Preparation of (0 2 0)-oriented BaTi2O5 thick films and their dielectric responses

Barium dititanate (BaTi₂O₅) thick films were prepared on a Pt-coated Si substrate by laser chemical vapor deposition, and ac electric responses of (0 2 0)-oriented BaTi₂O₅ films were investigated using several equivalent electric circuit models. BaTi₂O₅ films in a single phase were obtained at a Ti/Ba molar ratio (m_Ti/Ba) of 1.72–1.74 and deposition temperature (T_dep) of 908–1065 K as well as m_Ti/Ba = 1.95 and T_dep = 914–953 K. (0 2 0)-oriented BaTi₂O₅ films were obtained at m_Ti/Ba = 1.72–1.74 and T_dep = 989–1051 K. BaTi₂O₅ films had columnar grains, and the deposition rate reached 93 μm h^?1. The maximum relative permittivity of the (0 2 0)-oriented BaTi₂O₅ film prepared at T_dep = 989 K was 653 at 759 K. The model of an equivalent circuit involving a parallel combination of a resistor, a capacitor, and a constant phase element well fitted the frequency dependence of the interrelated ac electrical responses of the impedance, electric modulus, and admittance of (0 2 0)-oriented BaTi₂O₅ films.     

The effects of the post-annealing time on the growth mechanism of Bi2Sr2Ca1Cu2O8+∂ thin films produced on MgO (100) single crystal substrates by pulsed laser deposition (PLD)

Bi₂Sr₂Ca₁Cu₂O_8+∂ thin films were deposited on MgO (100) substrates by pulsed laser deposition (PLD). The effects of post-annealing time on the phase formation, the structural and superconducting properties of the films have been investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), temperature dependent resistivity (R–T), atomic force microscopy (AFM), and DC magnetization measurements. The films deposited at 600 °C were post-annealed in an atmosphere of a gas mixture of Ar (93%) and O₂ (7%), at 860 °C for 10, 30, and 60 min. All films have demonstrated a mainly single phase of 2212 with a high crystallinity (FWHM≈0.159°) and c-axis oriented. The critical temperature, T_C, of the films annealed for 10, 30, and 60 min were obtained as 77, 78, and 78 K, respectively. The highest critical current density, J_C, was calculated as 3.34×10⁷ A/cm² for the film annealed at 860 °C for 30 min at 10 K.     

The structures and positive magnetoresistance of metallic Sr2CrWO6 epitaxial thin film

Double perovskite Sr₂CrWO₆ films have been prepared on SrTiO₃ (111) substrates by pulsed laser deposition in high vacuum (10^?5 Pa). X-ray diffraction patterns indicate that the films are (111)-oriented. Both atomic force microscopy (AFM) and cross-section transmission electron microscopy (TEM) images prove that the films have very smooth surfaces. Detailed microstructures given by high resolution transmission electron microscopy (HRTEM) further confirm that the films are epitaxial with sharp and coherent substrate/film interface. Well saturated magnetization–magnetic field hysteresis loop is observed with the saturation magnetization of 1.2 μ_B/formula unit at 10 K. The films show metallic transport behavior and large positive magnetoresistance (～180% at 10 K). The structure–property relationship is discussed in detail.     

Preparation of highly oriented β-SiC bulks by halide laser chemical vapor deposition

Highly oriented β-SiC bulks with high hardness were fabricated by halide laser chemical vapor deposition (HLCVD) using SiCl₄, CH₄ and H₂ as precursors. The effects of total pressure (P_tot) and deposition temperature (T_dep) on the preferred orientation, microstructure, deposition rate (R_dep) and micro-hardness were investigated. The 〈110〉-oriented β-SiC bulks were obtained at low P_tot (2–4 kPa), non-oriented β-SiC bulks were obtained at mediate P_tot (6 kPa), and 〈111〉-oriented β-SiC bulks were obtained at high P_tot (10–40 kPa), exhibiting faceted, cauliflower-like and six-fold pyramid-like microstructure, respectively. The maximum R_dep of 〈111〉- and 〈110〉-oriented β-SiC bulks were 3600 and 1300 μm/h at, respectively. The activation energy obtained by the plot of lgR_dep-T_dep⁻¹ is 170 to 280 kJ mol⁻¹, showing an exponential relation with P_Si. The Vickers micro-hardness of β-SiC bulks increased with increasing P_tot and showed the highest value of 35 GPa at P_tot = 40 kPa with a complete 〈111〉 orientation.     

Effect of substrate temperature on microstructure and optical properties of nanocrystalline alumina thin films

Aluminum oxide (Al₂O₃) thin films were deposited on silicon (100) and quartz substrates by pulsed laser deposition (PLD) at an optimized oxygen partial pressure of 3.0×10^?3 mbar in the substrate temperatures range 300–973 K. The films were characterized by X-ray diffraction, transmission electron microscopy, atomic force microscopy, spectroscopic ellipsometry, UV–visible spectroscopy and nanoindentation. The X-ray diffraction studies showed that the films deposited at low substrate temperatures (300–673 K) were amorphous Al₂O₃, whereas those deposited at higher temperatures (≥773 K) were polycrystalline cubic γ-Al₂O₃. The transmission electron microscopy studies of the film prepared at 673 K, showed diffuse ring pattern indicating the amorphous nature of Al₂O₃. The surface morphology of the films was examined by atomic force microscopy showing dense and uniform nanostructures with increased surface roughness from 0.3 to 2.3 nm with increasing substrate temperature. The optical studies were carried out by ellipsometry in the energy range 1.5–5.5 eV and revealed that the refractive index increased from 1.69 to 1.75 (λ=632.8 nm) with increasing substrate temperature. The UV–visible spectroscopy analysis indicated higher transmittance (>80%) for all the films. Nanoindentation studies revealed the hardness values of 20.8 and 24.7 GPa for the films prepared at 300 K and 973 K respectively.     

Superconducting properties of homoepitaxial CVD diamond

Superconductivity was achieved above 10 K in heavily boron-doped diamond thin films deposited by the microwave plasma-assisted chemical vapor deposition (CVD) method. Advantages of the CVD method are the controllability of boron concentration in a wide range, and a high boron concentration, compared to those obtained using the high-pressure high-temperature method. The superconducting transition temperatures of homoepitaxial (111) films are determined to be 11.4 K for T_C onset and 8.4 K for zero resistance from transport measurements. In contrast, the superconducting transition temperatures of (100) films T_C onset = 6.3 K and T_C zero = 3.2 K were significantly suppressed.     

Preparation of LaNiO3/SrTiO3/LaNiO3 capacitor structure through sol–gel process

In this paper, SrTiO₃/LaNiO₃ (STO/LNO) bilayer films were prepared on lanthanum aluminate (LAO) substrates by use of the sol–gel technique. An array of LNO electrodes with diameters of 200 μm was prepared on the surface of STO/LNO bilayer films. Therefore, LaNiO₃/SrTiO₃/LaNiO₃ (LNO/STO/LNO), a capacitor structure with symmetrical top and bottom electrodes, was obtained. The XRD analysis showed that the obtained capacitor structure has a biaxial texture. The dielectric test suggested that the relative dielectric constant of the LNO/STO/LNO structure is symmetric, has a high tunability, and has a low dielectric dissipation factor (tan δ) in response to varying electric field bias. As the temperature decreased, the relative dielectric constant of the STO film increased, the tunability increased, and the tan δ decreased. At test conditions of 80 K and 100 kH, the tunability and the figure of merit (FOM) reached 56% and 107, respectively.     

Textured YBCO-Film Formation by Sol–Gel Process and Post Annealing

Thick YBCO-films were prepared on single crystalline SrTiO₃(001) and LaAlO₃(001) substrates by the dip-coating process starting with Y-, Ba- and Cu-acetylacetonates. When a characteristic temperature during transformation of the precursor into YBa₂Cu₃O_x is exceeded a strong biaxial texture with FWHM = 1·2° for 1 μm thick fims results. When the O₂ partial pressure of the annealing atmosphere is lowered the characteristic temperature decreases and films with increased current density (j_c) are obtained. The values of J_c achieved for films deposited onto SrTiO₃(001) at 77 K in self field are as high as 50·000 A cm⁻² with T_cs of 90K.     

Heteroepitaxial growth of Cu2O films on Nb-SrTiO3 substrates and their photovoltaic properties

In this paper, p-type Cu₂O thin films have been epitaxially grown on n-type semiconducting (001) oriented Nb-SrTiO₃ (NSTO) substrates with different Nb doping concentration by pulsed laser deposition technique. X-ray diffraction and high resolution transmission electron microscopy reveal a cube-on-cube epitaxial relationship between Cu₂O and NSTO. It is found that the deposition temperature, the thickness of Cu₂O films and the Nb doping concentration of NSTO substrates have critical impact on the photovoltaic (PV) properties of the Cu₂O/NSTO heterojunction devices. A maximum PV performance is observed in ITO/Cu₂O/NSTO device when the deposition temperature, film thickness and Nb doping concentration of NSTO are 550 °C, 76 nm, and 0.7 wt% NSTO, respectively. The optimized PV output corresponds to the open circuit voltage, short-circuit current density, fill factor and photovoltaic conversion efficiency about 0.45 V, 1.1 mA/cm², 46% and 0.23%,respectively. This work offers an insight into the strategy for developing and designing novel optoelectronics of NSTO-based oxide heterostructures.     

Piezoelectric properties and phase transition temperatures of the solid solution of (1 − x)(Bi0.5Na0.5)TiO3–xSrTiO3

The phase diagram of (1 ? x)(Bi_0.5Na_0.5)TiO₃–xSrTiO₃ was completed and investigations on polarization and strain in this system were carried out. (1 ? x)(Bi_0.5Na_0.5)TiO₃–xSrTiO₃-ceramics were prepared by conventional mixed oxide processing. The depolarization temperature (T_d), the temperature of the rhombohedral–tetragonal phase transition (T_r–t) and the Curie temperature (T_m) were determined by measuring the temperature dependence of the relative permittivity. All solid solutions of (1 ? x)(Bi_0.5Na_0.5)TiO₃–xSrTiO₃ show relaxor behavior (A-site relaxor). From XRD-measurements a broad maximum of the lattice parameter can be observed around x = 0.5 but no structural evidence for a morphotropic phase boundary was found. SEM-analysis revealed a decrease of the grain size for increasing SrTiO₃-content. At room temperature a maximum of strain of about 0.29% was found at x = 0.25 which coincides with a transition from a ferroelectric to an antiferroelectric phase. The temperature dependence of the displacement indicates an additional contribution from a structural transition (rhombohedral–tetragonal), which would be of certain relevance for the existence of a morphotropic phase boundary.     

Mosaicity,dislocations and strain in heteroepitaxial diamond grown on iridium

The present study provides a multi-scale investigation of the crystalline quality and the structural defects present in heteroepitaxial diamond films grown on iridium/SrTiO₃ (001) substrates by microwave plasma assisted chemical vapor deposition. X-ray diffraction, Raman spectroscopy and low temperature cathodoluminescence are combined to accurately characterize the mosaicity, the density of dislocations and the residual strain within the films. X-ray diffraction and Raman results confirm a structural quality at the state-of-the-art according to the epitaxial relationship 〈100〉diamond(001) // 〈100〉iridium(001) // 〈100〉SrTiO₃ (001). In addition, Raman and cathodoluminescence observations on cross-sections reveal the presence of local strain.     

Pulsed-laser deposited amorphous-like PZT thin-films: Microstructure and optical properties

Amorphous lead–zirconate–titanate (Pb_0.97Nd_0.02(Zr_0.55Ti_0.45)O₃, PNZT) thin-films were grown on single-crystal MgO(1 0 0) substrates at room temperature by pulsed laser deposition (PLD). Part of PNZT films was left as-deposited amorphous and others were post-annealed at temperatures from 100 to 400 °C. X-ray diffraction (XRD) and scanning probe microscopy (SPM) were used to characterize the microstructure. Optical properties were analyzed using spectrophotometry at UV–vis–NIR and prism-coupler method at 633 nm wavelengths. Initially, films were amorphous with a broad XRD peak around 2θ ≈ 29.7°. As the post-annealing temperature increased above 250 °C, the amorphous peak started to shift towards lower 2θ-angles and got narrower indicating of decreasing interatomic spacing and possible glass transformation. At the same time, the transmittance at all wavelengths increased remarkably, although no crystal structure was detected by XRD. Also, sharp optical TE₀ modes with full-width half-maximum (FWHM) values of Δβ ≈ 0.00067 could be coupled into these films.     

Hydrothermal and electrochemical growth of complex oxide thin films for electronic devices

Thin film growth of complex oxides including BaTiO₃, SrTiO₃, BaZrO₃, SrZrO₃, KTaO₃, and KNbO₃ were studied by the hydrothermal and the hydrothermal–electrochemical methods. Hydrothermal–electrochemical growth of ATiO₃ (A = Ba, Sr) thin films was investigated at temperatures from 100 to 200 °C using a three-electrode cell. Current efficiency for the film growth was in the range from ca. 0.6 to 3.0%. Tracer experiments revealed that the ATiO₃ film grows at the film/substrate interface. Thin films of AZrO₃ (A = Ba, Sr) were also prepared on Zr metal substrates by the hydrothermal–electrochemical method. By applying a potential above ca. +2 V versus Ag/AgCl to the Zr substrates, AZrO₃ thin films were formed uniformly. Thin films of KTaO₃ and KNbO₃ were prepared on Ta metal substrates by the hydrothermal method. Perovskite-type KTaO₃ thin films were formed in 2.0 M KOH at 300 °C. Pyrochlore-type K₂Ta₂O₆ thin films were formed at lower temperatures and lower KOH concentrations.     

Enhanced electrical properties in lead-free NBT–BT ceramics by series ST substitution

Comprehensive electrical properties of 0.94(Na_1/2Bi_1/2)TiO₃–0.06BaTiO₃ lead-free ceramics by doping series SrTiO₃ were investigated. High piezoelectric constant of 205 pC/N and electromechanical coupling factor of 0.34 were obtained due to the forming of the rhombohedral–tetragonal morphotropic phase boundary at x=0.02–0.06. Very large recoverable strain of 0.34% was obtained at x=0.10 due to the coexistence of ferroelectric and relaxor pseudocubic phases. A large electrocaloric effect (ΔT_max=1.71 K and ΔT/ΔE=0.34 K mm kV⁻¹ at 50 kV cm⁻¹) which determined by indirect measurements method was obtained at 120 °C at x=0.02, which is significantly higher than that of lead-free ferroelectric ceramics reported so far. Moreover, lower operating temperatures of 50 °C and 30 °C were proposed when x=0.10 and 0.20 with ΔT_max=0.79 K and 0.6 K, respectively. These properties added together indicate a promising material for applications in cooling systems and actuators.     

Frequency dependence of dielectric properties of metallodielectric SrTiO3–Pt composites

The frequency dependence of dielectric properties of SrTiO₃–Pt (platinum) composites has been investigated. The SrTiO₃–Pt composites sintered at 1300 °C for 2 h was prepared by using conventional ceramic fabrication method. The dense metallodielectric composites without chemical reaction between SrTiO₃ and Pt during sintering process were obtained. The relative permittivity (ɛ_r) of SrTiO₃–Pt composites was increased with increase of amount of Pt. The maximum ɛ_r of 2150 at 1 MHz was obtained for the composite of SrTiO₃–27 vol.% Pt. Moreover, the SrTiO₃–Pt composites exhibited high values of ɛ_r at low frequencies than at high frequencies due to space charge polarization. The loss tangent (tan δ) of SrTiO₃–Pt composites increased with increasing amount of Pt, and decreased with increasing frequency up to 1 MHz. However, the tan δ of SrTiO₃–Pt composites was significantly increased at microwave frequency due to abrupt increase of ac conductivity of composite.     

Influence of the composition-induced structure evolution on the electrocaloric effect in Bi0.5Na0.5TiO3-based solid solution

The present work investigated the influence of the composition induced structure evolution on the electrocaloric effect in lead-free (0.935−x)Bi_0.5Na_0.5TiO₃–0.065BaTiO₃–xSrTiO₃ (BNBST, BNBSTx) ceramics. It was found that broad ∆T peak could be observed for all compositions and the electrocaloric strength α (α=ΔT_max/ΔE) in BNBST0.02 could reach as high as 0.27 K mm/kV. The increase of the SrTiO₃ concentration led to a shift of ∆T_max to a lower temperature, resulting in a large near room-temperature electrocaloric strength α of 0.17 K mm/kV in BNBST0.22.     

Studies of heteroepitaxial growth of diamond

Large-scale heteroepitaxial growth of diamond depends critically on the development of a suitable lattice-matched substrate system. Oxide substrates, notably MgO and SrTiO₃, on which thin epitaxial films of iridium serve as a nucleation layer for diamond have already shown considerable promise. We describe here improvements in the growth of single crystal diamond by low-pressure microwave plasma-enhanced CVD. Oxide substrates with flat, low-index surfaces form the initial basis for the process. Iridium was deposited on heated substrates in a UHV electron-beam evaporation system resulting in epitaxial films, typically 150–300 nm thick, with Ir (1 0 0) parallel to the surface of all substrates as confirmed by X-ray and electron backscattering diffraction. Following Ir deposition, the samples were transferred to a CVD reactor where a bias-enhanced nucleation step induced a dense condensate that completely covered the Ir surface. Uniform nucleation densities of order 10¹² cm⁻² were observed. Interrupted growth studies, carried out at intervals from seconds to minutes subsequent to terminating the nucleation step, revealed a rapid coalescence of grains. One hour of growth resulted in a smooth, nearly featureless, (0 0 1) diamond film. For extended growth runs, slabs of diamond were grown with thickness as great as 38 μm and lateral dimensions near 4 mm. The crystals were transparent in visible light and cleaved on (1 1 1) planes along 〈1 1 0〉 directions, similar to natural diamond. Of particular significance is the successful use of sapphire as an underlying substrate. Its high crystalline perfection results in epitaxial Ir films with X-ray linewidths comparable to those grown on SrTiO₃. However, Al₂O₃ possesses superior interfacial stability at high temperatures in vacuum or in a hydrogen plasma with a better thermal expansivity match to diamond. Since sapphire is available as relatively inexpensive large diameter substrates, these results suggest that wafer-scale growth of heteroepitaxial diamond should be feasible in the near future.     

Spark plasma sintering of Al2O3–Ni nanocomposites using Ni nanoparticles produced by rotary chemical vapour deposition

Al₂O₃–Ni nanocomposites were fabricated by spark plasma sintering (SPS) using Ni nanoparticle produced by rotary chemical vapour deposition. Carbon-free Ni nanoparticles were prepared by reacting NiCp₂ with O₂ to form NiO and then reducing to Ni by H₂ for 30 min at 823 K. The highest Ni content and grain size were 7.8 wt.% and 47.7 nm, respectively, using a NiCp₂ supply rate (R_s) of 1.67 × 10⁻⁶ kg s⁻¹. At a sintering temperature (T_SPS) of 1573 K, the hardness of Al₂O₃–3.8 wt.% Ni was 20.5 GPa, around 1 GPa higher than that of monolithic Al₂O₃ sintered at the same temperature. The tensile strength of Al₂O₃–4.6 wt.% Ni was 170 MPa, 60 MPa higher than that of Al₂O₃ sintered at 1573 K.     

Optical scattering of nanocrystalline Pb(ZrxTi1−x)O3 films

Optical characterization methods, like spectrophotometry at UV–vis-NIR wavelengths and prism-coupler method, were applied to polycrystalline Pb(Zr_xTi_1?x)O₃ thin films at various thicknesses. Thin films were deposited at room temperature by pulsed laser deposition on MgO (1 0 0) substrates and post-annealed at different temperatures. X-ray diffraction and atomic force microscopy were used to characterize the crystal structure and surface morphology of the thin films, respectively.Well oscillating transmission with a sharp fall near the absorption edge was found in films with high orientation and low surface roughness. Changes in the surface morphology and crystal orientation were found to modulate optical interference maxima and minima of the transmittance spectra and to increase the width of the TE₀ mode (Δβ ≈ 0.06) indicating an increase in the scattering losses of the films. Single-phase oriented films had sharpest coupling values (Δβ ≈ 0.005) of the TE₀ mode.     

Study of polar relaxation processes in Sr(1−1.5x)LaxTiO3 ceramics by using field-induced thermally stimulated currents

Polar relaxation processes in Lanthanum doped SrTiO₃ (STO) ceramics, with general formulae Sr_(1−1.5x)La_xTiO₃, were studied by undertaking field-induced thermally stimulated currents measurements below room temperature.The experimental results obtained for doped ceramic (x = 0.0133) were analysed by using dipolar and space-charge relaxation thermally stimulated depolarization currents (TSDC) models in order to determine the nature of the relaxation processes involved.Our results reveal the existence of different relaxation processes in the temperature range 60–300 K. Whereas at low temperature, a relaxation mechanism of a dipolar type was disclosed within the temperature interval centred around 100 K, a space-charge relaxation process could be identified in the temperature range 120–300 K. The temperature dependence of the relaxation parameters will be also discussed in detail.