Patent Description:
Piezoelectric devices, which make use of the piezoelectric effect of substances, have conventionally been adopted. The piezoelectric effect is a phenomenon in which microscopic polarization is produced in response to a mechanical stress applied to a substance. Using the piezoelectric effect, various sensors including pressure sensors, acceleration sensors, and acoustic emission (AE) sensors for detecting elastic waves are commercially available.

In recent years, touch panels have been used as input interfaces of electronic devices such as smartphones, and piezoelectric devices are often applied to the touch panels. A touch panel is integrated into the display device of electronic equipment. In this case, it would be necessary to the piezoelectric layer to be highly responsive to pressure in order to accurately detect manipulations by a finger. Applications to biological sensors are also expected to detect biological signals such as pulse rate or respiratory rate. High sensitivity is again required for such applications.

A sintered film of a piezoelectric material is known, where a chemical solution containing a compound with a wurtzite crystal structure to which an alkaline earth metal such as magnesium (Mg), calcium (Ca) or the like is added is applied by a sol-gel process and sintered (see, for example, Patent Document <NUM> presented below).

Another conventional technique is to apply a paste of a mixture of MgO and varnish onto an oriented ZnO film formed by a vapor transport method, and to diffuse Mg into the oriented ZnO film by thermal diffusion (see, for example, Patent Document <NUM> presented below). In this method, the paste is applied such that the weight ratio of MgO to the oriented ZnO becomes <NUM> to <NUM> wt%. Non-Patent Document <NUM> (presented below) discloses a piezoelectric device and manufacturing method thereof according to the preamble of independent claims <NUM> and <NUM>, respectively. In particular, a solidly mounted resonator, a type of film bulk acoustic resonator for microwave operation applications is described, wherein a ternary compound magnesium doped zinc oxide (MgxZn<NUM>-xO) piezoelectric film is introduced and investigated, which has a flexible frequency response and a higher acoustic velocity. X-ray diffraction scans of MgxZn<NUM>-xO films are investigated versus different RF magnetron sputtering deposition conditions. Residual stresses are studied to ensure a reliable fabrication of solidly mounted resonators. A typical device based on MgxZn<NUM>-xO piezoelectric film with x=<NUM>% shows a resonant frequency at <NUM> with a return loss of -<NUM> dB, a quality factor of <NUM> and a squared coupling coefficient of <NUM>%. Non-Patent Documents <NUM> and <NUM> present further investigations on sputtered MgxZn<NUM>-xO thin films.

In general, a piezoelectric device has a structure in which a piezoelectric layer is sandwiched between a pair of electrodes. With this configuration, it is desired to efficiently convert the vibration in the thickness direction of the piezoelectric layer into electrical energy, or conversely, to efficiently convert the applied electrical energy into mechanical deformation. One of the objectives of the present invention is to provide a piezoelectric device having a satisfactory mechanical/electric conversion efficiency and a method of manufacturing the same.

In the present invention, a piezoelectric device having a high conversion ability between electrical energy and mechanical energy is provided by adding a predetermined amount of metal to the piezoelectric layer.

In one aspect of the invention, a piezoelectric device according to independent claim is provided.

In another aspect of the invention, a method of manufacturing a piezoelectric device according to independent claim <NUM> is provided.

Further advantageous features are set out in the dependent claims.

With an above-described configuration, a piezoelectric device having a high conversion efficiency between electrical energy and mechanical energy is provided.

<FIG> is a schematic diagram of a piezoelectric device 10A. The piezoelectric device 10A may be used as a piezoelectric sensor that produces an electric signal proportional to an externally applied pressure.

The piezoelectric device 10A has a structure in which a first electrode <NUM>, a piezoelectric layer <NUM>, and a second electrode <NUM> are stacked in this order on a substrate <NUM>. In the embodiment, the piezoelectric layer <NUM> has a wurtzite crystal structure doped with a predetermined amount of a metal.

For the substrate <NUM>, any suitable material such as a glass, a plastic, or a ceramic may be used. When a plastic substrate is used, a flexible substrate that can impart flexibility to the piezoelectric device 10A may be used. Such a plastic substrate includes, but is not limited to polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polycarbonate (PC), acrylic resin, cycloolefin polymer, polyimide (PI), and so on.

Among these materials, PET, PEN, PC, acrylic resin, and cycloolefin polymer are colorless and transparent materials, and suitably used when light transmission is required for the piezoelectric device 10A. When light transmission is not essential for the piezoelectric device 10A, for example, in applications to healthcare products including a pulse rate monitor and or a heart rate monitor, or to pressure sensor sheets for vehicles, a semitransparent or opaque plastic material may be used.

For the first electrode <NUM>, any conductive material can be used. In applications that require light transmission, a transparent oxide conductive film such as indium tin oxide (ITO), indium zinc oxide (IZO), indium zinc tin oxide (IZTO), indium gallium zinc oxide (IGZO), or the like may be used. When transparency is not required, a good conductor or a metal such as Au, Pt, Ag, Ti, Al, Mo, Ru, or Cu may be used.

From the viewpoint of suppressing roughness or crystal grain boundary at the interface between the first electrode <NUM> and the piezoelectric layer <NUM>, the oxide conductor film used for the electrode may be an amorphous film. By using an amorphous film, the surface roughness or crystal grain boundaries of the first electrode <NUM>, which may cause leakage current paths, can be reduced. Besides, the piezoelectric layer <NUM> can grow with a good crystal orientation on the first electrode <NUM>, with less influence from the crystal orientation of the underlaid first electrode <NUM>.

The piezoelectric layer <NUM> is made of an inorganic piezoelectric material which has a wurtzite crystal structure. The thickness of the piezoelectric layer <NUM> is not particularly limited; however, the thickness may be <NUM> or more. When the thickness of the piezoelectric layer <NUM> is less than <NUM>, it may be difficult to exhibit sufficient piezoelectric characteristics (or polarization proportional to applied stress).

Wurtzite crystal has a hexagonal unit cell and its polarization vector is parallel to the c-axis. Wurtzite piezoelectric materials include, but are not limited to zinc oxide (ZnO), zinc sulfide (ZnS), zinc selenide (ZnSe), zinc telluride (ZnTe), aluminum nitride (AlN), gallium nitride (GaN), cadmium selenide (CdSe), cadmium telluride (CdTe), silicon carbide (SiC), etc. Among these materials, only a single component may be used, or a combination of two or more components may be used. When two or more components are combined, the respective layers of the selected components may be stacked one by one. Such a single component or a combination of two or more components selected from the above-describe group may be used as the main component, and some other substance(s) may be optionally added as subcomponent(s) to the main component.

When ZnO, ZnS, ZnSe, and ZnTe are used as the wurtzite crystal, an alkaline earth metal such as Mg, Ca, Sr, V, Ti, Zr, or Li may be added at a predetermined ratio. These elements do not cause the piezoelectric layer to exhibit conductivity even when these elements enter the Zn site of the wurtzite crystal, and rather, these elements improve the value of the electromechanical coupling coefficient.

According to the present invention, ZnO is used as the wurtzite crystal, and Mg is added as the metal dopant, wherein the amount of Mg with respect to the total amount of Zn and Mg is <NUM> atom% to <NUM> atom%, and more preferably <NUM> atom% to <NUM> atom%. When Ca is added as an alkaline earth metal, outside the present invention, the amount of Ca with respect to the total amount of Zn and Ca is preferably <NUM> atom% to <NUM> atom%, more preferably <NUM> atom% to <NUM> atom%, and even more preferably <NUM> atom% to <NUM> atom%.

The second electrode <NUM> can be formed of any conductive material. When the piezoelectric device 10A requires light transmission, the second electrode <NUM> may be a transparent oxide conductive film such as ITO, IZO, IZTO, or IGZO. When light transmission is not required, a metal electrode may be made of a good conductor such as Au, Pt, Ag, Ti, Al, Mo, Ru, Cu, etc..

<FIG> is a schematic diagram of a sample <NUM> prepared for evaluation of the characteristics of the piezoelectric device of the embodiment. In the sample <NUM>, a Ti film <NUM> with a thickness of <NUM> is formed by DC sputtering on the substrate <NUM> of a quartz glass. Then, a ZnO piezoelectric layer <NUM> with a thickness of <NUM> is formed by RF magnetron sputtering on the Ti film <NUM>. Multiple samples <NUM> are prepared by changing the material and the amount of dopant added to the ZnO piezoelectric layer <NUM>. Then, Au film <NUM> with a thickness of <NUM> is provided as a top electrode on the piezoelectric layer <NUM>.

The samples <NUM> having ZnO piezoelectric layers <NUM> with different dopants added, each dopant being added at different ratios, are fabricated in the same size under the same conditions (except for the material and the amounts of dopants added). As a reference, a sample of a ZnO layer without dopant added (which may be called "pure-ZnO") is also prepared.

When Mg is used as the dopant, the content of Mg added to the piezoelectric layer is varied as described below. When Ca is used as the dopant, the Ca content in the piezoelectric layer is also varied within a predetermined range.

When Mg is added, a ZnO sintered target to which a predetermined ratio of MgO has been added in advance may be used for sputtering, or alternatively, a ZnO target and an MgO target may be used in a multi-element sputtering system for simultaneously and independently sputtering ZnO and MgO to carry out film formation at a desired doping ratio. When Ca is added, a ZnO sintered target to which a predetermined ratio of CaO has been added in advance may be used for sputtering, or alternatively, a ZnO target and a CaO target may be used in a multi-element sputtering system for simultaneously and independently sputtering ZnO and CaO to carry out film formation at a desired doping ratio.

The composition ratio of each of the samples is evaluated using Quantum <NUM>, which is an instrument for electron spectroscopy for chemical analysis (ESCA) manufactured by ULVAC-PHI, Inc. Particularly, the piezoelectric layer <NUM> is irradiated with X-rays emitted from a monochrome Al Kα radiation source at <NUM> kV and <NUM> W for analysis. Further, composition analysis is performed up to the depth of <NUM>, while etching the layer by Ar ion beams of acceleration voltage of <NUM> kV, and the average value thereof is used as the film composition ratio (or contents ratio) of the fabricated film.

In the evaluation test, an AC voltage is applied to each sample <NUM> using a network analyzer (manufactured by Agilent Technologies, Inc. ), and conversion loss of the piezoelectric layer <NUM> is measured. Specifically, the tip of the probe connected to the terminal of the network analyzer is pressed against the Au film <NUM> provided onto the top surface of the sample <NUM> to apply an AC voltage, and the conversion loss is measured by the network analyzer based upon the longitudinal sound wave (ultrasonic wave) generated inside the piezoelectric layer <NUM>. Then the electromechanical coupling coefficient kt (or its squared value kt<NUM>) in the thickness vibration mode of the piezoelectric layer <NUM> is estimated by comparing the measured conversion loss with the theoretical curve simulated by Mason's equivalent circuit model.

In addition, FWHM of the X-ray rocking curve (XRC) of the piezoelectric layer <NUM> of each sample having a different composition ratio is measured. The FWHM value of XRC (hereinafter referred to as "XRC-FWHM") is an index of the c-axis orientation of the piezoelectric layer <NUM>. The smaller the XRC-FWHM, the better the crystal orientation in the c-axis direction.

<FIG> shows the frequency dependence of the conversion loss of the reference sample with the pure ZnO piezoelectric layer <NUM> to which no dopant is added. The open circles represent the measured values, and the solid line is the theoretical curve simulated by Mason's equivalent circuit model. A dashed line with a very gentle slope near the horizontal axis of the chart indicates the propagation loss.

The conversion loss is expressed as the power ratio (dB) of the output frequency to the input frequency. Because the electromechanical coupling coefficient is expressed by the square root of the mechanical energy with respect to the supplied electric energy, the electromechanical coupling coefficient and the conversion loss correlate each other.

In <FIG>, the experimental values of the open circles exhibit the same tendency as the calculated values from Mason's equivalent circuit model. Elastic resonance of the primary mode occurs at <NUM>, and the conversion loss at this frequency is <NUM> dB. A semi-resonance occurs in the vicinity of <NUM> where the conversion loss reaches the peak.

The smaller the conversion loss, and the farther the separation between resonance frequency and the semi-resonance frequency, the greater the electromechanical coupling coefficient kt in the thickness direction is. Assuming that the resonance frequency is "fr" and the semi-resonant frequency is "fa", the squared value kt<NUM> of the electromechanical coupling coefficient is expressed by, for example,<MAT>.

For all the other samples with dopants of different materials added at different ratios to ZnO, the resonance frequency at which the conversion loss becomes the minimum and the semi-resonant frequency indicating the peak of the conversion loss are similarly determined using the network analyzer, and the squared values (kt<NUM>) of the electromechanical coupling coefficient in the thickness of the piezoelectric layers <NUM> are estimated.

<FIG> shows the kt<NUM> values when Mg is added as a dopant, compared with the kt<NUM> value of pure ZnO without dopant added. The kt<NUM> value of the undoped ZnO piezoelectric layer <NUM> is <NUM>%. When adding Mg of <NUM> atom% with respect to the total of Zn and Mg, the kt<NUM> value is improved to <NUM>%. With Mg of <NUM> atom% with respect to the total of Zn and Mg , the kt<NUM> value is <NUM>%, and with Mg of <NUM> atom% with respect to the total Zn and Mg, the kt<NUM> value is <NUM>%.

The kt<NUM> value increases by about <NUM>% to <NUM>%, compared with the pure ZnO without metal dopant added. It is understood from this analysis that, by adding Mg of <NUM> atom% to <NUM> atom%, the squared value kt<NUM> of the electromechanical coupling coefficient in the thickness direction of the piezoelectric layer <NUM> can be improved. In particular, when the Mg content is <NUM> atom% to <NUM> atom%, the kt<NUM> value increases to <NUM>% or more, compared with the undoped ZnO.

<FIG> shows XRC-FWHM values and kt<NUM> values measured while the Mg content in the piezoelectric layer is varied over a range broader than <FIG>. With the Mg content in the fabricated piezoelectric layer (relative to the total amount of Zn and Mg) ranging from <NUM> atom% to <NUM> atom%, the kt<NUM> values are <NUM>% or more, and sufficient resonance can be acquired. The XRC-FWHM values are also satisfactory <NUM>° or less.

When the Mg content is <NUM>%, the XRC-FWHM is good, but the kt<NUM> value decreases to <NUM>%. Meanwhile, when the doping ratio of Mg during film formation is <NUM> atom%, resonance could not be obtained and the kt<NUM> value cannot be estimated.

By adding Mg in an appropriate composition range, the kt<NUM> value increases nearly <NUM>%, compared with pure ZnO without metal dopant added, and the XRC-FWHM can be maintained small. From <FIG>, the appropriate range of Mg contained in the piezoelectric layer is <NUM> atom% to <NUM> atom%, more preferably, <NUM> atom% to <NUM> atom%.

<FIG> shows kt<NUM> value and XRC-FWHM when Ca is added as a dopant, compared with the kt<NUM> value and the XRC-FWHM of the pure ZnO without dopant added. Similar to <FIG>, the kt<NUM> value of the undoped ZnO piezoelectric layer <NUM> is <NUM>%, whereas the kt<NUM> value increases to <NUM>% by adding Ca of <NUM> atom% with respect to the total of Zn and Ca. With the Ca content of <NUM> atom% and <NUM> atom% with respect to the total of Zn and Ca, the kt<NUM> value is <NUM>%. When the Ca content with respect to the total amount of Zn and Ca is changed to <NUM> atom% and to <NUM> atom%, the kt<NUM> values are <NUM> atom% and <NUM> atom%, respectively. Within this range of Ca content, the XRC-FWHM is as good as <NUM>° or less.

On the other hand, when the Ca content is reduced to <NUM> atom%, the kt<NUM> value decreases. When the Ca content is increased to <NUM> atom%, the XRC-FWHM is good, but the resonance generated is insufficient.

By setting the Ca content in the piezoelectric layer to <NUM> atom% to <NUM> atom%, the kt<NUM> value is improved and the XRC-FWHM is reduced, compared with pure ZnO without metal dopant added. More preferably, by setting the Ca content to <NUM> atom% to <NUM> atom%, the kt<NUM> value can be improved to about <NUM>% to <NUM>% of the pure ZnO without metal dopant, while reducing the XRC-FWHM. By adding Ca in the appropriate range, both the c-axis orientation and the electromechanical coupling coefficient of the thickness vibration mode are improved.

The fact that the electromechanical coupling coefficient of the thickness vibration mode is increased in <FIG>, and <FIG> indicates that the piezoelectric characteristics of the piezoelectric device <NUM> of the embodiments are improved. By adding such a metal material that does not cause the zinc oxide-based sputtered film having a wurtzite crystal structure to exhibit conductivity, the squared value (kt<NUM>) of the electromechanical coupling coefficient of the thickness vibration mode can be increased to <NUM>% or more, and more preferably, to <NUM>% or more. The acquired kt<NUM> values are increased by <NUM>% to <NUM>%, compared with the kt<NUM> value of the undoped reference sample.

<FIG> is a schematic diagram of a piezoelectric device 10B, which is a modification of the piezoelectric device 10A. The piezoelectric device 10B has an orientation control layer <NUM> under the piezoelectric layer <NUM>. The orientation control layer <NUM> is an amorphous layer provided to improve the c-axis orientation of the piezoelectric layer <NUM>. The thickness of the orientation control layer <NUM> is <NUM> to <NUM>.

The orientation control layer <NUM> can be formed of an inorganic substance, an organic substance, or a mixture of an inorganic substance and an organic substance. As the inorganic substance, silicon oxide (SiOx), silicon nitride (SiN), aluminum nitride (AlN), aluminum oxide (Al<NUM>O<NUM>), gallium nitride (GaN), gallium oxide (Ga<NUM>O<NUM>), and so on can be used. Alternatively, ZnO with Al<NUM>O<NUM> and SiOx added (which is referred to as "SAZO" representing aluminum/silicon-added zinc oxide), or GaN, AlN, ZnO or other base material to which at least one of Al<NUM>O<NUM>, Ga<NUM>O<NUM>, SiOx and SiN is added can be used.

Examples of the organic substance include, but is not limited to, an acrylic resin, a urethane resin, a melamine resin, an alkyd resin, and a siloxane-based polymer. In particular, a thermosetting resin composed of a mixture of a melamine resin, an alkyd resin and an organic silane condensate can be preferably used as the organic substance. Using the above-described materials, an amorphous film can be formed by vacuum deposition, sputtering, ion plating, coating, or other appropriate methods. The orientation control layer <NUM> may be a single layer, or a multilayer with two or more films stacked. In the case of multilayer configuration, an inorganic thin film and an organic thin film may be laminated.

The amorphous orientation control layer <NUM> formed of the above-described materials is superior in surface smoothness, and the c-axis of the upper layer wurtzite crystal aligns vertical to the substrate (along the stacking direction). Further, the gas barrier performance is high. When a plastic substrate is used as the substrate <NUM>, adverse influence of the gas generated from the plastic during the film formation process can be reduced. In particular, when the orientation control layer <NUM> is formed of a thermosetting resin, highly smooth amorphous layer is formed. When a melamine resin is used for the orientation control layer <NUM>, the film density is high owing to the three-dimensional crosslinked structure and the barrier performance is good.

The orientation control layer <NUM> is not necessarily <NUM>% amorphous, and it may contain a non-amorphous portion as long as the c-axis orientation of the piezoelectric layer <NUM> can be enhanced. The proportion of the amorphous part in the orientation control layer <NUM> is preferably <NUM>% or more, and more preferably, <NUM>% or more. In this case, sufficient degree of c-axis orientation control effect a, etc. are used as the wurtzite crystal, alkaline earth metals such as Mg, Ca, or Sr, or alternatively, metals of other groups such as V, Ti, Zr, Li, etc. can be added in a predetermined composition range.

According to the present invention, Mg is added as a dopant to ZnO, wherein the Mg content with respect to the total of Zn and Mg in the film is <NUM> atom% to <NUM> atom%, and more preferably, <NUM> atom% to <NUM> atom%. When Ca is used as the dopant in ZnO, outside the present invention, Ca content with respect to the total of Zn and Ca in the film is <NUM> atom% to <NUM> atom%, and more preferably, <NUM> atom% to <NUM> atom%.

By doping Mg or Ca within above-described range, the electromechanical coupling coefficient of the thickness vibration mode can be improved, compared with undoped ZnO having a wurtzite crystal structure.

Also outside the present invention, a metal other than Mg or Ca, such as V, Ti, Zr, Sr, or Li selected from the above-described metals, can also improve the piezoelectric characteristics in the thickness direction of the zinc oxide-based material having a wurtzite crystal structure. A mixture of the above-described metals may also be used as the dopant.

By providing the amorphous orientation control layer <NUM> as an underlayer, the c-axis orientation of the piezoelectric layer <NUM> is improved, and consequently, the piezoelectric characteristics of the piezoelectric device 10B are further improved.

Regarding the substrate <NUM>, any material including glass, plastic, ceramic, and so on can be used. For the first electrode <NUM>, any conductive material can be used. When light transmission is required depending on the applications, a transparent conductive film may be used. When light transmission is not required, a metal electrode may be used. In the configuration with a transparent electrode, the first electrode <NUM> may be formed of an amorphous oxide conductor.

<FIG> is a schematic diagram a piezoelectric device 10C, which is another modification of the piezoelectric device 10A. In the piezoelectric device 10C, an adhesive layer <NUM> is provided at the interface between the piezoelectric layer <NUM> and the second electrode <NUM>.

The piezoelectric layer <NUM> is formed of a wurtzite crystal material to which a metal dopant is added. The metal dopant is one that does not cause the piezoelectric layer <NUM> to exhibit conductivity when added. When ZnO, ZnS, ZnSe, ZnTe, or the like is used as the wurtzite crystal, alkaline earth metals such as Mg, Ca, or Sr, or alternatively, metals of other group such as V, Ti, Zr, Li, etc. may be added in a predetermined composition range.

According to the present invention, Mg is added as a dopant to ZnO, wherein the Mg content with respect to the total of Zn and Mg in the film is <NUM> atom% to <NUM> atom%, and more preferably, <NUM> atom% to <NUM> atom%. When Ca is used as the dopant in ZnO, outside the present invention, the Ca content with respect to the total of Zn and Ca in the film is <NUM> atom% to <NUM> atom%, and more preferably, <NUM> atom% to <NUM> atom%. By doping Mg or Ca within the above-described range, the electromechanical coupling coefficient of the thickness vibration mode can be improved, compared with undoped ZnO having a wurtzite crystal structure.

The adhesive layer <NUM> suppresses leakage current paths caused by cracks or pinholes produced in the piezoelectric layer <NUM>. When metal grain boundaries or protrusions exist at the interface between the piezoelectric layer <NUM> and the first electrode <NUM>, or at the interface between the piezoelectric layer <NUM> and the second electrode <NUM>, a leakage current path will be formed between the electrodes due to cracking or the like. Such leakage current path extinguishes the polarization. By inserting the adhesive layer <NUM>, occurrence of leakage current paths is suppressed, and the piezoelectric characteristics of the piezoelectric layer <NUM> are maintained satisfactory.

The manufacturing process for the piezoelectric device 10C is as follows. As a first part of the device, the first electrode <NUM> is formed on the substrate <NUM>, and the piezoelectric layer <NUM> to which a metal dopant is added at a predetermined ratio is formed on the first electrode <NUM>. Meanwhile, a second electrode <NUM> is formed on a substrate <NUM> to provide a second part of the device. Any material can be used as the substrate <NUM>, and for example, a plastic substrate may be used. The piezoelectric layer <NUM> and the second electrode <NUM> are brought so as to face each other, and the first part and the second part are bonded together with the adhesive layer <NUM>. Thus, the multilayer structure of the piezoelectric device 10C is fabricated.

The piezoelectric device 10C has a large electromechanical coupling coefficient of the thickness vibration mode, while preventing leakage current paths from occurring between the electrodes, and it has good piezoelectric characteristics.

A piezoelectric device <NUM> according to the invention is not limited to a device that utilizes the piezoelectric effect, such as a force sensor for a touch panel, a pressure sensor, an acceleration sensor, or an acoustic emission sensor, but is also applicable to a speaker, a transducer, a high frequency filter device, or the like that utilizes the inverse piezoelectric effect. In the latter case, the conversion efficiency from electrical energy to mechanical energy is high, and large deformation is created in the thickness direction.

The configuration of the piezoelectric device <NUM> is not limited to the above-described examples of piezoelectric devices 10A to 10C. In the configurations of <FIG>, the second electrode <NUM> may be formed as a transparent conductive film. The configurations of <FIG> may be combined, such that the orientation control layer <NUM> may be inserted between the piezoelectric layer <NUM> and the first electrode <NUM>, and that the piezoelectric layer <NUM> and the second electrode <NUM> are bonded together by the adhesive layer <NUM>. In this case, the first part is fabricated by forming the first electrode <NUM> on the substrate <NUM>, then forming the orientation control layer <NUM> on the first electrode <NUM>, and then forming the piezoelectric layer <NUM> to which a selected dopant is added at a predetermined ratio on the orientation control layer <NUM>. Meanwhile, the second part is fabricated by forming the second electrode <NUM> on the substrate <NUM>. The piezoelectric layer <NUM> and the second electrode <NUM> are brought so as to face each other, and bonded together by the adhesive layer <NUM>.

In either case, the piezoelectric layer <NUM> has good piezoelectric characteristics in the thickness direction, and satisfactory conversion efficiency can be achieved.

Claim 1:
A piezoelectric device (<NUM>) comprising:
a first electrode layer (<NUM>);
a second electrode layer (<NUM>); and
a piezoelectric layer (<NUM>) provided between the first electrode layer (<NUM>) and the second electrode layer (<NUM>),
wherein the first electrode layer (<NUM>), the piezoelectric layer (<NUM>), and the second electrode layer (<NUM>) are stacked in this order on a substrate (<NUM>),
wherein the piezoelectric layer (<NUM>) is formed of a ZnO-based material having a wurtzite crystal structure to which a metal that does not cause the piezoelectric layer (<NUM>) to exhibit conductivity is added and wherein the metal is Mg,
characterised in that
a Mg content with respect to a total amount of Zn and Mg is <NUM> atom% to <NUM> atom%, and a squared value of an electromechanical coupling coefficient in thickness vibration mode, kt<NUM>, is <NUM>% or more.