Superconducting and structural properties of YBaCuO thin films deposited by inverted cylindrical magnetron sputtering

Thin films of YBaCuO have been deposited onto Al₂O₃, MgO, SrTiO₃ and ZrO₂ substrates by inverted cylindrical magnetron sputtering. The main advantage of this preparation technique is the high reproducibility of the results allowing systematic studies of the film properties as a function of deposition parameters. Optimum preparation conditions were a low discharge voltage (100 V), a high oxygen partial pressure (2×10^–1 Torr) in an oxygen argon mixture, and substrate temperatures around 800°C. The films grow highly textured on all substrates yielding single crystalline growth on SrTiO₃ and MgO as proved by X-ray diffraction and channeling. The zero resistance values of the sharp transtions usually are around 90 K. highest critical current densities were determined in films grown on MgO and SrTiO₃ with values up to 5.5×10⁶ A/cm² at 77 K.     

Laser assisted modification and chemical metallization of electron-beam deposited ceria thin films

Excimer laser processing is applied for tailoring the surface morphology and phase composition of CeO₂ ceramic thin films. E-beam evaporation technique is used to deposit samples on stainless steel and silicate glass substrates. The films are then irradiated with ArF* excimer laser pulses under different exposure conditions. Scanning electron microscopy, optical spectrophotometry, X-ray diffractometry and EDS microanalysis are used to characterize the non-irradiated and laser-processed films. Upon UV laser exposure there is large increase of the surface roughness that is accompanied by photo-darkening and ceria reduction. It is shown that the laser induced changes in the CeO₂ films facilitate the deposition of metal nano-aggregates in a commercial copper electroless plating bath. The significance of laser modification as a novel approach for the production of CeO₂ based thin film catalysts is discussed.     

Optical properties of YBaCuO thin films at Far-Infrared monochromatic light

By using monochromatic sources of radiation we measured the reflectivity and transmission vs temperature behavior of YBaCuO thin film in the Far Infrared region in the range 300-20 K. A smooth change of optical properties is observed from critical temperature down to 40 K with an evident reduction of the losses for all the frequencies we examined. To analyze the use of YBaCuO films as semireflecting elements in submillimeter devices we made a preliminary study of the optical properties of the substrate in the same conditions. The permanence of the sample in a superconducting phase can be deduced from its room-temperature transmittance, as it changes drastically if some degeneracy occurs in the film.Partially supported by the National Research Council of Italy, CNR, under the Progetto Finalizzato Superconductive and Cryogenics Technologies     

Electrochemically deposited bismuth telluride thin films

Thin films of bismuth telluride grown by electrochemical deposition technique on conducting glass and Mo sheet substrates, were characterized for their structural, morphological, optical and compositional analysis. These studies revealed polycrystalline anisotropic and layered structure of these films with different compositional stoichiometry. In the present work electrochemical deposition of bismuth telluride thin films is studied as a dopant material in II–VI group absorber materials for photovoltaic application since it has a narrow optical energy band gap of 0.13 eV. In this deposition process different film growth parameters were optimized to get good quality of compositionally uniform bismuth telluride thin film. XRD analysis revealed a hexagonal symmetry with large c-axis lattice constants (Bi₂Te₃, Bi_2+XTe_3−X).     

原子层沉积HfO_2薄膜的激光损伤特性分析

以目前激光惯性约束聚变中应用最广泛的高折射率材料二氧化铪(HfO2)为研究对象,在熔石英基底上分别采用TEMAH和HfCl4前驱体制备了HfO2薄膜,沉积温度分别为100,200和300℃。采用椭偏仪和激光量热计对薄膜的光学性能进行了测量分析,采用X射线衍射仪(XRD)对薄膜的微结构进行了测量。最后在小口径激光阈值测量平台上按照1-on-1测量模式对薄膜的损伤阈值进行了测试,并采用扫描电子显微镜(SEM)对损伤形貌进行了分析。研究表明,用同一种前驱体源时,随着沉积温度升高,薄膜折射率增加,吸收增多,损伤阈值降低。在相同温度下,采用有机源制备的薄膜更容易在薄膜内部形成有机残留,导致薄膜吸收增加,损伤阈值降低。采用HfCl4作为前驱体源在100℃制备氧化铪薄膜时,损伤阈值能够达到31.8J/cm2(1064nm,3ns)。     

Macrostructure of high-temperature superconducting YBaCuO thin films grown by laser ablation

The superconducting parameters and macrostructure of YBaCuO thin films grown by laser ablation on SrTiO₃ substrates are influenced by the substrate temperature during growth. In a study of this influence it is found that the macrostructure has significant bearing on the superconducting parameters of the films and on the critical current, in particular. For J _c⩽10⁵ A/cm² the films have a distinctly pronounced granular structure, while for J _c>10⁵ A/cm² the films do not have any kind of block structure. Zh. Tekh. Fiz. 68, 48–51 (February 1998)     

Strains and rotations in thin deposited films

Thin films deposited on misfitting substrates exhibit distortions produced by the superposition of coherency strains and the elastic fields of interfacial defects. These distortions become homogeneous strains, ?, and rotations, φ, beyond a characteristic distance from the interface, z, and are partitioned between the film and substrate. Residual strain arises when the density of interfacial defects is insufficient to compensate the intrinsic coherency strain, and is partitioned in a manner depending on the relative thicknesses of the two layers, d. However, rotations are not partitioned in this way. Expressions for the magnitude and partitioning of ? and φ are derived for the case of elastically isotropic materials. Calculated values are shown to be in excellent agreement with experimental measurements for a variety of technologically relevant cases.     

C-axis oriented growth of magnetron sputtered YBaCuO thin films on MgO substrates

Highly textured YBaCuO thin films were sputtered on MgO (100)-oriented single crystal substrates at ambient temperature followed by an anneal in oxygen for 1 h at temperatures up to 920 °C. X-ray diffractograms of the highly textured films indicate an orientation of the c-axis of the YBaCuO lattice perpendicular to the substrate surface. There are strong indications that the oriented c-axis growth is due to a CuO self-flux effect. Auger measurements reveal a copper diffusion profile into the substrate down to a depth of more than 400 nm.     

Composition of the sputter deposited W-Ti thin films

Tungsten-titanium (W-Ti) thin film was deposited by dc Ar⁺ sputtering of W(70 at.%)-Ti(30 at.%) target. The thin film composition, determined by X-ray photoelectron spectroscopy (XPS) depth profiling, is W_(0.77±0.07)Ti_(0.08±0.03)O_(0.15±0.03). The presence of oxygen in the deposit is due to the rather poor vacuum conditions during the deposition, while significant deficiency of Ti, as compared to the sputtering target composition cannot be explained straightforwardly. Monte Carlo simulations of both, transport of sputtered particles from target to the substrate through the background gas (SRIM 2003 program) and thin film sputtering during the XPS depth profiling (program TRIDYN_FZR) are presented. The simulations show that the particle transport through the background gas is mainly responsible for the Ti depletion: the estimated composition of the thin film is W_0.61Ti_0.16O_0.23. Additional apparent Ti depletion occurs due to the preferential sputtering during the thin film composition analysis. The simulation of the sputtering process show that the surface concentration measured by XPS should be about W_0.74Ti_0.11O_0.15. The discrepancy between the estimated surface composition and the actual experimental result is in the range of the experimental error.     

Crystallization of sputter deposited amorphous metal thin films

We have studied the crystallization of DC sputter deposited amorphous W-Si alloys under three separate conditions: free standing films, amorphous films deposited on Si substrates, and films with one of four different overlayers. In the case of the free standing films the crystallization temperature versus composition was measured by differential thermal analysis (DTA) for films with Si contents from 5 at% to 38 at%. Films with Si concentrations of 38% to 22% were amorphous as deposited, while those with less Si were crystalline. The crystallization temperature was a strong function of composition with a maximum T_c of 915°C (at a heating rate of 20 K/min) in alloys with 28% Si. Overlayers of W, Cu, Au and Al were investigated. Both the W and Cu overlayers had little effect on the stability of the underlying W-Si, while both the Au and Al reduced the crystallization temperature by at least 100°C. The results reported here reinforce the observation that the choice of overlayer plays a critical role in determining the overall stability of metallization systems that include amorphous layers as diffusion barriers.     

Laser scribing of polycrystalline thin films

We have investigated the use of several different types of lasers for scribing of the polycrystalline materials used for thin-film solar cells: CdTe, CuInGaSe₂ (CIGS), ZnO, SnO₂, Mo, Al, and Au. The lasers included four different neodymium–yttrium–aluminum garnet (Nd:YAG) (both 1064 and 532 nm wavelengths), a Cu vapor (511/578 nm), an XeCl excimer (308 nm), and a KrF excimer (248 nm). Pulse durations ranged from 0.1 to 250 ns. We found that the fundamental and frequency-doubled wavelengths of the Nd:YAG systems work well for almost all of the above materials except for the transparent conductor ZnO. The diode-laser-pumped Nd:YAG was particularly convenient to use. For ZnO the uv wavelengths of the two excimer lasers produced good results. Pulse duration was found generally not to be critical except for the case of CIGS on Mo where longer pulse durations (≥250 ns) are advantageous. The frequently observed problem of ridge formation along the edges of scribe lines in the semiconductor films can be eliminated by control of intensity gradients at the film through adjustment of the focus conditions.     

Q factor of megahertz LC circuits based on thin films of YBaCuO high-temperature superconductor

High-frequency properties of resonant structures based on thin films of YBa₂Cu₃O_7–δ high-temperature superconductor are studied experimentally in the frequency range 30–100 MHz. The structures planar induction coils with a self-capacitance fabricated on neodymium gallate and lanthanum aluminate substrates. The unloaded Q factor of the circuits exceeds 2 × 10⁵ at 77 K and 40 MHz. Possible loss mechanisms that determine the Q factor of the superconducting resonant structures in the megahertz range are considered.     

原子层沉积HfO2薄膜的激光损伤特性分析

以目前激光惯性约束聚变中应用最广泛的高折射率材料二氧化铪(HfO2)为研究对象,在熔石英基底上分别采用TEMAH和HfCl4前驱体制备了HfO2薄膜,沉积温度分别为100,200和300 ℃。采用椭偏仪和激光量热计对薄膜的光学性能进行了测量分析,采用X射线衍射仪(XRD)对薄膜的微结构进行了测量。最后在小口径激光阈值测量平台上按照1-on-1测量模式对薄膜的损伤阈值进行了测试,并采用扫描电子显微镜（SEM）对损伤形貌进行了分析。研究表明,用同一种前驱体源时,随着沉积温度升高,薄膜折射率增加,吸收增多,损伤阈值降低。在相同温度下,采用有机源制备的薄膜更容易在薄膜内部形成有机残留,导致薄膜吸收增加,损伤阈值降低。采用HfCl4作为前驱体源在100℃制备氧化铪薄膜时,损伤阈值能够达到31.8 J/cm2 (1064 nm,3 ns)。     

Nanostructured silver thin films deposited by pulsed laser ablation

Nanostructured thin films were deposited by excimer laser ablation of silver targets in controlled atmospheres of He and Ar. The film structural properties were investigated by means of scanning electron microscope and transmission electron microscope imaging. The film growth mechanism was identified as the result of coalescence of nanometric clusters formed during plume flight. Cluster formation involves plume confinement as a consequence of the increased collisional rate among plasma species. Fast photography imaging of the laser-generated silver plasma allowed to identify plasma confinement, shock wave formation and plasma stopping.     

Harmonic generation in ZnO nanocrystalline laser deposited thin films

ZnO plasma produced by third harmonic 355 nm of Nd:YAG laser at various ambient pressures of oxygen was used for depositing quality nanocrystalline ZnO thin films. Time and space resolved optical emission spectroscopy is used to correlate the plasma properties with that of deposited thin films. The deposited films showed particle size of 8 and 84 nm at ambient oxygen pressure of 100 and 900 mTorr, respectively. Third harmonic generation observed in ZnO thin films deposited under 100 mTorr of ambient oxygen is reported.     

Spatial variations in the stoichiometry of sputtered YBaCuO thin films: theory and experiment

A Monte Carlo simulation of the sputtering process is applied to the problem of the thickness and stoichiometry variations in the sputtering of YBaCuO superconducting targets. The theory predicts no strong spatial variations in stoichiometry due to the different scattering behaviour of the various constituents of the superconductor. The theory is compared with a systematic experimental study using both RF and DC sputtering in either argon or oxygen gases. The ratios Ba: Y and Cu: Y were determined as a function of radial distance by inductively coupled plasma analysis (ICP). In argon, both DC and RF sputtering gave a thickness distribution in reasonable agreement with theory. In oxygen, RF sputtering gave a thickness distribution very different to theory and gave no net deposition in an annular central region. Stoichiometry variations are interpreted in terms of etching processes. The role of oxygen ions in etching is discussed and a case made for the importance of oxygen positive ions in RF sputtering in oxygen.     

Laser short-pulse heating of silicon-aluminum thin films

Energy transport in silicon-aluminum thin films is examined during the laser short-pulse irradiation subjected to the silicon film. The silicon film is considered to be at the top of the aluminum film. Thermal boundary resistance at the interface of the films is incorporated in the analysis. The absorption of laser radiation in the silicon and aluminum films is modeled using the transfer matrix method. Since the silicon film is dielectric, the phonon radiative transport basing the Boltzmann transport equation is incorporated to determine equivalent equilibrium temperature in the film while modified two-equation model is used to account for the non-equilibrium energy transport due to thermal separation of electron and phonon sub-systems in the aluminum film during the laser short-pulse heating process.     

Laser scribing of indium tin oxide (ITO) thin films deposited on various substrates for touch panels

In this study, a Nd:YAG laser with wavelength of 1064 nm is used to scribe the indium tin oxide (ITO) thin films coated on three types of substrate materials, i.e. soda-lime glass, polycarbonate (PC), and cyclic-olefin-copolymer (COC) materials with thickness of 20 nm, 30 nm, and 20 nm, respectively. The effect of exposure time adjusted from 10 μs to 100 μs on the ablated mark width, depth, and electrical properties of the scribed film was investigated. The maximum laser power of 2.2 W was used to scribe these thin films. In addition, the surface morphology, surface reaction, surface roughness, optical properties, and electrical conductivity properties were measured by a scanning electron microscope, a three-dimensional confocal laser scanning microscope, an atomic force microscope, and a four-point probe. The measured results of surface morphology show that the residual ITO layer was produced on the scribed path with the laser exposure time at 10 μs and 20 μs. The better edge qualities of the scribed lines can be obtained when the exposure time extends from 30 μs to 60 μs. When the laser exposure time is longer than 60 μs, the partially burned areas of the scribed thin films on PC and COC substrates are observed. Moreover, the isolated line width and resistivity values increase when the laser exposure time increases.     

Pulsed laser deposited iron fluoride thin films for lithium-ion batteries

Iron fluoride thin films were successfully grown by Pulsed Laser Deposition (PLD), and their physico-chemical properties and electrochemical behaviours were examined by adjusting the deposition conditions, such as the target nature (FeF₂ or FeF₃), the substrate temperature (T_s ≤ 600 °C), the gas pressure (under vacuum or in oxygen atmosphere) and the repetition rates (2 and 10 Hz). Irrespective of the FeF₂ or FeF₃ target nature, iron fluoride thin films, deposited at 600 °C under vacuum, showed X-ray diffraction (XRD) patterns corresponding to the FeF₂ phase. On the other hand, iron fluoride thin films deposited at room temperature (RT) from FeF₂ target were amorphous, whereas the thin films deposited from FeF₃ target consisted of a two-phase mixture of FeF₃ and FeF₂ showing sharp and broad diffraction peaks by XRD, respectively. Their electrochemical behaviour in rechargeable lithium cells was investigated in the 0.05-3.60 V voltage window. Despite a large irreversible capacity on the first discharge, good cycling life was observed up to 30 cycles. Finally, their electrochemical properties were compared to the ones of iron oxide thin films.