Picosecond nonlinear optical response of Ba0.5Sr0.5TiO3 thin films fabricated by pulsed laser deposition

Well-crystallized Ba_0.5Sr_0.5TiO₃ thin films with good surface morphology were prepared on MgO(1 0 0) substrates by pulsed laser deposition technique at a deposition temperature of 800 °C under the oxygen pressure of 2 × 10⁻³ Pa. X-ray diffraction and atomic force microscopy were used to characterize the films. The full width at half maximum of the (0 0 2) Ba_0.5Sr_0.5TiO₃ rocking curve and the root-mean-square surface roughness within the 5 μm × 5 μm area were 0.542° and 0.555 nm, respectively. The nonlinear optical properties of the films were determined by a single beam Z-scan method at a wavelength of 532 nm with laser duration of 55 ps. The results show that Ba_0.5Sr_0.5TiO₃ thin films exhibit a fast third-order nonlinear optical response with the nonlinear refractive index and nonlinear absorption coefficient being n₂ = 5.04 × 10⁻⁶ cm²/kW and β = 3.59 × 10⁻⁶ (m/W), respectively.     

Large blue shift in the optical band-gap of sol-gel derived Ba0.5Sr0.5TiO3 thin films

In this paper, we report and analyze the large blue shift in the optical band-gap of sol-gel derived Ba_0.5Sr_0.5TiO₃ (BST) thin films. BST films of different thickness (150 nm, 320 nm and 480 nm respectively) were deposited layer by layer onto fused quartz substrates by a spin coating technique. The drying temperature for individual layers (pre-sintering temperature) was varied as 400, 500 and 600 ^°C. A large blue shift in the band-gap was observed (with a value 4.70 eV compared to the bulk value of 3.60 eV) for films pre-sintered at 400 ^°C, which decreased with increase in the pre-sintering temperature. To date such blue shifts have been attributed to grain size reduction, stress and the amorphous nature of the films. Here, the blue shift has been correlated with the presence of charge carriers generated by oxygen vacancies and explained on the basis of the Burstein-Moss effect.     

Thickness-dependent tunable characteristics of (Ba0.5Sr0.5)0.925K0.075TiO3 thin films prepared by pulsed laser deposition

The thickness-dependent dielectric properties and tunability of pulsed laser deposited (Ba_0.5Sr_0.5)_0.925K_0.075TiO₃ (BSKT) thin films with different thickness ranging from 80 to 300 nm has been investigated. Dielectric properties of the BSKT thin films are substantially improved as the BSKT film thickness increases, which can be explained by the model of a low-permittivity dead layer that is connected in series with the bulk region of the film. The estimated values of thickness and the average dielectric constant for the dead layer are 2.4 nm and 23.5, respectively, in a Pt/BSKT/Pt capacitor structure. The tunability and figure of merit increased with increasing film thickness, which are attributed to the change in lattice parameter and the dead layer effect.     

Self-assembled monolayers and chemical derivatization of Ba0.5Sr0.5TiO3 thin films: Applications in phase shifter devices

Thin films of barium strontium titanate (Ba_1−xSr_x TiO₃ (BSTO)) have been used in coupled microstrip phase shifters (CMPS) for possible insertion in satellite and wireless communication platforms primarily because of their high dielectric constant, low loss, large tunability, and good structural stability. In an attempt to improve the figure of merit K (phase shift °/dB of loss) of phase shifters, modification of the metal/BSTO interface of these devices has been done through surface modification of the BSTO layer using a self-assembled monolayer approach. The impact of this nanotechnology promises to reduce RF losses by improving the quality of the metal/BSTO interface. In this study, compounds such as 3-mercaptopropyltrimethoxysilane (MPS), 16-mercaptohexadecanois acid (MHDA) and 3-mercaptopropionic acid (MPA) were used to form the self-assembled monolayers on the BSTO surface. As a result of the previous modification, chemical derivatization of the self-assembled monolayers was done in order to increase the chain length. Chemical derivatization was done using 3-aminopropyltrimethoxysilane (APS) and 16-mercaptohexadecanoic acid. Surface chemical analysis was done to reveal the composition of the derivatization via X-ray photoelectron spectroscopy (XPS) and Fourier Transform Infrared (FT-IR). Low and high frequencies measurements of phase shifters were done in order measure the performance of these devices for insertion in antennas. X-ray photoelectron spectroscopy characterization of modified BSTO thin films with MPS showed a binding energy peak at 162.9 eV, indicative of a possible SO interaction: sulfur of the mercapto compound, MPS, used to modify the surface with the oxygen site of the BSTO thin film. This interaction is at higher binding energies compared with the thiolate interaction. This behavior is observed with the other mercapto compounds such as: MHDA and MPA. An FT-IR analysis present a band at 780 cm⁻¹, which is characteristic of an OSC stretching and reveals the modification of the BSTO thin film by the coupling of the O of the BSTO with the S of the mercapto compound. All the modification using mercapto compounds is through sulfur to the BSTO thin film. MHDA SAM on BSTO thin film was chemically derivatized using APS shown by XPS and FT-IR. The SAMs modified phase shifters showed an improvement in performance with respect to those phase shifters fabricated with standard methods.     

Ba0.5Sr0.5TiO3有序构型的第一性原理研究

采用第一性原理计算了Ba_0.5Sr_0.5TiO₃三种有序构型的晶格结构和对应的电子结构,晶格结构的详细分析结果表明BST{100}有序构型为四方相,Ti-O八面体中Ti原子和Ba-Sr平面上的O原子沿[100]方向分别偏心位移0.040 Å和0.065 Å,八面体畸变导致反平行自发极化出现,构型处于反铁电态. BST{110}构型也是四方相,并且(110)和(1 关键词： 钛酸锶钡 第一性原理 有序结构 铁电性     

Photoluminescence at room temperature in Ba0.5Sr0.5TiO3 ceramics

We present in this paper the experimental results of photoluminescence spectra of Ba_0.5Sr_0.5TiO₃ ceramics. An emission band centered at about 920 nm has been observed at room temperature. When we change the amount of oxygen vacancies in these samples by thermal treatment, these samples show an enhancement of luminescence. Our experimental results indicate that the origin of the photoluminescence is related to the oxygen vacancies in these samples.     

Domain configuration and dielectric properties of Ba0.6Sr0.4TiO3 thin films

The microstructure and electrical properties of Ba_0.6Sr_0.4TiO₃ thin films have been investigated. Nanometer-sized domains, ranging from 8 to about 30 nm, were observed by piezoresponse force microscopy (PFM). The critical size, below which only single domains exist, is found to be about 31 nm. The film exhibits ferroelectric behavior characterized by polarization hysteresis loop and capacitance-voltage curve.     

Microstructure and electrical properties of Ba0.5Sr0.5TiO3 thin films prepared on copper foils with La2O3 buffer layers

Ba_0.5Sr_0.5TiO₃ (BST) thin films were deposited on copper foils via sol-gel method with La₂O₃ as a buffer layer. The films were processed in almost inert atmosphere so that the substrate oxidation was avoided while allowing the perovskite film phase to crystallize. The existence of a La₂O₃ buffer layer between the BST thin film and Cu foil improved the dielectric constant and reduced the leakage current density of the BST thin film. Meanwhile, the BST thin film exhibited ferroelectric character at room temperature, which was contrast to the para-electric behavior of the film without the buffer layer. Effects of La₂O₃ buffer layer on the crystallizability and microstructure of BST thin films were also investigated.     

High pyroelectric Ba0.65Sr0.35TiO3 thin films with Ba0.65Sr0.35RuO3 seeding-layer for monolithic ferroelectric bolometer

Ba_0.65Sr_0.35TiO₃ (BST) thin films were deposited by RF sputtering with a very thin Ba_0.65Sr_0.35RuO₃ (BSR) seeding-layer on Pt/Ti/SiO₂/Si substrate. The crystallization of BST thin films and the surface morphology of BSR seeding-layer were characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM), respectively. XRD patterns show that the BSR seeding-layer affected the orientation of BST thin film, which is highly a-axis textured. It was also found that the BSR seeding-layer had a marked influence on the dielectric properties of BST thin films. Comparing with BST thin films directly deposited on Pt electrode, the dielectric relaxation can be suppressed and dielectric constant increased due to a possible reduction of interface oxygen vacancies at BST/BSR interface. Moreover, J–V measurement indicates that the leakage current density of BST thin films on BSR seeding-layer were greatly reduced compared with that of BST thin films directly on Pt electrodes. The pyroelectric coefficient of BST thin films with BSR seeding-layer is 7.57 × 10⁻⁷ C cm⁻² K⁻¹ at 6 V/μm at room temperature (RT). Our results reveal that high pyroelectric property of BST thin film could be achievable using BSR seeding-layer as a special buffer.     

Effect of deposition temperature on orientation and electrical properties of (K0.5Na0.5)NbO3 thin films by pulsed laser deposition

Lead-free ferroelectric K_0.5Na_0.5NbO₃ (KNN) thin films have been prepared on Pt/TiO₂/SiO₂/Si substrates by pulsed laser deposition process. The structures, crystal orientations and electrical properties of thin films have been investigated as a function of deposition temperature from 680 °C to 760 °C. It is found that the deposition temperature plays an important role in the structures, crystal orientations and electrical properties of thin films. The crystallization of thin films improves with increasing deposition temperature. The thin film deposited at 760 °C exhibits strong (0 0 1) preferential orientation, large dielectric constant of 930 and the remnant polarization of 8.54 μC/cm².     

The size effect of Ba0.6Sr0.4TiO3 thin films on the ferroelectric properties

Barium strontium titanate (BST) thin films were prepared by RF magnetron sputtering. The dielectric constant-voltage curves and the hysteresis loops of BST thin films with different grain sizes and film thicknesses were investigated. When the grain size increases from 12 nm to 35 nm, remarkable increases in dielectric constant and tunability were observed. Above 12 nm, the BST films exhibited size effects, i.e. a decrease in maximal polarization (P_m) and an increase in coercive electric field (E_c) with reduction in grain size. In our investigation, the dielectric constant, tunability and maximal polarization increased as the film thickness increased. Furthermore, the size dependence of the dielectric constant and tunability of Ba_0.6Sr_0.4TiO₃ thin films is determined by that of the maximal polarization and the coercive electric field.     

First-principles study on the structure instability and electronic structure of cubic Ba0.5Sr0.5TiO3

We present first-principles calculations on the structure instability and the electronic structure properties of cubic Ba_0.5Sr_0.5TiO₃ (BST). The calculated total energy result shows that the Sr ions have a more important effect on the structure instability of BST system than the Ba ions. The off-center displacement of the Sr ions will lower the system energy and makes it instable. In order to understand the interaction between ions, the density of states and the charge density distribution were calculated. From the analysis of the density of states, we conclude that the hybridization between Ba p and O p is stronger than that between Sr p and O p. This is consistent with the analysis of the charge density distribution.     

Ba0.5Sr0.5TiO3电子结构和光学性质的第一性原理研究

利用第一性原理研究了Ba_0.5Sr_0.5TiO₃的能带结构和光学性质.结果表明,导带和价带都来源于钛原子3d轨道和氧原子2p轨道的杂化.导带主要由钛原子的3d轨道贡献,价带主要由氧原子的2p轨道贡献.吸收系数为10⁵ cm^-1量级,且吸收主要集中在低能区.折射率为n（0）=2.1,结果与实验符合. 关键词： 第一性原理 能带结构 光学性质     

Vibrational spectroscopic studies on Ba0.8Sr0.2TiO3 thin films prepared by RF sputtering technique

Thin films of Ba_0.8Sr_0.2TiO₃ have been deposited on p-type Si substrate by radio frequency magnetron sputtering. Polycrystalline bulk Ba_0.8Sr_0.2TiO₃ sample has also been studied for comparison. X-ray diffraction patterns reveal that both the bulk sample and thin films are polycrystalline without any preferential orientation and belong to paraelectric cubic phase. We have compared the room temperature Raman and IR spectra of powder and thin films (both annealed and as-deposited) of Ba_0.8Sr_0.2TiO₃. The extra feature in the Raman spectrum for the annealed film has been explained as due to the presence of intergrain stresses from the submicron size grains in it.     

Phase diagram and dielectric behavior of Ba0.6Sr0.4TiO3 thin films grown on orthorhombic substrates

Using Landau-Devonshire (LD)-type phenomenological model, we investigate the phase diagrams and dielectric behaviors of single-domain single-crystal Ba_0.6Sr_0.4TiO₃ films deposited on orthorhombic substrates. An anisotropic strain factor is introduced to quantitatively calculate the effects of anisotropic in-plane misfit strains. Investigation indicates that anisotropic strains play a crucial role on formation of stable ferroelectric phases and dielectric properties. The anisotropic strains induce tetragonal phases which only contain one in-plane spontaneous polarization component. These phases do not exist in BST films of the same composition under isotropic strains. Moreover, permittivity and tunability of films can reach to maximum when the corresponding spontaneous polarization component disappears at the boundaries of structural phase transition.     

Tunable Ba0.5Sr0.5TiO3 film bulk acoustic resonators using SiO2/Mo Bragg reflectors

Tunable and switchable Ba 0.5 Sr 0.5 TiO 3 film bulk acoustic resonators（FBARs） based on SiO 2 /Mo Bragg reflectors are explored,which can withstand high temperature for the deposition of Ba x Sr 1 x TiO 3（BST） films at 800 C.The dc bias-dependent resonance may be attributed to the piezoelectricity of the BST film induced by an electrostrictive effect.The series resonant frequency is strongly dc bias-dependent and shifts downwards with dc bias increasing,while the parallel resonant frequency is only weakly dc bias-dependent and slightly shifts upwards at low dc bias（ 45 V） while downwards at higher dc bias.The calculated relative tunability of shifts at series resonance frequency is around 2.3% and the electromechanical coupling coefficient is up to approximately 8.09% at 60-V dc bias,which can be comparable to AlN FBARs.This suggests that a high-quality tunable BST FBAR device can be achieved through the use of molybdenum（Mo） as the high acoustic impedance layer in a Bragg reflector,which not only provides excellent acoustic isolation from the substrate,but also improves the crystallinity of BST films withstanding higher deposition temperature.     

Properties of K0.5Bi0.5TiO3 thin films deposited on different substrates

K_0.5Bi_0.5TiO₃ thin films were deposited on fused quartz, n-type Si(100) and Pt/TiO₂/SiO₂/Si substrates by repeated coating/dying cycles. X-ray diffraction analysis shows that the films annealed at 700 °C for 10 min present perovskite phase. Atomic force microscopy reveals that the surface morphology is smooth, the grain sizes of the films on Si(100) are quite larger than on fused quartz. The capacitance-voltage hysteresis loops at various sweeping speed are collected as are polarization types. The films in the ON and OFF states are relatively stable. The films also exhibit a hysteresis loop at an applied voltage of 4 V, with a remanent polarization of 9.3 μC/cm² and a coercive voltage of 2 V. The insulating property of negative bias voltage is better than that of positive bias voltage. The transmittance of the films is between 74 and 82% in the wavelength range of 200-2000 nm.     

Effect of annealing temperature on microstructure, optical and electrical properties of sputtered Ba0.9Sr0.1TiO3 thin films

Ba_0.9Sr_0.1TiO₃ (BST) thin films were deposited on fused quartz and Pt/TiN/Si₃N₄/Si substrates by radio frequency magnetron sputtering technique. Microstructure and chemical bonding states of the BST films annealed at 700 °C were characterized by field emission scanning electron microscopy, X-ray photoelectron spectroscopy, glancing angle X-ray diffraction and Raman spectrum. Optical constants including refractive indices, extinction coefficients and bandgap energies of the as-deposited BST film and the BST films annealed at 650, 700 and 750 °C, respectively, were determined from transmittance spectra by envelope method and Tauc relation. Dielectric constant and remnant polarization for the BST films increase with increasing annealing temperature. Leakage current density-applied voltage (J–V) data indicate that the dominant conduction mechanism for all the BST capacitors is the interface-controlled Schottky emission under the conditions of 14 V < V < 30 V and −30 V < V < −14 V. Furthermore, the inequipotential J–V characteristics for the BST films annealed at various temperatures are mainly attributed to the combined effects of the different thermal histories, relaxed stresses and strains, and varied Schottky barrier heights in the BST/Pt and Pt/BST interfaces.     

Effects of interface on the dielectric properties of Ba0.6Sr0.4TiO3 thin film capacitors

Ba_0.6Sr_0.4TiO₃ thin films were deposited on Pt/SiO₂/Si substrate by radio frequency magnetron sputtering. High-resolution transmission electron microscopy (HRTEM) observation shows that there is a transition layer at BST/Pt interface, and the layer is about 7-8 nm thickness. It is found that the transition layer was diminished to about 2-3 nm thickness by reducing the initial RF sputtering power. X-ray photoelectron spectroscopy (XPS) depth profiles show that high Ti atomic concentration results in a thick interfacial transition layer. Moreover, the symmetry ν of ?_r-V curve of BST thin film is enhanced from 52.37 to 95.98%. Meanwhile, the tunability, difference of negative and positive remanent polarization (P_r), and that of coercive field (E_C) are remarkably improved.     

The influence of Co doping on the dielectric, ferroelectric and ferromagnetic properties of Ba0.70Sr0.30TiO3 thin films

Ba_0.70Sr_0.30TiO₃ (BST) thin films doped by Co (BSTC) are fabricated by sol-gel method on a Pt/Ti/SiO₂/Si substrate. A strong correlation is observed among the microstructure, dielectric, ferroelectric, ferromagnetic properties and Co concentration. The dielectric constant of BST thin films can be tailored from 343 to 119 by manipulating the Co concentration. The dielectric loss of BSTC thin films are still kept below 0.020 and the tunability is above 30% at a dc-applied electric field of 500 kV/cm. With increasing Co doping up to 10 mol%, the coexistence of ferromagnetism and ferroelectrics is found. Suitable dielectric constant, low-dielectric loss, and high tunability of this kind of thin films can be useful for potential tunable applications.