Conductivity Effective Mass Research Articles

Predicting two-dimensional semiconductors using conductivity effective mass.

In this paper we investigate the relationship between the conductivity effective mass and exfoliation energy of materials to assess whether automatic sampling of the electron band structure can predict the presence of and ease of separating chemically bonded layers. We assess 22 976 materials from the Materials Project database, screen for only those that are thermodynamically stable and identify the 1000 materials with the highest standard deviation for p-type and the 1000 materials with the highest standard deviation for n-type internal conductivity effective mass tensors. We calculate the exfoliation energy of these 2000 materials and report on the correlation between effective mass and exfoliation energy. A relationship is found which is used to identify a previously unconsidered two-dimensional material and could streamline the modelling of other two-dimensional materials in the future.

Quasiparticle band structures of Cs2B+B3+Br6 lead-free halide double perovskites

Recently, lead-free halide double perovskites (DPs) have been proposed to be great alternative materials for perovskite solar cell (PSC). Therefore, we present a detailed understanding of the electronic structures of Cs2B+B3+Br6, where B+ are Ag and Cu, B3+ are Bi and Sb, by using the high accuracy first-principle method called quasiparticle self-consistent GW (QSGW). As a result, all bandgaps are indirect between X and L symmetry points, spanning from 1.13 to 2.33 eV. The band edges are composed from Br-4p, B+-sd and B3+-sp orbitals. Moreover, band structures including spin-orbit coupling (SOC) reveal that there are energy gaps of 1.29 eV between the first and second conduction bands in Bi-based compounds, resulting from the hybridization of Br-4p and Bi- 6p. Finally, the conductivity effective masses of electron (m*e) and hole (m*h) were calculated. We found that m*e of all materials are about 0.27 me, while m*h range from 0.32 me to 0.47 me. Overall, the effective masses of Cs2B+B3+Br6 are comparable to those of MAPbI3. In conclusion, Cs2B+B3+Br6 are predicted to be potential materials for photovoltaics and optoelectronics due to their bandgaps covering IR to UV range and have qualified electrical conductivity due to their small effective masses. We expected that our results will motivate researchers to further synthesize and characterize Cs2B+B3+Br6 and their related materials.

Tuning and interpretation of electronic transport properties with

Electronic transport coefficients such as the electrical conductivity, the termo-power, and the charge carrier concentration are routinely measured in a variety of application areas such as electronics or thermoelectric power-generation and cooling. Using fitting procedures, those measurements are used to infer microscopic features of the samples. The code ▪ facilitates the estimation of electronic structure parameters such as the effective masses and band energies improving the current approach by increasing the complexity of the band structure representation. The software is designed with a server-client architecture to enhance performance and scalability. The server is implemented in the Julia programming language. We illustrate the design and the efficiency of the code with selected applications, which are relevant to the optimization of thermoelectric materials. Program summaryProgram Title:▪CPC Library link to program files:https://doi.org/10.17632/494nj6ftbv.1Developer's repository link:https://github.com/marcofornari/etransport.gitLicensing provisions: GPLv3Programming language: Julia, Python.Nature of problem: Electronic transport measurements are routinely used to link the functional properties exploited in electronics and energy sciences applications with the underlying electronic structure of the material under investigation. Experimentalists collect data of derived and integrated physical quantities, from which they estimate properties and parameters, e.g., nature of the conduction mechanisms and effective mass. The interpretation is based on simplistic models that rarely represent the complexity of the band structure. The code ▪ improves the current state of the art and facilitates the estimation of effective masses and energies from experimental measurements of electrical conductivity, Seebeck coefficients, and carrier concentration using a multi-valley anisotropic parabolic band structure as a model for the unknown real band structure. Experimental data can be imported and visualized to facilitate the comparison with theoretical analysis.Solution method: The Boltzmann equation within the relaxation-time approximation is used to compute the electronic transport tensors from the electronic density of states characterized in terms of a set of effective masses tensors mi⁎ and critical energies ϵi. Numerical and analytical techniques are adopted to integrate the Boltzmann equation efficiently. By tuning the features of the density of states, temperature, Fermi level, and the functional expression for the relaxation time, the software streamlines the reconstruction of the underlying electronic properties from experimental data.Additional comments including restrictions and unusual features: The software is designed with a server-client architecture to enhance performance and scalability. The server is implemented in the Julia programming language as a stand-alone library. It performs all the numerical operations to compute the transport tensors from a set of multi-valley parabolic band structures. The client is implemented either as a command-line user interface (CLI) or a graphical user interface (GUI) written in the Python programming language. The communications between the server and clients are handled by a REST API, where the data are exchanged using JSON payloads.

Consistent optical and electrical determination of carrier concentrations for the accurate modeling of the transport properties of n-type Ge

A consistent methodology is presented to extract carrier concentrations in n-type Ge from measurements of the infrared dielectric function and the Hall effect. In the case of the optical measurements—usually carried out using spectroscopic ellipsometry—the carrier concentration is affected by the doping dependence of the conductivity effective mass, which is computed using a model of the electronic density of states that accounts for non-parabolicity and is fit to electronic structure calculations. Carrier concentrations obtained from Hall measurements require a knowledge of the Hall factor, which is arbitrarily set equal to unit in most practical applications. We have calculated the Hall factor for n-Ge using a model that accounts for scattering with phonons and with ionized impurities.We show that determinations of the carrier concentration n using our computed effective mass and Hall factor virtually eliminates any systematic discrepancy between the two types of measurement. We then use these results to compute majority carrier mobilities from measured resistivity values, to compare with measurements of minority carrier mobilities, and to fit empirical expressions to the doping dependence of the mobilities that can be used to model Ge devices.

Chemical trends in the high thermoelectric performance of the pyrite-type dichalcogenides ZnS2, CdS2 , and CdSe2

The thermoelectric (TE) properties of the three pyrite-type IIB-${\mathrm{VIA}}_{2}$ dichalcogenides (${\mathrm{ZnS}}_{2}, {\mathrm{CdS}}_{2}$, and ${\mathrm{CdSe}}_{2}$) are systematically investigated and compared with those of the prototype ${\mathrm{ZnSe}}_{2}$ to optimize their TE properties. Using the phonon Boltzmann transport equation, we find that they all have ultralow lattice thermal conductivities. By analyzing their vibrational properties, this is attributed to soft phonon modes derived from the loosely bound rattlinglike metal atoms and to strong anharmonicity caused by the vibrations of all atoms perpendicular to the strongly bound nonmetallic dimers. Additionally, by correlating those properties along the series, we elucidate several chemical trends. We find that the heavier atom masses, larger atomic displacement parameters, and longer bond lengths between metal and nonmetal atoms can be beneficial to the looser rattling behaviors of the metal atoms and therefore lead to the softer phonon modes and that the stronger nonmetallic dimer bonds can boost the anharmonicities, both leading to the lower thermal conductivities. Furthermore, we find that all three compounds have complex energy isosurfaces at valence and conduction band edges that simultaneously allow for large density-of-states effective masses and small conductivity effective masses for both $p$- and $n$-type carriers. Consequently, the calculated TE figures of merit ($ZT$) can reach large values for both $p$- and $n$-type doping. In this paper, we illustrate the effects of the rattlinglike metal atoms and the localized nonmetallic dimers on the thermal transport properties and the importance of different carrier effective masses to electrical transport properties in these pyrite-type dichalcogenides, which can be used to predict and optimize the TE properties of other TE compounds in the future.

Understanding the influence of Bi/Sb substitution on carrier concentration in Mg3Sb2-based materials: decreasing bandgap enhances the degree of impurity ionization.

Experimental results show an intriguing phenomenon that although Bi and Sb have the same number of valence electrons, Bi/Sb substitution increases the electron concentration of n-type Mg3Sb2-based materials. Using a combination of theoretical calculations and experimental synthesis, this work reveals the physical mechanism of the effect of Bi doping on carrier concentration. The increase in electron concentration mainly originates from the enhanced degree of ionization of donor impurity because of the decrease of conductivity effective mass and increase of dielectric constant caused by the narrowing of bandgap with Bi doping. Based on the collaborative optimization of the electrical and thermal transports, n-type Mg3.175Mn0.025Sb1.48Bi0.48Te0.04 exhibits the best thermoelectric performance with a peak zT of 1.85 at 725 K and an average zT of 1.21. This work demonstrates an effective strategy of bandgap engineering for the optimization of carrier concentration and provides insightful guidance for designing other thermoelectric materials.

Hydrogen roles approaching ideal electrical and optical properties for undoped and Al doped ZnO thin films

This paper distinguished hydrogen roles to improve electron mobility and carrier concentration in ZnO and Al doped ZnO sputtered films. By combining experimental evidences and theoretical results, we find out that hydrogen located at oxygen vacancy sites (HO) is the main factor gives rise to increase simultaneously mobility and carrier concentration which has not been mentioned before. Introducing appropriate hydrogen content during sputtering not only results in crystalline relaxation but also supports doping Al into ZnO, increasing carrier concentration and electron mobility in the film. First principles calculations confirmed hydrogen substitutional stability for oxygen vacancy, significantly reducing electron conductivity effective mass and hence increasing electron mobility. In particular, 0.8% hydrogen partial pressure ratio achieved 61 cm2V−1s−1 maximum electron mobility, optical transmittance above 82% in visible and near-infrared regions, and 2 × 1020 cm−3 carrier concentrations for HAl co-doped ZnO film. These values approach ideal electrical and optical properties for transparent conducting oxide films. The presence of one maximum electron mobility was attributed to competition between increasing mobility due to restoring effective electron mass and hydrogen passivation of native defects, and decreased electron mobility due to electron-phonon scattering.

Theoretical prediction of transport coefficients of antimony doped tin dioxide

Doped Tin (IV) Oxide has excellent potential in high-temperature thermoelectrics because of its large bandgap. It has been thoroughly experimentally studied for high-temperature thermoelectric application. Low electron mobility limits the thermoelectric performance of oxides. Understanding the temperature dependence of mobility help increase thermoelectric performance. This is rarely performed. In this work, we study antimony doped tin dioxide. Combining the calculated transport properties and the existing experimental results, we obtain other transport properties. Unlike in bulk materials, the grain boundary scattering competes against the acoustic phonon scattering. The grain size, temperature, and carrier concentration determine the mobility. The small grain, low carrier concentration, and high temperature are beneficial to the mobility. Antimony doping also causes the Fermi level into the conduction band as deep as 0.59 eV, making the large bandgap SnO2 metallic. Furthermore, the conductivity effective mass demonstrates the doping effect. These findings might help design new oxide thermoelectric materials by decrease the grain size.

Impact of Fermi Surface Shape Engineering on Calculated Electronic Transport Properties of Bi-Sb-Te

Using thermoelectric refrigerators can address climate change because they do not utilize harmful greenhouse gases as refrigerants. To compete with current vapor compression cycle refrigerators, the thermoelectric performance of materials needs to be improved. However, improving thermoelectric performance is challenging because of the trade-off relationship between the Seebeck coefficient and electrical conductivity. Here, we demonstrate that decreasing conductivity effective mass by engineering the shape of the Fermi surface pocket (non-parabolicity factor) can decouple electrical conductivity from the Seebeck coefficient. The effect of engineering the non-parabolicity factor was shown by calculating the electronic transport properties of a state-of-the-art Bi-Sb-Te ingot via two-band model with varying non-parabolicity. The power factor (the product of the Seebeck coefficient squared and electrical conductivity) was calculated to be improved because of enhanced electrical conductivity, with an approximately constant Seebeck coefficient, using a non-parabolicity factor other than unity. Engineering the non-parabolicity factor to achieve lighter conductivity effective mass can improve the electronic transport properties of thermoelectric materials because it only improves electrical conductivity without decreasing the Seebeck coefficient (which is directly proportional to the band mass of a single Fermi surface pocket and not to the conductivity effective mass). Theoretically, it is demonstrated that a thermoelectric figure-of-merit <i>zT</i> higher than 1.3 can be achieved with a Bi-Sb-Te ingot if the non-parabolicity factor is engineered to be 0.2. Engineering the non-parabolicity factor is another effective band engineering approach, similar to band convergence, to achieve an effective improvement in power factor.

Mobility Modeling in Advanced MOSFETs with Ultra-Thin Silicon Body under Stress

We use a two-band k·p model to describe the subband structure in strained silicon thin films. The model provides the dependence of the conductivity effective mass on strain and film thickness. The conductivity mass decreases along tensile stress in [110] direction applied to a (001) film. This conductivity mass decrease ensures the mobility enhancement in MOSFETs even with extremely thin silicon films. The two-band k·p model also describes the dependence of the non-parabolicity parameter on film thickness and strain. The influence of the subband structure modification on the mobility in advanced MOSFETs with strained ultra-thin silicon body is investigated. It is shown that an increase of subband non-parabolicity in thin films with strain reduces the mobility enhancement due to the conductivity mass modification, especially at higher strain values.

Chalcogenide Perovskite YScS3 as a Potential p-Type Transparent Conducting Material

Transparent conducting materials (TCMs) have been widely used in optoelectronic applications such as touchscreens, flat panel displays and thin film solar cells. These applications of TCMs are currently dominated by n-type doped oxides. High-performance p-type TCMs are still lacking due to their low hole mobility or p-type doping bottleneck, which impedes efficient device design and novel applications such as transparent electronics. Here, based on first-principles calculations, we propose chalcogenide perovskite YScS3 as a promising p-type TCM. According to our calculations, its optical absorption onset is above 3 eV, which allows transparency to visible light. Its hole conductivity effective mass is 0.48m0, which is among the smallest in p-type TCMs, suggesting enhanced hole mobility. It could be doped to p-type by group-II elements on cation sites, all of which yield shallow acceptors. Combining these properties, YScS3 holds great promise to enhancing the performance of p-type TCMs toward their n-type counterparts.

Long Wavelength InAs/InAsSb Infrared Superlattice Challenges: A Theoretical Investigation

InAs/InAsSb type-II superlattice focal plane arrays that demonstrate high operability and uniformity with cutoffs ranging from 5 μm to 13 μm have already been demonstrated. Compared to InAs/GaSb, the InAs/InAsSb superlattice is easier to grow and has longer minority carrier lifetimes, but requires a longer superlattice period to achieve long or very long wavelength cutoffs. A longer type-II superlattice period leads to smaller absorption coefficients and larger growth-direction hole conductivity effective masses. We explore by theoretical modeling some of the ideas aimed at addressing these challenges for the long and very long wavelength InAs/InAsSb superlattice. Increasing the Sb fraction in the InAsSb alloy can reduce the InAs/InAsSb superlattice period significantly, but this benefit can be negated by Sb segregation. Thin AlAsSb barrier layers can be inserted in InAs/InAsSb to form polytype W, M, and N superlattices in order to increase electron–hole wavefunction overlap for stronger optical absorption. However, this strategy can be unfavorable since the AlAsSb barriers increase the band gap, and thereby increase the superlattice period required to reach a given cutoff wavelength. Metamorphic growth on virtual substrates with larger lattice constants than GaSb can decrease the superlattice period needed to reach a specified cutoff wavelength, but this benefit should be weighed against the need for metamorphic buffer growth and the resulting higher defect density.

Simulation on the electric field effect of Bi thin film

We report our simulation on the electric field effect of Bi thin film. Band diagram and carrier concentrations of the Bi channel at different surface potentials have been obtained by numerically solving Poisson's equation. In the calculation, the anisotropic characteristic of effective mass for carrier concentration and conductivity have been considered. The carrier densities were calculated from Fermi-Dirac integral. The conductivity effective mass ratio of electron and hole have been calculated to verify how the gate bias voltage affects the conductance of the Bi channel. The result shows that the Debye length in Bi is ~10 nm and nearly independent of the bias voltage. The dependency of conductance on gate bias is also discussed.

Material Descriptors for the Discovery of Efficient Thermoelectrics

The predictive performance screening of novel compounds can significantly promote the discovery of efficient, cheap, and non-toxic thermoelectric materials. Large efforts to implement machine-learning techniques coupled to materials databases are currently being undertaken, but the adopted computational methods can dramatically affect the outcome. With regards to electronic transport and power factor calculations, the most widely adopted and computationally efficient method, is the constant relaxation time approximation (CRT). This work goes beyond the CRT and adopts the proper, full energy and momentum dependencies of electron-phonon and ionized impurity scattering, to compute the electronic transport and perform power factor optimization for a group of half-Heusler alloys. Then the material parameters that determine the optimal power factor based on this more advanced treatment are identified. This enables the development of a set of significantly improved descriptors that can be used in materials screening studies, and which offer deeper insights into the underlying nature of high performance thermoelectric materials. We have identified $n_v$$\epsilon_r$ / $D_o^2m_{cond}$ as the most useful and generic descriptor, a combination of the number of valleys, the dielectric constant, the conductivity effective mass, and the deformation potential for the dominant electron-phonon process. The proposed descriptors can accelerate the discovery of new efficient and environment friendly thermoelectric materials in a much more accurate and reliable manner, and some predictions for very high performance materials are presented.

Widely Tunable Optical and Thermal Properties of Dirac Semimetal Cd3As2

AbstractIn this paper, a detailed analysis of the temperature‐dependent optical properties of epitaxially grown cadmium arsenide (Cd3As2), a newly discovered 3D Dirac semimetal is reported. Fermi level tuning—instigated from Pauli‐blocking in the linear Dirac cone—and varying Drude response, generate large variations in the mid‐ and far‐infrared optical properties. Thermo‐optic shifts larger than those of traditional III–V semiconductors are demonstrated. Electron scattering rate, plasma frequency edge, Fermi level shift, optical conductivity, and electron effective mass analysis of Cd3As2 thin‐films are quantified and discussed in detail. The ab initio density functional study and experimental analysis of epitaxially grown Cd3As2 promise applications for nanophotonic and nanoelectronic devices, such as reconfigurable metamaterials and metasurfaces, nanoscale thermal emitters, and on‐chip directional antennas.

Photogenerated Carrier Transport Properties in Silicon Photovoltaics

Electrical transport parameters for active layers in silicon (Si) wafer solar cells are determined from free carrier optical absorption using non-contacting optical Hall effect measurements. Majority carrier transport parameters [carrier concentration (N), mobility (μ), and conductivity effective mass (m*)] are determined for both the n-type emitter and p-type bulk wafer Si of an industrially produced aluminum back surface field (Al-BSF) photovoltaic device. From measurements under 0 and ±1.48 T external magnetic fields and nominally “dark” conditions, the following respective [n, p]-type Si parameters are obtained: N = [(3.6 ± 0.1) × 1018 cm−3, (7.6 ± 0.1) × 1015 cm−3]; μ = [166 ± 6 cm2/Vs, 532 ± 12 cm2/Vs]; and m* = [(0.28 ± 0.03) × me, (0.36 ± 0.02) × me]. All values are within expectations for this device design. Contributions from photogenerated carriers in both regions of the p-n junction are obtained from measurements of the solar cell under “light” 1 sun illumination (AM1.5 solar irradiance spectrum). From analysis of combined dark and light optical Hall effect measurements, photogenerated minority carrier transport parameters [minority carrier concentration (Δp or Δn) and minority carrier mobility (μh or μe)] under 1 sun illumination for both n- and p-type Si components of the solar cell are determined. Photogenerated minority carrier concentrations are [(7.8 ± 0.2) × 1016 cm−3, (2.2 ± 0.2) × 1014 cm−3], and minority carrier mobilities are [331 ± 191 cm2/Vs, 766 ± 331 cm2/Vs], for the [n, p]-type Si, respectively, values that are within expectations from literature. Using the dark majority carrier concentration and the effective equilibrium minority carrier concentration under 1 sun illumination, minority carrier effective lifetime and diffusion length are calculated in the n-type emitter and p-type wafer Si with the results also being consistent with literature. Solar cell device performance parameters including photovoltaic device efficiency, open circuit voltage, fill factor, and short circuit current density are also calculated from these transport parameters obtained via optical Hall effect using the diode equation and PC1D solar cell simulations. The calculated device performance parameters are found to be consistent with direct current-voltage measurement demonstrating the validity of this technique for electrical transport property measurements of the semiconducting layers in complete Si solar cells. To the best of our knowledge, this is the first method that enables determination of both minority and majority carrier transport parameters in both active layers of the p-n junction in a complete solar cell.

Optical and electronic properties of high quality Sb-doped SnO2 thin films grown by mist chemical vapor deposition

Transparent Sb-doped epitaxial SnO2 (101) thin films were grown via mist chemical vapor deposition, a nonvacuum solution-based technique that involves the gas-assisted transport of ultrasonically-generated aerosols from simple aqueous Sn and Sb precursors. The electrical properties (at 300 K) of the resulting films could be varied from insulating to semimetallic via Sb doping, with a minimum resistivity of 8×10−4Ωcm, carrier concentrations up to 3.93×1020cm−3, and a maximum mobility of 48.1±0.1cm2V−1s−1, results similar to those achieved using molecular beam epitaxy and other more-sophisticated high vacuum techniques. Secondary ion mass spectrometry and Hall effect measurements revealed that 14% of Sb in the precursor solution incorporates into the Sb:SnO2 films, with almost all the Sb atoms forming shallow substitutional donors on the Sn-site. The increase in the carrier concentration with Sb doping produced a Burstein-Moss shift of the optical gap of 0.49 eV, without significantly reducing the more than 90% transparency of the films in the visible region. X-ray photoemission spectroscopy (hν=1486.6eV) showed an asymmetric Sn3d5/2 core-level emission characterized by a carrier concentration-dependent peak splitting. This effect was modeled in terms of the creation of an intrinsic plasmon loss satellite from which a conductivity effective electron mass of (0.49±0.11)me was determined.

Fermi surface complexity, effective mass, and conduction band alignment in n-type thermoelectric Mg3Sb2 – xBix from first principles calculations

Using first principles calculations, we study the conduction band alignment, effective mass, and Fermi surface complexity factor of n-type Mg3Sb2 – xBix (x = 0, 1, and 2) from the full ab initio band structure. We find that with an increase in the Bi content, the K and M band minima move away from the conduction band minimum CB1 while the singly-degenerate Г band minimum shifts rapidly downward and approaches the conduction band minimum. However, the favorable sixfold degenerate CB1 band minimum keeps dominating the conduction band minimum and there is no band crossing between the Г and CB1 band minima. In addition, we show that the connection of the CB1 carrier pockets with the energy level close to the band minimum M can strongly enhance the carrier pocket anisotropy and Fermi surface complexity factor, which is likely the electronic origin for the local maximum in the theoretical power factor. Our calculations also show that the density of states effective mass, Seebeck coefficient, and Fermi surface complexity factor decrease with an increase in the Bi content, which is unfavorable to the electrical transport. In contrast, reducing the conductivity effective mass with an increase in the Bi content is beneficial to the electrical transport by improving carrier mobility and weighted mobility as long as the detrimental bipolar effect is insignificant. As a result, in comparison with n-type Mg3Sb2, n-type Mg3SbBi shows higher power factors and a much lower optimal carrier concentration for the theoretical power factor at 300 K, which can be easily achieved by the experiment.

Maximum-entropy principle for ac and dc dynamic high-field transport in monolayer graphene

Using the maximum entropy principle, we present a general theory to describe ac and dc high-field transport in monolayer graphene within a dynamical context. Accordingly, we construct a closed set of hydrodynamic (HD) equations containing the same scattering mechanisms used in standard Monte Carlo (MC) approaches. The effects imputable to a linear band structure, the role of conductivity effective mass of carriers, and their connection with the coupling between the driving field and the dissipation phenomena are analyzed both qualitatively and quantitatively for different electron densities. The theoretical approach is validated by comparing HD results with existing MC simulations.

Thermoelectric Performance of IV-VI Compounds with Octahedral-Like Coordination: A Chemical-Bonding Perspective.

Thermoelectric materials provide a challenge for materials design, since they require optimization of apparently conflicting properties. The resulting complexity has favored trial-and-error approaches over the development of simple and predictive design rules. In this work, the thermoelectric performance of IV-VI chalcogenides on the tie line between GeSe and GeTe is investigated. From a combination of optical reflectivity and electrical transport measurements, it is experimentally proved that the outstanding performance of IV-VI compounds with octahedral-like coordination is due to the anisotropy of the effective mass tensor of the relevant charge carriers. Such an anisotropy enables the simultaneous realization of high Seebeck coefficients, due to a large density-of-states effective mass, and high electrical conductivity, caused by a small conductivity effective mass. This behavior is associated to a unique bonding mechanism by means of a tight-binding model, which relates band structure and bond energies; tuning the latter enables tailoring of the effective mass tensor. The model thus provides atomistic design rules for thermoelectric chalcogenides.