Degenerate Semiconductors Research Articles

Chemical Vapor Deposition Growth of Degenerate p-Type Mo-Doped ReS2 Films and Their Homojunction

Substitutional doping of transition metal dichalcogenide two-dimensional materials has proven to be effective in tuning their intrinsic properties, such as band gap, transport characteristics, and magnetism. In this study, we realized substitutional doping of monolayer rhenium disulfide (ReS2) with Mo via chemical vapor deposition. Scanning transmission electron microscopy demonstrated that Mo atoms are successfully doped into ReS2 by substitutionally replacing Re atoms in the lattice. Electrical measurements revealed the degenerate p-type semiconductor behavior of Mo-doped ReS2 field effect transistors, in agreement with density functional theory calculations. The p-n diode device based on a doped ReS2 and ReS2 homojunction exhibited gate-tunable current rectification behaviors, and the maximum rectification ratio could reach up to 150 at Vd = -2/+2 V. The successful synthesis of p-type ReS2 in this study could largely promote its application in novel electronic and optoelectronic devices.

Unified Compact Model for Nanowire Transistors Including Quantum Effects and Quasi-Ballistic Transport

We present a surface potential-based compact model for nanowire FETs, which considers 1-D electrostatics along with the effect of multiple energy subbands. The model is valid for any semiconductor material, cross-sectional geometry, and any channel length with transport regimes varying from drift-diffusive to quasi-ballistic. The model captures the phenomenon of quantum capacitance and the effect of temperature. We have validated it with numerical simulations and experimental data for Si, Ge, and InAs nanowires of different geometries. Circuit simulation has also been performed with the model. The physics-based model is accurate and can be used as a tool for analysis and prediction of the effects of geometry scaling, material dependence, and temperature variation on device and circuit characteristics. To the best of our knowledge, this is the first time a compact model for nanowire FETs is being presented, which includes multiple subbands along with geometry scaling while being valid for different degenerate and nondegenerate semiconductor materials.

On the Lorenz number of multiband materials

There are many exotic scenarios where the Lorenz number of the Wiedemann-Franz law is known to deviate from expected values. However, in conventional semiconductor systems, it is assumed to vary between the values of ~1.49x10^{-8} W {\Omega} K^{-2} for non-degenerate semiconductors and ~2.45x10^{-8} W {\Omega} K^{-2} for degenerate semiconductors or metals. Knowledge of the Lorenz number is important in many situations, such as in the design of thermoelectric materials and in the experimental determination of the lattice thermal conductivity. Here we show that, even in the simple case of two and three band semiconductors, it is possible to obtain substantial deviations of a factor of two (or in the case of a bipolar system with a Fermi level near the midgap, even orders of magnitude) from expectation. In addition to identifying the sources of deviation in unipolar and bipolar two-band systems, a number of analytical expressions useful for quantifying the size of the effect are derived. As representative case-studies, a three-band model of the materials of lead telluride (PbTe) and tin sellenide (SnSe), which are important thermoelectric materials, is also developed and the size of possible Lorenz number variations in these materials explored. Thus, the consequence of multi-band effects on the Lorenz number of real systems is demonstrated.

Thermophysical and magnetic properties of p- and n-type Ti-Ni-Sn based half-Heusler alloys

Abstract A total of 5 different half-Heusler alloys, two p-type and two n-type with the fifth a charge compensated alloy have been designed and synthesized. The thermophysical properties of these alloys have been investigated in the range 10 K–1000 K while the magnetic behavior has been studied up to 300 K. The electrical resistivity of all the alloys varies within the range 0.06–5 mΩ cm indicating that they are in the degenerate semiconductor limits. The temperature dependence of p-type alloys exhibits a transition from metallic to semiconducting behavior, typical of topological insulators. The transition is found to occur in the range 300 K–500 K. The n-type and compensated alloys exhibit a weak metallic behavior in the complete temperature range. The Seebeck coefficient in the p-type alloys increases with temperature reaching a maximum value of 50 μV K −1 while that of the n-type alloys increases continuously reaching a value of 45 μV K −1 at ∼800 K. The corresponding power factor of the n-type alloy reaches 900 μW m −1 K −2 at ∼900 K compared to a maximum of ∼250 μW m −1 K −2 at 700 K for the p-type alloy. Magnetically the p-type and n-type alloys are found to be paramagnetic while the compensated alloy exhibits a ferromagnetic behavior.

Ferromagnetic resonance features of degenerate GdN semiconductor

Using X-band Ferromagnetic Resonance (FMR) Spectroscopy, we demonstrate the microscopic ferromagnetic resonance features of degenerated GdN semiconductor. The FMR spectrum suggests a single resonance mode below 10 K; interestingly, this particular structure is found to exhibit a peculiar magnetic resonance (PMR) on the top of the uniform FMR while temperature increases from 12–36 K. The low-field PMR mode attributed to the differently magnetized part of the film with an easy in-plane axis. The narrow-field gap between PMR and uniform FMR suggests the strong coupling owning to the differently magnetized part with easy in-plane axis and the magnetized part with an out-of-plane axis. The saturation magnetization, cubic magnetocrystalline anisotropy, and uniaxial anisotropy of GdN epitaxial film have been evaluated by the angular-dependence FMR.

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

(Pr1−z Yb z ) y Fe4−x Co x Sb12 (z = 0.25, 0.75, y = 0.8, and x = 0, 0.5, 1.0) skutterudites were prepared by using encapsulated melting, annealing, and hot pressing, and the effects of Pr/Yb partial double filling and Co charge compensation on the microstructure, charge transport and thermoelectric properties were investigated. All specimens were transformed to the skutterudite phase by using melting and annealing processes, and a few secondary phases such as marcasite FeSb2 formed together with the skutterudite phase. However, with Co substitution, the intensities of the FeSb2 peaks decreased, and proportion of the FeSb2 phase decreased with increasing Co content. Pr and Yb were confirmed to be partially double filled in the voids, and Co was substituted at Fe sites because for the Pr/Yb and Fe/Co substitution, the lattice constant changed with the filling ratio. All specimens exhibited degenerate semiconductor characteristics and p-type conduction at temperatures from 323 K to 823 K, and the charge transport properties showed a slight difference with changing filling fraction due to the difference in the valences of Pr and Yb. With increasing Pr and Co contents, the carrier concentration decreased. Consequently, the Seebeck coefficient increased, and the electrical conductivity decreased. The electronic thermal conductivity dominated the thermal conductivity, and the lattice thermal conductivity was decreased because the phonon scattering was enhanced by the Pr and Yb partial double-filling. Although the lattice thermal conductivity of partially double-filled specimens was lower than that of completely double-filled specimens, the partially double-filled specimens showed higher thermal conductivity because of the increase in electronic thermal conductivity caused by the high carrier concentration. The maximum dimensionless figure of merit, ZT = 0.85, was obtained at 723 K for (Pr0.75Yb0.25)0.8Fe3CoSb12.

Thermoelectric and mechanical properties of Bi0.4Sb1.6Te3 prepared by using encapsulated melting and hot extrusion

Bi0.4Sb1.6Te3 solid solutions were synthesized by using encapsulated melting (EM) and were consolidated by using hot extrusion (HE), and their thermoelectric and mechanical properties were examined and compared with those of specimens prepared by using hot pressing (HP). The relative densities of all hot-extruded specimens were higher than 97%. The basal plane (00l) was preferentially orientated parallel, rather than perpendicular, to the HE direction. The values of the orientation factor (F) were as follows: F max = 0.082 for specimens hot-extruded at 723 K and F max = 0.115 for specimens hot-pressed at 723 K. Thus, the crystallographic anisotropy was not high. From the pole figure and the electron backscatter diffraction analyses, the microstructures and the crystallinities of the HE specimens were found to be similar and comparable to those of the HP specimens. A bending strength of 57 MPa and a Vickers hardness of 74 Hv were obtained for Bi0.4Sb1.6Te3 hot-extruded at 698 K, which were superior to the corresponding properties of the HP specimens. However, the mechanical properties degraded with increasing HE temperature owing to the occurrence of grain growth. All specimens exhibited a negative temperature dependence for the electrical conductivity, indicating a degenerate semiconductor behavior. All Seebeck coefficients showed positive signs (p-type conduction); they attained peak values at 373 K and then decreased with increasing temperature owing to intrinsic conduction. The HE specimens exhibited lower values of the power factor than the HP specimens did because of the lower electrical conductivity and Seebeck coefficient of the former. The thermal conductivity was lower than 1.0 Wm−1K−1 at room temperature, and it increased slightly with increasing temperature. The maximum figure of merit, ZT max = 1.04, and the average figure of merit, ZT ave = 0.87, were achieved at 323 K for Bi0.4Sb1.6Te3 hot-extruded at 698 K.

Charge transport and thermoelectric properties of Ce1−z Nd z Fe4−x Co x Sb12 skutterudites

Ce1−z Nd z Fe4−x Co x Sb12 skutterudites were prepared by encapsulated melting, quenching, annealing, and hot pressing. The skutterudite phase could be successfully synthesized; however, a small amount of the secondary phase FeSb2 was also obtained. Scanning electron microscopic analysis indicated the presence of (Ce,Nd)Sb2 phases in the Co-substituted specimens (x = 0.5). The electrical conductivity decreased with an increase in temperature, implying a degenerate semiconductor behavior. The electrical conductivity also decreased with an increase in the Nd and the Co contents. All specimens exhibited p-type characteristics with positive values for the Hall coefficient and the Seebeck coefficient. The Seebeck coefficients increased with an increase in temperature, with the maximum values obtained at temperatures between 723 K and 823 K. The power factor (PF) increased with a decrease in the Co content, and Ce0.75Nd0.25Fe4Sb12 exhibited a maximum PF value of 3.1 mWm−1K−2 at 823 K. The electronic thermal conductivity decreased with an increase in the Nd and the Co contents, and the lattice thermal conductivity decreased with a decrease in the Ce and the Co contents. The thermal conductivity increased at temperatures above 623 K because of bipolar conduction. A maximum dimensionless figure of merit (ZT) value of 0.74 was obtained at 723 K for Ce0.25Nd0.75Fe4Sb12.

Li-decorated porous graphene as a high-performance hydrogen storage material: A first-principles study

Development of novel carbon-based nanoporous materials with high reversible capacity and excellent cycling stability is a hot topic in the field of hydrogen delivery and storage. In this work, first-principles calculations are carried out to discuss the hydrogen storage properties of Li-decorated porous graphene (Li-PG). The binding energies, electronic structures, storage capacities of hydrogen on different sites are investigated in details. The computational results show that with the increase of lithium doping concentration, the electron concentration of donor atoms exceeds the Nc value, and as a consequence, the PG changes from the p-type semiconductor to the n-type degenerate semiconductor. The maximum hydrogen adsorption configurations of H1a-H'1b and H2a-H'2b systems are obtained, and the average binding energy of per H2 molecule is 0.245 eV and 0.263 eV, respectively. Furthermore, ab inito MD simulation results show that the H1-H'1 and H2-H'2 systems can hold up to sixteen and fifteen H2 molecules, which corresponds to a hydrogen storage capacity of 10.89 wt% and 10.79 wt% at T = 300 K (no external pressure), respectively.

Electrical Transport Properties of Single-Crystalline β-Zn4Sb3 Prepared Through the Zn-Sn Mixed-Flux Method

β-Zn4Sb3 is a promising p-type thermoelectric material for utilization in moderate temperatures. This study prepares a group of single-crystalline β-Zn4Sb3 samples using the Zn-Sn mixed-flux method based on the stoichiometric ratios of Zn4+x Sb3Sn y . The effect of Zn-to-Sn proportion in the flux on the structure and electrical transport properties is investigated. All samples are strip-shaped single crystals of different sizes. The actual Zn content of the present samples is improved (>3.9) compared with that of the samples prepared through the Sn flux method. Larger lattice parameters are also obtained. The carrier concentration of all the samples is in the order of over 1019 cm−3. With increasing Sn rate in the flux, this carrier concentration decreases, whereas mobility is significantly enhanced. The electrical conductivity and Seebeck coefficients of all the samples exhibit a behavior that of a degenerate semiconductor transport. Electrical conductivity initially increases and then decreases as the Sn ratio in the flux increases. The electrical conductivity of the x:y = 5:1 sample reaches 6.45 × 104 S m−1 at 300 K. Benefitting from the electrical conductivity and Seebeck coefficient, the flux proportion of the x:y = 7:1 sample finally achieves the highest power factor value of 1.4 × 10−3 W m−1 K−2 at 598 K.

Phase formation and stability in TiO x and ZrO x thin films: Extremely sub-stoichiometric functional oxides for electrical and TCO applications

Abstract Most functional materials are thermodynamic equilibrium phases representing minima in the thermodynamic phase space. However, it is expected that many metastable phases with highly interesting properties also exist. Here, we report on a systematic approach to prepare thin-films of such non-equilibrium phases based on the gas phase deposition methods sputtering and pulsed laser deposition (PLD). Our synthetic strategy is to deposit a “precursor phase” which is amorphous or already a crystalline non-equilibrium phase. Subsequent heat treatment leads to the nucleation of crystalline phases which again may be metastable or stable compounds. In the present paper we focus on the binary systems Ti–O and Zr–O, both systems being widely applied and technologically relevant. Highly oxygen-deficient titanium oxide (TiO1.6) and zirconium oxide (ZrO) films prepared by pulsed laser deposition at room temperature are optically absorbing and possess electronic conductivities in the range of 10 S/cm. Both materials are metastable in respect to both composition and structure. For TiO1.6 we find an amorphous matrix with embedded grains of cubic titanium monoxide (γ-TiO) directly after deposition. Upon annealing nanocrystalline grains of metallic Ti are formed in the amorphous matrix due to an internal solid-state disproportionation whereas the electrical conductivity of the films increases and comes close to metal-like conductivity (1000 S/cm) at about 450 °C. Congruently, room temperature deposited ZrO films with an average composition of Zr:O= 1:1 contain small ZrO nanocrystals within an amorphous matrix. Heat treatment again leads to an internal disproportionation reaction whereas small crystals of Zr2O and ZrO2 precipitate at temperatures as low as 75 °C. Increasing the temperature then results in the crystallization of metastable tetragonal ZrO2 at about 400 °C. Sputter deposition allows a subtler control of the oxygen partial pressure. Slightly non-stoichiometric TiO2−x films form a degenerate semiconductor with room temperature conductivities as high as 170 S/cm. Moreover, controlling both, the doping level and the vacancy concentration of these films allows to control the phase formation and the transition temperature between the rutile and anatase TiO2 polymorphs. Niobium doping of sputter deposited TiO2 can lead to films with very high electrical conductivities while maintaining a high optical transmittance demonstrating the potential of the material as an alternative transparent conducting oxide (TCO) with extraordinary properties.

Effects of V heavy doping on the magnetic and optical properties in anatase TiO2

A half-metal diluted magnetic semiconductor (DMS) can be formed in heavy V-doped TiO2. Contradictory experimental results in the literature have reported about the absorption spectra blueshift and redshift results in heavy V-doped TiO2. This study aims to reveal the mechanism of half-metal DMS in heavy V-doped TiO2 and solve the problem of absorption spectra blueshift and redshift in the doping system. In this study, models of the unit cells of pure anatase TiO2 and two V heavy-doped supercells of Ti[Formula: see text]V[Formula: see text]O2 and Ti[Formula: see text]V[Formula: see text]O2 were constructed based on density functional theory, which uses the first-principles plane-wave ultrasoft pseudopotential method. All models were obtained through geometry optimization. Local density approximation [Formula: see text] was used to calculate the band structure, density of states (DOS), orbital charge and absorption spectrum of the doping system. The calculated results under the condition of electron spin showed that in the heavy doping concentration range, the volume of supercells increases, the total energy and formation energy decrease and the stability of the supercells increases as V doping concentration increases. Furthermore, the interaction of [Formula: see text]–[Formula: see text] states is weaker than that of [Formula: see text]–[Formula: see text] states, which results in the valence band maximum shifting toward the low-energy region, and also the optical bandgap becomes narrower as well as the redshift and intensity of the absorption spectrum become more notable. Noticeably, the hybrid coupling effect of Ti-3[Formula: see text] and V-3[Formula: see text] states becomes stronger, and the magnetic moment increases. The Fermi levels of spin-up band structure within the conduction band, which form the [Formula: see text]-type degenerate semiconductors, and the Fermi levels of spin-down band structure within the bandgap indicate that the doping system has semiconductor features. Therefore, V-doped anatase TiO2 is an extremely promising DMS because of its high electron polarizability of nearly 100%. The calculation results are consistent with the experimental data; these results explain the problems reasonably and adequately. Therefore, the research findings can help solve the contradiction of the redshift and blueshift in the preparation of photocatalysts and half-metal diluted magnetic semiconductors of V heavy-doped anatase TiO2.

Metal-insulator transition in tin doped indium oxide (ITO) thin films: Quantum correction to the electrical conductivity

Tin doped indium oxide (ITO) thin films are being used extensively as transparent conductors in several applications. In the present communication, we report the electrical transport in DC magnetron sputtered ITO thin films (prepared at 300 K and subsequently annealed at 673 K in vacuum for 60 minutes) in low temperatures (25-300 K). The low temperature Hall effect and resistivity measurements reveal that the ITO thin films are moderately dis-ordered (kFl∼1; kF is the Fermi wave vector and l is the electron mean free path) and degenerate semiconductors. The transport of charge carriers (electrons) in these disordered ITO thin films takes place via the de-localized states. The disorder effects lead to the well-known ‘metal-insulator transition’ (MIT) which is observed at 110 K in these ITO thin films. The MIT in ITO thin films is explained by the quantum correction to the conductivity (QCC); this approach is based on the inclusion of quantum-mechanical interference effects in Boltzmann’s expression of the conductivity of the disordered systems. The insulating behaviour observed in ITO thin films below the MIT temperature is attributed to the combined effect of the weak localization and the electron-electron interactions.

Effect of Ni doping on optical and magnetic properties of ZnO

Nowadays, the experimental results of absorption spectrum distribution of Ni doped ZnO suffer controversy when the mole fraction of impurity is in a range from 2.78% to 6.25%. However, there is still lack of a reasonable theoretical explanation. To solve this problem, the geometry optimizations and energies of different Ni-doped ZnO systems are calculated at a state of electron spin polarization by adopting plane-wave ultra-soft pseudo potential technique based on the density function theory. Calculation results show that the volume parameter and lattice parameter of the doping system are smaller than those of the pure ZnO, and they decrease with the increase of the concentration of Ni. The formation energy in the O-rich condition is lower than that in the Zn-rich condition for the same doping system, and the system is more stable in the O-rich condition. With the same doping concentration of Ni, the formation energies of the systems with interstitial Ni and Ni replacing Zn cannot be very different. The formation energy of the system with Ni replacing Zn increases with the increase of the concentration of Ni, the doping becomes difficult, the stability of the doping system decreases, the band gap becomes narrow and the absorption spectrum is obviously red shifted. The Mulliken atomic population method is used to calculate the orbital average charges of doping systems. The results show that the sum of the charge transitions between the s state orbital and d state orbital of Ni2+ ions in the doping systems Zn0.9722Ni0.0278O, Zn0.9583Ni0.0417O and Zn0.9375Ni0.0625O supercells are all closed to +2. Thus, it is considered that the valence of Ni doped in ZnO is +2, and the Ni is present as a Ni2+ ion in the doping system. The ionized impurity concentrations of all the doping systems exceed the critical doping concentration for the Mott phase change of semiconductor ZnO, which extremely matches the condition of degeneration, and the doping systems are degenerate semiconductors. Ni-doped ZnO has a conductive hole polarization rate of up to nearly 100%. Then the band gaps are corrected via the LDA (local density approximation)+U method. The calculation results show that the doping system possesses high Curie temperature and can achieve room temperature ferromagnetism. The magnetic moment is derived from the hybrid coupling effect of p-d exchange action. Meanwhile, the magnetic moment of the doping system becomes weak with the increase of the concentration of Ni. In addition, the absorption spectrum of Ni-interstitial ZnO is blue-shifted in the ultraviolet and visible light bands.

Size effect in thermoelectric power factor of nondegenerate and degenerate low-dimensional semiconductors

Low-dimensional materials have been known to give high thermoelectric (TE) performance by reducing the confinement length of the materials. Recently, we have shown that the TE power factor of low-dimensional semiconductors depends not only on the confinement length, but also on the thermal de Broglie wavelength of electrons or holes [Phys. Rev. Lett. 117, 036602 (2016)], in which the calculation was performed by assuming the semiconductors to be nondegenerate, i.e., we approximated the Fermi energy to lie only within the energy band gap, or in other words, the low doping approximation. Now, in this work, we generalize the previous results considering the degenerate case, in which the Fermi energy can exist in the valence or conduction bands, thus enabling a full consideration of heavy doping. An analytical formula for the TE power factor is derived to describe the size effect in the power factor of the low-dimensional semiconductors. We find that for both nondegenerate and degenerate cases, the TE power factor is enhanced in one- and two-dimensional semiconductors when the confinement length is smaller than the thermal de Broglie wavelength of the semiconductors, with Fermi energy around top (bottom) of valence (conduction) band for the p-type (n-type) semiconductors.

Effect of Mo doping concentration on the physical properties of ZnO studied by first principles

The experimental results of red-shift and blue-shift in absorption spectrum of Mo-doped ZnO are in mutual contradiction, and this phenomenon has not been explained rationally so far. For explaining this phenomenon, we analyze the energy band structure, state density, and absorption-spectrum distributions for each of Zn0.9583Mo0.0417O, Zn0.9375Mo0.0625O and Zn14Mo2O by first-principles calculation. The results show that within a limited doping amount range of 2.08 at%-3.13 at%, the higher Mo doping amount results in higher doping system volume, higher formation energy, lower system stability, and more difficult to dope. Meanwhile, all doping systems are converted into n-type degenerate semiconductors. Compared with the band gap of pure ZnO, the band gap of each doping system becomes narrow and the absorption spectrum shows red-shift. The higher the Mo doping amount, the weaker the narrowing of band gap becomes and the weaker the red-shift in absorption spectrum as well as the lower the electronic effective mass and the lower the electronic concentration; the lower the electronic mobility, the lower the electronic conductivity is; the lower the electronic magnetic moment is. The Curie temperature of doping system can reach a temperature higher than room temperature.

Effects of high magnetic field assisted annealing on structure and optical, electric properties of electrodeposited ZnO films

Electrodeposited ZnO films have been annealed at 300 °C for 2 h under 12 T high magnetic field (HMF) with the directions of parallel and perpendicular to the films, respectively. The structural, optical and electric properties were characterized by scanning electron microscopy, X-ray diffraction, photoluminescence (PL) spectra, X-ray photoemission spectroscopy (XPS) and Seebeck coefficient/electrical resistance measuring system. The results show that HMF has a significant effect on the growth of ZnO films along c-axis and leads to hexagonal platelets of ZnO growing parallel to the direction of HMF. Furthermore, the hexagonal platelets become bulky platelets with an obvious trendy rotating their c-axis parallel to the substrate. The PL spectra of all the films exhibits the UV and blue emission, moreover, the blue emission plays the main role. The resistivity of ZnO films increases with the increase of measure temperature, which shows a typical degenerate semiconductor characteristic. HMF reduces significantly the intensity of whole emission peaks and the resistivity of ZnO films. These may be attribute to the significant changes of the structure and morphology of ZnO films, leading to various amounts of the defects in the ZnO crystal.

Effect of Ga doping and point defect on magnetism of ZnO

The combined influence mechanism of Ga doping and Zn vacancy or O vacancy on magnetism of ZnO is studied using the first-principle calculation. The coexistence of Ga doping and Zn vacancy can achieve a Curie temperature higher than room temperature and the Ga doped ZnO system is a p-type diluted degenerate semiconductor with metalized ferromagnetism. The magnetism of the doping system of Ga doping and Zn vacancy is mainly contributed by double-exchange interaction through the holes of Zn vacancy taking carrier as medium. However, the system of Ga doping and O vacancy is non-magnetic. In the coexistence of Ga doping and Zn vacancy or O vacancy, a close relative distance between doping and vacancy will reduce the formation energy of the doping system but increase the easiness of doping and vacancy, as well as enhance the stability of the doping system.

The effects of degeneracy of the carrier ensemble on the energy loss rate and the high field mobility characteristics under the conditions of low lattice temperatures

The rate of loss of energy of the non-equilibrium electrons to the acoustic mode lattice vibration in a degenerate semiconductor is obtained under the condition, when the lattice temperature is low enough, so that the traditional approximations like the elastic nature of the electron-phonon collisions and the truncation of the phonon distribution to the equipartition law are not valid any more. Using the results of the energy loss rate, the non-ohmic mobility is then calculated. Evaluating the loss rate and the non-ohmic mobility in degenerate samples of Si and Ge we find that significant changes in both the characteristics have been effected compared to that in the non-degenerate samples, in the regime of lower energy and for relatively lower fields. The effected changes are more significant the lower the lattice temperature is.

Effects of Ag-Doping on Thermoelectric Properties of Ca(2−x)AgxSi Alloys

Ca(2−x)AgxSi (0 ≤ x ≤ 0.1) with 47.5% excess of Ca alloys were fabricated by melting in a tantalum tube and hot pressing technique. Phase structures of the samples were studied by means of x-ray diffraction. The electrical conductivity and Seebeck coefficient of Ca(2−x)AgxSi alloys were studied in the temperature range of 300–873 K. The electrical conductivity of the Ag-doped samples increases within the whole test temperature range. All samples show p-type semiconductor behavior. The electrical conductivity decreases with increasing temperature from 300 K to 873 K, which is typically observed for a degenerate semiconductor. Compared with the undoped samples, Ag-doping (x = 0.04–0.1) results in decreases of Seebeck coefficient, especially Ca(2−x)AgxSi with x = 0.1. The thermal conductivity of the doped samples gradually increases with increasing the Ag-doping content. The Ca(2−x)AgxSi with x = 0.02 sample exhibits the lowest thermal conductivity within the whole test temperature range. The ZT values of Ca(2−x)AgxSi with x = 0.02 sample have an enhancement in the temperature range of 300–873 K by contrast with those of the Ca2Si sample. The maximum ZT value is 0.16 at 837 k, which is observed for the Ca(2−x)AgxSi with x = 0.04 sample.