Comparative analysis of the growth of SmBa2Cu3O7-x thin films by off-axis pulsed laser deposition on different substrates

The growth of SmBa₂Cu₃O_7-x superconducting thin films by off-axis pulsed laser deposition on different substrates (SrTiO₃, MgO, LaAlO₃, and YSZ) has been analyzed by means of resistance vs. temperature and X-ray diffraction measurements. The onset and width of the resistive transition depend on the substrate type and are in the ranges (89-80) K and (1-9) K, respectively. X-ray diffraction spectra show only the 00l reflections, from which the lattice parameter c can be estimated. Moreover, the rocking curves of the 005 peaks give an indication of the films' crystallinity and oxygen stoichiometry.     

A novel route to aligned nanotubes and nanofibres using laser-patterned catalytic substrates

We describe the generation of aligned carbon nanotube bundles and films by pyrolysis of solid organic precursors (for example 2-amino-4,6-dichloro-s-triazine, s-triaminotriazine) at 950-1050 °C over laser-patterned thin metal (Fe, Co, Ni) films, deposited on silica substrates. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) studies reveal that surface roughness of the laser-etched catalytic substrates plays a key role in achieving control of nanotube growth. We believe that, during the etching process, the energised (ablated) metal clusters condense and recrystallise evenly, possibly as the metal oxide, within the edges or surface of the eroded regions. During pyrolysis these catalytic particles, embedded in the silica substrates, are responsible for carbon agglomeration and subsequent tube axial growth, suggesting that nanotube alignment strongly depends upon the etching conditions (for example laser power, pulse duration, and focal distance). The pyrolysed products (usually nanotubes or nanofibres) were characterised by SEM, high-resolution transmission electron microscopy (HRTEM), electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDX). Samples containing only small amounts of amorphous carbon and other carbonaceous particles are notably absent. We observe that the degree of graphitisation is dependent upon the catalyst and the organic precursor. Interestingly, a nitrogen content З% was detected within the nanofibres, which exhibit corrugated graphite-like morphologies. This pyrolytic method may be used to advantage in generating aligned heteroatomic nanostructures such as B_xC_yN_z systems.     

The glass transition of thin polymer films: some questions, and a possible answer

A simple and predictive model is put forward explaining the experimentally observed substantial shift of the glass transition temperature, T_g, of sufficiently thin polymer films. It focuses on the limit of small molecular weight, where geometrical `finite size' effects on the chain conformation can be ruled out. The model is based on the idea that the polymer freezes due to memory effects in the viscoelastic eigenmodes of the film, which are affected by the proximity of the boundaries. The elastic modulus of the polymer at the glass transition turns out to be the only fitting parameter. Quantitative agreement is obtained with our experimental results on short chain polystyrene (M_W = 2 kg/mol), as well as with earlier results obtained with larger molecules. Furthermore, the model naturally accounts for the weak dependence of the shift of T_g upon the molecular weight. It explains why supported films must be thinner than free standing ones to yield the same shift, and why the latter depends upon the chemical properties of the substrate. Generalizations for arbitrary experimental geometries are straightforward.     

Quantitative ferroelectric characterization of single submicron grains in Bi-layered perovskite thin films

The local polarization state and the electromechanical properties of ferroelectric thin films can be probed via the converse piezoelectric effect using scanning force microscopy (SFM) combined with a lock-in technique. This method, denominated as piezoresponse SFM, was used to characterize at the nanoscale level ferroelectric SrBi₂Ta₂O₉ and Bi₄Ti₃O₁₂ thin films, grown by pulsed laser deposition. Two types of samples were studied: polycrystalline films, with grains having random orientations, and epitaxial films, consisting of (100)_orth- or (110)_orth-oriented crystallites, 100 nm to 2 7m in lateral size, which are embedded into a (001)-oriented matrix. The ferroelectric domain structure was imaged and the piezoelectric response under different external conditions was locally measured for each type of sample. Different investigation procedures are described in order to study the ferroelectric properties via the electromechanical response. A distinct ferroelectric behavior was found for single grains of SrBi₂Ta₂O₉ as small as 200 nm in lateral size, as well as for 1.2 7m쏿 nm crystallites of Bi₄Ti₃O₁₂. By probing separately the crystallites and the matrix the investigations have demonstrated at the nanoscale level that SrBi₂Ta₂O₉ has no spontaneous polarization along its crystallographic c-axis, whereas Bi₄Ti₃O₁₂ exhibits a piezoelectric behavior along both the a- and c-directions. The electrostriction coefficients were estimated to be 3᎒^-2 m⁴/C² for polycrystalline SrBi₂Ta₂O₉ and 7.7᎒^-3 m⁴/C² for c-orientedBi₄Ti₃O₁₂. Quantitative measurements at the nanoscale level, within the experimental errors give the same values for remanent polarization and coercive field as macroscopic ferroelectric measurements performed on the same samples.     

Low-temperature growth of epitaxial ZnO films on (001) sapphire by ultraviolet- assisted pulsed laser deposition

ZnO thin films have been grown on thin Si₃N₄ membranes and (001) sapphire substrates by an ultraviolet-assisted pulsed laser deposition (UVPLD) technique. The microstructure of the films grown on Si₃N₄ membranes, investigated by transmission electron microscopy, showed that crystalline and textured films can be grown by UVPLD at a substrate temperature of only 100 °C. For deposition temperatures higher than 400 °C, ZnO films grown on sapphire substrates were found to be epitaxial by Rutherford backscattering (RBS) and X-ray diffraction measurements. The minimum yield of channeling RBS spectra recorded from films deposited at 550 °C was around 2% and the FWHM of the rocking curve for the (002) diffraction peak was 0.17°; these values are similar to those recorded from ZnO layers grown by conventional PLD at 750 °C.     

Call for papers Special issue on "Properties of thin polymer films"

In recent years, there have been numerous reports of anomalous behavior of polymers in thin films. These observations, encompassing both structural and dynamical behavior, have attracted a great deal of interest. Measurements showing such anomalous behavior include density (irreversible and reversible changes after annealing below the bulk glass transition temperature), film stability and dewetting, glass transition temperature, diffusion coefficient and chain conformation and relaxation. The lack of a definitive M_w dependence in many of these studies means that it is not yet clear if the molecular size is a defining parameter in these observations. Alternatively, the surface to volume ratio may make such systems dependent on the (equally poorly understood) properties of polymer surfaces and interfaces. Despite a growing literature in this area, it is becoming increasingly obvious that a clear understanding of thin film properties has not yet been reached. In particular in glassy thin polymer films a possible cause for the anomalous behavior may originate from sample preparation. The widely used process of spin coating can potentially produce samples that are far from equilibrium. While most studies use thin films that have been annealed above the bulk glass transition, it is not clear if this represents sufficient annealing because the terminal relaxation times may be much larger than typical annealing times. One of the key questions is: Are these deviations from equilibrium important enough to cause any measurable effects? What are the consequences of film preparation and the resulting non-equilibrium conformations of the polymers on measurements of film stability, glass transition temperature, diffusion coefficient, etc.? Alternatively, if these non-equilibrium effects are not responsible for the anomalies observed in the structural and dynamical properties of thin polymer films, then other explanations must be considered. In the search for a unifying concept to explain all aspects simultaneously (i.e., density, film stability, glass transition temperature, ...) and in order to provide a forum for an open and possibly controversial discussion, The European Physical Journal E - Soft Matter invites regular articles and comments on this topic. In addition we invite submission of well-founded conjectures which may be published together with complementary views (solicited by the Editors). Papers to be considered for this special issue should be submitted by August 15, 2001 to the Editorial Office of the European Physical Journal E Véronique Condé, Editorial Office of The European Physical Journal E, Université Paris Sud, bâtiment 510, 91405 Orsay Cedex, France Tel.: 33 (0)1 69 15 59 76, Fax: 33 (0)1 69 15 59 75, e-mail: conde@edpsciences.org and marked clearly to the attention of either Günter Reiter (Editor) or James Forrest (Guest Editor). We look forward to receiving your submission.     

Off-axis pulsed laser deposition system for epitaxial oxide growth on substrates up to 2 inches in diameter

A novel off-axis pulsed-laser deposition (PLD) system for ferroelectric oxide thin films has been developed. The substrates are mounted "upside-down" and are rotating. The maximum substrate size is 2 inches in diameter. The optical and structural properties of the grown BaTiO₃ films are compared to the films produced by an on-axis PLD system. The stoichiometry and thickness were checked with Rutherford backscattering spectrometry (RBS). The crystalline quality and orientation were investigated with X-ray diffraction (XRD) and Rutherford backscattering spectrometry in channeling configuration (RBS/C). Using atomic force microscopy, the rms surface roughness was measured. The BaTiO₃ films grown on MgO form a planar optical waveguide. The optical losses and the refractive indices of these waveguides were determined with a prism coupling setup. Films grown on 10᎒ mm² MgO (100) substrates in on-axis geometry show optical waveguide losses less than 3 dB/cm.     

LaNiO3 under-electrode layers for the growth of textured (Pb0.4Zr0.6)TiO3 thin films using pulsed laser deposition

We investigated the structural properties of LaNiO₃ thin films of three different thicknesses deposited by pulsed laser deposition on Si(001) mainly by using a synchrotron X-ray scattering measurement. The LaNiO₃ thin films were grown with the (00l) preferred growth direction, showing completely random distribution in the in-plane direction. In the early stage of the growth, the film was almost unstrained. However, as the film grew further, tensile strain was markedly involved. Also its surface became rougher but its crystalline quality improved significantly with increasing film thickness. A completely (00l)-oriented (Pb_0.4Zr_0.6)TiO₃ thin film was successfully grown on such a LaNiO₃/Si(001) substrate at a substrate temperature of 350 °C by using the same pulsed laser deposition. Our results show that the LaNiO₃ film can serve effectively as a bottom electrode layer for the preparation of a well-oriented (Pb_0.4Zr_0.6)TiO₃ thin film on Si substrates.     

Room-Temperature Ferromagnetism in Zn1-xMnxO Thin Films Deposited by Pulsed Laser Deposition

Zn1-xMnxO （x = O.Olq3.1） thin films with a Curie temperature above 300K are deposited on Al2O3 （0001） substrates by pulsed laser deposition. X-ray diffraction （XRD）, ultraviolet （UV）-visible transmission and Raman spectroscopy are employed to characterize the microstructural properties of these films. Room temperature ferromagnetism is observed by superconducting quantum interference device （SQUID）. The results indicate that Mn doping introduces the incorporation of Mn^2＋ ions into the ZnO host matrix and the insertion of Mn^2＋ ions increases the lattice defects, which is correlated with the ferromagnetism of the obtained films. The doping concentration is also proven to be a crucial factor for obtaining highly ferromagnetic Zn1-xMnxO films.     

Magnetic Properties and Nanostructures of FePtCu：C Thin Films with FePt Underlayers

Magnetic properties and nanostructures of FePtCu：C thin films with FePt underlayers （ULs） are studied. The effect of FePt ULs on the orlentation and magnetic properties of the thin films are investigated by adjusting FePt UL thicknesses from 2nm to 14nm. X-ray diffraction （XRD） scans reveal that the orientation of the films is dependent on FePt UL thickness. For a 5-nm FePtCu：C nanocomposite thin film with a 2-nm FePt UL, the coercivity is 6.S KOe, the correlation length is 59 nm, the desired face-centred-tetragonal （fct） ordered structure [Llo phase] is formed and the c axis normal to the film plane [（001） texture] is obtained. These results indicate that the beffer orientation and magnetic properties of the films can be tuned by decreasing the thockness of the FePt UL.     

Pulsed laser deposition of tantalum pentoxide film

We report thin tantalum pentoxide (Ta₂O₅) films grown on quartz and silicon substrates by the pulsed laser deposition (PLD) technique employing a Nd:YAG laser (wavelength 5=532 nm) in various O₂ gas environments. The effect of oxygen pressure, substrate temperature, and annealing under UV irradiation using a 172-nm excimer lamp on the properties of the grown films has been studied. The optical properties determined by UV spectrophotometry were also found to be a sensitive function of oxygen pressure in the chamber. At an O₂ pressure of 0.2 mbar and deposition temperatures between 400 and 500 °C, the refractive index of the films was around 2.18 which is very close to the bulk Ta₂O₅ value of 2.2, and an optical transmittance around 90% in the visible region of the spectrum was obtained. X-ray diffraction measurements showed that the as-deposited films were amorphous at temperatures below 500 °C and possessed an orthorhombic (#-Ta₂O₅) crystal structure at temperatures above 600 °C. The most significant result of the present study was that oxygen pressure could be used to control the composition and modulate optical band gap of the films. It was also found that UV annealing can significantly improve the optical and electrical properties of the films deposited at low oxygen pressures (<0.1 mbar).     

Study of laser-deposited metallic thin films by a combination of high-resolution ex situ and time-resolved in situ experiments

Laser-deposited metallic alloys and multilayers were studied in detail by a combination of high-resolution ex situ and time-resolved in situ experiments. The purpose of these experiments is to better understand the special properties of laser-deposited metallic films in comparison with conventionally prepared thin films. During deposition, thickness, resistance, and electron diffraction (THEED) experiments show that the film surface is resputtered, local mixing at the interfaces of multilayers on a nanometre scale occurs, and metastable phases up to large film thicknesses are formed. After deposition, a compressive stress of 1-2 GPa was measured using four-circle diffractometry, and growth defects were observed on an atomic scale by electron microscopy (HRTEM) and field ion microscopy (FIM). The obtained structural details of the metallic films can be explained by an implantation model for the laser deposition process.     

Transport Behaviour of La0.8Sr0.2AlO3 Thin Film on Oxygen Deficient SrTiO3 Substrate

La_0.8Sr_0.2AlO₃ (LSAO) thin films are grown on SrTiO₃ (STO) and MgO substrates by laser molecular beam epitaxy. The LSAO thin film on oxygen deficient STO substrate exhibits metallic behaviour over the temperature range of 80--340K. The optical transmittance spectrum indicates that theLSAO thin films on MgO substrate are insulating at room temperature. The transport properties of LSAO thin films on STO substrates deposited in different oxygen pressure are compared. Our results indicate that oxygen vacancies in STO substrates should be mainly responsible for the transport behaviour of LSAO thin films.     

Synthesis and characterization of laser ablated TiNi films

TiNi thin films with BaTiO₃ and PbZr_0.52Ti_0.48O₃ (PZT) as buffer layers were deposited on Si(100) substrates by the pulsed laser deposition (PLD) method. Buffer layers (BaTiO₃ and PZT) were deposited at 600 °C in oxygen (O₂) environment and TiNi films were deposited on the top of the buffer layer in presence of 15 mTorr nitrogen (N₂) at various deposition temperatures (50, 300, and 500 °C). Synthesis and characterization of TiNi films were investigated from the crystallographic point of view by using X-ray diffractometer (XRD) and atomic force microscope (AFM) techniques. It is found that buffer layer of BaTiO₃ and PZT have improved the crystallinity of TiNi films deposited at higher temperatures. The TiNi/PZT film was uniform compared to TiNi/BaTiO3 film with the exception of agglomerates that appeared throughout the layer.     

Low-temperature growth of Y2O3 thin films by ultraviolet-assisted pulsed laser deposition

Thin Y₂O₃ films have been grown on (100) Si using an in-situ ultraviolet-assisted pulsed laser deposition (UVPLD) technique. When compared to conventional pulsed laser deposited (PLD) films under similar conditions, the UVPLD-grown films exhibited better structural and optical properties, especially those grown at lower substrate temperatures, from 200 °C to 400 °C. X-ray diffraction investigations showed that the films grown were highly crystalline and textured. According to X-ray photoelectron spectroscopy and Rutherford backscattering spectrometry investigations, UVPLD-grown Y₂O₃ films have a better overall stoichiometry and contain less physisorbed oxygen than the conventional PLD-grown films. The refractive index values, measured in the range 300-750 nm by using variable-angle spectroscopic ellipsometry, were similar to those of a reference Y₂O₃ film.     

Pulsed laser deposition of semiconducting crystalline double-doped barium titanate thin films on nickel substrates

We synthesized by pulsed laser deposition (Ba,Sr,Y)TiO₃ and (Ba,Pb,Y)TiO₃ thin films on mechanically polished nickel substrates.The synthesized thin films were analyzed for: crystalline structure by X-ray diffractometry, morphology and surface topography by atomic force microscopy, optical and scanning electron microscopy, and elemental composition by energy dispersive X-ray spectroscopy and electrical properties by electrical measurements.We have shown that film properties were determined by the dopants, target composition, and deposition parameters (oxygen pressure, substrate temperature and incident laser fluence). All films exhibited a semiconducting behavior, as proved by the decrease of electrical resistance with heating temperature.     

Epitaxial Properties of Co-Doped ZnO Thin Films Grown by Plasma Assisted Molecular Beam Epitaxy

High quality Co-doped ZnO thin films are grown on single crystalline Al2O3（0001） and ZnO（0001） substrates by oxygen plasma assisted molecular beam epitaxy at a relatively lower substrate temperature of 450℃. The epitaxial conditions are examined with in-situ reflection high energy electron diffraction （RHEED） and ex-situ high resolution x-ray diffraction （HRXRD）. The epitaxial thin films are single crystal at film thickness smaller than 500nm and nominal concentration of Co dopant up to 20%. It is indicated that the Co cation is incorporated into the ZnO matrix as Co^2＋ substituting Zn^2＋ ions. Atomic force microscopy shows smooth surfaces with rms roughness of 1.9 nm. Room-temperature magnetization measurements reveal that the Co-doped ZnO thin films are ferromagnetic with Curie temperatures Tc above room temperature.     

Adjusting chemical bonding of hard amorphous CSixNy thin films by N*-plasma-assisted pulsed laser deposition

Hard amorphous carbon silicon nitride thin films have been grown by pulsed laser deposition (PLD) of various carbon silicon nitride targets by using an additional nitrogen RF plasma source on [100] oriented silicon substrates at room temperature. The influence of the number of laser shots per target site on the growth rate and film surface morphology was studied. Up to about 30 at. % nitrogen and up to 20 at. % silicon were found in the films by Rutherford backscattering spectroscopy (RBS) and X-ray photoelectron spectroscopy (XPS). The XPS of the films showed a clear correlation of binding energy to the variation of PLD parameters. The films show a universal hardness value up to 23 GPa (reference value for silicon substrate 14 GPa) in dependence on target composition and PLD parameters. The results emphasise the possibility of variation of chemical bonding and corresponding properties, such as nanohardness, of amorphous CSi_xN_y thin films by the plasma-assisted PLD process.     

A comparative study of the physical properties of Sb2S3 thin films treated with N2 AC plasma and thermal annealing in N2

As-deposited antimony sulfide thin films prepared by chemical bath deposition were treated with nitrogen AC plasma and thermal annealing in nitrogen atmosphere. The as-deposited, plasma treated, and thermally annealed antimony sulfide thin films have been characterized by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy, scanning electron microscopy, atomic force microscopy, UV-vis spectroscopy, and electrical measurements. The results have shown that post-deposition treatments modify the crystalline structure, the morphology, and the optoelectronic properties of Sb₂S₃ thin films. X-ray diffraction studies showed that the crystallinity of the films was improved in both cases. Atomic force microscopy studies showed that the change in the film morphology depends on the post-deposition treatment used. Optical emission spectroscopy (OES) analysis revealed the plasma etching on the surface of the film, this fact was corroborated by the energy dispersive X-ray spectroscopy analysis. The optical band gap of the films (E_g) decreased after post-deposition treatments (from 2.36 to 1.75 eV) due to the improvement in the grain sizes. The electrical resistivity of the Sb₂S₃ thin films decreased from 10⁸ to 10⁶ Ω-cm after plasma treatments.     

Enhanced quantum-well photoluminescence in InGaAs/InGaAsP heterostructures following excimer-laser-assisted surface processing

The influence of excimer-laser-assisted surface processing on quantum-well photoluminescence (QW PL) has been investigated in InGaAs/InGaAsP heterostructures capped with a thin layer of In_0.53Ga_0.47As. The PL mapping measurements carried out on samples before they were processed in a rapid thermal annealer indicated no significant differences in the QW PL signal intensity measured at the excimer-laser-processed sites and in their vicinity. However, a large difference in the QW PL signal, with its intensity significantly enhanced at the laser-processed sites, was observed after 10 s of rapid thermal annealing (RTA) at 750 °C. The largest contrast in the PL signal (an intensity difference up to about 100%) was obtained for a site processed with 50 pulses and a laser fluence of 123 mJ/cm². The changes in the QW PL signal intensity have been found to coincide with the changes in the surface chemical composition that were detected with Auger electron spectroscopy. The main difference concerns the development of a GaO_x layer on the laser-processed In_0.53Ga_0.47As surface.