Reactive Crossed-beam Laser Ablation Research Articles

Tracing the plasma interactions for pulsed reactive crossed-beam laser ablation

Pulsed reactive crossed-beam laser ablation is an effective technique to govern the chemical activity of plasma species and background molecules during pulsed laser deposition. Instead of using a constant background pressure, a gas pulse with a reactive gas, synchronized with the laser beam, is injected into vacuum or a low background pressure near the ablated area of the target. It intercepts the initially generated plasma plume, thereby enhancing the physicochemical interactions between the gaseous environment and the plasma species. For this study, kinetic energy resolved mass-spectrometry and time-resolved plasma imaging were used to study the physicochemical processes occurring during the reactive crossed beam laser ablation of a partially 18O substituted La0.6Sr0.4MnO3 target using oxygen as gas pulse. The characteristics of the ablated plasma are compared with those observed during pulsed laser deposition in different oxygen background pressures.

Pulsed laser deposition and characterization of nitrogen-substituted SrTiO 3 thin films

Nitrogen-substituted cubic perovskite-type SrTiO 3 thin films were deposited in a one-step process using pulsed reactive crossed beam laser ablation (PRCLA) and RF-plasma assisted pulsed laser deposition (RF-PLD). Both techniques yield preferentially oriented films on SrTiO 3(0 0 1), LaAlO 3(0 0 1) and MgO(0 0 1) substrates with the unit cell parameters within 0.390(5) < a < 0.394(9) nm. The nitrogen content is higher in films deposited by PRCLA (0.84–2.40 at.%) as compared to films deposited by RF-PLD with nitrogen plasma (0.10–0.66 at.%). PRCLA with an ammonia gas pulse leads to a higher nitrogen content compared to the films grown with a nitrogen gas pulse, while films deposited by RF-PLD with ammonia plasma reveal only minor nitrogen contents (<0.10 at.%). The amount of the incorporated nitrogen can be tuned by adjusting the deposition parameters. Films deposited by PRCLA have a lower roughness of 1–3 nm compared to 12–18 nm for the films grown by RF-PLD. PRCLA yields partially reduced films, which exhibit electronic conductivity, while films deposited by RF-PLD are insulating. There is also a pronounced influence of the substrate material on the resistivity of the films deposited by PRCLA: films grown on SrTiO 3 substrates exhibit a metallic-like behaviour, while the corresponding films grown on MgO and LaAlO 3 substrates reveal a metal-to-semiconductor/insulator transition. Nitrogen incorporation into the SrTiO 3 films results in an increased optical absorption at 370–500 nm which is associated with N(2p) localized states with the energy about 0.7 eV higher than the valence band energy in strontium titanate. The optical band gap energies in the studied N-substituted SrTiO 3 films are 3.35–3.40 eV.

Characterization of sol‐gel SiO2:Ce,Tb powder and pulsed laser deposited thin film phosphors

AbstractThin films of SiO2:Ce,Tb (Ce = Tb = 0.5 mol%) nanoparticulate phosphor (nanophosphor) were ablated on Si (100) substrates using either the conventional pulsed laser deposition (PLD) or pulsed reactive crossed beam laser ablation (PRCLA) technique. Morphology and surface topography of the films were analysed with scanning electron microscopy (SEM) and atomic force microscopy (AFM) respectively. The chemical composition and thicknesses of the films were determined by Rutherford backscattering spectroscopy (RBS). Luminescence properties of the films were studied with cathodoluminescence (CL) spectroscopy and a 325 nm HeCd laser. Enhanced green cathodoluminescence and photoluminescence (PL) associated with 5D4→7F4 transitions in Tb3+ was observed at ∼540 nm in powder and thin film samples, and is attributed to non‐radiative energy transfer from co‐doped Ce3+. Luminescence intensities of the films and that of the SiO2:Ce,Tb powder used to make the films are compared. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Structure, microstructure, and high-temperature transport properties of La1−xCaxMnO3−δ thin films and polycrystalline bulk materials

Single-phase samples of La1−xCaxMnO3−δ (LCMO), x≈0.3, prepared by pulsed reactive crossed beam laser ablation on SrTiO3 (STO) substrates, and soft chemistry synthesized powders were studied by various methods. The precise study of the crystal structure and microstructures by a combination of electron diffraction and high-resolution electron microscopy revealed a monoclinic distortion of the GdFeO3-type structure, P21/c space group, in both types of materials, i.e., the thin films and powder compound. The analysis of the LCMO/STO interface showed nonhomogeneous stress states and a composition that results in a different superstructure from the usual detected structure. The temperature-dependent thermoelectric power in the case of thin films presented an anomalous behavior compared to those from the powder compound. A structural transition at high temperature (T≈750 K) influences the thermopower behavior as well as the thermal conductivity values.

One-step preparation of N-doped strontium titanate films by pulsed laser deposition

Perovskite-type oxynitrides exhibit promising electrical and optical properties and can possibly be used in the future as functional materials for electrical, photo-, and electrochemical applications. Continuous heterovalent substitution of oxygen ions by nitrogen ions allows tuning of the desired optical and/or electronic properties to the application specifications. In the present work deposition of SrTiO3:N films by pulsed reactive crossed beam laser ablation was studied in order to examine the influence of different deposition parameters on the film crystallinity and composition. The deposited films exhibit a perovskite-type crystal structure and reveals epitaxial growth on MgO(100) substrates. The unit cell parameters of the deposited SrTiO3:N films range within $3.9\underline{11}<a<3.9\underline{19}$ , which is slightly larger than for polycrystalline SrTiO3 (a=3.905). The studied films reveal an oxygen content in the range of (2.70-2.98)±0.15. The relative N content (vs. O) can be tuned within the range of 1.0–3.0% by adjusting the deposition parameters. The N:O concentration ratio increases with increasing laser fluence and target-to-substrate distances, while the substrate temperature has a more complex influence on the nitrogen concentration. In the range of 580–650 °C the [N]/[O] ratio increases while further heating results in a gradual decrease of the N content.

Analysis of the electronic configuration of the pulsed laser deposited La 0.7Ca 0.3MnO 3 thin films

The electronic properties of La 0.7Ca 0.3MnO 3− δ thin films grown by the pulsed reactive crossed beam laser ablation method are investigated. The effects of post-deposition annealing of epitaxial La 0.7Ca 0.3MnO 3− δ thin films have been investigated using X-ray photoelectron spectroscopy (surface sensitive) and hard X-ray absorption spectroscopy (bulk sensitive). The films deposited in the high vacuum are oxygen deficient and contain mostly Mn 3+. High temperature annealing in a flowing oxygen atmosphere partially changes the Mn oxidation state from +3 towards +3.4. These changes should favor a metal-like conduction and a ferromagnetic double exchange transport mechanism in the annealed thin films.

Preparation of epitaxial La0.6Ca0.4Mn1−xFexO3 (x=0, 0.2) thin films: Variation of the oxygen content

Abstract Perovskite thin films with a nominal composition of La0.6Ca0.4Mn1−xFexO3 (x = 0, 0.2) were deposited by pulsed reactive crossed beam laser ablation. The film properties, such as electrical conductivity and magnetoresistance are studied as a function of the oxygen content and substrate type. The oxygen content of the thin films was determined by Rutherford Backscattering and controlled by varying the background gas pressure, pressure of the gas pulse and by using alternatively O2 and N2O as the gas pulse. LaAlO3 and SrTiO3 were used as substrates at deposition temperature of 650 °C. The grown films were analyzed by X-ray diffraction in order to optimize the growth conditions, i.e. to obtain epitaxial thin films. Thin films doped with 20% Fe were grown under the same experimental conditions as the undoped LCMO films and the effect of the doping on the structural and transport properties of the thin films has been investigated. The temperature of the metal–insulator transition was measured as a function of the oxygen content and substrate type.

Structure and Composition of Nanoscopic Domains in Functional Perovskite-Type Materials

A- and B-site substituted cobaltate perovskites (ABO3) were prepared by soft chemistry and Pulsed laser Deposition (PLD) processes. The formation of nano-and microdomains was examined by means of transmission electron microscopy to determine the influence on the materials properties. The growth of thin epitaxial film son oxide substrates by pulsed reactive cross beam laser ablation proceeds through island formation in stranski-Krastanov fashion; after the islands reach a certain height they develop laterally to form a dense epitaxial film. Orientation relationships, interfacial strain and surface roughness depend on the misfit with the respective substrate. Oxygen deficient La/Ca cobaltates exhibit Brownmillerite and Ruddleston-Popper defects, and increasing O-deficiency results in increasing resistivity of the films. Local defects are found as well in the microstructure of Mn-doped cobaltates obtained by soft chemistry and sintered for long times at high temperatures. This perovskite phase exists in the orthorhombic phase at higher Mn-concentration and presents an array of inter-grown twin domains. Thin films of cobaltates with nominal composition of La0.6Ca0.4CoO3 exhibit catalytic activity for oxygen reduction and evolution in alkaline electrolytes. Control over the crystallinity of the thin film, achieved by pulsed reactive crossed beam laser ablation, was used to show that the catalytic activity varies with crystallinity. Single crystalline films exhibit the highest activity, followed by polycrystalline film and amorphous films. Even the orientation of single crystalline films has an influence on the catalytic activity.

Influence of experimental parameter on the Li-content of LiMn2O4 electrodes produced by pulsed laser deposition

Thin film deposition by pulsed reactive crossed beam laser ablation has often advantages such as no need for annealing after deposition over conventional pulsed laser deposition. However, when depositing material from a target containing a low atomic mass component such as lithium, the resulting films are frequently sub-stoichiometric in the light elements. This is most probably due to scattering of the plasma plume in the crossed gas pulse, which creates a high local pressure. The light species are scattered more than the heavier elements, which leads to Li-deficiencies in the resulting film. Depositing lithium-spinels without the use of a crossed gas pulse is still strongly dependent on the applied background pressure. A background pressure below 10 Pa results also in lithium-deficient films. However, this is not due to scattering effects but very probably to the evaporation of lithium oxide from the growing film. These boundary conditions allow only a window of conditions for the growth of stoichiometric LiMn2O4 films that is useful as test electrodes for Li-batteries research. A strong influence of the experimental parameters was observed. The films deposited at different background pressures and different substrate/target distances showed very different behaviors in the electrochemical measurements. Extraction and insertion of lithium was only detected for films deposited at a distance between 3 and 4 cm and at a background pressure of 20 Pa, whereas films deposited at other conditions did not show this electrochemical activity.

Structural characterization and magnetoresistance of manganates thin films and Fe-doped manganates thin films

Perovskites thin films with the composition La0.6Ca0.4MnO3 doped with 20% Fe, were prepared by pulsed reactive crossed beam laser ablation, where a synchronized reaction gas pulse interacts with the ablation plume. The films were grown on various substrates and the highest colossal magnetoresistance ratio (CMR) was detected by Hall measurements for films grown on LaAlO3 (1 0 0), which was selected as substrate for further investigations. Several growth parameters, such as substrate temperature and target to substrate distance were varied to analyze their influence on the film properties. The structure of the deposited thin films was characterized by X-ray diffraction and atomic force microscope, while Rutherford backscattering (RBS) was used to determine the film stoichiometry. The electrical properties were determined by Hall effect measurements in a magnetic field of 0.51 T. These measurements reveal that the amplitude of the CMR ratio depends strongly on the substrate and that the oxygen content influences the temperature where the transition from semiconductor to metal is observed. # 2005 Elsevier B.V. All rights reserved.

Pulsed reactive crossed beam laser ablation of La0.6Ca0.4CoO3 using 18O

Abstract The composition of thin perovskite films, especially the oxygen content, is a crucial parameter which influences many physical properties, such as conductivity and catalytic activity. Films produced by pulsed laser deposition are normally annealed in an oxygen atmosphere after deposition to achieve a desired oxygen content. In pulsed reactive crossed beam laser ablation, no annealing step is necessary, but a fundamental question regarding this deposition technique is still open: where does the oxygen in the films come from? There are three possibilities, i.e. from the target, from the gas background, or from the gas pulse. To answer this question two experiments were performed: 18 O 2 was used during the deposition process as background gas with 16 O anions in the target and 16 O 2 gas pulse, and a 18 O 2 gas pulse with 16 O from the target and background. These experiments revealed that the quantification of the oxygen origin is only possible, when no oxygen exchange occurs at the deposition temperature. The films are characterized after deposition by elastic recoil detection analysis (ERDA) to determine the 16 O/ 18 O ratio. Experiments with different oxidizing species in the gas pulse (N 2 O and O 2 ) confirm that the oxidizing potential (N 2 O > O 2 ) as well as the number of molecules are important.

Production and characterization of Ti:sapphire thin films grown by reactive laser ablation with elemental precursors

Crystalline Ti:sapphire (Ti:Al(2)O(3)) thin films were grown at low temperatures upon Al(2)O(3) (0001) substrates by reactive crossed-beam laser ablation at 248 nm by use of a liquid Ti-Al alloy target and O(2) . The films were investigated ex situ by x-ray diffraction, x-ray photoelectron spectroscopy, and Rutherford backscattering spectrometry. Low-temperature luminescence was identical to that for Ti(3+) ions in bulk samples of Al(2)O(3) .

RHEED analysis of interface growth modes of TiN films on Si(001) produced by crossed beam laser ablation

Epitaxial growth of TiN thin films on Si(001) using reactive crossed beam laser ablation has been investigated for substrate temperatures between 250 and 800°C. In situ RHEED analysis of the growing film has revealed two distinct interface growth regimes. Under about 550±50°C, TiN(001) grows epitaxially on Si(001) in a Volmer–Weber type 4-on-3 cube-on-cube mode and produces compressive biaxial stress in-plane, whereas at higher temperatures, the growth mode changes to a Stranski–Krastinov 5-on-4 cube-on-cube type with dilation of the strained interface layer.