Search

HfO 2 films were deposited at low temperature (∼400 °C) by UV assisted injection metal-organic chemical vapor deposition (UVI-MOCVD). A three-step process was used for this study, consisting of (A) Pre-deposition anneal for nitridation; (B) Deposition step; (C) Post-deposition annealing in oxygen. Special attention was paid to the effect of UV exposure during these steps. Films were characterized by physical, optical and electrical techniques. Thickness was determined by different methods (X-ray Reflectrometry (XRR), spectroscopic ellipsometry and transmission electron microscopy) and a good agreement was found for all samples. The HfO 2 permittivity, equivalent oxide thickness ( E OT ), flat-band voltage ( V fb ) and total charge ( Q t ) were extracted from the CV response at high frequency taking into account the HfO 2 and SiO 2 thicknesses obtained by XRR. The calculated permittivity values were in the range 7–13, i.e. lower than theoretical values for the monoclinic phase. Explanations are suggested in the context of the other characterizations. JE eff characteristics were constructed taking into account the E OT values ( E eff = V / E OT ). Effective breakdown fields range between 8.7 and 16.9 MV/cm. No dependence of E eff with UV exposure was found.

SrTiO 3 thin films (STO), were deposited on Si(100) covered by 2 nm of SiO 2 , at different temperatures from 450 °C to 850 °C using liquid injection MOCVD, the bimetallic precursor being Sr 2 Ti 2 (OiPr) 8 (tmhd) 4 . The STO films were analysed by XRD, FTIR, SIMS and TEM. An amorphous layer was observed between STO and SiO 2 /Si. The nature and thickness of the interlayer were determined, as well as the most favourable conditions for a good quality crystalline STO film, and a reduced interlayer.

The electrical properties of SrTiO3 thin films grown on Si substrates by liquid injection MOCVD have been investigated as a function of films thickness (from 4 to 200 nm). Using an heterometallic precursor and optimised low pressure conditions, highly textured SrTiO3 films were obtained in a 30-150 nm thickness range. A continuous increase of the SrTiO3 dielectric permittivity with increasing films thickness is deduced, and an interface state density minimum value, lower than 1 × 1011 cm-2 eV-1 is observed for 30 nm films. The lowest gate current is obtained for 30-50 nm films: in those samples, the conduction is identified as a Schottky emission type mechanism. For thinner films, the gate current increases when the thickness decreases as classically observed for amorphous oxides. For thicker samples, the leakage current is enhanced by the polycrystalline microstructure of the SrTiO3 films.

As the sharpness of the metal-oxide interface is of major importance to develop efficient structures, we have studied the thermal stability of interfaces in a series of thin film samples where the metallic component was either Pt, Al, Pd, Ni, Nb or Ti deposited on a thin HfO2 film. Thermodynamic considerations have been carried out to determine the possible products of metal-oxide reaction at a temperature of 500 °C. The evolution of the as-deposited thin film structures as a function of annealing temperature has been analysed by combining various techniques and specifically using X-ray reflectometry. This particular technique allows one to detect very thin embedded interfaces, distinguish small density variations and determine accurately the thickness, the normal roughness and the density of each layer. In addition, reflectometry results have been correlated with atomic force microscopy and transmission electron microscopy observations of selected samples. Our results indicate that platinum, palladium and nickel are stable up to 500 °C on hafnium oxide. Annealed aluminium, an interfacial layer forms, whereas niobium and titanium present a diffuse interface.

Epitaxial thin films of RMnO 3 (R=Sm, Eu, Gd, Dy) were prepared using liquid injection MOCVD and studied by XRD. The formation of high-temperature hexagonal polymorphic structure of DyMnO 3 and non-existing-as-bulk hexagonal EuMnO 3 , GdMnO 3 and SmMnO 3 (traces) was observed on ZrO 2 (Y 2 O 3 ) (111) substrates at 900 °C due to epitaxial stabilization. For the thickness exceeding the critical one the oriented stable perovskite form grew on the hexagonal phase. Parallel deposition on perovskite or non-coherent substrates gave only the stable perovskite phase, epitaxially grown in the former case or oriented in the latter.

La1−xSrxMnO3 films were grown at 450 °C by pulsed liquid-injection metalorganic chemical vapor deposition on silicon substrates. The films were amorphous and exhibited a high resistivity (> 10 kΩ cm). Different kinds of annealing (650 °C for 1, 3, 6, and 10 h, 800 °C for 15 mn, 900 °C for 5 mn, and rapid thermal annealings) were performed in order to achieve crystallization through both variations of nucleation and growth rates. X-ray diffraction and transmission electron microscopy were used to characterize the different microstructures obtained. After annealing, the perovskite La1−xSrxMnO3 (x∼0.3) phase was obtained. The resistivity of the films ranged within two orders of magnitude depending on the annealing temperature and time. The magnetoresistive properties were also recovered and similar to those obtained on as-grown polycrystalline films. The rapid thermal annealing process proved to be efficient: the amorphous films exhibited, after a 90 s flash-annealing, an ordering temperature of 340 K, a res...

High quality epitaxial La1−xSrxMnO3 films were deposited by pulsed injection metal organic chemical vapor deposition, using as precursor materials metal-2,2,6,6-tetramethyl-3,5-heptandionates dissolved in monoglyme. The influence of various deposition conditions, substrate material and film thickness on film properties was investigated. The best films were deposited on LaAlO3 substrates at 825 °C: films exhibited a sharp semiconductor–metal transition and high magnetoresistance (∼40%) at a close-to-room temperature and in a rather low field of 1.5 T. In-situ or ex-situ high-temperature annealing in oxygen increased the temperature of semiconductor–metal transition, but decreased the magnetoresistance and the temperature coefficient of resistance. Biaxial strain imposed by the lattices’ mismatch was clearly observed in thinner La1−xSrxMnO3 films on perovskite substrates. Tensile strain was present in the films on SrTiO3 and compressive strain in the films on LaAlO3 (and less clearly on NdGaO3). Both tensile and compressive strains decreased the temperature of electric transition and the values of magnetoresistance. The strain was completely relaxed in the films more than ∼100 nm thick.

Structural transformations of an epitaxial La0.8MnO3−δ thin film deposited on MgO (100) substrate by metal-organic chemical vapor deposition were conjointly studied by X-ray diffraction using a conventional 4-circles-diffractometer and by transmission electron microscopy. As reported in a previous work [Pignard et al., J. Appl. Phys., 82(9), 4445 (1997)] changes of physical properties of this material have been observed through annealing treatments: Films deposited at 650°C exhibit temperatures of semiconductor-metal and para-ferromagnetic transitions at 130 and 200 K while after successive annealings between 600 and 800°C in air, these transition temperatures increase up to 310 and 330 K, respectively. Two different film structures related to the evolution of these properties were determined at room temperature in their paramagnetic state. As a result, the film structures were found to be monoclinic and arranged in twinned microdomains. Several structural transformations resulting of annealing treatments were deduced: (i) The as-deposited film is almost single-phased with a I2/b space group structure and transforms into a mixing of two phases of I2/b and P21/a space groups in a 82/18% ratio, respectively; (ii) Mn octahedra become more regular, with significant increases of Mn–O–Mn bond angles, close to 180°. These structural changes are interpreted in a discussion related to an increase of the Mn4+/Mn3+ ratio. Besides, as a surprising feature, observations by transmission electron microscopy on La0.8MnO3–δ/MgO interfaces showed that MgO exhibits domains of Fd 3 cubic lattice and a cell parameter twice this of normal MgO. This seems to be the result of a strain occurring between the film and the MgO substrate rather than a chemical diffusion.

The single liquid source pulsed injection MOCVD technique was used to deposit high quality heteroepitaxial structures containing YBa 2 Cu 3 O 7-x (YBCO), SrTiO 3 (STO) and La 1-x Sr x MnO 3 (LSMO) layers. Y, Cu, Sr, La, Mn thd complexes (thd-2,2,6,6-tetramethylheptane-3,5-dionate), Ba(thd) 2 (phen) 2 (phen-1,10-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 grown and studied.

Dielectric multilayers of (BaTiO 3 /SrTiO 3 ) 15 were synthesised on various substrates by a Metal Organic Chemical Vapour Deposition (MOCVD) technique using a pulsed liquid injection source. A single precursor solution was used per compound. The X ray diffraction analysis of the satellite diffraction peaks indicated that the superlattices periodicity can be precisely controlled by the number of injections. Sets of samples were prepared with varying interlayer thickness on doped Nb-SrTiO 3 substrates (0.5 atom %). Electrical measurements in the range 20 -500 K and 20 Hz - 100 kHz were performed on the multilayers, using the STO:Nb substrate as a bottom electrode. As the thickness of the bilayer of the (BaTiO 3 /SrTiO 3 ) 15 superlattices increased (hence the total thickness), the dielectric constant increased as well, reaching values of 650 for (BaTiO 3 36 u.c / SrTiO 3 36 u.c) 15 .

YBa 2 Cu 3 O 7-x (YBCO) and La 1-x Sr x MnO 3 (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 LaAlO 3 and MgO substrates was studied. Thin YBCO layers deposited at 825 °C on LaAlO 3 substrates exhibited epitaxial quality, sharp superconducting transition (∼0.2 K) and a high critical current density (= 4MA/cm 2 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 LaAlO 3 and SrTiO 3 substrates.

We illustrate the capabilities of pulsed-liquid injection metal–organic chemical vapor deposition in the nano-engineering of complex oxides with results obtained on (La0.7Sr0.3MnO3/SrTiO3)15 (LSMO/STO) superlattices. Superlattices with ultrathin films of a few monolayers have been successfully prepared, as shown by synchrotron radiation diffraction experiments. Laue oscillations were observed between the satellite peaks, indicating a smooth upper surface. Sets of superlattices with varying LSMO or STO thickness were synthesized on different substrates (LaAlO3, SrTiO3, MgO). By varying the relative thickness of each compound and changing the substrate, the strain in the stackings could be changed.

Using injection, powder-flash and band-flash MOCVD techniques, epitaxial thin films of Nd, Ho, Y, Tm, Dy and Lu manganites were prepared. The formation of high-pressure phase NdMn 7 O 12 as a thin surface layer on Pnma Nd 1− x MnO 3+δ ( x ∼0.15) in an Nd-deficient system and HoMnO 3 , YMnO 3 , TmMnO 3 and LuMnO 3 as perovskite phases (known in the bulk only as high-pressure phases) on a LaAlO 3 substrate were observed. The remarkable reduction in the pressure needed for formation of the phases in epitaxial films in comparison with bulk samples is a manifestation of the epitaxial stabilisation discussed in the paper. The details of the crystal and domain structure of the films as observed by X-ray diffraction (XRD), selected-area electron diffraction (SAED) and high-resolution electron microscopy (HREM) techniques are described.

[La 0.7 Sr 0.3 MnO 3 /SrTiO 3 ] 15 superlattices (LSMO/STO) with different thickness of STO or LSMO layers were grown on the (001) surface of single crystal substrates (LaAlO 3 , SrTiO 3 and MgO) by pulsed liquid-injection MOCVD. Structural properties of the deposited superlattices were investigated by X-ray diffraction analysis. Θ/2Θ scans indicated that LSMO and STO layers are highly (001) oriented and showed the satellite peaks characteristic of a superlattice structure. It was observed that the strain state in the superlattices depends on the relative thickness of the LSMO and STO layers and on the substrate nature.

Single liquid source pulsed injection MOCVD technique was applied for the deposition of epitaxial SrRuO 3 films and SrRuO 3 /YBa 2 Cu 3 O 7 heterostructures on various monocrystalline substrates (LaAlO 3 , SrTiO 3 , NdGaO 3 , 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 SrRuO 3 /YBa 2 Cu 3 O 7 heterostructures which were in-situ deposited at 825 °C exhibited rather sharp superconducting transition at about 91 K and critical current density >10 6 A/cm 2 at 77 K.

BaTiO3 (BTO) and SrTiO3 (STO) and BaxSr1-xTiO3 (x=0–1) (BST) thin films have been epitaxially grown on LaAlO3 and SrTiO3:Nb at a substrate temperature of 800°C using a new liquid source delivery technique called injection MOCVD. A X-ray study evidenced FWHMs of 0.16° and 0.45° for SrTiO3 and BaTiO3 respectively. In a next step the feasibility of BaTiO3/SrTiO3 superlattices was studied. The multilayers obtained were epitaxially grown on LaAlO3 as well as on SrTiO3:Nb. The structural properties were studied using X-ray diffraction as well as XPS, proving the low interface roughness of 1nm. The XPS study also confirmed the absence of carbon contamination in the film.

YBa2Cu3O7-x /BaxSr1-xTiO3 /LaAlO3 heterostructures can be used as a basis for devices with voltage control in microwave circuits. BaxSr1-xTiO3 (x=0–0.1) (BST) thin films have been epitaxially grown on LaAlO3 substrates using injection MOCVD. The excellent crystalline quality of the obtained BST films can be proven by a FWHM of

Using injection MOCVD technique a series of epitaxial thin "lms of La and Nd manganites were prepared. Epitaxial stabilisation of high-pressure phase NdMn 7 O 12 in thin "lms was observed for low Nd content. For ceramic samples reduced in the #owing Ar with increase of La-vacancy concentration Mn2‘ appearance and collapse of static JT-distortion were observed. Oxidation of manganites stabilises vacancies in A-sublattice of perovskite, which results in the broadening of homogeneity area. For La/Mn and Nd/Mn cation-stoichiometric composition 1 : 1 a shift of oxygen solubility boundary was observed. ( 2000 Elsevier Science B.V. All rights reserved.

Recent results on the study of high-TC superconductors using the e ‐ perturbed angular correlation technique are presented. The basic features of the experimental equipment and its installation at the ISOLDE facility are briefly described. Results obtained from 197m Hg implanted into high quality Y1Ba2Cu3O6+ epitaxy thin films are presented and discussed.

The effects of the microstructure on the magnetoresistance of polycrystalline manganite (La1−xSrxMnO3) thin films have been investigated. The films have been grown by metalorganic chemical vapor deposition on silicon (001) substrates covered with a native silica layer. Different microstructures were obtained by varying the substrate temperature in the range (550–750 °C). The films exhibit a ferromagnetic/paramagnetic transition around 325 K. The grain size is in the range 20–100 nm and, for deposition temperature lower than 650 °C, weak links appear in the films as the grains are mostly noncohesive. The low-field contribution to the magnetoresistance is independent of the grain size in the studied range. A factor of ∼1.8 is gained in this contribution as the substrate temperature is lowered below 650 °C, which can be related to the occurrence of noncohesive grains in the films. The largest low-field magnetoresistance obtained is about 20% at 22 K (under 0.2 T). The high-field contribution to the total mag...

Bilayers composed from a YSZ or CeO 2 buffer layer and a YBCO film were in situ grown on monocrystalline sapphire, MgO. silicon, vicinally polished sapphire and MgO substrates by single source pulsed injection CVD. Zr. Ce. Y. Ba and Cu 2,2,6,6-tetramethyl-3,5-heptandionates dissolved in organic solvent were used for film deposition. CeO 2 /YBCO bilayers on sapphire were grown epitaxially and exhibited critical current densities up to 10 6 A/cm 2 at 77 K. YSZ and CeO 2 buffer layers on MgO substrates did not improved the properties of YBCO layers compared with YBCO films deposited directly on MgO surface; however, interesting microstructural properties of these heterostructures were found. Homogeneous YSZ films were also deposited on large Si substrates (3 inches in diameter).

The in situ synthesis of thin YBa2Cu3O7–x (YBCO) films and YBCO/CeO2 heterostructures was carried out on (001) Zr(Y)O2 (YSZ) substrates from the organometallic vapor phase by a new pulsed injection CVD technique. The secondary ion mass spectrometry (SIMS) concentration profiles revealed a diffusion of Zr from the unbuffered substrate to the films. The films had a dominating (001) texture and a dominating 45° in-plane orientation, but other orientations (0°, 9°, 36°) were also not infrequent in the films. The vapor phase composition was found to have a great influence on the film microstructure and critical parameters. The critical temperatures (Tc) of the best YBa2Cu3O7–x films were about 90 K, the critical current densities (Jc) being ∼4 × 105 A cm−2 at 77 K. A thin epitaxial CeO2 layer grown in situ before YBCO deposition stopped the Zr diffusion from the substrate and dramatically improved the epitaxy and critical parameters of YBCO films. The Tc (ΔTc) and Jc of the deposited YSZ/CeO2/YBCO heteroepitaxial structures equaled ∼90 K (∼0.3 K) and ∼4 × 106 A cm−2 at 77 K. This result shows that a simple deposition of an intermediate CeO2 layer before the growth of YBCO makes YSZ substrates very competitive with perovskite substrates.

The activation energy in several (YBCO){sub m}/(PBCO){sub n} superconducting multilayers was deduced from resistivity measurements in magnetic fields up to 16 T, as a function of the angle between the magnetic field and the ab planes. The activation energy U{sub 0}(B, {theta}) scales with a reduced field B{sub red} = B(sin{sup 2}{theta} + cos{sup 2}{theta}/I{sup 2}){sup 1/2}, with an angle dependent anisotropy parameter {Gamma}. A crossover from U{sub 0}{proportional{underscore}to}log B{sub red}, at low B{sub red}, to U{sub 0}{proportional{underscore}to}(B{sub red}){sup {alpha}} at high B{sub red}, is also observed. The implications of these results on vortex dynamics and on the coupling between the superconducting layers will be discussed.

BaTiO 3 (BTO) and SrTiO 3 (STO) layers were deposited by injection MOCVD using both a mixed Ba 2 Ti 2 precursor (dissolved in hexane) and a Sr(thd) 2 , Ba(thd) 2 and Ti(OPr) 2 (thd) 2 precursor system (dissolved in monoglyme). Films have been deposited at different temperatures between 600°C and 850°C. The microstructural properties of the films obtained on different kinds of substrates MgO (100). LaAlO 3 (012), sapphire (1-102), Si (100), Pt on Si and YBa 2 Cu 3 O 7 (YBCO) on LaAlO 3 are compared.

The possibilities of a new single source injection metalorganic chemical vapor deposition technique were demonstrated for in situ growth of YBa 2 Cu 3 O 7 films of very high crystalline and transport quality. Precisely controlled microamounts of organometallic β-diketonates dissolved in an organic solvent were injected sequentially into the evaporator by means of a computer-driven injector and the resultant vapor was transported into the deposition zone. Accuracy of microdosing (within 5%) and nonselective flash-like evaporation of the precursors were the main factors for obtaining a reproducible vapor phase composition in spite of poor stability of the Ba precursor. The influence of vapor phase composition on film properties was investigated. After the optimisation of deposition conditions, the YBa 2 Cu 3 O 7 films with very high critical current density up to (6–7)×10 6 A/cm 2 at 77 K were reproducibly grown on LaAlO 3 (0 0 1).

Epitaxial YBa 2 Cu 3 O 7 layers with 45° in-plane orientation have been grown by Injection CVD on MgO substrates polished off-axis to within 1.4–1.9° of the [100] direction. This new single-source CVD process is based on computer-controlled injection of precise microdoses of a metal–organic precursor solution into a CVD reactor. A wide range of solution compositions was tested to investigate compositional effects on phase purity, surface morphology, texturing and superconducting properties of the prepared films. The highest quality films with pure 45° texture had a smooth surface, zero resistance T c ( R =0) of 88–89 K, and critical current density J c (77 K) above 10 6 A/cm 2 .

YBCO thin (about ) films were deposited at on (001) by single-source injection CVD. Precisely controlled microamounts of organometallic -diketonates dissolved in an organic solvent were injected sequentially into the evaporator by means of a computer-driven injector and the resultant vapour was transported into the deposition zone. The influence of the vapour phase composition on films' properties was investigated. A mixture of and -oriented YBCO crystallites exists in all deposited films and its ratio depends on the vapour phase composition. For both a and c perpendicular crystallites only in-plane orientation with respect to substrate axes was found. Bidirectional twinning was established in the crystallites of both types. of the films (about 90 K) was almost independent of the vapour phase composition in the studied range. However, the critical current density depended clearly on the vapour phase composition in relation to the ratio variation. of the films varied in the range .

A new liquid source metalorganic chemical vapor deposition process has been developed in order to control precisely the amount of precursors vapors that is produced. Films of self-doped La0.8MnO3−δ have been deposited on LaAlO3 (012) substrates. X-ray diffraction measurements reveal an epitaxial growth. The as-deposited films exhibit both a ferromagnetic (Tc=200 K) and a metal–insulator (Tρ=130 K) transition. Postannealing experiments have been carried out and the results indicate the resistivity is considerably reduced, the temperature of transitions is raised to 320 K, and a magnetoresistance Δρ/ρ0=20% per tesla is obtained at 300 K in the 0–2 T range.

A new principle for stable generation of precursors from volatile vapours in a CVD reactor was developed for in-situ deposition of bilayers CeO 2 /YBa 2 Cu 3 O 7 on sapphire (11¯02) substrates. This principle is based on computer controlled injection of precise microdoses of liquid precursors or solutions of solid precursors in an organic solvent in an evaporator of CVD reactor, “flash evaporation” and rapid vapour transport to the deposition zone. Application of several injections sources permits in-situ deposition of multilayered oxide structures. Heteroepitaxially grown CeO 2 (≈ 0.2 μm)/YBa 2 Cu 3 O 7 (0.5–1 μm)/Al 2 O 3 structures exhibited a sharp superconducting transition at about 90 K (Δ T = 0.6-1 K) and had a critical current density of about 10 5 A/cm 2

Some YBa2Cu3O7-δ films and heterostructures prepared by Chemical Vapor Deposition (CVD) were analyzed in our laboratories by EPMA-EDX or WDX, RBS, SNMS and AES. It was found that in some cases the results of composition analysis can significantly deviate from each other. At least two main reasons for these deviations exist: the different lateral resolution and the application of different reference samples for the calibration.

Copyright (c) 1997 Elsevier Science S.A. All rights reserved.A study of the anisotropy of the critical current in YBCO films is reported. The films have been grown on LaAlO 3 substrates by metal organic chemical vapour deposition. Current-voltage (I-V) measurements have been performed on microbridges using a single pulsed method. The angular magnetic field dependence of the critical current density J c () has been measured for temperatures close to T c and down to 40 K. Films having different fraction of a-axis grains are compared. Significant differences that may be attributed to the differences in microstructure are stressed out. In particular films with a larger amount of a-axis grains exhibit an enhanced pinning when H is parallel to the substrate normal, indicating a flux pinning mechanism that is directionally oriented.

Epitaxial YBCO thin films have been grown on MgO, SrTiO 3 and NdGaO 3 substrates by a new process (injection MOCVD). Droplets (few μ1) of a solution of Y, Ba and Cu β-diketonates in monoglyme are injected into an evaporator through a fast micro electro-valve driven by a computer. Growth rates more than ten times higher than those obtained by the conventional MOCVD process have been obtained in this first step of experiments (>3 μ m h −1 ). The best films obtained so far have T c >90 K and J c >3×10 6 A cm −2 .

A new process of injection-MOCVD has been used to synthesize barium hexaferrite thin films on Al 2 O 3 (0001) substrates. This new technique uses a liquid source of precursors dissolved in a convenient solvent. X-ray diffraction measurements have been performed to observe thin films lattices with respect to the orientation of the substrate: hexaferrite film epitaxy is observed with c-axis perpendicular to the substrate. Magnetic measurements have been performed in the plane and perpendicular to the film plane: they confirm the preferential orientation of films.

Thin and thick YBCO Films have been grown by Aerosol Assisted as well as by thermal MOCVD. The Aerosol Assisted MOCVD technique allows the growth of YBCO films from a single liquid source at deposition rates of up to 10 {mu}m/h. Transport measurements (I-V) on etched microbridges using a single pulse technique have been performed. The angular magnetic field dependence J{sub c}({Theta}) of the critical current density from T{sub c} down to 50 K has been measured. Transport properties are reported and discussed with respect to the microstructural features as determined by X-ray diffraction and electron microscopy.

The pyrosol spraying deposition technique has been used for the synthesis of SnO 2 , SnO 2 (CuO) and SnO 2 (NiO) polycrystalline films. The growth kinetics, composition, microstructure and electrical properties of the films have been investigated and correlated with the deposition conditions. Copper and nickel introduction results in a significant increase in the film resistance.

The interaction of SnO 2 (CuO) polycrystalline films with H 2 S + N 2 gas mixtures has been investigated by conductivity measurements, Auger electron spectroscopy, secondary neutral mass spectrometry and X-ray diffraction, A significant effect of film conductivity increase in the presence of H 2 S has been found. It is suggested that the reversibility of the electrical properties of the doped films may be produced by a change in the copper state in the tin dioxide by means of selective chemical reactions with H 2 S and oxygen accordingly.

We describe field modulated microwave absorption measurements on YBa2Cu307 / PrBa2Cu3O7 multilayers with PrBa2Cu3O7 thickness in the range 2.5 to 10 nm and with a fixed YBa2Cu3O7 thickness of 10 nm. The Fourier spectrum of the reflected microwave power reveals one fundamental frequency which appears at Tc and even harmonics of the modulation field frequency ω at lower temperatures. The determination of the irreversibility line near Tc for different values of the PrBa2Cu3O7 thickness and inferred from the vanishing of the nonlinear response signal at 2ω in the presence of superimposed dc and modulation fields is reportedit.

A new process for stable generation of precursor's vapour pressure for CVD thin layers synthesis is experimented. The vapour pressure is controlled by sequential computer-driven injection of precise micro amounts of liquid in an evaporator, where flash volatilization occurs. During deposition, the precursors are maintained at room temperature under inert gases in an hermetically closed vessel: even thermally unstable precursors may be used. The results reported here are focused on the OMCVD deposition of Ta 2 O 5 . The growth rate increases up to 11 μm/h at 650°C (amorphous layers), and decreases above (crystallized layers). In a second part, we demonstrate the feasibility of Ta 2 O 5 /SiO 2 multilayers using two injectors.

We have examined the properties of thin epitaxial CeO 2 films prepared by aerosol MOCVD. The films were deposited on (1102) sapphire at deposition temperatures between 500 °C and 900°C. The best properties were observed for the film grown at high deposition temperatures. The films have thickness ∼ 0.2 μm and the full width at half maximum of the rocking curve 0.3° - 0.4°. The minimal yield measured by backscattering spectrometry in the channeling mode was 5.5%, confirming high preferred orientation of the deposited films. The epitaxial character of the CeO 2 films was revealed by the measurement of the φ-scan. The aerosol MOCVD CeO 2 films were found to be suitable as a buffer layer for a preparation of superconducting high-T c films. YBa 2 Cu 3 O 1 superconducting films deposited on the CeO 2 /(1102) sapphire exhibit superconducting transition temperature T 0(R=0) = 86 K

Un reacteur de depot chimique en phase vapeur a partir de precurseurs organometalliques (EPVOM) a ete specialement concu pour la synthese de couches alternees du type supraconducteur/isolant. Le systeme etudie (YBa2Cu3O7-x/PrBa2Cu3O7-x) permet la realisation de structures epitaxiees. Les films minces, deposes entre 750 et 900 ○C sur MgO $\{100\}$, SrTiO3 $\{100\}$ et LaAlO$_3\{012\}$, sont obtenus a partir de la decomposition thermique des precurseurs (tetramethylheptanedionates de Pr, Y, Ba et Cu). Les couches d'YBa2Cu3O7-x deposees dans ce reacteur ont des densites de courant critique a 77 K de quelques 106 A.cm-2. La variation de la resistivite des films de PrBa2Cu3O7-x en fonction de la temperature montre un comportement semiconducteur. L'effet de la temperature de croissance sur l'interdiffusion aux interfaces des multicouches sera etudie dans le cas de deux echantillons elabores a des temperatures de depot differentes (750 et 900 ○C). Malgre que la diffusion des elements s'amplifie avec la temperature de depot, les caracteristiques supraconductrices ne sont pas alterees.

Nous presentons une etude sur l'influence des defauts vis-a-vis du piegeage des lignes de flux magnetique dans des monocristaux de Bi2Sr2Ca1Cu2Ox. Ces monocristaux, non irradies ou irradies sur l'accelerateur du GANIL avec des ions (Pb56+) d'energie 6 GeV ont ete decores dans l'etat supraconducteur selon la technique de Bitter. Les reseaux de lignes de flux magnetique ont ete caracterises par microscopie electronique a balayage et avec un analyseur d'images. Les resultats montrent que les reseaux de lignes de flux magnetique sont fortement dependant de la densite de defauts resultant de l'irradiation. Pour caracteriser l'etat de desordre, nous avons applique une methode de calcul de termes d'energie elastique des reseaux de lignes de flux magnetique. Cette methode ne peut etre appliquee que si la transformee de Fourier des reseaux de vortex presente des reflexions discretes.

Pure and Pd-doped polycrystalline SnO2 thin films have been synthesized by a modified chemical vapour deposition process using the pyrolysis of an aerosol generated by ultrahigh frequency spraying of a volatile precursor solution. The depositions have been carried out on oxidized 〈100〉 silicon. Growth rate, microstructure and composition of the films are well-controlled by the deposition temperature (460–560 °C) and the moisture of the carrier gas. Palladium particles (metal or oxide) are dispersed on SnO2 grains. At low temperature the moisture increases the growth rate of tin oxide without increasing the palladium one. The influence of adsorption phenomena on the surface composition has been studied by annealing under ethanol and CO, and afterwards by analysing Sn, O and C contents by Auger spectroscopy. The heat treatments induce an increase of O and C concentration. The introduction of palladium in the films largely increases the gaseous adsorption, showing a maximum at low temperature. These results are correlated with electrical conductance measurements, showing the influence of CO adsorption on electrical properties.

The critical current density (Jc) for zero magnetic field has been measured as a function of temperature in c-oriented thin layers of YBa2CU3O7-x prepared by different synthesis methods. One direct and two indirect Jc(T) methods were employed for each film: transport measurements on patterned film constrictions, DC remanent magnetization studies using the Bean model and non-linear AC screening analysis. Results from the indirect methods were calibrated against transport data. Possible origins of observed dependences are suggested and calibration procedures are discussed. The investigation demonstrated the feasibility of using AC screening analysis in rapid determinations of Jc(T) close to Tc.

Thin‐film growth and compositional effects of c‐axis oriented YBa2Cu3O7−x (YBCO) thin films synthesized by metalorganic chemical vapor deposition have been investigated. The formation of single cation films using tetramethylheptanedionate precursors was shown to be mass controlled, exhibiting a ratio of deposited to evaporated species in the increasing order Ba, Y, and Cu. The physical properties of off‐stoichiometric YBCO films deposited on MgO substrates were measured in the compositional range 1.1≤Ba/Y≤2.3 and 1.5≤Cu/Ba≤4.6. While structural properties such as c‐axis values and rocking curves appeared unaffected to variations in cation stoichiometry, morphology was observed to be extremely sensitive even to slight changes in composition. Off‐stoichiometric layers with Cu/Ba≳1.5 were observed to exhibit Cu‐rich precipitates embedded in a 1:2:3 YBCO film matrix. The zero‐resistivity temperatures were above 77 K for all cation film compositions measured. However, sharp ac‐susceptibility transitions were r...

Transport currents in epitaxial thin layers of YBa 2 Cu 3 O 7 have been measured by a pulse technique which avoids heating at the contacts. We give a complete description of the experimental set up, which allows the measurement of critical currents as a function of temperature, applied field and angle. Thin films have been obtained by Metalorganic Chemical Vapor Deposition on different kinds of substrates: (100) SrTiO 3 and (100) MgO. These films have been patterned by chemical etching and bridges of typically 100μm width have been obtained. The microbridges for I-V measurements can be as wide as 100μm since the contact heating during critical current measurements is considerably reduced in the pulse method and large current intensities (1A and over) can be used without damage. Typical results of Jc(T, H) and of critical current anisotropy will be given for selected MOCVD samples.

Magnetic properties and field modulated microwave absorption have been studied on epitaxial YBa 2 Cu 3 O 7 thin films prepared by MOCVD. Possible correlations between the irreversibility line and the properties of harmonics of the modulation field frequency in the reflected microwave power are discussed.

Epitaxial thin layers of YBa 2 Cu 3 O 7−x are synthesised by thermal decomposition (750 – 830 °C) of tetramethylheptanedionates of yttrium, barium and copper in the presence of oxygen. Argon is used as a carrier gas and the partial pressures of the different precursors are monitored via a careful control of the sources temperatures. The superconducting films with thicknesses ranging between 40 nm and 200 nm are grown on (100) SrTiO 3 , (012) LaA1O 3 or (100) MgO. The growth rate varies between 2.7 nm/min and 4 nm/min. The layers are analysed by scanning and transmission electron microscopy, x-ray diffraction and Rutherford backscattering spectrometry. The normal — superconductor transition is investigated via DC and AC resistance, magnetization and AC susceptibility measurements as a function of temperature. Magnetisation hysteresis loops recordings, I–V measurements on microbridges and non linear susceptibility analysis are used to explore the irreversible properties of the layers. Typical parameters for MOCVD films grown on LaA1O3 are as follows: T c = 90 K, Δ T c = 0.4 K and J c (77 K) = 2 10 6 A cm −2 .

Electrical transport and optical properties of transition-metal silicides are reviewed. They are integrated with thermal properties of single-crystal silicides. Most of these compounds behave as metals while some of them behave as semiconductors. The former show an increasing electrical resistivity ρ with increasing temperature. Several of them show a non-classical deviation of ρ(T) from linearity in the high-temperature limit. This deviation, related to intrinsic properties of the compound, can be affected both in sign and in amount by the presence of foreign atoms (impurities) and structural defects. Moreover, defects dominate the electrical transport at low temperatures both in metallic and semiconducting compounds. Therefore, the interpretation of the electrical properties measured as a function of temperature may give a non-realistic description of silicide intrinsic properties. Since also other physical properties, like thermal and optical ones, can be strongly affected by impurities and defects, results about single-crystal silicides will be first illustrated. Single-crystal preparation and structural characterization are described in detail, with emphasis on crystalline quality in terms of residual resistivity ratio. The electrical quantities, resistivity and magnetoresistance, are measured as a function of temperature and along the main crystallographic directions. The effect of impurities and defects on the transport properties is then evaluated by examining the electrical transport of polycrystalline thin-film silicides. The different contributions to the total resistivity are measured by changing: (i) film stoichiometry, (ii) impurity concentration, (iii) texture growth and (iv) film thickness. Hall-coefficient measurements are briefly discussed with the main purpose to evidence that great caution is necessary when deducing mobility and charge-carrier density values from these data. The theoretical models currently used to interpret the low- and high-temperature resistivity behavior of the metallic silicides are presented and used to fit the experimental resistivity curves. The results of these studies reveal that in several cases there are well-defined temperature ranges in which a specific electron—phonon scattering mechanism dominates. This allows a more detailed study of the microscopic processes. The optical functions from the far-infrared to the vacuum ultraviolet, derived from Kramers—Kronig analysis of reflectance spectra or directly measured by spectroscopic ellipsometry, are presented and discussed for some significant metallic disilicides, both single crystals and polycrystalline films. Different physical phenomena are distinguished in the spectra: intraband transitions at the lowest photon energies, interband transitions at higher energies, and collective oscillations. In particular, the free-carrier response derived from this analysis is compared with the transport results. The interpretation of the experimental spectra is based on the calculated electronic structures or optical functions. Moreover, it is shown how the optical studies contribute to assess definitively the semiconducting character of some disilicides. Specific-heat measurements on single crystals between 0.1 and 8 K are reported. The Debye temperature and the density of electronics states at the Fermi surface are deduced from the lattice and electronic contributions, respectively. Some silicides have been found superconductors with small electron—phonon coupling constants. Emphasis is given to the comparison between the properties deduced from these studies and those obtained from the analysis of electrical transport data. The final part of this review is devoted to the calculation of some microscopic physical quantities, as for example the electron mean free path, the charge-carrier density, the Fermi velocity. The parameters of the best fit to the experimental resistivity curves, the free-carrier parameters obtained from infrared spectra and the density of electronic states at the Fermi surface determined from specific-heat measurements were used in such evaluations.