Preparation, Microstructure And Photovoltaic Properties Of Bi-based Ferroelectric Epitaxial Thin Films

In recent years, a variety of physical properties of Bi based ferroelectric materials with its intrinsic crystal structure showed by extensive and in-depth study on the people. In various forms of ferroelectric materials: single crystal, thin film and ceramic block, thin film materials has been an important direction of research in the field of microelectronics. In some research of the ferroelectric thin film material properties, when explored the ferroelectric, magnetic properties and micro structure and properties, also finding the unique characteristics of PV, so the photovoltaic effect in ferroelectric thin films has become one of the hotspots in the field of ferroelectric- photoelectronics.As an important member of the ferroelectric photovoltaic materials, BiFeO₃?BFO? has become the research focus since it was discovered the photovoltaic effect, studies and reports on the BFO photovoltaic effect is endless and constantly enriched and improved. In the further mechanism study of the BFO photovoltaic effect, the researchers have been inclined to find answers from the semiconductor heterostructure and bandgap theory, however, the impact of the dynamic change on the optical bandgap of BFO photovoltaic effect has not been reported. In this context, the first major part of content in this paper reveals the regulation impact of dynamic strain on BFO photovoltaic effect. The second important part of this thesis is the?Bi₂O₂?²⁺(A_m-1B_mO_3m+1)^2- Structural System Bi-based ferroelectric materia Bi₅Fe_1-xCo_xTi₃O₁₅?x =0, 0.25, 0.5??BFTO and BFCTO?, this section explores the preparation of three kinds of epitaxial films and studys the nature of their microstructure and photovoltaic properties.1. In situ dynamic control on BFO epitaxial films:To study the effect of dynamic strain regulation on the photovoltaic effect of BFO epitaxial thin films, we selected PMN-PT single crystal with excellent piezoelectric properties as substrate, and epitaxially grown 22 nm thickness LSMO conductive layer and the BFO thin films using pulsed laser deposition?PLD? method on the PMN-PT single crystal substrate respectively, and eventually got the Pt/BFO/LSMO/ PMN-PT structure heterojunction devices. Via the corresponding microstructure analysis, the BFO thin film is highly crystalline epitaxial growth and is provided with well ferroelectric properties with remnant polarization Pr =52 ?C/cm2. Under violet irradiation, the photovoltaic effect significantly enhanced after the BFO thin film polarized.After +10 k V/cm electric field is applied to the PMN-PT single crystal substrate, the photovoltaic effect BFO thin films significantly enhanced, the open-circuit voltage Voc increased from 0.11 V to 0.21 V and the short-circuit current Isc increased from 44 n A to 80 n A, in order to quantitatively understanding the influence of the piezoelectric strain on the photovoltaic effect, we introduce the gauge factor??Voc/Voc?/??xx,??Isc/Isc?/??xx and???/??/??xx strain to characterize the effectiveness of regulation, these coefficients represent the open-circuit voltage Voc, the relative change in short-circuit current Isc and the conversion efficiency ? with respect to the rate of change of the in-plane strain, the??Voc/Voc?/ ??xx,??Isc/Isc?/??xx and???/??/??xx were calculated to be 758,642 and 1817. Such large strain coefficients suggest BFO film photovoltaic response of the lattice strain is very sensitive. By enhancing the photovoltaic effect of strain generated by research, we have come to the corresponding enhancement mechanism: under strong electric field generated by external voltage, the lattice constant of PMN-PT single crystal substrate increases, resulted in reverse piezoelectric effect,then strain is transferred to BFO film through the LSMO buffer layer. As a result, the lattice constant of BFO film will also increase, causing the optical band gap of the BFO film decreases under the same lighting conditions, which can stimulate more light carriers, resulting in significantly enhanced photovoltaic effect.2. The preparation process,microstructure research and photovoltaic performance of Bi₅Fe_1-xCo_xTi₃O₁₅?x =0, 0.25, 0.5?epitaxial film:Bi₅Fe_1-xCo_xTi₃O₁₅?x=0, 0.25, 0.5? is Aurivillius phase?Bi₂O₂?²⁺((A_m-1B_mO_3m+1)^2- layered perovskite material architecture. The main part of the present study is based on SrTiO₃ single crystal substrate for the preparation of the three kinds of components of the epitaxial film, since the lattice constant of c-axis direction is large, the growth conditions of epitaxial films is more complex. Take BFTO films as an example, after repeated exploration, we found that 30 PaO₂ atmosphere is the best film deposition conditions. In this O₂ pressure, we explored the temperature growth conditions for BFTO epitaxial films,and got that the films quality under 780? is best. Similarly, we explored the growth temperature of Bi₅Fe_1-xCo_xTi₃O₁₅?x = 0.25, 0.5? film, the optimum growth temperature conditions were 735? and 650?. By the HR-XRD, SEM and TEM microstructure analyzing methods, we further confirmed that Bi₅Fe_1-xCo_xTi₃O₁₅?x = 0, 0.25, 0.5? films have highly epitaxial relationship between the NSTO substrates.In the three photovoltaic performance, the photovoltaic effect of BFTO film is weak, at 220 m W violet irradiation with Voc = 0.07 V, Isc = 680 n A. For Bi₅Fe_1-xCo_xTi₃O₁₅?x = 0.25, 0.5?, the photovoltaic effect was significantly enhanced,Voc and Isc are 0.18 V, 1500 n A and 0.2 V, 900 n A, respectively,which suggests that Co doping can significantly improve BFTO photovoltaic properties, which may be related to increased photo-carrier concentration after Co doping. In the thin film optical response test, the corresponding irradiation light wavelength for the strongest photovoltaic effects to the three films are 380 nm, 400 nm and 365 nm.