Thermoelectric Power Coefficient Research Articles

Some transport measurements of WSe2-x single crystals

Electrical resistivity and its anisotropy, thermoelectric power and temperature coefficient of resistivity of WSe2-x crystals have been measured (0.01 ≦x≦ 0.1) in the temperature range 300–448 K. Variations of these physical properties with respect to temperature and composition have been examined. The observed results are discussed in the light of semiconducting nature of the material.

Electric, galvanomagnetic and electrothermal properties of vacuum-evaporated antimony films

The electrical resistivity, temperature coefficient of resistivity, Hall constant and thermoelectric power of antimony films were measured in situ. Measurements of the Hall mobility and the thermoelectric power indicated that holes were the majority carriers in these films. The values of the fundamental parameters describing the electron transport processes were derived. The effect of grain boundary scattering was studied critically.

Thermoelectric detection of 10 µm energy using point-contact Schottky-barrier diodes operating at zero bias

Zero-biased-point contact Schottky-barrier diodes operating as detectors of 10 µm radiation are analyzed. It is found that a themioelectric process is responsible for the detected voltage observed experimentally. This process dominates all other detection mechanisms due to the high thermoelectric power coefficient of semiconductors. The time constants associated with the process are of the order of 0.4 µs.

Transport properties of SnTe interpreted by means of a two valence band model

Electrical conductivity, thermoelectric power and Hall coefficient of SnTe have been measured between 300 K and 160 K. The temperature variation of thermoelectric power and the carrier concentration estimated from the Hall coefficient have been explained with a two valence band model. The variation of mobility with temperature has been calculated from the above measurements. The mobility of holes in the upper valence band is found to follow the usual T -3 2 law, whereas the mobility in the lower valence band increases with temperature in a peculiar manner which cannot be explained with the help of a parabolic band.

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.

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.

Preparation and conduction mechanism of n-type semiconducting chalcogenide glasses chemically modified by bismuth

Two serieses of Bi containing glasses Ge 20Bi xSe 80−x (x≦13) and Ge 20Bi xSe 70−xTe 10 (x≦11) in at.% were prepared, and their resistivity, thermoelectric power, and optical absorption coefficient were measured. The resistivity at 25°C of Ge 20Bi xSe 80−x glasses decreased by about four orders of magnitude between x=9 and 10, and remained constant for x≧10 ; accompanied with this abrupt decrease, the conduction type changed from p to n in the thermoelectric power measurement. In Ge 20Bi xSe 70−xTe 10 glasses the conduction type changed from p to n at practically the same value of x as in Ge 20Bi xSe 80−x glasses; the resisitivity was nearly the same for both serieses of glasses with x≧10 , while for x<10 Te containing glasses showed lower resistivities by about four orders of magnitude. Optical absorption measurements revealed that the band gap decreased with the incorporation of a small amount of Bi, while remained nearly constant for the further increase in Bi content, indicating that the conduction type was controlled independently of the band gap. These results were discussed on the basis of chemical bonds present in the glasses, and the shift of the Fermi energy toward the conduction band was related to the formation of negatively charged Bi and positively charged dangling bonds.

Electrical and optical properties of n-type semiconducting chalcogenide glasses in the system Ge-Bi-Se

The glass-forming region was determined for the system Ge-Bi-Se; 13 at.% Bi was found to be incorporated at its maximum content into glasses, the Ge content of which was 20 at.%. Electrical resistivity, thermoelectric power, and optical absorption coefficient were measured mainly on a series of Ge20BixSe80−x glasses (x=0 –13). The resistivity decreased by about four orders of magnitude between x=9 and 10, and remained almost constant for x?10. The thermoelectric measurement showed the change of conduction type from p to n, accompanied by the above-mentioned abrupt decrease of resistivity. In n-type glasses, hopping conduction in the tail of localized states was proposed in parallel with conduction in extended states. The optical band gap was very slightly changed with the incorporation of more than 2.5 at.% Bi, in contrast to the remarkable decrease in the activation energy for conduction between x=9 and 10. The discussion based on the concentration of covalent bonds formed in the glasses led to the conclusion that the formation of a fairly large number of Bi-Se bonds and the disappearance of Se-Se bonds near x=10 were closely related to the composition dependence of the electrical and optical properties of the glasses in the present system.

Kinetic coefficients in solid solutions of the (Agx/2Pb1−xSbx/2)Te type

en

Thermoelectric and thermomagnetic properties of graphite—I: The cylindrical band model

The cylindrical band model of graphite is used to obtain the degeneracy parameters necessary for the analysis of the basal conductivity, thermoelectric power and Nernst-Ettingshausen coefficient (drift-diffusion terms). Densities and mean mobilities of electrons and holes are obtained from galvanomagnetic properties, then the thermoelectric power and Nernst-Ettingshausen coefficient are computed. Several features of the thermoelectric power curves with temperature can be qualitatively understood with this model, particularly differences in the behavior of pyrolytic and natural, or Kish graphites. A minimum in the Nernst-Ettinghausen coefficient depends strongly on the type of scattering law assumed, but the model cannot account realistically for the size of this effect near 300°K, where phonon-drag terms are thought to be very small.

Thermoelectric power and temperature coefficient of resistivity of thin metal films

The dependence of the electrical conductivity, σ(T) and thermopower, S(T), of Ni-SiO2 thin films on temperature, as a function of themetal content, for Ni concentration values neat" the percolation threshold has been investigated. The cermet films are deposited by r.f. sputtering, in Ar atmosphere (p ≈ 10-2 mbar), using composite targets. The samples are thermally treated in vacuum, at 600 and 700 K, respectively, in order to stabilize their electrical properties. The dependence of the samples' conductivity on the temperature has been studied between 200 and 500 K and it shows a metal-insulator transition when the Ni content in the films decreases below 50%. In order to study the thermopower of Ni-SiO2. lihns, we prepared Pt/(Ni-SiO2) thin-film test thermocouples, on alumina substrates, also by r.f. sputtering. The dependence of S on temperature does not show the metal-insulator transition for the entire compositional range we investigated. The films also present a good resistance to the oxidation agents. The values of the TCR (temperature coefficient of resistance) and S(T) obtained the near the percolation compositional range make Ni-SiO2 thin lilms interesting for applications in the field of temperature sensors.

Two formulations of semiconductor transport equations

Transport equations for semiconductors can be written with the diffusion term as qD 1∇ n or as q∇( D 2 n), where D 1 and D 2 are diffusion coefficients. In a recent note in this journal the issues arising from this ambiguity were clarified and the possibility of a more general treatment, valid for arbitrary densities of states and current carriers, was announced. The details are given in this paper which includes magnetic field effects and covers also thermal conductivity, thermoelectric power and Soret coefficients in the two formulations. A special effort is made to develop a consistent macroscopic scheme of equations separately from their microscopic interpretations. A number of new equations and convenient connecting formulae between the two formulations are given; for example four general Einstein relations are found: for the electric and heat currents in the D 1- and the D 2-formulations.

Thermoelectric power of gold-silver alloy films

Thermoelectric power, resistivity, and temperature coefficient of resistance of annealed Au−Ag alloy films, covering the entire composition range, have been measured. Using size effects in the electron transport theory, the energy dependence of the mean free path U and the Fermi surface area V have been determined.

Hall-thermopower sign anomaly in chalcogenide glasses

Recent models for the electronic structure of multicomponent chalcogenide glasses have led to the conclusion that the Fermi level is essentially pinned near the center of the gap by a relatively low-energy defect center. Expressions for the conductivity, thermoelectric power, and Hall coefficient of these glasses are calculated within such a model. When the effective mass of holes in the valence band is larger than that of electrons in the conduction band, it is found that a $p$-type thermopower and an $n$-type Hall coefficient can coexist over a wide temperature range, an anomaly which has been reported for a variety of glasses. In addition, the model explains several salient features of the transport coefficients measured in some of these glasses.

Electronic and Ionic Conduction in Cu2-δSe, Cu2-δS and Cu2-δ(Se, S)

Using the galvanic cell Cu|CuBr·C 6 H 12 N 4 CH 3 Br|Cu 2-δ VI|Pt, the deviation δ from the stoichiometric composition is varied up to 0.2 for Cu 2-δ Se, Cu 2-δ S and their alloys. Electronic conductivity, thermoelectric power and Hall coefficient are measured as functions of e.m.f. of the cell, E , and of temperature, T , (20°C∼200°C). The phase diagrams are constructed front these results, especially from E vs T plots, and the electrical properties arc interpreted by the use of a simple model. Further, the ionic conductivity is measured with the use of the cell in low temperature phase as well as in high temperature one. The value of E seems not to represent the Fermi kinetic energy even in the high temperature phase having the average structure. This is because of the largeness of δ, and can be interpreted as being due to the screening effect of carriers (holes) on copper ion vacancies.

Practical SQUID Technique for the Precise Measurements of Transport Coefficients

A practical system of SQUID voltmeter for precise measurements of thermoelectric power and other transport coefficients in metals and alloys at low temperatures is described. The cryostat is designed to allow temperature control of the sample continuously from 1.3 K to 8.5 K, while the SQUID and the standard resistors are kept at a fixed temperature of 4.2 K. The sensitivity was found to reach the Johnson noise level for a source resistance lower than 0.02 Ω; typically 0.9×10-13 V Hz-1/2 for 0.4×10-4 Ω at 4.2 K. A simple noise analysis indicates that the system has an ultimate sensitivity of at least 1.5×10-14 V with a time constant of 1 sec for a source resistance lower than 10-6 Ω at helium temperatures.

Electrical resistivity, TCR and thermoelectric power of annealed thin copper films

The electrical resistivity, temperature coefficient of resistance (TCR) and thermoelectric power are measured on systematically annealed copper films formed by the vacuum evaporation method. From the measured quantities the scattering coefficient and the bulk mean free path of the conduction electrons are calculated. The electron mean free path is found to be 410+or-100 AA. The energy dependence of mean free path U and the energy dependence of Fermi surface area V are calculated from the thermoelectric power measurements as a function of thickness. Both U and V are found to be negative and thickness-dependent.

Resistivity, thermoelectric power, and Hall coefficient of liquid mercury using at-matrix formalism

The electrical resistivity, thermoelectric power, and Hall coefficient of liquid mercury have been calculated in a $t$-matrix formulation of Ziman's nearly-free-electron theory. The results are in reasonable agreement with experiment and indicate that the anomalously large electrical resistivity of liquid mercury is due to a large $d$ contribution to the electron-ion scattering cross section. The $t$-matrix "form factor" has been compared with the pseudopotential form factors obtained earlier.

Transport properties of MoTe 2− x and MoSe 2− x compounds between 130 and 300°K

Thermoelectric power and Hall coefficient values were measured on compact polycrystalline samples from the MoTe 2− x and MoSe 2− x systems. It appears that qualitatively the temperature dependence of the Seebeck coefficient is not very different from that of crystalline semiconductors. The Seebeck coefficient is positive indicating hole conduction whilst Hall coefficient measurements on the same samples show negative values implying that conduction is by electrons. Singularities in the graphs plotted as a function of the composition x are found for both MoTe 2− x and MoSe 2− x systems. These results are discussed in terms of a quasi amorphous semi-conductor model.

Some electrical properties of silicon and germanium on a pseudopotential approach

A three-parameter model pseudopotential is proposed for semiconductors. A calculation is carried out to obtain the form factor, the binding energy, the monovacancy resistivity, the liquid-state resistivity, and the coefficient of thermoelectric power for silicon and germanium. Satisfactory agreement with the experimental data is obtained in most cases.