Temperature Dependence Of Mobility Research Articles

Charge Density and Mobility of Charge Density Waves in the Quasi-One-Dimensional Conductor NbS3

Three charge density waves (CDWs), two of which are formed above room temperature, are observed in the NbS3 monoclinic phase (NbS3-II). The charge density and mobility in the high-field limit have been determined for each of three CDWs in this work using the synchronization effect of CDWs in high-frequency fields. It has been found that the mobility of each CDW in this limit is approximately equal to the normal-state mobility of quasiparticles condensed in it. Furthermore, correlation has been observed between the temperature dependences of mobilities of CDWs and quasiparticles. The results of this work refresh problems of a mechanism of the limit conductivity of CDWs and of the distribution of CDWs between atomic chains in the unit cell.

Carrier mobility of two-dimensional Dirac materials: the influence of optical phonon scattering.

We developed an analytical formula to calculate the influence of optical phonons on the mobility of two-dimensional Dirac materials at arbitrary temperature and arbitrary doping concentration. The method was combined with first-principles calculations to show that the effect of optical phonons on mobility is not negligible for typical Dirac materials such as graphene even though the occupation number of optical phonons is relatively small. Unlike the treatment of electron-acoustic phonon coupling, the energy change of electrons in the scattering process with optical phonons is crucial, which leads to a non-power temperature dependence of mobility under weak doping. The formalism was applied to calculate and analyze the mobility of two well-known Dirac materials, α-graphyne and the VCl3 monolayer, which differs by one to two orders of magnitude.

Evolution of extremely large magnetoresistance in a Weyl semimetal, WTe2 with Ni-doping

${\mathrm{WTe}}_{2}$, a type-II Weyl semimetal, exhibits extremely large magnetoresistance (XMR) that is attributed to perfect electron-hole compensation. Here, we studied the evolution of the electronic properties of ${\mathrm{WTe}}_{2}$ with Ni substitution at W sites. Transport measurements exhibit a metal-to-insulator transition at low temperatures with the application of magnetic field; the field-induced effect is significantly reduced in the doped sample. The XMR of about ${10}^{6}$% at 2 K in ${\mathrm{WTe}}_{2}$ is reduced to ${10}^{4}$% in the 10% Ni-doped sample. The magnetoresistance shows an ${H}^{2}$ dependence and follows the temperature dependence of classical carrier mobility. We observe a deviation from Kohler's rule in both cases, indicating the presence of multiple types of charge carriers. Quantum oscillation data exhibit a signature of four Fermi pockets (two electron and two hole pockets) in both cases, and the nontrivial nature of the electron and hole bands. Interestingly, the electron-hole compensation is nearly perfect even in the doped compound, although the size of the Fermi pockets and the charge-carrier interaction parameters are slightly changed with doping. These results reveal intriguing insight into the XMR behavior of this class of materials and the possibilities of tuning the properties of charge carriers within the XMR regime.

Electron transport properties in thin InN layers grown on InAlN

We present a comprehensive analysis of structural and charge transport properties of high-quality N-polar InN/In0.57Al0.43N heterostructures grown by metalorganic chemical vapour deposition (MOCVD) on on-axis c-plane sapphire substrate and off-axis c-plane sapphire substrate misoriented by 4° toward the a-plane. Instead of a typical GaN template, we included an In-rich InAlN buffer layer to mitigate the lattice mismatch between InN and sapphire. We observed a strong correlation between the structural quality of InN and the electron mobility that was measured in a broad temperature range. Experimental results showed that InN grown at 550 °C had a higher density of edge-type dislocations, weaker temperature dependence of electron mobility, and higher electron concentration compared with InN grown at 600 °C. Our results suggest that acoustic phonon piezoelectric scattering dominated between 150 and 300 K, while dislocations and interface scattering prevailed from 10 to 150 K. The highest electron mobility of 714 cm2/Vs at room temperature was obtained for InN grown at 600 °C on the on-axis substrate, which corresponded with the lowest density of edge-type dislocations (2.7 ✕ 1010 cm−2) and the best surface morphology with a root-mean-square roughness of 1.4 nm.

Temperature-Dependent Rotational Dipole Mobility and Devitrification of the Rigid Amorphous Fraction in Unpoled and Poled Biaxially Oriented Poly(vinylidene fluoride)

Temperature-Dependent Rotational Dipole Mobility and Devitrification of the Rigid Amorphous Fraction in Unpoled and Poled Biaxially Oriented Poly(vinylidene fluoride)

Intrinsic Channel Mobility Associated with Extended State Transport in IGZO TFTs

There is wide variation in the understanding of the interaction and/or independence of channel mobility and charge trapping in IGZO TFTs. A new interpretation of transport behavior proposes an intrinsic channel mobility with distinctive temperature dependence. Bottom-gate TFTs were fabricated with a 50 nm IGZO film sandwiched between a 50 nm gate oxide and a 50 nm passivation oxide layer. The working metal was molybdenum for the gate electrode and source/drain contacts. A passivation anneal was done and a capping layer was added to promote electrical stability; full process details are provided in a previous report. Devices were tested using a Lakeshore cryogenic probe station, with transfer characteristic measurements taken from room temperature to below 100 K.Electrical measurements made on long-channel devices (L = 12 µm) has revealed temperature-dependent behavior that is not explained by existing TCAD models employed for defect states and carrier mobility. An IGZO TFT device model has been recently developed using Silvaco Atlas, which accounts for the role of donor-like oxygen vacancy defects, acceptor-like BTS, acceptor-like interface traps, and a temperature-dependent intrinsic channel mobility. The model demonstrates a remarkable match to transfer characteristics measured at T = 150 K to room temperature. The temperature-dependent mobility follows a power-law relationship resembling behavior consistent with ionized defect scattering; the source of which is proposed. Details of the material and device model, and associated defect distribution parameters will be presented. Figure 1

A Compact Model of Amorphous InGaZnO TFTs to Predict Temperature-Dependent Characteristics

In amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs), the conduction mechanism of percolation and trap-limited conduction prevails within the limits of gate voltage. In these limits, the gate voltage-dependent mobility follows a power law. The temperature dependence of gate-functioned mobility is modeled by introducing very few empirical parameters (P and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\gamma $ </tex-math></inline-formula> ), where P follows the Arrhenius equation and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\gamma $ </tex-math></inline-formula> is found to be constant for varying temperature ranges. The proposed model effectively incorporates the combined effects of mobility as a function of temperature and gate voltages of the device using a single formulation for accurate thermal characterization. This model reproduces the measured characteristics of oxide TFT for a wide temperature range with an average error less than 2.5%. The model is implemented in Verilog-A to facilitate circuit simulations. Further, an 11-stage Ring-Oscillator is simulated with the proposed model. These results are compared with the measured circuit response under similar testing conditions to validate the model accuracy in the complete region of operation over a wide range of temperature variations. These simulation results show a good agreement with measured results within 8% accuracy, reinforcing the model validation.

A Unified Distribution Form of the Density of States in Disordered Organic Semiconductors

An accurate density of states (DOS) is essential for understanding charge carrier transport properties and then better developing the electronic and optoelectronic devices. Currently, Gaussian and exponential forms are the two most general DOS, which can be unified in the tail state of the state. However, Gaussian DOS at high concentrations is not applicable, as well as exponential DOS at low concentrations. To avoid the disadvantage of Gaussian and exponential form, based on the Poisson flow, a unified distribution form of the DOS have been developed to discuss the charge carrier transport. The proposed DOS is more consistent with the actual situation and exhibits better rationality under the whole range of carrier density. Otherwise, we for the first time introduced the entropy to analyze the disorder parameters. Based on the proposed DOS, the concentration and temperature dependence of mobility has been discussed in detail.

Impact of dopant-induced band tails on optical spectra, charge carrier transport, and dynamics in single-crystal CdTe

Cadmium telluride (CdTe) semiconductors are used in thin-film photovoltaics, detectors, and other optoelectronic applications. For all technologies, higher efficiency and sensitivity are achieved with reduced charge carrier recombination. In this study, we use state-of-the-art CdTe single crystals and electro-optical measurements to develop a detailed understanding of recombination rate dependence on excitation and temperature in CdTe. We study recombination and carrier dynamics in high-resistivity (undoped) and arsenic (As)-doped CdTe by employing absorption, the Hall effect, time-resolved photoluminescence, and pump-probe in the 80–600 K temperature range. We report extraordinarily long lifetimes (30 µs) at low temperatures in bulk undoped CdTe. Temperature dependencies of carrier density and mobility reveal ionization of the main acceptors and donors as well as dominant scattering by ionized impurities. We also distinguish different recombination defects. In particular, shallow AsTe and deep VCd−AsCd acceptors were responsible for p-type conductivity. AX donors were responsible for electron capture, while nonradiative recombination centers (VCd−AsTe, As2 precipitates), and native defects (VCd−TeCd) were found to be dominant in p-type and n-type CdTe, respectively. Bimolecular and surface recombination rate temperature dependencies were also revealed, with bimolecular coefficient T−3/2 temperature dependence and 170 meV effective surface barrier, leading to an increase in surface recombination velocity at high temperatures and excitations. The results of this study allowed us to conclude that enhanced crucible rotation growth of As-doped CdTe is advantageous to As activation, leading to longer lifetimes and larger mobilities and open-circuit voltages due to lower absorption and trapping.

A coupled crystal-plasticity and phase-field model for understanding fracture behaviors of single crystal tungsten

The brittle-ductile transition (BDT) of tungsten depends heavily on temperature-dependent dislocation mobility. To well understand the underlying mechanism of tungsten fracture behavior, the dislocation activities need to be well described, but the related model remains limited. In this work, a coupled crystal-plasticity and phase-field model (CP-PFM) is developed based on a unified thermodynamic framework, which considers the thermal activated kink-pair mechanism of screw dislocation motion, the evolution of dislocation density, and the coupling effects between the plasticity and the crack propagation. The developed model can well predict the experimental results and is used to study the BDT process of tungsten. Four typical fracture processes are disclosed depending on the temperature, including the brittle fracture, semi-brittle fracture, micro-ductile fracture and ductile fracture. They exhibit different micro-cleavage crack features, induced by the competition between the shielding effect of the plastic zone and the crack propagation near the crack tip zone. In addition, the fracture process of tungsten is found to be significantly influenced by non-Schmid effect in the low and medium temperature regimes.

Practicality assessment: Temperature-governed performance of CO2-containing Li–O2 batteries

Practical lithium–oxygen batteries require a shift from pure O2 to air. CO2 traces, however, fundamentally alter the O2 electrochemistry towards Li2CO3 formation via peroxocarbonate intermediates in highly-solvating electrolytes e.g., tetraethylene glycol dimethyl ether (G4). Here, we reveal that operating temperatures (0∼70 °C) critically dictate Li2CO3-associated cell overcharges (4.60∼3.77 V), by determining temperature-dependent reaction kinetics and evolving species mobility, as analogously witnessed under pure O2-conditions. Against what is observed for Li2O2 formation in the Li–O2 cell, however, cell temperatures do not govern the crystallinity of the Li2CO3 discharge product. In agreement with experimental observations, comprehensive density functional theory calculations also uncover the effect of the temperature on the Li2CO3 precipitation mechanism and shed light on the dwindling stabilization of metastable peroxocarbonate intermediates during discharge at increasing cycling temperatures. On the other hand, during extended operation, the temperature-dependent reactants mobility in the viscous G4 electrolyte and remnant Li-deficient carbonate surfaces from precedent recharge steps play equally prominent roles on the Li2CO3 precipitation mechanism and the resulting cell capacity. Our study highlights the complexity of practical Li–O2 cells with temperature-dependent performances.

Transport phenomena in copper doped cadmium telluride: calculation from the first principles

In the presented work, the method of determining the energy spectrum, the wave function of the heavy hole and the crystal potential in CdTe at an arbitrarily given temperature is considered. Using this approach within the framework of the supercell method the temperature dependences of the ionization energies of various types of defects caused by the introduction of copper impurity in cadmium telluride are calculated. Also the proposed method makes it possible to define the temperature dependence of the optical and acoustic deformation potentials, as well as the temperature dependence of the scattering parameters of heavy holes on ionized impurities, polar optical, piezooptical and piezoacoustic phonons. Within the framework of short-range scattering models, the temperature dependences of the heavy hole mobility and Hall factor are considered.

A control system for performing automated time-domain NMR measurements in Bruker Minispec spectrometers

Time-Domain Nuclear Magnetic Resonance (TD-NMR) is a powerful method used for general characterization of a variety of materials as well as to online or in-situ monitoring of process. Because of that industries increasingly benefit from its use in process analytical technology (PAT) for production control and quality assurance. However, there is an increasing number of applications that requires automated signal acquisition as a function of temperatures, a resource that is not always available in many of the commercial equipments. We describe an automation procedure using the Bruker Minispec mq20 spectrometer with the BVT3000 temperature control, building software and hardware to execute TD-NMR experiments as a function of the temperature as well as the temperature calibration in an automatic form. Despite being developed for this specific equipment, the general idea can be used in other TD-NMR equipments that could benefit from this type of automation. As a demonstration, the system was used to monitor the temperature dependence of molecular mobility in polymers. All developed codes are shared in an open repository on GitHub.

Scrutinizing thermally stimulated current transients originating from trapped charges in organic semiconductors: A drift-diffusion study

Thermally stimulated current (TSC) is a widely used technique to assess trap states and extract their density, energy, and capture rate using analytical expressions. In many cases, the latter are derived from physical models pertaining to inorganic semiconductors stipulating the absence of space charge or constant lifetime of free charge carriers. Especially for organic semiconductors, the validity of these equations can, therefore, be argued. Here, we investigate the validity range of this approach by fitting the classical equations to synthetic TSC data obtained from drift-diffusion simulation using representative input parameters for organic semiconductors. We find that the equation derived for slow recapture rate as well as the initial rise method provide excellent trap parameter predictions. On the other hand, the equation using the temperature of the peak current as well as the one derived for fast retrapping have a limited range of validity. An important merit of drift-diffusion modeling is the possibility to access local variables such as charge carrier density, electric field, and recombinaton. We unravel that a small fraction of traps nearby the electrode cannot be emptied even at high temperature due to the diffusion of charge carriers from the electrode into the semiconductor. Additionally, we find that an important electrostatic factor relates the extracted charge carriers measured by the external circuit and the input trap density. For the homogeneously distributed trap states used here, this factor is precisely two. Finally, extensions of the model are analyzed by implementing temperature and field dependent mobility into the drift-diffusion model.

Thermodynamic assessment of the Co–Cu–Fe system and diffusion study of its fcc phase

The Co–Cu–Fe ternary system was thermodynamically reassessed with the CALPHAD method in the present work. The miscibility gap of the fcc phase at 1373 K was experimentally determined. The phase relationship among the liquid, Cu-rich and Cu-depleted fcc phases was established. Based on the new experiment data and available data in the literature, the thermodynamic optimization of the Co–Cu–Fe system was carried out. The experimental phase equilibria over the whole composition range can be reproduced. Additionally, in order to optimize the temperature-dependent mobility parameters, the interdiffusion coefficients at 1373 K and 1473 K were measured with the diffusion-couple method. Based on the updated thermodynamic description and atomic mobilities in the binary subsystems, the atomic mobilities of Co, Cu and Fe in the fcc phase were re-optimized. The measured composition profiles and diffusion paths of diffusion couples can be well reproduced using the present mobility parameters.

Phonon-Limited Mobility in h-BN Encapsulated AB-Stacked Bilayer Graphene.

The weak acoustic phonon scattering in graphene monolayer leads to high mobilities even at room temperatures. We identify the dominant role of the shear phonon mode scattering on the carrier mobility in AB-stacked graphene bilayer, which is absent in monolayer graphene. Using a microscopic tight-binding model, we reproduce experimental temperature dependence of mobilities in high-quality boron nitride encapsulated bilayer samples at temperatures up to ∼200 K. At elevated temperatures, the surface polar phonon scattering from boron nitride substrate contributes significantly to the measured mobilities of 15 000 to 20000 cm^{2}/Vs at room temperature and carrier concentration n∼10^{12} cm^{-2}. A screened surface polar phonon potential for a dual-encapsulated bilayer and transferable tight-binding model allows us to predict mobility scaling with temperature and band gap for both electrons and holes in agreement with the experiment.

Electrical and galvanomagnetic properties of black phosphorus single crystals

Black phosphorus (b-P) single crystals having the n-type electrical conductivity produced in a high pressure set-up (~1 GPa) with six diamond anvils at 800 °C for 12 h have been studied. The electrical conductivity σ(Т,В) and the Hall constant Rh(Т,В) have been analyzed within one-band and two-band models as functions of temperature in the 2 &lt; Т &lt; 300 K range and magnetic field in the 0 &lt; В &lt; 8 T range. Fitting of the experimental σ(Т,В) and Rh(Т,В) curves suggests the following key properties of the crystals: (1) intrinsic conductivity type, (2) approximately equal electron and hole concentrations and mobilities, (3) anisotropic behavior of electron and hole conductivities, concentrations and mobilities and (4) combination of negative and positive contributions to magnetoresistance (magnetoresistive effect, MR). In a zero magnetic field the anisotropy coefficient α = [σа(Т) – σс(Т)]/σс(Т) below 50—70 K is positive whereas above 220 K its sign changes to negative due to a specific combination of the temperature dependences of carrier concentration and mobility. It has been shown that the negative sign of relative MR (negative magnetoresistive effect) dominates at T &lt; 25 K and B &lt; 6 T and is presumably caused by the effects of strong localization resulting from structural disorder. The positive MR sign (positive magnetoresistive effect) is associated with the Lorentz mechanism of carrier movement and exhibits itself above 25 K in 6–8 T magnetic fields.

Simulation and validation of a planar HPGe detector signal database for use in pulse shape analysis

Due to their excellent spectroscopic capabilities and sensitivity to the position of γ-ray interactions, segmented High-Purity Germanium (HPGe) detectors are frequently used in γ-ray imaging applications. The quality of produced images is heavily dependent on the precision of the identification of interaction position within the detector that can be achieved through segmentation and the application of pulse shape analysis (PSA) techniques. In this work, a simulated database of signals was produced for a HPGe planar detector used in a multi-tiered Compton camera system. The impurity concentration and temperature-dependent mobility parameters of the simulation were optimised and validated by evaluating the pulse-shape response at select positions using experimentally-measured collimated γ-beam data. A grid-search algorithm was developed for analysis of single and double-site γ-ray interactions using signal comparison PSA and is shown to improve the position resolution within the detector.

Influence of numerous Moiré superlattices on transport properties of twisted multilayer graphene

Band structure engineering by changing the parameters of Moiré superlattices enables creation of materials with outstanding properties for twistronics, straintronics and quantum nanoelectronics applications. Herein, twisted multilayer graphene (tMLG) with various combinations of twist angles (θ) and numerous Moiré superlattices was synthesized. The strong dependence of the transport properties of tMLG on the structural characteristics was demonstrated using synchrotron-based core-level spectroscopy methods combined with micro-Raman spectroscopy. It has been shown that tMLG with a large content of the twisted phase with the twist angles between the adjacent graphene layers of θ = 26–30° over the entire tMLG thickness is characterized by a perfect structure of the single layer graphene (SLG) and extraordinary transport characteristics. At low temperatures, the asymmetry of charge carriers and atypical temperature dependence of average carrier mobility μ were observed. In such tMLG, μ was higher than 105 cm2V−1s−1. Decrease in the content of the twisted phase with θ = 26–30°, as well as increase in the contents of other twist angles and AB-stacked phase (θ = 0°), leads to a significant decrease in μ. Hence, tMLGs are a better choice for the nanoelectronics industry because, having outstanding transport properties, they are, contrary to SLG, insensitive to the specific features of process procedures.

Enhancement of channel mobility in 4H-SiC trench MOSFET by inducing stress at SiO2/SiC gate interface

To investigate the impact of stress at the SiO2/SiC gate interface on the channel mobility of 4H-SiC trench MOSFETs, we fabricated trench MOSFETs with two variations of stress in the channel region by changing the deposition temperature of polycrystalline silicon used for gate electrodes. The results found that effective mobility was enhanced by several dozen MPa of tensile stress toward induced by the polycrystalline silicon. Based on the temperature dependence of mobility, all scattering mobilities were enhanced. It was supposed that the electron effective mass decreased because of tensile stress induced at the channel region.