, causing reverse sputtering. In contrast, when the DC voltage reaches its positive maximum value, it corresponds to a parallel resonance state between the substrate 2 and the stray capacitance of the high-frequency matching box 134 including the rotating shaft 131, and negative charged particles are attracted to the substrate 2. In this embodiment, by adjusting and maintaining the reactance to the latter value, it is possible to suppress re-evaporation of Li atoms deposited on the substrate 2 due to reverse sputtering, and to promote epitaxial growth of the deposited thin film.

[0036] The epitaxial thin film manufacturing apparatus 1 of this embodiment further includes a DC power supply 135 that is connected to the rotating shaft 131 and applies a DC voltage to the electrically floating rotating shaft 131. The DC power supply 135 may apply either a negative voltage or a positive voltage, but it is preferable to apply a positive voltage from the viewpoint of promoting epitaxial growth of the deposited thin film.

[0037] In this embodiment, the target 121 is positioned so as not to directly face the substrate 2. That is, as shown in FIG. 1 , the target 121 is positioned offset with respect to the rotation axis 131, and the surface normal PL1 of the target 121 is inclined at an angle α of 15° to 75° with respect to the substrate normal PL2 of the substrate 2. The DC negative bias generated on the surface of the target 121 accelerates negative ions in the plasma and causes the negative ions to be emitted along the surface normal L1 of the target 121. Therefore, by not facing the substrate 2 and the target 121, direct collision of negative ions with the substrate 2 can be avoided, and the impact of the negative ions can be alleviated. This can promote the epitaxial growth of the deposited thin film.

[0038] The epitaxial thin film manufacturing apparatus 1 of this embodiment includes a shielding plate 122 that reduces the impact of negative ions emitted from the target 121 along the surface normal PL1 of the target 121. The shielding plate 122 of this embodiment is disposed in front of the target 121 so as to shield the portion where a group (aggregate) of the surface normals PL1 of the target 121 intersects with the surface of the substrate 2. This makes it possible to suppress the negative ions emitted along the surface normal PL1 of the target 121 from impacting the substrate 2, further promoting the epitaxial growth of the deposited thin film.

[0039] The epitaxial thin film manufacturing apparatus 1 of this embodiment includes a perforated metal plate 123 and a perforated quartz plate 124. As shown in FIG. 1 , the perforated metal plate 123 and the perforated quartz plate 124 of this embodiment are provided on the inner wall of the film formation chamber 111, in front of the target 121, with the perforated quartz plate 124 disposed in front of the perforated metal plate 123. The perforated metal plate 123 is electrically coupled to the plasma and provides electrons to the plasma. The potential of the perforated metal plate 123 may be ground, or a controlled negative potential may be applied to more actively provide electrons. Furthermore, to increase the efficiency of the power consumed in the actual sputtering phenomenon by the high-frequency power input to the target 121, multiple perforated metal plates 123 may be stacked to increase their surface area. The perforated quartz plate 124 prevents sputter particles from accumulating on the perforated metal plate 123 and maintains a constant supply of electrons. The porous quartz plate 124 also has the function of preventing the material of the porous metal plate 123, which is sputtered when a negative potential is applied, from being deposited back onto the substrate 2.

[0040] The epitaxial thin film manufacturing apparatus 1 of this embodiment further includes a gas supply device 125, which supplies argon gas and oxygen gas to the film formation chamber 111 as process gases during film formation.

[0041] Figure 2 is a diagram showing another embodiment of the epitaxial thin film manufacturing apparatus 1 of the present invention. The epitaxial thin film manufacturing apparatus 1 of the embodiment shown in Figure 2 differs from the epitaxial thin film manufacturing apparatus 1 of the embodiment shown in Figure 1 in that it is equipped with two sputtering electrodes 12A, 12B and two targets 121A, 121B. Since the components other than those related to these differences are the same as those of the epitaxial thin film manufacturing apparatus 1 of the embodiment shown in Figure 1, the same reference numerals are used in Figure 2 and the description thereof is incorporated herein by reference.

[0042] The epitaxial thin film manufacturing apparatus 1 of the embodiment shown in Fig. 2 includes two sputtering electrodes 12A and 12B, one of which is equipped with a target 121A, and the other of which is equipped with a target 121B. When the surface of the substrate is made of LN single crystal or sapphire single crystal, a target of sintered LN powder is used as the target 121A. When the surface of the substrate is made of LT single crystal or sapphire single crystal, a target of sintered LT powder is used. These targets of sintered LN powder or sintered LT powder contain lithium oxide (Li). ２ O may be contained to a certain extent.

[0043] In contrast, the other target 121B is a sintered powder target of any one of the alkali metal elements sodium, potassium, rubidium, cesium, and francium, regardless of whether the substrate surface is LN single crystal, LT single crystal, or sapphire single crystal.

[0044] Then, high frequency power is simultaneously applied to one sputtering electrode 12A to which one target 121A is attached and the other sputtering electrode 12B to which the other target 121B is attached, to generate plasma, and positive ions accelerated by a negative DC bias generated on the surfaces of the targets are caused to collide with the targets. As a result, particles sputtered from the surfaces of the respective targets 121A and 121B are used as material for the epitaxial thin film.

[0045] 3 is a cross-sectional view showing an embodiment of an epitaxial thin film according to the present invention. In the illustrated example, an epitaxial thin film 22 of potassium-doped lithium niobate is formed on an LN single crystal 21. In deposition (epitaxial growth) using the epitaxial thin film manufacturing apparatus 1 of this embodiment described above, a target 121 shown in FIG. 1 is formed with a composition formula Li ｘ K （１－ｘ） NbO ｙThe sputtering was performed using a potassium-added LN powder sintered body in which x was adjusted to x = 0.950, with a sputtering rate of 0.3 angstroms / second, an argon gas to oxygen gas flow ratio of 50:50, a pressure in the film formation chamber 111 of 0.3 to 0.5 Pa, and an epitaxial thin film thickness of 250 to 300 nm. The heating temperature of the substrate 2 by the heater 133 was set arbitrarily to a temperature below 400°C as shown below.

[0046] 4 to 6 are graphs showing the results of X-ray diffraction (XRD) measurements of an epitaxial thin film of potassium-doped lithium niobate formed using the epitaxial thin film manufacturing apparatus 1 of this embodiment. The X-ray diffractometer used was a Smart Lab manufactured by Rigaku Corporation, and ω-2θ was measured to evaluate the out-of-plane crystallinity. To verify the effect of potassium addition, the XRD results of an epitaxial thin film of lithium niobate formed using a target of a sintered powder compact of lithium niobate without potassium addition are also shown. A 128Y-cut lithium niobate substrate was used as the substrate 2 for forming the epitaxial thin film.

[0047] 4 is a graph showing the results of X-ray diffraction (XRD ω-2θ measurement) of a comparative example (FIGS. 4(a) and (b)) in which an epitaxial thin film of lithium niobate was formed on a 128Y-cut lithium niobate substrate with the heating temperature of the substrate 2 set to 300° C., and an example (FIGS. 4(c) and (d)) in which an epitaxial thin film of potassium-doped lithium niobate was formed. FIGS. 4(b) and (d) are graphs in which the diffraction peak of the (014) plane in FIGS. 4(a) and (c) is enlarged, respectively.

[0048] From the results of Figures 4(a) and (c), it was confirmed that each epitaxial thin film does not have any diffraction peaks other than the diffraction peak corresponding to the 128Y cut plane (lattice plane (104)). From the results of Figures 4(b) and (d), the diffraction peaks of the substrate 21 (Substrate) and the diffraction peaks derived from the thin film accompanied by intensity oscillations (fringes) can be distinguished, and it was confirmed that each thin film 22 (Epilayer) is an epitaxial thin film. At the same time, the fringes indicate the presence of an interface between the substrate 21 and the thin film 22, suggesting that the crystal structures of the substrate 21 and the thin film 22 are strictly different. This is because the difference between the ilmenite structure and the LiNbO ３ This may be due to differences in the content of the structure. The clarity of the fringes differs between the two, which is presumably due to the influence of the surface roughness of the thin film 22. Evaluation of the surface roughness using an atomic force microscope (AFM, Dimension Icon manufactured by Bruker) revealed that the surface roughness of the epitaxial thin film of lithium niobate was Ra = 0.8 nm, and the surface roughness of the epitaxial thin film of potassium-doped lithium niobate was Ra = 1.6 nm.

[0049] 5 is a pole figure showing the results of XRD pole measurement (2θ=23.715) of comparative examples ((a) and (b)) in which an epitaxial thin film of lithium niobate was formed on a 128Y-cut lithium niobate substrate at a heating temperature of the substrate 2 set to 250° C. and 300° C., and examples ((c) and (d)) in which an epitaxial thin film of potassium-doped lithium niobate was formed. The diffraction spots displayed on the pole figure are those of LiNbO ３ This corresponds to diffraction from the (012) crystal lattice plane.

[0050] The results are compared below. First, comparing Fig. 5(a), which shows the diffraction pattern of an epitaxial thin film of lithium niobate formed by setting the heating temperature of the substrate 2 at 300°C, with Fig. 5(c), which shows the pattern of an epitaxial thin film of potassium-doped lithium niobate, both show two-fold symmetric spots that indicate a single single crystal structure. When combined with the results of Fig. 4, it was confirmed that a single crystal film not containing a twin structure had been formed.

[0051] Next, in Fig. 5(b), which is the diffraction pattern of an epitaxial thin film of lithium niobate formed by setting the heating temperature of the substrate 2 to 250°C, an "additional" spot (rotation twin part) appears, which indicates the presence of a rotation twin in the thin film. In contrast, in Fig. 5(d), which is the diffraction pattern of an epitaxial thin film of potassium-doped lithium niobate, a 2-fold symmetric pattern is shown, similar to the case where the heating temperature of the substrate 2 is set to 300°C, and an "additional" spot (rotation twin part) which indicates the presence of a rotation twin in the thin film is not shown. Therefore, it was confirmed that when a film is formed using a sputtering target made of potassium-doped lithium niobate, an epitaxial film without a rotation twin can be formed at a lower temperature than when a film is formed using a sputtering target made of lithium niobate.

[0052] FIG. 6 shows the results of XRD pole measurement (2θ=19.200) in which the diffraction spots on the polar axis were observed for the 128Y-cut lithium niobate substrate 2 itself ( FIG. 6( a)), comparative examples ((b) and (c)) in which an epitaxial thin film of lithium niobate was formed on a 128Y-cut lithium niobate substrate with the heating temperature of the substrate 2 set to 250° C. and 300° C., and examples ((d) and (e)) in which an epitaxial thin film of potassium-added lithium niobate was formed.

[0053] LiNbO ３ In the case of lithium niobate having the ilmenite structure, diffraction at 2θ=19.200, which is the (003) reflection, is prohibited, and no diffraction spot is normally observed. However, since a diffraction spot was observed in Fig. 6(a) where the substrate itself was measured, it was confirmed that the 128Y-cut lithium niobate substrate used in this example and this comparative example was lithium niobate containing an ilmenite structure therein.

[0054] Next, in Fig. 6(b), which shows the diffraction spots of an epitaxial thin film of lithium niobate formed with the heating temperature of the substrate 2 set to 300°C, small changes in the spread of the diffraction spots were observed, confirming that the content of the ilmenite structure in the thin film was small. In contrast, in Fig. 5(c), which shows the diffraction spots of an epitaxial thin film of lithium niobate formed with the heating temperature of the substrate 2 set to 250°C, the spread of the diffraction spots was large, confirming that the thin film contained a larger amount of the ilmenite structure than the diffraction spots at 300°C shown in Fig. 6(b).

[0055] Next, in Figures 6(d) and (e), which show the diffraction spots of the epitaxial thin film of potassium-doped lithium niobate formed at the heating temperatures of the substrate 2 set to 300°C and 250°C, broadening of the diffraction spots was observed, confirming the presence of the ilmenite structure. However, since the broadening of the diffraction spots for both films at different heating temperatures was similar, it was confirmed that the content of the ilmenite structure was similar. In other words, it was confirmed that the epitaxial thin film of potassium-doped lithium niobate obtained the same crystal quality as the epitaxial thin film of lithium niobate without potassium doping, even when the substrate temperature was set lower.

[0056] FIG. 7 shows the results of observations of the surfaces of a 128Y-cut lithium niobate substrate 2 itself ( FIGS. 7( a) and (b) ), an epitaxial thin film of lithium niobate formed on this substrate 2 by setting the heating temperature of the substrate 2 to 300° C. ( FIGS. 7( c) and (d) ), and an epitaxial thin film of potassium-doped lithium niobate formed by setting the heating temperature of the substrate to 300° C. ( FIGS. 7( e) and (f) ) using a piezoelectric microscope (PFM). The piezoelectric microscope used was a Dimension Icon manufactured by Bruker. FIGS. 7( b), (d), and (f) are histograms of the piezoelectric responses of FIGS. 7( a), (c), and (e), respectively. FIGS. 7( d) and (f) are histograms of the piezoelectric response amplitude normalized by the mode of the piezoelectric response amplitude signal of the substrate itself shown in FIG. 7(b).

[0057] It was confirmed that the mode of the piezoelectric response amplitude of the epitaxial thin film of potassium-doped lithium niobate shown in Fig. 7(f) is larger than the mode of the piezoelectric response amplitude of the epitaxial thin film of lithium niobate shown in Fig. 7(d) and is also larger than the mode of the piezoelectric response amplitude of the substrate itself shown in Fig. 7(b). Therefore, it was confirmed that the lithium niobate thin film doped with a different alkali metal element formed in the example has an excellent piezoelectric response.

[0058] REFERENCE SIGNS LIST 1... epitaxial thin film manufacturing apparatus 11... chamber 111... film formation chamber 112... exhaust port 113... exhaust device 12, 12A, 12B... sputtering electrode 121, 121A, 121B... target 122... shielding plate 123... perforated metal plate 124... perforated quartz plate 125... gas supply device 13... susceptor 131... rotation shaft 132... drive device 133... heater 134... high frequency matching box 135... DC power supply PL1... target surface normal PL2... substrate surface normal α... angle between target surface normal and substrate surface normal 2... substrate 21... LN single crystal 22... epitaxial thin film

Claims

1. A method for manufacturing an epitaxial thin film in which an epitaxial thin film is formed by adding one of the alkali metal elements sodium, potassium, rubidium, cesium, or francium to lithium niobate as a base material and depositing the epitaxial thin film on the surface of a substrate by radio frequency sputtering.

2. The method for producing an epitaxial thin film according to claim 1, wherein the surface of the substrate has a surface of lithium niobate single crystal or sapphire single crystal.

3. A method for producing an epitaxial thin film according to claim 1 or 2, wherein a target containing lithium, niobium, oxygen and any one of the alkali metal elements is sputtered.

4. A method for producing an epitaxial thin film according to claim 1 or 2, wherein a first target containing lithium, niobium, and oxygen and a second target containing any one of the alkali metal elements are simultaneously sputtered.

5. A target containing lithium, niobium, oxygen, and any one of the alkali metal elements, having the composition formula Li ｘ A （１－ｘ） NbO ｙ 4. The method for producing an epitaxial thin film according to claim 3, wherein x satisfies the relationship 0.920≦x≦0.995, where A is any one of the alkali metal elements.

6. A method for manufacturing an epitaxial thin film, in which an epitaxial thin film is formed by adding one of the alkali metal elements sodium, potassium, rubidium, cesium, or francium to lithium tantalate as a base material, and depositing the epitaxial thin film on the surface of a substrate by radio frequency sputtering.

7. The method for producing an epitaxial thin film according to claim 6, wherein the surface of the substrate has a surface of lithium tantalate single crystal or sapphire single crystal.

8. A method for producing an epitaxial thin film according to claim 6 or 7, wherein a target containing lithium, tantalum, oxygen and any one of the alkali metal elements is sputtered.

9. A method for producing an epitaxial thin film according to claim 6 or 7, wherein a first target containing lithium, tantalum, and oxygen and a second target containing any one of the alkali metal elements are simultaneously sputtered.

10. A target containing lithium, tantalum, oxygen, and any one of the above alkali metal elements, with the composition formula Li ｘ A （１－ｘ） Ta O ｙ 9. The method for producing an epitaxial thin film according to claim 8, wherein x satisfies the relationship 0.920≦x≦0.995, where A is any one of the alkali metal elements.

11. The method for producing an epitaxial thin film according to any one of claims 1 to 10, wherein the temperature of the substrate is set to less than 450°C and epitaxial growth is carried out by the high frequency sputtering method.

12. An apparatus for producing an epitaxial thin film using the method for producing an epitaxial thin film according to any one of claims 1 to 11.

13. An epitaxial thin film formed on the surface of a lithium niobate single crystal or a sapphire single crystal, in which one of the alkali metal elements sodium, potassium, rubidium, cesium, or francium is added to lithium niobate as a base material.

14. An epitaxial thin film formed on the surface of a lithium tantalate single crystal or a sapphire single crystal, in which one of the alkali metal elements sodium, potassium, rubidium, cesium, or francium is added to lithium tantalate as a base material.

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