Metastable states of superconducting indium films. II

Supercooling fields of In films condensed at 295 and 77 K have been studied. Each series of films defines a different bulk material, whose Ginzburg-Landau (GL) parameter can be obtained from the measured supercooling fields. These bulk metals satisfy the Gor'kov-Goodman relation between the GL parameter and resistivity. Furthermore, this relation is also satisfied by GL parameters of individual type II films condensed at 77 K, while other layers condensed at 295 K show deviations ascribed to nonlocal effects. The importance of a careful study of transport properties of the films for a correct interpretation of experiments is stressed.     

Metastable states of superconducting indium films

A new technique (division of the film into very many small squares) allows the observation of supercooling and superheating of superconducting layers under both parallel and perpendicular fields. The possible existence of edge nucleation in thick films, analogous to surface nucleation for semiinfinite samples, is shown. The superconducting parameters determined for films extrapolate correctly to known bulk values. Critical thicknesses for passage from type I to type II behavior agree with theoretical predictions. Nonlocal corrections to the penetration depth allow one to explain deviations from the Gorkov-Goodman relation for bulk metals.     

Microwave characterization of laser ablated Y1Ba2Cu3O7−x thin films

In the present study we report the measurements of microwave surface resistance (R_s) of YBCO thin films on LaAlO₃ substrate as a function of temperature, thickness and magnetic field by microstrip resonator technique. The T_c(R = 0) of the films is 90 K and J_c > 10⁶ A/cm² at 77 K. The microwave surface resistance has been measured for films of various thicknesses. The value of R_s has been found to be initially decreased with increasing film thickness due to increase in number of defects. A minimum microwave surface resistance has been obtained for film thickness of about 300 nm. The increase of R_s with film thickness above 300 nm is possibly due to degradation of the film microstructure as observed with Atomic Force Microscopy. Temperature dependence of surface resistance has been studied for best quality films. The field induced variations of surface resistance are also investigated by applying dc magnetic field perpendicular to stripline structure and surface of the film. A general linear and square field dependence of R_s at low and high value of fields has been observed with critical field value of 0.4 T which confirms the microwave dissipation induced by flux flow in these resonators at 10 GHz frequency. The hysteresis of R_s in dc field observed for field value above critical field shows the higher value of surface resistance in decreasing field than in increasing field which is in agreement with one state critical model and is a characteristic of homogeneous superconductors.     

Pair and Quasiparticle States of YBa2Cu3O7−x Films Deduced from the Surface Impedance and a Comparison with Nb3Sn

Surface impedance data at 19 and 87 GHz of high-quality epitaxial YBCO films on different substrates are compared with data for Nb₃Sn films on sapphire in terms of pair and quasiparticle (qp) transport. Surface resistance R _s and penetration depth λ of YBCO are strongly affected by temperature dependent qp scattering, which is depressed in films with enhanced lattice strain. All films showed a comparable residual resistance R _res(19 GHz)～90 μΩ constituting a qp reservoir which is likely to be caused by the electronic configuration and by impurities. Subtracting R _res from R _s (T) revealed activated behavior with a reduced energy gap Δ₀/k _B T _c～0.9 for a film on sapphire, but power-law behavior for the other films. The penetration depth did not reveal power-law dependences at T≤0.5 ·T _c, but was consistent with a reduced energy gap of 0.45 for a film on MgO. The increase of λ(T) at T≥0.5 · T _c was related to qp scattering, which also caused an extremal conductivity σ₁(T). A shoulder in λ(T) at T=(0.6–0.7) · T _c confirmed evidence for the existence of two superconducting bands. The magnetic-field induced recovery of λ(B) of various YBCO films hinted for an important role of magnetic scattering. The results are in contradiction to a d-wave symmetry of the order parameter, at least for the chain band.     

Magnetic field penetration in niobium- and vanadium-based Josephson junctions

Measurements on the temperature dependence of the magnetic field penetration in Nb-Nb_xO_y-Pb and V-V_xO_y-Pb Josephson junctions have been performed. Results on the zero-temperature penetration depth in niobium films are far above the bulk values although consistent with other measurements on junctions reported in the literature. For vanadium junctions anomalously large penetration depth values are obtained at low temperatures. Nevertheless, the temperature dependence is in reasonable agreement with the local dirty limit model.     

Transition from type I to type II behavior and the density of states in thin indium films

We have studied thin indium films in a perpendicular magnetic field by electron tunneling. From measurements of the density of states and critical field we conclude that the films undergo a transition from type I to type II behavior at a thickness of 2700 Å at 0.4 K. In thicker films the number of flux quanta in a bundle increases smoothly with thickness and we observe supercooling effects associated with type I. The thin films all have the same density of states and are reversible, indicating type II behavior. A comparison is made with other determinations of the critical thickness. The densities of states in type II lead, indium, and aluminum films are found to be different but show a consistent trend with transition temperature. Calculations only exist for bulk, dirty type II materials and are not applicable to these bulk type I materials.     

Transport Properties of Over-doped Epitaxial NdCeCuO Films

The transport properties of slightly overdoped Nd_2?x Ce _x CuO_4??? (NCCO) c-axis oriented thin films with x=0.17 have been investigated in the temperature range from 2.5 K to 300 K and in magnetic fields up to 6 T applied perpendicular to the CuO₂ planes. The films have been grown by a dc sputtering technique in on-axis configuration. They have been optimized on (001)-oriented SrTiO₃ substrates and successively annealed in pure Argon at 900?°C. Structural and compositional analyses were carried out by means of X-ray diffraction and scanning electron microscopy equipped with a wavelength dispersive spectroscopy detector. Current-voltage characteristics of the NCCO films have been measured and the temperature and the magnetic field dependence of the critical current density J _c(T,H) has been obtained. Finally, an increase of the low-temperature normal state resistance with the field has also been observed.     

Tailoring the stress-depth profile in thin films; the case of γ’-Fe4N1-x

Homogeneous γ’-Fe₄N_1-x thin films were produced by gas through-nitriding of iron thin films (thickness 800 nm) deposited onto Al₂O₃ substrates by Molecular Beam Epitaxy. The nitriding parameters were chosen such that the nitrogen concentration within the γ’ thin films was considerably lower (x ≈ 0.05) than the stoichiometric value (x = 0). X-ray diffraction stress analysis at constant penetration depths performed after the nitriding step revealed the presence of tensile stress parallel to the surface; the tensile stress was shown to be practically constant over the entire film thickness. For further nitriding treatments, the parameters were adjusted such that nitrogen enrichment occurred near the specimen surface. The depth-dependent nitrogen enrichment could be monitored by evaluating the strain-free lattice parameter of γ’ as a function of X-ray penetration depth and relating it to the nitrogen concentration employing a direct relation between lattice parameter and nitrogen concentration. The small compositional variations led to distinct characteristic stress-depth profiles. The stress changes non-monotonously with depth in the film as could be shown by non-destructive X-ray diffraction stress analysis at constant penetration depths. This work demonstrates that by a specific choice of a first and a subsequent nitriding treatment (employing different nitriding potentials and/or different temperatures for both treatments) controlled development of residual stress profiles is possible in thin iron-nitride surface layers.     

Elaboration of heterojunction solar cells by the deposit of tin oxide thin films on silicon by APCVD

We present in this paper the experimental results concerning the deposition of tin oxide SnO₂ on silicon substrate by the technique of Atmospheric Pressure Chemical Vapour Deposition (APCVD). The obtained Si-SnO₂ heterostructure is used for photovoltaic application. The properties of tin oxide thin films deposited by APCVD technique depends on three parameters which are the deposition temperature, the deposition time and the oxygen pressure. We have obtained the optimal value of each parameter by the measurement of the open-circuit voltage of the obtained Si-SnO₂ heterostructure. So, at the temperature of 490 °C during 12 min of deposition time under oxygen pressure of 1 bar we have obtained tin oxide thin layers exhibiting the best optoelectronic and morphology characteristics. These thin films are polycrystalline and present a resistivity of 1.3 · 10^− 3 Ω cm and a refractive index of 1.72. The Si-SnO₂ heterojunction solar cell that has an area of 2 × 1.5 cm² is characterised by the current-voltage I(V) measurement. It gives an open circuit voltage of 0.45 V and a short circuit current of 74 mA.     

Penetration of a magnetic field into superconducting lead and lead-indium alloys

The temperature dependence of the magnetic field penetration depth of superconducting lead and lead-indium alloys has been studied over the temperature range between about 2 K andT _c. Data are analyzed in terms of the microscopic theory. The difficulties of a unique analysis of the penetration data are pointed out and a strategy for the best analysis is discussed. The penetration depth atT=0 K for pure lead is determined as 525 Å. This value, though higher than the previously accepted value for lead, is nevertheless consistent with the strong coupling character of lead.     

On the annealing temperature, penetration depth of oxygen and film thickness on the DC and AC electrical properties and nano-structure of Ti thin films

Ti films of different thickness ranging from 12.3 to 246.2 nm were deposited, using resistive heat method and post-annealed at different temperatures with a flow of 5 cm³ s⁻¹ oxygen. The nano-structures of the films were obtained using X-ray diffraction (XRD) and atomic force microscopy (AFM). The results showed an initial reduction of the grain size at 373 K annealing temperature and increase of the grain size at higher temperatures. The cause of this was due to the reaction of oxygen with Ti atoms which breaks up the Ti grains and hence needle-like features form. The enhancement of activation processes at higher temperatures results in larger grains. The analysis of XRD in conjunction with AFM images showed that those films containing (004) line of anatase phase and sub-oxide phases of titanium oxide also show two types of grains in the AFM images. The resistivity of the film increased with annealing temperature, which is due to competition between increased diffusion rate and the increased reaction rate of oxygen with Ti atoms. The Hall coefficient R_H and the mobility μ decreased with increasing film thickness at all annealing temperatures, while R_H increases and μ decreases with increasing the annealing temperature. The carrier concentration increased with film thickness and decreased with annealing temperature. The impedance spectroscopy showed that all films have a pure RC behaviour, where the magnitude of R depends on the annealing temperature and film thickness. The apparent activation energies E_a, obtained from three different methods, namely σ, R_H and grain size showed good agreement within 0.30-0.46 eV for the range of film thickness examined in this work. It was found that films with thickness less than 70 nm can be recognized as Ti-oxide films while thicker films are only surface-oxidised Ti films.     

Film thickness dependence of microstructures and magnetic properties in single-layered FePt films by in-situ annealing

The FePt films with various thicknesses (t) of 5 to 50 nm are deposited on Si(100) substrate without any underlayer by in-situ annealing at substrate temperature (Ts) of 620 °C. A strong (001) texture of L1₀ FePt film is obtained and presents high perpendicular magnetic anisotropy as the film thickness increases to 30 nm. By further increasing the thickness to exceed 30 nm, the (111) orientation of L1₀ FePt is enhanced greatly, indicating that the quality of perpendicular magnetic anisotropy degrades when the thickness of the FePt film is greater than 30 nm. The single-layered FePt film with thickness of 30 nm by in-situ depositing at 620 °C shows good perpendicular magnetic properties (perpendicular coercivity of 1033 kA/m (13 kOe), saturation magnetization of 1.08 webers/m² and perpendicular squareness of 0.91, respectively), which reveal its significant potential for perpendicular magnetic recording media.     

Influence of magnetic annealing on high-frequency magnetic properties of FeCoNd films

FeCoNd thin film with thickness of 166 nm has been fabricated on silicon (1 1 1) substrates by magnetron co-sputtering and annealed for one hour under magnetic field at different temperatures (T_a) from 200 °C to 700 °C. The As-deposited and annealed FeCoNd film samples at T_a ≤ 500 °C were amorphous while the ones obtained at T_a ≥ 600 °C were crystallized. We found that the perpendicular anisotropy field gradually decreases as the annealing temperature increases from room temperature to 300 °C. A well induced in-plane uniaxial anisotropy is achieved at the annealing temperature between 400 and 600 °C. The variation of the dynamic magnetic properties of annealed FeCoNd films can be well explained by the Landau-Lifshitz equation with the variation of the anisotropy field re-distribution and the damping constant upon magnetic annealing. The magnetic annealing might be a powerful post treatment method for high frequency application of magnetic thin films.     

Microwave Measurements of Overdoped Y0.9Ca0.1Ba2Cu3O7−δ Thin Films

In the present report we show, for the first time, that a complex order parameter is not just a surface effect, but a bulk property in overdoped Y_0.9Ca_0.1Ba₂Cu₃O_7– thin films. The penetration depth change versus temperature is very well fitted to a complex order parameter of the form d_x ^{2-y 2} + id _xy , where = 14.5 meV and = 2meV. This result contrasts our previous results for optimally doped YBCO thin films where a power law for the penetration depth, versus temperature was reported, corresponding to a d_x ^{2-y 2}-wave order parameter. The penetration depth behavior is accompanied by a rapid reduction of the scattering rate at low temperatures, in contrast with a flattening of this quantity for the optimally doped YBCO films.     

Analysis of the lattice thermal conductivity of thin films by means of a modified Mayadas-Shatzkes model: The case of bismuth films

The lattice thermal conductivity λ_øx(d, T) of bismuth films in a direction of the film plane (x direction) with the thicknesses d ranging from 20 to 400 nm was determined in the temperature range 80 K ⩽ T ⩽ 400 K from the measured total thermal conductivity λ_x(d, T) and the calculated charge carrier contribution λ_ex(d, T). A modified Mayadas-Shatzkes model of phonon scattering on polycrystalline films is presented. Following this model the thickness and temperature dependence of λ_øx(d, T) can be interpreted.     

High field critical current densities and resistive transition of NbCN superconducting films

We studied the influence of the carbon density on the critical current densities of niobium carbonitride films in high magnetic fields. The samples were about 2 200 Å thick and were obtained by reactive sputtering either from a Nb-C target in a residual Ar + N₂ atmosphere or from a bulk Nb target in a residual Ar + N₂ + CH₄ atmosphere. They all had sufficiently high critical temperature, around 15 K, which implies that the ratio of carbon atoms over the total supplementary nitrogen and carbon atoms lay in the range 0.06 to 0.35. The samples were immersed in the liquid helium at 4.2 K. The registered V(I) curves either showed an hysteretic feature or not, depending on the composition and/or the applied magnetic field. The best critical current densities, about several 10¹⁰ Am^?2 in a zero field were still more than 3 × 10⁸ in a 14 T field, irrespective of direction, transverse or perpendicular. In the former case, some samples were tested up to 18 T, a value for which the best critical density was 10⁷ Am^?2. Heterogeneous samples which have periodic variations in carbon concentration, have still greater critical densities in high transverse fields. The J_c(B) curves show a plateau or a slight hump when the field is such that the flux line lattice parameter fits the period of the carbon ratio variations. The resistivities of the different films are also high enough, more than 10^?6 Ωm. So these alloys might be of interest in a superconducting switchgear, using the resistive transition to the normal state in order to limit a short circuit current.     

Ginzburg-Landau theory for three-dimensional Josephson junction arrays

We have derived a mean-field theory and a Ginzburg-Landau (GL) theory for classical three-dimensional Josephson junction arrays. We show that the full GL theory for arrays is formally equivalent to the GL theory for a bulk superconductor, demonstrating that for low fields and close to the critical temperature, the two systems are electrodynamically equivalent. We derive expressions for the penetration depth and coherence length, the effective superfluid density, critical fields, and the current-phase relationship for arrays. Using the Ginzburg criterion, we show that the critical region is very wide and that three-dimensional critical fluctuations should be experimentally accessible. Although the wide critical region casts doubt on the strict validity of the GL theory, suitable modifications of the coefficients should allow it to be used at lower temperatures, as is done for bulk superconductors.     

First- and second-order superconducting phase transitions of tin whiskers in a magnetic field

Using a sensitive inductance technique, first- and second-order phase transitions in tin (type I) whiskers have been observed in both parallel and perpendicular magnetic fields. ForT much less thanT _c the samples exhibit considerable superheating and supercooling from which values of the Ginzburg-Landau parameter are calculated. However, these values are not as low as those obtained for single spheres (diameters 10–30 µ), probably due to mean free path and size effects. Close to the transition temperature it is observed that the hysteresis vanishes indicating the onset of a second-order phase transition. Good qualitative agreement between these results and theoretical predictions based on the Ginzburg-Landau theory are obtained. Size effects on the ratioH _C3/H _C2 H _SC /H _C2 are clearly illustrated.     

Effect of ion irradiation on the structural and magnetic properties of sputtered CoPt alloy

We are interested in studying thin layers of CoPt alloy prepared by co-deposition sputtering. The CoPt layers are deposited on a MgO(001) substrate with a Pt(001) buffer layer. We obtain a layer with the L1₀ tetragonal structure, ordered in the growth direction. This equiatomic L1₀ phase is a “natural” multilayer which consists in a stacking along the (001) direction of pure Co and pure Pt monolayers. In this case, we get an easy magnetization direction perpendicular to the layer plane, which is the required configuration for the perpendicular magneto-optic recording. The order state of these thin films has been modified locally by a low energy ion irradiation. The structural and magnetic properties have changed jointly: with a 40-keV energy and a 4×10¹⁶-ions/cm² flux, the chemical order decreases (the long-range-order (LRO) parameter changed from S=0.80 to S=0.50), the coercive field diminished from 0.090 T down to 0.052 T. Such results show that it is possible to locally change the magnetization easy axis, which is promising for applications.     

The electrical conductivity of copper and tin thin films vacuum deposited in a lateral electric field

Copper and tin thin films of different thicknesses in the coalescence thickness range (160, 210 and 260 Å) were grown by vacuum deposition onto clean glass substrates at room temperature under a pressure of 2 × 10^?5 Torr in different electric fields between 0 and 200 V cm^-1 aligned parallel to the substrates. The electrical conductivities of the films were measured immediately after formation both in situ and after letting air into the system.We found that the application of an electric field of less than 60 V cm^-1 decreased the resistance of the films. However, the application of stronger electric fields produced an increase in the film resistance. This is attributed to a disturbance of the electrostatic forces between the growing islands by the redistribution of charges in the presence of the applied electric field which causes an increased rate of coalescence of the islands in the film.