Pulsed 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 characterisation of perovskite-type LaTiO3−xNx thin films

LaTiO3−xNx films with high nitrogen contents (x≈0.4–0.8) have been grown on (001)-oriented MgO and LaAlO3 substrates by pulsed reactive crossed-beam laser ablation from a La2Ti2O7 target in a one-step process. These films have a perovskite-type structure, similar to bulk polycrystalline LaTiO2N. The nitrogen content in the films can be changed by selecting the nitriding source, i.e. NH3 or N2 gas pulse, and by varying the substrate temperature. Films deposited on MgO show a preferential (001) and (112) orientation, whereas films on LaAlO3 are predominantly (001) oriented. N(2p) orbitals contribute to the top of the valence band, leading to a considerable decrease in the band gap energy from ∼4.0eV for the parent LaTiO3.5 to ∼2.4–2.9eV for LaTiO3−xNx (x≈0.4–0.8) films. This results in a strong visible light absorption at wavelengths below ∼500nm. The band gap energies in LaTiO3−xNx films decrease with increasing nitrogen content. The values of the band gap energies for LaTiO3−xNx as a function of the N content agree well with other titanate-based perovskite-type oxynitrides, which confirms the proposed band gap model.

ZnO(101) films by pulsed reactive crossed-beam laser ablation

We have employed pulsed reactive crossed-beam laser ablation (PRCLA) to deposit a (101) oriented ZnO film. In this method, a supersonic jet of oxygen pulse is made to cross the laser plume from a zinc metal target while being carried to the Si(111) substrate. The obtained deposit was nanocrystalline ZnO as confirmed by a host of characterization techniques. When the substrate was held at varying temperatures, from room temperature to 900°C, the crystallinity of the obtained films increased as expected, but importantly, the crystallographic orientation of the films was varied. High substrate temperatures produced the usual (001) oriented films, while lower substrate temperatures gave rise to increasingly (101) oriented films. The substrate held at room temperature contained only the (101) orientation. The film morphology also varied with the substrate temperature, from being nanoparticulate to rod-like deposits for higher deposition temperatures. Surprisingly, the (101) orientation showed reactivity with acetone forming carbonaceous nanostructures on the surface.

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.

Thermoelectric properties of LaCo 1− xNi xO 3 polycrystalline samples and epitaxial thin films

Epitaxial thin films of LaCo 1− x Ni x O 3 were deposited on MgO (100) substrates by pulsed reactive crossed-beam laser ablation (PRCLA). The electrical transport and thermopower of the epitaxial La(Co,Ni)O 3 thin films were measured in a broad temperature range and their thermoelectric activity was compared with bulk samples. The measured films show positive Seebeck coefficient values indicative of p-type conduction. The thin films display a better thermoelectric performance at high temperatures compared to the polycrystalline samples.

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)

The effect of the fluence on the properties of La–Ca–Mn–O thin films prepared by pulsed laser deposition

Thin films of La 0.6Ca 0.4MnO 3−δ were deposited on SrTiO 3(100) by PRCLA (Pulsed Reactive Crossed-Beam Laser Ablation). The dependence of the structural and transport properties of the films on the laser fluence and different target to substrate distances during the growth are studied. Both parameters have a direct influence on the films thickness and velocity of the ions arriving at the substrate, which influence the film properties directly. The surface roughness of the La 0.6Ca 0.4MnO 3−δ thin films is depending mainly on the laser fluence and less on the target-substrate distance. Lower laser fluences and therefore lower growth rates yield film with lower roughness, i.e. in the range of 0.2 nm. The electronic transport measurements show a decrease of the transition temperature from metal to semiconductor with an increase of the target to substrate distance. This is related to an increase of the films thickness and therefore decrease of the strain in the films due to the lattice mismatch with the substrate. The magnetoresistance values are also strongly affected by the tensile strain, i.e. they increase for higher strained films.

Perovskite thin films deposited by pulsed laser ablation as model systems for electrochemical applications

Thin films of a bifunctional electrocatalyst with the compositions La0.6Ca0.4CoO3−δ and La0.7Ca0.3CoO3−δ have been deposited by a variation of pulsed laser deposition, i.e. pulsed reactive crossed-beam laser ablation. These perovskite phases have been used as catalysts for oxygen reduction and evolution in re-chargeable Zn/air batteries. The utilization of a synchronized reactive gas pulse with N2O or O2 allows the preparation of perovskite films with almost ideal oxygen content without additional annealing steps and to control the oxygen content of the films. The films with higher oxygen content reveal a lower resistivity. These compositions have been selected to study the influence of the texture on the electrocatalytical activity for oxygen reduction and evolution of the films. Amorphous films, or films with mixed or single orientation can be obtained by varying the target–substrate distance and substrate temperature without changing the composition of the films. A clear influence of the crystallinity on the catalytic activity, i.e. smaller overpotential for the two oxygen reactions, is observed. The amorphous films reveal the largest overpotential, followed by the polycrystalline films with one or more orientations, and the single crystalline films with (100) orientation.

Can thin perovskite film materials be applied as model systems for battery applications?

The catalytic performance of carbon-based perovskite gas diffusion electrodes and thin films of identical perovskite catalyst phases (La 0.6Ca 0.4CoO 3− δ (LCCO), La 0.7Ca 0.3MnO 3− δ (LCMO) and La 0.7Ca 0.3Mn 0.9Ni 0.1O 3− δ (LCMNO)) has been compared. The thin films were deposited on MgO(1 0 0), MgO(1 1 0), and MgO(1 1 1) by pulsed reactive crossed-beam laser ablation (PRCLA). The quality of the films was determined by analyzing the composition as well as the crystallographic phase structure and texture of the electrodes. The catalytic activity for both the oxygen reduction and oxygen evolution reactions is evaluated from polarization curves for the selected electrodes. The compositional analysis of the films indicates that the ablation is congruent mainly for the LCCO, and that the films are nearly stoichiometric in their composition. The crystallographic orientation of the grown films is affected by the substrate. Under the selected deposition conditions it was not possible to grow parallel epitaxial films on MgO(1 1 0) and MgO(1 1 1). The key factors for epitaxy are besides the lattice misfit, also chemical/electronic interactions between film and substrate. The predominant crystallographic orientation is the one corresponding to the next lower surface/interfacial energy. The prepared films act as bifunctional catalysts, and the electrochemical experiments indicate that different exposed surfaces affect the performance of the electrode. The most active electrode was found to be the (1 0 0) textured film.

Pulsed laser deposition of atomically flat La1-xSrxMnO3 thin films using a novel target geometry

A new ablation target geometry is presented that was used to produce thin films of La1-xSrxMnO3 grown heteroepitaxially on SrTiO3 by pulsed reactive crossed-beam laser ablation. The films were grown in order to perform angle-resolved photoelectron spectroscopy, which demands that the surface be atomically flat. In situ and ex situ analysis shows that this condition was met, even after depositing to a thickness of over 100 nm.

Microstructures and film properties of La0.7Ca0.3CoO3 produced by pulsed reactive crossed-beam laser ablation

The quality and microstructure of La0.7Ca0.3CoO3 films produced by pulsed reactive crossed-beam laser ablation are analyzed considering composition, crystallographic orientation and morphology. The effects of cooling conditions, deposition with an oxygen background and of an additional N2O gas pulse are studied. The analysis indicates that the best film quality is obtained for films deposited in the presence of the gas pulse and the oxygen background, followed by a fast cooling. The presence of the different oxidizing sources influences the crystallographic orientation of the grown films.

Nucleation and growth of epitaxial La1−xCaxCoO3-δ films on single crystalline substrates by pulsed reactive crossed-beam laser ablation

To achieve a qualitative understanding of the heteroepitaxial perovskite film formation process, the synthesis and characterisation of La0.6Ca0.4CoO3 nanoscopic islands, ultrathin and thin films is described. These samples are produced by pulsed reactive crossed-beam laser ablation on single crystalline MgO substrate materials. Atomic force microscopic (AFM) and electron microscopic (EM) studies on the very initial growth steps on MgO reveal a three-dimensional (3D) growth mode forming 2 nm diameter perovskite-type islands. After further deposition the islands agglomerate to epitaxial dense films of a cubic perovskite-type metal oxide phase.

Analysis of the plasma produced by pulsed reactive crossed-beam laser ablation of La 0.6Ca 0.4CoO 3

Pulsed laser deposition (PLD) in conjunction with a synchronized pulsed gas source provides an extra degree of freedom in processing the ablation plume. This technique is called pulsed reactive crossed-beam laser ablation (PRCLA). The ablation plasma of La 0.6Ca 0.4CoO 3 (LCCO) produced by KrF excimer laser (248 nm) irradiation is analyzed by spatially and temporal resolved optical emission spectroscopy in the range of 300–800 nm in vacuum, with oxygen background and with an additional N 2O as a gas pulse. The spectra reveal higher intensities for all the peaks when the gas pulse is applied. No new peaks assigned to the interaction with the gas pulse, i.e. oxygen, nitrogen or diatomic species are observed. The maximum intensities of the plasma with the gas pulse are observed later in time and further away from the target.

From nitride to carbide: control of zirconium-based hard materials film growth and their characterization

High-quality thin films of ZrCyN1-y and the novel tribological material Zr0.8Al0.2CyN1-y have been grown by pulsed reactive crossed-beam laser ablation using Zr and Zr–Al ablation targets, respectively, and a pulsed gas. The gas mixture provided the carbon and nitrogen for the solid-solution films. Control of the stoichiometry (i.e. y) was determined by the relative partial pressures of the nitrogen- and carbon-containing gases. It was found that optimal control of the film chemistry was achieved by using the least thermally reactive gases. In this manner, it was possible to activate the gas species exclusively by collisions in the gas phase with the ablation-plume particles, thereby decoupling the chemistry from surface processes. The films were characterized for their chemical, crystallographic, optical, and tribological properties. All the films had very low impurity levels and a cubic rock salt crystal structure over the entire investigated temperature range between 100 and 600 °C. Exceedingly high quality epitaxial films could be grown on MgO (001) at 600 °C. Films grown on stainless steel were polycrystalline. The hardness of the films showed a maximum for both sets for stoichiometries predicted by a recent theoretical model for hardness based on band-structure calculations. In addition, all the films had an exceptionally low coefficient of friction versus steel.

Materials by design—exploiting the unique properties of pulsed laser deposition for the synthesis of novel hard materials and structures

Novel multilayer thin film structures with an (A–B–C–B) four-sublayer periodicity were synthesized using pulsed reactive crossed-beam laser ablation. The layers were based on transition metal carbonitrides in which one sublayer (A=TiC x N 1− x ) was optimized for its high hardness, another (C=ZrC x N 1− x ) for its low frictional properties, and a third (B=VC x N 1− x ) which acted as a barrier to dislocation propagation. Control of growth and stoichiometry was facilitated by using thermally stable gases as sources for the carbon and nitrogen which were activated by collisions with the ablation plasma. It was discovered that the ablation yields of Ti, V, and Zr were almost identical, so that the sublayer thickness was directly proportional to the number of ablated shots per sublayer metal. The four-sublayer structures were harder ( H ̄ =35 GPa) than corresponding bilayer structures ( H ̄ =30 GPa) in which the VC x N 1− x sublayers were missing. Further improvements are expected by optimizing the sublayer ratios and the absolute period thickness.

Pulsed laser deposition of La0.6Ca0.4CoO3 (LCCO) films. A promising metal-oxide catalyst for air based batteriesPresented at the LANMAT 2001 Conference on the Interaction of Laser Radiation with Matter at Nanoscopic Scales: From Single Molecule Spectroscopy to Materials Processing, Venice, 3–6 October, 2001.

La0.6Ca0.4CoO3 (LCCO) thin films were deposited on MgO(001) and stainless steel substrates by pulsed reactive crossed-beam laser ablation (PRCLA). The film stoichiometry was characterized by Rutherford backscattering spectrometry (RBS). The data confirmed that the material transfer from the target to the substrate is congruent. The thickness and surface roughness of the films is in the range of 200–500 and 2–12 nm, respectively, depending on the deposition conditions. The quality of the deposited film was analyzed by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). Epitaxial and single oriented thin films could be grown. In view of the importance of the surface composition in electrochemistry, X-ray photoelectron spectroscopy (XPS) measurements were performed. The electrochemical activity of the LCCO films is influenced by the crystallinity of the film.

Gas-phase mechanisms in the growth of ZrC yN 1− y thin films by pulsed reactive crossed-beam laser ablation

Superhard zirconium carbonitride films have been grown via pulsed reactive crossed-beam laser ablation (PRCLA) using zirconium metal and a nitrogen- and carbon-containing gas pulse mixture. The control of stoichiometry was much simplified by using the thermally stable gas-phase species N 2 and CH 4. The gas-phase processes are investigated using quadrupole mass spectroscopy and optical emission spectroscopy. The excitation of the ablation plume depends intimately on the collision partner of the gas pulse, in particular on its density of states and the probability of energy transfer to internal degrees of freedom.

Pulsed laser deposition of electrochemically active perovskite films

La 0.6Ca 0.4CoO 3 (LCCO) thin films were deposited on MgO(0 0 1) and stainless steel substrates by pulsed reactive crossed-beam laser ablation. High resolution X-ray diffraction (XRD) spectra of the films showed that their quality depends on the oxygen background during cooling. Rutherford backscattering spectrometry measurements reveal that under optimal conditions the films have the same stoichiometry as the original target. X-ray photoelectron spectroscopy (XPS) was applied to characterize the surface of the films, which revealed two oxygen and calcium species: the one at lower binding energy, corresponds to oxygen from the perovskite structure and the one at higher binding energy correspond to Ca–O surface species and adsorbed oxygen. The electrochemical activity of the LCCO films is influenced by the crystallinity of the perovskite electrodes. The structural and electrochemical properties depend on the preparation conditions and substrate.

Design and synthesis of novel thin films and structures by reactive crossed-beam laser ablation

As new materials are discovered, there is a strong drive to include them in thin film form as micro- and nanostructures. Established growth techniques may be unable to meet the demands required of these often chemically and crystallographically complex materials. An account is given of pulsed laser deposition (PLD) and its novel extension, pulsed reactive crossed-beam laser ablation (PRCLA) as possible alternative methods for the non-thermal synthesis and growth of novel advanced materials. After a brief description of the principles and applications of PLD, it is shown that adding a synchronized pulsed gas source can lend both technical and fundamental advantages. Examples of syntheses of thin films and multilayer structures of novel and technologically relevant materials are then described.

ZrN, ZrxAlyN and ZrxGayN thin films – novel materials for hard coatings grown using pulsed laser deposition

High-quality thin films of ZrN, ZrxAlyN and ZrxGayN have been grown by pulsed reactive crossed-beam laser ablation using Zr, Zr-Al and Zr-Ga ablation targets, respectively, and a N2 gas pulse. The films were characterized for their chemical, crystallographic and tribological properties. All the films had very low impurity levels and a cubic rock salt crystal structure over the entire investigated temperature range between room temperature and 600 °C. High-quality epitaxial films could be grown on Si (001) at 400 °C, though the crystallinity was disrupted at 525 °C by Si diffusion into the film bulk and the formation of ZrSi2 crystallites. Films grown on stainless steel were polycrystalline. The ratios of the metals in the alloy targets were in general not equal to those in the films: the Al content in the ZrxAlyN films was lower than the target value, which we attribute to differential scattering in the ablation plume. The Ga content in the ZrxGayN films fell with increasing substrate temperature, indicative of re-evaporation of Ga from the substrate surface. Those ZrxGayN films with the highest Ga content, grown at the lowest temperatures, were particularly nitrogen-deficient, which we attribute to the low reactivity of Ga with N2. The ZrxAlyN films had an exceptionally low coefficient of friction (0.20) versus steel and the greatest nanohardness of 28 GPa.