Dynamics of laser-induced phase switching in GeTe films

Phase switching in GeTe thin films (grown using a modified metal organic chemical vapor deposition system) upon pulsed femtosecond and nanosecond laser irradiation has been studied. Two in situ methods, i.e., optical microscopy and real-time reflectivity measurements, have been used in order to compare the optical response before and after phase change and to follow the phase change dynamics with a time resolution close to 400 ps. The results show that cycling is possible under irradiation with both fs and ns pulses using single pulses for amorphization and multiple pulses for crystallization. The use of ns pulses favors the crystalline-to-amorphous phase transformation, with a characteristic transformation time of ∼15 ns. The presence of the liquid phase was identified and temporally resolved, featuring a well-defined transient reflectivity state, in between those of the crystalline and amorphous phases. We have also studied the role of material configuration in the phase change dynamics and the mechanisms involved in the re-crystallization process. © 2011 American Institute of Physics.This work has been partially supported by the EU project CHEMAPH (IST-027561) and by the Spanish Ministry of Education and Science (Project TEC 2008-01183).Peer Reviewe

Effect of pulsed laser irradiation on the structure of GeTe films deposited by metal organic chemical vapor deposition: A Raman spectroscopy study

Phase changes between amorphous and crystallized states were induced by laser irradiation with nanosecond pulses in Gex Tey films grown by metal organic chemical vapor deposition. The different phases were obtained by adjusting the pulse energy and could be distinguished by their different optical reflectivities. The corresponding structural changes were studied by Raman spectroscopy, showing marked differences for the two phases. A clear correlation is found between optical reflectivity levels, crystallographic state and the evolution of Ge-Ge, Te-Te, and Ge-Te Raman bands. © 2009 American Institute of Physics.This work was partially supported by the European Project CHEMAPH, FP6, under Grant No. N 027561.Peer Reviewe

YBa2Cu3O7-x and La1-xSrxMnO3 thin films grown by pulsed injection MOCVD

YBa2Cu3O7-x (YBCO) and La1-xSrxMnO3 (LSMO) layers were grown by single liquid source pulsed injection MOCVD technique in hot wall vertical MOCVD reactors. Y, Ba, Cu, Sr, La, Mn thd complexes (thd - 2,2,6,6-tetramethylheptane-3,5-dionate) were used as precursor materials and monoglyme (1,2-dimethoxyethane) as a solvent. The influence of deposition temperature and thickness on the properties of films on LaAlO3 and MgO substrates was studied. Thin YBCO layers deposited at 825 °C on LaAlO3 substrates exhibited epitaxial quality, sharp superconducting transition (0.2 K) and a high critical current density (= 4 MA/cm2 at 77 K). Films on MgO substrates were of lower quality. Epitaxial LSMO films were deposited on various substrates, and the tensile or compressive strains and their influence on the film properties was obvous in thinner films on LaAlO3 and SrTiO3 substrates

Deposition of SrRuO3 films and SrRuO3/YBa2Cu3O7 heterostructures by pulsed injection MOCVD

Single liquid source pulsed injection MOCVD technique was applied for the deposition of epitaxial SrRuO3 films and SrRuO3/YBa2Cu3O7 heterostructures on various monocrystalline substrates (LaAlO3, SrTiO3, NdGaO3, MgO, YSZ, sapphire). Sr, Y, Ba, Cu 2,2,6,6-tetramethyl-3,5-heptanedionates and Ru 2,4-pentanedionate (acetylacetonate) were used as precursor materials, 1,2-dimethoxyethane as a solvent. Deposition conditions were optimised to obtain epitaxial films and heterostructures and the influence of substrate material on films' microstructure and electrical properties was studied. The best SrRuO3/YBa2Cu3O7 heterostructures which were in situ deposited at 825 °C exhibited rather sharp superconducting transition at about 91 K and critical current density >106 A/cm2 at 77 K

Current Distribution in Y-Ba-Cu-O Superconducting Microbridges Containing Π-Shaped Channel for Easy Vortex Motion

A mixed state in dc-biased thin films of II-type superconductors realizes the Abrikosov magnetic vortices/antivortices, which are the result of the current-self magnetic field penetration into the film at temperatures lower than its critical temperature $T_{c}$. A nucleation of vortices/antivortices at the superconducting film's edges, their motion perpendicular to the direction of biasing current, and the annihilation in the film's center originates from a current dissipation in the superconductor and expresses itself in experiments as a dc voltage. This work reports on the results of simulation of current density in a 50 μm wide, 100 μm long, and 0.3 μm thick $YBa_2Cu_3O_{7 - x}$ microbridges containing Π-shaped 5 μm wide single channel of easy vortex motion fabricated by means of laser-writing technique. Analyzing a two-dimensional-net of resistors and assuming that, due to the Meissner-Ochsenfeld effect, the magnetic flux penetration into superconducting film is nonlinear, we demonstrate that presence of a Π-shaped channel causes a non-homogeneous distribution of current in the microbridge

Hot-wire chemical vapor growth and characterization of crystalline GeTe films

Films of germanium telluride (GeTe) were grown by chemical vapor deposition (CVD) using pulsed liquid injection/flash evaporation principle to introduce the metalorganic precursor vapors. Simple thermal CVD with no additional process activation and CVD employing hot-wire remote catalytic activation of the precursor decomposition process were investigated and compared. Ge(NMe2)4 and Te(iPr)2 precursors in the form of diluted solutions in toluene were used as source materials for the depositions. Film composition was controlled changing the ratio of injected precursors, while the thickness was directly related to the number of injection pulses. Growth of GeTe films with a clearly better quality was demonstrated via the hot-wire-activated CVD process when compared to the standard thermal CVD. The influence of substrate temperature and deposition pressure on film crystallinity and morphology was studied. Reversible optical phase switching was demonstrated in 40-50 nm thick GeTe films on Si/SiO2 substrates. © 2008 Elsevier B.V. All rights reserved.This work has been partially supported by the EU Project CHEMAPH (IST-027561) within the 6th Framework Programme. W.G. acknowledges the I3P-CSIC Program contract (co-funded by the European Social Fund).Peer Reviewe

Pulsed injection MOCVD of heteroepitaxial multilayers of perovskite oxides

The single liquid source pulsed injection MOCVD technique was used to deposit high quality heteroepitaxial structures containing YBa2Cu3O7-x(YBCO), SrTiO3 (STO) and La1-xSrxMnO3 (LSMO) layers. Y, Cu, Sr, La, Mn thd complexes (thd-2,2,6,6-tetramethylheptane-3,5-dionate), Ba(thd)2(phen)2 (phen-1,l0-phenanthroline) and Ti(i-OPr)2(thd)2 or Ti(i-OEt)2(thd)2 were used as precursor materials and monoglyme (1,2- dimethoxyethane) as a solvent. The deposition of heterostructures was performed in situ or ex situ in a hot wall vertical MOCVD reactor. Various heteroepitaxial structures (STO/LSMO, LSMO/STO/LSMO, STO/YBCO, LSMO/YBCO, STO/LSMO/YBCO, LSMO/STO/YBCO, STO/LSMO/STO/YBCO) were gown and studied

Investigation of spin-polarized carrier injection effect in|YBa2Cu3O7/SrTiO3/La0.67/Sr0.33MnO3 and YBa2Cu3O7/La0.67Sr0.33MnO3 heterostructures

We report preparation and electrical properties of the YBa2Cu3O7SrTiO3(d ≈ 5.0 nm)/La0.67Sr0.33MnO3 (YBCO/STO/LSMO) and YBCO/LSMO multilayer structures grown heteroepitaxially on LaAlO3(100) substrates by single source pulsed injection MOCVD. The heterostructures were patterned in a cross-strip geometry to perform electrical measurements at T=78/300 K under magnetic field μ0H= 0/16 T using 4-probe dc method. Suppression of a supercurrent of the patterned YBCO stripes was indicated for the YBCO/LSMO heterostructure by injecting tunneling spin-polarized current fiom the underlying FM layer. The transport effects were modelled taking into account current redistribution in the adjacent conducting HTS and FM layers

Effectiveness of visible and ultraviolet light emitting diodes for inactivation of Staphylococcus aureus, Pseudomonas aeruginosa,and Escherichia coli: a comparative study

The rapid use of ultraviolet light emitting diodes (UV-LEDs) in various disinfection applications is growing tremendously due to their advantages unachievable using UV lamps. In this study, a comparison of standard LED at 460 nm wavelength and UVA LED at 385 nm was conducted to determine their effectiveness in disinfection of frequently isolated pathogens in hospitals (Staphylococcus aureus, Pseudomonas aeruginosa, and Escherichia coli). Determination of disinfection efficiency was carried out by measuring inhibition zone. Effects of varied exposure time on the inactivation of pathogenic microorganisms was studied. The results demonstrated that LED does not have germicidal activities. The highest inactivation for UVA LED was achieved for Pseudomonas aeruginosa. Linear relationship was found between exposure time and log reduction. This study showed that UVA LEDs can effectively inactivate significantly higher number of microorganisms hence can be used in disinfection of various applications

Anisotropic resistance change of La0.7Sr0.3MnO3 thin films effected by in-plane axial strain

Thin films of La0.7Sr0.3MnO3 grown on (001) NdGaO3 substrate using vertical hot wall Injection CVD reactor were investigated by means of standard fish-bellied beam inducing uniaxial compressive (or tensile) strain of the films. The measurements were carried out in current direction both parallel (longitudinal resistance R1) and perpendicular (transversal resistance R1) to the strain direction. The compression along [100], [110], [010] and [110] directions on films having 8 nm thickness showed the strong anisotropy of strain induced resistance change. The compression along [010] and [l10] directions caused the decrease of the film resistance while the compression along [100] and [110] directions increased this resistance. The maximum obtained relative resistance change was about 27% at strain 0.005. This effect appeared in both longitudinal and transversal directions of the electric current. The experimental results are discussed in terms of external strain influence on anisotropically strained La0.7Sr0.3 MnO3 film lattice produced as a result of large mismatch between substrate and film lattice constants