Preparation and structural properties of YBCO films grown on GaN/c-sapphire hexagonal substrate

Epitaxial YBCO thin films have been grown on hexagonal GaN/c-sapphire substrates using DC magnetron sputtering and pulsed laser deposition. An MgO buffer layer has been inserted between the substrate and the YBCO film as a diffusion barrier. X-ray diffraction analysis indicates a c-axis oriented growth of the YBCO films. Φ-scan shows surprisingly twelve maxima. Transmission electron microscopy analyses confirm an epitaxial growth of the YBCO blocks with a superposition of three a-b YBCO planes rotated by 120° to each other. Auger electron spectroscopy and X-ray photoelectron spectroscopy reveal no surface contamination with Ga even if a maximum substrate temperature of 700 °C is applied.     

Effect of CeO2 buffer layer thickness on the structures and properties of YBCO coated conductors

Biaxially textured YBa₂Cu₃O_7−x (YBCO) films were grown on inclined-substrate-deposited (ISD) MgO-textured metal substrates by pulsed laser deposition. CeO₂ was deposited as a buffer layer prior to YBCO growth. CeO₂ layers of different thickness were prepared to evaluate the thickness dependence of the YBCO films. The biaxial alignment features of the films were examined by X-ray diffraction 2θ-scans, pole-figure, ?-scans and rocking curves of Ω angles. The significant influence of the CeO₂ thickness on the structure and properties of the YBCO films were demonstrated and the optimal thickness was found to be about 10 nm. High values of T_c = 91 K and J_c = 5.5 × 10⁵ A/cm² were obtained on YBCO films with optimal CeO₂ thickness at 77 K in zero field. The possible mechanisms responsible for the dependence of the structure and the properties of the YBCO films on the thickness of the CeO₂ buffer layers are discussed.     

Effect of deposition conditions on YBa2Cu3O7−δ thin films by inverted cylindrical magnetron sputtering and substrate effects

The dependence of YBCO thin film properties on the deposition conditions was studied for different substrates. The deposition conditions were optimized for the epitaxial growth of high quality YBCO thin films of 1500 Å thickness onto single crystal (100-oriented) SrTiO₃ (STO), MgO and LaAlO₃ (LAO) substrates by DC Inverted Cylindrical Magnetron Sputtering (ICMS). The samples were investigated in detail by means of X-ray diffraction analysis (XRD), EDX, AFM, ρ-T, magnetic susceptibility and current-voltage (I-V) characterizations. The samples show strong diamagnetic behavior and sharp transition temperatures of 89-91 K with ΔT<0.5 K. XRD of the samples exhibited highly c-axis orientation. The full width at half maximum (FWHM) values of the rocking curves were ranging typically from 0.22 to 0.28°. The samples have smooth surfaces as shown from AFM micrographs. The surface roughness, R_a, changed between 5-7 nm. I-V characteristics were obtained from the 20 μm-wide microbridges, which were patterned by a laser writing technique. The critical current densities (J_c, 1.06×10⁶ for LAO-based YBCO, 1.39×10⁶ for MgO-based YBCO, 1.67×10⁶ A/cm² for STO based YBCO) of the microbridges were evaluated from I-V curves at 77 K.     

Ferroelectric properties of highly c-axis oriented Bi4−xLaxTi3O12 film-based capacitors

Highly c-axis oriented lanthanum-modified bismuth titanate (Bi_4−xLa_xTi₃O₁₂) films having a variety of lanthanum (La) contents were grown on Pt/TiO₂/SiO₂/Si(100) substrates using metal-organic sol deposition and subsequent annealing at 650 °C for 1 h. After systematically examining the ferroelectric properties of Bi_4−xLa_xTi₃O₁₂ films as a function of the La-content, it was concluded that the film with x=0.85 had the largest remanent polarization in the direction parallel to the c-axis. The Pt/Bi_3.15La_0.85Ti₃O₁₂/Pt capacitor showed a well-saturated polarization-electric field (P-E) switching curve with the switching remanent polarization (2P_r) value of 33 μC/cm² and the coercive field (E_c) of 68 kV/cm at an applied voltage of 10 V. More importantly, the capacitor exhibited fatigue-free behavior up to 6.5×10¹⁰ read/write switching cycles at a frequency of 1 MHz. The capacitor also demonstrated an excellent charge-retaining ability and a strong resistance against the imprinting failure.     

Pulsed-laser crystallization and epitaxial growth of metal-organic films of Ca-doped LaMnO3 on STO and LSAT substrates

Ca-doped LaMnO₃ (LCMO) thin films have been successfully prepared on SrTiO₃ (STO) and [(LaAlO₃)_0.3-(SrAlTaO₆)_0.7] (LSAT) substrates using the excimer laser assisted metal-organic deposition (ELAMOD) process. The crystallization and the epitaxial growth of the amorphous metal-organic LCMO thin films have been achieved using a KrF excimer laser irradiation while the substrates were kept at constant temperature of 500 °C. Epitaxial films were obtained using laser fluence in the interval of 50-120 mJ/cm². The microstructure of the LCMO films was studied using cross-section transmission electron microscopy. High quality of LCMO films having smooth surfaces and sharp interfaces were obtained on both the STO and the LSAT substrates. The effect of the laser fluence on the temperature coefficient of resistance (TCR) was investigated. The largest values of TCR of the LCMO grown on the LSAT and the STO substrates of 8.3% K⁻¹ and 7.46% K⁻¹ were obtained at different laser fluence of 80 mJ/cm² and 70 mJ/cm², respectively.     

Enhanced ferroelectric property of (Pb0.95Ca0.05)(Nb0.02Zr0.80Ti0.20)O3 thin films prepared by RF magnetron sputtering

(Pb_0.95Ca_0.05)(Nb_0.02Zr_0.80Ti_0.20)O₃ [PCNZT] thin films were deposited on the Pt(1 1 1)/Ti/SiO₂/Si(1 0 0) substrates by RF magnetron sputtering with and without a LaNiO₃ [LNO] buffer layer. Ca and Nb elements in PZT films enhance the ferroelectric property, LaNiO₃ buffer layer improves the crystal quality of the PCNZT thin films. PCNZT thin films possess better ferroelectric property than that of PZT films for Ca and Nb ion substitution, moreover, PCNZT thin films with a LNO buffer layer possess (1 0 0) orientation and good ferroelectric properties with high remnant polarization (P_r = 38.1 μC/cm²), and low coercive field (E_c = 65 kV/cm), which is also better than that of PCNZT thin films without a LNO buffer layer (P_r = 27.9 μC/cm², E_c = 74 kV/cm). The result shows that enhanced ferroelectric property of PZT films can be obtained by ion substitution and buffer layer.     

Preparation of YBCO film for microwave filter using a hybrid route

Pure (0 0 l)-textured CeO₂ buffer layers were deposited on single crystal r-plane Al₂O₃ (1–102) substrate by a hybrid process which was combined with magnetron sputtering for the seed layer and metal–organic deposition for the subsequent layer. Strongly c-axis oriented YBCO films were deposited on the CeO₂ buffered r-cut Al₂O₃ (1–102) substrates. Atomic force microscope and scanning electronic microscopy results show that the prepared buffers and YBCO films are relatively dense and smooth. The critical current of the YBCO films exceeds 1.5 MA/cm² at 77 K with the superconducting transition temperature of 90 K. The surface resistivity is as below as 14 μΩ at 1 GHz frequency. The results demonstrate that the hybrid route is a very promising method to prepare YBCO films for microwave application, which can combine the sputtering advantage for preparing of highly c-axis oriented CeO₂ buffer layers and the advantages of metal–organic deposition with rapid processing, low cost and easy preparation of large-area YBCO films.     

Enhancement of L10 ordered FePt by Pt buffer layer

L₁₀-ordered FePt thin films prepared by molecular-beam epitaxy on MgO (0 0 1) substrate at 320 °C with different thickness of Pt buffer layer have been investigated. The out-of-plane coercivity increases with increasing thickness of Pt buffer. The maximum values of the long-range order parameter and uniaxial magnetic anisotropy energy are 0.72 and 1.78×10⁷ erg/cm³, respectively, for films with 12 nm thick Pt buffer layer, where the c/a ratio (0.976) shows the minimum value. The reason for the enhancement in ordering is due to the proper lattice strains Pt buffer bestows on FePt layer, these strains are equal to the contraction in lattice parameter c and the expansion in a. Studies of angular-dependent coercivity revealed that the magnetization reversal behaviour shifts from a domain-wall motion dominated case towards a near rotational mode with increasing thickness of Pt buffer layer.     

Optimal growth conditions for GdBa2Cu3O7 thin-film coated conductors characterized by polarized Raman scattering spectroscopy

High temperature superconducting GdBa₂Cu₃O₇ (GdBCO) thin films were grown by pulsed laser ablation. Textured MgO on metal substrates was used as a template for second generation wire applications. Growth conditions of GdBCO thin films were investigated for substrate temperature (T_s) and oxygen partial pressure (PO₂) during deposition. Superconducting critical currents of the films were obtained in the films grown at 790–810 °C of T_s and at 100–700 mTorr of PO₂. Scanning electron micrographs of the films revealed uniform and well-connected grains with some outgrown structures. X-ray θ–2θ scans of the films grown at 810 °C and 300–500 mTorr exhibited c-axis oriented texture. In-plane alignment and c-axis mosaic spread of the films were determined from X-ray Φ scans and rocking curves, respectively. Polarized Raman scattering spectroscopy was used to characterize optical phonon modes, oxygen content, cation disorder, and some possible second phases of the films. The Raman spectra of the films with large critical current density showed modes at 326–329 cm⁻¹, 444–447 cm⁻¹, 500–503 cm⁻¹ related to vibration of oxygen atoms. Origin of small peaks near 600 cm⁻¹ will be discussed as well. The information obtained from Raman scattering measurements will be useful for quality control of the conductors as well as optimization of the process conditions.     

Laser-induced voltage characteristics of Bi2Sr2Co2Oy thin films on LaAlO3 substrates

We report the laser-induced voltage (LIV) effects in c-axis oriented Bi₂Sr₂Co₂O_y thin films grown on (0 0 1) LaAlO₃ substrates with the title angle α of 0°, 3°, 5° and 10° by a simple chemical solution deposition method. A large open-circuit voltage with the sensitivity of 300 mV/mJ is observed for the film on 10° tilting LaAlO₃ under a 308 nm irradiation with the pulse duration of 25 ns. When the film surface is irradiated by a 355 nm pulsed laser of 25 ps duration, a fast response with the rise time of 700 ps and the full width at half maximum of 1.5 ns is achieved. In addition, the experimental results reveal that the amplitude of the voltage signal is approximately proportional to sin 2α and the signal polarity is reversed when the film is irradiated from the substrate side rather than the film side, which suggests the LIV effects in Bi₂Sr₂Co₂O_y thin films originate from the anisotropic Seebeck coefficient of this material.     

Compositional dependence of ferroelectric properties of highly c-axis oriented Bi4−xNdxTi3O12 film capacitors

Highly c-axis oriented neodymium-modified bismuth titanate (Bi_4−xNd_xTi₃O₁₂) films having a variety of neodymium (Nd) contents were successfully grown on Pt/TiO₂/SiO₂/Si(100) substrates using metal-organic sol decomposition. After systematically examining ferroelectric properties of the c-axis oriented Bi_4−xNd_xTi₃O₁₂ film capacitors as a function of the Nd-content, we concluded that the capacitor with x=0.85 had the largest remanent polarization. The Bi_3.15Nd_0.85Ti₃O₁₂ capacitor fabricated using a top Pt electrode showed well-saturated polarization-electric field (P-E) switching curves with the remanent polarization (P_r) of 51 μC/cm² and the coercive field (E_c) of 99 kV/cm at an applied voltage of 10 V. More importantly, the Pt/Bi_3.15Nd_0.85Ti₃O₁₂/Pt capacitor exhibited fatigue-free behavior up to 4.5×10¹⁰ read/write switching cycles at a frequency of 1 MHz. The capacitor also demonstrated an excellent charge-retaining ability and a strong resistance against the imprinting failure.     

Luminescent characteristics of CaTiO3:Pr thin films prepared by pulsed laser deposition method with various substrates

CaTiO₃:Pr³⁺ films were deposited on different substrates such as Al₂O₃ (0 0 0 1), Si (1 0 0), MgO (1 0 0), and fused silica using pulsed laser deposition method. The crystallinity and surface morphology of these films were investigated by XRD and SEM measurements. The films grown on the different substrates have different crystallinity and morphology. The FWHM of (2 0 0) peak are 0.18, 0.25, 0.28, and 0.30 for Al₂O₃ (0 0 0 1), Si (1 0 0), MgO (1 0 0), and fused silica, respectively. The grain sizes of phosphors grown on different substrates were estimated by using Scherrer's formula and the maximum crystallite size observed for the thin film grown on Al₂O₃ (0 0 0 1). The room temperature PL spectra exhibit only the red emission peak at 613 nm radiated from the transition of (¹D₂ → ³H₄) and the maximum PL intensity for the films grown on the Al₂O₃ (0 0 0 1) is 1.1, 1.4, and 3.7 times higher than that of the CaTiO₃:Pr³⁺ films grown on MgO (1 0 0), Si (1 0 0), and fused Sillica substrates, respectively. The crystallinity, surface morphology and luminescence spectra of thin-film phosphors were highly dependent on substrates.     

High Curie temperature in B-site ordered Sr2CrWO6 epitaxial thin films

Epitaxial and c-axis oriented double perovskite Sr₂CrWO₆ thin films were prepared on SrTiO₃ (100) and LaAlO₃ (100) substrates by pulsed-laser deposition. Structural, magnetic and transport properties were found to be sensitive to the gas conditions employed during the deposition. A small amount of oxygen along with Ar during the deposition was found to be essential for B-site ordering; such films displayed lattice parameters close to the bulk value and display ferromagnetic metallic behavior. The Curie temperature observed above 500 K in these films is higher than bulk Sr₂CrWO₆ samples. Films grown without oxygen were observed to have long c-parameter and no B-site ordering; they were non-magnetic and semiconducting.     

A new method to preserve the c-axis growth of thick YBa2Cu3O7–δ films grown by pulsed laser deposition

YBa₂Cu₃O_7–δ (YBCO) films were prepared on (1 0 0) MgO substrates by pulsed laser deposition (PLD) method. In order to eliminate the a-axis growth, which is commonly observed in the YBCO film thicker than a critical value, we developed a new PLD target that was sintered at a temperature far below YBCO 123 phase formation. The surface analysis made by AFM technique confirmed that very fine particles of around 20 nm size could be ejected from the new target to the substrate. The fine oxide clusters could be easily moved and incorporated into the YBCO phase thus benefited the c-axis growth even in the thick films. For instance, only the c-axis growth in the new film with a thickness of about 650 nm was larger than a critical thickness of the a-axis growth. However, in the standard film of the same thickness, there is 24.5% of the a-axis growth accompanying the main c-axis growth. Therefore, the c-axis growth could be preserved in the very thick YBCO film by a non-superconducting target.     

Fast epitaxial growth of a-axis- and c-axis-oriented YBa2Cu3O7−δ films on (1 0 0) LaAlO3 substrate by laser chemical vapor deposition

a-axis- and c-axis-oriented YBa₂Cu₃O_7−δ (YBCO) films were epitaxially grown on (1 0 0) LaAlO₃ substrates by laser chemical vapor deposition. The preferred orientation in the YBCO film changed from the a-axis to the c-axis with increasing laser powers from 77 to 158 W (the deposition temperatures from 951 to 1087 K). The a-axis-oriented YBCO film showed in-plane epitaxial growth of YBCO [0 0 1]//LAO [0 0 1], and the c-axis-oriented YBCO film showed that of YBCO [0 1 0]//LAO [0 0 1]. A c-axis-oriented YBCO film with a high critical temperature of 90 K was prepared at a deposition rate of 90 μm h⁻¹, about 2-1000 times higher than that of metalorganic chemical vapor deposition.     

利用脉冲激光沉积技术在双轴织构的Ni基带上外延CeO2薄膜

利用脉冲激光沉积技术在氢还原气氛下成功地在双轴织构的Ni基带上外延了高质量的CeO₂薄膜. x射线衍射θ—２θ扫描和ω扫描结果表明，CeO₂薄膜在Ni基带上呈c轴方向生长，存在很强的平面外织构；极图和φ扫描显示它具有良好的平面内织构. Ni基片上织构的CeO₂薄膜为进一步在其上外延高质量的YBa₂Cu₃O_7-x超导薄膜提供了很好的模板. 关键词： 双轴织构的Ni基带 2薄膜')" href="#">CeO₂薄膜 脉冲激光沉积     

Investigation of SiO2/Si3N4 films prepared on sapphire by r.f. magnetron reactive sputtering

Sapphire is a desired material for infrared-transmitting windows and domes because of its excellent optical and mechanical properties. However, its thermal shock resistance is limited by loss of compressive strength along the c-axis of the crystal with increasing temperature. In this paper, double layer films of SiO₂/Si₃N₄ were prepared on sapphire (α-Al₂O₃) by radio frequency magnetron reactive sputtering in order to increase both transmission and high temperature mechanical performance of infrared windows of sapphire. Composition and structure of each layer of the films were analyzed by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), respectively. Surface morphology and roughness of coated and uncoated sapphire have been measured using a talysurf. Flexural strengths of sapphire sample uncoated and coated with SiO₂/Si₃N₄ have been studied by 3-point bending tests at different temperatures. The results show that SiO₂/Si₃N₄ films can improve the surface morphology and reduce the surface roughness of sapphire substrate. In addition, the designed SiO₂/Si₃N₄ films can increase the transmission of sapphire in mid-wave infrared and strengthen sapphire at high temperatures. Results for 3-point bending tests indicated that the SiO₂/Si₃N₄ films increased the flexural strength of c-axis sapphire by a factor of about 1.4 at 800 °C.     

Influence of the laser wavelength on the epitaxial growth and electrical properties of La0.8Sr0.2MnO3 films grown by excimer laser-assisted MOD

Epitaxial La_1−xSr_xMnO₃ (LSMO) films were prepared by excimer laser-assisted metal organic deposition (ELAMOD) at a low temperature using ArF, KrF, and XeCl excimer lasers. Cross-section transmission electron microscopy (XTEM) observations confirmed the epitaxial growth and homogeneity of the LSMO film on a SrTiO₃ (STO) substrate, which was prepared using ArF, KrF, and XeCl excimer lasers. It was found that uniform epitaxial films could be grown at 500 °C by laser irradiation. When an XeCl laser was used, an epitaxial film was formed on the STO substrate at a fluence range from 80 to 140 mJ/cm² of the laser fluence for the epitaxial growth of LSMO film on STO substrate was changed. When the LaAlO₃ (LAO) substrate was used, an epitaxial film was only obtained by ArF laser irradiation, and no epitaxial film was obtained using the KrF and XeCl lasers. When the back of the amorphous LSMO film on an LAO substrate was irradiated using a KrF laser, no epitaxial film formed. Based on the effect of the wavelength and substrate material on the epitaxial growth, formation of the epitaxial film would be found to be photo thermal reaction and photochemical reaction. The maximum temperature coefficient of resistance (TCR) of the epitaxial La_0.8Sr_0.2MnO₃ film on an STO substrate grown using an XeCl laser is 4.0%/K at 275 K. XeCl lasers that deliver stabilized pulse energies can be used to prepare LSMO films with good a TCR.     

Enhanced ferroelectric properties of vanadium doped bismuth titanate (BTV) thin films grown by pulsed laser ablation technique

Bi_3.99Ti_2.97V_0.03O₁₂ (BTV) thin films were grown by pulsed laser deposition at substrate temperatures ranging between 650 and 750 °C. The structural phase, and orientation of the deposited films were investigated in order to understand the effect of the deposition parameters on the properties of the BTV films. As the substrate temperature was increased to 700 °C, the films started showing a tendency of assuming a c-axis preferred orientation, while at lower temperatures polycrystalline films were formed. The Au/BTV/Pt capacitor showed an interesting dependence of the remnant polarization (P_r) as well as dc leakage current values on the growth temperature. The film deposited at 675 °C showed a very large 2P_r of 42 μC cm⁻², which is the largest for BTV thin films among the values reported so far.     

Superconducting molybdenum nitride epitaxial thin films deposited on MgO and α-Al2O3 substrates by molecular beam epitaxy

Molybdenum nitride Mo₂N_x films were grown on MgO(0 0 1) and on α-Al₂O₃(0 0 1) substrates by molecular beam epitaxy under nitrogen radical irradiation. X-ray photoelectron spectroscopy revealed that the composition of the film varied in the range of Mo₂N_1.4-Mo₂N_2.8 depending on the growth temperature. The deposition at 973 K gave well-crystallized films on both substrates. The high-resolution reciprocal space mapping by X-ray diffraction showed that the nitrogen-rich γ-Mo₂N crystalline phase (the composition: Mo₂N_1.4) was epitaxially grown on MgO at 923 K with a slight tetragonal distortion (a = 0.421 and c = 0.418 nm) to fit the MgO lattice (a = 0.421 nm). On α-Al₂O₃(0 0 1), nitrogen-rich γ-Mo₂N (Mo₂N_1.8) was grown at 973 K with (1 1 1) planes parallel to the substrate surface. X-ray diffraction analysis with a multi-axes diffractometer revealed that the γ-Mo₂N on α-Al₂O₃(0 0 1) had a slight rhombohedral distortion (a = 0.4173(2) and α = 90.46(3)°). Superconductivity was observed below 2.8-3 K for the films grown at 973 K on MgO and on α-Al₂O₃(0 0 1).