Classifications

• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10N30/00—Piezoelectric or electrostrictive devices
  • H10N30/50—Piezoelectric or electrostrictive devices having a stacked or multilayer structure
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10N30/00—Piezoelectric or electrostrictive devices
  • H10N30/704—Piezoelectric or electrostrictive devices based on piezoelectric or electrostrictive films or coatings
  • H10N30/706—Piezoelectric or electrostrictive devices based on piezoelectric or electrostrictive films or coatings characterised by the underlying bases, e.g. substrates
  • H10N30/708—Intermediate layers, e.g. barrier, adhesion or growth control buffer layers
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10N30/00—Piezoelectric or electrostrictive devices
  • H10N30/80—Constructional details
  • H10N30/85—Piezoelectric or electrostrictive active materials
  • H10N30/853—Ceramic compositions
  • H10N30/8548—Lead-based oxides
  • H10N30/8554—Lead-zirconium titanate [PZT] based
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10N30/00—Piezoelectric or electrostrictive devices
  • H10N30/01—Manufacture or treatment
  • H10N30/07—Forming of piezoelectric or electrostrictive parts or bodies on an electrical element or another base
  • H10N30/074—Forming of piezoelectric or electrostrictive parts or bodies on an electrical element or another base by depositing piezoelectric or electrostrictive layers, e.g. aerosol or screen printing
  • H10N30/076—Forming of piezoelectric or electrostrictive parts or bodies on an electrical element or another base by depositing piezoelectric or electrostrictive layers, e.g. aerosol or screen printing by vapour phase deposition
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10N30/00—Piezoelectric or electrostrictive devices
  • H10N30/01—Manufacture or treatment
  • H10N30/07—Forming of piezoelectric or electrostrictive parts or bodies on an electrical element or another base
  • H10N30/074—Forming of piezoelectric or electrostrictive parts or bodies on an electrical element or another base by depositing piezoelectric or electrostrictive layers, e.g. aerosol or screen printing
  • H10N30/079—Forming of piezoelectric or electrostrictive parts or bodies on an electrical element or another base by depositing piezoelectric or electrostrictive layers, e.g. aerosol or screen printing using intermediate layers, e.g. for growth control

Definitions

• This disclosure relates to piezoelectric elements and actuators.
• Perovskite oxides such as lead zirconate titanate (Pb(Zr,Ti) O3 , hereinafter referred to as PZT) are known as materials with excellent piezoelectric and ferroelectric properties.
• Piezoelectrics made of perovskite oxides are used as piezoelectric films in piezoelectric elements that have a lower electrode, a piezoelectric film, and an upper electrode on a substrate. These piezoelectric elements are being deployed in a variety of devices, including memories, inkjet heads (actuators), micromirror devices, angular velocity sensors, gyro sensors, ultrasonic elements (PMUTs: Piezoelectric Micromachined Ultrasonic Transducers), and vibration power generation devices.
• actuators inkjet heads
• micromirror devices angular velocity sensors
• gyro sensors ultrasonic elements
• PMUTs Piezoelectric Micromachined Ultrasonic Transducers
• JP 2013-80886 A proposes a piezoelectric element in which a first electrode, an Nb-added PZT film, a second electrode, an Nb-added PZT film, and a third electrode are stacked in this order. It is known that the spontaneous polarization of an Nb-added PZT film is aligned upward relative to the substrate when it is formed. In other words, both of the two Nb-added PZT films in JP 2013-80886 A have spontaneous polarization that is aligned upward. In general, for a piezoelectric film with aligned spontaneous polarization, applying an electric field in the same direction as the spontaneous polarization will provide better piezoelectric performance.
• JP 2013-80887 A an electric field is applied to the two Nb-doped PZT films in the same direction as the spontaneous polarization, using a first driving method in which the second electrode is grounded and a positive voltage (+V) is applied to the first electrode and a negative voltage (-V) is applied to the third electrode, or a second driving method in which the first electrode is grounded and a negative voltage (-V) is applied to the second electrode and a negative voltage (-2V) with an absolute value greater than that of the second electrode is applied to the third electrode.
• This achieves a displacement that is roughly twice as large as that of a piezoelectric element with only one layer.
• JP 2013-80887 A proposes a piezoelectric element in which a first electrode, a first piezoelectric film, a second electrode, a second piezoelectric film, and a third electrode are stacked in this order, in which the spontaneous polarization of the first piezoelectric film is aligned in a different direction from the spontaneous polarization of the second piezoelectric film.
• the first piezoelectric film is an Nb-added PZT film
• the second piezoelectric film is a PZT film without Nb addition (hereinafter referred to as non-doped PZT).
• the spontaneous polarization of the Nb-added PZT film is aligned without poling treatment, but the spontaneous polarization of the non-doped PZT film is not aligned without poling treatment. Therefore, a poling process is performed on the non-doped PZT film so that the spontaneous polarization is aligned in the opposite direction to the spontaneous polarization of the Nb-doped PZT film, thereby making the spontaneous polarization of the first piezoelectric film different from the spontaneous polarization of the second piezoelectric film.
• the first electrode and the third electrode of the piezoelectric element are at the same potential, and the second electrode is used as the driving electrode, so that an electric field is applied to the first piezoelectric film and the second piezoelectric film in the same direction as the spontaneous polarization of each film.
• the piezoelectric performance of two layers can be obtained with a voltage large enough to drive one piezoelectric film, so high piezoelectric performance can be obtained at a low voltage.
• the piezoelectric element of JP 2013-80887 A provides very good piezoelectric performance.
• the first piezoelectric film and the second piezoelectric film must be made of piezoelectric films containing different materials, for example, the first piezoelectric film must be an Nb-doped PZT film and the second piezoelectric film must be a non-doped PZT film. Therefore, two different targets are required to form the first piezoelectric film and the second piezoelectric film, and at least one of the piezoelectric films must be subjected to a poling process, which makes it difficult to achieve sufficient cost reduction.
• piezoelectric elements that can achieve high piezoelectric performance are expensive or require the application of high voltages, and it has not been possible to develop low-cost piezoelectric elements that can achieve high piezoelectric performance at low voltages.
• the purpose of this disclosure is to provide piezoelectric elements and piezoelectric actuators that provide high piezoelectric performance at low voltages at low cost.
• a piezoelectric element includes a first electrode, a first piezoelectric film, a second electrode, a second piezoelectric film, and a third electrode, in this order, on a substrate; the first piezoelectric film and the second piezoelectric film each contain a perovskite-type oxide as a main component;
• the perovskite oxide is a first perovskite oxide
• a seed layer mainly composed of a second perovskite oxide that is lattice-matched to the first perovskite oxide is provided only between the first electrode and the first piezoelectric film or between the second electrode and the second piezoelectric film; one of the first piezoelectric film and the second piezoelectric film, which is not provided on the seed layer, is polarized in a thickness direction due to a spontaneous internal electric field;
• the other of the first and second piezoelectric films, which is the piezoelectric film provided on the seed layer has a coercive voltage on the positive side of
• the positive coercive voltage Vcf + and the negative coercive voltage Vcf- may have the same sign.
• the perovskite oxide that is the main component of the first piezoelectric film and the perovskite oxide that is the main component of the second piezoelectric film may be composed of the same constituent elements.
• the first perovskite oxide is Pb a ⁇ (Zr x Ti 1-x ) 1-y M y ⁇ O 3 M is a metal element selected from the group consisting of V, Nb, Ta, Sb, Mo and W; 0 ⁇ x ⁇ 1, 0 ⁇ y ⁇ 1, 0.9 ⁇ a ⁇ 1.2 It is preferable that:
• the second perovskite oxide is preferably conductive.
• the second perovskite oxide preferably has a lattice constant of 0.39 nm to 0.405 nm when considered as a pseudocubic crystal.
• the second perovskite oxide is preferably SrRuO3 or BaRuO3 .
• a seed layer is provided between the first electrode and the first piezoelectric film, and that the first piezoelectric film is the other piezoelectric film.
• the metal element M in the first perovskite oxide is Nb, and that at least y in the composition ratio of the first perovskite oxide contained in the first piezoelectric film and the second piezoelectric film is the same.
• an electric field having the same direction as the polarization direction of one piezoelectric film is applied to one piezoelectric film, and an electric field having the opposite direction to the electric field applied to one piezoelectric film is applied to the other piezoelectric film.
• the second electrode may be maintained at ground potential, and the first electrode and the third electrode may be drive electrodes for applying a drive voltage to the first piezoelectric film and the second piezoelectric film.
• the first electrode and the third electrode may be maintained at ground potential, and the second electrode may be a drive electrode for applying a drive voltage to the first piezoelectric film and the second piezoelectric film.
• the first electrode and the third electrode may be connected.
• the actuator of the present disclosure is an actuator including the piezoelectric element of the present disclosure and a drive circuit that applies a drive voltage to the piezoelectric element,
• the drive circuit applies an electric field to one of the piezoelectric films in the same direction as the polarization direction of the one piezoelectric film, and applies an electric field to the other piezoelectric film in the opposite direction to the electric field applied to the one piezoelectric film.
• the technology disclosed herein makes it possible to provide piezoelectric elements and actuators that provide high piezoelectric performance at low voltages at low cost.
• FIG. 2 is a cross-sectional view of a piezoelectric element according to an embodiment.
• FIG. 2A is a graph showing the polarization-voltage hysteresis curve of one piezoelectric film
• FIG. 2B is a graph showing the polarization-voltage hysteresis curve of the other piezoelectric film.
• FIG. 11 is a cross-sectional view of a piezoelectric element according to a modified example.
• FIG. 2 is a diagram showing a schematic configuration of an actuator.
• 13A and 13B are diagrams illustrating a schematic configuration of an actuator according to a modified example.
• 13A and 13B are diagrams showing a schematic configuration of an actuator including a piezoelectric element according to a modified example.
• FIG. 11 is a cross-sectional view of a piezoelectric element according to a modified example.
• 1A to 1C are diagrams for explaining problems with a multi-layer piezoelectric element.
• 9A is a hysteresis curve of the first piezoelectric film 114
• FIG. 9B is a hysteresis curve of the second piezoelectric film 118
• FIG. 9C is a diagram showing the amount of displacement of the piezoelectric element 101 relative to the voltage.
• 5A to 5C are diagrams illustrating the effect of the piezoelectric element according to the present embodiment.
• 13 is a diagram showing polarization-voltage hysteresis curves of the first piezoelectric film and the second piezoelectric film of Comparative Example 1.
• FIG. 4 is a diagram showing polarization-voltage hysteresis curves of the first piezoelectric film and the second piezoelectric film of Example 1.
• FIG. 1 is a graph showing the voltage dependence of the piezoelectric constant d31 in the low voltage region for the piezoelectric elements of the examples and the comparative examples.
• FIG. 1 is a schematic cross-sectional view showing the layer structure of a piezoelectric element 1 according to one embodiment.
• the piezoelectric element 1 includes a first electrode 12, a first piezoelectric film 14, a second electrode 16, a second piezoelectric film 18, and a third electrode 20, arranged in this order on a substrate 10.
• the substrate 10 is not particularly limited, and examples thereof include substrates such as silicon, glass, stainless steel, yttrium-stabilized zirconia, alumina, sapphire, and silicon carbide.
• the substrate 10 may be a laminate substrate such as a silicon substrate with a thermally oxidized film in which a SiO2 oxide film is formed on the surface of a silicon substrate.
• the substrate 10 may also be a resin substrate such as PET (polyethylene terephthalate), PEN (polyethylene naphthalata), and polyimide.
• the first electrode 12 is formed on the substrate 10.
• the main component of the first electrode 12 is not particularly limited, and examples thereof include metals or metal oxides such as Au (gold), Pt (platinum), Ir (iridium), Ru (ruthenium), Ti (titanium), Mo (molybdenum), Ta (tantalum), and Al (aluminum), as well as combinations of these.
• ITO Indium Tin Oxide
• ITO Indium Tin Oxide
• the second electrode 16 is laminated on the first piezoelectric film 14, and the third electrode 20 is laminated on the second piezoelectric film 18.
• the first electrode 12 and the second electrode 16 are paired to apply a voltage to the first piezoelectric film 14.
• the second electrode 16 and the third electrode 20 are paired to apply a voltage to the second piezoelectric film 18.
• the main components of the second electrode 16 and the third electrode 20 are not particularly limited, and may include the materials exemplified for the first electrode 12, as well as electrode materials commonly used in semiconductor processes, such as Cr, and combinations of these.
• the thickness of the first electrode 12, the second electrode 16, and the third electrode 20 is not particularly limited, but is preferably about 50 nm to 300 nm, and more preferably 100 nm to 300 nm.
• the first piezoelectric film 14 and the second piezoelectric film 18 each mainly comprise a perovskite oxide represented by the general formula ABO3 .
• the main component refers to a component that occupies 80 mol% or more. It is preferable that the first piezoelectric film 14 and the second piezoelectric film 18 each comprise a perovskite oxide that occupies 90 mol% or more, and it is more preferable that the first piezoelectric film 14 and the second piezoelectric film 18 are made of a perovskite oxide (however, containing inevitable impurities).
• the perovskite oxide that is the main component of the first piezoelectric film 14 and the perovskite oxide that is the main component of the second piezoelectric film 18 may be made of different constituent elements, or the constituent elements of each may be the same. Note that “the constituent elements are the same” means that the elements contained are the same, but the composition ratio may be different.
• the first piezoelectric film 14 and the second piezoelectric film 18 are preferably mainly composed of a perovskite oxide containing Pb (lead) at the A site, and Zr (zirconium), Ti (titanium) and a metal element M at the B site.
• This perovskite oxide is represented by the following general formula. Pb a ⁇ (Zr x Ti 1-x ) 1-y M y ⁇ O 3
• the metal element M is one or more elements selected from V (vanadium), Nb (niobium), Ta (tantalum), Sb (antimony), Mo (molybdenum) and W (tungsten).
• Pb a ⁇ (Zr x Ti 1-x ) 1-y M y ⁇ O 3 is referred to as M-added PZT.
• M-added PZT when the metal element M is Nb, it is referred to as Nb-added PZT.
• the molar ratio a of Pb is 1 as a standard, but if it is in the range of 0.9 ⁇ a ⁇ 1.2, a perovskite structure can be obtained.
• the metal element M may be a single element such as only V or only Nb, or it may be a combination of two or more elements such as a mixture of V and Nb, or a mixture of V, Nb and Ta. When the metal element M is one of these elements, it can be combined with the A-site element Pb to achieve an extremely high piezoelectric constant.
• Pb a ⁇ (Zr x Ti 1-x ) 1-y M y O 3 , in which the metal element M is Nb, is optimal.
• y>0.1 a higher piezoelectric constant can be obtained.
• a piezoelectric film is formed by a vapor phase growth method such as sputtering using Nb-added PZT, in which M is Nb, a piezoelectric film with an extremely high piezoelectric constant can be obtained.
• both the first piezoelectric film 14 and the second piezoelectric film 18 have Nb-added PZT as the main component.
• the Nb composition ratios of the Nb-added PZT, which is the main component of the first piezoelectric film 14, and the Nb-added PZT, which is the main component of the second piezoelectric film 18, are the same.
• the composition ratio being the same means that they are equal within the range of measurement error.
• ICP Inductively Coupled Plasma
• XRF X-ray Fluorescence
• PZT-based perovskite oxides exhibit high piezoelectric properties at or near the morphotropic phase boundary (MPB).
• the MPB composition is when Zr:Ti (molar ratio) is near 52:48, and in the above general formula, the MPB composition or its vicinity is preferable.
• MPB or its vicinity refers to the region where a phase transition occurs when an electric field is applied to the piezoelectric film.
• the thickness t1 of the first piezoelectric film 14 and the thickness t2 of the second piezoelectric film 18 are preferably 0.2 ⁇ m or more and 5 ⁇ m or less, and more preferably 1 ⁇ m or more.
• the thicknesses of the first piezoelectric film 14 and the second piezoelectric film 18 may be the same or different.
• the thicknesses of the first piezoelectric film 14 and the second piezoelectric film 18 are preferably 2 ⁇ m or less.
• the piezoelectric element 1 of this embodiment has a seed layer 13 that is mainly composed of a second perovskite oxide between the first electrode 12 and the first piezoelectric film 14.
• the second perovskite oxide is lattice-matched with the first perovskite oxide.
• the second perovskite oxide is a perovskite oxide with a different composition from the first perovskite oxide (the perovskite oxide that is the main component of the first piezoelectric film 14 and the perovskite oxide that is the main component of the second piezoelectric film 18).
• the crystallinity of the piezoelectric film (here, the first piezoelectric film 14) formed on the upper layer can be improved.
• the piezoelectric film formed on the seed layer 13 becomes an epitaxial film that has been epitaxially grown.
• the second perovskite oxide preferably has a lattice constant of 0.39 nm to 0.405 nm when considered as a pseudocubic crystal.
• the lattice constant is in this range, it is lattice-matched to the perovskite oxide made of M-doped PZT, so when the first perovskite oxide is M-doped PZT, the crystallinity of the piezoelectric film formed in the upper layer can be improved.
• the second perovskite type oxide examples include SrRuO 3 , BaRuO 3 and PbTiO 3. These have electrical conductivity and, when considered as pseudocubic, have a lattice constant of 0.39 nm to 0.405 nm.
• the second perovskite oxide which is the main component of the seed layer 13, is preferably conductive.
• being conductive means that the electrical resistivity is sufficient to function as an electrode, and here, a material is considered to be conductive when its electrical resistivity at 20° C. is 10 ⁇ 5 ⁇ m or less. If the seed layer 13 is conductive, it can function as an electrode for applying a drive voltage to the piezoelectric film together with the electrode (the first electrode 12 in this example) below the seed layer 13.
• Figure 2A is a schematic diagram of a P-V hysteresis curve obtained by grounding the second electrode 16, which is the lower electrode of the pair of electrodes 16, 20 sandwiching the second piezoelectric film 18, using the third electrode 20, which is the upper electrode, as the drive electrode, and applying a sweep voltage to the second piezoelectric film 18.
• Figure 2B is a schematic diagram of a hysteresis curve obtained by grounding the first electrode 12, which is the lower electrode of the pair of electrodes 12, 16 sandwiching the first piezoelectric film 14, using the second electrode 16, which is the upper electrode, as the drive electrode, and applying a sweep voltage to the first piezoelectric film 14.
• the second piezoelectric film 18 is one of the piezoelectric films, and in order to distinguish it from the second piezoelectric film 18 in the piezoelectric element 2 in FIG. 3 described later, the second piezoelectric film 18 in the piezoelectric element 1 is shown as the second piezoelectric film 18f.
• the first piezoelectric film 14 is one of the piezoelectric films, and in order to distinguish it from the first piezoelectric film 14 in the piezoelectric element 2 in FIG. 3 described later, the first piezoelectric film 14 in the piezoelectric element 1 is shown as the first piezoelectric film 14r.
• the second piezoelectric film 18 is a film in which spontaneous polarization is aligned upward in the film thickness direction when no external electric field is applied, and is polarized upward in the film thickness direction.
• Arrow P2 in FIG. 1 indicates the direction of polarization, and will be referred to as polarization direction P2 below.
• the substrate 10 is used as the reference, and the direction away from the substrate 10 is defined as up, and the direction toward the substrate 10 is defined as down.
• Whether the spontaneous polarization in the piezoelectric film is aligned and the direction in which the spontaneous polarization is aligned can be confirmed by measuring the P-V hysteresis curve (or P-E hysteresis curve), which indicates the polarization-voltage characteristics (or polarization-electric field characteristics) of the piezoelectric film.
• the second piezoelectric film 18 is a film in which the spontaneous polarization direction is aligned immediately after film formation without being subjected to poling, and is polarized in the film thickness direction.
• the reason why the spontaneous polarization direction is aligned in the piezoelectric film without being subjected to poling and without an external electric field is thought to be because an electric field (hereinafter referred to as a spontaneous internal electric field) caused by distortion or defects in the crystal structure is generated in the piezoelectric film.
• the second piezoelectric film 18 has spontaneous polarization aligned in the film thickness direction due to such spontaneous internal electric field.
• the second piezoelectric film 18 is a piezoelectric film polarized in the film thickness direction when no external electric field is applied because the spontaneous polarization is aligned in the film thickness direction.
• the piezoelectric film is composed of a large number of spontaneously polarized domains.
• spontaneous polarization aligned in a specific direction means a state in which the spontaneous polarization components in a specific direction are relatively more than the spontaneous polarization components in other directions. If the spontaneous polarization component in a particular direction in a piezoelectric film is relatively greater than the spontaneous polarization components in other directions, the film is polarized in that particular direction.
• the spontaneous polarization component in a particular direction is the largest component in the entire film (i.e., if the film is polarized in a particular direction), the spontaneous polarization is considered to be aligned in a particular direction even if spontaneous polarizations in various directions exist in the piezoelectric film.
• the polarization direction P2 may be referred to as the direction P2 in which the spontaneous polarization is aligned.
• the P-E hysteresis curve (or P-V hysteresis curve) is shaped so that its center coincides with the origin.
• the direction of spontaneous polarization is aligned with the spontaneous internal electric field, so the center of the hysteresis curve deviates (shifts) from the origin.
• the amount of shift of the center of the hysteresis curve from the origin is proportional to the degree of alignment of the spontaneous polarization, and the greater the shift, the stronger the spontaneous internal electric field. Furthermore, the direction in which the spontaneous polarization is aligned can be determined by the shift direction of the hysteresis curve from the origin. In the case of a P-V hysteresis curve, the center of the hysteresis curve is defined as the midpoint between two coercive voltages, which will be described later.
• the coercive voltage is the voltage at which the polarization becomes zero in the hysteresis curve, and as shown in Figures 2A and 2B, one hysteresis curve has two coercive voltages.
• the positive coercive voltage refers to the coercive voltage on the relatively positive voltage side (right side in the figure) of the two coercive voltages
• the negative coercive voltage refers to the coercive voltage on the relatively negative voltage side (left side in the figure).
• the positive coercive voltage is Vcf + and the negative coercive voltage is Vcf- .
• the positive coercive voltage Vcf + and the negative coercive voltage V cf - are both positive (+) signs.
• the positive coercive voltage and the negative coercive voltage having the same sign means that the P-V hysteresis does not include the origin in the inner region of the hysteresis curve, as shown in Fig. 2A.
• the two coercive voltages have the same sign in the second piezoelectric film 18f, which is one of the piezoelectric films, but it is also possible that the positive coercive voltage is a positive value and the negative coercive voltage is a negative value.
• the absolute value of the difference in coercive voltages Vcf + -Vcf - is the hysteresis width ⁇ Vcf, and half the sum of the coercive voltages (Vcf + +Vcf - )/2 indicates the center Hcf of the hysteresis.
• the center Hcf of the hysteresis is shifted from the origin toward the positive voltage side, which indicates that the second piezoelectric film 18f has a spontaneous internal electric field.
• the shift of the center Hcf of the hysteresis toward the positive voltage side indicates that a spontaneous internal electric field is generated in the opposite direction to the electric field when a positive voltage is applied.
• the spontaneous internal electric field is in the upward direction from the substrate side in the film thickness direction, and the spontaneous polarization is aligned in the upward direction in the film thickness direction.
• FIG. 2A shows the case where a positive potential is applied to the third electrode 20, which is the upper electrode of the second piezoelectric film 18f, as a positive voltage, and the case where a negative potential is applied as a negative voltage.
• a positive voltage is a state in which an electric field is generated from the third electrode 20 toward the second electrode 16, i.e., a downward electric field toward the substrate 10 is generated in the second piezoelectric film 18f
• a negative voltage is a state in which an electric field is generated from the second electrode 16 toward the third electrode 20, i.e., an upward electric field away from the substrate 10 is generated in the second piezoelectric film 18f.
• a positive voltage is a state in which an electric field is generated from the second electrode 16 to the first electrode 12, i.e., a downward electric field is generated in the first piezoelectric film 14r toward the substrate 10
• a negative voltage is a state in which an electric field is generated from the first electrode 12 to the second electrode 16, i.e., an upward electric field is generated in the first piezoelectric film 14r away from the substrate 10.
• the first piezoelectric film 14r is provided on the seed layer 13.
• Vcr + and the negative coercive voltage is Vcr ⁇
• the unit of coercive voltage is [V].
• the right side of the above equation is the absolute value of the difference in coercive voltage, and indicates the width ⁇ Vcr of the hysteresis curve.
• the center of the hysteresis curve is expressed as (Vcr ++ Vcr - )/2. Therefore, the absolute value of (Vcr ++ Vcr - )/2 indicates the amount of shift of the hysteresis curve.
• the first piezoelectric film 14r is polarized in the film thickness direction, and the direction of polarization P1 is opposite to the direction of polarization P2 of the second piezoelectric film 18f. That is, in the first piezoelectric film 14, the spontaneous polarization is aligned in the opposite direction to the direction of the spontaneous polarization of the second piezoelectric film 18f.
• the first piezoelectric film 14 is a film whose spontaneous polarization is aligned by a poling process and is polarized in the opposite direction to the polarization direction P2 of the second piezoelectric film 18f.
• the center Hcr of the hysteresis curve of the first piezoelectric film 14r is shifted from the origin toward the positive voltage side, indicating that it has a spontaneous internal electric field.
• the first piezoelectric film 14r has an internal spontaneous electric field, similar to the second piezoelectric film 18f.
• the width of the hysteresis curve of the first piezoelectric film 14 is larger than the width of the hysteresis curve of the second piezoelectric film 18 . That is, the first piezoelectric film 14 and the second piezoelectric film 18 are
• the hysteresis curve of the first piezoelectric film 14 includes the origin and has a large width, so it can be polarized by performing a poling process in which an electric field in the opposite direction to the internal spontaneous electric field is applied.
• polarization residual polarization Pr
• the first piezoelectric film 14, which is the other piezoelectric film formed on the seed layer 13, is a piezoelectric film that is polarized in the same direction as the polarization direction of the second piezoelectric film 18, which is the other piezoelectric film, due to a spontaneous internal electric field when it is provided directly on the first electrode 12 without going through the seed layer 13.
• the first piezoelectric film 14 becomes a film that exhibits the hysteresis shown in FIG. 2B by being formed on the seed layer 13, but when it is formed on the seed layer 13 and formed on the first electrode 12, it is made of a piezoelectric material that becomes a film that exhibits the same or similar hysteresis as that shown in FIG. 2A.
• the second piezoelectric film 18f and the first piezoelectric film 14r having the hysteresis characteristics shown in Figures 2A and 2B can be obtained by sputtering using the same target of Nb-doped PZT, for example. Even when the same target is used, piezoelectric films with different crystallinity can be obtained by depositing the first piezoelectric film 14r on the seed layer 13, depositing the second piezoelectric film 18f, and depositing them on the second electrode 16 without the seed layer 13.
• the piezoelectric film deposited on the seed layer 13 has higher crystallinity than a piezoelectric film deposited on an electrode without the seed layer 13, has a perovskite structure with high orientation, and shows a hysteresis curve with a wide hysteresis width as shown in Figure 2B.
• the piezoelectric element 1 is driven by applying a voltage to the second piezoelectric film 18f in the area surrounded by a two-dot chain line in FIG. 2A, and to the first piezoelectric film 14r in the area surrounded by a two-dot chain line in FIG. 2B.
• an electric field is applied to both the second piezoelectric film 18f and the first piezoelectric film 14r in the same direction as the spontaneous polarization, so that good piezoelectric performance can be obtained in the low voltage region.
• the piezoelectric element 1 is composed of two piezoelectric films, each of which is mainly composed of a perovskite oxide, stacked with an electrode between them, one of the two piezoelectric films (in this example, the second piezoelectric film 18) is formed on an electrode (in this example, the second electrode 16), and the other piezoelectric film (in this example, the first piezoelectric film 14) is formed on a seed layer 13 which is formed on an electrode (in this example, the first electrode 12).
• One of the piezoelectric films has spontaneous polarization aligned in the film thickness direction due to a spontaneous internal electric field, and the other piezoelectric film has a hysteresis curve which includes the origin inside the loop and has a hysteresis width which is wider than the hysteresis width of the one of the piezoelectric films.
• the piezoelectric element 1 when the piezoelectric element 1 is driven by applying an electric field in the same direction as the polarization to one piezoelectric film (second piezoelectric film 18 in this example) and an electric field in the opposite direction to the electric field applied to one piezoelectric film to the other piezoelectric film (first piezoelectric film 14 in this example), good piezoelectric performance can be obtained in the low voltage region.
• the low voltage region is a voltage region suitable for incorporation into consumer devices, specifically a voltage region with an absolute value of 12 V or less. It is preferable to obtain high piezoelectric performance at a voltage of 7 V or less, and even 5 V or less.
• the first piezoelectric film 14 and the second piezoelectric film 18 are
• the hysteresis width of one piezoelectric film (here, the second piezoelectric film 18f) formed on an electrode without a seed layer is 9 V or less, the shift amount is 3 V or less, and the hysteresis width is large and narrow
• the hysteresis width of the other piezoelectric film (here, the first piezoelectric film 14r) formed on the seed layer is 1.4 times or more and 2.0 times or less than the hysteresis width of one piezoelectric film (see the examples below).
• the hysteresis width of one piezoelectric film (here, the second piezoelectric film 18f) formed on an electrode without a seed layer is 9 V or less
• the shift amount is large and the hysteresis width is narrow, such as 3 V or less
• the effect of improving piezoelectric performance in the low voltage region is particularly high compared to the case where two piezoelectric films having the same hysteresis curve are provided.
• the first piezoelectric film 14 and the second piezoelectric film 18 are both primarily composed of perovskite oxide made of M-doped PZT, so the first piezoelectric film 14 and the second piezoelectric film 18 can be formed using a single target.
• costs can be reduced compared to using different targets.
• the piezoelectric element 1 has a seed layer 13 between the first piezoelectric film 14 and the first electrode 12, but as in the piezoelectric element 2 shown in FIG. 3, the seed layer 13 may not be provided between the first piezoelectric film 14 and the first electrode 12, and a seed layer may be provided between the second piezoelectric film 18 and the second electrode 16.
• the same components as those in FIG. 1 are given the same reference numerals.
• the first piezoelectric film 14 formed on the first electrode 12 without the seed layer 13 is a piezoelectric film polarized in the direction indicated by the arrow P21 due to the spontaneous polarization being aligned in the film thickness direction due to the spontaneous internal electric field.
• the first piezoelectric film 14 exhibits the hysteresis characteristic shown in FIG. 2A.
• the second piezoelectric film 18 formed on the seed layer 13 is a piezoelectric film exhibiting the hysteresis characteristic shown in FIG. 2B.
• the second piezoelectric film 18 is polarized in the direction P22 opposite to the polarization direction 21 of the first piezoelectric film 14.
• the spontaneous polarization of the second piezoelectric film 18 is aligned in the opposite direction to the direction P21 in which the spontaneous electrodes of the first piezoelectric film 14 are aligned.
• the first piezoelectric film 14 corresponds to one of the piezoelectric films in the claims
• the second piezoelectric film 18 corresponds to the other piezoelectric film.
• the first piezoelectric film 14, which is one of the piezoelectric films may be referred to as the first piezoelectric film 14f
• the second piezoelectric film 18, which is the other piezoelectric film may be referred to as the second piezoelectric film 18r.
• the piezoelectric element 1 shown in FIG. 1 it is more preferable to provide a seed layer 13 under the first piezoelectric film 14 arranged on the substrate 10 side.
• the crystallinity of the first piezoelectric film 14 is improved, thereby suppressing surface roughness. If the surface roughness of the first piezoelectric film 14 arranged on the substrate side is small, the surface roughness of the second piezoelectric film 18 formed thereafter can also be suppressed. The smaller the surface roughness of the first piezoelectric film 14 and second piezoelectric film 18, the higher the piezoelectric characteristics tend to be.
• FIG. 4 shows a schematic configuration of an actuator 5 equipped with a piezoelectric element 1.
• the actuator 5 includes a piezoelectric element 1 and a drive circuit 30.
• the drive circuit 30 is a means for supplying a drive voltage to the first piezoelectric film 14r and the second piezoelectric film 18f sandwiched between the electrodes in order to drive the piezoelectric element 1.
• the second electrode 16 is connected to the ground terminal (GND) of the drive circuit 30, and the first electrode 12 and the third electrode 20 are connected to the drive voltage output terminal (-V) of the drive circuit 30.
• the drive circuit 30 applies electric fields in opposite directions to the first piezoelectric film 14r and the second piezoelectric film 18f.
• the drive circuit 30 applies an electric field Ef in the same direction as the direction (polarization direction) P2 in which the spontaneous polarization is aligned to the second piezoelectric film 18f, which is one of the piezoelectric films, and applies an electric field Er in the opposite direction to the electric field Ef applied to the second piezoelectric film 18f to the first piezoelectric film 14r, which is the other piezoelectric film.
• the drive circuit 30 is a negative drive circuit that gives a negative potential to the drive electrode (here, the second electrode 16).
• the second electrode 16 of the piezoelectric element 1 is at ground potential, and the first electrode 12 and the third electrode 20 are used as drive electrodes.
• a negative potential (-V) is applied to the drive electrodes (here, the first electrode 12 and the third electrode 20).
• an electric field Ef in the same direction as the aligned direction (polarization direction) P2 of the spontaneous polarization is applied to the second piezoelectric film 18, which is one of the piezoelectric films, and an electric field Er in the opposite direction to the electric field Ef applied to the second piezoelectric film 18 is applied to the first piezoelectric film 14, which is the other piezoelectric film.
• first electrode 12 and the third electrode 20 of the piezoelectric element 1 are connected. If the first electrode 12 and the third electrode 20 are connected, drive control is easy.
• the drive circuit 32 may include a positive drive circuit that applies a positive potential to the drive electrode.
• the first electrode 12 and the third electrode 20 are connected to the ground terminal of the drive circuit 32, and the second electrode 16 is connected to the drive voltage output terminal of the drive circuit 32.
• the first electrode 12 and the third electrode 20 are at ground potential, and the second electrode 16 functions as a drive electrode.
• the drive circuit 32 can apply an electric field Ef to the second piezoelectric film 18f in the same direction as the direction (polarization direction) P2 in which the spontaneous polarization is aligned, and apply an electric field Er to the first piezoelectric film 14r in the opposite direction to the electric field Ef applied to the second piezoelectric film 18f.
• the actuators 5 and 6 are equipped with only one polarity drive circuit as the drive circuits 30 and 32, and can be realized at low cost.
• the actuators 5 and 6 are equipped with the above-mentioned piezoelectric element 1, so they can obtain high piezoelectric performance in the low voltage range.
• the actuator 7 shown in FIG. 6 includes the piezoelectric element 2 shown in FIG. 3.
• the piezoelectric element 2 includes a first piezoelectric film 14f, which is one of the piezoelectric films having the hysteresis curve shown in FIG. 2A, and a second piezoelectric film 18r, which is the other of the piezoelectric films having the hysteresis curve shown in FIG. 2B. In this case, as shown in FIG.
• an electric field Ef in the same direction as the polarization direction P21 is applied to the first piezoelectric film 14f, which is one of the piezoelectric films, and an electric field Er in the opposite direction to the electric field Ef applied to the first piezoelectric film 14f is applied to the second piezoelectric film 18r, which is the other of the piezoelectric films, thereby making it possible to obtain good piezoelectric performance in the low voltage region using a drive circuit of one polarity.
• the first electrode 12 and the third electrode 20 are connected to the ground terminal of the drive circuit 34, and the second electrode 16 is connected to the drive voltage output terminal of the drive circuit 34. That is, the first electrode 12 and the third electrode 20 are at ground potential, and the second electrode 16 functions as a drive electrode.
• the drive circuit 34 is a negative drive circuit that applies a negative potential to the second electrode 16, which is a drive electrode, to perform negative drive.
• the drive circuit 34 applies an electric field Ef in the same direction as the polarization direction P21 to the first piezoelectric film 14f, which is one of the piezoelectric films, and applies an electric field Er in the opposite direction to the electric field Ef applied to the first piezoelectric film 14f to the second piezoelectric film 18r, which is the other piezoelectric film.
• the actuator may be configured to include a piezoelectric element 2 and a positive drive circuit, with the second electrode 16 connected to a ground terminal to provide a ground potential, and the first electrode 12 and the third electrode 20 connected to a drive power output terminal to function as drive electrodes.
• the piezoelectric elements 1 and 2 described above are both two-layer laminated piezoelectric elements in which two layers of piezoelectric films are laminated, but the piezoelectric element of the present disclosure is not limited to two layers and may have three or more layers of piezoelectric films.
• a first piezoelectric film 14 (14r) formed on the seed layer 13 and a second piezoelectric film 18 (18f) formed on the second electrode 16 may be alternately provided in multiple places, or a first piezoelectric film 14 (14f) formed on the first electrode 12 and a second piezoelectric film 18 (18r) formed on the seed layer 13 may be alternately provided in multiple places.
• first electrode 7 has a first electrode 12, a seed layer 13, a first piezoelectric film 14r, a second electrode 16, a second piezoelectric film 18f, a third electrode 20, a seed layer 13, a first piezoelectric film 14r, a second electrode 16, a second piezoelectric film 18f, and a third electrode 20 laminated in this order on a substrate 10.
• one piezoelectric film here, the second piezoelectric film 18
• the other piezoelectric film here, the first piezoelectric film 14 having the hysteresis curve shown in FIG. 2B may be provided in multiple layers, alternately arranged with electrodes interposed between them.
• the piezoelectric performance of a piezoelectric element 101 will be described, which has a laminated structure similar to that of the piezoelectric element 1 shown in FIG. 1, but in which the first piezoelectric film 114 and the second piezoelectric film 118 are both piezoelectric films that exhibit the hysteresis curve shown in FIG. 2A.
• the piezoelectric element 101 shown in FIG. 8 does not have a seed layer 13 under either the first piezoelectric film 114 or the second piezoelectric film 118, and both the first piezoelectric film 114 and the second piezoelectric film 118 have spontaneous polarization that is aligned upward in the film thickness direction due to a spontaneous internal electric field, and the polarization directions P31 and P32 are both upward in the film thickness direction.
• the first piezoelectric film 114 and the second piezoelectric film 118 are piezoelectric films whose main component is a first perovskite oxide, for example, a Nb-added PZT film.
• the drive circuit 30 which is a negative drive circuit, for example, as shown in FIG. 8
• the second electrode 16 is set to ground potential
• the first electrode 12 and the third electrode 20 are set to drive electrodes. This results in electric fields being applied to the first piezoelectric film 114 and the second piezoelectric film 118 in opposite directions.
• the polarization gradually decreases as the driving potential changes from 0 to -V, and then, after the polarization is reversed by the coercive voltage, the polarization in the same direction as the electric field Er gradually increases. Therefore, as shown by the dashed line in FIG. 9C, the first piezoelectric film 114 is displaced in the opposite direction up to the coercive voltage at which the polarization value becomes zero.
• the change in the displacement amount of the piezoelectric element 101 behaves as the sum of the displacement amounts of both the first piezoelectric film 114 and the second piezoelectric film 118.
• the piezoelectric element 101 as a whole can obtain a very large displacement amount on the high voltage side, but the displacement amount in the low voltage region is smaller than the displacement amount obtained with only one piezoelectric film.
• the piezoelectric element 101 there was a problem in that the piezoelectric characteristics in the low voltage region were low.
• the piezoelectric element 1 having the configuration shown in FIG. 1 has a seed layer 13 below the first piezoelectric film 14.
• the first piezoelectric film 14r is formed on the seed layer 13, which improves the crystallinity and increases the hysteresis width compared to the hysteresis curve obtained when the film is formed directly on the electrode.
• the first piezoelectric film 14r has a larger hysteresis width compared to the second piezoelectric film 18f, as shown in FIG. 2B, and shows a hysteresis curve that includes the origin.
• the first piezoelectric film 14r of the piezoelectric element 1 is in a poled state, and the spontaneous polarization is aligned in a direction P1 opposite to the direction P2 in which the spontaneous polarization of the second piezoelectric film 18f is aligned (see FIG. 1). Therefore, an electric field Er in the same direction as the direction P1 in which the spontaneous polarization is aligned is also applied to the first piezoelectric film 14r. Since there is no need to reverse the polarization direction P1 in the first piezoelectric film 14r, the piezoelectric element 1 can obtain higher piezoelectric characteristics than the piezoelectric element 101 in the low voltage region.
• the dashed line shows the change in the amount of displacement when the piezoelectric element 101 in Figure 8 is driven by applying a driving potential of 0 to -V to the driving electrode
• the solid line shows the change in the amount of displacement when the piezoelectric element 1 in Figure 3 is driven by applying a driving potential of 0 to -V to the driving electrode.
• Figure 10 is an enlarged view of the low voltage region surrounded by the two-dot chain line in Figure 9C. As shown in Figure 10, the piezoelectric element 1 can obtain a larger displacement in the low voltage region compared to the piezoelectric element 101.
• a piezoelectric element (see Figure 8) was fabricated that had a first electrode, a first piezoelectric film, a second electrode, a second piezoelectric film, and a third electrode arranged in that order on a substrate.
• a piezoelectric element (see Figure 1) was fabricated that had a first electrode, a seed layer, a first piezoelectric film, a second electrode, a second piezoelectric film, and a third electrode arranged in that order on a substrate.
• piezoelectric elements (see FIG. 3) were fabricated that had a first electrode, a first piezoelectric film, a second electrode, a seed layer, a second piezoelectric film, and a third electrode in that order on a substrate, as shown in FIG. 3.
• a sputtering device was used to deposit each layer.
• the deposition conditions for each layer were as follows:
• a silicon substrate with a thermal oxide film was used as the substrate 10.
• a first electrode was formed on the substrate by sputtering. Specifically, as the first electrode, a 50 nm thick TiW layer and a 200 nm thick Ir layer were laminated in this order on the substrate.
• the sputtering conditions for each layer were as follows:
• Target-substrate distance 100 mm
• Target input power 600W
• Ar gas pressure 0.1 Pa
• Substrate temperature setting 350° C.
• a seed layer having a thickness of 30 nm was formed under the following sputtering conditions on the first electrode in Examples 1 to 4, and on the second electrode in Examples 5 to 8.
• the seed layer in each example was as shown in Table 1.
• SRO is SrRuO3
• BRO is BaRuO3
• PTO is PbTiO3 .
• Target-substrate distance 100 mm
• Target input power 500W Degree of vacuum: 0.3 Pa
• Ar/ O2 mixed atmosphere O2 volume fraction 10%
• Substrate temperature setting 600° C.
• the film thickness of the first piezoelectric film was as shown in Table 1 for each example and comparative example. The film thickness was adjusted by changing the film formation time.
• a Nb-doped PZT film with 12 at % Nb doping in the B site was formed as the second piezoelectric film on the second electrode in the comparative example and examples 1 to 4, and on the seed layer in the examples 5 to 8.
• the target and sputtering conditions were the same as those for the first piezoelectric film.
• the laminates for the comparative example and the example were produced using the above process.
• evaluation sample 1- A rectangular piece measuring 2 mm ⁇ 25 mm was cut out from the laminate to prepare a cantilever as evaluation sample 1.
• the polarization-voltage (P-V) hysteresis curve was measured using the evaluation sample 2.
• a voltage was applied until the polarization was saturated under the condition of a frequency of 1 kHz, and measurements were performed.
• the first electrode 12 was grounded, and a sweep voltage was applied to the first piezoelectric film 14 using the second electrode 16 as a driving electrode.
• the second electrode 16 was grounded, and a sweep voltage was applied to the second piezoelectric film 18 using the third electrode 20 as a driving electrode.
• FIG. 11 shows the P-V hysteresis curves for the first and second piezoelectric films of Comparative Example 1.
• the hysteresis curve of the first piezoelectric film is shown by a dashed line
• the hysteresis curve of the second piezoelectric film is shown by a solid line.
• the positive coercive voltage Vc1 + is 7.9V
• the negative coercive voltage Vc1- is -0.7V.
• Comparative Example 1 is an example that does not include a seed layer, for convenience, in Table 2, the second piezoelectric film is one piezoelectric film and the first piezoelectric film is the other piezoelectric film.
• Fig. 12 shows P-V hysteresis curves for the first and second piezoelectric films of Example 1.
• the hysteresis curve for the first piezoelectric film is shown by a dashed line
• the hysteresis curve for the second piezoelectric film is shown by a solid line.
• the positive coercive voltage Vc1 + is 13V
• the negative coercive voltage Vc1- is -4.2V
• the positive coercive voltage Vc2 + is 7.3V
• the negative coercive voltage Vc1- is -1.1V.
• the first piezoelectric film corresponds to the other piezoelectric film
• the second piezoelectric film corresponds to one of the piezoelectric films. That is, in Examples 1 to 4, the positive side coercive voltage Vc1 + of the first piezoelectric film corresponds to the positive side coercive voltage Vcr + of the other piezoelectric film, and the negative side coercive voltage Vc1- of the first piezoelectric film corresponds to the positive side coercive voltage Vcr- of the other piezoelectric film.
• the positive side coercive voltage Vc2 + of the second piezoelectric film corresponds to the positive side coercive voltage Vcf + of one of the piezoelectric films
• the negative side coercive voltage Vc2- of the second piezoelectric film corresponds to the positive side coercive voltage Vcf- of one of the piezoelectric films.
• the first piezoelectric film corresponds to one of the piezoelectric films
• the second piezoelectric film corresponds to the other piezoelectric film.
• the coercive voltage Vc1 + of the positive side of the first piezoelectric film is the coercive voltage Vcf + of the positive side of one piezoelectric film
• the coercive voltage Vc1- of the negative side of the first piezoelectric film is the coercive voltage Vcf- of the positive side of one piezoelectric film
• the coercive voltage Vc2 + of the positive side of the second piezoelectric film is the coercive voltage Vcr + of the positive side of the other piezoelectric film
• the coercive voltage Vc2- of the negative side of the second piezoelectric film is the coercive voltage Vcr- of the positive side of the other piezoelectric film.
• Table 2 shows Vc1 + , Vc1 - , Vc2 + , and Vc2 - for each example.
• Table 2 also shows the results of calculating
• the piezoelectric constant d31 was measured. Measurement of the piezoelectric constant d 31 was carried out using Evaluation Sample 1. The piezoelectric constant d 31 was measured according to the method described in I. Kanno et al. Sensor and Actuator A 107(2003)68. For Comparative Example 1 and Examples 1 to 4, the second electrode 16 was grounded, and the first electrode 12 and the third electrode 20 were used as driving electrodes, and the piezoelectric constant d 31 was measured. For Examples 5 to 8, the first electrode 12 and the third electrode 20 were grounded, and the second electrode 16 was used as a driving electrode, and the piezoelectric constant d 31 was measured.
• the piezoelectric constant d 31 was measured when the driving potential (applied potential) was -1, -3, -5, -7, -10, and -15 V.
• the piezoelectric constant d 31 when the applied potential was -1 V was measured by applying a driving signal obtained by adding a sine wave with an amplitude of 0.5 V to a bias voltage of -0.5 V to the second electrode 16. The measurement results are shown in Table 3.
• FIG. 13 is a graph showing the relationship between the driving potential and the piezoelectric constant d 31 for each piezoelectric element.
• a piezoelectric constant significantly larger than the piezoelectric constant d 31 of the comparative example was obtained when driven in the applied potential range of 0 to -15 V.
• the applied potential was 0 to -10 V
• the difference in the piezoelectric constant between the examples and the comparative example was large, and in the range of 0 to -5 V, the difference between the two was particularly noticeable.
• a first electrode, a first piezoelectric film, a second electrode, a second piezoelectric film, and a third electrode are provided on a substrate in this order;
• the first piezoelectric film and the second piezoelectric film each contain a perovskite-type oxide as a main component;
• a seed layer mainly composed of a second perovskite oxide that is lattice-matched to the first perovskite oxide is provided only between the first electrode and the first piezoelectric film or between the second electrode and the second piezoelectric film; one of the first piezoelectric film and the second piezoelectric film, which is not provided on the seed layer, is polarized in a thickness direction due to a spontaneous internal electric field;
• the other of the first and second piezoelectric films, which is disposed on the seed layer has a polarization-voltage characteristic such that, when a positive coercive voltage is Vc
• Appendix 2 2. The piezoelectric element according to claim 1, wherein a positive coercive voltage Vcf + and a negative coercive voltage Vcf- in a hysteresis curve showing the polarization-voltage characteristics of one of the piezoelectric films have the same sign.
• the first perovskite oxide is Pb a ⁇ (Zr x Ti 1-x ) 1-y M y ⁇ O 3 M is a metal element selected from the group consisting of V, Nb, Ta, Sb, Mo and W; 0 ⁇ x ⁇ 1, 0 ⁇ y ⁇ 1, 0.9 ⁇ a ⁇ 1.2 5.
• the piezoelectric element according to claim 1, wherein the second perovskite oxide has a lattice constant of 0.39 nm to 0.405 nm when considered as a pseudocubic crystal.
• An actuator comprising: a piezoelectric element according to any one of claims 1 to 14; and a drive circuit that applies a drive voltage to the piezoelectric element, An actuator in which a drive circuit applies an electric field to one piezoelectric film in the same direction as the polarization direction of the other piezoelectric film, and applies an electric field to the other piezoelectric film in the opposite direction to the electric field applied to the one piezoelectric film.

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