Liubov A. Delimova

Porous ferroelectric lead zirconate titanate (PZT) films are a promising material for various electronic applications. This study focuses on understanding how the structure-directing agent, polyvinylpyrrolidone, can alter the structure and electrical properties of porous PZT films prepared through chemical solution deposition. Films with various porosities of up to ~40 vol.% and pore connectivities from 3-0 to 3-3 were prepared and studied by capacitance–voltage, dielectric hysteresis, transient current, photocurrent, and local current techniques. We have found that a linear decrease in material volume in a porous film is not the only factor that determines film properties. The creation of new internal grain boundaries plays a key role in changing electrical properties. This research expands the understanding of physical phenomena in porous ferroelectric films and may facilitate the development of new materials and devices.

Porous ferroelectric lead zirconate-titanate (PZT) films are a promising material for various electronics applications. This study focuses on understanding how structure-directing agent (polyvinylpyrrolidone) can change structure and electrical properties of porous PZT films prepared by chemical solution deposition. The films with various porosity up to ~ 40 vol.% and pore connectivity changing from 3-0 to 3-3 were prepared and studied by capacitance-voltage, dielectric hysteresis, transient current, photocurrent, and local current techniques. We have found that the linear decrease in the material volume in a porous film is not the only factor governing film properties. The key role in changing electrical properties plays the creation of new internal grain boundaries. This research expands the understanding of physical phenomena in porous ferroelectric films and may enable the development of new materials and devices.

A model of non-stationary charge carrier transport in M/PZT/M ferroelectric structures has been developed. It is assumed that, at room temperature, electrons generated by oxygen vacancies are captured by Ti+3 levels and move between them under the action of electric fields caused by the external bias and polarization. The polarization distribution in a PZT film is described with varying degrees of complexity, from a constant value outside the defect layers to one determined by the equation following from the Landau–Ginzburg theory. The numerical simulation performed made it possible to explain the reasons and conditions for the appearance of current–voltage characteristics with unusual current peaks, to show the existence of several solutions in the Landau–Ginzburg model for a film with Schottky barriers, and to reveal the response of charged domain walls to an applied voltage.

A model of non-stationary charge carrier transport in M/PZT/M ferroelectric structures has been developed. It is assumed that at room temperature, electron

The properties of ferroelectric sol–gel PZT films deposited on a silicon-on-sapphire (SOS) substrate are compared with those of PZT films formed on a Si substrate. The crystalline structure, asymmetry of the hysteresis loops, polarization dependences of the transient current, short-circuit photocurrent, and open-circuit photovoltage, as well as the substrate bending have been studied. The PZT-on-SOS films are textured in a single (111) direction and exhibit symmetric hysteresis loops with a strong remanent polarization. The PZT films on Si are textured in the main (111) and weaker (100) directions, have weaker polarization, and exhibit an asymmetry of the hysteresis loops, which is reflected in the magnitude of the transient current and photocurrent. It is shown that the sapphire substrate has a convex bending that causes a compressive stress in the film plane, which weakens the effect of the lattice mismatch between PZT and Pt. By contrast, the Si substrate has a concave curvature, which causes the film to stretch. The deformations and mechanical stresses within the films were estimated. For PZT-on-Si, an estimate of the strain gradient along the (111) axis was obtained, which makes it possible to relate the asymmetry of the hysteresis loops to the flexoelectric polarization, with the flexoelectric coefficient for sol–gel PZT films found to be 0.0154 μC/cm. The results obtained show that the sapphire substrate provides a better quality of thin PZT films.

The properties of ferroelectric sol-gel PZT films deposited on a silicon-on-sapphire (SOS) substrate are compared with those of PZT films formed on a Si substrate. The crystalline structure, asymmetry of the hysteresis loops, polarization dependences of the transient current, short-circuit photocurrent, and open-circuit photovoltage, as well as the substrate bending have been studied. The PZT on SOS films are textured in a single (111) direction and exhibit symmetric hysteresis loops with a strong remanent polarization. The PZT films on Si are textured in the main (111) and weaker (100) directions, have weaker polarization, and exhibit an asymmetry of the hysteresis loops, which is reflected in the magnitude of the transient current and photocurrent. It is shown that the sapphire substrate has a convex bending that causes a compressive stress in the film plane, which weakens the effect of the lattice mismatch between PZT and Pt. By contrast, the Si substrate has a concave curvature, which causes the film to stretch. The deformations and mechanical stresses within the films were estimated. For PZT on Si, an estimate of the strain gradient along the (111) axis was obtained, which makes it possible to relate the asymmetry of the hysteresis loops to the flexoelectric polarization, with the flexoelectric coefficient for sol-gel PZT films found to be 0.0154 µC/cm. The results obtained show that the sapphire substrate provides a better quality of thin PZT films.

A drift-diffusion model of unsteady carrier transport in M/PZT/M structure is proposed to account for the formation of the current peak in the current –voltage curves, which is not caused by the domain switching and observed only when the bias and polarization directions coincide. In the model, electrons generated by oxygen vacancies are trapped by titanium deep centers at room temperature and can move by hopping between titanium atoms in the electric field. The polarization is constant across the film thickness while it is zero within defective layers near the contacts. It is shown that a pronounced current peak is formed when an accumulated space-charge layer appears near one of the contacts under the action of the polarization and this effect is purely unstationary.

A drift-diffusion model of unsteady carrier transport in an M/lead zirconate titanate (PZT)/M structure is proposed. It is assumed that the PZT film has electronic conductivity. Electrons are generated by oxygen vacancies and trapped by Ti⁺³ deep centers. It is assumed that electrons move in the electric field by hopping between titanium atoms, with an effective mobility that is considered constant. To account for the polarization, it is believed that, near the contacts, there are thin defective layers in which the polarization is zero, while outside these layers, the polarization does not vary across the film thickness and depends on the applied bias. The model was used to account for the formation of the current peak in the current–voltage curves, which is not caused by the domain switching and observed in epitaxial films only when the bias and polarization directions coincide. It is shown that a pronounced current peak is formed when (а) an accumulated space-charge layer appears near one of the contacts under the action of polarization and (b) this contact is cathode, which is only possible when the polarization and bias directions coincide. As a result, electrons flow between space-charge layers, and the film resistance first decreases and then starts to increase again, and this gives rise to a current peak. It is shown that this effect is purely nonstationary. The model also made it possible to estimate the basic parameters of the structure: electron mobility, density of oxygen vacancies, dielectric constant, defect layer thickness, and barrier height.

A new method of two-stage crystallization of lead zirconate–titanate (PZT) films using a seed sublayer with a low excess lead content has been proposed and realized. A seed layer with a strong...