Sequential Pulsed Laser Deposition Research Articles

Strain-engineering of magnetic anisotropy in CoxNi1−x−SrTiO3/SrTiO3 (001) vertically assembled nanocomposites

Ultrathin ${\mathrm{Co}}_{x}{\mathrm{Ni}}_{1\ensuremath{-}x}$ alloy nanowires vertically embedded in ${\mathrm{SrTiO}}_{3}/{\mathrm{SrTiO}}_{3}\phantom{\rule{0.16em}{0ex}}(001)$ thin films were grown using a self-assembly approach based on sequential pulsed laser deposition. Due to vertical epitaxial coupling of the metallic and oxide phases, a large average tensile strain of up to 4% arises within the nanowires, which is evidenced using a combination of x-ray diffraction and transmission electron microscopy. Macroscopic magnetometry experiments are used to demonstrate that this huge deformation allows us to enhance the uniaxial anisotropy of the nanowires, leading to saturation field in excess of 1 T in the hard direction, large coercive field at low temperature along the easy axis, and to a blocking temperature exceeding 600 K in the case of nanowires with a diameter of 5 nm and 78% Co content. These data are complemented with angular dependent x-ray magnetic circular dichroism measurements at the Co and Ni ${L}_{2,3}$ edges. The value of the magnetic moment was extracted from these measurements by applying sum rules and the anisotropy of the orbital moment was investigated.

Optical properties of differing nanolayered structures of divalent europium doped barium fluoride thin films synthesized by pulsed laser deposition

Optically-active thin films are employed in a variety of applications, such as LEDs and photovoltaics, due to their ability to act as up- or down-photon energy converters. Their performance depends critically on their composition and structure; thus, the use of novel synthesis techniques that allow for their control at the nanoscale level can result in improved efficiency and practicality. Layered thin films consisting of Eu2+- doped BaF2 nanocrystalline layers separated by amorphous Al2O3 were synthesized via sequential pulsed laser deposition using three separate targets for the different components; this synthesis technique provides precise control of layer thickness at the nanoscale along with dopant distribution within the film. Cross-sectional transmission electron microscopy analysis verified the desired nano-layering. Post-deposition heat treatments in a nitrogen atmosphere resulted in samples exhibiting steady emission with a broad peak ranging from 400 to 600 nm and a shoulder at 410 nm. The CIE 1931 chromaticity coordinates are x = 0.26–0.29 and y = 0.32–0.35 and vary as a function of the sample configuration. Because the chromaticity coordinates are close to those of a pure white light (x = 0.33,y = 0.33), these films demonstrate advantageous properties for applications with UV-pumped white light LEDs.

Strain, magnetic anisotropy, and composition modulation in hybrid metal–oxide vertically assembled nanocomposites

Self-assembled vertically aligned nanocomposites (VANs) have recently emerged as a novel playground for strain engineering of physical properties in nanostructures. In contrast to thin films obtained by classical planar heteroepitaxy, VANs consist of two (or more) intertwined phases, coupled along vertical interfaces. Their unique nanoarchitecture, which can be tuned by choosing appropriate growth conditions, results in deformations that cannot be easily attained in traditional flat geometries. In this article, we show how nanometer-sized acicular inclusions of magnetic 3d metals in various oxide host matrices can be obtained via sequential pulsed laser deposition. We discuss the distinct sources of magnetic anisotropy in such metal–oxide VANs and demonstrate how to use strain to accurately control the magnetic properties of the nanocomposites. We finally present possible extensions of this approach to more than one embedded metallic phase and sketch some of the remaining challenges that must be overcome to create novel functional nanoarchitectures.

Artificially engineered nanostrain in FeSexTe1-x superconductor thin films for supercurrent enhancement

Although nanoscale deformation, such as nanostrain in iron-chalcogenide (FeSexTe1−x, FST) thin films, has attracted attention owing to its enhancement of general superconducting properties, including critical current density (Jc) and critical transition temperature, the development of this technique has proven to be an extremely challenging and complex process thus far. Herein, we successfully fabricated an epitaxial FST thin film with uniformly distributed nanostrain by injection of a trace amount of CeO2 inside an FST matrix using sequential pulsed laser deposition. By means of transmission electron microscopy and geometric phase analysis, we verified that the injection of a trace amount of CeO2 forms nanoscale defects, with a nanostrained region of tensile strain (εzz ≅ 0.02) along the c-axis of the FST matrix. This nanostrained FST thin film achieves a remarkable Jc of 3.5 MA/cm2 under a self-field at 6 K and a highly enhanced Jc under the entire magnetic field with respect to those of a pristine FST thin film.

Direct In Situ Growth of Centimeter-Scale Multi-Heterojunction MoS2/WS2/WSe2 Thin-Film Catalyst for Photo-Electrochemical Hydrogen Evolution.

To date, the in situ fabrication of the large‐scale van der Waals multi‐heterojunction transition metal dichalcogenides (multi‐TMDs) is significantly challenging using conventional deposition methods. In this study, vertically stacked centimeter‐scale multi‐TMD (MoS2/WS2/WSe2 and MoS2/WSe2) thin films are successfully fabricated via sequential pulsed laser deposition (PLD), which is an in situ growth process. The fabricated MoS2/WS2/WSe2 thin film on p‐type silicon (p‐Si) substrate is designed to form multistaggered gaps (type‐II band structure) with p‐Si, and this film exhibits excellent spatial and thickness uniformity, which is verified by Raman spectroscopy. Among various application fields, MoS2/WS2/WSe2 is applied to the thin‐film catalyst of a p‐Si photocathode, to effectively transfer the photogenerated electrons from p‐Si to the electrolyte in the photo‐electrochemical (PEC) hydrogen evolution. From a comparison between the PEC performances of the homostructure TMDs (homo‐TMDs)/p‐Si and multi‐TMDs/p‐Si, it is demonstrated that the multistaggered gap of multi‐TMDs/p‐Si improves the PEC performance significantly more than the homo‐TMDs/p‐Si and bare p‐Si by effective charge transfer. The new in situ growth process for the fabrication of multi‐TMD thin films offers a novel and innovative method for the application of multi‐TMD thin films to various fields.

Phase separation and surface segregation in Co–Au–SrTiO3 thin films: Self-assembly of bilayered epitaxial nanocolumnar composites

Phase separation and surface segregation are powerful levers that allow one to synthesize nanocomposites via self-assembly. In the present work, we combine these concepts with vertical epitaxial growth and study Co-Au-${\mathrm{SrTiO}}_{3}$ thin films as a model system. We demonstrate that ${\mathrm{SrTiO}}_{3}$, Co, and Au undergo phase separation during sequential pulsed laser deposition, giving rise to a dense array of ultrathin bilayered Co-Au nanowires (NWs) with highly anisotropic optical and magnetic properties. A detailed analysis of the structure of the embedded metallic NWs reveals stabilization of a Co fcc phase and pronounced coupling to the matrix, which leads to large magnetoelastic effects. We discuss possible growth mechanisms yielding bilayer phase separation in nanocolumnar composites and show how the present results can be used to estimate a lower bound for the Co-Au interface energy.

Sequential pulsed laser deposition of homoepitaxial SrTiO3 thin films

The control of thin film stoichiometry is of primary relevance to achieve desired functionality. Pulsed laser deposition ablating from binary-oxide targets (sequential deposition) can be applied to precisely control the film composition, offsetting the importance of growth conditions on the film stoichiometry. In this work, we demonstrate that the cation stoichiometry of SrTiO3 thin films can be finely tuned by sequential deposition from SrO and TiO2 targets. Homoepitaxial SrTiO3 films were deposited at different substrate temperatures and Ti/Sr pulse ratios, allowing the establishment of a growth window for stoichiometric SrTiO3. The growth kinetics and nucleation processes were studied by reflection high-energy electron diffraction and atomic force microscopy, providing information about the growth mode and the degree of off-stoichiometry. At the optimal (stoichiometric) growth conditions, films exhibit atomically flat surfaces, whereas off-stoichiometry is accommodated by crystal defects, 3D islands, and/or surface precipitates depending on the substrate temperature and the excess cation. This technique opens the way to precisely control stoichiometry and doping of oxide thin films.

Ferroelectricity in Rare-Earth Modified Hafnia Thin Films Deposited by Sequential Pulsed Laser Deposition

Room temperature ferroelectricity in pulsed laser deposited rare-earth doped hafnium oxide (HfO2) thin films is discussed. Maximum values of remnant polarizations (Pr) ∼13.5 and 12 μC/cm2 along with coercive fields (EC) ∼334 and 384 kV/cm are observed in 6 mol. % of rare-earth (Sm or Gd) doped-HfO2 thin films (Sm:HfO2 and Gd:HfO2), respectively. Piezoresponse force microscopy measurements confirmed ferroelectric nature of films by showing phase hysteresis and butterfly amplitude loops. It is noticed that wake-up cycles improved the remnant polarization and found to be necessary for the forming of well saturated hysteresis loops. Our results showed potential toward realization of future highly scaled non-volatile ferroelectric memories.

Surface plasmon resonances of Ag-Au alloy nanoparticle films grown by sequential pulsed laser deposition at different compositions and temperatures

We studied localized surface plasmon resonances (LSPR) at different compositions, substrate temperatures, and mass thicknesses of Ag-Au alloy nanoparticle films grown by sequential pulsed laser deposition. The LSPRs were pronounced at all compositions of the films grown at high substrate temperature of about 300 °C as compared to those grown at room temperature. The alloy formation and composition of the films were determined using X-ray photoelectron and energy dispersive spectroscopy. Films' mass thickness and compositional uniformity along the thickness were determined using X-ray reflectometry and secondary ion mass spectroscopy. Atomic force microscopic analysis revealed the formation of densely packed nanoparticles of increasing size with the number of laser ablation pulses. The LSPR wavelength red shifted with increasing either Au percentage or film mass thickness and corresponding LSPR tuning was obtained in the range of 450 to 690 nm. The alloy dielectric functions obtained from three different models were compared and the optical responses of the nanoparticle films were calculated from modified Yamaguchi effective medium theory. The tuning of LSPR was found to be due to combined effect of change in intrinsic and extrinsic parameters mainly the composition, morphology, particle-particle, and particle-substrate interactions.

Stoichiometry control of complex oxides by sequential pulsed-laser deposition from binary-oxide targets

To have precise atomic layer control over interfaces, we examine the growth of complex oxides through the sequential deposition from binary targets by pulsed laser deposition. In situ reflection high-energy electron diffraction (RHEED) is used to control the growth and achieve films with excellent structural quality. The growth from binary oxide targets is fundamentally different from single target growth modes and shows more similarities to shuttered growth by molecular beam epitaxy. The RHEED intensity oscillations of non-stoichiometric growth are consistent with a model of island growth and accumulation of excess material on the surface that can be utilized to determine the correct stoichiometry for growth. Correct monolayer doses can be determined through an envelope frequency in the RHEED intensity oscillations. In order to demonstrate the ability of this growth technique to create complex heterostructures, the artificial n = 2 and 3 Srn+1TinO3n+1 Ruddlesden-Popper phases are grown with good long-range order. This method enables the precise unit-cell level control over the structure of perovskite-type oxides, and thus the growth of complex materials with improved structural quality and electronic functionality.

Cadmium(1−x)zinc(x)telluride thin films deposited by sequential pulsed laser deposition

In this work, sequential pulsed laser deposition was used for the deposition of cadmium zinc telluride (CZT) thin films. CZT is a ternary II–VI compound semiconductor with a tunable band gap between 1.51 and 2.26eV. In this work, three different CZT film compositions were achieved at room temperature by sequential deposition of nanometric layers with a precise number of laser shots on the cadmium telluride (CdTe) and zinc telluride (ZnTe) targets. XPS, XRD and UV–vis transmittance techniques were used to characterize the CZT films. The atomic content of zinc ranged from 60% down to 13%. This represents an enlargement of the lattice constant from 6.19 to 6.41Å, and a band gap decrement from 1.94 to 1.55eV. In addition, the CZT film resistivity can be modulated between the CdTe (4.1×107Ω-cm) and ZnTe (2.8×105Ω-cm) values. Our results demonstrated that the sequential pulsed laser deposition can be used to obtain several CZT film compositions with precise control of its stoichiometry and can be extended to the production of other ternary compounds.

Out of plane superferromagnetic behavior of quasi two-dimensional Fe/Al2O3 multilayer nanocomposites

The magnetic properties of low filling factor Fe-nanoparticle monolayers separated by relatively thick Al2O3 layers were investigated in parallel and perpendicular external magnetic field. The thin film nanocomposites were prepared by sequential pulsed laser deposition on (100) Si substrates, and the monolayers contain single-domain, dispersive nanoparticles. When the magnetic field is oriented parallel to the layers, the composite exhibits superparamagnetism. However, in perpendicular field, the superferromagnetic order sets in, as revealed by the increase of the magnetic moment, the hysteresis persisting at high temperatures, and a smaller relaxation rate. The complex out of plane behavior of our nanocomposites in perpendicular field is attributed to quasi two-dimensionality and to the dispersion of the in-plane nanoparticle separation.

Improving the uncommon (110) growing orientation of Al-doped ZnO thin films through sequential pulsed laser deposition

High quality Al-doped ZnO (AZO) films with uncommon (110) orientation are obtained on amorphous substrate by using Sequential Pulsed Laser Deposition technique. The dependence of the structural, optical and electrical properties with dopant concentration and oxygen deposition pressure was investigated systematically. We note a transition from the (002) preferential orientation of crystallites to an uncommon (110) orientation due to a combined effect of doping concentration and deposition pressure decreasing. For constant deposition pressure of 5Pa the film crystallinity is changed from preferential (002) to polycrystalline when increasing dopant concentration. For the maximum dopant concentration that we have investigated (i.e., 4.4% at.) structural properties of AZO films are changed from a polycrystalline phase to a (110) preferential orientation when the deposition pressure decreases. This uncommon growth mode is accompanied by a change of the morphology from a densely packed granular structure to a more rarefied one. Moreover, the band gap widens up to 3.88eV and the electrical resistivity drops to 5.4×10−2Ωcm. The structural changes were attributed to two mechanisms: a first one, responsible for the (002) phase suppression as a consequence of aluminum ion bombardment during the doping process and, a second one, in charge with (110) phase growth as the diffusion rates of zinc and oxygen atoms are affected by the dopant incorporation and by the decrease of deposition pressure.

Photovoltaic effect and enhanced magnetization in 0.9(BiFeO3)–0.1(YCrO3) composite thin film fabricated using sequential pulsed laser deposition

We report on the photovoltaic effect and multiferroic properties of a 0.9(BiFeO3)–0.1(YCrO3) composite thin film deposited on a Pt/TiO2/SiO2/Si substrate by sequential ablation of BiFeO3 and YCrO3 ceramic targets using pulsed laser deposition. The desired composition of the composite was achieved by controlling the ablation time of respective targets. As confirmed by the x-ray diffraction pattern the resultant film was found to be polycrystalline in nature and composed of a mixture of both rhombohedral BiFeO3 and orthorhombic YCrO3 phases. Interesting multiferroic properties in terms of an enhanced saturation magnetization of ∼14 emu cm−3 and the remnant polarization of ∼4.5 µC cm−2 were observed where the enhancement in magnetization as compared to pristine BiFeO3 could be attributed to the super-exchange interaction between Fe and Cr-ions. The photovoltaic properties of the composite thin film were studied under white light illumination in both top–bottom and lateral electrode configurations. Short circuit current densities (JSC) = 1.48 µA cm−2 and 0.44 µA cm−2, and open circuit voltages (VOC) = 0.51 V and 0.32 V were observed in top–bottom and lateral electrode configurations, respectively.

Quantum corrections to temperature dependent electrical conductivity of ZnO thin films degenerately doped with Si

ZnO thin films degenerately doped with Si (SixZn1−xO) in the concentrations range of ∼0.5% to 5.8% were grown by sequential pulsed laser deposition on sapphire substrates at 400 °C. The temperature dependent resistivity measurements in the range from 300 to 4.2 K revealed negative temperature coefficient of resistivity (TCR) for the 0.5%, 3.8%, and 5.8% doped SixZn1−xO films in the entire temperature range. On the contrary, the SixZn1−xO films with Si concentrations of 1.0%, 1.7%, and 2.0% showed a transition from negative to positive TCR with increasing temperature. These observations were explained using weak localization based quantum corrections to conductivity.

Nitrogen-doped and gold-loaded TiO2 photocatalysts synthesized by sequential reactive pulsed laser deposition

Nitrogen-doped titanium oxide thin films covered by gold metal nanoparticles were grown on (001) SiO2 quartz substrates by pulsed laser deposition. A KrF* excimer laser source (λ = 248 nm, τFWHM ≤ 25 ns, ν = 10 Hz) was used for the irradiation of TiO2 and gold metal targets. The experiments were performed in controlled reactive oxygen or nitrogen atmosphere. The layers were grown for photocatalytic applications. Evaluation of photocatalytic activity was performed by photodegradation of methyl orange under near-UV light irradiation. Our results show that nitrogen doping and addition of gold nanoparticles have complementary effects, photoactivity being significantly improved as compared to that of pure titanium oxide.

A Simple Approach for the Magnetic Relaxation in Systems of Weakly-Interacting, Dispersive Nanoparticles in Applied Magnetic Field

We investigated the magnetic relaxation of weakly interacting, dispersive, single domain Fe nanoparticles in Fe/Al2O3 composites prepared by sequential pulsed laser deposition on (100) Si substrates. The external magnetic field was oriented parallel to the substrate and applied in zero-field cooling conditions. It was found that the relax- ation of the irreversible magnetic moment mirr is similar to the well-known vortex-creep process in disordered super- conductors with poor supercurrent redistribution across the sample, leading to a power-law dependence of mirr(t), with a well-defined time t exponent.

Experimental study and modeling of laser plasma ion implantation for WSex/57Fe interface modification

A multilayer WSe1.7/57Fe/54Fe structure was created by sequential pulsed laser deposition on a Si substrate of 54Fe-rich, 57Fe-rich, and WSe1.7 thin films subjected to laser plasma ion implantation (LPII). Electric pulses of negative polarity with amplitude 45kV were applied to the 54Fe laser target. The turn-on time of the pulse, with a duration of ~10μs, was varied in the range of 0–9μs after the laser pulse. Modeling of the LPII was carried out using the particle in the cell method in a one-dimensional approximation. The initial input characteristics of the plasma (velocity, temperature, concentrations of ions) were measured experimentally. The model adequately describes the ion pulses and results in terms of energy distribution of the ions. The LPII regime, providing an efficient implantation of 54Fe+2 ions with energies up to 90keV, was used. Before and after LPII the structure was studied by Rutherford backscattering spectroscopy of 4He+ ions and conversion-electron Mössbauer spectroscopy. LPII treatment induced mixing of W, Se and 57Fe atoms. 57Fe–Se phases and non-equilibrium 57Fe–W compound were detected in a local structure of the interface layer. Thus, LPII may be considered as a promising method in combined laser technology, allowing deposition of coatings and modification of the interface, which may provide an enhanced adhesion and improved tribological properties

Studies on temperature dependent semiconductor to metal transitions in ZnO thin films sparsely doped with Al

For a detailed study on the semiconductor to metal transition (SMT) in ZnO thin films doped with Al in the concentration range from 0.02% to 2%, we grew these films on (0001) sapphire substrates using sequential pulsed laser deposition. It was found that the Al concentration in the films increased monotonically with the ratio of ablation durations of the alumina and ZnO targets used during the deposition. Using x-ray photo electron spectroscopy, it was found that while most of the Al atoms occupy the Zn sites in the ZnO lattice, a small fraction of the Al also gets into the grain boundaries present in the films. The observed SMT temperature decreased from ∼270 to ∼50 K with increase in the Al concentration from 0.02% to 0.25%. In the Al concentration range of ∼0.5% to 2%, these doped ZnO films showed metallic behavior at all the temperatures without undergoing any SMT. A theoretical model based on thermal activation of electrons and electron scatterings due to the grain boundaries, ionic impurities, and phonons has been developed to explain the observed concentration and temperature dependent SMT.

Magneto-optical study of slanted Co nanowires embedded in CeO2/SrTiO3(0 0 1)

Sequential pulsed laser deposition of CoO and CeO2 at 650°C under vacuum leads to the formation of a slanted Co nanowires assembly embedded in CeO2/SrTiO3(001) epilayers. High temperature magneto-optical Faraday measurements were performed, which revealed a Faraday ellipticity of 1.3° at a wavelength of 450nm for 300nm thick samples and which allowed to access the magnetic properties. From the analysis of the coercivity dependence on temperature, it is shown that the magnetic anisotropy of the slanted Co nanowires is dominated by shape anisotropy and that their magnetization reversal is localized.