A ferroelectric film of PZT or the like formed by using a chemical solution deposition (CSD) method represented by a sol-gel method cannot be used as a piezoelectric body immediately after being formed, and a poling process is necessarily performed in order to use the ferroelectric film in a gyro sensor or the like. In a case of using this ferroelectric film in a sensor such as a pyroelectric sensor or a gyro sensor, a performance index g of a piezoelectric film (ferroelectric film) used is represented by the following Formula (1).g(V·m/N)=d31/ε33  (1)
In Formula (1), d31 represents a piezoelectric constant and ε33 represents a dielectric constant.
That is, in a case of using a ferroelectric film of PZT or the like in a sensor such as a pyroelectric sensor or a gyro sensor, it is generally desirable that a piezoelectric constant of a film be great and a dielectric constant or a dielectric loss (tan δ) of a film be low. In addition, it is desirable that polarization directions of a film be aligned immediately after a film is formed, from the viewpoints of stability of polarization and needlessness of a polarization step.
In a case of using such a film in an actuator of an ink jet head or the like, the film is used by applying a high voltage, and accordingly, a poling process is not always necessary. This is because, in a case of using such a film by applying a high voltage, polarization is performed with a driving voltage, even when polarization directions of the film are not aligned immediately after the film is formed, for example. Even if a poling process is performed, depolarization may occur at the time of a thermal treatment such as a reflow process after the poling process.
Regarding such problems, self-poling has been investigated and a phenomenon in which polarization directions are aligned to one direction immediately after a film is formed has been reported. A specific mechanism regarding this self-poling phenomenon is not clear, but it has been reported that charge trapped in internal electric fields or electrode interfaces in a film is one of the reasons (for example, see NPL 1).
Regarding a piezoelectric constant, it is known that, when Nb is added at the time of forming a PZT thin film represented by PbZrxTi1-xO3 by using a sol-gel method, piezoelectric properties are improved (for example, see NPL 2). NPL 2 discloses a research result regarding a {100}-oriented PZT thin film grown by doping Nb on a seed layer of PbTiO3 formed by using a CSD method. Specifically, NPL 2 discloses a research result when a {100}-oriented Pb1.1Zr0.52Ti0.48O3 thin film having a thickness of 1 μm is doped with Nb within a range of 0 atom % to 4 atom %. NPL 2, for example, discloses that a high degree of {100} orientation of 97% is obtained in the entire film or maximum polarization, residual polarization, squareness, and a saturated holding force of the entire PZT thin film are decreased together with a doping level of Nb, due to the incorporation of a thin seed layer of Pb1.05TiO3 having a thickness of several nm. In addition, NPL 2 discloses that a PZT thin film doped with 3% of Nb shows the highest piezoelectric constant −e31.f of 12.9 C/cm2, which is higher than other thin films having other doping levels by 5% to 15%.