| The ferroelectric and piezoelectric properties of the lead zirconate-titanate(PZT) thin film were intensively investigated in the past 20 years. Recently, the photovoltaic effect of the PZT film has gradually attracted research attention due to its potential applications for optical detection and wireless actuation. There were a few studies related to the photovoltaic effect of the PZT films, and photocurrent can be observed in various metal/PZT film heterostructures. Various factors that may affect the photovoltaic output have been investigated, including light intensity and wavelength, Schottky barrier and Ohmic contact, poling voltage magnitude and direction, and film composition and thickness. However, the physical mechanism of photovoltaic effect in ferroelectrics is not completely understood. In this thesis, the Pb(Zr0.20Ti0.80)O3 (PZT) films were deposited on Pt/Ti/SiO2/ Si substrates by sol-gel method. For electrical and photocurrent measurements, about 40 nm thick Pt dots with a diameter of 0.28 mm were sputtered onto the top surface of films by magnetron sputtering at room temperature, forming a typical Pt/PZT/Pt (MFM) capacitor structure. The photoelectric properties in the MFM structure were then tested, and the mechanism behind the origin of the photocurrent was also investigated.Firstly, we try to study the physical mechanism of photovoltaic effect in the PZT films by changing the film's thickness. The thicknesses of prepared films were controlled to be about 270, 360, 450, and 540 nm, respectively. The photocurrent of these films have been measured after being poled by different dc electric fields. We find that the photocurrent is inversely related to the film thickness. Moreover, both the internal electric field originated from the aligned polarization and Schottky barriers at the PZT/Pt interfaces contribute to the photocurrent. Then, a simple model was suggested to evaluate the contribution of the barrier and remanent polarization effects on the short-circuit photocurrent observed in Pt sandwiched PZT films. It was demonstrated that the photocurrent originated from the aligned polarization was proportion to the 2Pr values of the PZT film, while the one originated from the built-in field of the Schottky barriers was inversely related to the film thickness. This work has been published in Applied Physics Letters.As we well known, in most cases, polycrystalline PZT films are defective system, in which space charges exist among the grain boundaries and/or electrode/film interface. These space charges may also have a strong influence on the photovoltaic behavior. In this thesis, the annealing temperature and atmosphere during film preparation were changed, with an attempt to control the densities of space charges in the polycrystalline PZT films. By testing the short-circuit photocurrent after poling the films with different dc voltage similar to the ferroelectric hysteresis (P-V) loop's poling method, we can get a photocurrent vs poling voltage (PC-V) loop. Comparing the PC-V with P-V loop, we find two points: on the one hand, they are very similar, indicating that the photocurrent is related to the magnitude of polarization; on the other hand, the two kinds of loops may exhibit some difference, due to the fact that the polarization loop is obtained in a dynamic way and the other a static way where the space charges may have different response. When a poling filed is applied on the PZT film, the polarization bound charges are oriented arrangement, in addition, the movable space charges existing in the films are also pushed to the top or bottom metal/film interfaces, which can modify the height of Schottky barriers in the top and bottom interface. As a result, the photocurrent due to Schottky barrier can also be affected. This study also indicates that PC-V loops can provide the additional features which are absent in traditional P-V loops for Pt/PZT/Pt capacitors. One is the density of movable space charges; and the other the relative net Schottky barrier height judged from the magnitude of PCo. This work has been published in Journal of Physics Letters. |
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