Function Of Carrier Concentration Research Articles

Revealing large room-temperature Nernst coefficients in 2D materials by first-principles modeling.

Two-dimensional (2D) materials have attracted significant attention owing to their distinctive electronic, thermal, and mechanical characteristics. Recent advancements in both theoretical understanding and experimental methods have greatly contributed to the understanding of thermoelectric properties in 2D materials. However, thermomagnetic properties of 2D materials have not yet received the same amount of attention. In this work, we select promising 2D materials guided by the physics of the Nernst effect and present a thorough first-principles study of their electronic structures, carrier mobilities, and Nernst coefficients as a function of carrier concentration. Specifically, we reveal that trilayer graphene with an ABA stacking exhibits an exceptionally large Nernst coefficient of 112 μV (KT)-1 at room temperature. We further demonstrate that monolayer graphene, ABC-stacked trilayer graphene, and trilayer phosphorene (AAA stacking) have large Nernst coefficients at room temperature. This study establishes an ab initio framework for the quantitative study of the thermomagnetic effects in 2D materials and demonstrates high fidelity with previous experimental data.

A first-principles study of electronic, optical and thermoelectric properties of TlXF3 (X: Zn, Sr) perovskite crystal structure

This study communicates the structural, electronic, optical, and thermoelectric properties of a perovskite TlXF3 (X: Zn, Sr) crystal structure using first-principles calculations. Both compounds were found structurally as well as thermodynamically stable in cubic phase. To check the kinetic stability, the phonon frequancy band structure is calculated which indicates that there is no negative frequency in the phonon spectrum, which confirms the dynamic stability of TlXF3. The bandgaps of both compounds belong to insulating region. The TlSrF3 exhibits direct bandgap of 4.37 eV at X symmetry point while TlZnF3 possess indirect bandgap of 3.95 eV (M-X). Using HSE functional, the calculated electronic bandgaps of TlSrF3 and TlZnF3 are 5.74 eV(X-X) and 5.36 eV (M-X), respectively. The analysis of electronic states in band structure shows that for TlSrF3, the top of valence band is composed of the F-p states whereas the states in lower potion of conduction band are impacted by the significant contribution of Tl-p orbital states. In case of TlZnF3, the states near the valence band maxima and conduction band minima are coming mainly form Tl atom. The effect of spin–orbit coupling (SOC) on the electronic band structure is also considered. The SOC in the valance band is negligible, while in the conduction band it shows a dominant effect at Γ-point in both TlSrF3 and TlZnF3 compounds. The dependence of thermoelectric properties such as electrical conductivity σ, Seebeck coefficient (S), Power factor (PF), electronic thermal conductivity (κ) and figure-of-merit (ZT) as a function of carrier concentration and temperature are investigated. The optical properties were also studied to understand the response of TlXF3 to incident photons of energies upto 14 eV. We conclude that the TlXF3 are promising candidates for electronic, thermoelectric and optoelectronic devices.

Ab initio theory of free-carrier absorption in semiconductors

The absorption of light by free carriers in semiconductors results in optical loss for all photon wavelengths. Since free-carrier absorption competes with optical transitions across the band gap, it also reduces the efficiency of optoelectronic devices such as solar cells because it does not generate electron-hole pairs. In this work, we develop a first-principles theory of free-carrier absorption taking into account both single-particle excitations and the collective Drude term, and we demonstrate its application to the case of doped Si. We determine the free-carrier absorption coefficient as a function of carrier concentration and we obtain excellent agreement with experimental data. We identify the dominant processes that contribute to free-carrier absorption at various photon wavelengths, and analyze the results to evaluate the impact of this loss mechanism on the efficiency of Si-based optoelectronic devices.

Thermoelectric power factor of MnSb2X4 (X = S, Se) spinel chalcogenides – A DFT study

Recently, spinel chalcogenides have been found to be promising class of materials in thermoelectric power generation. In this line, the complete understanding about electronic and thermoelectric properties of MnSb2S4 and MnSb2Se4 is crucial. With the help of first-principles calculations, its interesting characteristics are explored, including bonding nature, ground state magnetic ordering, mechanical stability, etc. A systematic study on these materials revealed their antiferromagnetic and semiconducting behaviour. The Seebeck coefficient as a function of carrier concentration has its maximum at 600 and 681 μV K−1 (T = 300 K) for hole doped MnSb2S4 and MnSb2Se4 respectively. Besides the coupled behaviour of Seebeck coefficient and electrical conductivity, an optimal power factor of 1.82 and 0.96 mW m−1 K−2 is attained at 300 K for MnSb2S4 and MnSb2Se4 respectively. This study suggests that both the spinels can be potential p-type thermoelectric materials for near room temperature power generation applications and enables the path for their experimental investigations.

Giant effective g factor and anisotropy in Sn1−xEuxTe: Contribution from spin-orbit and s/p − f hybridization

We consider the effective equation of motion starting from the fundamental six-level double group basis as stated by Mitchell-Wallis under effective mass representation in the presence of spin-orbit interaction, external magnetic field, and impurity. Here, the hybridized exchange type interactions due to magnetic impurities with carrier and external magnetic field are incorporated in the k⃗⋅π⃗ perturbation technique. Finally, we perform a systematic analysis of the effective g factor and its anisotropy in Sn1−xEuxTe as functions of carrier concentration and impurity at T = 300 K. We report very high values of effective g factor; g = 1028 for n − Sn1−xEuxTe and g = 997 for p − Sn1−xEuxTe with large anisotropies around x = 0.02 as compared to Pb1−xEuxTe in the concentration range 0.01 × 1019cm−3 to 0.1 × 1019cm−3. Our calculated results and trends are very similar to others’ work, previously reported for similar systems. The presence of a large effective g factor renders this material a potential choice for spintronics applications.

Theoretical Prediction of the Monolayer Hf2Br4 as Promising Thermoelectric Material

The stability, electronic structure, electric transport, thermal transport and thermoelectric properties of the monolayer Hf2Br4 are predicted by using first principle calculations combined with Boltzmann transport theory. The dynamic stability of the monolayer Hf2Br4 is verified by phonon band dispersion, and the thermal stability is revealed by ab initio molecular dynamics simulations. The electronic structure calculation indicates that the monolayer Hf2Br4 is an indirect band gap semiconductor with a band gap of 1.31 eV. The lattice thermal conductivity of the monolayer Hf2Br4 is investigated and analyzed on phonon mode level. The calculation results of the electric transport explore the excellent electric transport properties of the monolayer Hf2Br4. The thermoelectric transport properties as a function of carrier concentration at three different temperatures are calculated. The study indicates that the monolayer Hf2Br4 can be an alternative, stable two-dimensional material with potential application in the thermoelectric field.

Electronic structure and electrical transport properties of MoS2single-walled nanotubes based on first principles

The work theoretically calculated the electronic structure and electrical transport properties of two configurations of single-walled MoS2nanotubes: armchair nanotubes (ANTs) and zigzag nanotubes (ZNTs) based on the density functional theory and Boltzmann transport method. ANTs have an indirect one. while ZNTs have a direct bandgap structure. The Seebeck coefficient ([Formula: see text]), electrical conductivity ([Formula: see text] and power factor ([Formula: see text] were calculated as a function of carrier concentration, chemical potential and temperature using the Boltzmann transport method. The calculated power factor ([Formula: see text]) indicates that the most promising electronic properties were exhibited by [Formula: see text]-type ANTs and [Formula: see text]-type ZNTs. The [Formula: see text] of narrow bandgap (6, 6) (7, 7) (8, 8) semiconductors reached [Formula: see text], [Formula: see text] and [Formula: see text]WK[Formula: see text]m[Formula: see text] at room-temperature, respectively. (7, 7) nanotube have a maximum power factor of [Formula: see text]WK[Formula: see text]m[Formula: see text] at 950 K, and the maximum power factor of ANTs is almost twice that of ZNTs.

Investigation on the optical and electrical properties of undoped and Sb-doped SnO2 nanowires obtained by the VLS method

In this work, we report the effects of Sb doping on the optical and electrical properties of the SnO2 nanowires obtained by the vapor-liquid-solid (VLS) method. The absorption edges were found to be 3.30 eV and 3.66 eV for undoped SnO2 and Sb-doped SnO2 (ATO) nanowires, respectively. The energy shift was related to the Burstein-Moss effect taking place in the doped nanowires. We studied the ATO optical bandgap (ΔE = 0.36 eV) shift as a function of carrier concentration. The incorporation of Sb caused the resistivity to decrease three orders of magnitude for single-nanowire ATO devices. In addition, it was found that undoped SnO2 nanowires exhibit semiconductor characteristics while a metal-insulator transition (MIT), around 170 K, was observed in the ATO nanowires.

Characterization of tellurium and silicon as n-type dopants for GaAsBi

Films of n-GaAs1-xBix films were grown via molecular beam epitaxy using both Si and Te as dopant sources. Electron mobility was characterized by Hall effect measurements as a function of carrier concentration and Bi content for films with bismuth fractions of x = 0.02 and x = 0.06. While GaAsBi:Te shows lower majority carrier mobility than GaAsBi:Si at low Bi concentrations, the two become comparable as Bi content increases. Furthermore, it was observed that in the presence of bi-metallic Bi-Ga droplets on the film surface, films doped with Si display p-type behavior, likely due to Si preferentially occupying group-V sites. The use of Te as a dopant always resulted in n-type epilayers, making it a more reliable dopant choice for high Bi content films. Finally, ex situ annealing was studied as a method to improve majority carrier mobility in GaAs0.98Bi0.02:Te films, with a 10 min anneal at 350 °C resulting in a 30% improvement in electron mobility. Improvement of film quality was confirmed through spectroscopic ellipsometry examination of film optical properties. Annealing at higher temperatures resulted in electrical, optical, and structural degradation of the GaAsBi films.

Computational determination of structural, electronic, optical, thermoelectric and thermodynamic properties of hybrid perovskite CH3CH2NH3GeI3: An emerging material for photovoltaic cell

Owing to high power conversion efficiency and low-cost, methyl-ammonium lead-based halide (viz. CH3NH3PbI3) Perovskites have been emerging as the innovative candidate in the development of optoelectronic devices. However, the toxic lead in these materials is a major hurdle in its commercialization. Thus, there is an urgent need to replace lead with an appropriate element. Ethyl-ammonium based lead-free hybrid halide perovskites may be an alternative photovoltaic (PV) absorber material with appropriate band gap, high stability and non-toxic properties. Herein, we have investigated structural, electronic, optical, thermoelectric and thermodynamic properties of ethyl-ammonium germanium iodide (CH3CH2NH3GeI3 or EAGeI3) by full-potential augmented plane wave (FP-LAPW) method as implemented in the WIEN2k code within the density functional theory (DFT). In this paper, we have found that EAGeI3 has direct band gap of 1.3 eV and high absorption coefficient greater than 104 cm−1 indicating its suitability as PV absorber material. We have also calculated thermoelectric coefficients as a function of carrier concentration, chemical potential and temperature. The thermodynamic calculations have been done within the quasi-harmonic approximation. As EAGeI3 has been studied first time for PV applications, the present study may open a new vista for more exhaustive experimental and theoretical investigations in search of non-toxic and eco-friendly PV materials.

First principles study of electronic structure and thermoelectric transport in tin selenide and phase separated tin selenide–copper selenide alloy

The electronic structure and thermoelectric transport in SnSe and its alloy with Cu2Se have been studied using the first principles technique and semi classical Boltzmann transport theory. Our study reveals that SnSe is p-type with indirect band gap of 0.66 eV, while the alloy is phase separated and n-type with negligible indirect band gap of 0.064 eV. In both cases, two fold degeneracy in band extrema have been observed within the range of 25 meV. Delocalization of Se lone pair has been observed due to Cu substitution in Sn sites, which is supposed to lower its lattice thermal conductivity. A chemical potential map has been generated obeying thermodynamic restrictions to predict the possible existence of secondary phases. Our study shows the existence of SnSe2 as a secondary phase, while the possibility of Cu2Se as a secondary phase is negligible due to its higher formation energy. We calculated the transport coefficients as a function of carrier concentration and temperature to understand the range of optimized thermoelectric performance. The transport coefficients are similar along in plane direction whereas significant deviation is observed along the cross plane direction due to anisotropy in effective masses in SnSe. The effective masses are more isotropic in alloy than SnSe, thus transport properties show less anisotropy along three directions. Significant contribution of bipolar transport is observed in SnSe, while that is not noticed in the alloy. The behaviors of the Seebeck coefficients in both cases are discussed in terms of Mott’s theory and density of states modification near Fermi energy. Electron mobilities limited by acoustic phonon, ionized impurities, alloy scattering and inter carrier scattering have been examined relying on deformation potential approach and effective mass theory. The results indicate that acoustic phonon scattering is dominant scattering mechanism in SnSe over inter carrier scattering, whereas for the alloy the former contribute very weakly. Ionized impurity scattering and inter carrier scattering are most dominant in the alloy. Alloy scattering with U = 2 eV also contribute significantly.

Investigation of structural, electronic, optical and thermoelectric properties of Ethylammonium tin iodide (CH3CH2NH3SnI3): An appropriate hybrid material for photovoltaic application

Hybrid halide Perovskites have been emerging as the innovative candidate in the development of optoelectronic devices. Ethyl-ammonium based hybrid halide perovskites have been admirable compounds regarding low band gap, high stability and non-toxic properties instead of methyl-ammonium based hybrid halide perovskites. Herein, we have investigated structural, electronic, optical and thermoelectric (TE) properties of ethyl-ammonium tin iodide (CH3CH2NH3SnI3) by full-potential linearized augmented plane wave plus local orbital (FP-LAPW + lo) method as implemented in the WIEN2k code within the density functional theory (DFT). The PBE-GGA, PBE-sol-GGA and TB-mBJ-GGA exchange-correlation potentials have been used to calculate the density of states and band structure of ethyl ammonium tin iodide (EASnI3) and found that it is a direct band gap semiconductor. We have calculated optical parameters such as dielectric function, absorption coefficient, refractive index, extinction coefficient, Eloss, optical reflectivity and optical conductivity for photon energies in the range 0–25 eV. We have also calculated transport coefficients (Seebeck coefficient, the figure of merit, power factor, electrical conductivity, thermal conductivity) as a function of carrier concentration, chemical potential and temperature. Most of the investigated parameters are being reported for the first time. The present study establishes that ethyl-ammonium tin iodide (EASnI3) offers high absorption coefficient, high Seebeck coefficient, etc. This makes it a prospective material for cost effective photovoltaic device applications to overcome environmental instabilities.

Polarization mechanism and Shockley Read Hall recombination on Quantum Efficiency of InGaN/GaN Blue LED

InxGa1-x N/GaN blue LEDs faces significant efficiency droop issue. The causes of efficiency droop are Shockley Read Hall recombination (SRH), Auger recombination (AR), carrier delocalization (CD) and electron leakage (EL). The SRH, Auger and electron leakage are functions of carrier concentration and temperature. InGaN/GaN superlattice has polarization electric field at interface. In this work we explore effect of built in polarization (BIP) on efficiency droop. It is shown that polarization field enhances SRH coefficient resulting in more droop in internal quantum efficiency of blue LED. Thus, for improvement in efficiency, polarization field required to be minimized which requires growth of the material in m-plane instead of c-plane.

Improvement of transparent conductive properties of GZO/Cu/GZO tri-layer films by introducing H2 into sputtering atmosphere

Abstract For preparation of transparent conductive oxide (TCO)/metal/TCO tri-layer films, Cu and Ga-doped ZnO (GZO) films were respectively used as the metal and TCO layers and were deposited by magnetron sputtering at room temperature. The effect of film thickness on transparent conductive properties of single Cu films was first investigated in order to better understand the properties of tri-layer films. The results show that the resistivity, sheet resistance (Rs), average transmittance in visible light range (TVis), and figure of merit (FOM) of Cu films decrease with increasing film thickness. The investigations of effect of H2 flux on the transparent conductive properties of GZO films show that hydrogenated GZO (HGZO) films have lower resistivity and higher TVis than those of GZO films. On basis of the investigation of single Cu and GZO films, the GZO/Cu/GZO (GcG) and HGZO/Cu/HGZO (HcH) tri-layer films were prepared, and their transparent conductive properties were investigated as a function of Cu and GZO (or HGZO) layer thickness. The results show that these two kinds of tri-layer films have the best FOM at Cu layer thickness of 7 nm and GZO (or HGZO) layer thickness of 40 nm. Compared with GcG tri-layer films, HcH tri-layer films have better FOM, indicating that introducing H2 into sputtering atmosphere plays a significant role on the transparent conductive properties of the tri-layer films. In addition, bandgap (Eg) of single GZO and tri-layer films was discussed as a function of carrier concentration in the paper.

Effect of strain on internal quantum efficiency of InGaN/GaN blue LED

In this work, effect of strain on efficiency droop of InGaN/GaN blue LEDs is investigated. This blue LEDs faces significant efficiency droop issue. The causes of efficiency droop are Shockley Read Hall recombination (SRH), Auger recombination (AR), carrier delocalization and electron leakage. The SRH, Auger and electron leakage are functions of carrier concentration and temperature. InGaN/GaN superlattice has strain at each interface. This strain generates piezoelectric polarization charges which modify optical and electrical properties of the material InGaN. It is shown that polarization charges enhance droop in internal quantum efficiency of blue LED. Thus, for improvement in efficiency, polarization field required to be minimized which requires growth of the material in m-plane instead of c-plane.

Discovery of colossal Seebeck effect in metallic Cu2Se

Both electrical conductivity σ and Seebeck coefficient S are functions of carrier concentration being correlated with each other, and the value of power factor S2σ is generally limited to less than 0.01 W m−1 K−2. Here we report that, under the temperature gradient applied simultaneously to both parallel and perpendicular directions of measurement, a metallic copper selenide, Cu2Se, shows two sign reversals and colossal values of S exceeding ±2 mV K−1 in a narrow temperature range, 340 K < T < 400 K, where a structure phase transition takes place. The metallic behavior of σ possessing larger magnitude exceeding 600 S cm−1 leads to a colossal value of S2σ = 2.3 W m–1 K–2. The small thermal conductivity less than 2 W m−1 K−1 results in a huge dimensionless figure of merit exceeding 400. This unusual behavior is brought about by the self-tuning carrier concentration effect in the low-temperature phase assisted by the high-temperature phase.

X‐Ray Scattering Reveals Ion‐Induced Microstructural Changes During Electrochemical Gating of Poly(3‐Hexylthiophene)

AbstractThe heterogeneous microstructure of semicrystalline polymers complicates the relationship between their electrical conductivity and carrier concentration. Charge transport models typically describe conductivity with an assumption of uniform doping throughout the material. Here, the evolution in morphology and optoelectronic properties of poly(3‐hexylthiophene) (P3HT) is reported as a function of carrier concentration in an organic electrochemical transistor using a polymeric ionic liquid (PIL) as the gate insulator. Operando grazing incidence X‐ray scattering reveals that negatively charged ions from the dielectric first infiltrate the amorphous regions of the semiconductor, and then penetrate the crystalline regions at a critical carrier density of 4 × 1020 cm−3. Upon infiltration, the crystallites expand by 12% in the alkyl stacking direction and compress by 4% in the π–π stacking direction. The change in crystal structure of P3HT correlates with a sharply increasing effective carrier mobility. UV–visible spectroscopy reveals that holes induced in P3HT first reside in the crystalline regions of the polymer, which verifies that a charge carrier need not be in the same physical domain as its associated counterion. The dopant‐induced morphological changes of P3HT rationalize the dependence of mobility on carrier concentration, suggesting a phase transition of crystalline regions at high carrier concentration.

(Invited) RF Pump-Probe Modulation Spectroscopy of Silicon Nanocrystals: Determination of the Carrier Dynamics and Quantum Efficiency

We present our latest work on the development of an all-optical (pump-probe) technique for estimating the quantum efficiency (QE) of silicon nanocrystals. Our experiment combines measurement of micro-photoluminescence and reflectivity when driven with an RF frequency modulated pump. By varying this frequency over a wide (5 decade) range, we are able to determine the dynamics of the free carrier concentration (multi-component in these systems) and thus retrieve the PL decay rate(s). Interestingly, we find that, in addition to the dominant (10’s-us) component (typically noted from direct measurement of the PL decay), a very slow component persists (in the ms regime), which we attribute to the recombination of ‘trapped’ charges. The technique provides a method for estimating the relative QE as a function of carrier concentration, providing clues as to the efficiency limiting processes in these materials. We present our most recent data for silicon nanocrystals formed in glass cover-slips after ion implantation and thermal annealing, and we discuss how the technique can be applied to luminescent materials more generally, e.g. to study the efficiency ‘droop’ mechanism observed at high carrier concentration in nitride based LED materials.

Spin relaxation and proximity effect in WS2/graphene/fluorographene non-local spin valves

The mechanisms leading to spin relaxation in graphene and its heterostructures continue to be debated. Control of the spin relaxation in graphene-based structures is necessary to achieve the envisioned utility of graphene in future spintronic devices beyond Moore's law. Proximity induced spin relaxation caused by contact to a high spin-orbit material, such as WS2, offers a promising avenue to manipulate the spin lifetime. We demonstrate the operation of WS2/graphene/fluorographene non-local spin valves and extract the spin lifetimes for a range of carrier concentrations by Hanle effect measurements. Four-terminal charge transport measurements allow us to calculate the momentum relaxation time as a function of carrier concentration and compare it to the spin lifetime. These data show that the D'yakonov-Perel’ mechanism is the dominant spin relaxation mechanism for WS2/graphene/fluorographene devices, while, for reference graphene/fluorographene devices, linear scaling between the spin and momentum lifetimes points to spin-flip scattering during strong elastic scattering events where the scattering event is strongly coupled to the electron spin. We attribute the change in spin relaxation type in part with the inclusion of WS2 as a substrate to proximity induced spin-orbit coupling due to the adjacent WS2 layer, and we compare our data to the literature.

Superfluid density and carrier concentration across a superconducting dome: The case of strontium titanate

We present a study of the lower critical field, \hc1, of \STO as a function of carrier concentration with the aim of quantifying the superfluid density. At low carrier concentration (i.e. the underdoped side), superfluid density and the carrier concentration in the normal state are equal within experimental margin. A significant deviation between the two numbers starts at optimal doping and gradually increases with doping. The inverse of the penetration depth and the critical temperature follow parallel evolutions as in the case of cuprate superconductors. In the overdoped regime, the zero-temperature superfluid density becomes much lower than the normal-state carrier density before vanishing all together. We show that the density mismatch and the clean-to-dirty crossover are concomitant. Our results imply that the discrepancy between normal and superconducting densities is expected whenever the superconducting gap becomes small enough to put the system in the dirty limit. A quantitative test of the dirty BCS theory is not straightforward, due to he multiplicity of the bands in superconducting strontium titanate.