Reactive Thermal Coevaporation Research Articles

Spectroscopic ellipsometry study of xPbO–(1 − x)TiO2 thin films elaborated by mixed reactive thermal co-evaporation

In this paper, the refractive index, extinction coefficient and optical band gap of xPbO–(1 − x)TiO2 systems are determined by spectroscopic ellipsometry in the spectral range of wavelength 250–850 nm. All films are elaborated by mixed reactive thermal co-evaporation on a SiO2/Si substrate. The Tauc–Lorentz model is used to extract the optical responses and characteristics of the layers. The best values of the fitted parameters are reported. The wavelength-by-wavelength numerical inversion, carried out without considering any fitting parameter, is also represented as another way to derive the optical constants of the layers. The refractive index and the extinction coefficient depend on the PbO content in xPbO–(1 − x)TiO2 systems. The obtained values of the optical band gap are found to change between 2.54 and 3.38 eV. It is demonstrated here that the xPbO–(1 − x)TiO2 systems with the studied compositions have an indirect optical band gap.

STRUCTURAL PROPERTIES OF PbTiO3 FILMS GROWN BY MIXED REACTIVE THERMAL CO-EVAPORATION

ABSTRACT This paper describes the growth conditions and the structural characterization of thin films PbTiO3 elaborated by a mixed reactive thermal co-evaporation. In a same chamber, in oxygen atmosphere, the titanium source is deposited by electron beam evaporation of TiO2 and the lead source is brought by thermal evaporation of PbO. Crystallographic properties are presented using X-ray diffractometry, morphological and optical characterization aspects are shown by means of electronic, atomic force microscopes (MEB and AFM) and Raman spectroscopy. The effect of post-annealing on morphology is presented.

Optical and electrical properties of Vanadium doped Indium oxide thin films

A new transparent conducting indium vanadium oxide (IVO) thin film is developed by using a modification-specific reactive thermal coevaporation method. Electrical and optical characteristics of IVO films were studied with different vanadium doping concentration, which shows good optical transmittance in the visible spectra range and a minimum electrical resistivity of 7.95 x 10(-4) Omega.cm corresponding to a carrier density of 2.28 x 10(20) cm(-3) and a Hall mobility of 34.5 cm(2) V(-1) s(-1), respectively. Using IVO film as the anode, OLED shows a reduced turn-on voltage and significantly enhanced luminance and electroluminescence efficiency with respect to the device with an ITO anode. Our results indicate that IVO is a promising transparent conducting oxide material, and a suitable electrical contact for hole injection in OLEDs.

PbTiO 3 thin films grown by mixed reactive thermal co-evaporation

This paper describes the growth conditions and the structural characterization of thin films PbTiO 3 elaborated by a mixed reactive thermal co-evaporation. In a same chamber, in oxygen atmosphere, the titanium source is deposited by electron beam evaporation of TiO 2 and the lead source is brought by thermal evaporation of PbO. Crystallographic properties are presented using X-ray diffractometry, morphological aspects are shown by means of electronic and atomic force microscopes (MEB and AFM). The effect of post-annealing on morphology is presented.

Test of a 1 kA Superconducting Fault Current Limiter for DC Applications

A low voltage fault current limiter (FCL) having a nominal current of 1 kA has been set up using switching elements based on YBCO thin films fabricated by reactive thermal co-evaporation. The films showed critical current densities exceeding 1 MA/cm/sup 2/ @ 77 K. After patterning and contacting the films, the elements have been assembled in a closed cryostat for operation in a liquid nitrogen bath. The FCL model was successfully tested using prospective fault currents from 25 kA to 150 kA. The electric data show a peak current of 2.7 kA within 1 ms and a limitation to approximately nominal current within 5 ms. Due to this fast response of the YBCO switching elements, FCL coupled grids can be instantaneously decoupled during a fault, leaving the faultless part of the grid practically unimpaired.

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

Abstract The effect of magnesium oxide (MgO) surface conditions on in-plane grain orientation and critical current density of epitaxial YBa 2 Cu 3 O 7 (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.

RE-123 thin films for microwave applications

Large area REBa 2Cu 3O 7 (RE-123; RE=rare earth) thin films have been grown by reactive thermal coevaporation. The work was targeted to identify compounds with superior high frequency properties compared to Y-123 films. As a general trend we found that with increasing RE ionic radius the transition temperature as well as the tendency for cracking increased. No such trend was recognizable for the surface resistance. Dy-123 and Nd-123 films exhibit lower surface resistance at 77 K than Y-123 films at the same thickness. On sapphire substrates however Y-123 films are advantageous because they are less susceptible to thermal cracking.

YBCO coated conductors by reactive thermal co-evaporation

Coated tape conductors of YBCO require a deposition process allowing to obtain a high volume growth rate in order to produce long lengths of tape in a reasonable amount of time. We present our tape coating system where 15 parallel loops of travelling tape of 1 cm width can be coated simultaneously by reactive thermal co-evaporation. For high critical current densities, in-plane alignment of the YBCO film is necessary. Inclined substrate deposition (ISD) is a technique that allows to deposit in-plane oriented buffer layers suitable for YBCO growth at high deposition rates. We present results obtained for YBCO films grown on MgO-ISD buffer layers deposited by e-gun evaporation onto metallic tape substrates.

YBCO films on metal substrates with biaxially aligned MgO buffer layers

We successfully deposited high quality YBCO films on metal tapes with biaxially aligned MgO buffer layers. e-gun evaporation on inclined substrates with a very high deposition rate of up to 250 nm/min was used for the deposition of the buffer layers. The MgO layers are biaxially textured with an in-plane full width at half maximum of 80. The MgO [001]-axis is not parallel to the substrate normal but tilted significantly towards the deposition direction. YBCO films were deposited on ISD buffer layers by reactive thermal co-evaporation. The critical current density of the films was found to be anisotropic due to the tilt of the [001]-axis. Critical current densities up to 7.9/spl times/10/sup 5/ A/cm/sup 2/ at 77 K were achieved.

Large area YBCO films on polycrystalline substrates with very high critical current densities

We deposited large-area YBCO films by reactive thermal co-evaporation on biaxially textured, Y/sub 2/O/sub 3/ stabilized ZrO/sub 2/ (YSZ) buffer layers fabricated by an IBAD technique (ion beam assisted deposition). Polycrystalline, partially stabilized ZrO/sub 2/ (PSZ) with a size of 10/spl times/10 cm was used as a substrate. Homogeneous, biaxially aligned IBAD buffer layers were prepared by using large ion guns and substrate rotation during the IBAD process. YBCO films with thicknesses of up to 1400 nm were deposited. The biaxial alignment of the buffer layer and of the YBCO film was characterized by X-ray diffraction. The FWHM of YSZ [111] /spl phi/-scans varied laterally within a range of 15/spl deg/ to 20/spl deg/. The FWHM of the YBCO [103] was always several degrees better with value of only 7/spl deg/ to 9/spl deg/. The YSZ thickness dependence of this improvement was analyzed. We achieved a mean critical current density of 1.9 MA/cm/sup 2/ and a maximum critical current density of 2.1 MA/cm/sup 2/ on a substrate area of 10/spl times/10 cm.

Deposition of YBCO and NBCO films on areas of 9 inches in diameter

High quality YBa/sub 2/Cu/sub 3/O/sub 7/ thin films can be grown by reactive thermal co-evaporation. Combined with a rotating disk heater this method allows to fabricate even double sided homogeneous films on substrate areas up to 9 diameter. A scanning inductive j/sub c/ probe is used to monitor the homogeneity of critical current densities of typically >2 MA/cm/sup 2/. Surface resistance values are found to be below 500 /spl mu//spl Omega/ at 10 GHz. On biaxially aligned buffer layers on polycrystalline substrates j/sub c/ values of 1.3 MA/cm/sup 2/ are achieved as well. The properties of Nd/sub 1-x/Ba/sub 2+x/Cu/sub 3/O/sub 7/ films grown with the same system are presented and evaluated.

Continuous YBa/sub 2/Cu/sub 3/O/sub 7-δ/ film deposition by optically controlled reactive thermal co-evaporation

We built a thermal co-evaporation system to deposit YBCO films on large areas in a continuous process which requires some basic modifications to the established standard co-evaporation setup. An optical rate control system based on atomic absorption spectroscopy has been developed to measure the individual metal fluxes, in addition the metal sources can be reloaded in-situ for long times. The performance of these new components is discussed and results of first YBCO films grown on small samples are shown.

Electrical behaviour of thin ZrO2 films containing some ceramic oxides

Zirconia was stabilized with MgO, CeO2 and Gd2O3 by a reactive thermal coevaporation process using an electron gun. Electrical measurements were performed in the frequency range 10 KHz to 10 MHz and for various temperatures ranging from 27–600°C. For the ZrO2MgO system, the dielectric constant ε and the a.c. electrical conductivity σ were found to increase up to maximum values for the sample containing 15 mol.% MgO, then decrease for MgO content in the alloy higher than 15 mol.%. The ZrO2CeO2 was found to be partially stabilized in a cubic lattice. The dielectric constant ε and σ increased linearly with the CeO2 content in the range of concentration studied (8–36 mol.%). For the ZrO2Gd2O3 system σ and ε were optimized for the 8 mol.% Gd2O3 sample. The three systems investigated displayed a conduction process thermally activated and controlled by the hopping of charge carriers over the barrier separating two localized sites (Correlated Barrier Hopping).

Inductive measurements of λ(T) of bare YBCO films and the proximity effect in YBCO/normal metal bilayers

Inductive measurements were performed on YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) films and superconducting/normal metal bilayers using Au, PrBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// and (Y/sub 0.4/Pr/sub 0.6/)Ba/sub 2/Cu/sub 3/O/sub 7-/spl delta// as normal material in order to investigate their screening properties between T/sub c/ and 2.2 K. YBCO films were prepared using high pressure dc and hollow cathode sputtering, laser ablation, MOCVD and reactive thermal coevaporation and the penetration depth, /spl lambda/, and the c-axis lattice parameter measured. Comparison of the temperature dependence of the superfluid density, n/sub s/(T), for the YBCO films showed a significant difference in the shape of n/sub s/(T) depending on the deposition method used. This could have implications for preparing stable passive microwave devices. We have also demonstrate that SN-bilayers, using YBCO as the superconductor, exhibit an enhanced screening over the entire temperature range compared with bare YBCO films. >

Studies on MgO-stabilized zirconia thin films in the UV-visible region

ZrO 2-MgO optical coatings were deposited by reactive thermal coevaporation. The nickel substrate optical constants were determined by ellipsometry. Film reflectance measurements at normal incidence were made in the region 350 nm–800 nm in order to determine the Fresnel reflection coefficient. The film's refractive index was computed using the Fresnel formalism. It was found that it varies with the amount of MgO in the alloy. The maximum values were 2.29 ± 0.12, 2.18 ± 0.10 and 2.06 ± 0.07 respectively for λ = 0.37 μm, λ = 0.63 μm and for λ = 0.75 μm. These maxima were obtained from the sample containing 15 mol.% MgO in the alloy. At molar percentages of MgO greater than 20% a monotonic decrease of index is noted. The investigated samples displayed a large dispersion of film refractive index. We found a film packing density ranging from 0.75 to 0.98. As a function of MgO mole fraction, this important factor related to film structure determines the refractive index behaviour. These investigations aimed to produce thin optical coatings with variable optical constants for optimizing the performance of an optical device for a certain wavelength or wavelength region.

High-quality YBa2Cu3O7 films on 4-in. Wafers of sapphire, gallium arsenide, and silicon

Our technique of reactive thermal co-evaporation has been extended to fabricate large films (up to 4 in.) of YBa2Cu3O7 with high quality. A rotating substrate holder is used to separate the deposition and oxidation processes. This allows free access of the metal vapors. As large substrate wafers we use Al2O3, Si, and GaAs with buffer layers of CeO2, YSZ, and MgO, respectively. On all substrates, the uniformity of thickness and composition was better than 2%. Inductively measuredTc andjc(77 K) were 87.5±0.2 K and >1×106 A/cm2, respectively, across the full wafer area. This holds also for GaAs substrates due to a new procedure of capping by Si3N4.

Epitaxial YBa2Cu3O7 growth on KTaO3 (001) single crystals

Epitaxial films of YBa2Cu3O7 have been grown on KTaO3 (100) single-crystal substrates by means of reactive thermal coevaporation at temperatures between 560 and 750 °C. The orientation of the YBa2Cu3O7 crystallographic c axis in these films was perpendicular to the substrate surface throughout the complete range of deposition temperatures even though the KTaO3 lattice constant represents a better match to the YBa2Cu3O7 c axis than to the a and b axes. Since the nucleation and growth of YBa2Cu3O7 films are known to be relatively sensitive to the formation of a-axis-oriented grains at imperfections on the substrate material, an investigation of the influence of the substrate surface finish on the film properties was carried out. Using substrates with appropriately prepared surfaces, YBa2Cu3O7 films exhibited a high degree of crystalline perfection, zero resistance at temperatures of 89 K and critical current densities up to 3×106 A/cm2.

TEM study of epitaxial YBa 2Cu 3O 7−x thin films deposited on GaAs with MgO buffer layers

Epitaxial, c-oriented YBa 2Cu 3O 7− x (YBCO) thin films yielding large critical transport currents ( j c > 1.2×10 6 A/cm 2 at 77 K) were deposited on GaAs using MgO buffer layers. The MgO was deposited by electron gun evaporation at a substrate temperature of 460°C whereas the YBa 2Cu 3O 7− x was deposited at 620°C by reactive thermal coevaporation. The low deposition temperatures which can be applied for the thermal co-evaporation deposition to obtain high-quality YBCO thin films, are particularly important for the deposition on GaAs substrates. High-resolution transmission electron microscopy was used to image the various interfaces and to investigate the quality of the deposited films. In spite of a significant mismatch between MgO and GaAs ( a MgO=0.4213 nm and a GaAs=0.565 nm) the MgO film grows in single orientation with a 〉100〈 MgO∥〉100〈 GaAs orientational relationship. the GaAsMgO interface was very smooth and had a roughness of 1–2 nm. The MgO buffer layer was significantly strained and contained planar defects parallel to the (100) plane. The MgOYBa 2Cu 3O 7− x interface was surprisingly smooth and planar. In the c-oriented film, stacking faults, dislocations and a-oriented areas a few nm in size were found.

Epitaxial YBa 2Cu 3O 7−δ films on silicon using combined YSZ/Y 2O 3 buffer layers : A comprehensive study

Epitaxial YBa 2Cu 3O 7−δ films with excellent structural and electronic properties were grown on silicon by reactive thermal coevaporation employing a combination of yttria-stabilized zirconia (YSZ) and Y 2O 3 as a buffer layer. To provide optimum conditions, the YSZ growth was studied over a wide range of deposition parameter space. For the first time the effectiveness of the buffer has been derived quantitatively from SNMS profiles. Although the buffer layer can solve the problem of interdiffusion, the large differential thermal expansion between silicon and YBa 2Cu 3O 7−δ cannot be compensated in any way. In many cases this results in a fracture of films thicker than 70 nm and a fast degradation which imposes severe restrictions on the technical applications of silicon substrates. A thorough study of crack formation led to the conclusion that the theoretical limitation has not yet been reached and that there is still hope to overcome this problem.

Structural properties of Y2O3 stabilized ZrO2 films deposited by reactive thermal coevaporation

Films of Y2O3 stabilized ZrO2 were obtained by reactive thermal coevaporation using an electron gun. These films were deposited on nickel substrates at substrate temperature to 500 °C, while the oxygen pressure was held to 4×10−5 Torr. The film structure was characterized by x-ray diffraction, the film composition has been determined by electron probe microanalysis. Postdeposition x-ray analysis showed the cubic phase of ZrO2. The cubic phase’s lattice parameter presented a monotonic increase with increased Y2O3; the mole fraction of Y2O3 was varied from 0.6% to 18.5%.