Large-area pulsed laser deposition of YBCO thin films: homogeneity and surface

YBCO thin films and buffer layers are deposited by a special PLD setup with an 8-cm line focus on cylindrical targets and substrate scanning perpendicular to it. Different kinds of substrates (SrTiO₃, MgO, LaAlO₃, Y-ZrO₂and sapphire) as large as 7᎜ cm² were coated with YBCO. Two important aspects of the presented PLD setup will be discussed in detail: the method of substrate heating and the variation of the angle between the incident laser beam and the target surface ("wobbling"). The surface of the target material has been investigated by SEM. The influence of target "wobbling" on the time stability of the plasma will be discussed. The homogeneity of the deposited YBCO films with respect to structural and electrical properties has been investigated by XRD, RBS/channeling, and spatially resolved inductive measurements of T_c and j_c. The values of j_c on 7᎜ cm²in situ buffered sapphire substrates are 2.0 MA/cm² at (77 K, 0 T) with a j_c variation of less than ᆨ%.     

Highly reproducible large-area and double-sided pulsed laser deposition of HTSC YBCO:Ag thin films for microwave applications

Large-area pulsed laser deposition (PLD) producing high-quality YBa₂Cu₃O_7-x (YBCO) thin films on both sides of R-plane sapphire substrates with CeO₂ buffer layer is used routinely to optimize planar microwave stripline filters for satellite and mobile communication systems. A relatively simple PLD arrangement with fixed laser plume and rotating substrate, with an offset between the laser plume and the center of the substrate is employed to deposit laterally homogeneous 3-inch-diameter Ag-doped YBCO thin films. The YBCO:Ag films are about 250 nm thick and have laterally homogeneous critical current densities of more than 3.5 MA/cm² at 77 K and homogeneous maps of microwave surface resistance R_s of about 40 m Q at 145 GHz and 77 K. The R_s at 8.5 GHz and 77 K, determined in the center position of the YBCO:Ag films, remains constant at about 370 7 Q up to a microwave surface magnetic field of about 10 mT. After experience with more than 700 double-sided 3-inch-diameter films, a high degree of homogeneity and reproducibility of j_c and R_s is reached. The PLD-YBCO:Ag films are suitable for microwave applications envisaged for future communication systems.     

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.     

Structural and electrical properties of La0.5Sr0.5CoO3 films on SrTiO3 and porous a-Al2O3 substrates

Films of La_0.5Sr_0.5CoO₃ (LSCO) have been deposited on specially treated TiO₂-terminated (001) SrTiO₃ substrate surfaces and on macroporous polycrystalline !-Al₂O₃ substrates, having a mean pore diameter of 80 nm, by pulsed laser deposition. The films deposited on SrTiO₃ are good conducting, (001) textured, and exceptionally smooth (1-2 Å for 100 nm thick films). LSCO films deposited on porous !-Al₂O₃ are polycrystalline and exhibit good crystallographic and electrical properties despite the large substrate roughness and the differences in lattice parameters and crystal structure between the film and the substrate. Different growth modes have been observed on the porous !-Al₂O₃ substrates depending on the oxygen pressure during film deposition. Films grown at an oxygen pressure of 10^-1 mbar are macroporous, whereas films grown at 10^-2 mbar completely cover the substrate pores. In the latter case, strain effects lead to film cracking.     

Growth of oriented Pb(ZrxTi1-x)O3 thin films on glass substrates by pulsed laser deposition

Oriented crystalline Pb(Zr_xTi_1-x)O₃ (x=0.53) (PZT) thin films were deposited on metallized glass substrates by pulsed laser deposition (1060-nm wavelength Nd:YAG laser light, 10-ns pulse duration, 10-Hz repetition rate, 0.35-J/pulse and 25-J/cm² laser fluence), from a commercial target at substrate temperatures in the range 380-400 °C. Thin films of 1-3 7m were grown on Au(111)/ Pt/NiCr/glass substrates with a rate of about 1 Å/pulse on an area of 1 cm². The deposited PZT films with perovskite structure were oriented along the (111) direction, as was revealed from X-ray diffraction spectra. Fourier transform infrared spectroscopy (FTIR) was performed on different PZT films so that their vibrational modes could be determined. Piezoelectric d₃₃ coefficients up to 30 pC/N were obtained on as-deposited films. Ferroelectric hysteresis loops at 100 Hz revealed a remanent polarization of 20 7C/cm² and a coercive field of 100 kV/cm.     

Dependence of functional properties on processing in sol-gel (Pb,La)TiO3 thin films

Pb_0.88La_0.08TiO₃ films were processed on Si-based substrates by a diol-based sol-gel route from solutions with variable content of PbO excess. Crystallisation was performed at heating rates of 10 °C min^-1 and higher than 500 °C min^-1 (rapid heating). The pyroelectric coefficient was measured after poling the samples by two methods: applying a sinusoidal wave and applying a train of square pulses, with the latter showing a higher poling efficiency. The piezoelectric d₃₃ coefficient was determined by double-beam interferometry. Strain vs. field measurements provided evidence of 90° domain orientation in these films. Those crystallised at 10 °C min^-1 showed the highest functional properties (%=1.7᎒^-2 7C cm^-2 K^-1 and d₃₃=57 pm V^-1). This is a consequence of the higher stability of the 90° domains oriented during poling, caused by the lower tensile stress arising during preparation. The voltage responsivity of these films also benefited from the lower permittivity arising from their higher porosity. These films are good candidates for applications in infrared detectors and microelectromechanical devices.     

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.     

Electrical and optical properties of indium tin oxide thin films grown by pulsed laser deposition

Indium tin oxide (ITO) thin films (200-400 nm in thickness) have been grown by pulsed laser deposition (PLD) on glass substrates without a post-deposition anneal. The electrical and optical properties of these films have been investigated as a function of substrate temperature and oxygen partial pressure during deposition. Films were deposited at substrate temperatures ranging from room temperature to 300 °C in O₂ partial pressures ranging from 0.1 to 100 mTorr. For 300 nm thick ITO films grown at room temperature in oxygen pressure of 10 mTorr, the electrical conductivity was 2.6᎒^-3 Q^-1cm^-1 and the average optical transmittance was 83% in the visible range (400-700 nm). For 300 nm thick ITO films deposited at 300 °C in 10 mTorr of oxygen, the conductivity was 5.2᎒^-3 Q^-1cm^-1 and the average transmittance in the visible range was 87%. Atomic force microscopy (AFM) measurements showed that the RMS surface roughness for the ITO films grown at room temperature was ~7 Å, which is the lowest reported value for the ITO films grown by any film growth technique at room temperature.     

Pulsed-laser deposition of Ta-doped PZT ferroelectric films for memory applications using conductive oxide La0.25Sr0.75CoO3 and SrRuO3 electrodes

New methods for fabricating highly 𘚡¢-oriented and complete 𘜏¢-textured Pb(Ta_0.05Zr_0.48Ti_0.47)O₃ (PTZT) films on Pt/TiO₂/SiO₂/Si(001) substrates by pulsed-laser deposition have been developed using conductive oxide La_0.25Sr_0.75CoO₃ and SrRuO₃ electrodes. The 𘚡¢-preferred orientated PTZT ferroelectric capacitor was not subjected to loss of its polarization after 1᎒¹⁰ switching cycles at an applied voltage of 5 V and a frequency of 1 MHz, and the 𘜏¢-textured PTZT film capacitor retains 94.7% of its polarization after 1.5᎒¹⁰ switching cycles at 5 V and 50 kHz. The PTZT capacitors using these conductive oxide electrodes have low leakage current dominated by Schottky field emission mechanism.     

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.     

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.     

Multilayer Au/TiO2Composite Films with Ultrafast Third-Order Nonlinear Optical Properties

We report on the ultrafast third-order optical nonlinearity in multilayer Au/TiO2 composite films fabricated on quartz substrates by pulsed laser deposition technique. The linear optical properties of the films are determined and optical absorption peaks due to surface plasmon resonance of Au particles are observed at about 590hm. The third-order optical nonlinearities of the films are investigated by z-scan method using a femtosecond laser （50 fs） at the wavelength of 800 nm. The sample showed fast nonlinear optical responses with nonlinear absorption coefficient and nonlinear refractive index being -3.66 × 10^-10 m/W and -2.95 × 10^-17 m^2/W, respectively. The results also show that the nonlinear optical effects increase with the increasing Au concentration in the composite films.     

Preferential orientation of modified SrBi2Nb2O9 ferroelectric thin films prepared by pulsed laser deposition

We report on the deposition of SrBi₂Nb₂O₉ and Sr_1-xNa_xBi_2-xTe_xNb₂O₉ ferroelectric thin films on Pt/TiO₂/SiO₂/(100)Si substrates using the pulsed laser deposition technique. Deposition on substrates heated to 600-700 °C produces {11l} film texture and dense films with grain sizes up to about 500 nm. The recrystallization at 700 °C of amorphous films deposited at lower temperatures enhances the contribution of the {100} and {010} orientations. These films show smaller grain size, namely 50-100 nm. {11l}-oriented Sr_1-xNa_xBi_2-xTe_xNb₂O₉ films have remnant polarization P_rۆ 7C/cm², a coercive field E_c䏐 kV/cm and dielectric constant, )𪓴. The low value of P_r is probably related to the low fraction of grains with the ferroelectric axis in the direction of the applied field, E. The recrystallized films have more grains with the ferroelectric axis parallel to E; however, they have a low resistivity which so far has prevented electrical characterization.     

Laser irradiation of SrTiO3 single crystals

Excimer laser irradiation (248 nm, 34 ns) of SrTiO₃(100)single crystals was studied in order to resolve the feasibility of obtaining well-defined patterned structures. Spots irradiated with a single pulse in the 0.4-1 J/cm² fluence range presented a pattern of cracks along crystallographic directions, which do not propagate beyond the borders, and no ejected material was observed on the non- irradiated areas. Arrays were patterned by translating the single crystal, and different morphologies were found depending on the width of the tracks. Tracks 10 7m wide or more developed a pattern of cracks, whereas 3 7m wide tracks did not. Artificial arrays in magnetoresistive La_2/3Sr_1/3MnO₃ thin films were prepared by using SrTiO₃(100) single-crystal substrates in which arrays had been previously patterned. The interfaces originated a substantial low-field tunnel magnetoresistance.     

Role of bismuth precursor in crystallization of SrBi2Ta2O9 thin films

Crystallization of SrBi₂Ta₂O₉ (SBT) thin films was studied as a function of viscosity of bismuth precursor and baking temperature, in order to fabricate capacitors with improved ferroelectric properties. SBT thin films were deposited on to Pt substrates using a chemical solution deposition (CSD) technique. Post-deposition anneal at 750 °C for 1 h in oxygen atmosphere revealed a significant influence of baking temperature and the viscosity of bismuth precursor on the microstructure and the ferroelectric properties of SBT thin films. A high baking temperature (350 °C) and a low viscosity of bismuth precursor (8 cp) yielded larger amounts of Bi₂O₃ secondary phase, smaller SBT grains (104 nm), and lower remanent polarization (P_r=2.0 7c/cm²). Additionally, these films exhibited a very high rate of ageing (>45% reduction in P_r after 7 days). A modified CSD process is suggested, which could suppress the formation of Bi₂O₃ secondary phase. Films fabricated using modified CSD technique exhibited a much larger grain size of 165 nm, higher P_r of 7.2 7c/cm², and significantly improved ageing characteristics (<1% reduction in P_r after 7 days). A qualitative model to describe the ageing in SBT-based capacitors is also suggested.     

Ablation of transparent solids during self-limited deposition of diamond-like films from liquid hydrocarbons

A novel effect is studied of self-limitation of the diamond-like film thickness during laser irradiation of the interface of transparent substrates with liquid aromatic hydrocarbons. The interface is exposed through the transparent substrate to radiation of a copper vapor laser (wavelength of 510.6 nm, pulse duration of 20 ns). The thickness of diamond-like film increases linearly to 80-100 nm with the number of laser pulses and then saturates, while the substrate is ablated with nearly constant rate. This ablation rate depends on the thermal expansion coefficient of the substrate (glass, fused silica, sapphire, or CaF₂). The absorption of extinction coefficient of deposited films measured by ellipsometry is of order of 10⁴ cm^-1 and is sufficient to cause the significant heating of the interface. The ablation of the transparent substrates is due to their unequal thermal expansion compared to the diamond-like film having different thermal expansion coefficient. The measured ablation rates scale from 0.2 Å/pulse for glass to 4.5 Å/pulse for CaF₂. A 7m spatial resolution of the ablation process has been demonstrated for fused silica.     

Flexible Cu（In,Ga）Se2 Thin-Film Solar Cells on Polyimide Substrate by Low-Temperature Deposition Process

The electrical and structural properties of polycrystalline Cu（In, Ga）Se2 films grown on polyimide （PI） substrates below 400℃ via one-stage and three-stage co-evaporation process have been investigated by x-ray diffraction spectra （XRD）, scanning electron microscopy （SEM） and Hall effect measurement. As shown by XRD spectra, the stoichiometric CIGS films obtained by one-stage process exhibit the characteristic diffraction peaks of the （In0.68Ga0.32）2Se3 and Cu（In0.7Ga0.3）2Se. It is also found that the film structures indicate more columnar and compact than the three-stage process films from SEM images. The stoichiometric CIGS films obtained by three-stage process exhibit the coexistence of the secondary phase of （In0.68Ga0.32）2Se3, Cu2-xSe and Cu（In0.7Ga0.3）2Se. High net carrier concentration and sheet conductivity are also observed for this kind of film, related to the presence of Cu2-xSe phase. As a result, when the CIGS film growth temperature is below 400℃, the three-stage process is inefficient for solar cells. By using the one-stage co-evaporation process, the flexible CIGS solar cell on a PI substrate with the best conversion efficiency of 6.38% is demonstrated （active area 0.16cm^2）.     

Deposition of Ti:sapphire thin films by reactive pulsed laser ablation using liquid metals and oxygen

Ti:sapphire films were grown using molten Al-Ti alloy ablation targets with either O₂ gas pulses or O₂ background reactive medium on sapphire (0001) substrates. The films were characterized by the use of XRD, RHEED, AFM, and XPS. While the films deposited at a substrate temperature of 650 °C showed three-dimensional epitaxial growth, the films deposited at 1000 °C exhibited a two-dimensional structure. Annealing of the low-temperature deposited films improved the crystal quality but failed to improve the surface morphology. Ti exists in the host sapphire lattice in the form of Ti³⁺ for films deposited at lower temperatures, whereas it assumes the tetravalent form in the high-temperature deposited films. The valence states of Ti identified by XPS studies are in agreement with low-temperature luminescence results.     

Influence of MgO surface conditions on the in-plane crystal orientation and critical current density of epitaxial YBCO films

The effect of magnesium oxide (MgO) surface conditions on in-plane grain orientation and critical current density of epitaxial YBa₂Cu₃O₇ (YBCO) films was systematically investigated. The MgO substrates were either “as received” or stored for some time, cleaned using different methods and lithographically prepared for our step-edge junction devices. The YBCO films were grown via reactive thermal co-evaporation by Theva, GmbH. The surface characterisation of MgO substrates was studied using X-ray photoelectron spectroscopy (XPS). The in-plane grain orientation of the YBCO films was studied by means of X-ray diffraction (XRD) φ-scan and the critical current density was measured for the XRD scanned samples. The surface condition of the MgO substrates was found to have a strong influence on the in-plane grain orientation and the critical current density of the YBCO films. The MgO substrates with a degraded or contaminated surface gave rise to 45° grain misorientation in YBCO films and reduced the critical current density. A final process step using a low energy Ar ion beam etching (IBE) of the MgO substrates prior to the YBCO film deposition was found effective in removing the in-plane grain misorientation and promoting the growth of perfectly aligned c-axis YBCO films.     

Ruthenium oxide and strontium ruthenate electrodes for ferroelectric thin-films capacitors

Metallic ruthenium and ruthenium oxides, such as SrRuO₃ and RuO₂, are potential electrode materials for ferroelectric capacitors. The electrical properties (e.g. leakage currents) of such thin film devices are dependent on the electronic properties of the electrode/ferroelectric junctions and therefore also on the electrode work functions. During growth and processing of film-electrode layer structures the formation of sub-oxides within the electrode is possible, with their work functions being unknown. In order to obtain information for predicting device properties, we have systematically analysed the valence bands and work functions of RuO_x and SrRuO_y thin films with different oxidation states by using photoelectron spectroscopy techniques. The results suggest that Ru⁰ and Ru⁴⁺ ions are present in co-existence at the surfaces of oxygen-deficient polycrystalline films (inhomogeneous oxidation). For both oxygen-deficient materials the work function coincides with that of metallic ruthenium (4.6ǂ.1 eV). Only for fully oxidised ruthenium oxide and strontium ruthenate films (no Ru⁰ present at the surface) is the work function increased to 5.0 or 4.9 eV, respectively. As an example of importance for new dynamic random access memory applications, the junctions of Ba_1-xSr_xTiO₃ with SrRuO_y and RuO_x are discussed.