Thickness dependent magnetic and ferroelectric properties of LaNiO3 buffered BiFeO3 thin films

BiFeO₃ (BFO) thin films with thickness increasing from 40 to 480 nm were successfully grown on LaNiO₃ (LNO) buffered Pt/Ti/SiO₂/Si(100) substrate and the effects of thickness evolution on magnetic and ferroelectric properties are investigated. The LNO buffer layer promotes the growth and crystallization of BFO thin films. Highly (100) orientation is induced for all BFO films regardless of the film thickness together with the dense microstructure. All BFO films exhibited weak ferromagnetic response at room temperature and saturation magnetization is found to decrease with increase in film thickness. Well saturated ferroelectric hysteresis loops were obtained for thicker films; however, the leakage current dominated the ferroelectric properties in thinner films. The leakage current density decreased by three orders of magnitude for 335 nm film compared to 40 nm film, giving rise to enhanced ferroelectric properties for thicker films. The mechanisms for the evolution of ferromagnetic and ferroelectric characteristics are discussed.     

Effect of film thickness on interface and electric properties of BiFeO3 thin films

Bismuth ferrite (BFO) thin films were fabricated by RF-magnetron sputtering deposition method on Pt/Ti/SiO₂/Si(1 0 0) substrate. The effect of the thickness of BFO films varying from 85 to 280 nm on electrical properties was investigated. Saturated coercive fields were found to increase with the BFO film thickness. The dielectric constant of BFO thin films measured at 1 kHz decreased with decreasing thickness from 98 to 86, while tangent losses increased from 0.013 to 0.021. The presence of bismuth oxide at the interface between BFO films and Pt bottom electrodes was responsible for the high leakage currents in thin BFO thin films as was demonstrated by X-ray diffraction, grazing-incident X-ray diffraction, and secondary ion mass spectroscopy analysis.     

Effect of bilayer structure and a SrRuO3 buffer layer on ferroelectric properties of BiFeO3 thin films

Effects of the BiFe_0.95Mn_0.05O₃ thickness and a SrRuO₃ (SRO) buffer layer on the microstructure and electrical properties of BiFeO₃/BiFe_0.95Mn_0.05O₃ (BFO/BFMO) bilayered thin films were investigated, where BFO/BFMO bilayered thin films were fabricated on the SRO/Pt/Ti/SiO₂/Si(100) substrate by a radio frequency sputtering. All thin films are of a pure perovskite structure with a mixture of (110) and (111) orientations regardless of the BFMO layer thickness. Dense microstructure is demonstrated in all thin films because of the introduction of BFMO layers. The SRO buffer layer can also further improve the ferroelectric properties of BFO/BFMO bilayered thin films as compared with those of these thin films without a SRO buffer layer. The BFO/BFMO bilayered thin film with a thickness ratio of 220/120 has an enhanced ferroelectric behavior of 2P _r??165.23???C/cm² and 2E _c??518.56?kV/cm, together with a good fatigue endurance. Therefore, it is an effective way to enhance the ferroelectric and fatigue properties of bismuth ferrite thin films by constructing such a bilayered structure and using a SRO buffer layer.     

Influence of oxygen pressure on the ferroelectric properties of BiFeO3 thin films on LaNiO3/Si substrates via laser ablation

BiFeO₃(BFO) thin films of about 200 nm in thickness have been successfully grown on oxide bottom electrode, LaNiO₃(LNO), via pulsed laser ablation. X-ray diffraction spectrum of the as-deposited BFO film reveals a (100) preferred textured structure. The morphology of the BFO film is found to be strongly dependent on oxygen partial pressure in laser ablation. A saturated hysteresis loop with remanent polarization of 42 μC/cm² and coercive field of 100 kV/cm is obtained at the film deposition at 50 mTorr. The dielectric properties have also been obtained based on the influence of the oxygen pressure.     

Modification in structure of La and Nd co‐doped epitaxial BiFeO3 thin films probed by micro Raman spectroscopy

The influence of La and Nd co‐substitution on structure, electric and magnetic properties of epitaxial thin films of BiFeO₃ (BFO) was examined. We demonstrate structural phase transition in co‐doped La and Nd BFO thin films using Raman spectroscopy. Based on group theoretical analysis of the number and symmetry of Raman lines, we provide strong experimental evidence that the structure has been changed from rhombohedral to monoclinic due to co‐doping in BFO. The change in structure was also reflected in morphology of these films. Room temperature magnetic hysteresis curves showed that doped films exhibit enhanced ferromagnetic properties with remnant magnetization of ~10 emu/cm³ and coercive field of 1.2 kOe. The enhanced magnetic properties highlight the potential applications of doped BLNFO thin film for smart devices. Copyright © 2015 John Wiley & Sons, Ltd.     

Electric field driven phase transition and possible twining quasi-tetragonal phase in compressively strained BiFeO3 thin films

Highly compressively strained BiFeO₃ thin films with different thickness are epitaxially grown on (001) LaAlO₃ substrates and characterized using various techniques. The quasi-tetragonal phase with a giant axial ratio of ?? 1.25 and its thickness-dependent evolution are investigated. An interesting twining structure of the quasi-tetragonal phase is evidenced in thicker films through detailed reciprocal space mapping, which becomes more pronounced with increasing film thickness. Moreover, an interesting electric-field driven phase transition was evidenced in the film with a thickness of 38 nm, in which the quasi-tetragonal and rhombohedral phases are close to each other in energy landscape.     

Thickness dependent phase transformation of magnetron-sputtered Ni–Mn–Sn ferromagnetic shape memory alloy thin films

In this study, the influence of film thickness on the first-order martensite–austenite phase transformation of Ni–Mn–Sn ferromagnetic shape memory alloy thin films has been systematically investigated. Different thicknesses of the Ni–Mn–Sn films (from ~100 to 2,500 nm) were deposited by DC magnetron sputtering on Si (100) substrates at 550 °C. X-ray analysis reveals that all the films exhibit austenitic phase with the L2₁ cubic crystal structure at room temperature. The grain size and crystallization extent increase with the increase in film thickness, but the films with thickness above ~1,400 nm show structural deterioration due to the formation of MnSn₂ and Ni₃Sn₄ precipitates. The improvement in the crystallinity of the film with thickness is attributed to the decrease in film–substrate interfacial strain resulting in preferred oriented growth of the films. Temperature-dependent magnetization measurements as well as electrical measurements demonstrate the complete absence of phase transformation for the film of thickness of ~120 nm. For thickness greater than 400 nm, film exhibits the structural transformation, and it occurs at higher temperature with better hysteresis as film thickness is increased up to ~1,400 nm, after which degradation of phase transformation phenomenon is observed. This degradation is attributed to the disorders present in the films at higher thicknesses. Film with thickness ~1,400 nm possesses the highest magnetization with the smallest thermal hysteresis among all the films and therefore best suited for the actuators based on first-order structural phase transformation. Nanoindentation measurements reveal that the higher values of hardness and elastic modulus of about 5.5 and 215.0 GPa obtained in film of 1,014 nm thickness can considerably improve the ductility of ferromagnetic shape memory alloys (FSMA) and their applicability for MEMS applications. The exchange bias phenomenon is also found to be present in the films of thickness 1014, 1412, and 2022 nm exhibiting prominent martensitic transformation. Film of thickness 2,022 nm exhibits maximum exchange bias of ~50 Oe and higher exchange bias blocking temperature of 70 K as compared to other films.     

Magnetic field annealing effects on self-oriented BiFeO3 thin films prepared by chemical solution deposition

Self-oriented BiFeO₃ (BFO) thin films are prepared via chemical solution deposition method with magnetic field in-situ annealing process. The effects of magnetic annealing on the microstructure, magnetic and dielectric properties as well as magnetoelectric coupling effect of the BFO thin films are investigated. With increasing the annealing magnetic field, the crystallization quality, texture, grain boundary connectivity and densification of the films are enhanced, which is attributed to the improvement of connection and diffusion of components. The magnetization of the field-annealing films and dielectric constant as well as remanent polarization increases with increasing the strength of annealing magnetic field. In addition, it is observed that magnetocapacitance value of the magnetic-field-annealing BFO thin film is higher than the non-field-annealing one. Moreover the BFO thin films annealed at 3 kOe magnetic field show the magnetoelectric effect with 4% under 2 kOe at room temperature.     

Influence of thickness on the domain structure of barium-strontium-titanate films on MgO substrates

Ba_0.8Sr_0.2TiO₃/MgO(001) heterostructures are studied using atomic-force spectroscopy in several modes (the contact and semicontact modes and that of piezoelectric response) to obtain information on the relief and the distribution of the electric potential on the surface. Based on data obtained for films with different thicknesses, the correlation between the domain sizes and orientations and the film thickness and the surface relief is established. It is shown that the films with thicknesses less than 36 nm contain only aa domains whose dimensions decrease with increasing film thickness. The films with thicknesses greater than 36 nm contain only c domains whose dimensions increase with increasing film thickness.     

Bi1-xLaxFeO3±δ薄膜的快速制备及铁电性

样品的制备是对影响样品质量的各个工艺参数进行优化的过程. 传统的试错法是对各个参数逐个进行尝试, 需要的周期较长. 与传统的单参数尝试法相比, 高通量样品制备方法可以对参数实现并行筛选, 因而极大地缩短了优化工艺所需的时间. 本工作借助高通量制备方法成功实现系列镧掺杂BiFeO₃薄膜的快速优化, 包括对烧结温度、镧元素含量和高温固态反应气氛等关键工艺参数的快速筛选, 同时分析了不同生长条件下样品的结构并测试了其铁电性. 实验结果表明: 1) 560 ℃ 烧结可得到单相薄膜; 2)测量不同La含量样品的铁电性, 发现当E=75 kV/cm时, La=15%的样品剩余极化值(2Pr)最大, 约为26.7 μC/cm²; 3) 在纯氧气氛下烧结有助于得到结晶性更好的单相Bi_0.75La_0.25FeO_3±δ 薄膜, 并且能够提高薄膜的铁电性.     

Photoluminescence from ZnO-SiO2 opals with different sphere diameters and thicknesses

We systematically investigated the photoluminescence (PL) and transmittance characteristics of ZnO-SiO₂ opals with varied positions of the stop-band and film thicknesses. An improved ultraviolet (UV) luminescence was observed from ZnO-SiO₂ composites over pure ZnO nanocrystals under 325 nm He-Cd laser excitation at room temperature. The UV PL of ZnO nanocrystals in SiO₂ opals with stop-bands center of 410 nm is sensitive to the thickness of opal films, and the UV PL intensity increases with the film thickness increasing. The PL spectra of ZnO nanocrystals in SiO₂ opals with stop-bands center of 570 nm show a suppression of the weak visible band. The experimental results are discussed based on the scattering and/or absorbance in opal crystals.     

Study on solid structure of pentacene thin films using Raman imaging

We present a 532‐nm excited Raman imaging study of pentacene thin films (thickness, 2, 5, 10, 20, 50, 100, and 150 nm) prepared on an SiO₂ surface. The structure of the pentacene films has been investigated by images and histograms of the ratio (R) of intensity of the 1596‐cm⁻¹ band (b_3g) to that of the 1533‐cm⁻¹ band (a_g), which can be used as a marker of solid‐state phases: 1.54‐nm and 1.44‐nm phases. The Raman images showed that island‐like 1.44‐nm phase domains are grown on the 1.54‐nm phase layer from 50 nm, and all the surface of the 1.54‐nm phase layer is covered with the 1.44‐nm phase layer from 100 nm. The structural disorders have been discussed on the basis of the full width at half maximum of a band in the histogram of the R values for each film. Copyright © 2012 John Wiley & Sons, Ltd.     

单晶Si表面离子束溅射沉积Co纳米薄膜的研究

采用离子束溅射沉积法，在单晶Si基片上制备了不同厚度(1—100nm)的Co纳米薄膜.利用原子力显微镜、X射线光电子能谱(XPS)仪和X射线衍射仪对不同厚度的Co纳米薄膜进行了分析和研究.结果表明：当薄膜厚度为1—10nm时，沉积颗粒形态随薄膜厚度增加将由二维生长的细长胞状过渡到多个颗粒聚集成的球状.当膜厚大于10nm时，小颗粒球聚集成大颗粒球，颗粒球呈现三维生长状态.表面粗糙度随膜厚的增加呈现先增加后减小的趋势，在膜厚为3nm时出现极值.XPS全程宽扫描和窄扫描显示：薄膜表面的元素成分为Co，化学态分别 关键词： 离子束沉积 纳米薄膜 X射线光电子能谱 X射线衍射     

Improvement of corrosion resistance of carbon steel using chemical vapor deposition from Cr(CO)6 with an ArF excimer laser

Corrosion resistance of carbon steel coated with thin film deposited from Cr(CO)₆ using an ArF excimer laser (193 nm) has been evaluated by an electrochemical method as a function of laser beam intensity. The carbon steel coated with the film formed at higher beam intensity shows higher corrosion resistance. Microstructure, composition, and thickness of the films have also been investigated. SEM micrographs show that the films consist of small grains which decrease in size with increasing beam intensity. Auger electron spectroscopy (AES) combined with Ar⁺ beam sputtering reveals that the films deposited at higher beam intensity give higher chromium content, and that the thickness at a fixed total irradiation energy increases up to the intensity of 10 MW cm^–2, falling above this intensity. In addition, the change of film thickness by addition of buffer gases (Ar, CO, and H₂O) has been investigated. The thickness is 10 times smaller under the addition of H₂O, and twice smaller under the addition of Ar or CO than without the addition of gases. A deposition mechanism based on photolysis of Cr(CO)₆ in the gas phase is proposed related to the experimental data after the discussion of several possible mechanisms.     

Probing the ferroelectric phase transition in sol–gel‐derived polycrystalline bismuth ferrite thin films

Polycrystalline BiFeO₃ (BFO) thin films were successfully grown on Pt/Ti/SiO₂/Si(100) and SrTiO₃ (STO) (100) substrates using the chemical solution deposition (CSD) technique. X‐ray diffraction (XRD) patterns indicate the polycrystalline nature of the films with rhombohedrally distorted perovskite crystal structure. Differential thermal analysis (DTA) was performed on the sol–gel‐derived powder to countercheck the crystal structure, ferroelectric (FE) to paraelectric (PE) phase transition, and melting point of bismuth ferrite. We observed a significant exothermic peak at 840 °C in DTA graphs, which corresponds to an FE–PE phase transition. Raman spectroscopy studies were carried out on BFO thin films prepared on both the substrates over a wide range of temperature. The room‐temperature unpolarized Raman spectra of BFO thin films indicate the presence of 13 Raman active modes, of which five strong modes were in the low‐wavenumber region and eight weak Raman active modes above 250 cm⁻¹. We observed slight shifts in the lower wavenumbers towards lower values with increase in temperature. The temperature‐dependent Raman spectra indicate a complete disappearance of all Raman active modes at 840 °C corresponding to the FE–PE phase transitions. There is no evidence of soft mode phonons. Copyright © 2008 John Wiley & Sons, Ltd.     

Influence of oxygen depletion layer on the properties of tin oxide gas-sensing films fabricated by atomic layer deposition

In this paper we report on the influence of film thickness on the electrical and gas-sensing properties of tin oxide thin films grown by atomic layer deposition (ALD) technique. The nature of the carrier and post-flow gases used in ALD was found to have a dramatic influence on the electrical conductance of the deposited films. Up to a film thickness of 50 nm the sheet conductance of the films increased with the thickness, and above 50 nm the sheet conductance was not significantly influenced by the film thickness. This effect was attributed to oxygen depletion at the film surface. When the depth of oxygen depletion (d _dep) was greater than or equal to the film thickness (t), the sheet conductance was thickness dependant. On the other hand, when d _dep≤t, the sheet conductance was independent of the film thickness but depended on the depth of the oxygen depletion. This proposed explanation was verified by subjecting the films to different lengths of post-annealing in an oxygen depleted atmosphere. Gas-sensing functionality of the films with various thicknesses was examined. It was observed that the film thickness had a significant influence on the gas-sensing property of the films. When the thickness was greater than 40 nm, the sensitivity of the films to ethanol was found to follow the widely reported trend, i.e., the sensitivity decreases when the film thickness increases. Below the film thickness of 40 nm the sensitivity decreases as film thickness decreases, and we propose a model to explain this observation based on the increase in resistance due to multiple grain boundaries.     

Organic film thickness influence on the bias stress instability in?sexithiophene field effect transistors

In this paper, the dynamics of bias stress phenomenon in Sexithiophene (T6) Field Effect Transistors (FETs) has been investigated. T6 FETs have been fabricated by vacuum depositing films with thickness from 10 to 130 nm on Si/SiO₂ substrates. After the T6 film structural analysis by X-ray diffraction and the FET electrical investigation focused on carrier mobility evaluation, bias stress instability parameters have been estimated and discussed in the context of existing models. By increasing the film thickness, a clear correlation between the stress parameters and the structural properties of the organic layer has been highlighted. Conversely, the mobility values as a result are found to be almost thickness independent.     

Coherent phonon generation and detection in ultrathin SrTiO3 grown directly on silicon

Time‐resolved two color pump‐probe polarization spectroscopy was performed at room temperature on SrTiO₃ films grown directly on Si with film thickness varying from 2 nm to 7.8 nm. An E‐symmetry mode with a characteristic frequency of 0.2 THz is impulsively generated and measured in these coherently strained tetragonal phase SrTiO₃ thin films. A superimposed exponentially decaying signal observed indicates the possible relaxational hopping of Ti ion between double potential wells. The dependence of the coherent phonon signal on pump and probe laser polarization helps to identify the symmetry of the phonon modes.     

Stability of polymer films as a 2D system

According to classical theory of phase transition, fluctuations in systems with low dimensions are so violent that the phase boundary between unstable and metastable states would be smeared. In this experiment, we measure the growth of surface fluctuations on an unstable polymer film with a thickness, h₀ = 5.1 nm, which is much less than the spinodal thickness, h _sp (= 243 nm) thereby the film is in the very deep unstable region. We find the film to show rupturing behavior markedly different from that of an unstable film. Specifically, nucleation of holes – a characteristic rupturing feature of metastable films – is prominent, which is surprising for a film in the very deep unstable region even provision is given to thin films being two-dimensional and hence are susceptible to broadening of the phase boundary by fluctuations. Monte Carlo simulation shows that the nucleated holes can be caused by stochastic thermal fluctuations. Our result thus confirms the broadening of the phase boundary in thin films by fluctuations to be extremely large. As a consequence, the phase behavior of thin films cannot be predicted by the mean-field calculated phase boundary, which however has been the general practice so far.