دانلود کتاب پیشرفت‌ها در سرامیک‌های الکترونیکی

Ferroelectric thin films exhibit diverse properties, such as ferroelectricity, piezoelectricity, dielectricity, pyroelectricity, and photoelectric effects, rendering them suitable for various applications including ferroelectric memory, piezoelectric drivers, infrared detectors, and optoelectronic devices [1–5]. Traditional ferroelectric materials, predominantly lead-based compounds, are exemplified by the widely used lead zirconate titanate (PZT) in commercial applications due to their robust ferroelectric and piezoelectric characteristics [6,7]. However, the environmental and health concerns arising from the volatilization of lead in lead-based materials necessitate the exploration of lead-free alternatives. Bismuth-based titanate and bismuth ferrite emerge as promising candidates for lead-free piezoelectric materials. Notably, bismuth titanate (Bi4Ti3O12, BIT) exhibits favorable properties, including a small coercive field, a low leakage current density, excellent fatigue resistance, and applicability in non-volatile memory [8–10]. BIT, classified as a typical three-layer bismuth layered ferroelectric (BLSF) material, consists of a bismuth oxide layer (Bi2O3)2+ and a perovskite-like layer (An−1BnO3n+1)2− along the c-axis direction [11,12]. Characterized by strong anisotropy, BIT demonstrates spontaneous polarization of approximately 50 μC/cm2 along the a-axis and 4 μC/cm2 along the c-axis. Additionally, it features a low film deposition temperature and a high Curie temperature (Tc = 675 ◦C) [13,14]. Despite its favorable properties, BIT faces challenges such as instability in the oxidation state of Ti ions and volatility in Bi ions during the sintering process. These issues lead to defects, resulting in a high leakage current and domain pinning, thereby affecting its practical applications [9,15]. To enhance its ferroelectric properties, researchers have explored the substitution of Bi3+ ions in the perovskite-like layer (Bi2Ti3O12)2+ with trivalent rare-earth ions such as Pr3+, Nd3+, Sm3+, Eu3+, and La3+ [1,16–19]. In 1999, Park et al. reported a ferroelectric thin film of La-doped BIT, Bi3.25La0.75Ti3O12 (BLT) [11]. Wu et al. demonstrated that the application of tensile or compressive stress on the surface of BLT thin films in the high-field region (>75 kV/cm) effectively mitigates the leakage current [20]. Furthermore, substituting high-valent cations for Ti4+ at the B site proves instrumental in eliminating oxygen vacancy defects, thereby enhancing remanent polarization and reducing the leakage current. BIT thin films co-doped with Sm and Ta, denoted as BSTTO, exhibited markedly improved ferroelectric properties, boasting a higher remanent polarization (2Pr = 46.2 μC/cm2) compared to BIT thin films (2Pr = 26 μC/cm2) [21]. Various techniques, including pulsed laser deposition (PLD) [22], magnetron sputtering (MS) [23], metal–organic chemical vapor deposition (MOCVD) [24], and the sol–gel method [25], have been employed for the preparation of BLT ferroelectric thin films. Among these, the sol–gel process stands out for its cost-effectiveness, facile stoichiometry control, and uniform deposition over large areas, making it versatile for widespread applications. Notably, factors such as the precursor solution, annealing conditions, film thickness, and substrate properties exert substantial influence on film orientation [25–27]. Optimal film layer thicknesses of 30, 50, and 100 nm corresponded to preferentially oriented thin films with (001), (100), and (117) orientations, respectively [28]. By adjusting the grain size of BLT through annealing temperature modulation, Yang et al. achieved a higher energy storage density, rendering BLT ferroelectric thin films suitable for high-energy-density storage devices, thereby expanding their application scope [29]. In addition, information storage technology stands as one of the most rapidly advancing fields within the realm of integrated circuits. Among its components, memory stands as the pivotal core in information storage technology. However, conventional storage devices have reached a point where they can no longer meet the burgeoning demands of information storage technology. Consequently, the pursuit of developing a new generation of memristors has captured academic interest in recent years. The development of ferroelectric materials with resistive switching characteristics holds significant importance for memristors. In this study, lead-free BLT thin films were synthesized using the sol–gel method, and the adjustment of spin coating times yielded BLT thin films with exceptional ferroelectric properties. The dielectric and ferroelectric characteristics of thin films subjected to varying spin coating times were systematically compared across different electric field strengths and frequencies. The microscopic ferroelectricity of the BLT film and the microscopic evolution of the domain wall of the BLT film were studied through PFM observation. The resistor characteristics of the BLT4 thin film were studied, demonstrating its potential application in memristor.