Electron Mean Free Path Research Articles

Resistivity and thermal conductivity of an organic insulator β′–EtMe3Sb[Pd(dmit)2

A finite residual linear term in the thermal conductivity at zero temperature in insulating magnets indicates the presence of gapless excitations of itinerant quasiparticles, which has been observed in some candidate materials of quantum spin liquids (QSLs). In the organic triangular insulator β′–EtMe3Sb[Pd(dmit)2]2, a QSL candidate material, the low-temperature thermal conductivity depends on the cooling process and the finite residual term is observed only in samples with large thermal conductivity. Moreover, the cooling rate dependence is largely sample dependent. Here we find that, while the low-temperature thermal conductivity significantly depends on the cooling rate, the high-temperature resistivity is almost perfectly independent of the cooling rate. These results indicate that in the samples with the finite residual term, the mean free path of the quasiparticles that carry the heat at low temperatures is governed by disorders, whose characteristic length scale of the distribution is much longer than the electron mean free path that determines the high-temperature resistivity. This explains why recent X-ray diffraction and nuclear magnetic resonance measurements show no cooling rate dependence. Naturally, these measurements are unsuitable for detecting disorders of the length scale relevant for the thermal conductivity, just as they cannot determine the residual resistivity of metals. Present results indicate that very careful experiments are needed when discussing itinerant spin excitations in β′–EtMe3Sb[Pd(dmit)2]2.

Optimized BaZrO3 nanorod density in YBa2Cu3O matrix for high field applications

To maximize the flux pinning in high-temperature superconductor (HTS) thin film applications, we have experimentally studied the effect of BaZrO3 (BZO) nanorod density within the YBa2Cu3O (YBCO) lattice. Even though the BZO decreases the self-field critical current density and the absolute at high fields is observed being the highest for 4% BZO doped YBCO, the maximized pinning property is observed at the level of 10% of BZO, when the distance between the outer edge of the nanorods is in the order of the diameter of the nanorod. In general, as also theoretically calculated, the flux pinning is increased even above 10% of BZO, but the improvement is limited by disturbance of the nanorod growth, weakening the flux pinning and decreasing the absolute drastically. The results evidently show that by maximizing the flux pinning using higher BZO doping concentration than earlier expected and taking care of the maximum self-field , which is strongly dependent on the electron mean free path, would offer the keys to resolve the challenges in the future HTS power applications.

Multiple elastic scattering of electrons in condensed matter (new version announcement)

Multiple elastic scattering of electrons in condensed matter (new version announcement)

Attenuation of Photoelectron Emission by a Single Organic Layer.

We report an in situ study of the thin-film growth of cobalt-phthalocyanine on Ag(100) surfaces using photoelectron emission microscopy (PEEM) and the Anderson method. Based on the Fowler–DuBridge theory, we were able to correlate the evolution of the mean electron yield acquired with PEEM for coverages up to two molecular layers of cobalt-phthalocyanine to the global work function changes measured with the Anderson method. For coverages above two monolayers, the transients measured with the Anderson method and those obtained with PEEM show different trends. We exploit this discrepancy to determine the inelastic mean free path of the low-energy electrons while passing through the third layer of CoPc.

Roles of electron mean free path and flux pinning in optimizing the critical current in YBCO superconductors

We present a way to reach the maximum possible critical current density, , for YBa2Cu3O (YBCO) thin films. This value is found to be around ten times the currently reached values. It is found that the (0 T) is governed by the mean free path of the electrons, as is the critical temperature, . The in field, on the other hand, is governed by flux pinning sites and can be enhanced by optimizing the size and distribution of the non-superconducting nanoinclusions. By optimizing both the mean free path and the pinning structure, the maximum values can be reached.

Patterning challenges for direct metal etch of ruthenium and molybdenum at 32 nm metal pitch and below

Ruthenium and molybdenum are candidate materials to replace Cu as the back-end-of-line interconnect metal for the tightest pitch features for future technology nodes. Due to their better figure of merit ρ0 × λ (ρ0 bulk resistivity, λ electron mean free path), it is expected that the resistance of <10 nm wide Ru and Mo metal lines can be significantly reduced compared to Cu. An important advantage for Ru and Mo is that both materials, in contrast to Cu, can be patterned by means of so-called direct metal etch, through reactive ion etching or atomic layer etching and can potentially be implemented without barrier. An integration scheme with direct metal etch instead of damascene patterning could simplify the overall patterning flow and eventually opens the possibility for exploring new integration concepts and patterning approaches. However, the learning on direct metal etch of Ru and Mo in the literature is scarce, especially at the relevant dimensions of today's interconnects. In this work, we will focus on the major patterning challenges we have encountered during the development of direct metal etch processes for Ru at 18 nm pitch and Mo gratings at 32 nm pitch. We have observed that the direct metal etch of Ru at these small dimensions is impacted by the growth of an oxidized layer on the sidewalls of the hard mask, which originates from the sputtering of the hard mask in combination with the O2-based Ru etch chemistry. This results in a narrowing of the trenches to be patterned and can easily lead to an etch stop in the smallest features. We will discuss several mitigation mechanisms to remove this oxidized layer, as well as to avoid the formation of such a layer. For patterning Mo with a Cl2/O2-based chemistry, the major patterning challenges we encountered are the insufficient sidewall passivation and the oxidation of the patterned Mo lines. The sidewall passivation issue has been overcome with an in situ thin SiO2-like deposition after partial Mo etch, while a possible mitigation mechanism for the Mo oxidation could be the in situ encapsulation immediately after Mo patterning.

Enhancement of breakdown strength and electromigration reliability for cobalt lines lightly doped with boron

Conventional copper interconnect is facing a considerable rise in electrical resistivity for applications in nanoscale devices due mainly to its significant-high electron mean free path. Cobalt, a metal with a shorter electron mean free path, has been suggested as the most promising interconnecting material for the next-generation device. Thus, this work studies alloying cobalt film with a small amount of boron as an electromigration retarder without sacrificing its electrical conductivity through evaluating its electrical properties, electromigration resistance, bonding structure, and thermal stability. Examining composition of the boron lightly-doped cobalt films shows that it can be intentionally controlled by the deposition power, yielding the incorporating boron not significantly increasing its electrical resistivity. Additionally, boron incorporation strongly intensifies the breakdown strength and electromigration reliability of the cobalt film. However, boron addition leads to a higher surface energy, and thus causes a degradation of the thermal stability. Nevertheless, the studied boron lightly-doped cobalt film is still thermally stable up to 600 °C, making it a promising interconnecting material for application in nanoscale devices.

Role of electronic energy loss on defect production and interface stability: Comparison between ceramic materials and high-entropy alloys

High-entropy alloys (HEAs) and some complex alloys exhibit desirable properties and significant structural stability in harsh environments, including possible applications in advanced reactors. Energetic ion irradiation is often used as a surrogate for neutron irradiation; however, the impact of ion electronic energy deposition and dissipation is often neglected. Moreover, differences in recoil energy spectrum and density of cascade events on damage evolution must also be considered. In many chemically complex alloys, the mean free path of electrons is reduced significantly, thus their decreased thermal conductivity and slow dissipation of localized radiation energy can have noticeable effects on displacement cascade evolution that is greatly different from metals with high thermal conductivity. In this work, nanocrystalline HEAs of Ni20Fe20Co20Cr20Cu20 and nonequiatomic (NiFeCoCr)97Cu3, both having much lower room-temperature thermal conductivity than pure Ni or Fe, are chosen as model HEAs to reveal the role that electronic energy loss during ion irradiation has in complex alloys. The response of nanocrystalline HEAs is investigated under irradiation at room temperature using MeV Ni and Au ions that have different ratios of electronic energy to damage energy, which is the energy dissipated in displacing atoms. Different from previously reported amorphization of nanocrystalline SiC, experimental results on these HEAs show that, similar to the process in nanocrystalline oxide materials, both inelastic thermal spikes via electron–phonon coupling and elastic thermal spikes via collisions among atomic nuclei contribute to the overall grain growth. The growth follows a power law dependence with the total deposited ion energy, and the derived value of the power-exponent suggests that the irradiation-induced instability at and near grain boundaries leads to local rapid atomic rearrangements and consequently grain growth. The high power-exponent value can be attributed to the sluggish diffusion and delayed defect evolution arising from the chemical complexity intrinsic to HEAs. This work calls attention to quantified fundamental understanding of radiation damage processes beyond that of simplified displacement events, especially in simulating neutron environments.

Finite-size and quantum effects in plasmonics: manifestations and theoretical modelling [Invited

The tremendous growth of the field of plasmonics in the past twenty years owes much to the pre-existence of solid theoretical foundations. Rather than calling for the introduction of radically new theory and computational techniques, plasmonics required, to a large extent, application of some of the most fundamental laws in physics, namely Maxwell’s equations, albeit adjusted to the nanoscale. The success of this description, which was triggered by the rapid advances in nanofabrication, makes a striking example of new effects and novel applications emerging by applying known physics to a different context. Nevertheless, the prosperous recipe of treating nanostructures within the framework of classical electrodynamics and with use of macroscopic, bulk material response functions (known as the local-response approximation, LRA) has its own limitations, and inevitably fails once the relevant length scales approach the few- to sub-nm regime, dominated by characteristic length scales such as the electron mean free path and the Fermi wavelength. Here we provide a review of the main non-classical effects that emerge when crossing the border between the macroscopic and atomistic worlds. We study the physical mechanisms involved, highlight experimental manifestations thereof and focus on the theoretical efforts developed in the quest for models that implement atomistic descriptions into otherwise classical-electrodynamic calculations for mesoscopic plasmonic nanostructures.

Near-Infrared Optical Response and Carrier Dynamics for High Photoconversion in Tellurene

Materials for applications in solar cells require a combination of features including an appropriate band gap and long relaxation times for photoexcited hot carriers. On the basis of ab initio many-body perturbation theory, including the spin–orbit interaction, we investigate the photocarrier generation and dynamics in α-tellurene. We show that photoexcited electrons are mainly generated in the near-infrared range, starting at 0.89 eV and forming excitons that are strongly bound, compared to its bulk counterpart, with a binding energy of 0.31 eV. We also explore the role of the electron–phonon interaction, finding that the electronic states in the first conduction band minimum couples weakly with phonons, yielding longer hot electron lifetimes (up to 70 fs) and mean free paths up to 37 nm. We also show that the extraction of hot holes may result in a challenging task as these carriers possess sub-3 nm mean free paths. We finally estimate that 1-nm-thick α-Te provides a short-circuit current density of 6.7 mA/cm2 and a maximum power conversion efficiency of 4.4%, which highlights its potential for efficient photovoltaic device development.

Influence of Size, Shape, and Scattering on Electrical Resistivity of Metal Nanowires

A simple quantitative model has been proposed for exploring the combined effect of size, shape, and electron scattering on the electrical resistivity of metallic nanowires. In the present model, the effect of different cross-sectional shapes of nanowires has been comprised on the surface and grain boundary scattering. For understanding electrical behavior at the nanolevel, the incorporation of specularity parameter (p) with different cross-sectional shapes of nanowires is essential. It is responsible for the reduction in the mean free path of electrons; which generates the favorable condition for enhancing the surface scattering, consequently contributing to increment of electrical resistivity. The applicability of the proposed model has been investigated for copper, nickel, silver, and aluminum metallic nanowires of four different cross-sectional shapes (rectangular, triangular, square, and spherical) along with different values of reflection coefficient (R). Calculated results have been compared with the available experimental data and it is observed that the results are in close agreement, which proves the validity of the proposed model. The proposed model shows the collective effect of size, scattering, and crosssectional shape factor (δ) on electrical resistivity in a very simple and straightforward manner and able to reduce the complexity of existing models up to great extent.

High-Energy Electron Scattering in Thick Samples Evaluated by Bright-Field Transmission Electron Microscopy, Energy-Filtering Transmission Electron Microscopy, and Electron Tomography.

Energy-filtering transmission electron microscopy (TEM) and bright-field TEM can be used to extract local sample thickness $t$ and to generate two-dimensional sample thickness maps. Electron tomography can be used to accurately verify the local $t$. The relations of log-ratio of zero-loss filtered energy-filtering TEM beam intensity ($I_{{\rm ZLP}}$) and unfiltered beam intensity ($I_{\rm u}$) versus sample thickness $t$ were measured for five values of collection angle in a microscope equipped with an energy filter. Furthermore, log-ratio of the incident (primary) beam intensity ($I_{\rm p}$) and the transmitted beam $I_{{\rm tr}}$ versus $t$ in bright-field TEM was measured utilizing a camera before the energy filter. The measurements were performed on a multilayer sample containing eight materials and thickness $t$ up to 800 nm. Local thickness $t$ was verified by electron tomography. The following results are reported:•The maximum thickness $t_{{\rm max}}$ yielding a linear relation of log-ratio, $\ln ( {I_{\rm u}}/{I_{{\rm ZLP}}})$ and $\ln ( {I_{\rm p}}/{I_{{\rm tr}}} )$, versus $t$.•Inelastic mean free path ($\lambda _{{\rm in}}$) for five values of collection angle.•Total mean free path ($\lambda _{{\rm total}}$) of electrons excluded by an angle-limiting aperture.•$\lambda _{{\rm in}}$ and $\lambda _{{\rm total}}$ are evaluated for the eight materials with atomic number from $\approx$10 to 79.The results can be utilized as a guide for upper limit of $t$ evaluation in energy-filtering TEM and bright-field TEM and for optimizing electron tomography experiments.

Combined first-principles-Monte Carlo analysis to evaluate the effect of surface hydrogen on the secondary electron yield of nickel

Secondary electron yield (SEY) modeling of Ni(110) surface has been carried out with and without the inclusion of wavevector-dependent harmonic corrections (which alter both the inelastic mean free path and stopping power) and is compared to available experimental data. The correction is shown to improve predictions of the inelastic electron mean free path in Ni and yield better agreement with experimental SEY data. It is found that the SEY is strongly dependent on the presence of adsorbates on surfaces. An increase of hydrogen on the surface, for example, is predicted to result in a significant enhancement in the secondary electron yield, with the positional placement of hydrogen layers on or near the Ni surface influencing the SEY. Using first-principles calculations, the permittivities work function and adsorption energy of various Ni systems have also been calculated, and have shown to compare favorably with available experimental data, and have been used in the present Monte Carlo calculations of electron transport.

Electron–neutral collisions effects on Langmuir probe in the lower E-region ionosphere

We present the first set of particle-in-cell simulations including Monte Carlo collisions between charged and neutral particles used to simulate a cylindrical Langmuir probe in the electron saturation regime with a collisional electron sheath. We use a setup focused on the E-region ionosphere; however, the results of these simulations are analyzed in a general sense using dimensionless values. We find that the electron currents get enhanced as the collision frequency for electrons increases and the values of λe/λD→1, where λe is the electron mean free path and λD is the electron Debye length. In addition, we apply the simulation results to a sounding rocket experiment and show how we can correct the currents for the Investigation of Cusp Irregularities-4 sounding rocket due to collisions while it flies through the E-region.

Photon, neutron absorption capabilities of Y2O3-Al2O3-P2O5 glasses

In this study, the shielding properties of (Y2O3)0.31 −z(Al2O3)z(P2O5)0.69, where , in glasses, have been examined to be applied in radiation protection applications. Therefore, we used the Monte Carlo simulation Geant4 code to simulate the mass attenuation coefficient for A, B, C, D, E, F glass samples prepared according to the formula (Y2O3)0.31 −z(Al2O3)z(P2O5)0.69. The obtained simulated mass attenuation (µm) results were compared with those obtained theoretically by the XCOM database within photon energies 0.03–2 MeV. The simulated µm results were found to be in good agreement with the XCOM received data. The linear attenuation coefficients(µ), the effective atomic number (Zeff), the effective electron density (Neff), half-value layer (Δ0.5), and mean free path (MFP) were calculated based on the simulated µm. The µm was decreased with the increase of photon energy, and with the increase of (mol%) fraction of Y2O3, and the decrease of (mol%) fraction of Al2O3. It is found that the F glass with 0.364(wt.%)concentration of Y2O3 has superior gamma-ray shielding effectiveness due to its higher values of mass attenuation coefficient, effective atomic number, and electron densities, and lower values of Δ0.5 and MFP. Also, in this work, the neutron shielding ability of the selected glasses was determined. The effective removal cross-section (cm− 1) was calculated by using the Monte Carlo simulation Geant4 code. Finally, it is found that the B Glass with 0.173 (wt.%)concentration of Al2O3has better neutron shielding properties than the other selected glass samples; due to it has the highest value of effective removal cross-section and has the lowest value of HVL at energy 1MeV.

Application of the Point-Like Contact Model: Resistance Oscillations of the Domain Wall in Magnetic Nanowires and Junctions Due to Mean Free Path Effects

This work is focused on determining the electrical resistance, which is induced by single domain wall (DW) in magnetic nanowire (NW) with a negligible defect. The provided model covers a wide range of NW diameters. The obtained result demonstrates a few-order rapid reduction of the DW resistance accompanied by its possible deviations versus the diameter growth ranging from 1.2[Formula: see text]nm to 15.2[Formula: see text]nm. The origin of these deviations, which are also identified as oscillations, is referred to the nonuniform electron scattering on the DW due to the intermixing electron scattering conditions: ballistic for one spin channel and quasi-ballistic for other one with opposite spin direction. It may happen when the DW width by value is approximately in between two lengths: a mean free path (MFP) with the spin down and spin up. The indirect evidence of this finding also comes from the fact that homogeneous NWs show the most valuable DW resistance oscillations by magnitude rather than segmented magnetic NWs. In addition to the approach, where DW width is constant, the other reasonable model is used when the DW can be constrained for some conditions. The same results are valid for magnetic junctions with DW. Finally, resistance simulation in the diffusive range, when a diameter of the NW (or point-like junction) is larger than any of spin-resolved MFP of electrons, successfully follows experimental data for the single and double DW resistances available in literature.

Edge and Bulk Transport in a Two-Dimensional Topological Insulator Based on a CdHgTe Quantum Well

The transport response of a CdHgTe quantum well with a thickness of 11.5 nm is investigated. The behavior of the local and nonlocal resistance in the temperature range from 0.1 to 20 K is examined. It is shown that the system under study is a two-dimensional topological insulator. In comparison with traditional two-dimensional topological insulators implemented in 8-nm-thick HgTe quantum wells, the investigated one is characterized by a significantly smaller energy gap and, at the same time, a higher carrier mobility. The data are analyzed using computer simulations taking into account the actual geometry of the sample, as well as scattering between edge and bulk carrier states. It is shown that the backscattering probability of topological electrons within the edge states is nearly independent of temperature. In contrast, the probability of scattering from the edge channels into the bulk depends exponentially on the temperature, and fitting this dependence with a standard activation formula is the most accurate way to determine the mobility gap in the system under study. Even at the highest temperature, the probability of scattering between the counter-propagating states of the same edge exceeds the probability of scattering into the bulk by an order of magnitude. Therefore, this mechanism is dominant and determines the mean free path of edge electrons.

Evaluation of carrier concentration dependence of phonon and electron mean free path in Si single crystals

In this study, we evaluated the carrier concentration dependence of phonon and electron mean free paths in single crystal Si. The phonon group velocity was derived using the nanoindentation method, and the thermal conductivity was derived using the laser spot periodic heating method. The phonon mean free path was derived by combining them. In addition, electron mean free paths were calculated, and the carrier concentration dependence on phonon and electron mean free paths were evaluated. As a result, both phonon and electron mean free paths were exhibited in the order of 10-8 -10-9 m, and were found to show a similar behavior depending on the carrier concentration.

Modeling and Analysis of Carbon-Nanotube Interconnections for Future Nanotechnology Interconnections between High Speed CMOS Integrated Circuits using FDTD Method

The size reduction of copper interconnects degrades their performances due to increased surface scattering, which significantly reduces the effective electron mean free path. Unlike Cu, CNTs support ballistic electron flow with a lower value of mean free, which highly induces researchers to change copper by carbon nanotubes. In this way, this paper presents an accurate method based on the finite difference method describing the behavior of carbon nanotube interconnects in the time domain. The proposed algorithm is implemented in the MATLAB tool. The crosstalk between the interconnects and the induced delays are studied as a function of their length and the technology node (45nm, 32nm, 22nm and 16nm). The values obtained by the suggested method are compared with those of the PSPICE simulation tool. A good agreement between these results is observed, which demonstrates that CNT interconnects are more efficient than copper ones in terms of crosstalk induced delays.

Влияние поглощения матрицы на ближнеполевые и спектральные характеристики рассеяния плазмонных сферических наночастиц

Regularities of a size dependence of extinction and absorption efficiency factors as well as of near-zone and far-zone scattering efficiency factors are studied in the spectral range of the surface plasmon resonance of absorption (SPRA) for silver spherical nanoparticles placed into absorbing matrixes with complex refraction index nm+ikappam. Numerical simulation was made with the use of the Mie theory for absorbing matrixes. Approximation of the electron mean free path limitation was used in order to take into account the intrinsic size effects into a metal nanoparticle which consist in a size dependence of its dielectric characteristics. We shown that a growth of matrix absorbency leads to a strong suppression of resonances of extinction (Qext) and near-zone scattering (QNF) efficiency factors without any essential changing of their spectral position. We established that for any fixed wavelength in the spectral range of the SPRA the values of Qext and QNF depend on metal nanoparticle sizes nonmonotonically at kappam = 0–0.1. The RNP optimal value, which corresponds to the maximal values of efficiency factors for the given material of plasmonic nanoparticles, increases at the matrix absorbency growth. The intrinsic size effects contribution to the efficiency factors resonance suppression decreases at the enhancement of nanoparticle sizes and/or matrix absorption. As an example of a real absorbing matrix with a dispersion of relraction index we considered the nickel phtalocyanine matrix (NiPc), which is interesting from the point of view of photovoltaic applications.