Degenerate Semiconductors Research Articles

Mechanical and thermoelectric properties of Bi2−xSbxTe3 prepared by using encapsulated melting and hot pressing

Bi2-xSbxTe3 (x = 1.4 - 1.7) solid solutions were synthesized by using encapsulated melting (EM) and were consolidated by using hot pressing (HP). The lattice constants decreased with increasing Sb content, which indicated that solid solutions were successfully synthesized. The relative densities of all the hot-pressed specimens were higher than 97.9 %. X-ray diffraction patterns, pole figures and electron backscattered diffraction spectra of all the hot-pressed specimens indicated randomly oriented textures. Very low values of the orientation factor (F) were obtained, Fmin = 0.008 for Bi0.4Sb1.6Te3 hot-pressed at 673 K and Fmax = 0.115 for Bi0.4Sb1.6Te3 hot-pressed at 723 K, which implied that the microstructures were highly isotropic. A bending strength of 46 MPa and a Vickers hardness of 94 Hv were attained for Bi0.4Sb1.6Te3 hot-pressed at 648 K. However, the mechanical properties were degraded with increasing HP temperature owing to grain growth. An increased HP temperature did not affect the electrical properties significanctly. The carrier concentration increased with increasing Sb content, and the specimen with x = 1.4 showed nondegenerate semiconductor behavior whereas those with x ≥ 1.5 behaved as degenerate semiconductors. All specimens showed p-type conduction, which was confirmed from the positive signs of the Seebeck coefficient and the Hall coefficient. The increased Sb content caused a shift in the peak values of the Seebeck coefficient to higher temperatures and an enhancement of the power factor. The electronic thermal conductivity decreased, but the lattice thermal conductivity increased, with increasing Sb content. The lowest theraml conductivity of 0.83 Wm -1K -1 was obtained at 373 K for Bi0.4Sb1.6Te3. The maximum dimensionless figure of merit, ZTmax = 1.1, was achieved at 323 K for Bi0.4Sb1.6Te3.

A realistic analysis of the phonon growth characteristics in a degenerate semiconductor using a simplified model of Fermi-Dirac distribution

The phonon growth characteristic in a degenerate semiconductor has been calculated under the condition of low temperature. If the lattice temperature is high, the energy of the intravalley acoustic phonon is negligibly small compared to the average thermal energy of the electrons. Hence one can traditionally assume the electron-phonon collisions to be elastic and approximate the Bose-Einstein (B.E.) distribution for the phonons by the simple equipartition law. However, in the present analysis at the low lattice temperatures, the interaction of the non equilibrium electrons with the acoustic phonons becomes inelastic and the simple equipartition law for the phonon distribution is not valid. Hence the analysis is made taking into account the inelastic collisions and the complete form of the B.E. distribution. The high-field distribution function of the carriers given by Fermi-Dirac (F.D.) function at the field dependent carrier temperature, has been approximated by a well tested model that apparently overcomes the intrinsic problem of correct evaluation of the integrals involving the product and powers of the Fermi function. Hence the results thus obtained are more reliable compared to the rough estimation that one may obtain from using the exact F.D. function, but taking recourse to some over simplified approximations.

Indium tin oxide as a semiconductor material in efficient p-type dye-sensitized solar cells

Indium tin oxide (ITO) is a well-known n-type degenerate semiconductor. Herein, mesoporous ITO is utilized as a photocathode material for p-type dye-sensitized solar cells in place of the commonly applied p-type semiconductors, such as nickel oxide. In conjunction with [Fe(acac)3]0/− as redox mediator and a new sensitizer, an impressive energy conversion efficiency of 1.96±0.12% was achieved. Photoelectron spectroscopy in air revealed that ITO exhibits a significant local density of states arising below −4.8 eV, which enables electron transfer to occur from ITO to the excited dye, giving rise to the sustained photocathodic current.

Thermoelectric properties of p-type partially double-filled (Pr1−z Nd z ) y Fe4−x Co x Sb12 skutterudites

p-Type partially double-filled (Pr1−zNdz)yFe4−xCoxSb12 (z = 0.25, 0.75, y = 0.8, 0.9, and x = 0, 0.5, 1.0) skutterudites were synthesized by encapsulated melting and annealing and consolidated by hot pressing. The effects of Pr/Nd partial double-filling and Co substitution for Fe (charge compensation) on the electronic transport and the thermoelectric properties of the skutterudites were examined. A few secondary phases such as marcasite FeSb2 were formed together with the skutterudite phase, but the formation was suppressed with increasing Co content. Unlike for complete filling with Pr/Nd, the (Pr,Nd)Sb2 phases were not formed by partial filling with Pr/Nd. Successful Pr/Nd double filling in the voids and Co substitution for Fe were confirmed because the lattice constant increased with increasing filling fraction and Pr/Nd ratio, and decreased with increasing Co content. The electrical conductivity slightly decreased with increasing temperature, showing degenerate semiconductor characteristics, while the Hall and the Seebeck coefficients showed positive signs, indicating that holes were the major carriers (p-type conductions). The Seebeck coefficient increased due to a decrease in the carrier concentration with increasing Pr and Co contents while the electrical conductivity decreased. The lattice thermal conductivity decreased because phonon scattering was enhanced by Pr and Nd double filling, which led to low thermal conductivity, but partially double-filled specimens did not have a significant effect on the reduction of lattice thermal conductivity compared with the completely double-filled specimens. The maximum dimensionless figure of merit, ZT = 0.84, was obtained at 723 K for (Pr0.75Nd0.25)0.8Fe3CoSb12.

Thermoelectric Properties of Bi2Te3−y Se y :I m Prepared by Mechanical Alloying and Hot Pressing

Bi2Te3−ySey:Im (y = 0.15–0.6 and m = 0.0025–0.01) solid solutions were prepared by mechanical alloying and hot pressing. The lattice constants that were measured from x-ray diffraction patterns decreased linearly with increasing Se content, but they were not changed remarkably by I doping. The average relative densities of the hot-pressed specimens are higher than 97%. All of the specimens exhibited n-type conductions in the measured temperature range from 323 K to 523 K, and their electrical conductivity decreased slightly with increasing temperature, indicating degenerate semiconductor behaviors. The electrical conductivity decreased with increasing Se content, whereas it was increased by I doping, and this is in contrast with the Seebeck coefficient; this resulted from the changes of the electron concentrations due to the Se substitution and the I doping. The thermal conductivity decreased with increasing Se content, and this is the result of both the decreased electronic thermal conductivity due to the decreased carrier concentration and the decreased lattice thermal conductivity due to the increased alloy scattering. The maximum dimensionless figure of merit for Bi2Te2.4Se0.6, ZTmax = 0.84 at 473 K, is due to its low thermal conductivity and high Seebeck coefficient.

ChemInform Abstract: Searching for New Thermoelectric Materials: Some Examples Among Oxides, Sulfides and Selenides

Different families of thermoelectric materials have been investigated since the discovery of thermoelectric effects in the mid-19th century, materials mostly belonging to the family of degenerate semi-conductors. In the last 20 years, new thermoelectric materials have been investigated following different theoretical proposals, showing that nanostructuration, electronic correlations and complex crystallographic structures (low dimensional structures, large number of atoms per lattice, presence of 'rattlers'…) could enhance the thermoelectric properties by enhancing the Seebeck coefficient and/or reducing the thermal conductivity. In this review, the different strategies used to optimize the thermoelectric properties of oxides and chalcogenides will be presented, starting with a review on thermoelectric oxides. The thermoelectric properties of sulfides and selenides will then be discussed, focusing on layered materials and low dimensional structures (TiS2 and pseudo-hollandites). Some sulfides with promising ZT values will also be presented (tetrahedrites and chalcopyrites).

Effect of Zr doping on the electrical and optical properties of ZnO

Within 0.02083–0.04167 Zr doping amount, there are contradictory reports on the experimental results of ZnO doping system in absorption spectrum distribution. However, there is no reasonable theoretical explanation until now. To solve this problem, density functional theory-based generalized gradient approximation plane wave ultra-soft pseudopotential GGA+U method is utilized in this paper and the first-principles are adopted to construct the supercell models with three different doping amounts, Zn0.97917Zr0.02083O, Zn0.96875Zr0.03125O and Zn0.95833Zr0.04167O. On the basis of geometrical optimization of all the models, band structure distribution, density of states distribution and absorption spectrum distribution are calculated and the calculation results show that within the limited doping amount, the higher the Zr doping amount is, the higher the doping system volume is, the higher the total energy is, the lower the system stability is, the higher the formation energy is, and the more difficult doping is. With all the doping systems converted into n-type degenerate semiconductor, the wider the doping system band gap is, the more significant the absorption spectrum blueshift is, the lower the absorption intensity is, the higher the electronic effective mass is, the higher the electronic concentration is, the lower the electronic mobility is, the higher the electronic conductivity is, and the more significant the doping system conductivity is. The calculation results and the experiment results are consistent.

Examination of the temperature dependent electronic behavior of GeTe for switching applications

The DC and RF electronic behaviors of GeTe-based phase change material switches as a function of temperature, from 25 K to 375 K, have been examined. In its polycrystalline (ON) state, GeTe behaved as a degenerate p-type semiconductor, exhibiting metal-like temperature dependence in the DC regime. This was consistent with the polycrystalline (ON) state RF performance of the switch, which exhibited low resistance S-parameter characteristics. In its amorphous (OFF) state, the GeTe presented significantly greater DC resistance that varied considerably with bias and temperature. At low biases (<1 V) and temperatures (<200 K), the amorphous GeTe low-field resistance dramatically increased, resulting in exceptionally high amorphous-polycrystalline (OFF-ON) resistance ratios, exceeding 109 at cryogenic temperatures. At higher biases and temperatures, the amorphous GeTe exhibited nonlinear current-voltage characteristics that were best fit by a space-charge limited conduction model that incorporates the effect of a defect band. The observed conduction behavior suggests the presence of two regions of localized traps within the bandgap of the amorphous GeTe, located at approximately 0.26–0.27 eV and 0.56–0.57 eV from the valence band. Unlike the polycrystalline state, the high resistance DC behavior of amorphous GeTe does not translate to the RF switch performance; instead, a parasitic capacitance associated with the RF switch geometry dominates OFF state RF transmission.

Tailoring p- and n- type semiconductor through site selective oxygen doping in Cu3N: density functional studies

Using ab initio density functional calculations, we have investigated the stability and electronic structure of pure and oxygen doped semiconducting Cu3N. The oxygen can be accommodated in the system without structural instability as the formation energy either decreases when oxygen substitutes nitrogen, or remains nearly same when oxygen occupies the interstitial position. The interstitial oxygen (OI) prefers to stabilize in the unusual charge neutral state and acts as an acceptor to make the system a p-type degenerate semiconductor. In this case the hole pockets are formed by the partially occupied OI-p states. On the other hand, oxygen substituting nitrogen (OS) stabilizes in its usual −2 charge state and acts as a donor to make the system an n-type degenerate semiconductor. The electron pockets are formed by the conducting Cu-p states. In the case of mixed doping, holes are gradually compensated by the donor electrons and an intrinsic gap is obtained for stoichiometry. Our calculations predict the nature of doping as well as optical band gap variation in experimentally synthesized copper oxynitride. While interstitial doping contracts the lattice and increases the substitutional doping increases both lattice size and Mixed doping reduces Additionally we show that a rare intra-atomic d–p optical absorption can be realized in the pristine Cu3N as the Fermi level lies in the gap between the Cu-d dominated anti-bonding valence state and Cu-p conducting state.

Electrical and optical properties of Cu–Cr–O thin films fabricated by chemical vapour deposition

We present electrical and optical properties of CuCrO2 thin films deposited by chemical vapour deposition, as well as the influence of depositions' parameters on these properties. Oxygen partial pressure and precursor's concentrations have the greatest influence on optical and electrical properties of the films. Values of conductivities ranging from 10−4 to 10S/cm were obtained using different deposition conditions. The conductivity is thermally activated with an activation energy ranging from 57 to 283meV. Thermoelectric measurements confirm the p-type conduction, and demonstrate high carrier concentration typical for a degenerate semiconductor. The as-deposited films show a medium degree of crystallinity, a maximum optical transmission up to 80% in the visible range with a corresponding band gap around 3.2eV.

Many-body, Pauli blocking and carrier-impurity interaction effects on the band gap of aluminum doped zinc oxide thin films: A new method to evaluate both hole and electron effective masses of degenerate semiconductors

In this study, the role of many-body interactions, Pauli blocking, and impurity-carrier interactions on the band gap variation were used to analyze the observed blue shift of the Al doped ZnO thin films. The optical constants and the band gap variations of our samples were obtained using spectroscopic ellipsometry method. Tauc-Lorentz model and effective medium approximation were used to model the dielectric function. We demonstrated for the first time that the combination of the Tauc formalism for absorption and the proposed model by Jain et al. for the band gap variation, can be used to determine both electron and hole effective masses. Reasonable results were obtained for carriers effective mass and their dependence on carrier concentration.

Direct optical band gap measurement in polycrystalline semiconductors: A critical look at the Tauc method

The direct optical band gap of semiconductors is traditionally measured by extrapolating the linear region of the square of the absorption curve to the x-axis, and a variation of this method, developed by Tauc, has also been widely used. The application of the Tauc method to crystalline materials is rooted in misconception–and traditional linear extrapolation methods are inappropriate for use on degenerate semiconductors, where the occupation of conduction band energy states cannot be ignored. A new method is proposed for extracting a direct optical band gap from absorption spectra of degenerately-doped bulk semiconductors. This method was applied to pseudo-absorption spectra of Sn-doped In2O3 (ITO)—converted from diffuse-reflectance measurements on bulk specimens. The results of this analysis were corroborated by room-temperature photoluminescence excitation measurements, which yielded values of optical band gap and Burstein–Moss shift that are consistent with previous studies on In2O3 single crystals and thin films.

Weak localization in CdO thin films prepared by sol–gel method

Abstract This paper reports the study of the magnetotransport properties of polycrystalline CdO thin films prepared by the sol–gel method. Both the sheet resistance and electron density as a function of temperature indicate that our samples are degenerate semiconductors. The weak localization (WL) effect is observed in the low temperature range. Applying the two-dimensional WL theory, we have extracted the dephasing length and a relatively large value (the maximum gets to 151 nm at 2 K) is obtained. The temperature dependence of the extracted dephasing length can be well described by the electron–electron scattering process, indicating that the electron–electron scattering is main dephasing mechanism for electrons.

Effect of Proximity in Arrays of Plasmonic Nanoantennas on Hot Spots Density: Degenerate Semiconductors vs. Conventional Metals

Using rigorous finite element electromagnetic calculations, we study the performance of a localized surface plasmons resonance array infrared sensor, from an uncommon perspective, by analyzing electromagnetic hot spots density. We show that the hot spots density strongly depends on near-field interaction between plasmonic nanoantennas. We provide important theoretical evidence that the degree of the interaction depends on both material and geometrical properties of an array. To this end, we consider arrays of conventional metal (CM) and plasmonic transparent conducting oxide (TCO) at different nanoantennas density. We establish that a single CM nanoantenna has higher field intensity enhancement than a TCO counterpart. On the other hand, an array of TCO nanoantennas provides 30 times larger density of the hotspots and 15 times higher signal-to-noise ratio, as compared to a CM array.

Charge transport and thermoelectric properties of double-filled Nd1–z Yb z Fe4–x Co x Sb12 skutterudites

p-Type Nd1–zYbzFe4–xCoxSb12 (z = 0.25, 0.5, 0.75 and x = 0, 0.5, 1.0) skutterudites were synthesized by using encapsulated melting and hot pressing. The effects of Nd/Yb double filling and Co substitution for Fe (charge compensation) on the microstructure, the charge transport, and the thermoelectric properties of the skutterudite specimens were investigated. All specimens were transformed to the skutterudite phase by the annealing process, and a few secondary phases such as marcasite FeSb2 were formed together with the skutterudite phase, although their formation was suppressed with increasing Co content. The lattice constant changed with the filling ratio of Nd/Yb and Fe/Co substitution, which meant that the double filling of Nd/Yb and the substitution of Co for Fe were successfully performed. All specimens showed p-type conduction at temperatures ranging from 323 K to 823 K and exhibited degenerate semiconductor characteristics, in which the electrical conductivity decreased and the Seebeck coefficient increased with increasing temperature. The Seebeck coefficient increased with increasing Nd and Co contents due to the decreased carrier concentration while the electrical conductivity and the thermal conductivity decreased. The maximum Seebeck coefficient was obtained at temperatures in the range from 723 K to 823 K, and the thermal conductivity significantly increased at temperatures above 623 K due to bipolar conduction. The dimensionless figure of merit, ZT, showed maximum values at temperatures ranging from 723 K to 823 K due to the decrease in the Seebeck coefficient (or the decrease in the power factor) and the increase in the thermal conductivity at high temperatures. The maximum ZT = 0.81 was obtained for Nd0.75Yb0.25Fe4Sb12 at 823 K and for Nd0.75Yb0.25Fe3CoSb12 at 723 K.

Dispersion of nanoscale oxides in MnSi1.73 fabricated by solid state reaction and pulsed electric current sintering

We have investigated a relationship between the fabrication process and the resultant crystal structure of a MnSi1.73 thermoelectric material in order to understand the effect of microstructures on the thermoelectric properties. Local crystal structure of the matrix phase is identified as a modulated Mn15Si26 phase based on the Ye and Amelinckx method [J. Solid State Chem. 61, 8 (1986)]. The Mn15Si26 phase is a degenerate semiconductor showing metal like behavior. In addition, nanoscale SiO2 particles are dispersed. The crystal structure of SiO2 is amorphous, and the SiO2 particles reduce thermal conductivity because of its inherent characteristic. A relatively high figure of merit of 0.38 is obtained, resulting from the multiple microstructures of Mn15Si26 and SiO2.

Flexible n-Type High-Performance Thermoelectric Thin Films of Poly(nickel-ethylenetetrathiolate) Prepared by an Electrochemical Method.

Flexible thin films of poly(nickel-ethylenetetrathiolate) prepared by an electrochemical method display promising n-type thermoelectric properties with the highest ZT value up to 0.3 at room temperature. Coexistence of high electrical conductivity and high Seebeck coefficient in this coordination polymer is attributed to its degenerate narrow-bandgap semiconductor behavior.

The influence of boundary conditions on the electrical conductivity of a thin cylindrical wire

Within the kinetic approach, the high-frequency electrical conductivity of a thin semiconductor circular cross-section wire is calculated. The radius of the wire is assumed to be small compared with the characteristic skin depth that allows neglecting the skin effect. A model taking into account the dependence of the reflectivity coefficient on the surface roughness factor and the angle of incidence of the charge carriers at the inner border of the wire was used as the boundary conditions for the nonequilibrium distribution function of the charge carriers The extreme cases of a degenerate semiconductor and a nondegenerate one are considered. The results obtained are compared to the calculations for the model of Fuchs diffuse-specular boundary conditions.

Structural, characterization and electrical properties of AgPbmSbTem+2 compounds synthesized through a solid-state microwave technique

AgPbmSbTe2+m compounds are prepared through solid-state microwave synthesis method with different elements ratio of m (0.0 ≤ m ≤ 10). The X-ray diffraction method confirmed that all of the synthesized samples are polycrystalline with a rock salt NaCl-type crystalline structure. The lattice constant of the prepared AgPbmSbTe2+m increases from 6.089 Å to 6.467 Å with the increase in the mole fraction (m) from 0.0 to 10, respectively. Scanning electron microscopy images showed that increasing the mole fraction allows the continuous growth of multigrains, and volume of these grains increased with m value. The electrical conductivity (σ) measurements of AgPbmSbTe2+m prepared with a different m ratio exhibited a degenerate semiconductor behavior for all samples, and the sample prepared with m = 0 showed a higher σ of 482.96 S/cm at 300 K compared by the others. All of the samples had a negative Seebeck coefficient (S), indicating that the AgPbmSbTe2+m compounds are of n-type conductivity. Moreover, the maximum value of S is −419.69 μV/K at 338 K that obtained for the sample prepared with m = 8 while the minimum value of S was ˗6.8 μV/K recorded when the composition ratio of 4.

Sol–gel versus sputtering indium tin oxide films as transparent conducting oxide materials

The aim of this paper is the replacing of the expensive sputtering method with the low cost sol–gel one in TCO applications. To this end two sets of indium tin oxide (ITO) thin films are compared and discussed in this paper: one obtained by r.f. sputtering and one by the sol–gel technique and dip-coating. For each of these sets of samples, a series of deposition parameters have been varied in an effort to obtain the most promising optical and electrical properties. Comparative structural, morphological and opto-electrical characterization of sol–gel and sputtered ITO-based films was performed by X-ray diffraction, Scanning electron microscopy, Atomic force microscopy, Spectroellipsometry, UV–VIS Spectroscopy and Hall Effect measurements in order to establish whether the chemical deposition method could lead to thin films with competitive properties as those obtained through the physical method. Comparable, high transmittance (85–90 %) in the VIS–NIR range (250–1050 nm) and carrier concentration values (1020–1021 cm−3) were obtained between sputtered and sol–gel ITO films. The sputtered ITO film in 75 % N2, annealed at 500 °C and the sol–gel 0.1 M ITO film with 10 layers deposited on SiO2/glass exhibit degenerate semiconductor behavior.