High thermoelectric figure-of-merit in kondo insulator nanowires at low temperatures

We predict a large thermoelectric figure-of-merit in Kondo insulator nanowires at low temperatures. The high ZT values are due to the Kondo effect for electrons and boundary scattering on phonons. We simulated the electron properties of the bulk Kondo insulators within the framework of dynamical mean field theory and found that electrons have short mean free path. In nanowire structures, electron transport is hardly affected by the boundary scattering due to their small intrinsic mean free paths while phonons are strongly scattered due to classical size effect. The results suggest that the nanostructures of Kondo insulators can be designed for high performance thermoelectric cooling devices at low temperatures.     

Oxidation of thin titamium films in ambient conditions

The room temperature oxidation of vapor-deposited titanium films was investigated as a function of film thickness by resistivity and ellipsometric measurements. The thickness of the films ranged from 3.0 to 120 nm. The electron mean free path in the films varied from 11.3 to 26.0 nm, and the product of the bulk resistivity and the electron mean free path was 1.5 × 10^-10Ω cm². The values of the surface electron scattering parameter in the films were between 0.2 and 0.6 and decreased to 0.18 at the onset of oxidation. The best value for the complex refractive index of titanium was found to be 3.61-i4.06. The refractive index of the oxide film was 2.75. After a 1 d exposure of the films to room air at atmospheric pressure both the resistance and ellipsometric measurements indicated a residual oxide thickness of 2.6±0.3 nm, regardless of the original metal film thickness. These results disagree with the theoretical and experimental results of Mindel and Pollack, which implied a rapid decrease in oxide thickness with decreasing titanium film thickness.     

Hall effect in chromium-copper alloy films

Chromium-copper alloy was deposited in thin film form by vapor deposition at room temperature onto well cleaned glass substrates at a pressure of 10⁻⁵ Pa. The polycrystalline alloy thus formed was vacuum annealed up to a temperature of 525 K. Hall coefficient R_H and Hall mobility μ have been measured for annealed polycrystalline chromium-copper alloy films at 300 K. Below 50 nm, a film thickness dependence (size effect) was observed. Data on polycrystalline alloy film agree well with Sondheimer theory for perfect diffuse scattering so that the best fit mean free path value can be calculated. The calculated mean free path value (37 nm) agrees fairly well with the mean free path value reported from electrical resistivity data (37.6 nm) and temperature coefficient of resistivity data (36.3 nm) for perfect diffuse scattering. The decline of the Hall mobility is expected theoretically. The bulk Hall coefficient and bulk mobility of the alloy can be predicted from the experimental data and comes out to be −5.2 × 10⁻⁵ cm³/coulomb and 13.57 cm²/v-sec respectively. The results are discussed.     

A new method for studying surface scattering of electrons in thin metallic plates

A determination of the surface scattering, mean free path, and bulk resistivity of electrons in a specimen requires three independent experiments. A new method is proposed making use of the measurement of the first and second terms in the expansion of a decaying eddy current in a thin plate. The decay rates have been calculated and tabulated using the free electron model and assuming an isotropic electron mean free path.     

Electrical properties of thin oxidized aluminium films

Thin aluminium films of thickness 40 to 200 nm were deposited on to glass substrates at 573 K in a high vacuum. The deposition was carried out layer by layer and the interfaces between these layers were exposed to oxygen. The electrical resistivity was studied as a function of the film thickness, annealing time, annealing temperature and oxygen pressure. The temperature coefficient of resistivity and the activation energy for the conduction electrons were studied as a function of the film thickness and oxygen pressure. Fuchs-Sondheimer theory for electrical conduction was applied to the experimental results. The mean free path of the conduction electrons was calculated as a function of temperature and agreed well with the theoretical relation.     

Linear variations in conductivity with thickness of thin polycrystalline films

Starting from a three-dimensional scattering model and from the mean free path representing the effect of electronic scattering on external surfaces, a linear analytic expression is proposed for thin film conductivity and resistivity. Good agreement with experiment is found.     

Étude de la conductibilité électrique de couches minces de chrome préparées par évaporation thermique. Détermination du libre parcours moyen et de la densité des électrons de conduction

The electrical conductivity of chromium thin films evaporated in vacuum on glass substrates is explained by means of a simple model which enables the mean free path and the density of the conduction electrons in chromium to be determined. It is supposed that the films are composed of agglomerates connected by metallic bridges and that the lateral dimensions of the agglomerates are constant for thicknesses between 50 and 1600 Å, whereas the height is equal to that of the film. Taking into account the variation of the resistivity of the films due to adsorption of the residual gases of the vacuum chamber and applying Fuchs-Sondheimer's theory, values for the mean free path and the density of the conduction electrons in bulk chromium are obtained, respectively equal to 320 Å and 5.3 × 10²¹ electrons/cm³.     

Description of electronic transport properties of monocrystalline films by a statistical model

Grain boundaries in monocrystalline films are represented by a two- dimensional array of scatterers. The effects of electronic scattering are calculated by means of Matthiessen's rule starting from the electronic mean free path related to a particular type of scattering (bulk, grain boundary, external surface).

Theoretical expressions for the film resistivity and its temperature coefficient of resistivity and thermoelectric power are proposed; comparison with previously reported experiments gives a satisfactory fit.     

Electron scattering and the lattice thermal conductivity of superconducting transition metal alloys

The mechanism of heat energy transport by phonon carriers which are primarily scattered by conduction electrons is re-examined for concentrated alloys in the normal and superconducting phases. The large increase in the normal state lattice thermal conductivity observed experimentally for concentrated niobium alloys, compared with the pure metal, is explained qualitatively as due to the diminished importance of scattering by d-electrons, compared with s-electrons, when the mean free path for the former becomes less than the typical phonon wavelength. Evidence is presented for the existence of a gap-less type of superconductivity in the concentrated alloys for both s and d-electrons.     

Angular and energy distribution of backscattered electrons simulated by Monte-Carlo—Assessment by experiment I

The angular and energy distributions of backscattered electrons for the primary energies of 0.2, 0.5 and 1 keV and for Cu and Au were calculated by Monte-Carlo method and compared to measurement. The elastic differential cross-sections were calculated by ELSEPA code. For inelastic scattering a modified Tougaard's model of universal cross-section was used. We fit inelastic mean free path (IMFP) obtained from energy loss distribution, given by the modified Tougaard's model, in order they agreed with the value of IMFP from the TPP-2 formula. By this way we obtained an energy-dependent correction of the constants in Tougaard's formula. A very good agreement was found for the calculated and measured values of angular distribution of elastically reflected electrons. For the energy distribution of backscattered electrons some differences between experiment and calculation appeared, which were probably caused mainly by very low energy losses, e.g. losses at exiting of bremsstrahlung radiation.     

Random media characterization using the analysis of diffusing light data on the basis of an effective medium model

The transport properties of dense random media such as rutile powder layers and polyball suspensions are analyzed in visible and near infrared on the basis of experimental data on coherent backscattering, diffuse transmittance, and low-coherence interferometry. The developed technique of retrieval of the transport parameters of examined scattering media allows the evaluation of the transport mean free path l* and the effective refractive index n(ef) of the medium without a priori knowledge of the optical properties of the scattering particles. It is found that with decreasing wavelength lambda(0) the value of localization parameter 2pin(ef)l*/lambda(0) of the studied rutile samples abruptly drops and approaches approximately 2.6 at 473 nm. This peculiarity is caused by the very large scattering efficiency of scatterers in the vicinity of the first Mie resonance.     

Size effects in double-layer metallic films

An analysis is made of the electrical conductivity of double-layer, continouos, metallic films using the free-electron Boltzmann equation with elastic and isotropic bulk scattering. The electron-surface scattering is assumed to be specular. The analysis shows that a size effect can be expected when the thickness of the double-layer is of the same order of magnitude as the mean free path of the conduction electrons in the better conductor. The theoretical results are compared with the results obtained using the Fuchs-Sondheimer formalism and assuming diffuse electron-boundary scattering. The theoretical results are also related to published experimental results with superimposed metallic films.     

Sputtering yields of thin overlayer films by attenuation of the substrate Auger electron signal

The sputtering yield S of the first oxide layers of 304 stainless steel has been measured by monitoring the bulk MVV 48 eV Auger electron intensity of elemental iron during ion etching. For comparison the same procedure has been applied to a gold layer of thickness 65 Å on a copper substrate for 920 eV copper Auger electrons. Assuming literature values of the mean free path, the experimental S values for stainless steel and gold under 6 keV Ar⁺ ion bombardment with a 30° angle of incidence are 1 and 7 respectively.

The same technique gives information about overlayer film continuity and non-uniformity.     

Energy dependence of transport parameters derived from correlated variations in the thermoelectric power and temperature coefficient of resistivity of polycrystalline metal films

Literal and general expressions of the thermoelectric power (TEP) of polycrystalline metal films are derived in the framework of the recently proposed three-dimensional model of conduction. Theoretical plots of the thickness dependence of film TEP are given and show that the TEP of infinitely-thick polycrystalline film markedly depends on the grain parameter. These theoretical results, as well as the general relation which expresses the film TEP in terms of the film temperature coefficient of resistivity (TCR), suggest two convenient ways for graphically determining the energy dependence of the bulk mean free path and the Fermi surface area.     

Size and temperature dependence of thermoelectric power and electrical resistivity of vacuum-deposited antimony thin films

Thin antimony films of thicknesses in the range 30 to 200 nm have been vacuum deposited on glass substrates at room temperature. After annealing for about an hour at 500 K, the thermoelectric power and electrical resistivity were measured in vacuum as a function of temperature. The thermoelectric power and electrical conductivity data were combined and simultaneously analysed using the effective mean free path theory of size effect in thin films developed by Tellier and Pichard et al. In addition, their temperature dependence was also analysed. It was found that the thermoelectric power is positive and increases with increasing temperature and is inversely proportional to the thickness of the film. The electrical resistivity was found to be temperature dependent with the temperature coefficient of resistivity being positive, and inversely proportional to the thickness of the film. Analysis combining the data from the thermoelectric power and electrical conductivity measurements has led to the determination of mean free path, carrier concentration, effective mass, Fermi energy and the parameter The data were analysed for least squares fitting by local functions, such as the spline functions, which eliminates possible errors in conventional least squares fitting of data using non-local functions valid throughout the range.     

One-dimensional spin-glass

A model of a one-dimensional spin-glass is studied; this is a system of localized spins with indirect exchange interaction via the conduction electrons. It is assumed that the electrons are scattered at defects and their mean free path is less than the average interspin distance. Specific heat and magnetic susceptibility are calculated in various temperature regions. The properties in a strong magnetic field are determined. The case of a long-range interaction is briefly discussed.     

Penetration depth for diffusing-wave spectroscopy

The depth at which diffusing photons are assumed to be deposited in a random scattering medium has traditionally been treated as a phenomenological parameter comparable to the photon transport mean free path. We show how to average properly over an exponential distribution of depositions weighted additionally by the transmission probability, and compare our prediction for the autocorrelation of intensity fluctuations in the transmitted light with experimental data on an ideal system. The improved correlation function, where distinguishable from the prior form, provides slightly better agreement with data as long as the sample is thicker than approximately 10 transport mean free paths. However, in contrast with static transmission, proper averaging over a range of penetration depths does not extend the validity of diffusing-wave spectroscopy to significantly smaller slab thicknesses. The most significant errors in the theory must therefore arise from approximations other than the treatment of the source of diffusing photons.     

Thermoelectric properties of the flash-evaporated n-type Bi2Te2.4Se0.6 thin films

The electronic transport properties of the flash-evaporated n-type Bi₂Te_2.4Se_0.6 thin films have been investigated. The thickness dependence of Seebeck coefficient and electrical resistivity has been analyzed by the effective mean free path model. It was found that both the Seebeck coefficient and the electrical resistivity have a linear relation with the reciprocal of film thickness. The data analyzed by the effective mean free path model have been combined to evaluate important material parameters such as mean free path, its energy dependence, and Fermi energy. Annealing effects on the electronic transport properties have been also examined in conjunction with the antistructure defects. It was found that the annealing treatment is necessary for Bi₂Te_2.4Se_0.6 thin films to reduce the antistructure defects and to improve their thermoelectric properties.     

The electrical conductivity of inhomogeneous thin metallic films

This paper reports the following results: (i) an explicit for the electrical conductivity of a thin metal film where the electron mean free path is not constant through its thickness, and (ii) calculations showing how in this case an illusory fit to the Fuchs-Sondheimer result can be obtained with a dependence of the specularity parameter on thickness or angle. A survey of experimental results is also presented.     

The effect of physical vapor deposition parameters on the thermoelectric power of thin film molybdenum-nickel junctions

The effect of various physical vapor deposition parameters on the thermoelectric power of thin film (10 000 Å) Mo-Ni junctions deposited by electron beam evaporation was investigated in this study. The deposition parameters of interest were the substrate temperature, the deposition rate and the vapor source purity. The thermoelectric power of the thin film Mo-Ni junctions was dependent on the structure of the Mo thermoelectric element of the couple, the characteristics of which were significantly altered by varying the deposition parameters. Varying the deposition parameters caused a change in lattice imperfections in the metal, which changed the mean free path of the conduction electrons and the thermoelectric power of the couple. The parameters having the greatest effect on the thermoelectric power were the deposition rate and the substrate temperature. Couples deposited at a high deposition rate (10 000 Å min^-1) and a high substrate temperature (? 300 °C) demonstrated e.m.f. characteristics closest to bulk values.