Rumen I. Tomov

Growth of Nd:potassium gadolinium tungstate thin-film waveguides by pulsed laser deposition

Thin Nd-doped potassium gadolinium tungstate [KGW or KGd(WO4)2] films are grown by pulsed laser deposition by ablation of a stoichiometric monocrystal target. Rutherford backscattering, x-ray diffraction, atomic force microscopy, and waveguide propagation analyses are performed. The as-grown films are optically active, as evidenced by the photoluminescence spectra centered at 1.068 μm. In some of the films, fine photoluminescence spectra between Stark levels are observed.

Sr-ferrite thin films grown on sapphire by pulsed laser deposition

High-quality epitaxial strontium-hexaferrite (SrFe12O19) thin films were grown by pulsed laser deposition (PLD) on c-cut sapphire using KrF∗ excimer laser at a fluency of 2 J/cm2 and substrate temperature of 800°C in 100 mTorr oxygen environment. The X-ray diffraction (XRD) and morphology analyzes showed films with excellent crystalline structure and flat surface. The thickness was found to influence considerably the surface morphology and magnetic properties of the as-deposited films. The highest orientation and the best morphology with smooth surface and fine grain structure was obtained for the film having a thickness of 750 nm. The highest coercive force of 1453 Oe was measured for this film in perpendicular to the plane direction.

Pulsed laser deposition of Mn-Zn ferrite thin films

Mn-Zn ferrite thin films were deposited on sapphire substrates by pulsed laser deposition from sintered Mn1 − xZnxFe2O4 ceramic targets. A full stoichiometric transfer from targets to substrates was achieved. Magnetic inplane measurements in two perpendicular directions were carried out and the macromagnetic properties of films were determined. The hysteresis loops obtained are rectangular and the values of the coercive force, the saturation, and the remanent magnetization are comparable to the same parameters of the bulk Mn-Zn ferrite. The films were characterized using a vibrating sample magnetometer (VSM), a scanning electron microscope (SEM) and by X-ray photoelectron spectroscopy (XPS).

Plasma optical emission studies of high-Tc superconducting and buffer thin film physical vapour deposition

Abstract The non-intrusive technique of optical emission spectroscopy was used to identify the plasma species active in two different physical vapour deposition techniques: sputtering and laser ablation, when applied for high T c superconducting (HTSC) and buffer thin film deposition. Peak identification of the plasma optical emission spectra taken for targets of YBa 2 Cu 3 O 7 − x (YBCO) , BiSrCaCu 2 O 7 − x (BiSCCO 1112) , Bi 2 Sr 3 Ca 3 Cu 4 O 7 −x ( BiSCCO 2334), SrTiO 3 , BaTiO 3 , Y 2 O 3 -stabilized ZrO 2 (YSZ) and CeO 2 was made from published values and measured results for the constituting elements (Cu) and elemental oxides (Y 2 O 3 , BaCO 3 , CuO). Aiming at determination of an appropriate criterion for in situ assessment of ambient oxygen sufficiency, the plasma optical emission was related to the deposited HTSC film characteristics. During YBCO RF single target off-axis sputtering and pulsed laser ablation the gas-phase preoxidation, witnessed by YO-optical emission, could be used as a qualitative criterion ensuring high quality film deposition. During dc magnetron sputtering with two-opposedtargets, the oxygen-to-argon peak actinometric ratio was used as a criterion instead. SEM investigation of the ablation-induced surface modification of targets of HTSC and buffer materials revealed specific topography for different target structure and density, which can influence the deposited film thickness uniformity. An extensive table is presented of the plasma emission lines and band heads for YBCO compiled from this work and that of other authors.

Experimental and theoretical investigation of cumulative laser irradiation effects in YBCO thin film pulsed laser deposition

Abstract Compositional and morphological modifications of superconducting YBCO targets under prolonged laser irradiation were investigated by EDS and SEM. Rotated and fixed targets were ablated at different process parameters - laser fluence and oxygen environment. Optical emission spectroscopy was employed for investigation of the evolution of the laser-induced plasma plume. The deposited film thickness distribution over the substrate surface area was examined through the film absorbance. The results can be consistently explained by assuming the existence of an additional effective ablation threshold imposed by the modified surface relief. A one-dimensional theoretical model of the laser beam-target interaction was developed involving temperature-dependent thermophysical parameters and the laser-induced plasma absorption. Reasonable agreement between the experimental and theoretical dependencies of the eroded depth per shot on the pulse energy is found. The model results show a possible influence of the post-pulse evaporation on the target surface stoichiometric deviation found in the experiments.

Prolonged laser ablation effects of YBCO ceramic targets during thin film deposition: Influence of processing parameters

Cumulative laser irradiation during high-Tc superconducting thin film pulsed laser deposition (PLD) may have a detrimental effect on film characteristics. Initial decrease of deposition rate and gradual shift of the center of the deposited material spot towards the incoming laser beam were registered on cold glass substrates. Their absorbance was used for evaluation of the film thickness distribution over the substrate area. At the initial stage, two components of the spot could be distinguished along its short axis: central (∼cosn θ, n≫1) and peripherial (∼cos θ), while with cumulative irradiation the thickness followed an overall cosm θ (m<n) angular distribution. Two stages of the laser-induced plasma optical emission evolution were observed, according to target surface morphology modification. Compositional and morphology changes of the ceramic YBCO target under prolonged XeCl laser irradiation were studied by EDAX and SEM for different processing parameters — laser fluence and oxygen environment. The results can be consistently explained suggesting the existence of an additional effective ablation threshold imposed by the modified surface relief.

Influence of cumulative laser irradiation on YBCO thin film pulsed laser deposition

Morphological and stoichiometric changes of YBCO ceramic targets under cumulative XeCl laser irradiation are investigated by SEM and EDAX. The impact of taget modifications on laser-induced plasma plume optical emission and on deposited thin films thicknes distribution is studied.

Growth and characterization of pulsed laser-deposited Mn–Zn ferrite thin films ☆

Abstract The magnetic behavior of Mn–Zn ferrite thin films deposited on silicon and sapphire substrates with and without a buffer layer of MgO was studied. The films were grown in a vacuum chamber using a KrF ∗ excimer laser. The magnetic and structural properties of the films deposited were examined. The magnetic behaviour of the film deposited on Si with a buffer layer of MgO was studied comparatively to that deposited on other substrates, using Kerr effect. The large coercive force obtained is related to the microstructure, which was investigated by AFM. The films were smooth and homogeneous. XRD-analyses revealed the formation of Mn–Zn ferrite phase.

Plasma assisted in situ laser deposition of Y1Ba2Cu3O7−x superconducting thin films with laser heating and annealing

Abstract Single-chamber laser processing of high temperature Y 1 Ba 2 Cu 3 O 7−x thin films on SrTiO 3 (100), ZrO 2 (100) and MgO(100) substrates was carried out using plasma assisted in situ laser deposition. The films were deposited by XeCl excimer laser ablation at substrate temperatures as low as 450–550°C. A dc oxygen plasma at a partial pressure of up to 200 mtorr forced oxygen incorporation during the film growth at a low substrate temperature. The problem of firing and maintaining the discharge was solved by appropriate electrode geometry. Direct cw CO 2 laser irradiation of the substrate surface and excimer laser plume were used to reduce the density of particulates. The films were characterized by: X-ray diffraction, four-point probe ac electrical resistivity measurements, SEM and XMA. A T c onset of ∼88–90 K and J c ∼ 7 × 10 5 A cm −2 at 4.2 K were measured.

Ultraviolet annealing of thin Y2O3 films grown by pulsed laser deposition

We report on the results of ultraviolet (UV)-assisted annealing of thin Y2O3 films produced by pulsed laser deposition. An excimer XeCl laser was used for ablation of Y2O3 ceramic target. The films were grown on (001) SiO2 substrates at 500 °C and oxygen pressure of 0.05 mbar. The effect of UV-assisted laser annealing on the structure, morphology, and optical properties was investigated. The UV-assisted annealing was performed by the same laser. The beam was directed parallel or toward the surface of the as-deposited films. The influence of the ambient gas (O2 or N2O) is explored. The ambient atmosphere has an influence on the preferential (cubic or monoclinic) phase of gowth while it has no significant effect on the surface morphology. The absorption coefficient in the VIS range has a lower value for the films annealed with laser directed parallel to the surface independently on the gas environment. Annealing of the films with laser beam directed at the film surface slightly increases the refractive index, independently of the gas ambient.