The effect of parallel electric fields on the orientation of polycrystalline zinc oxide thin films produced by reactive evaporation and oxidation of zinc

The effect of an electric field (detached electrodes) on the crystal orientation of zinc oxide thin film was investigated. Two methods, direct and indirect, were utilized to produce zinc oxide thin films. In the direct method (reactive evaporation) oxygen was introduced into the zinc vapour stream and ZnO films were deposited on cold substrates in a vacuum system. Parallel electric fields were applied during the reaction process. It was observed that the electric field had no effect on the preferred c-axis orientation of the ZnO crystals. There was no significant difference in the crystal size or surface texture of ZnO samples subjected to electric fields with respect to the sample that experienced no electric field. In the indirect method (oxidation) electric fields were applied during the evaporation of zinc then the samples were oxidized in air in a furnace at 600 C. Here the application of electric field improved the c-axis orientations. The crystal size remained unchanged, but the surface morphology was affected by the application of the electric field. Whenever the c-axis orientation improved, crystals on the surface became rounded and a more ordered microstructure was observed.     

Critical current density and microstructure of YBa2Cu3O7 thin films post-annealed at a low oxygen partial pressure

Post-annealing of thin films of YBa₂Cu₃O₇ (YBCO) has been performed at 29 Pa and 750°C. For films 0.6 m thick, a critical current density >1 MA cm^–2 is obtained at 77 K, with a sharp eddy current response at 25 MHz. Microstructural investigation of these films by crosssectional and planar transmission electron microscopy reveals that the YBCO film has thec-axis normal to the plane of the substrate in a continuous sheet of varying thickness, frequently covering the entire thickness of the film. Mutually perpendicular rods with thec-axis in the plane of the LaAlO₃ substrate are also seen. The microstructure and critical current density of these films are compared with those of previously reported films post-annealed in atmosphericpressure oxygen.     

Growth of oriented aluminium nitride films on silicon by chemical vapour deposition

Aluminium nitride films were grown on silicon substrates using the chemical vapour deposition (CVD) method. The properties of the films were studied by scanning electron microscopy (SEM), atomic force microscope (AFM) measurements, X-ray diffraction and Raman scattering. The resulting films were strongly textured and had a preferential orientation with the c-axis normal to the surface, the Raman spectra showed two peaks at 607 and 653 cm^–1 and two large bands at 750 and 900 cm^–1 of smaller intensity. Both the macro- and micro-Raman spectra showed the same peaks.     

Structural and electrical properties of the Y-Ba-Cu-O superconducting films prepared by spray pyrolysis

High-T _c , superconducting YBa₂Cu₃O_7– thin films have been grown on (100) MgO substrates by a chemical spray pyrolysis method. The crystal structure and surface morphology have been studied by X-ray diffraction and scanning electron microscopy, respectively. The assprayed films were amorphous and insulating, but upon annealing the films became superconducting and show a textured surface morphology with an average grain size of the order of 5–15m. The films were highly oriented with thec-axis being perpendicular to the substrate surface. Three different microstructures were recorded: long rod-shaped grains, platelet or rounded-shape grains, and a melting-like growth. Electrical measurements were carried out in a low-temperature cryostat using a standard d.c. four-probe technique. The onset transition temperature was around 83–86 K, and the completion of the transition to zero resistance was in the range 73–78 K. The magnitude of the measured critical current density was in the range 750–3750 A/cm² at 30 K. A correlation between the resistance of the tunnelling junctions and the critical current density was found from the theoretical models.     

Microstructures ofa-axis oriented YBa2Cu3O7-δ thin films prepared by low temperature and template processes

Highly a-axis oriented epitaxial thin films of YBa₂Cu₃O_7– have been prepared by both a low temperature process and also a self-template technique. Films deposited at low temperature showed good crystallinity whereas films grown on a template exhibited a high transition temperature into the superconducting state. Detailed transmission electron microscopic investigations have been performed on these two kinds ofa-axis oriented films. Significant differences have been found in the microstructures of these films. The dominant defects formed in these films are misoriented grains which mainly show ac-axis orientation. The origin of the nucleation of misoriented grains is attributed to surface defects of the substrate. No boundary between the template layer and upper layer could be distinguished for films made by the self-template process. A thinc-axis intermediate layer with a thickness of 2–5 unit cells was observed at the interface between thea-axis film and the SrTiO₃ substrate for both kinds of films. The formation and influence of such an intermediate layer needs further study.     

Magnetic field dependence of the critical current density for the bismuth-based bulk high-Tc superconductors

Three types of bismuth-based bulk samples were prepared through uniaxial pressing at room temperature, hot isostatic pressing (HIP) and drawing and rolling. Transport current properties were characterized in a steady field up to 1.12 T at 77 K (T/T _c=0.75). The Josephson weak-link decoupling fields have been found to be 5 mT for the cold-pressed pellet and 30 mT for the HIPed pellet and the rolled tape. At the decoupling field the transport critical current density,J _c, drops 80% from 124 (OT) to 29 A cm^–2 (5 mT) for the cold-pressed pellet, 80% from 582 (OT) to 126 A cm^–2 (30 mT) for the HIPed sample and 50% from6500 (0 T) to 2850 A cm^–2 (30 mT) for the rolled tape. In the flux flow regime, whereB is perpendicular to thec-axis a modified Kim's modelJ _c=(/B ₀)/[(1+B/B ₀)] ⁿ can be used to describe the field dependence of the critical current density, J_c, in the field range 0.2–1.12 T. The effective upper critical fields were estimated to be 0.98, 1.54 and 1.94 T for the three types of samples, respectively. An adjustable range ofB _c2 for bismuth-based bulk highT _c superconductors is given. Flux shear may operate in these materials. The prediction of this pinning mechanism is yielded from fitting the equation qualitatively. WhenB is parallel to thec-axis, the absence of strongly intragranular flux-pinning is emphasized by the poor flux flow regime for the rolled tape sample.     

Anisotropy of optical constants of YBCO

Otto configuration attenuated total reflection (ATR) measurements of the excitation of surface plasmons in the infrared have been carried out on YBCO films deposited on MgO (100) substrates. The dielectric constants for YBCO at 3.392m are determined to be –10+15i forc-axis material. The anisotropic nature of the cuprate is seen from films with other orientations: nearlya-axis material has constants of 4.0+7.0i. It is thus not metallic in its optical response along thec-axis which lies parallel to the substrate plane. Ellipsometric measurements in the visible onc-axis material point to a maximum surface plasmon energy of 1 eV.     

Effect of processing on the properties of Bi3.15Nd0.85Ti3O12 thin films

Various crystallization parameters were studied during the fabrication of Bi_3.15Nd_0.85Ti₃O₁₂ (BNdT) thin films on Pt/Ti/SiO₂/Si (100) by metal organic solution decomposition method. The effect of crystallization processes, crystallization ambients on the properties of BNdT thin films such as orientation, ferroelectric properties were examined. By adopting different fabrication processes, it is possible to get both highly c-axis oriented as well as randomly oriented thin films. Highly c-axis oriented BNdT thin film showed a large remnant polarization (2P_r) of 70 μC/cm² at an applied voltage of 10 V and exhibited a fatigue free behavior unto 2 × 10⁹ switching cycles. The improved ferroelectric properties of BNdT thin films suggest their suitability for high density ferroelectric random access memory applications.     

An electron microscopic study on MnBi thin films

The structure and orientation of MnBi thin films prepared by sequential evaporation of bismuth and manganese on glass substrates were studied by transmission electron microscopy and electron diffraction. Results indicate that these films develop a preferred orientation with thec-axis perpendicular to the film plane. This preferred orientation is due to the formation of MnBi from a highly oriented bismuth layer, i.e., a layer with thec-axis perpendicular to the film plane. Trace amounts of elemental bismuth, manganese and MnO are found in these MnBi films. There is evidence of close parallel alignment between the MnBi and the bismuth lattices.     

Effects of growth ambient on electrical properties of Al-N co-doped p-type ZnO films

p-Type zinc oxide thin films with c-axis orientation were prepared in N₂O-O₂ atmosphere by an Al-N co-doping method using reactive magnetron sputtering. Secondary ion mass spectroscopy (SIMS) measurements indicate that as-grown ZnO films were co-doped with Al and N. Hall effect measurements show a dependence of types of conduction, carrier concentration and mobility of as-grown ZnO films on N₂O partial pressure ratios. p-Type ZnO thin films deposited in a N₂O partial ratio of 10% show the highest hole concentration of 1.1×10¹⁷ cm⁻³, the lowest resistivity of about 100 Ω cm, and a low mobility of 0.3 cm² V⁻¹ s⁻¹.     

Space-charge-limited current analysis in amorphous InSe thin films

The study of space-charge-limited currents (SCLC) in amorphous InSe thin films is presented. The temperature-dependent current–voltage (J–V) measurements were carried out for TO/a-InSe/Au sandwich structures in the range of 200–320 K. For all samples, ohmic behavior was observed up to an electric field strength of about 2×10⁵ V cm^–1. From the temperature dependence of conductivity data, the position of the thermal equilibrium Fermi level E _fo is determined as 250 meV above the valence band E _v. At higher electric field strength values in the SCLC regions, the proportionality constant of voltage changes is between 2 and 2.9 with temperature. The analysis of J–V characteristics using the SCLC method and analytical approach for the determination of density of states (DOS) in the energy range of 190–250 meV shows that DOS changes between 3.8×10¹⁷–1.7×10¹⁸ eV^–1cm^–3 with energy. The energy distribution of DOS is temperature independent indicating that the SCLC in these amorphous films is related to the bulk, not to the surface layer between the contact and the film.     

Deposition of low-resistivity gallium-doped zinc oxide films by low-temperature radio-frequency magnetron sputtering

The transparent and conductive gallium-doped zinc oxide (GZO) film was deposited on 1737F Corning glass using the radio-frequency (RF) magnetron sputtering system with a GZO ceramic target. (The Ga₂O₃ contents are approximately 5 wt. %). In this study, the effect of the sputtering pressure on the structural, optical and electrical properties of GZO films upon the glass or polyester film (PET) substrate was investigated and discussed in detail. The GZO film was grown under a steady RF power of 400 W and a lower substrate temperature from room temperature up to 200 °C. The crystal structure and orientation of GZO thin films were examined by X-ray diffraction. All of the GZO films under various sputtering pressures had strong c-axis (002)-preferred orientation. Optical transparency was high (> 80%) over a wide spectral range from 380 nm to 900 nm. According to the experimental data, the resistivity of a single-layered GZO film was optimized at  8.3 × 10^− 4 Ω cm and significantly influenced by the sputtering pressure. In further research, the sandwich structure of the GZO film/Au metal/GZO film was demonstrated to improve the electrical properties of the single-layered GZO film. The resistivity of the sandwich-structured GZO film was around 2.8 × 10^− 4 Ω cm.     

Microstructural study of YBa2Cu3O7/SrTiO3/YBa2Cu3O7 heteroepitaxial trilayer films grown on (100) SrTiO3 substrates

Detailed transmission electron microscopic study has been carried out on heteroepitaxial YBa₂Cu₃O₇/SrTiO₃/YBa₂Cu₃O₇ trilayer thin films grown on (100)SrTiO₃ substrates prepared by DC and RF magnetron sputtering. The microstructural results showed the existence of somea-axis-oriented YBCO grains 20–90 nm wide in thec-axis-oriented YBCO matrix. Some of thea-axis grains in the lower YBCO thin film layer have protruded into the above SrTiO₃ layer, which may cause short circuit between the two YBCO superconducting layers. This is unsuitable for the application of trilayer thin films for microelectronic devices. The defects on the surface of the substrates would also influence the growth quality of the YBCO thin films.     

Influence of the substrate temperature on the optical and electrical properties of Ga-doped ZnO thin films fabricated by pulsed laser deposition

Ga-doped (5 wt%) zinc oxide (GZO) thin films were fabricated on corning 1737 substrates at a fixed oxygen pressure of 200 mTorr at various substrate temperatures (100–300 °C) by using pulsed laser deposition (PLD) in order to investigate the microstructure, optical, and electrical properties of the GZO thin films. It was observed that all the thin films exhibit c-axis orientation and exhibit only a (002) diffraction peak. The GZO thin film, which was fabricated at 200 mTorr and 300 °C, showed the highest (002) orientation, and the full width at half maximum (FWHM) of the (002) diffraction peak was 0.38°. The position of the XRD peak shifted to a higher angle with increase in the substrate temperature. The optical transmittance in the visible region was greater than 85%. The Burstein-Moss effect, which causes a shift toward a high photon energy level, was observed. The electrical property indicated that the highest carrier concentration (2.33 × 10²¹ cm⁻³) and the lowest resistivity (3.72 × 10⁻⁴ Ωcm) were obtained in the GZO thin film fabricated at 200 mTorr and 300 °C.     

Critical current density and microstructure of YBa2Cu3O7 thin films post-annealed at a low oxygen partial pressure

Post-annealing of thin films of YBa₂Cu₃O₇ (YBCO) has been performed at 29 Pa and 750°C. For films 0.6 μm thick, a critical current density >1 MA cm^?2 is obtained at 77 K, with a sharp eddy current response at 25 MHz. Microstructural investigation of these films by crosssectional and planar transmission electron microscopy reveals that the YBCO film has thec-axis normal to the plane of the substrate in a continuous sheet of varying thickness, frequently covering the entire thickness of the film. Mutually perpendicular rods with thec-axis in the plane of the LaAlO₃ substrate are also seen. The microstructure and critical current density of these films are compared with those of previously reported films post-annealed in atmosphericpressure oxygen.     

Investigation of the temperature dependence of electron and phonon Raman scattering in single crystal YBa2Cu3O6.952

Detailed Raman-scattering measurements have been performed on high-quality YBa₂Cu₃O_6.952 single crystal (T _c =93 K, T _c =0.3 K). A sharp (FWHM 7.2 cm^–1 at 70 K and 10.0 cm^–1 at 110 K) 340 cm^–1phonon mode has been observed inB _1g polarization. An electronic scattering peak at 500 cm^–1 in theB _1g polarization extends down to 250 cm^–1. These FWHM values determine the upper limit of the homogeneous linewidth of the phonon and electronic excitations. The start of the electronic spectral function renormalization and of the 340 cm^–1 mode anomalies (frequency softening, linewidth sharpening, and intensity increase) have been observed to occur approximately 40 K aboveT _c . The 340 cm^–1 mode Fano shape analysis has been performed and the temperature dependences of the Fano shape parameters have been estimated. All 340 cm^–1 mode anomalies have been explained by the electronic spectral function renormalization.This work was supported by Swedish Natural Sciences Research Council (G.B. and L.B.) and by the National Science Foundation (DMR 91-20000) through the Science and Technology Center for Superconductivity (G.B. and M.V.K.).     

Superconducting behavior of high-Tc YBa2Cu3O7 thin films with BaO impurity produced by pulsed laser deposition

We have investigated the superconducting behavior of high-T _c YBa₂Cu₃O₇ (YBCO) thin films containing BaO impure phase produced by pulsed laser deposition. The thin films were characterized by the standard four-probe method, X-ray diffraction (XRD), and scanning electron microscopy (SEM). XRD showed that all these thin films contained BaO impurity, with thec-axis normal to the surface of the substrates. The presence of impurity existed from substrate temperatureT _s of 727 to 796°C. When these thin films with BaO impurity were measured under the magnetic fields, it was found that the critical current densityJ _c increased slightly with increase in magnetic fieldB within the range ofB500 G, in the case ofB perpendicular to thec-axis of the film.     

RF diode reactive sputtering of n- and p-type zinc oxide thin films

We present the relationship between parameters of reactive RF diode sputtering from a zinc oxide (ZnO) target and the crystalline, electrical and optical properties of n-/p-type ZnO thin films. The properties of the ZnO thin films depended on RF power, substrate temperature and, particularly, on working gas mixtures of Ar/O₂ and of Ar/N₂. Sputtering in Ar+O₂ working gas (up to 75% of O₂) improved the structure of an n-type ZnO thin film, from fibrous ZnO grains to columnar crystallites, both preferentially oriented along the c-axis normally to the substrate (〈0 0 2〉 direction). These films had good piezoelectric properties but also high resistivity (ρ≈10³ Ω cm). ZnO:N p-type films exhibited nanograin structure with preferential 〈0 0 2〉 orientation at 25% N₂ and 〈1 0 0〉 orientation for higher N₂ content. The presence of nitrogen N_O at O-sites forming N_O-O acceptor complexes in ZnO was proven by SIMS and Raman spectroscopy. A minimum value of resistivity of 790 Ω cm, a p-type carrier concentration of 3.6×10¹⁴ cm⁻³ and a Hall mobility of 22 cm² V⁻¹ s⁻¹ were obtained at 75% N₂.     

Preparation and properties of ferroelectric Pb1 − xCaxTiO3 thin films produced by the polymeric precursor method

Pb_{1 – x} Ca _x TiO₃ thin films with x = 0.24 composition were prepared by the polymeric precursor method on Pt/Ti/SiO₂/Si substrates. The surface morphology and crystal structure, and the ferroelectric and dielectric properties of the films were investigated. X-ray diffraction patterns of the films revealed their polycrystalline nature. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) analyses showed the surface of these thin films to be smooth, dense and crack-free with low surface roughness. The multilayer Pb_{1 – x} Ca _x TO₃ thin films were granular in structure with a grain size of approximately 60–70 nm. The dielectric constant and dissipation factor were, respectively, 174 and 0.04 at a 1 kHz frequency. The 600-nm thick film showed a current density leakage in the order of 10^–7 A/cm² in an electric field of about 51 kV/cm. The C-V characteristics of perovskite thin films showed normal ferroelectric behavior. The remanent polarization and coercive field for the deposited films were 15 C/cm² and 150 kV/cm, respectively.