Range Of Carrier Concentrations Research Articles

TTEP: A code for efficient calculation of the thermal transport from constant electron-phonon coupling approximation

A Fortran code named TTEP (Thermal Transport from constant Electron-Phonon coupling approximation) is presented in this work. Within the framework of first-principles calculation, this code utilizes the deformation potential approximation to replace the electron–phonon (EP) coupling matrix element in the phonon scattering due to EP interaction. By adopting this approximation, our method achieves a balance between computational efficiency and accuracy, enabling the exploration of the thermal transport from EP interaction with much reduced computational consumption. One of the key features of TTEP is its versatility in handling a broad range of carrier concentration, including both hole and electron doping. We also extend the applicability of TTEP to actual doping case and capture EP effect over a wide temperature range. Other scattering mechanisms, such as grain boundary scattering and phonon–phonon interaction, have been included in the calculation of lattice thermal conductivity through the Matthiessen’s rule. Several test cases are presented in this work to demonstrate the above features.

Directional solidification induced compositional gradient for a quick evaluation of transport properties in p-PbTe thermoelectrics

Revelation of transport properties in a broad range of carrier concentration is indispensable for thermoelectrics to locate its performance optimum. This signifies inevitable efforts on the syntheses of many samples with various concentrations of dopants. Moreover, the realization of low carrier concentration is usually a great challenge for a precise control. Directional solidification can thermodynamically enable a spontaneous gradient in composition, therefore, naturally offers a convenient pathway to continuously manipulate the dopant concentration thus the carrier concentration by one synthesis. In this work, gradiently sodium-doped PbTe ingots with ∼10 mm in diameter are successfully synthesized by a vertical gradient solidification (VGS) technique. Such a composition gradient leads to an efficient manipulation of hole concentration in a broad range as well as a continuous variation in transport properties along the solidification direction. The comparable transport properties to the literatures demonstrate the strategy used here to be highly efficient for the evaluation of transport properties, which is believed to be equally applicable for efficient research of both existing and novel thermoelectric materials.

Magneto-transport in the monolayer MoS2 material system for high-performance field-effect transistor applications

Electronic transport in monolayer MoS2 is significantly constrained by several extrinsic factors despite showing good prospects as a transistor channel material. Our paper aims to unveil the underlying mechanisms of the electrical and magneto-transport in monolayer MoS2. In order to quantitatively interpret the magneto-transport behavior of monolayer MoS2 on different substrate materials, identify the underlying bottlenecks, and provide guidelines for subsequent improvements, we present a deep analysis of the magneto-transport properties in the diffusive limit. Our calculations are performed on suspended monolayer MoS2 and MoS2 on different substrate materials taking into account remote impurity and the intrinsic and extrinsic phonon scattering mechanisms. We calculate the crucial transport parameters such as the Hall mobility, the conductivity tensor elements, the Hall factor, and the magnetoresistance over a wide range of temperatures, carrier concentrations, and magnetic fields. The Hall factor being a key quantity for calculating the carrier concentration and drift mobility, we show that for suspended monolayer MoS2 at room temperature, the Hall factor value is around 1.43 for magnetic fields ranging from 0.001 to 1 Tesla, which deviates significantly from the usual value of unity. In contrast, the Hall factor for various substrates approaches the ideal value of unity and remains stable in response to the magnetic field and temperature. We also show that the MoS2 over an Al2O3 substrate is a good choice for the Hall effect detector. Moreover, the magnetoresistance increases with an increase in magnetic field strength for smaller magnetic fields before reaching saturation at higher magnetic fields. The presented theoretical model quantitatively captures the scaling of mobility and various magnetoresistance coefficients with temperature, carrier densities, and magnetic fields.

Flexural and acoustic phonon-drag thermopower and electron energy loss rate in silicene

We have performed a comprehensive numerical and analytical examination of two crucial transport aspects in silicene: the phonon-drag thermopower, Sp , and the electron’s energy loss rate, Fe . Specifically, our investigation is centered on their responses to out-of-plane flexural phonons and in-plane acoustic phonons in silicene, a two-dimensional allotrope of silicon as a function of electron temperature, T, and electron concentration, n, upto the room temperature. It is found that the calculated quantities have a non-monotonic dependence for the phonon modes for both parameters (T and n) considered while analytical results predict definite dependencies up to the complete low-temperature Bloch–Gruneisen (BG) regime. To provide a more comprehensive picture, we contrast the complete numerical outcomes with the approximated analytical BG results, revealing a convergence within a specific range of temperature and carrier concentration. In light of this convergence, we put forth suggestions to elucidate the underlying factors responsible for this behavior. A comparison based on the magnitude of the calculated quantities can be made from the figures between the two considered phonon modes, which clearly shows that the out-of-plane flexural phonons are effective throughout the considered temperature range. This observation leads us to posit that the dominating contribution of the out-of-plane flexural phonon modes hinges upon the deformation potential constant and phonon energy associated with the phonon mode. Our study carries significant implications for estimating the electrical and thermal properties of silicene and provides valuable insights for the development of devices based on silicene-based technologies.

Electronic, Mechanical, and thermoelectric properties of Ge/Zn co-doped SnSe: first-principles calculations

Materials based on tin selenide have attracted significant attention due to their unique properties, particularly their high ZT value. This study investigates the impact of co-doping Germanium and Zinc on the electronic, mechanical, and thermoelectric properties of SnSe crystal using first-principles calculations. The doped structure demonstrated a p-type semiconducting behavior with a triclinic stable structure, which was predicted by the calculated elastic constants. Thermoelectric properties were studied for both doped and undoped systems across a wide range of carrier concentrations and temperatures. Results showed that co-doping SnSe with Ge/Zn reduced electronic thermal conductivity at room temperature while simultaneously doubling the Seebeck coefficient. This promising combination of features suggests high thermoelectric performance for the material.

Dopant Control of Solution-Processed CuI:S for Highly Conductive p-Type Transparent Electrode.

Copper iodide (CuI) has garnered considerable attention as a promising alternative to p-type transparent conducting oxides owing to its low cation vacancy formation energy, shallow acceptor level, and readily modifiable conductivity via doping. Although sulfur (S) doping through liquid iodination has exhibited high efficacy in enhancing the conductivity with record high figure of merit (FOM) of 630 00MΩ-1, solution-processed S-doped CuI (CuI:S) for low-cost large area fabrication has yet to be explored. Here, a highly conducting CuI:S thin-film for p-type transparent conducting electrode (TCE) is reported using low temperature solution-processing with thiourea derivatives. The optimization of thiourea dopant is determined through a comprehensive acid-base study, considering the effects of steric hindrance. The modification of active groups of thioureas facilitated a varying carrier concentration range of 9×1018-2.52×1020cm-3 and conductivities of 4.4-390.7Scm-1. Consequently, N-ethylthiourea-doped CuI:S exhibited a FOM value of 7 600MΩ-1, which is the highest value among solution-processed p-type TCEs to date. Moreover, the formulation of CuI:S solution for highly conductive p-type TCEs can be extended to CuI:S inks, facilitating high-throughput solution-processes such as inkjet printing and spray coating.

Carrier Screening Controls Transport in Conjugated Polymers at High Doping Concentrations.

Transport properties of doped conjugated polymers (CPs) have been widely analyzed with the Gaussian disorder model (GDM) in conjunction with hopping transport between localized states. These models reveal that even in highly doped CPs, a majority of carriers are still localized because dielectric permittivity of CPs is well below that of inorganic materials, making Coulomb interactions between carriers and dopant counterions much more pronounced. However, previous studies within the GDM did not consider the role of screening the dielectric interactions by carriers. Here we implement carrier screening in the Debye-Hückel formalism in our calculations of dopant-induced energetic disorder, which modifies the Gaussian density of states (DOS). Then we solve the Pauli master equation using Miller-Abrahams hopping rates with states from the resulting screened DOS to obtain conductivity and Seebeck coefficient across a broad range of carrier concentrations and compare them to measurements. Our results show that screening has significant impact on the shape of the DOS and consequently on carrier transport, particularly at high doping. We prove that the slope of Seebeck coefficient versus electric conductivity, which was previously thought to be universal, is impacted by screening and decreases for systems with small dopant-carrier separation, explaining our measurements. We also show that thermoelectric power factor is underestimated by a factor of ∼10 at higher doping concentrations if screening is neglected. We conclude that carrier screening plays a crucial role in curtailing dopant-induced energetic disorder, particularly at high carrier concentrations.

The electron–phonon scattering and charge transport of the two-dimensional (2D) polar h-BX(X = P, As, Sb) monolayers

In this paper, the electron–phonon scattering and phonon-limited transport properties of the two-dimensional polar h-BX(X = P, As, Sb) have been studied through first-principles calculations in combination with Boltzmann transport theory. The electron–phonon scattering in these three systems is systematically assessed. Remarkably, intravalley scattering and intervalley scattering are separately investigated, of which the contribution to total scattering is found to be relatively comparable. The carrier mobility is determined over a broad range of carrier concentrations. The results indicate that h-BX (BP, BAs, BSb) simultaneously possess ultrahigh electron mobilities (4097 cm2 V−1 s−1, 4141 cm2 V−1 s−1, 12 215 cm2 V−1 s−1) and hole mobilities (7563 cm2 V−1 s−1, 7606 cm2 V−1 s−1, 22 282 cm2 V−1 s−1) at room temperature as compared to the most known two-dimensional (2D) materials. Additionally, it is discovered that compressive strain can induce a further increase in carrier mobility. The exceptional charge transport properties exhibited by these 2D semiconductors are attributed to the small effective masses in combination with the significant suppression of scattering due to high optical longitudinal optical- and transverse optical-phonon frequencies. This is the first time that we have provided a systematic interpretation of the reason for the exceptional charge transport properties exhibited by the 2D h-BX(X = P, As, Sb) semiconductors. Our finding can provide a theoretical perspective regarding the search for 2D materials with the high carrier mobility.

Growth and Photoelectrochemical Characterization of Epitaxial ZnTe Photocathodes for Carbon Dioxide Reduction

Harnessing solar energy to drive photocatalytic carbon dioxide reduction reactions (CO2RR) provides an appealing pathway to generate hydrocarbon and oxygenated fuels without an external power source. Zinc telluride (ZnTe) is a II-VI semiconductor which has been identified as a promising photocathode material due to its suitable band gap alignments to CO2 reduction reaction potentials, chemical stability, and strong p-type character. Using molecular beam epitaxy (MBE), single crystal and epitaxial layers are synthesized to gain a deeper understanding of fundamental charge transport and reactivity mechanisms between the single crystal ZnTe thin film and the CO2-saturated electrolyte. These findings are of fundamental interest and are also critical to the design of efficient tandem solar fuels generators for unassisted photoelectrochemical CO2R. Epitaxy allows for highly controlled doping of the thin films over a large range of carrier concentrations. This work focuses largely on the synthesis and characterization of nitrogen doped p-type ZnTe via MBE. Films grown in the temperature range of 340–360ºC on GaAs of (100) orientation with a 200 nm undoped ZnTe buffer layer and a 100 nm doped ZnTe layer have been characterized by RHEED, XRD, AFM, and Hall effect measurements. Doping concentrations between 1020 cm-3 and 1018 cm-3 have been achieved. Dark current-voltage measurements have been used to indicate stability of the electrode in aqueous conditions in less than -0.5 V vs RHE. Future work will include further investigations into carrier dynamics via transient absorption spectroscopy and continual development of a tandem, ZnTe-based photocathode.

Anomalous conductance quantization of a one-dimensional channel in monolayer WSe2

Among quantum devices based on 2D materials, gate-defined quantum confined 1D channels are much less explored, especially in the high-mobility regime where many-body interactions play an important role. We present the results of measurements and theory of conductance quantization in a gate-defined one-dimensional channel in a single layer of transition metal dichalcogenide material WSe2. In the quasi-ballistic regime of our high-mobility sample, we report conductance quantization steps in units of e2/h for a wide range of carrier concentrations. Magnetic field measurements show that as the field is raised, higher conductance plateaus move to accurate quantized values and then shift to lower conductance values while the e2/h plateau remains locked. Based on microscopic atomistic tight-binding theory, we show that in this material, valley and spin degeneracies result in 2 e2/h conductance steps for noninteracting holes, suggesting that symmetry-breaking mechanisms such as valley polarization dominate the transport properties of such quantum structures.

Ab initio predictions of the structural, electronic, optical, elastic, and thermoelectric properties of quasi-two-dimensional BaFZnP

We studied the structural parameters, elasticity, electronic structure, linear optical spectra, and thermoelectric coefficients of quasi-two-dimensional BaFZnP using ab initio methods. The computed equilibrium coordinates of atomic positions and lattice parameters were in good agreement with existing results. We derived numerical values for the elastic moduli and related physical parameters, such as the independent monocrystalline elastic constants, modulus of compressibility, modulus of shear, Poisson's ratio, Young's modulus, isotropic sound velocities, Debye temperature, ductility/brittleness nature, and elastic anisotropy. We examined the electronic properties including the energy band dispersions, density of states, and charge carrier effective masses, and concluded that BaFZnP is a semiconductor with a direct bandgap. The linear optical functions were explored in an energy window of 0–15 eV for incident electromagnetic waves polarized parallel to the [100] and [001] crystal directions. Numerical evaluation of the dependencies of the thermoelectric coefficients on charge carrier concentration and temperature shows that the thermoelectric characteristics of the p-doping BaFZnP are better than the corresponding ones of the n-doping BaFZnP over the charge carrier concentration range of 1016−1021cm−3. The p-doping BaFZnP with a hole concentration of around 1.81×1020 cm−3 possesses a figure of merit of 1.16 at 900 K, making it suitable for thermoelectric application.

Seebeck-induced anomalous Nernst effect in van der Waals MnBi2Te4 layers

Magnetic semiconductors with an anomalous Hall conductivity σ xy ≠ 0 near the Fermi energy are expected to have a large anomalous Nernst coefficient N owing to the Seebeck term, which is the product of the Hall angle ratio and Seebeck coefficient. In this study, we examined the typical cases of ∣N∣ ≥ 20 μV K–1 in the ferrimagnetic phase of semiconducting van der Waals layers MnBi2Te4 using first-principles calculations. A large enhancement in ∣N∣ was obtained by the Seebeck term for a wide range of carrier concentrations. The present results motivate further studies on the anomalous Nernst effect in intrinsically or doped magnetic semiconductors.

Optoelectronic, mechanical, and thermoelectric properties of Na/I co-doped SnSe via ab initio calculations

Tin selenide-based materials have attracted much attention recently because of their unique properties. This study investigates the effect of Sodium and Iodine co-doping on the optoelectronic, mechanical, and thermoelectric properties of orthorhombic SnSe via First-principles calculations. Na/I co-doped crystal was found to have a triclinic structure, and its electronic bandgap is 0.325 ​eV, whereas the calculated band gap of the pristine SnSe is 0.824 ​eV using SCAN functional. Na/I co-doping alters the Fermi level of SnSe up to its conduction bands, resulting in an n-type system. Furthermore, the static dielectric constant shows that the doped system could be suitable for capacitors and solar cell applications. According to the calculated elastic constants, the doped system is stable. Moreover, it has a negative Poisson's ratio value suggesting it's auxetic sensor material. The thermoelectric performance is examined from 300 ​K to 800 ​K across a broad range of carrier concentrations for the doped and undoped SnSe systems. We have found that Na/I co-doping enhances the electrical conductivity and the Seebeck coefficient of SnSe. The highest power factor calculated for the doped system was 27.9μV/Kcm at carrier concentration of n≅−3×1020cm−3.

Doping of dilute nitride compounds grown by liquid phase epitaxy

The doping of dilute nitrides is an important point of the growth and processing technology for different optoelectronic devices based on these compounds. In this paper, both intentional and nonintentional doping of InGaAsN and GaAsSbN have been investigated by temperature-dependent Hall effect measurements. Dilute nitrides layers have been grown by low-temperature (Tcryst < 600 °C) liquid-phase epitaxy (LPE). The chemical elements Sn and Mg have been chosen as n-type and p-type dopants, respectively. All nominally undoped InGaAsN layers are n-type with free carrier concentration about one order of magnitude higher than layers not containing nitrogen and grown by LPE. This makes it difficult to obtain epitaxial layers with p-type conductivity with these compounds. However, high-quality n-type, p-type and nearly compensated GaAsSbN layers have been successfully grown covering a large range of carrier concentrations from 1015 to 6×1018 cm−3. The quality of the GaAsSbN layers has been evidenced by a good photoresponse with a low energy threshold extended down to 1.2 eV as obtained by surface photovoltage spectroscopy.

Unravelling Disorder Effects on Thermoelectric Properties of Semicrystalline Polymers in a Wide Range of Doping Levels.

Thermoelectric (TE) performance of a specific semicrystalline polymer is studied experimentally only in a limited range of doping levels with molecular doping methods. The doping level is finely controlled via in situ electrochemical doping in a wide range of carrier concentrations with an electrolyte ([PMIM]+ [TFSI]- )-gated organic electrochemical transistor system. Then, the charge generation/transport and TE properties of four p-type semicrystalline polymers are analyzed and their dynamic changes of crystalline morphologies and local density of states (DOS) during electrochemical doping are compared. These polymers are synthesized based on poly[(2,5-bis(2-alkyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophene-2-yl)benzo[c][1,2,5]thiadiazole)] by varying side chains: With oligoethylene glycol (OEG) substituents, facile p-doping is achieved because of easy penetration of TFSI- ions into the polymer matrix. However, the charge transport is hindered with longer OEG chains length because of the enhanced insulation. Therefore, with the shortest OEG substituents the electrical conductivity (30.1 S cm-1 ) and power factor (2.88 µW m-1 K-2 ) are optimized. It is observed that all polymers exhibit p- to n-type transition in Seebeck coefficients in heavily doped states, which can be achieved by electrochemical doping. These TE behaviors are interpreted based on the relation between the localized DOS band structure and molecular packing structure during electrochemical doping.

Carrier concentration mediated enhancement in thermoelectric performance of various polymorphs of hafnium oxide: a plausible material for high temperature thermoelectric energy harvesting application

The optimization of figure of merit by tuning carrier concentrations is an effective way to realize efficient thermoelectrics (TEs). Recently, the feasibility of high p-type carrier concentration (order of ∼1022cm−3) is experimentally demonstrated in various polymorphs of hafnium oxide (HfO2). In light of these studies, using the first-principles calculation combined with the semi-classical Boltzmann transport theory and phonon dynamics, we realized high TE performance in various polymorphs of HfO2 in a range of carrier concentrations at high temperatures. The phonon dispersion calculations confirm the dynamical stability of all polymorphs. The observed values of the Seebeck coefficient are 945.27 mV K−1, 922.62 mV K−1, 867.44 mV K−1, and 830.81 mV K−1 for tetragonal (t), orthorhombic (o), monoclinic (m), and cubic (c) phases of HfO2, respectively, at 300 K. These values remain positive at all studied temperatures which ensures the p-type behaviour of HfO2 polymorphs. The highest value of electrical conductivity (2.34 × 1020 Ω−1m−1s−1) observed in c-HfO2 at 1200 K, and the lowest value of electronic thermal conductivity (0.37 × 1015 W mK s−1) observed in o-HfO2 at 300 K. The lattice thermal conductivities at room temperature are 5.56 W mK−1, 2.87 W mK−1, 4.32 W mK−1, and 1.75 W mK−1 for c-, m-, o- and t- HfO2, respectively which decrease to 1.58 W mK−1, 0.92 W mK−1, 1.12 W mK−1, 0.53 W mK−1 at 1200 K for respective phases. The low lattice thermal conductivities lead to the high values of the figure of merit, i.e. 0.97, 0.87, 0.83, and 0.77 at 1200 K for the m-, o-, t-, and c- HfO2, respectively, at the optimized carrier concentrations (∼1021 cm−3). The predicted optimized carrier concentrations for various phases are in close agreement with the experimental reports. The estimated high figure of merit can make HfO2 a potential material for TE energy harvesting applications at elevated temperatures.

Percolating Superconductivity in Air‐Stable Organic‐Ion Intercalated MoS2

AbstractWhen doped into a certain range of charge carrier concentrations, MoS2 departs from its pristine semiconducting character to become a strongly correlated material characterized by exotic phenomena such as charge density waves or superconductivity. However, the required doping levels are typically achieved using ionic‐liquid gating or air‐sensitive alkali‐ion intercalation, which are not compatible with standard device fabrication processes. Here, the emergence of superconductivity and a charge density wave phase in air‐stable organic cation intercalated MoS2 crystals are reported. By selecting two different molecular guests, it is shown that these correlated electronic phases depend dramatically on the intercalated cation, demonstrating the potential of organic ion intercalation to finely tune the properties of 2D materials. Moreover, it is found that a fully developed zero‐resistance state is not reached in few‐nm‐thick flakes, indicating the presence of 3D superconductive paths that are severed by the mechanical exfoliation. This behavior is ascribed to an inhomogeneous charge carrier distribution, which is probed at the nanoscale using scanning near‐field optical microscopy. The results establish organic‐ion intercalated MoS2 as a platform to study the emergence and modulation of correlated electronic phases.

Low-temperature growth of In2O3 films on a-plane sapphire substrates by pulsed laser deposition

High-quality body-centered cubic In2O3 films were grown on a-plane sapphire substrates at a temperature range of 100–600 °C by pulsed laser deposition. The structure, optical, and electrical properties were studied by X-ray diffraction, Raman spectroscopy, spectrophotometer, and Hall effect. The obtained In2O3 films have a bcc structure, a high carrier concentration range of 1 × 1019–9 × 1020 cm−3, a mobility range of 8 to 30 cm2V−1s−1, and a wide bandgap of 3.7–3.9 eV. The change of substrate temperature affects the crystalline quality, transmittance, carrier concentration, mobility, and bandgap value of In2O3 films. We believe that the low-temperature growth of high-quality In2O3 films can make In2O3 more widely used in various semiconductors devices.

Anisotropic Behavior of Thermoelectric Power in Zigzag and Armchair 2D Phosphorene

Anisotropic thermopower, S, in zigzag and armchair 2D phosphorene is investigated using Boltzmann transport formalism. Anisotropic behavior of thermopower shows that thermopower along armchair direction is higher than that of zigzag direction. The theoretical studies of S and its dependence on the temperature, T, carrier concentration, n s, and phonon mean free path, λ, bring out the features of anisotropic thermopower. The limitations of the often used Mott expression for the analysis of thermopower are brought out. For the range of carrier concentration, 1016 &lt; n s &lt; 1018 m−2, the anisotropic ratio of S varies from 3.1 to 2.3 at room temperature. The modification in the behavior of S from anisotropic to low anisotropic for higher carrier concentration and at lower temperature is noticed. These theoretical investigations are in good agreement with recent experimental results.

The prediction of electronic and thermoelectric performance of bulk and monolayer Sb2TeSeS

The thermoelectric (TE) properties of the bulk and monolayer Sb2TeSeS have been studied by the first-principles calculations and the Boltzmann Transport theory. Due to the greater band gap, the absolute values of Seebeck coefficient for TB-mBJ potential at the vicinity of Fermi level are larger than that for GGA approach. It is found that the electronic thermal conductivity of the layered structure of Sb2TeSeS is relatively lower than that of the bulk compounds. It is stable for monolayer Sb2TeSeS on account of the phonon structure without imaginary frequency, and the phonon dispersion is comparatively flatter. It is expected the ultra-low lattice thermal conductivity, which is verified by the result of the calculated lattice thermal conductivity. For monolayer Sb2TeSeS, the maximum value of figure of merit, which is procured from p-type doping within the reasonable range of carrier concentration, is larger than the optimal ZT value of other well-known materials. Therefore, p-type doping of monolayer Sb2TeSeS is a potential candidate for TE device applications.