Thermal Excitation Of Electrons Research Articles

Process effect analysis on nitride trap distribution in silicon-oxide-nitride-oxide-silicon flash memory based on charge retention model

This paper analyzes data retention characteristics to determine process effects on the trap energy distribution of silicon nitride in silicon-oxide-nitride-oxide-silicon (SONOS) flash memory devices. Nitride films were prepared by low-pressure chemical vapor deposition (LPCVD) and plasma-enhanced chemical vapor deposition (PECVD). PEVCD films embedded with silicon nanocrystals (Si-NCs) were also compared. The flat band voltage shift in the programmed device was measured at high temperatures to observe the thermal excitation of electrons from the nitride traps in retention mode. The trap energy distribution was extracted using the charge decay rates, and the experimental results showed that nitride fabricated by PECVD has a shallower trap than nitride fabricated by LPCVD. In nitride with Si-NCs, increased trap sites were observed in the range of 1.14 eV to 1.24 eV.

Semiconductor thermionics for next generation solar cells: photon enhanced or pure thermionic?

Semiconductors have been used in solar energy conversion for decades based on the photovoltaic effect. An important challenge of photovoltaics is the undesired heat generated within the device. An alternative approach is thermionics, which uses the thermal excitation of electrons from an emitter to a collector across a vacuum gap. If the emitter is a p-type semiconductor, the photogeneration-induced quasi-Fermi level splitting can reduce the effective barrier for electron emission—a mechanism used by a photon enhanced thermionic emission device. Here, we evaluate the prospects of this alternative solar conversion technology considering different semiconductor materials and thermionic device configurations. We also reveal that whether such a device operates in the photon enhanced or purely thermionic mode, depends on the complex interplay among materials properties, device physics and solar concentration level.

Thermodynamic properties ofε-Fe with thermal electronic excitation effects on vibrational spectra

The thermodynamic properties of hcp-iron ({\epsilon}-Fe) are essential for investigating planetary cores' internal structure and dynamic properties. Despite their importance to planetary sciences, experimental investigations of {\epsilon}-Fe at relevant conditions are still challenging. Therefore, ab initio calculations are crucial to elucidating the thermodynamic properties of this system. Here we use a free energy calculation scheme based on the phonon gas model compatible with temperature-dependent phonon frequencies. We investigate the effects of electronic thermal excitations, which introduces a temperature dependence on phonon frequencies, and the implication for the thermodynamic properties of {\epsilon}-Fe at extreme pressure (P) and temperature (T) conditions. We disregard phonon-phonon interactions, i.e., anharmonicity and their effect on phonon frequencies. Nevertheless, the current scheme is also applicable to T-dependent anharmonic frequencies. We conclude that the impact of thermal electronic excitations on vibrational properties is not significant up to ~ 4,000 K at 200 GPa but should not be ignored at higher temperatures or pressures. However, the static free energy, Fst, must always include the effect of thermal excitation fully in a continuum of Ts. Our results for isentropic equations of state show good agreement with data from recent ramp compression experiments up to 1,400 GPa conducted at the National Ignition Facility (NIF).

Two-Dimensional Cold Electron Transport for Steep-Slope Transistors.

Room-temperature Fermi-Dirac electron thermal excitation in conventional three-dimensional (3D) or two-dimensional (2D) semiconductors generates hot electrons with a relatively long thermal tail in energy distribution. These hot electrons set a fundamental obstacle known as the "Boltzmann tyranny" that limits the subthreshold swing (SS) and therefore the minimum power consumption of 3D and 2D field-effect transistors (FETs). Here, we investigated a graphene (Gr)-enabled cold electron injection where the Gr acts as the Dirac source to provide the cold electrons with a localized electron density distribution and a short thermal tail at room temperature. These cold electrons correspond to an electronic refrigeration effect with an effective electron temperature of ∼145 K in the monolayer MoS2, which enables the transport factor lowering and thus the steep-slope switching (across for three decades with a minimum SS of 29 mV/decade at room temperature) for a monolayer MoS2 FET. Especially, a record-high sub-60-mV/decade current density (over 1 μA/μm) can be achieved compared to conventional steep-slope technologies such as tunneling FETs or negative capacitance FETs using 2D or 3D channel materials. Our work demonstrates the potential of a 2D Dirac-source cold electron transistor as a steep-slope transistor concept for future energy-efficient nanoelectronics.

Electrical response with temperature and magnetoelectric coupling of multiferroic nanocomposites

Magnetoelectric Zn0.5Co0.5Fe2O4 – PbZr0.58Ti0.42O3 nanocomposites are synthesized considering chemical reaction process at low temperature and solid-state reaction technique. Structural characterization is performed by X-ray diffraction analysis, which shows two types of nanometric grains coexisting simultaneously. The materials surface morphology displays that the fabrication method is quite effective for preparation of nanocomposites. Crystallite size of different constituent phases is observed to in manometer range. The dependency of temperature on electrical response and dielectric constant of nanocomposites are analyzed with a variation of frequency. The dielectric constant with temperature decreases attributing the thermal excitation of electrons. Moreover, we have evaluated the magnetoelectric coefficient in two configurations (longitudinal and transverse) with increasing applied magnetic field. The coupling coefficient is observed to ∼0.8 mV/cm-Oe in transverse configuration, whereas in longitudinal configuration it is increasing in behavior with measured magnetic field. At room temperature, the observing magnetoelectric coupling may be as a cause of magnetic field dependent strain transfer to the piezoelectric phase to the piezomagnetic phase of the nanocomposites.

First-principles investigation of electrons’ thermal excitations in UN, UAl2 and ThN

We investigated the electronic heat capacity and energetics of thermal excitations of electrons around Fermi energy of selected metallic compounds (UN, UAl2 and ThN) using first-principles methods. The effect of magnetism was evaluated for UN and UAl2. The generalized gradient approximation (GGA) of the Perdew, Burke, and Ernzerhof functional, developed for solids (/PBEsol) as implemented in Quantum ESPRESSO (QE) was used for UN and UAl2 while the earlier PBE was used for ThN. We found that electrons' thermal excitations would only slightly affect the equilibrium lattice constants in considered compounds except for non-magnetic UN, where it needs to be taken into account. The electron density of states at Fermi energy is larger and therefore an increased effect was found. The calculated electronic heat capacity i predicted to be the largest for non-magnetic UN and very small for non-magnetic ThN. The electronic heat capacity is lower for ferromagnetic UN and UAl2 and larger for all compounds in a non-magnetic state when the lattice constant decreases. Electronic heat capacity increases with temperature and becomes more significant at higher temperatures. The predicted γ coefficients are smaller than the evaluated experimentally at low temperatures, but is in a better agreement with the lower values evaluated at 300 K–1700 K temperature range. Our evaluations of the respective electronic energy correction parameters due to electrons’ thermal excitations show that except for non-magnetic UN, they are at least one order of magnitude smaller than the Grüneisen parameter for ferromagnetic UN, both non-magnetic and ferromagnetic UAl2 and ThN.

Quantum Monte Carlo determination of the principal Hugoniot of deuterium

We present Coupled Electron-Ion Monte Carlo results for the principal Hugoniot of deuterium together with an accurate study of the initial reference state of shock wave experiments. We discuss the influence of nuclear quantum effects, thermal electronic excitations, and the convergence of the energy potential surface by wave function optimization within Variational Monte Carlo and Projection Quantum Monte Carlo methods. Compared to a previous study, the new calculations also include low pressure-temperature (P,T) conditions resulting in close agreement with experimental data, while our revised results at higher (P,T) conditions still predict a more compressible Hugoniot than experimentally observed.

Investigation on elastic and thermodynamic properties of Fe25Cr20NiMnNb austenitic stainless steel at high temperatures from first principles

Temperature-dependent properties are very useful in modeling the behavior of materials servicing at high temperature such as austenitic stainless steels. Besides, for predicting the mechanical properties of such alloys, experimental data at high temperature are very important, but scarce. In this work, we use first-principles calculations combined with a quasi-harmonic Debye model to evaluate the Helmholtz free energy of paramagnetic Fe25Cr20NiMnNb austenitic stainless steel as a function of temperature and volume. The contributions due to electronic excitations, magnetic fluctuations, and lattice vibrations are taken into account. Elastic and other thermodynamic properties of the material, such as the equilibrium lattice parameter, elastic moduli, entropy, are derived from the free energy in the temperature range from 800 to 1100 K and compared with available experimental data. The influence of Nb concentration, as well as the effects of electronic excitations and lattice vibrations, on the elastic and thermodynamic properties are discussed in detail. The elastic moduli are found to decrease with increasing temperature. This elastic softening phenomenon is mainly due to thermal expansion; the contribution of thermal electronic excitations is found to be relatively small.

Carrier leakage via interface-roughness scattering bridges gap between theoretical and experimental internal efficiencies of quantum cascade lasers

When conventionally calculating carrier leakage for state-of-the-art quantum cascade lasers (QCLs), that is, LO-phonon-assisted leakage from the upper laser level via electron thermal excitation to high-energy active-region (AR) states, followed by relaxation to low-energy AR states, ∼18%-wide gaps were recently found between calculated and experimentally measured internal efficiency values. We incorporate elastic scattering [i.e., interface-roughness (IFR) and alloy-disorder scattering] into the carrier-leakage process and consider carrier leakage from key injector states as well. In addition, the expressions for LO-phonon and IFR-triggered carrier-leakage currents take into account the large percentage of thermally excited electrons that return back to initial states via both inelastic and elastic scattering. As a result, we find that the gaps between theoretical and experimental internal efficiency values are essentially bridged. Another finding is that, for the investigated state-of-the-art structures, IFR scattering causes the total carrier leakage to reach values as much as an order of magnitude higher than conventional inelastic scattering-only leakage. The developed formalism opens the way to significantly increase the internal efficiency (i.e., to more than 80%) via IFR-scattering engineering, such that maximum wall-plug efficiencies close to projected fundamental, both-facets values (e.g., 42% at λ = 4.6 μm) can be achieved. By employing this formalism, we reached a 4.6 μm-emitting-QCL preliminary design for suppressing IFR-triggered carrier leakage, which provides an internal efficiency of 86% as well as a projected single-facet wall-plug efficiency value of 36% at a heatsink temperature of 300 K.

Ideal strengths and thermodynamic properties of W and W-Re alloys from first-principles calculation

Abstract First-principles calculation, quasi-harmonic approximation and thermal electronic excitation have been combined to examine ideal strengths and thermodynamic properties of W and W-Re alloys. It is found that the ideal tensile and shear strengths of W are decreased due to Re alloying, with the easy cleavage plane of both W and W-Re alloys found in (100) plane. Moreover, the derived temperature-dependent elastic properties explain the “Re softening effect”, and provide a deep understanding of the ductility of W-Re alloys as a function of the temperature. The results also reveal that Re alloying significantly increases the ductility of W, which is rare at low temperature in the temperature range 0−2000 K. The calculated results agree well with similar experimental observations in the literatures, and could explore their fundamental properties as well as various potential applications.

Thermal regulation mechanism of photoluminescence in intrinsic acceptor-rich ZnO microtube

<sec>Thermal effect is one of the most important factors limiting the photoluminescence performances of semiconductor devices. With the increase of temperature, the PL intensity decreases gradually due to the effect of thermal quenching. However, the abnormal negative thermal quenching effect has been found in many semiconductor materials in recent years, e.g. ZnO, BiFeO<sub>3</sub>, InPBi, etc. This effect is generally considered as the sign of the existence for middle/local energy state in the electron-hole recombination process, which usually needs to be confirmed by the temperature-dependent PL spectra.</sec> <sec>Here, we report the thermal regulation mechanism of photoluminescence in intrinsic acceptor-rich ZnO (A-ZnO) microtubes grown by the optical vapour supersaturated precipitation method. The grown A-ZnO microtube with a length of 5 mm and diameter of 100 μm has regular hexagonal cross-section morphology. Its optical band gap at room temperature is about 3.30 eV. With the increase of temperature, the PL intensity of A-ZnO microtube exhibits an abnormal behavior from the thermal quenching to the negative thermal quenching and then to the thermal quenching. The thermal quenching effect at 80–200 K is associated with regurgitation/ionization of shallow donor, thermal ionization of free exciton, and conversion of neutral acceptor bound exciton. The negative thermal quenching effect at 200–240 K is associated with thermal excitation of electrons in a deep level trap of 488 meV below the conduction band minimum (CBM). The thermal quenching effect at 240–470 K is related to Shockley-Read-Hall recombination based on the non-radiative recombination center of 628 meV below the CBM. The non-radiative recombination center and trap level are far from the acceptor level of A-ZnO microtube, which may be related to the deep-level defect of oxygen vacancy in the intrinsic A-ZnO microtube. This work establishes the temperature-dependent transition model of photo-generated carriers and reveals the thermal regulation mechanism of PL for the A-ZnO microtubes. It provides a novel platform for designing the high-temperature and high-efficiency ZnO-based photoelectric devices.</sec>

Composite transport mechanism enhancing thermoelectric performance of Ag-doped Mg3Sb2

The first principle calculations are employed to investigate the Ag-doped Mg3Sb2. Comparing the formation energy and the bonds length between the Ag atom and its nearest Sb atoms, it is found that Ag atom is more favorable to substitute Mg atom in the anionic [Mg2Sb2]2− layers. Comparing the electronic energy band structures between the un-doped and Ag-doped Mg3Sb2, the Femi level across the valence band through Ag doping. The double transport mechanism are induced by the doping, at low temperature, materials exhibit metallic properties, and when the temperature is higher than 422 K, the thermal excitation of electrons increases gradually, metallic and semiconductor transport mechanism coexist. As the temperature continue rising, the metallic conduction mechanism returns to dominant. The composite transport mechanism are verified in our calculations, and are found in temperature dependence of the electronic conduction and Seebeck coefficient. Our calculations explain the previous experiments.

Lattice dynamics for isochorically heated metals: A model study

The electron-excitation induced bond strength variations in metals have been predicted from density-functional theory calculations and observed experimentally, while the microscopic mechanism has yet to be elucidated. Here, we present a minimal model that reproduces the phonon hardening and softening for fcc- and bcc-structured metals as a result of the electron thermal excitation. For the cases of bcc-structured metals, a delicate balance between the first and second nearest neighbor force constants leads to a phonon softening at the N point.

An alternative approach to predict Seebeck coefficients: Application to La3−xTe4

A thermodynamic understanding of Seebeck coefficient was demonstrated in terms of electrochemical potential. It divided the contributions to the Seebeck coefficient into two contributions: the effect of thermal electronic excitations due to Fermi distribution and the effect of charge carrier gradient due to thermal expansion. The procedure is illustrated within the rigid band approximation in terms of the electronic density-of-states and the quasiharmonic approximation in terms of the phonon density-of-states. Numerical results were given using the n-type high temperature thermoelectric material La3-xTe4 at x = 0, 0.25, and 0.33 as the prototype at a variety of carrier concentrations.

Er-doped antimonite Sb2O3−PbO−ZnO/ZnS glasses studied by low-temperature photoluminescence spectroscopy

Antimonite glasses Sb2O3−PbO−ZnO/ZnS doped with ErCl3 within the range of 0.25–1.0 mol% were prepared and investigated by means of transmission, photoluminescence excitation and photoluminescence (PL) spectroscopy. In the studied glasses, the wavelength of 522 nm was found to be the most suitable for PL excitation of doped-in Er3+ ions in both the NIR and VIS spectral ranges. The PL spectra excited by wavelengths at 522 and/or 514.5 nm were measured at room and low temperatures (4 K) in the wavelength range of 400–1700 nm. Low-temperature PL spectroscopy allowed us to observe both the broad luminescence of the host glass centred at around 1050 nm and the narrow emissions caused by Er3+ ions. Special attention was given to the wavelengths where the broad-band luminescence of the host overlaps with 4f–4f transitions within Er3+ ions. A broad PL band of the host glass was observed together with a superimposed narrow emission band at 985 nm (4I11/2 → 4I15/2) and a narrow absorption dip at about 970 nm (4I15/2 → 4I11/2). This absorption dip is a direct experimental evidence of energy transfer from the electronic structure of the host glass to 4f states of doped-in Er3+ ions. The strongest PL band at 1530 nm, representing transition from the lowest excited manifold 4I13/2 to the ground state manifold 4I15/2 was measured at 300 K and 4 K. The broadening at elevated temperature occurs primarily in the high energy part of the spectrum due to thermal excitation of electrons within the 4I13/2 manifold.

Thermo-mechanical properties of Ni-Mo solid solutions: A first-principles study

The mechanical strength of Ni-based single-crystal superalloys under service condition is related to the thermo-mechanical properties of the disordered γ matrix. Here we use density functional theory and quasi-harmonic approximation to determine the temperature-dependent bulk moduli and generalized stacking fault energies (GSFEs) of Ni-Mo solid solutions. We show that the increasing temperatures between 1000 K and 1400 K cause evident reductions in the bulk moduli and planar fault energies of Ni-Mo alloys. Furthermore, their negative slopes versus temperature are gradually diminished with increasing Mo concentration except that of the unstable stacking fault energy. Adopting recent theoretical models for twinning based on GSFE, increasing temperature enhances the twinnability of low-Mo alloys but has limited influences in the case of high-Mo alloys. The composition-dependent thermal expansion, the thermal electronic excitation and the magnetic transition are shown to be the main factors rendering the complex variations in the elastic properties and twinning behavior of Ni-Mo solid solution with temperature.

Nonequilibrium optical properties of transition metals upon ultrafast electron heating

Femtosecond laser excitation of metals triggers swift modifications of the electronic distribution within the band structure. This has direct consequences on optical transitions transiently modifying the optical properties of materials. Influencing in real time the action of the pulse, these changes lead to substantial variations of the amount and the distribution in the energy deposited during the laser irradiation. The effect of the laser pulse can be described considering electrons heated to a range of electronic temperatures. In order to evaluate the dielectric response of ultrafast heated electrons, we performed ab initio molecular dynamic simulations coupled to the Kubo–Greenwood formalism and determined electronic temperature dependent optical properties. A series of representative transition metals was investigated: Cu, Ni, Cr, W, Ti, and Fe. The evolution of the optical properties is optically-pumped based on electronic redistribution within the density of electronic states. The proposed interpretation rely on modifications of the energy range of occupied states undergoing optical electronic transitions. It is found that the degree of filling and the shape of the d-block drive the dynamics of optical processes. Nonequilibrium optical indices, reflectivities and skin depths are reported for electron thermal excitation relevant to near-threshold laser ablation regimes. The effect of the electron temperature on optical properties allows to reconstruct and model ultrafast excitation dynamics in time-resolved diagnostics with relevance in laser micro- and nano-processing.

Temperature-induced modification of the Dirac cone in the tetradymite topological insulator Bi2Te2Se

The thermal excitation of electrons to higher, unoccupied states leads in certain cases to the paradox situation that the chemical potential needs to be shifted to lower energies. Here, a manipulation of Dirac fermions through the temperature dependence of the chemical potential is analyzed while maintaining sufficient insulating character in the bulk. The appearance of a bulk conduction band and consequently, a remarkable energy shift of the Dirac point is observed in ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{2}\mathrm{Se}$ at low temperatures (20 K), compared to the high temperatures (200--300 K), as revealed by hard x-ray photoelectron spectroscopy as well as momentum-resolved photoelectron microscopy. The temperature-induced shift of the Dirac cone and the appearance of bulk bands are related to the paradoxical shift of the chemical potential. The experiments are completely reversible, i.e., repeated cooling and heating of the sample up to room temperature recovers the original spectra. Such bulk related energy shifts must be considered not only in photoelectron spectroscopy but also in analysis of other measurable physical quantities and while designing devices for applications.

Effect of anti-site point defects on the mechanical and thermodynamic properties of MgZn2, MgCu2 Laves phases: A first-principle study

A theoretical investigation on the effect of anti-site point defects on mechanical and thermodynamic properties of MgZn2 and MgCu2 based on the first-principles calculations has been implemented. The results show that Mg anti-site defect on Zn or Cu site plays a reinforcing role, while exhibiting a more brittle behavior. However, the defect phase of Cu anti-sites on Mg sublattice shows a ductile tendency. The temperature-dependent thermodynamic properties are also predicted along with Debye-Grüneisen model including Debye temperature, thermal expansion coefficient and vibrational heat capacity as well as vibrational entropy. Taking into account the contribution of the lattice vibration and thermal electronic excitations, the Helmholtz free energy F can be calculated, suggests that anti-site defects caused by the occupancy of Mg at the Zn or Cu site are thermodynamically unstable compared to the defect-free phases. In addition, the total amount of charge transfer, the overall and the local difference charge densities are further discussed to analyze the mechanism of mechanical properties.

Thermal annealing and transient electronic excitations induced interfacial and magnetic effects on Pt/Co/Pt trilayer

Present study investigates the importance of thermal annealing and transient electronic excitations (using 100 MeV oxygen ions) in assisting the interfacial atomic diffusion, alloy composition, and magnetic switching field distributions in Pt/Co/Pt stacked trilayer. X-ray diffraction analysis reveals that thermal annealing results in the formation of the face centered tetragonal L1°CoPt phase. The Rutherford back scattering spectra shows a trilayer structure for as-deposited and as-irradiated films. Interlayer mixing on the thermally annealed films further improves by electronic excitations produced by high energy ion irradiation. Magnetically hard face centered tetragonal CoPt alloy retains its hard phase after ion irradiation and reveals an enhancement in the structural ordering and magnetic stability. Enhancement in the homogeneity of alloy composition and its correlation with the magnetic switching field is evident from this study. A detailed investigation of the contributing parameters shows that the magnetic switching behaviour varies with the type of thermal annealing, transient electronic excitations of ion beams and combination of these processes.