Temperature Dependence Of Carrier Density Research Articles

Robustly Boosting Thermoelectric Performance of N-Type PbSe via Lattice Plainification and Dynamic Doping.

Ideal thermoelectrics shall possess a high average ZT, which relies on high carrier mobility and appropriate carrier density at operating temperature. However, conventional doping usually results in a temperature-independent carrier concentration, making performance optimization over a wide temperature range be challenging. This work demonstrates the combination of lattice plainification and dynamic doping strategies is an effective route to boost the average ZT of N-type PbSe. Because Sn and Pb have similar ionic radii and electronegativity, this allows Sn to fill the intrinsic Pb vacancies and effectively improves the carrier mobility of PbSe to 1300 cm2 V-1 s-1. Furthermore, a trace amount of Cu is introduced into the Sn-filled PbSe to optimize the carrier concentration. The extra Cu is situated in the interstitial sides of the lattice, which undergoes a dissolution-precipitation process with temperature, leading to a strongly temperature-dependent carrier density in the material. This dynamic doping effectively improves the electrical transport properties and is also valid to suppress the lattice thermal conductivity. Ultimately, the resulting PbSn0.004Se+3‰Cu obtains a maximum ZT of ≈1.7 at 800 K and an average ZT of ≈1.0, with a 7.7% power generation efficiency in a single-arm device, showing significant potential for commercial application.

Interstitials in Thermoelectrics.

Defect structure is pivotal in advancing thermoelectric performance with interstitials being widely recognized for their remarkable roles in optimizing both phonon and electron transport properties. Diverse interstitial atoms are identified in previous works according to their distinct roles and can be classified into rattling interstitial, decoupling interstitial, interlayer interstitial, dynamic interstitial, and liquid interstitial. Specifically, rattling interstitial can cause phonon resonance in cage compound to scatter phonon transport; decoupling interstitial can contribute to phonon blocking and electron transport due to their significantly different mean free paths; interlayer interstitial can facilitate out-of-layer electron transport in layered compounds; dynamic interstitial can tune temperature-dependent carrier density and optimize electrical transport properties at wide temperatures; liquid interstitial could improve the carrier mobility at homogeneous dispersion state. All of these interstitials have positive impact on thermoelectric performance by adjusting transport parameters. This perspective therefore intends to provide a thorough overview of advances in interstitial strategy and highlight their significance for optimizing thermoelectric parameters. Finally, the profound potential for extending interstitial strategy to various other thermoelectric systems is discussed and some future directions in thermoelectric material are also outlined.

Weak antilocalization and topological edge states in PdSn4

Here, we report a successful synthesis of single crystals of a topological semimetal (TSM) candidate, PdSn4, using a self-flux route. The synthesized crystal is well characterized through x-ray diffraction (XRD), field emission scanning electron microscopy, and x-ray photoelectron spectroscopy. A detailed Rietveld analysis of the powder XRD pattern of PdSn4 confirmed the same to crystallize in an Aea2 space group instead of reported Ccce. A large magnetoresistance (MR) along with Shubnikov–de Haas (SdH) oscillations have been observed in magneto-transport measurements at 2 K. The presence of weak antilocalization (WAL) effect in the synthesized PdSn4 crystal is confirmed and analyzed using Hikami Larkin Nagaoka formalism, being applied on magnetoconductivity of the same at a low magnetic field. An extended Kohler's rule is implemented on MR data to determine the role of scattering processes and temperature-dependent carrier density on the transport phenomenon in PdSn4. Furthermore, the non-trivial band topology and the presence of edge states are shown through density functional theory-based theoretical calculations. All calculations are performed considering the Aea2 space group symmetry. The calculated Z2 invariants (0; 0 1 0) suggest the presence of weak topological insulating properties in PdSn4. A clear evidence of topological edge states at Γ point is visible in calculated edge states spectra. This is the first report on PdSn4, showing the presence of SdH oscillation in magneto-transport measurements. Also, the presence of a weak insulating phase in PdSn4 is described for the first time on the basis of calculated Z2 invariants.

Temperature-driven changes in the Fermi surface of graphite

We report on temperature-dependent size and anisotropy of the Fermi pockets in graphite revealed by magnetotransport measurements. The magnetoresistances obtained in fields along the c-axis obey an extended Kohler's rule, with the carrier density following prediction of a temperature-dependent Fermi energy, indicating a change in the Fermi pocket size with temperature. The angle-dependent magnetoresistivities at a given temperature exhibit a scaling behavior. The scaling factor that reflects the anisotropy of the Fermi surface is also found to vary with temperature. Our results demonstrate that temperature-driven changes in Fermi surface can be ubiquitous and need to be considered in understanding the temperature-dependent carrier density and magnetoresistance anisotropy in semimetals.

Extended Kohler’s Rule of Magnetoresistance

A notable phenomenon in topological semimetals is the violation of Kohler$^,$s rule, which dictates that the magnetoresistance $MR$ obeys a scaling behavior of $MR = f(H/\rho_0$), where $MR = [\rho_H-\rho_0]/\rho_0$ and $H$ is the magnetic field, with $\rho_H$ and $\rho_0$ being the resistivity at $H$ and zero field, respectively. Here we report a violation originating from thermally-induced change in the carrier density. We find that the magnetoresistance of the Weyl semimetal, TaP, follows an extended Kohler$^,$s rule $MR = f[H/(n_T\rho_0)]$, with $n_T$ describing the temperature dependence of the carrier density. We show that $n_T$ is associated with the Fermi level and the dispersion relation of the semimetal, providing a new way to reveal information on the electronic bandstructure. We offer a fundamental understanding of the violation and validity of Kohler$^,$s rule in terms of different temperature-responses of $n_T$. We apply our extended Kohler$^,$s rule to BaFe$_2$(As$_{1-x}$P$_x$)$_2$ to settle a long-standing debate on the scaling behavior of the normal-state magnetoresistance of a superconductor, namely, $MR$ ~ $tan^2\theta_H$, where $\theta_H$ is the Hall angle. We further validate the extended Kohler$^,$s rule and demonstrate its generality in a semiconductor, InSb, where the temperature-dependent carrier density can be reliably determined both theoretically and experimentally.

Ga composition effects on the electrical parameters of (Al,Ga)Sb/InAs two-dimensional electron gas

The effects of Ga composition on the electrical parameters of (Al,Ga)Sb/InAs two-dimensional electron gas have been investigated. The (Al,Ga)Sb/InAs structures are grown on GaAs (001) substrate by molecular-beam epitaxy with various Ga compositions in Al1−x Ga x Sb bottom barrier. The sheet resistance shows a crossover with the variation of temperature, and the temperature T* corresponding to this crossover decreases with increasing the Ga composition. The temperature dependence of the electron mobility is similar for the samples with different Ga compositions, while the temperature dependence of the carrier density is obviously different. Qualitative analyses reveal that the temperature dependent portion of carrier density is mainly determined by the interface-related donors. A thermal activation model is used to fit the temperature dependence of carrier density, and the fitting results imply that the thermal activation energies of the carriers are significantly affected by the Ga composition. Our results provide useful information for understanding the sources of carriers in the (Al,Ga)Sb/InAs systems, and provide one pathway for the modulation of the temperature dependent carrier density.

Interaction between in-gap states and carriers at the conductive interface between perovskite oxides

The 2D electron systems of SrTiO3/NdGaO3 (STO/NGO) and amorphous-LaAlO3/SrTiO3/NdGaO3 (a-LAO/STO/NGO) heterojunctions were explored. An obvious interaction between in-gap states (IGSs) and carriers was found. The IGSs can trap a large number of carriers and enhance carrier scattering. As a result of the high density of IGSs in STO, the conductivity of STO/NGO was severely weakened. However, for a-LAO/STO/NGO heterojunctions, the high carrier density can reduce the effect of IGSs through the electrostatic screening effect. The competition between IGSs and the screening effect of carriers results in an insulator–metal transition and a strange temperature dependence of carrier density. We also explored the interaction between IGSs and carriers theoretically. A mathematical description was proposed and the calculated results showed good agreement with experimental findings.

Growth of Sb-Doped β-FeSi<sub>2</sub> Epitaxial Films and Optimization of Donor Activation Conditions

Sb-doped β-FeSi2 epitaxial films on Si(111) were grown by molecular beam epitaxy to control an electron density of β-FeSi2. After an optimization of donor activation conditions in the Sb-doped β-FeSi2, the electron density of 6 × 1018 cm-3 at 300 K was achieved by thermal annealing in a N2 ambient. In the temperature dependence of carrier density, the n-type conduction was changed to p-type conduction at low temperatures in the film annealed at high temperature (600 °C). Raman spectra of the annealed films showed that both Fe and Si sites were substituted by the doped Sb in β-FeSi2 lattice.

Carrier trapping and activation at short-period wurtzite/zinc-blende stacking sequences in polytypic InAs nanowires

We explore the effects of random and short-period crystal-phase intermixing in InAs nanowires (NW) on the carrier trapping and thermal activation behavior using correlated optical and electrical transport spectroscopy. The polytypic InAs NWs are grown by catalyst-free molecular beam epitaxy under different temperatures, resulting in different fractions of wurtzite (WZ) and zincblende (ZB) and variable short-period (\ensuremath{\sim}1--4 nm) WZ/ZB stacking sequences. Temperature-dependent microphotoluminescence $(\ensuremath{\mu}\text{PL})$ studies reveal that variations in the WZ/ZB stacking govern the emission energy and carrier confinement properties. The optical transition energies are modeled for a wide range of WZ/ZB stacking sequences using a Kronig-Penney type effective mass approximation, while comparison with experimental results suggests that polarization sheet charges on the order of \ensuremath{\sim}0.0016--0.08 C/m along the WZ/ZB interfaces need to be considered to best describe the data. The thermal activation characteristics of carriers trapped inside the short-period WZ/ZB structure are directly reproduced in the temperature-dependent carrier density evolution (4--300 K) probed by four-terminal (4T) NW-field effect transistor measurements. In particular, we find that activation of carriers in-between $\ensuremath{\sim}{10}^{16}\ensuremath{-}{10}^{17}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$ follows a two-step process, with activation at low temperature attributed to WZ/ZB traps and activation at high temperature being linked to surface states and electron accumulation at the InAs NW surface.

Two-Dimensional SnS: A Phosphorene Analogue with Strong In-Plane Electronic Anisotropy.

We study the anisotropic electronic properties of two-dimensional (2D) SnS, an analogue of phosphorene, grown by physical vapor transport. With transmission electron microscopy and polarized Raman spectroscopy, we identify the zigzag and armchair directions of the as-grown 2D crystals. The 2D SnS field-effect transistors with a cross-Hall-bar structure are fabricated. They show heavily hole-doped (∼1019 cm-3) conductivity with strong in-plane anisotropy. At room temperature, the mobility along the zigzag direction exceeds 20 cm2 V-1 s-1, which can be up to 1.7 times that in the armchair direction. This strong anisotropy is then explained by the effective mass ratio along the two directions and agrees well with previous theoretical predictions. Temperature-dependent carrier density determined the acceptor energy level to be ∼45 meV above the valence band maximum. This value matches a calculated defect level of 42 meV for Sn vacancies, indicating that Sn deficiency is the main cause of the p-type conductivity.

Nearly Constant Electrical Resistance over Large Temperature Range in Cu3NMx (M = Cu, Ag, Au) Compounds

Electrical resistance is a material property that usually varies enormously with temperature. Constant electrical resistivity over large temperature range has been rarely measured in a single solid. Here we report the growth of Cu3NMx (M = Cu, Ag, Au) compound films by magnetron sputtering, aiming at obtaining single solids of nearly constant electrical resistance in some temperature ranges. The increasing interstitial doping of cubic Cu3N lattice by extra metal atoms induces the semiconductor-to-metal transition in all the three systems. Nearly constant electrical resistance over 200 K, from room temperature downward, was measured in some semimetallic Cu3NMx samples, resulting from opposite temperature dependence of carrier density and carrier mobility, as revealed by Hall measurement. Cu3NAgx samples have the best performance with regard to the range of both temperature and doping level wherein a nearly constant electrical resistance can be realized. This work can inspire the search of other materials of such a quality.

Reversible room-temperature ferromagnetism in Nb-doped SrTiO3single crystals

The search for oxide-based room-temperature ferromagnetism has been one of the holy grails in condensed matter physics. Room-temperature ferromagnetism observed in Nb-doped SrTiO3 single crystals is reported in this Rapid Communication. The ferromagnetism can be eliminated by air annealing (making the samples predominantly diamagnetic) and can be recovered by subsequent vacuum annealing. The temperature dependence of magnetic moment resembles the temperature dependence of carrier density, indicating that the magnetism is closely related to the free carriers. Our results suggest that the ferromagnetism is induced by oxygen vacancies. In addition, hysteretic magnetoresistance was observed for magnetic field parallel to current, indicating that the magnetic moments are in the plane of the samples. The x-ray photoemission spectroscopy, the static time-of-flight and the dynamic secondary ion mass spectroscopy and proton induced x-ray emission measurements were performed to examine magnetic impurities, showing that the observed ferromagnetism is unlikely due to any magnetic contaminant.

Intrinsic Electron-Phonon Resistivity ofBi2Se3in the Topological Regime

We measure the temperature-dependent carrier density and resistivity of the topological surface state of thin exfoliated Bi(2)Se(3) in the absence of bulk conduction. When the gate-tuned chemical potential is near or below the Dirac point, the carrier density is strongly temperature-dependent, reflecting thermal activation from the nearby bulk valence band, while, above the Dirac point, unipolar n-type surface conduction is observed with negligible thermal activation of bulk carriers. In this regime, linear resistivity vs temperature reflects intrinsic electron-acoustic phonon scattering. A quantitative comparison with a theoretical transport calculation including both phonon and disorder effects gives the ratio of deformation potential to Fermi velocity D/ħν(F)=4.7 Å(-1). This strong phonon scattering in the Bi(2)Se(3) surface state gives intrinsic limits for the conductivity and charge carrier mobility at room temperature of ~550 μS per surface and ~10,000 cm(2)/V s.

Hole-polar phonon interaction scattering mobility in chain structured TlSe0.75S0.25crystals

In this study, the electrical resistivity, charge carriers density and Hall mobility of chain structured TlSe0.75S0.25 crystal have been measured and analyzed to establish the dominant scattering mechanism in crystal. The data analyses have shown that this crystal exhibits an extrinsic p-type conduction. The temperature-dependent dark electrical resistivity analysis reflected the existence of three energy levels located at 280 meV, 68 meV and 48 meV. The temperature dependence of carrier density was analyzed by using the single donor–single acceptor model. The carrier concentration data were best reproduced assuming the existence of an acceptor impurity level being located at 68 meV consistent with that observed from resistivity measurement. The model allowed the determination of the hole effective mass and the acceptor–donor concentration difference as 0.44m0 and 2.2 × 1012 cm–3, respectively. The Hall mobility of the TlSe0.75S0.25 crystal is found to be limited by the scattering of charged carriers over the (chain) boundaries and the scattering of hole–polar phonon interactions above and below 300 K, respectively. The value of the energy barrier height at the chain boundaries was found to be 261 meV. The polar phonon scattering mobility revealed the high-frequency and static dielectric constants of 13.6 and 15.0, respectively. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of oxygen content on the transport properties of La-doped SrTiO 3 thin films

We report on the study of the transport property of La-doped SrTiO 3 thin films grown by pulsed laser deposition (PLD) method under the oxygen pressures of 0.1 Pa, 1 Pa and 10 Pa. With the increase of oxygen pressure, resistivity of the samples is enhanced markedly. A metal-to-insulator transition appears in all the films at low temperatures. The Hall effect has been studied to obtain the information on carrier density and Hall mobility. An electron-like character and temperature-dependent carrier density have been found. It shows a monotonic decrease with the increase of oxygen pressure. This phenomenon can be ascribed to the cation vacancies induced by excessive oxygen in the films. The Hall mobility also exhibits obvious dependence on oxygen pressure. Due to the inelastic scattering strength of electrons, the transport behavior in metallic region can be described by the T 2 law or small-polaron coherent conduction model.

Determination of carrier effective mass, impurity energy levels, and compensation ratio in Ga4Se3S layered crystals by Hall effect measurements

AbstractIn this work, the dark electrical resistivity, charge carriers density and Hall mobility of Ga4Se3S single crystals have been measured and analyzed in the temperature region of 200–350 K. The data analyses have shown that this crystal exhibits an extrinsic n‐type of conduction. The temperature‐dependent dark electrical resistivity analysis reflected the existence of energy level as 0.31 eV. The temperature dependence of carrier density was analyzed by using the single donor‐single acceptor model. The latter analysis allowed the determination of electron effective mass as 0.38m0, hole effective mass as 0.42m0, donor impurity energy level as 0.45 eV, acceptor–donor concentration ratio (Na/Nd) as 0.97 and a donor–acceptor concentration difference as Nd – Na = 1.5 × 1011 cm–3. The Hall mobility of Ga4Se3S is found to increase with decreasing temperature following a power law of slope of ∼(–6.3). The abnormal behavior of mobility was attributed to the domination of phonon scattering and/or crystals anisotropy. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical Study of the Free-Carrier Response ofLaTiO3/SrTiO3Superlattices

We used infrared spectroscopic ellipsometry to investigate the electronic properties of LaTiO_{3}/SrTiO_{3} superlattices (SLs). Our results indicated that, independent of the SL periodicity and individual layer thickness, the SLs exhibited a Drude metallic response with sheet carrier density per interface approximately 3x10;{14} cm;{-2}. This is probably due to the leakage of d electrons at interfaces from the Mott insulator LaTiO3 to the band insulator SrTiO3. We observed a carrier relaxation time approximately 35 fs and mobility approximately 35 cm;{2} V-1 s;{-1} at 10 K, and an unusual temperature dependence of carrier density that was attributed to the dielectric screening of quantum paraelectric SrTiO3.

Continuous Laser-Excited Photothermal Spectrometry of CdSxSe1–x Doped Glasses

Photothermal lens measurements and finite element modeling are used to examine the physical changes taking place in optical filter glasses. Colored glass and neutral density filters are found to have a strong positive temperature-dependent refractive index change. The overall positive refractive index change is thought to be a consequence of complex counteracting factors: stress-induced birefringence, polarizability, structural network, and temperature-dependent carrier density changes in the CdSxSe1-x microcrystals that produce optical properties of these glasses. Finite element analysis (FEA) modeling is used to examine the temperature profiles and the goodness of the semi-infinite thermal diffusion solution normally used for thermal lens experiments. The results of FEA were used to optimize experimental parameters and calculate values of dn/dT for the glass by comparison with standard liquid samples.

Growth and small-polaron conduction of hole-dopedLaTiO3+δ∕2andNdTiO3+δ∕2thin films

$\mathrm{Re}\mathrm{Ti}{\mathrm{O}}_{3+\ensuremath{\delta}∕2}$ ($\mathrm{Re}=\mathrm{La}$ and Nd) thin films have been deposited on $(100)\mathrm{La}\mathrm{Al}{\mathrm{O}}_{3}$ substrates using the pulsed laser deposition technique. The oxygen content in the formula can be tuned to some extent by carefully adjusting the oxygen pressure during film deposition. When grown at higher oxygen pressures, the films tend to be in their fully oxidized insulating form ${\mathrm{Re}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ $(\ensuremath{\delta}=1,{\mathrm{Ti}}^{4+})$ with a twinned monoclinic structure of ${[01\overline{2}]}_{m}$ orientations. When grown in a background vacuum around $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}\phantom{\rule{0.3em}{0ex}}\mathrm{Pa}$, the films are lightly hole doped $(\ensuremath{\delta}\ensuremath{\sim}0,\ensuremath{\sim}{\mathrm{Ti}}^{3+})$ and retain the near-cubic perovskite structure with the ${[001]}_{c}$ axis perpendicular to the film surface, though high-density defects may exist. These two phases coexist in the films grown at intermediate oxygen pressures. For the vacuum-grown near-cubic films, the antiferromagnetic ordering dies in the lanthanum titanate due to oxygen doping but still survives in the neodymium titanate owing to the higher electron correlation strength in the latter. The transport behavior in the $\ensuremath{\sim}{\mathrm{Ti}}^{3+}$ Mott insulating films can be described well by the small-polaron hopping or variable range hopping model. In the hole-doped metallic films, a temperature-dependent carrier density and Hall mobility were observed, especially at low temperatures. The temperature dependence of the resistance can be almost perfectly fitted by a small-polaron coherent conduction model $[{R}_{s}(T)={R}_{s}(0)+C{\ensuremath{\omega}}_{\ensuremath{\alpha}}∕{\mathrm{sinh}}^{2}({T}_{\ensuremath{\omega}}∕T)]$.

Conductive mechanism in manganite materials

We describe a model in which f(T)=M(T)/M m a x represents both the fraction of the itinerant electron density in the double-exchange (DE) theory and the magnetization σ in the current carrier density collapse (CCDC) theory. With this model, we have checked the DE and CCDC theories with our experimental results of the transport behavior. The DE theory yields agreement with the experimental resistivity excellently, in which the conductivity is the sum of the polaronic and itinerant electronic conductivity for the insulator-metal transition regime. The fitting curves of the resistivity by the CCDC theory deviate from the experiment seriously. This might be caused by the improper assumption of the temperature-dependent carrier density and the temperature-independent carrier mobility. Therefore, it is concluded that the DE theory is more suitable to explain the conductive mechanism in perovskite manganites.