Nano-structured oriented carbon films grown by PLD and CVD methods

PLD and CVD methods, plasma or bias assisted, have been used to prepare thin films of nano-clustered graphite. The experimental conditions (vz. deposition apparatus, substrate temperature, working pressure, inert sustaining gases, parent species, and applied voltage) have been changed with the final aim of obtaining small graphene particles with the basal planes oriented along the growth vector and perpendicular to the n-Si100 substrate. Pulsed laser ablation (Nd:YAG, 2nd harmonic: =532 nm, h=2.33 eV, =7 ns, =10 Hz, 7 J/cm²), assisted by an RF-plasma, of a pyrolytic graphite target gave good results for nano-structure formation, provided high substrate temperature and high inert gas pressure are maintained. CVD methods, in the presence of high substrate temperature and a DC bias, showed a good attitude to drive a longitudinal growth of graphene layers and nano-wires from a reactive gas flow of Argon/Hydrogen and Methane. The morphology of the films grown at different conditions have been characterised by scanning electron microscopy (SEM). Film quality and nano-particle dimensions have been estimated by Raman spectroscopy. PACS 81.07.-b; 81.15.-z; 78.30.-j; 68.37.-d     

Crystalline growth of cubic (Eu, Nd):Y2O3 thin films on α -Al2O3 by pulsed laser deposition

In this paper we report on the growth of crystalline, europium- and neodymium-doped cubic yttria ((Eu,Nd):Y₂O₃) thin films on hexagonal corundum (-Al₂O₃ ) substrates using the pulsed laser deposition (PLD) technique. A KrF excimer laser was used to ablate material from ceramic (Eu, Nd):Y₂O₃ targets. The yttria films were deposited on the -Al₂O₃ (0001) substrates. X-ray diffractometry (XRD) revealed that the films grew in the Y₂O₃[111]-direction. The surface topography of the films was investigated using atomic force microscopy (AFM). PACS 81.15.Fg; 42.70.hj; 68.55.Jk     

Photoelectrochemical Characteristics Of α-Fe2O3 Nanocrystalline Semiconductor Thin Film

-Fe₂O₃ single crystal thin films have been prepared from 45nm diameter colloid. These thin nanocrystalline films exhibit a typical behavior of n-type semiconductor material because of the anodic photocurrent generation. The anodic photocurrent response upon illumination and the reversal spike of cathodic current upon the light switched off suggests that the electrons can flow in both directions and no space charge layer exists at the thin film/electrolyte interface. The decreased photocurrent responses of thicker films can be explained by the electric resistance effect and recombination effect. Moreover, the thicker film will lead to a poor photocurrent response for short wavelength light. Considering the use of sunlight, the thin film thickness should be controlled to an optimal value.     

Tailored internal coating of components by PLD and pulsed laser evaporation

An advanced PLD technique combining laser ablation and laser induced thermal evaporation was introduced to deposit thin films onto the inner surfaces of components.Beside the well known film properties of the classical PLD, like high reproducibility, smooth surface and high thickness precision, this novel combination of laser ablation and evaporation is characterized by improved or extended properties in terms of microstructure, deposition rate and possible layer thickness. The extension of the conventional PLD process allows for the co-deposition of dense, amorphous films as well as columnar grown or even porous films with thicknesses up to several tens of micrometers.In a first successful application of the novel process, tailored thermal barrier coatings (TBCs) for combustion chambers were realized. A remarkable result was that these coatings sustained very large strains of about 5% without notable delaminations. Under hot gas testing in model combustion chambers, the PLD-TBCs showed very promising behavior with excellent adhesion.Super hard, amorphous carbon films (DLC) deposited onto the inner surfaces of small components by this PLD technique are characterized by a Youngs modulus of about 400–500 GPa and a low friction coefficient of about 0.1. Due to its layered structure, with graded material density and hardness, this DLC exhibits relatively low internal compressive stresses of about -1...-2 GPa. PACS 81.15.Fg; 68.55-a; 46.50+a     

Growth of polycrystalline La0.5Sr0.5CoO3 films by femtosecond pulsed laser deposition

In this paper we present the growth of La_0.5Sr_0.5CoO₃ (LSCO) films on MgO, quartz, and silicon substrates by pulsed laser deposition (PLD) using a Ti:sapphire laser (50 fs, 800 nm wavelength). The morphology and the structure of the films were studied by X-ray diffraction, atomic force microscopy, and scanning electron microscopy. The films were polycrystalline and exhibit a good adherence to the Si substrate. Different deposition parameters such as substrate temperature, oxygen pressure, and laser fluence were varied to achieve good surface quality and low resistivity crystalline films. We also defined the optimum conditions in which the deposited film surface is particulate free. The best films (droplets free) were grown at 625 °C, in an ambient oxygen pressure of 6 mbar, with an incident laser fluence of 0.19 J/cm². This is a mandatory step in the complex work of fabricating La_0.5Sr_0.5CoO₃/BaTiO₃/La_0.5Sr_0.5CoO₃ heterostructures for the development of thin film capacitors for non-volatile ferroelectric access memory devices. PACS 81.15 Fg; 42.62-b; 68.65.Ac     

Investigation of laser-induced plasma deposition of boron nitride

Laser ionization mass spectrometry (LIMS) was applied to the investigation of the composition of laser plasmas of three different boron nitride modifications (hexagonal -boron nitride with graphite structure, cubic -boron nitride with diamond structure and hexagonal -boron nitride resembling the wurtzite type). Thin films in the 10 nm range were produced in the ion source of the laser mass spectrograph by laser-induced plasma deposition. So we could carry out the plasma diagnostic and the preparation of thin films under the same condition. The cluster distribution in laser plasma is independent of the structure of the BN target. An explanation of laser-induced plasma deposition of boron nitride from the results of mass spectrometric analysis of laser plasmas and of the analysis of deposited thin films (TEM, EELS) was derived.     

Magnetoconductivity of thin epitaxial silver films

The magnetoconductance (MC) of thin epitaxial Ag films on Si(111) surfaces is studied as a function of film thickness (1–125 monolayers (ML)) at 20 K under ultra high vacuum (UHV) conditions. Three different regimes of magnetoconductance are observed depending on the degree of disorder in the films which is controlled by film thickness and annealing procedures. Thick films (d>3 ML) with diffuse electron transport show in the case of large elastic scattering times ₀ a classical, negative MC B ² and in the case of small ₀ a positive MC due to weak localization effects. The MC of thin films (d<2 ML) which have a conductance smaller than e ²/h, i.e. localized electron states, is negative again.     

Optical CO gas sensing using nanostructured NiO and NiO/SiO2 nanocomposites fabricated by PLD and sol–gel methods

Several kinds of NiO-based nanostructured films were prepared by pulsed-laser deposition (PLD) and sol–gel method, and CO sensing properties (1%, balanced by N₂) of these films were studied. The sensitivity, defined as a difference of optical transmittance by gas atmospheric change (T=T(CO)-T(air)), increased with increasing NiO content for the sol–gel prepared films, and increased with the film thickness for the laser deposited NiO films. Sol–gel films exhibited shorter response time than NiO films prepared by PLD under low Ar pressure of 6.7×10^-2 Pa indicating a better gas permeability. A shorter response time was also obtained upon raising argon pressure from 6.7×10^-2 Pa to 8.0 Pa during laser ablation due to the morphological change. Covering a NiO film even with a very thin (0.8 nm) layer of SiO₂ by sputtering drastically reduced the CO sensitivity. The multilayered NiO/SiO₂ films were substantially less sensitive to the CO gas than NiO films due to the same reason. Sensing mechanism of the NiO films is due to catalytic CO oxidation that reduces the concentration of adsorbed O₂ species and results in optical transmittance increase upon change in the environment from air to CO. PACS 81.15.Fg; 81.20.Fw; 83.85.Gk     

Electrical conductivity of InAs films prepared at different temperatures

Conclusion The structure and electrical conductivity of thin InAs films, prepared by flash evaporation, are seen to be dependent on the substrate temperature, post-deposition heat treatment and thickness. The structure was found to improve from amorphous polycrystalline preferred orientation as the substrate temperature was increased. The corresponding micrographs showed increasingly bigger grain sizes. The post-deposition heat treatment was successfully employed to improve the structure from amorphous crystalline, which was reflected in the increased electrical conductivity. An interesting observation was that the room temperature electrical conductivities after heat treatment were the same for all films irrespective of the temperature at which they were prepared. The activation energies increased with increasing temperature but decreased with increasing substrate temperature and increasing thickness. These values throw important light on the conduction mechanism operative in the film in various temperature ranges.One of the authors (AKS) wishes to acknowledge the financial assistance provided by the CSIR, New Delhi, India.     

Hexatic ordering in freely suspended liquid crystal films

In this paper, we report results from synchrotron X-ray scattering studies of thefluid/hexactic/solid phases and phase transitions in both very thick and very thin, freely suspended films of tilted hexatic liquid crystals. Contrary to the thick film case, the higher Fourier coefficients describing the bond orientational order are suppressed in very thin films. This suppression is consistent with a two-dimensional bond orientational order parameter, ₆, rather than the three-dimensional bond orientational order parameter found in very thick films. For a film containing twently-three (23) smectic layers we find that ₆ is two-dimensional whereas the positional order in the crystallineS _J phase is three-dimensional. We present an analysis of the thick film data in terms of the three-dimensionalXY-model and a new mean field theory model which incorporates explicitly the quasi two-dimensional nature of bulk smectic phases.     

Dielectric properties of SrZrO3 thin films prepared by pulsed laser deposition

SrZrO₃ (SZO) thin films have been prepared on Pt-coated silicon substrates and directly on Si substrates by pulsed laser deposition (PLD) using a ZrSrO target at a substrate temperature of 400 °C in 20 Pa oxygen ambient. X-ray –2 scans showed that the as-deposited films remain amorphous at a substrate temperature of 400 °C. The dielectric constant of SZO has been determined to be in the range 24–27 for the Pt/SZO/Pt structure. Capacitance–voltage (C–V) characteristics of a metal-oxide-semiconductor (MOS) structure for SZO films deposited in 20 Pa oxygen ambient and 20 Pa nitrogen ambient (SZON) indicated that incorporation of nitrogen during the substrate heating and film deposition can suppress the formation of an interfacial SiO₂ layer, and the SZON films have a lower equivalent oxide thickness (EOT) than that of the SZO films. However, the leakage current of the SZON films is larger than that of the SZO films. The EOT is about 1.2 nm for a 5-nm SZON film deposited at 400 °C. The leakage-current characteristics of as-deposited SZON films and SZON films post-annealed in oxygen ambient by rapid thermal annealing (RTA) have been studied comparatively. The films post-annealed with RTA have a lower leakage current than the as-deposited SZON films. Optical transmittance measurements showed that the band gap of the films is about 5.7 eV. It is proposed that SrZrO₃ films prepared at 400 °C are potential materials for alternative high-k gate-dielectric applications. PACS 77.84.Bw; 77.84.-s; 77.55.+f     

Finslerian N-Spinors: Algebra

New mathematical objects called Finslerian N-spinors are discussed. The Finslerian N-spinor algebra is developed. It is found that Finslerian N-spinors are associated with an N²-dimensional flat Finslerian space. A generalization of the epimorphism SL(2, ) O ₊ (1, 3) to a case of the group SL(N,) is constructed. Particular examples of Finslerian N-spinors for N = 2, 3 are considered in detail.     

Effect of gas pressure on the growth of selenium thin films by pulsed laser deposition

The effect of an inert gas pressure on the structure of selenium thin films has been systematically investigated in the pulsed laser deposition process. The ablated material is deposited on Au (111) gold thin films for its characterization by atomic force microscopy (AFM). Analysis of the surface morphology shows that instead of the formation of a uniform Se thin film on top of Au (111) terraces, as it occurs in high vacuum, the film grows as two dimensional ellipsoid shaped aggregates. The size of these Se aggregates increases significantly with the gas pressure and reaches a maximum at pressures of 1.5 Torr, and subsequently decreases with further increase of the gas pressure. This effect is probably due to the fact that the kinetic energy of the impinging species decreases as pressure increases, thus impeding diffusion on the substrate surface. However, further increase in the pressure prevents the Se species from being deposited on the substrate resulting in a decrease in size of the aggregates. PACS 81.07; 68.37.P; 81.16.M     

A Perturbation Method for Complex Root Finding of Nonlinear Electromagnetic Waves

Aspects of the perturbation method are presented and implemented on the analytic function of the complex variable for the propagation of nonlinear electromagnetic surface guided by a thin film. The behavior of nonlinear TE waves guided by asymmetrical dielectric thin film, bounded by a generalizes nonlinear substrate with a permittivity of the form | E | is analyzed. The complex roots, which correspond to the propagation and attenuation coefficient are obtained and discussed.     

Growth and magnetism of Fe on Cr(001): a spin-polarized scanning tunneling spectroscopy and magnetic force microscopy study

The growth and magnetic domain structure of Fe nanoislands and films on Cr(001) are investigated by spin-polarized scanning tunneling microscopy and magnetic force microscopy (MFM). Topographic images of films grown at different substrate temperatures reveal that the highest film quality is obtained by evaporation at room temperature and subsequent annealing at 500 K for 4 min. Spin-resolved studies of the magnetic structure of submonolayer Fe films (coverage 0.2 ML) show the expected antiferromagnetic Fe–Cr coupling, i.e. any Fe island is magnetized antiparallel with respect to the underlying Cr(001) terrace. As the Fe coverage exceeds 0.2 ML the magnetic contrast decreases and completely vanishes for 0.4 ML. Only for 3 ML does a weak magnetic contrast reappear, which is interpreted in terms of a small spatial variation of the 90° coupling between the Cr substrate and the Fe overlayer. MFM reveals that the number of visible domain walls decreases with increasing film thickness and completely vanishes at 12 ML. PACS 68.55.-a; 75.60.Ch; 68.37.Ef     

Rapid thermal processing of lithium tantalite thin films prepared by a diol-based sol–gel process

Lithium tantalite (LiTaO₃) thin films have been deposited on Pt(111)/SiO₂/Si(100) substrates by means of sol–gel spin-coating technology. Using a diol-based precursor solution and rapid thermal processing (RTP), highly c-axis oriented LiTaO₃ thin films are obtained and the degree of orientation is increased with an increase of the heating rate. By changing the heating rate (600–3000 °C/min) and heating temperature (500–800 °C), the effects of various processing parameters on the growth of films are investigated. With the increase of heating rate, the grain size of LiTaO₃ thin films decreases markedly, and the relative dielectric constant (_r) increases from 28 up to 45.6. It was found that the dielectric loss factor (cos) decreased, and the ferroelectric properties were improved by the increase of heating rate. The figures of merit (F_v and F_m) indicate that the LiTaO₃ thin film with a heating rate of 1800 °C/min is suitable for application as a high-performance pyroelectric thin-film detector. PACS 81.20.Fw; 81.40.-z; 61.10.Eq; 77.84.-s     

The temperature dependence of the deformation strength of solid Cu-Al solutions

The temperature dependence of the deformation strength, of the parameters ₀ and K of Pötsch's equation, and of the discontinuous deformation in solid Cu-Al solutions of various concentrations have been studied. It was shown that the temperature dependence of and ₀ changes substantially when the alloy concentration is increased (under otherwise equal conditions). The nature of the processes which lead to the disappearance of the deformation discontinuity and to a sharp high-temperature recrystallization by process annealing is discussed.     

Influence of ambient oxygen pressure on the preferred orientation, microstructures, and dielectric properties of (Ba1-xSrx)TiO3 thin films with compositionally graded structures

Compositionally graded (Ba_1-xSr_x)TiO₃ (BST) thin films, with x decreasing from 0.25 to 0.0, were deposited on Pt(111)/Ti/SiO₂/Si(100) substrates by pulsed-laser ablation at 600 °C and under ambient oxygen pressures ranging from 50 to 400 mTorr. The influence of the ambient gas pressure on the preferred orientation, microstructures, and dielectric properties of compositionally graded BST films was investigated by X-ray diffraction, scanning electron microscopy, and dielectric frequency spectra, respectively. As the ambient oxygen pressure was increased, the preferred orientation evolved in the order: (100)+(110)(110)+(111) random orientation, and the surface roughness of the graded BST films also increased. The graded BST films deposited at high ambient oxygen pressures (300400 mTorr) exhibited a grainy structure with polycrystalline grains throughout the film thickness, whereas the graded films deposited at low ambient oxygen pressures (50200 mTorr) possessed a columnar structure. The evolution of the microstructure was ascribed to the different physical and chemical properties of the species that were incident onto the substrates at the various oxygen pressures. The dielectric properties of the graded BST films were dependent upon the ambient oxygen pressures. The graded BST films deposited at 200 mTorr exhibited the highest dielectric constant. PACS 77.55.+f; 77.22.Ch; 81.15.Fg     

Parametric dependence of the properties of pulsed-laser-deposited diamond-like carbon films

The properties of Pulsed-Laser-Deposited Diamond-Like Carbon (PLD DLC) films are studied as functions of the power density and the wavelength of the laser beam, and the incident angle of the beam relative to the normal of the target surface. All the films have a similar structure consisting of graphite particulates embedded in a continuous matrix, so the macroscopic performance of the films is determined by the overall contributions of the particulates and the matrix. The use of higher, shorter, or larger leads to an enhancement of the diamond-like characteristics and a simultaneous increase of the particulate density. These two effects give opposite contributions to the electrical conductivity _R, leading to the following results. (i) _R drops with increasing in the low range (region I) due to the stronger diamond-like nature of the matrix, but increases sharply after has exceeded a threshold _min as a result of the rapid increase in particulate density. (ii) In region I, the use of shorter or larger leads to a more diamond-like matrix, and this overwhelms the degradation effect caused by the slight increase in particulate density. The samples thus become more insulating. In the high region (region II), however, the use of shorter or larger gives rise to higher particulate density, thereby increasing the electrical conductivity.     

Present status of thin oxide films creation in a microwave plasma

This paper summarizes present knowledge of the creation of thin films in isotropic and magnetoactive plasma. It analyses conditions under which films in the microwave plasma can be created and shows how the growth rate and properties of films depend on microparameters of plasma. On the basis of plasma floating potential measurements it is shown why the creation of thin films in microwave discharges takes place at high electron plasma densities (N 10¹² cm^–3) only. Besides this it describes properties of created films, underlies negative role of fast electrons in forming of good quality films and gives recommendation how to avoid their generation. Considerable attention is devoted also to a comparison of films creation in pulse and continuous plasma. At the end possibility of films creation at low temperatures by a deposition technique utilizing a microwave excitation of molecular gases is given.The authors gratefully acknowledge many stimulating and fruitful discussions with Dr. F. áek of Institute of Plasma Physics of Czechoslovak Academy of Sciences. We also thank Dr. V. Malina of Institute of Radioelectronics of Czechoslovak Academy of Sciences for measurements of C—V curves of SiO₂ films.