Remote Charges Research Articles

Does the capacitor analogy model in fracture mechanics of elastic dielectrics constitute an appropriate approximation?

The analytical solution of an elliptic dielectric cavity in an infinite dielectric plate is taken as basis to investigate a Griffith crack problem which is obtained by letting the semi-minor axis tend towards zero. In the course of this, an erroneous conformal mapping, commonly employed in literature and correctly reproducing the electric field only in a part of the physical space, is rectified. Interpreting the elliptic interface as faces of a mechanically opened crack which is exposed to an oblique remote electric field, surface charges and electrostatic tractions are calculated. In contrast to the established capacitor analogy model, approximately yielding electric charge densities and Coulombic tractions from displacements and electric potentials in the undeformed crack configuration, the work at hand provides exact solutions accounting for different implications of the crack curvature and for the inclination of the electric field. Crack weight functions are finally used to calculate stress and electric displacement intensity factors. As turns out, the surface charges of the capacitor analogy represent an excellent substitute for the exact electric boundary conditions within a relevant range of parameters, whereas inaccurate Coulombic tractions in the vicinity of crack tips may lead to a significantly overestimated mode I stress intensity factor.

Spin accumulation in photo-induced potential dimples generated in semiconductors

Conventional ways of confining charges in semiconductors employ advanced lithographic and crystal-growth techniques. The construction of micro/nano-scale structures is also essential for manipulating spins. However, existing techniques are not always flexible enough to control spins in appropriate positions and timings. Here we report an alternative mechanism, which enables us to design temporal and reconfigurable low-dimensional potentials. The formation of photo-induced potential dimples is deduced from time and spatially-resolved Kerr rotation measurements performed on a GaAs quantum well. Two-dimensional images of spin distributions reveal that the photo-injected electron spins in a small area illuminated by a pump light survive for a time that is two orders of magnitude longer than typical recombination lifetimes. The Kerr rotation dependence on the pump laser conditions implies that the temporally generated dimple-shaped potential profile induced by remote charges effectively confines the electrons and enhances the spin lifetime determined by fluctuating spin-orbit effective magnetic fields.

The Role of Piezoeffect in Anomalous Dependence of the Conductivity of AlGaAs/GaAs Heterostructure with a Two-Dimensional Electron Gas on the Distance between Contacts

The electric conductivity of the AlGaAs/GaAs heterostructure with two-dimensional electron gas has been experimentally studied in a temperature range of 10–300 K. At low temperatures, electric resistance of the structure exhibited growth when the distance between contacts decreased from 100 to 20 μm. For explaining this anomalous behavior, numerical simulation of the influence of piezoeffect in the semiconductor on the channel conductivity was carried out, which showed that it is necessary to take into account the crystallographic orientation of the channel and the influence of remote piezoelectric charges on its potential.

Experimental evidence for long-range stabilizing and destabilizing interactions between charge and radical sites in distonic ions

Polarizable radical sites in distonic radical anions are stabilized by ostensibly remote negative charges. Computational evidence suggests bond dissociation energies of closed-shell precursors are significantly lowered by through-space interactions with a proximate negative charge, however direct experimental confirmation has proved challenging. Herein, we exploit two complementary tandem mass spectrometry strategies to probe the influence of a remote charge on the stability of nitroxyl radicals, and vice versa. Dissociation of negatively charge-tagged alkoxyamines reveals that the energetic onset of radical formation is dependent on the proximity and basicity of the charged group, thus providing direct evidence for a charge-induced stabilization of the nitroxyl radical. Complementary kinetic method experiments on a series of proton-bound dimers demonstrate that the presence of a nitroxyl radical decreases the proton affinity for a selection of proximate ionic groups. These data show excellent agreement with quantum-chemical calculations and provide a general framework to explore the magnitude and direction of charge-radical interactions through systematic exploration of the identity, polarity and the proximity of the ion to the radical site. These findings expand our fundamental understanding of radical ion energetics that underpin the application of distonic ions as models for neutral radical reactivity, and open new avenues for exploiting these interactions as chemical switches.

Electrostatic Effects on the Stability of Peptide Radicals.

We explored the influence of external electric fields (EEFs) on the stability of a glycine dipeptide model radical using high-level quantum chemical methods. Remotely located ions (Cl-/Na+) are used to implement EEF effects. The effects of these ions are reproduced using background point charges and oriented EEFs. Remote charges as far as 900 pm from the Cα radical center can be significantly stabilizing or destabilizing as a function of their relative orientation. The magnitude of these effects is also strongly dependent on the distance between the radical center and the charge location. After examining the strengths and weaknesses of some frequently used quantum mechanics methods in describing these effects properly, a comparison is made on the stability of dipeptide radicals bearing protonable or deprotonable side chains. In this group, the stability of the respective Cα radicals mainly depends on the preferred orientation of the charge-carrying side chain.

Electric-field tuning of the valley splitting in silicon corner dots

We perform an excited state spectroscopy analysis of a silicon corner dot in a nanowire field-effect transistor to assess the electric field tunability of the valley splitting. First, we demonstrate a back-gate-controlled transition between a single quantum dot and a double quantum dot in parallel which allows tuning the device into corner dot formation. We find a linear dependence of the valley splitting on back-gate voltage, from 880 μeV to 610 μeV with a slope of −45 ± 3 μeV/V (or equivalently a slope of −48 ± 3 μeV/(MV/m) with respect to the effective field). The experimental results are backed up by tight-binding simulations that include the effect of surface roughness, remote charges in the gate stack, and discrete dopants in the channel. Our results demonstrate a way to electrically tune the valley splitting in silicon-on-insulator-based quantum dots, a requirement to achieve all-electrical manipulation of silicon spin qubits.

Carrier scattering in high-κ/metal gate stacks

A significant degradation of the mobility has been repeatedly observed at low inversion density in high-κ/metal gate metal-oxide-semiconductor field-effect transistors. However, the scattering mechanisms responsible for this degradation are still debated. It is often assumed that the mobility is limited by remote charges (RCS) at the interface between SiO2 and HfO2. However, the amount of charges needed to reproduce the experimental mobilities is usually very high (a few 1013 cm−2), and does not seem to be consistent with the measured threshold voltages. Scattering by localized dipoles hardly solves these discrepancies. Here, we investigate the contribution from three alternative mechanisms in a non-equilibrium Green's functions framework: (i) scattering by band offset fluctuations at the SiO2/HfO2 interface, (ii) scattering by dielectric constant fluctuations in SiO2 and HfO2, and (iii) scattering by workfunction fluctuations in a granular metal gate. None of these mechanisms significantly shifts the threshold voltage. We show that mechanisms (i) and (iii) efficiently scatter the carriers at low inversion densities. This reduces the amount of RCS charges needed to reproduce the experimental data. RCS and these mechanisms show different dependences on the thickness of the HfO2 layer, which might help to identify the dominant contributions.

Screening of remote charge scattering sites from the oxide/silicon interface of strained Si two-dimensional electron gases by an intermediate tunable shielding electron layer

We report the strong screening of the remote charge scattering sites from the oxide/semiconductor interface of buried enhancement-mode undoped Si two-dimensional electron gases (2DEGs), by introducing a tunable shielding electron layer between the 2DEG and the scattering sites. When a high density of electrons in the buried silicon quantum well exists, the tunneling of electrons from the buried layer to the surface quantum well can lead to the formation of a nearly immobile surface electron layer. The screening of the remote charges at the interface by this newly formed surface electron layer results in an increase in the mobility of the buried 2DEG. Furthermore, a significant decrease in the minimum mobile electron density of the 2DEG occurs as well. Together, these effects can reduce the increased detrimental effect of interface charges as the setback distance for the 2DEG to the surface is reduced for improved lateral confinement by top gates.

Extremely high electron mobility in isotopically-enriched 28Si two-dimensional electron gases grown by chemical vapor deposition

Both depletion-mode and enhancement-mode two-dimensional electron gases (2DEGs) in isotopically enriched 28Si with extremely high mobility (522 000 cm2/V s) are presented. The samples were grown by chemical vapor deposition using enriched silane. The fraction of the spin-carrying isotope 29Si was reduced to the level of 800 ppm by 28Si enrichment, with the electron spin dephasing time expected to be as long as 2 μs. Remote impurity charges from ionized dopants and the Si/Al2O3 interface were suggested to be the dominant source for electron scattering in the enriched 28Si 2DEGs.

Very Low Electron Density in Undoped Enhancement-Mode Si/SiGe Two-Dimensional Electron Gases with Thin SiGe Cap Layers

We report the lowest electron density (4.9×1010 cm-2 and 1.1×1011 cm-2) and high mobility (~400,000 cm2/Vs and ~200,000 cm2/Vs) of undoped enhancement-mode Si/SiGe two-dimensional electron gases comparing to samples previously reported with similar thin SiGe cap thickness (55nm and 27nm). The dominant scattering mechanism over a wide range of two-dimensional electron density in both samples is the scattering from remote charges at oxide/silicon interface. In addition, a clear metal-insulator transition is observed in the sample with a 27-nm SiGe cap.

Remote charge scattering: a full Coulomb interaction approach and its impact on silicon nMOS FinFETs with HfO2 gate dielectric

Remote charge scattering (RCS) has become a serious obstacle inhabiting the performance of ultra- thin gate oxide MOSFETs. In this paper, we evaluate the impact of RCS by treating the real-space full Coulomb interaction between remote charges and inversion carriers. A new approach that can be simply incorporated in ensemble Monte Carlo (EMC) based simulations without any variation of the standard EMC simulator is developed. The charge-carrier (c-c) interaction model is based on a particle-mesh (PM) calculation method which resolves both the long-range and short-range Coulomb interactions by solving Poisson's equation on a re- fined mesh. The validity of our approach is verified by three-dimensional (3-D) resistor simulations, from which the obtained doping dependence of the low-field mobility agrees well with experimental results. The proposed approach is then used to study the impact of RCS on the drive current and carrier transport properties in the channel of a 20 nm silicon (Si) nMOS FinFET with HfO2 gate stack. We find that the influence of RCS is strongly localized in the vicinity of the remote charges and exhibits a granular nature, indicating the necessity to consider the full Coulomb interaction in RCS.

Remote carrier trapping in FinFETs with ONO buried layer: Temperature effects

The charge trapping mechanisms are studied at room and high temperature in advanced SOI FinFETs fabricated on SiO2–Si3N4–SiO2 (ONO) multi-layer buried insulator (BOX). By applying appropriate back-gate and/or drain bias, the buried nitride layer can trap charges. The coupling mechanism between front and back interfaces enables the front-channel drain current to reflect the presence or absence of trapped/injected charges in the BOX. In this work, the charge trapping/detrapping is induced by a constant drain or back-gate bias or during the scanning of the back-gate voltage. According to the polarity of the trapped charges and their location along the channel, various current levels are observed leading to a memory effect. In order to clarify the charge trapping and coupling mechanisms, the temperature of operation was used as additional experimental parameter. The amount of trapped charges depends not only on the bias conditions but also on temperature and fin geometry. We discuss the 3D coupling effects between the channel and the remote trapped charges.

Semiclassical Monte Carlo Analysis of Graphene FETs

We present a 3-D semiclassical Monte Carlo simulator for modeling transport in graphene metal-oxide-semiconductor field-effect transistors (MOSFETs). We have calibrated our material simulations by matching simulation results to experimental bulk velocity-field curves. We have included a full range of phonon-scattering mechanisms, intrinsic and oxide/extrinsic remote impurity charges, and carrier-carrier interactions from classical electrostatics. We have modeled Klein tunneling and, in device simulations, treated charged impurities as localized Coulomb centers within the self-consistent potential function rather than through a scattering rate approximation. The necessity of these latter two treatments is demonstrated through simulations of 80-nm channel-length graphene MOSFETs.

Thermionic tunneling through Coulomb barriers in charged self-assembled quantum dots

The temperature and electric-field dependence of the electron emission from charged semiconductor quantum dots is studied with transient capacitance spectroscopy. The self-assembled InAs quantum dots are embedded within Schottky diodes grown with molecular-beam epitaxy on GaAs(001). In accordance with the different activation energies the emission from the $s$ and the $p$ shell of the quantum dots takes place with strongly different rates. In addition, the emission rates depend on the charge state of the shells. The behavior can quantitatively be understood with a thermionic-tunneling model in which the tunnel barrier is assumed to consist of a Coulomb barrier arising from the charge within the dot and a triangular contribution from remote charges.

Sensitivity of Carbon Nanotube Transistors to a Charged Dielectric Coating

This paper investigates the electronic properties of single-walled carbon nanotube field-effect transistors (SWCNT-FETs) in which the SWCNT element is coated with a charged dielectric. The presence of remote charge on the surface of the dielectric is considered to effect carrier transport in the nanotube as a result of both carrier-scattering and gate screening. Nanotube device characteristics are simulated using the multi- subband Boltzmann transport method incorporating scattering from both phonons and remote charges. This allows assessment of the sensitivity of a nanotube FET to the presence of a charged dielectric coating during room temperature operation. Results show remote charge scattering affects the diameter (d) dependence of the peak conductance and peak field-effect mobility of carbon nanotube devices. Under phonon-limited transport conditions, these peak values increase as ~ d and ~ d2, respectively. When remote charge scattering is significant, peak values cease to vary with diameter once a critical diameter reached. Charge scattering is found to particularly degrade device current at gate voltages that allow carriers scattering into or out of a subband minimum. Furthermore, simulations show that intersubband scattering resulting from asymmetry in the circumferential remote charge density becomes increasingly important as the nanotube length decreases. The authors propose that remote charge scattering effects may be applicable in sensing devices allowing for the identification of the charge on a functionalized CNT coating.

Process optimization for high electron mobility in nMOSFETs with aggressively scaled HfO/sub 2//metal stacks

The performance of aggressively scaled (1.4nm<T/sub inv/<2.1nm) self-aligned HfO/sub 2/-based nMOSFETs with various metal gate electrodes (W, TaN, TiN, and TaSiN) is optimized. It is shown that high mobility values, competitive with oxynitride controls (SiON/poly-Si, T/sub inv//spl sim/1.8-2.1nm), can be achieved. Detailed studies of the role of interface states, remote charges in the HfO/sub 2/ layer, interfacial layer regrowth, and nitrogen-induced charge lead to the conclusion that high-temperature-induced structural modifications near the SiO/sub 2//HfO/sub 2/ interface substantially improve the electron mobility.

Electrostatic energy calculation for the interpretation of scanning probe microscopy experiments

We discuss the correct expression for the classical electrostatic energy used while analysing scanning probe microscopy (SPM) experiments if either a conducting tip or a substrate or both are used in the experiment. For this purpose a general system consisting of an arbitrary arrangement of finite metallic conductors at fixed potentials (maintained by external sources) and a distribution of point charges in free space are considered using classical electrostatics. We stress the crucial importance of incorporating into the energy the contribution coming from the external sources (the `battery'). Using the Green function of the Laplace equation, we show in a very general case that the potential energy of point charges which are far away from metals is equally shared by their direct interaction and the polarization interaction due to charge induced in metals by the remote charges (the image interaction). When the charges are located close to the metals, there is an additional negative term in the energy entirely due to image interaction. The exact Hamiltonian of a quantum system interacting classically with polarized metal conductors is derived and its application in the Hartree-Fock and the density functional theories is briefly discussed. As an illustration of the theory, we consider an interaction of several point charges with a metal plane and a spherical tip, based on the set-up of a real SPM experiment. We show the significance of the image interaction for the force imposed on the tip.

Formation of a two-dimensional electron gas in an inverted undoped heterostructure with a shallow channel depth

We investigated the dependence of transport characteristics of a two-dimensional electron gas (2DEG) on channel depth in an inverted undoped GaAs/AlGaAs heterostructure. We succeeded in forming a high-mobility 2DEG in a sample with 70 nm channel depth. We controlled the carrier density by varying the back-gate bias over a wide range. The highest mobility reached 2.3×106 cm2/Vs at 3.4×1011 cm−2. The relation between mobility and carrier density is determined: the mobility decreases in a low-carrier-density region as the channel depth decreases. This result suggests that the scattering due to the remote surface charges plays a more significant role.

Mobility and quantum lifetime in a GaAs/AlGaAs heterostructure: tuning of the remote-charge correlations

We demonstrate that different spatial correlations among remote impurity charges in the GaAs/AlGaAs quantum well lead to different values of (i) the transport-relaxation time, (ii) the single-particle relaxation time, and that (iii) the different width of the integer quantum Hall plateauxs could be achieved. These findings supply qualitatively new information about the important contribution of the non-random arrangement of remote charges to the transport properties of semiconductor heterostructures.

Two-dimensional electron gas mobility anomalies (and enhancement) in pseudomorphic AlGaAs/InGaAs/GaAs heterostructures

Variation of two-dimensional electron gas mobility with carrier concentration ns has been examined for different modulation-doped heterostructures of AlGaAs/InGaAs/GaAs. It is demonstrated that, depending on the method, electrons are distributed among remote impurity sites and different values of mobility can be achieved for the same ns and in the same heterostructure. The origin of this finding is associated with the spatial correlations of remote impurity charges which form dipolelike objects consisting positively charged Si donors and negatively charged DX− states of the same donor. The effect of correlations causes enhancement of mobility of the order of tens of percent and can explain the appearance of the maximum on a mobility versus ns dependence.