Surface resistance of large-area Tl2Ba2CaCu2O8 thin films at microwave and millimeter wave frequencies measured by three noncavity techniques

The surface resistanceR _s of Tl₂Ba₂CaCu₂O₈ films fabricated on LaAlO₃ wafers up to 3 inches (7.6 cm) in diameter through a post-deposition anneal process was measured over the frequency range 5.55–94.1 GHz by the following techniques: 5.55 and 27.5 GHz high-temperature superconductor (HTS)-sapphire resonators, 10 GHz parallel plate resonator, and 94.1 GHz scanning confocal resonator.R _s was found to exhibit a quadratic dependence on frequencyf at 77 K:R _s ∝f ^2.0±0.1. The highest-quality films yieldR _s =145±15μΩ at 10 GHz and 77 K. Scanning confocal resonator mapping ofR _s across a 2-inch (5.1 cm) diameter wafer yielded a base value forR _s of 16±1 mΩ at 77 K and 94.1 GHz (equivalent to 180±10μΩ at 10 GHz) and good uniformity inR _s across the wafer. HTS-sapphire resonator measurements ofR _s for fifteen 1.2 cm square parts cut from a 3-inch diameter wafer yieldedR _s values scaled to 10 GHz of 196±10μΩ at 80 K. Similar values were measured for Tl₂Ba₂CaCu₂O₈ films prepared on both sides of a 2-inch diameter wafer.Rs values at 10 GHz and 80 K of 147?214μΩ were maintained over the course of 40 independent and successive deposition runs and corresponding anneals under nominally identical film fabrication conditions. Surface resistance at 5.55 GHz remained below 80μΩ for maximum rf magnetic fields up to 85 Oe at 4.2 K and 7 Oe at 80 K, respectively. Results are compared with predictions of the two-fluid model. The relative advantages and disadvantages of the different techniques for measuring surface resistance are discussed.     

Investigation of germanium films and GeSi interface structure by transmission electron microscopy

The structure of germanium films (d≌0.3 μm) evaporated onto a silicon substrate and the GeSi interface have been investigated by transmission electron microscopy (TEM). Ge was evaporated in a vacuum of approximately 10^?6 Torr onto (111) Si. Epitaxial growth was observed at substrate temperatures T_s>500°C. The films grown at T_s = 520°–600°C (low temperature epitaxy) were characterized by a high density of stacking faults (SF), microtwin lamellae and dislocations which lay normal to the film surface. In this case the interface dislocations were regular and the dislocation density was equal to (3–4)×10¹⁰ cm^?2. At T_s = 700°–850°C (high temperature epitaxy) few stacking faults were discovered. Dislocations in the interior of the film formed three-dimensional networks as a result of interactions of different slip systems and dislocation climb. The formation of misfit dislocations was apparent at the interface. The region T_s = 630°–700°C is an intermediate region.     

Effect of substrate temperature on ac conduction properties of amorphous and polycrystalline GaSe thin films

X-ray diffraction analysis of GaSe thin films used in the present investigation showed that the as-deposited and the one deposited at higher substrate temperature are in amorphous and polycrystalline state, respectively. The alternating current (ac) conduction properties of thermally evaporated films of GaSe were studied ex situ employing symmetric aluminium ohmic electrodes in the frequency range of 120-10⁵ Hz at various temperature regimes. For the film deposited at elevated substrate temperature (573 K) the ac conductivity was found to increase with improvement of its crystalline structure. The ac conductivity (σ_ac) is found to be proportional to (ω^s) where s < 1. The temperature dependence of ac conductivity and the parameter, s, is reasonably well interpreted by the correlated barrier-hopping (CBH) model. The maximum barrier heights W_m calculated from ac conductivity measurements are compared with optical studies of our previous reported work for a-GaSe and poly-GaSe thin films. The distance between the localized centres (R), activation energy (ΔE_σ) and the number of sites per unit energy per unit volume N(E_F) at the Fermi level were evaluated for both a-GaSe and poly-GaSe thin films. Goswami and Goswami model has been invoked to explain the dependence of capacitance on frequency and temperature.     

Conduction and dielectric polarization in Se thin films

Se films were prepared by thermal evaporation technique in thickness range 150-8500 Å. X-ray diffraction measurements showed that Se films are in the amorphous state. The ac conductivity and dielectric properties of the amorphous Se films have been investigated in the frequency range 100-100 KHz and 100-400 K temperature range. The ac conductivity σ_ac(ω) is found to be proportional to ω^s where s<1. The temperature dependence of both ac conductivity and the parameter s is reasonably well interpreted by the correlated barrier (CBH) model. The dc conductivity at the room temperature was also studied in the same thickness range. It was concluded that the same mechanism of carrier motion might be dominant in both ac polarization and dc conduction. This carrier transport mechanism might be electronic.     

Electrical characterization of amorphous germanium dioxide films

Amorphous germanium dioxide (GeO₂) films have been deposited by electron beam evaporation onto different substrates including glass, SnO₂ conducting glass, evaporated gold and n-type silicon in order to examine the electrical behaviour of GeO₂ in metal/insulator/metal (MIM) and metal/insulator/semiconductor (MIS) structures. In MIM structures the as-deposited films are strongly influenced by electrode barriers but heat treatment at 600 K induced ohmic behaviour. The dielectric response of the films in the frequency range 0.1–100 kHz and the temperature range 180–350 K showed that the dielectric constant at 300 K was 9 and was virtually independent of frequency, while the a.c. conductivity follows the relation σ ∞ ω^s, where s is temperature dependent. Good agreement with a classical electronic hopping model is obtained. In MIS structures, GeO₂ on silicon gives rise to heterojunction behaviour at low voltages while, at higher voltages, the d.c. conduction is bulk dominated and exhibits space-charge-limited conduction. The dielectric response of MIS structures is strongly influenced by the depletion capacitance at the interface between GeO₂ and silicon.     

Measurement of the surface resistance of YBa2Cu3O7−x thin films using stripline resonators

We review methods of measuring surface resistance (R _s ) of thin films using stripline resonators, and present our measurements of theR _s of YBa₂Cu₃O_7?x films as a function of frequency, temperature, and r.f. magnetic field. The films were deposited on LaAlO₃ substrates by two methods: (1) electron-beam coevaporation of Y, BaF, and Cu followed by annealing in O₂, and (2) single-targetin situ sputtering. The measurements were obtained at frequencies from 0.4 to 10 GHz, temperatures from 4 to 90 K, and an r.f. magnetic field range from 0 to 30 Oe. At low temperature and low r.f. field at 0.4 GHz, theR _s values obtained for the two deposition methods are approximately 7×10^?6 and 4×10^?6 Ω, respectively.     

Parallel-plate resonator of variable spacer thickness for accurate measurements of surface impedance of high-T c superconductive films

Measurements of microwave surface impedance of high-T _c films at gigahertz frequencies and nitrogen temperature are performed. A simple technique employing a parallel-plate resonator with liquid nitrogen as a dielectric spaces is suggested. The use of a precise mechanical device provides smooth changing of distance between films from 200μm down to zero. Coupling to the resonator is accomplished by means of two small antennas-half-wave vibrators for frequency 10 GHz. The method for determining resistivity and magnetic field penetration depth was based on the analysis of spacer thickness dependences of the resonator quality factor and frequency. YBa₂Cu₃O₇ films produced by a laser deposition technique on CaNdAlO₄ substrates withT _c =91 K andj _c =10⁷ A/cm² and on NdGaO₃ substrates withT _c =91 K andj _c =10⁶ A/cm² are examined, and the valuesR _s =0.6 mΩ,λ=348 nm atf=8.97 GHz andR _s =0.5 mΩ,λ=250 nm atf=10.12 GHz, respectively, are obtained at 77 K.     

Dielectric losses and ac conductivity of Si–LiNbO3 heterostructures grown by the RF magnetron sputtering method

Single-axis <0001> textured polycrystalline LiNbO₃ films were grown on (001) Si substrates by the RF magnetron sputtering method. Dielectric losses that occur in the Si–LiNbO3 heterostructures are caused by the conductivity of the LiNbO3 films. Analysis of temperature and frequency dependence of ac conductivity in the frequency range f = 25/10⁵ Hz has demonstrated that it is expressed by the power law σ(ω) = Aω^s and is described in the framework of the correlated barrier-hopping model. Thermal annealing (TA) of the Si–LiNbO3 heterostructures causes an increase in the density of the localized states in the band gap of LiNbO3 from D = 7 × 10²⁴ m^?3 to D = 2 × 10²⁵ m^?3. The conduction mechanism is changed radically after TA and phonon-assisted tunneling influences ac conductivity at the frequency of up to 800 Hz. At high frequency (f > 800 Hz), dielectric relaxation predominates affecting frequency dependence σ(ω) on relaxation time τ = 6.6 × 10^?5 s.     

The electrical properties of CuInSe2 thin films deposited onto CaF2 substrates

CuInSe₂ thin films were deposited onto (111)-oriented CaF₂ substrates by flash evaporation in the substrate temperature range T_s = 650–890 K. Epitaxial growth was found at T_s = 770–800 K; at lower and higher substrate temperatures the films were partly polycrystalline. Films produced at T_s ? 675 K showed n-type conductivity due to a donor with an ionization energy of 78 ± 5 meV ascribed to indium interstitials. At T_s ? 725 K the films exhibited p-type conductivity due to a shallow acceptor with an ionization energy characteristic of indium vacancies. The important role of the substrate material in establishing the electrical parameters of CuInSe₂ thin films is proved by a comparative study of thin films deposited onto GaAs and CaF₂ substrates.     

Preparation and dielectric properties of Si3N4 thin films

Thin Si₃N₄ films of thickness 50–1500 nm were prepared by a low pressure, room temperature chemical vapour deposition process. The dielectric properties of the layers were studied in the frequency range from 10 Hz to 1 MHz and at temperatures between 77 and 400 K. We observed a σ'(ω, T) = A(T)ω^s(T) dependence with s?1 (where σ' is the real part of the a.c. conductivity). For samples prepared in various ways, A varied between a constant value and a superlinear dependence on temperature. At 77 K, however, σ' for all the samples was found to be approximately the same (about 8 × 10^?12 Ω^?1 cm^?1 at 1 kHz) irrespective of the preparation parameters.     

Ac conductivity and dielectric properties of Ge20Se75In5 films

Amorphous films of Ge₂₀Se₇₅In₅ chalcogenide glass were prepared using a thermal evaporation technique. The chemical composition of the deposited films was examined using energy dispersive X-ray spectroscopy (EDX). The ac conductivity and dielectric properties of the prepared films have been studied as a function of temperature in the range from 300 to 423 K and frequency in the range from 10² to 10⁵ Hz. The experimental results indicate that ac conductivity σ_ac(ω) is proportional to ω^s where s equals 0.902 at room temperature and decreases with increasing temperature. The results obtained are discussed in terms of the correlated barrier hopping (CBH) model. The density of localized states N(E_F) at the Fermi level is found to have values of the order 10¹⁹ eV⁻¹ cm⁻³, which increase with temperature. The dielectric constant ?₁ and dielectric loss ?₂ were found to decrease with increasing frequency and to increase with increasing temperature over the ranges studied. The maximum barrier height W_m was estimated from an analysis of the dielectric loss ?₂ according to Giuntini equation. Its value for the deposited films (0.43 eV) agrees with that proposed by the theory of hopping of charge carrier over a potential barrier as suggested by Elliott for chalcogenide glasses.     

Fabrication and characteristics of porous germanium films

Abstract

Porous germanium films with good adhesion to the substrate were produced by annealing GeO₂ ceramic films in H₂ atmosphere. The reduction of GeO₂ started at the top of a film and resulted in a Ge layer with a highly porous surface. TEM and Raman measurements reveal small Ge crystallites at the top layer and a higher degree of crystallinity at the bottom part of the Ge film; visible photoluminescence was detected from the small crystallites. Porous Ge films exhibit high density of holes (10²⁰ cm^?3) and a maximum of Hall mobility at ～225 K. Their p-type conductivity is dominated by the defect scattering mechanism.     

Effects of methane flow rate on the optical properties and chemical bonding of germanium carbon films deposited by reactive sputtering

Amorphous hydrogenated germanium carbon (a-Ge_1−xC_x:H) films were prepared by radio frequency (RF) reactive magnetron sputtering of a pure Ge (111) target in a CH₄ + H₂+Ar mixture and their composition, optical properties, chemical bonding were investigated as a function of gas flow rate ratio of CH₄/(Ar + H₂). The results showed that the deposition rate first increased and then decreased as gas flow rate ratio of CH₄/(Ar + H₂) was increased from 0.125 to 0.625. And the optical gap of the a-Ge_1−xC_x:H films increased from 1.1 to 1.58 eV accompanied with the increase in the carbon content and the decrease in the relative content of Ge–C bonds of the films as the CH₄ flow rate ratio was increased, while refractive index of the films decreased and the absorption edge shifted to high energy. Through the analysis of X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy, it was found that the formation of Ge–C bonds in the films was promoted by low CH₄ flow rate which is connected with relatively high H₂ concentration and Ge content. Especially in low CH₄ concentration, the formation of sp²-hybridised C–C bonds was suppressed considerably both due to the etching effect on weak bonds of H and the fact that chemical bonding for germanium can be only sp³ hybridization.     

Fabrication and characteristics of porous germanium films

Porous germanium films with good adhesion to the substrate were produced by annealing GeO₂ ceramic films in H₂ atmosphere. The reduction of GeO₂ started at the top of a film and resulted in a Ge layer with a highly porous surface. TEM and Raman measurements reveal small Ge crystallites at the top layer and a higher degree of crystallinity at the bottom part of the Ge film; visible photoluminescence was detected from the small crystallites. Porous Ge films exhibit high density of holes (10²⁰ cm⁻³) and a maximum of Hall mobility at ∼225 K. Their p-type conductivity is dominated by the defect scattering mechanism.     

Microwave surface resistance of epitaxial YBa2Cu3O7 thin films at 18.7 GHz measured by a dielectric resonator technique

We used a dielectric resonator technique for highly sensitive measurements of the temperature dependence of the microwave surface resistanceR _s of 1×1 cm² superconducting films at 18.7 GHz. It consists of a sapphire disc positioned on the film under investigation within a copper cavity which is acting as a radiation shield. In the TE_01δ oscillation mode the highly reproducible quality factor of about 10⁵ results in a sensitivity of ±50μΩ forR _s measurements. The temperature dependence ofR _s can be measured up to values as high as 1 Ω. We have investigated several YBa₂Cu₃O₇ thin films prepared by high oxygen pressure d.c. sputtering on LaAlO₃ and NdGaO₃. Our best films exhibit a pronounced nonlinear behavior of the d.c. resistivityρ(T) withρ(300K)/ρ(100K) values of about 3.7. Those films show, besides the initial fall-off just belowT _c , a further strong decrease ofR _s at low temperatures. This was observed both at 18.7 GHz and 87 GHz, as measured by a conventional cavity end plate replacement technique. ForT?T_c/2 these films exhibit an exp (?αT _c/T) dependence ofR _s withα-values around 0.4. These observations may be explained by a superconducting energy gap with 2Δ/kT _c≈0.8 for charge carriers localized in the CuO chains for YBa₂Cu₃O₇.     

Deposition temperature effect on the electric and dielectric properties of InSbSe3 thin films

Thin films of InSbSe₃ compound were obtained by thermal evaporation on to clean glass substrates maintained at various deposition temperatures from 423 to 593 K. At deposition temperature T_d?473 K, the films have an amorphous structure, while those prepared at T_d>473 K have a polycrystalline structure identified by X-ray diffraction analysis. The DC electrical conductivity of the films increases as T_d increases, whereas activation energy decreases with increasing T_d, which reflects a change in the degree of disorder. AC conductivity was studied as a function of frequency in the range (10²-10⁵ Hz) and as a function of deposition temperature. The dependence of T_d on the frequency exponent s in the conductivity-frequency relation confirmed that the mechanism of AC conductivity is correlated barrier hopping with a single polaron hopping mechanism. The discrepancy between DC and AC activation energies was studied as a function of deposition temperature. The maximum barrier height W_m and the density of defect states N were also determined. Finally, the dependence of dielectric constant and dielectric loss on T_d were studied. A Debye-like relaxation of dielectric behavior was observed for crystalline films and is found to be a thermally activated process. The position of maximum dielectric loss is shifted towards higher temperature with T_d treatment and there by reduces the relaxation time.     

Electrical relaxations in polymeric particulate composites of epoxy resin and metal particles

Composites of epoxy resins and nickel particles in various amounts were prepared and their dielectric spectra were measured in the frequency range 5 Hz–13 MHz and temperature interval from ambient to 140°C.The formalism of electric modulus proved to be efficient in analysing and interpreting obtained data.For these composites two relaxation processes are revealed in the frequency range and temperature interval of the measurements.One is an interfacial dielectric relaxation (Maxwell–Wagner–Sillars), MWS and the other is a conductivity relaxation. They both follow the Cole–Davidson approach with the exponent γ reflecting a distribution of relaxation times with the characteristics of each process.AC conductivity of these composites is frequency and temperature dependant, it generally follows the exponential law σ_ac∼ω^s and reveals a conductivity relaxation process, in the low frequencies.     

Dielectric behavior of polyaniline–CNTs composite in microwave region

Films of polyaniline (PANI) and polyaniline–CNTs composites have been synthesized by solution casting technique. Fourier transform infrared spectroscopic studies of PANI and PANI–CNTs composite films indicated the presence of interaction between CNTs and molecular chains of PANI. Dielectric properties of PANI and composite films have been investigated in the frequency range of 8.0–12.0 GHz. The real part of permittivity (ε′) and loss factor (tan δ) were found to be higher in PANI–CNTs composite films as compared to the PANI film. The increasing behavior of ε′ and tan δ has been attributed to the interaction present between CNTs and PANI molecular chains and increase of conductivity of PANI films after incorporation of CNTs.     

AC conductivity and dielectric properties of Sb2Te3 thin films

Sb₂Te₃ films of different thicknesses, in the thickness range 300-620 nm, were prepared by thermal evaporation. X-ray analysis showed that the as-deposited Sb₂Te₃ films are amorphous while the source powder and annealed films showed a polycrystalline nature. The AC conductivity and dielectric properties of Sb₂Te₃ films have been investigated in the frequency range 0.4-100 kHz and temperature range 303-373 K. The AC conductivity σ_AC(ω) was found to obey the power law ω^s where s?1 independent of film thickness. The temperature dependence of both AC conductivity and the exponent s can be reasonably well interpreted by the correlated barrier hopping (CBH) model. The experimental results of the dielectric constant ε₁ and the dielectric loss ε₂ are frequency and temperature dependent and thickness independent. The maximum barrier height W_M calculated from dielectric measurements according to the Guintini equation agrees with that proposed by the theory of hopping of charge carriers over a potential barrier as suggested by Elliott for chalcogenide glasses. The effect of annealing at different temperatures on the AC conductivity and dielectric properties was also investigated. Values of σ_AC, ε₁ and ε₂ were found to increase with annealing treatment due to the increase of the degree of ordering of the investigated films. The Cole-Cole plots for the as-deposited and annealed Sb₂Te₃ films have been used to determined the molecular relaxation time τ. The temperature dependence of τ indicates a thermally activated process.     

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.