Showing papers on "Pulsed laser deposition published in 2004"

Multiferroic BaTiO3-CoFe2O4 Nanostructures.

Abstract: We report on the coupling between ferroelectric and magnetic order parameters in a nanostructured BaTiO3-CoFe2O4 ferroelectromagnet. This facilitates the interconversion of energies stored in electric and magnetic fields and plays an important role in many devices, including transducers, field sensors, etc. Such nanostructures were deposited on single-crystal SrTiO3 (001) substrates by pulsed laser deposition from a single Ba-Ti-Co-Fe-oxide target. The films are epitaxial in-plane as well as out-of-plane with self-assembled hexagonal arrays of CoFe2O4 nanopillars embedded in a BaTiO3 matrix. The CoFe2O4 nanopillars have uniform size and average spacing of 20 to 30 nanometers. Temperature-dependent magnetic measurements illustrate the coupling between the two order parameters, which is manifested as a change in magnetization at the ferroelectric Curie temperature. Thermodynamic analyses show that the magnetoelectric coupling in such a nanostructure can be understood on the basis of the strong elastic interactions between the two phases.

Atomic-scale imaging of nanoengineered oxygen vacancy profiles in SrTiO3

Abstract: At the heart of modern oxide chemistry lies the recognition that beneficial (as well as deleterious) materials properties can be obtained by deliberate deviations of oxygen atom occupancy from the ideal stoichiometry. Conversely, the capability to control and confine oxygen vacancies will be important to realize the full potential of perovskite ferroelectric materials, varistors and field-effect devices. In transition metal oxides, oxygen vacancies are generally electron donors, and in strontium titanate (SrTiO3) thin films, oxygen vacancies (unlike impurity dopants) are particularly important because they tend to retain high carrier mobilities, even at high carrier densities. Here we report the successful fabrication, using a pulsed laser deposition technique, of SrTiO3 superlattice films with oxygen doping profiles that exhibit subnanometre abruptness. We profile the vacancy concentrations on an atomic scale using annular-dark-field electron microscopy and core-level spectroscopy, and demonstrate absolute detection sensitivities of one to four oxygen vacancies. Our findings open a pathway to the microscopic study of individual vacancies and their clustering, not only in oxides, but in crystalline materials more generally.

Annealing effect on the property of ultraviolet and green emissions of ZnO thin films

Abstract: The mechanism of ultraviolet (UV) and green emission of ZnOthin filmsdeposited on (001) sapphire substrates by pulsed laser deposition was investigated by using postannealing treatment at various annealing temperatures after deposition.Structural, electrical, and optical properties of ZnOfilms have been also observed. As the postannealing temperature increased, the intensity of UV (380 nm) peak and the carrier concentration were decreased while the intensity of the visible (about 490–530 nm) peak and the resistivity were increased. The role of oxygen in ZnOthin film during the annealing process was important to the change of optical properties. The mechanism of the luminescence suggested that UVluminescence of ZnOthin film was related to the transition from near band edge to valence band, and green luminescence of ZnOthin film was caused by the transition from deep donor level to valence band due to oxygen vacancies. The activation energy derived by using the variation of green emission intensity was 1.19 eV.

Dilute magnetic semiconducting oxides

Abstract: A review of recent results on transition metal doping of electronic oxides such as ZnO, TiO2, SnO2, BaTiO3, Cu2O, SrTiO3 and KTaO3 is presented. There is interest in achieving ferromagnetism with Curie temperatures above room temperature in such materials for applications in the field of spintronic devices, in which the spin of the carriers is exploited. The incorporation of several atomic per cent of the transition metals without creation of second phases appears possible under optimized synthesis conditions, leading to ferromagnetism. Pulsed laser deposition, reactive sputtering, molecular beam epitaxy and ion implantation have all been used to produce the oxide-based dilute magnetic materials. The mechanism is still under debate, with carrier-induced, double-exchange and bound magnetic polaron formation all potentially playing a role depending on the conductivity type and level in the material.

Pulsed laser ablation and deposition of thin films.

Abstract: Pulsed laser ablation is a simple, but versatile, experimental method that finds use as a means of patterning a very diverse range of materials, and in wide areas of thin film deposition and multi-layer research. Superficially, at least, the technique is conceptually simple also, but this apparent simplicity hides a wealth of fascinating, and still incompletely understood, chemical physics. This overview traces our current physico-chemical understanding of the evolution of material from target ablation through to the deposited film, addressing the initial laser–target interactions by which solid material enters the gas phase, the processing and propagation of material in the plume of ejected material, and the eventual accommodation of gas phase species onto the substrate that is to be coated. It is intended that this Review be of interest both to materials scientists interested in thin film growth, and to chemical physicists whose primary interest is with more fundamental aspects of the processes of pulsed laser ablation and deposition.

Blueshift of near band edge emission in Mg doped ZnO thin films and aging

Abstract: Pure and Mg doped ZnO thin films were deposited at 400 °C on glass substrates by pulsed laser deposition. An x-ray diffractometer (XRD) was used to investigate the structural properties of the thin films. It is found that all the thin films have a preferred (002) orientation. The peak position of (002) orientation is found to shift from 34.39° to 34.55°. The lattice constants of ZnO thin films were also obtained from XRD data. It is found that, with the increase of the dopant concentration, the lattice constant a decreases from 3.25 to 3.23 A, and c decreases from 5.20 to 5.16 A. From the spectrophotometer transmittance data, the band gap energies of the thin films were calculated by a linear fitting process. The band gap energy of Mg doped ZnO thin film increases with increasing dopant concentration. In photoluminescence (PL) spectra, two PL emission peaks are found in pure ZnO thin films, one is the near band edge (NBE) emission at 3.28 eV, and the other is green-yellow-red emission at around 2.4 eV. However, with the increase of the dopants, no green-yellow-red emissions are found in PL of Mg doped ZnO thin films. The NBE emission has a blueshift compared with that of pure ZnO thin film (as much as 0.12 eV). As time goes on, NBE emission in pure ZnO thin film is enhanced, and the green-yellow-red emissions disappear.

Structural and multiferroic properties of BiFeO3 thin films at room temperature

Abstract: BiFeO3 thin films have been prepared on Pt∕TiO2∕SiO2∕Si substrates under various oxygen pressures of 0.15–0.005Torr at a temperature of 450°C by pulsed-laser deposition. The effects of deposition pressure on their crystal structure and multiferroic properties have been investigated at room temperature. X-ray diffraction analysis (θ-2θ scans and 2-dimensional scans) shows that the BiFeO3 thin films consist of perovskite single phase with tetragonal crystal structure and space group P4mm. The c-axis lattice constant decreases (4.062–4.006A) and c∕a ratio of the films decreases from 1.032 to 1.014 with a decrease in the oxygen pressure. The surface roughness and grain size of the films depend dramatically on oxygen pressures. The dielectric constant of the films decreases with decreasing oxygen pressure. The film deposited at 0.05Torr shows a stable current density and well-saturated hysteresis loop with twice the remanent polarization (2Pr) of 136μC∕cm2 and coercive field (2Ec) of 109kV∕cm. The BiFeO3 thin ...

Band gap energy of pure and Al-doped ZnO thin films

Abstract: Pulsed laser deposition (PLD) technique is used to deposit pure and Al-doped ZnO thin films at different temperatures on glass substrates. From the transmission data from optical spectroscopy the band gap energy Eg of the films is derived. The dependences of Eg on the deposition temperatures of the pure and Al-doped ZnO films are different. The band gap energy of the pure ZnO increases and saturates with temperature. However, Eg of Al-doped ZnO shows an exponential decrease. Refractive indices of 1.9–2.1 in the VIS are determined by the spectroscopic ellipsometry (SE). Photoluminescence (PL) data reveal the strong near band emission by increasing the deposition temperature. # 2003 Elsevier Ltd. All rights reserved.

Epitaxial BiFeO3 thin films on Si

Abstract: BiFeO3 was studied as an alternative environmentally clean ferro/piezoelectric material. 200-nm-thick BiFeO3 films were grown on Si substrates with SrTiO3 as a template layer and SrRuO3 as bottom electrode. X-ray and transmission electron microscopy studies confirmed the epitaxial growth of the films. The spontaneous polarization of the films was ∼45μC∕cm2. Retention measurement up to several days showed no decay of polarization. A piezoelectric coefficient (d33) of ∼60pm∕V was observed, which is promising for applications in micro-electro-mechanical systems and actuators.

Generation of silicon nanoparticles via femtosecond laser ablation in vacuum

Abstract: We demonstrate that femtosecond laser ablation of silicon targets in vacuum is a viable route to the generation and deposition of nanoparticles with radii of ≈5–10 nm. The nanoparticles dynamics during expansion has been analyzed through their structureless continuum optical emission, while atoms and ions, also present in the plume, have been identified by their characteristic emission lines. Atomic force microscopy analysis of the material deposited at room temperature has allowed the characterization of the nanoparticles size distribution. Taking into account the emissivity of small particles we show that the continuum emission is a blackbody-like radiation from the nanoparticles. Our results suggest that nanoclusters are generated as a result of relaxation processes of the extreme material state reached by the irradiated target surface, in agreement with recently published theoretical studies.

Effects of Ti–W codoping on the optical and electrical switching of vanadium dioxide thin films grown by a reactive pulsed laser deposition

Abstract: Thin films of thermochromic VO2, V1−xWxO2 and V1−x−yWxTiyO2 (x=0.014, and y=0.12) were synthesized onto quartz substrates using a reactive pulsed laser deposition technique. The films were then characterized by x-ray diffraction and x-ray photoelectron spectroscopy. The W and Ti dopant effects on the semiconductor-to-metal phase transition of VO2 were investigated by measuring the temperature dependence of their electrical resistivity and their infrared transmittance. Remarkably strong effects of Ti–W codoping were observed on both the optical and electrical properties of V1−x−yWxTiyO2 films. The IR transmittance was improved, while the transition temperature could be varied from 36°C for W-doped VO2 film to 60°C for Ti–W codoped VO2 film. In addition, at room temperature, a higher temperature coefficient of resistance of 5.12%∕°C is achieved. Finally, both optical and electrical hysteresis are completely suppressed by Ti–W codoping the VO2 films.

Ultrahigh vacuum glancing angle deposition system for thin films with controlled three-dimensional nanoscale structure

Abstract: An ultrahigh vacuum apparatus for the deposition of thin films with controlled three-dimensional nanometer-scale structure is described. Our system allows an alternate, faster, cheaper way of obtaining nanoscale structured thin films when compared to traditional procedures of patterning and etching. It also allows creation of porous structures that are unattainable with known techniques. The unique feature of this system is the dynamic modification of the substrate tilt and azimuthal orientation with respect to the vapor source during deposition of a thin film. Atomic-scale geometrical shadowing creates a strong directional dependence in the aggregation of the film, conferring control over the resulting morphological structure on a scale of less than 10 nm. Motion can create pillars, helixes, zig–zags, etc. Significant features of the apparatus include variable substrate temperature, insertion and removal of specimens from atmospheric conditions without venting the deposition system, computer controlled p...

Effects of high-dose Mn implantation into ZnO grown on sapphire

Abstract: ZnO films grown by pulsed-laser deposition on c-plane Al2O3 substrates were annealed at temperatures up to 600 °C to produce n-type carrier concentrations in the range 7.5×1015–1.5×1020 cm−3. After high-dose (3×1016 cm−2) Mn implantation and subsequent annealing at 600 °C, all the films show n-type carrier concentrations in the range 2–5×1020 cm−3 and room temperature hysteresis in magnetization loops. The saturation magnetization and coercivity of the implanted single-phase films were both strong functions of the initial anneal temperature, suggesting that carrier concentration alone cannot account for the magnetic properties of ZnO:Mn, and that factors such as crystalline quality and residual defects play a role.

Transparent conducting Sb-doped SnO2 thin films grown by pulsed-laser deposition

Abstract: Antimony-doped tin oxide (SnO2:Sb) thin films (100–480 nm thick) have been deposited by pulsed-laser deposition on glass substrates without a postdeposition anneal. The structural, electrical, and optical properties of these films have been investigated as a function of doping amount, substrate temperature, and oxygen partial pressure during deposition. Films were deposited at temperatures ranging from 25 to 600 °C in O2 partial pressures ranging from 10 to 100 mTorr. The films (300 nm thick) deposited at 300 °C in 45 mTorr of oxygen show electrical resistivities as low as 9.8×10−4 Ω cm, an average visible transmittance of 90%, a refractive index of 1.98 (at 550 nm), and an optical band gap of 4.21 eV.

Review of polarity determination and control of GaN

Abstract: Polarity issues affecting III-V nitride semiconductors are reviewed with respect to their determination and control. A set of conditions crucial to the polarity control of GaN is provided for each of the following growth techniques; molecular beam epitaxy (MBE), pulsed laser deposition (PLD) and hydride vapor phase epitaxy (HVPE). Although GaN films might have been deposited by identical growth methods using the same buffer layer technologies, there is often a conflict between the resulting polarities achieved by different research groups. In this paper, we present the implications of the conditions used in each of the processes used for two-step metalorganic chemical vapor deposition (MOCVD), demonstrating systematic control of the polarity of GaN films on sapphire substrates. The potential for confusion in polarity control will be explained, taking into account the implications clarified in our studies. The correlation between the polarity and the growth conditions will be discussed in order to provide a mechanism for the determination and control of the crystal polarity during the growth of GaN films.

Electrochemical Reactions of Lithium with Transition Metal Nitride Electrodes

Abstract: Novel transition metal nitrides of Co3N and Fe3N thin films have been successfully fabricated by combining reactive pulsed laser deposition and DC discharge in a nitrogen ambient. The electrochemical reactions of lithium with Co3N and Fe3N thin film electrodes were first investigated by the galvanostatic discharge and charge, cyclic voltammetry, and the in-situ spectroelectrochemical measurements. These thin film electrodes exhibited reversible discharge capacities ranging from 324 mAh/g to 420mAh/g. The structure, morphology, and composition of the as-deposited, lithiated, and delithiated Co3N and Fe3N thin films have been characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). The observed diffraction peaks from metal Co(111) and Fe(110) in the lithiated thin films showed well crystallinity of transition metallic Co and Fe formed by Li reacted with Co3N and Fe3N, respectively, which could further be confirmed by their XPS spectra. We prop...

Three-dimensional heteroepitaxy in self-assembled BaTiO3–CoFe2O4 nanostructures

Abstract: Self-assembled BaTiO3–CoFe2O4 complex oxide nanostructures have been synthesized by pulsed laser deposition. A single ceramic target with a molar ratio of 0.62BaTiO3–0.38CoFe2O4 was used. Spinel CoFe2O4 and perovskite BaTiO3 phases spontaneously separated during heteroepitaxial growth on a single-crystal SrTiO3(001) substrate. The nanostructures are epitaxial in-plane as well as out-of-plane, with CoFe2O4 nanopillar arrays embedded in a BaTiO3 matrix. The CoFe2O4 nanopillars have uniform size and spacing and nearly circular cross section. As the substrate temperature increases from 750 to 950°C, the average diameter of the pillars increases from ∼9 to ∼70nm.

Ferromagnetism at room temperature with a large magnetic moment in anatase V-doped TiO2 thin films

Abstract: V-doped TiO2 thin films were grown by laser ablation on LaAlO3 substrates. In the chosen range of the growth conditions, all V:TiO2 films have an anatase structure and exhibit semiconducting and ferromagnetic behaviors at room temperature. V:TiO2 films have a giant magnetic moment and they seem to be far better ferromagnetic than Co/Fe/Ni-doped TiO2 films. This study has proved that a few percent of V substituting for Ti in TiO2 can result in a potential diluted magnetic semiconductor.

Band-gap modified Al-doped Zn1-xMgxO transparent conducting films deposited by pulsed laser deposition

Abstract: Al-doped Zn1−xMgxO films have been deposited on glass substrates at a substrate temperature of 200°C by a pulsed laser deposition system. A resistivity of 3×10−4Ωcm was obtained at x=0.06. Film resistivity was found to increase with further increases in Mg composition. The maximum band gap of films with a resistivity ρ⩽1×10−3Ωcm was found to be 3.97eV, demonstrating band-gap engineering possibilities in the range of Eg=3.5–3.97eV with a resistivity ρ⩽1×10−3Ωcm. The average transmittance of the films was higher than 90% in the wavelength region λ=400–800nm, a range suitable for transparent conducting film applications.

Insights into microstructural evolution from nanocrystalline Sn O 2 thin films prepared by pulsed laser deposition

Abstract: Low-dimensional nanostructures of SnO{sub 2} thin films with the interesting features of the tetragonal rutile structure have been prepared by pulsed laser deposition. The microstructural evolution of nanocrystalline SnO{sub 2} thin films has been investigated using x-ray diffraction, transmission electron microscopy, high-resolution transmission electron microscopy, and Raman spectroscopy. Experimental results indicate that the as-prepared SnO{sub 2} thin films appear to be of polycrystalline state, have a large amount of defects, such as oxygen vacancies, vacancy clusters, and local lattice disorder at the interface and surface, and the appearance of a new Raman peak. It suggests that this new Raman peak is closely related to a surface layer of nonstoichiometic SnO{sub x} with different symmetries than SnO{sub 2}, or in other words, the new peak marks an additional characteristic of space symmetry of the grain agglomeration of nanocrystalline SnO{sub 2}. The study of the microstructural evolution of nanocrystalline SnO{sub 2} is significant for the understanding of the whole structure feature of nanomaterials and for the fabrication of new nanomaterials with favorable properties.