Intervalley Phonon Research Articles

Effect of different loss mechanisms in SiGeSn based mid-infrared laser

We have analyzed the mid-infrared SiGeSn based Barrier-Well-Barrier Heterostracture and calculated the transparency carrier density and corresponding current density for the structure. The effects of different loss mechanisms like free carrier absorption, spontaneous recombination and Auger recombination processes on the transparency current density have been examined. It is shown that, the transparency current density increases significantly with the injected carrier density. Different scattering processes like acoustic phonon scattering and intervalley optical phonon scattering are taken into consideration for this analysis of free carrier absorption mechanisms.

Electron scattering in the Δ1 model of the conduction band of germanium single crystals

Electron scattering in the possible Δ1 models of the conduction band in germanium crystals formed by hydrostatic or uniaxial pressure is investigated. On the basis of the theory of anisotropic scattering, the temperature dependences of the anisotropy parameter of the relaxation times and electron mobility for these models under conditions of scattering at impurity ions, as well as at acoustic and intervalley phonons are obtained. Analysis of the temperature dependences indicates that, in the temperature range of 77–300 K, intervalley scattering becomes substantial. Only for the Δ1 model formed by uniaxial pressure along the crystallographic direction [100], the electron scattering at intervalley phonons, which correspond to the g transitions, is minor with respect to scattering at acoustic phonons (the intravalley scattering) and impurity ions.

The effects of drain scatterings on the electron transport properties of strained-Si diodes with ballistic and non-ballistic channels**Project supported by the National Natural Science Foundation of China (No. 61366001) and the National Basic Research Program of China (No. 2011CB706601).

The effects of multiple scattering on the electron transport properties in drain regions are numerically investigated for the cases of strained-Si diodes with or without scattering in the channel. The performance of non-ballistic (with scattering) channel Si-diodes is compared with that of ballistic (without scattering) channel Si-diodes, using the strain and scattering model. Our results show that the values of the electron velocity and the current in the strain model are higher than the respective values in the unstrained model, and the values of the velocity and the current in the ballistic channel model are higher than the respective values in the non-ballistic channel model. In the strain and scattering models, the effect of each carrier scattering mechanism on the performance of the Si-diodes is analyzed in the drain region. For the ballistic channel model, our results show that inter-valley optical phonon scattering improves device performance, whereas intra-valley acoustic phonon scattering degrades device performance. For the strain model, our results imply that the larger energy splitting of the strained Si could suppress the inter-valley phonon scattering rate. In conclusion, for the drain region, investigation of the strained-Si and scattering mechanisms are necessary, in order to improve the performance of nanoscale ballistic regime devices.

Computational Study of Effects of Surface Roughness and Impurity Scattering in Si Double-Gate Junctionless Transistors

Electron transport in Si double-gate junctionless transistors (JLTs) is simulated on the basis of the multisubband Monte Carlo method, considering acoustic phonons, intervalley phonons, ionized impurities (IIs), and surface roughness (SR) scattering. It is demonstrated that JLTs can actually minimize the mobility degradation caused by SR scattering and improve the current drivability. On the other hand, II scattering is confirmed to degrade the current drivability compared with conventional MOSFETs with an intrinsic channel. However, the performance degradation in JLTs due to II scattering is shown to be insignificant, owing to the screening effect of free carriers and the forward scattering properties of high-speed carriers. Furthermore, by extracting a backscattering coefficient, the screening effect and the forward scattering properties are shown to more mitigate II scattering as the gate voltage increases.

Comparison of Different Scattering Mechanisms in the Ge (111), (110), and (100) Inversion Layers of nMOSFETs With Si nMOSFETs Under High Normal Electric Fields

In this paper, we report, for the first time, that surface roughness scattering is not necessarily the dominant scattering mechanism in the high-normal-field region of Ge nMOSFETs. This statement is quite different from the well-recognized situation in Si MOSFETs. In Ge(100), phonon scattering is dominant in the high-field region. Thus, it is difficult to increase the high-normal-field mobility in Ge(100) nMOSFETs by controlling the interface roughness. In contrast, because Ge(111) and Ge(110) are free of intervalley phonon scattering, the high-field mobility in Ge(111) and Ge(110) nMOSFETs could be enhanced by the Ge interface engineering. Furthermore, different from that in Si nMOSFETs, mobility limited by surface roughness scattering in Ge nMOSFETs shows a strong temperature dependence due to the valley occupancy change of electrons. The results in this paper should facilitate efforts to increase the high-normal-field mobility in Ge nMOSFETs.

Influence of electron - phonon interaction on piezoresistance of single crystals n-Ge

Piezoresistance of uniaxially deformed along the crystallographic direction [100] single crystals n-Ge for different fixed temperatures has been investigated. Presence of the significant piezoresistance for given conditions of the experiment is explained by increase in the effective mass and decrease in the relaxation time for electrons at the expense of the inversion of (L1-∆1) type of an absolute minimum in n-Ge. Temperature dependencies of resistivity for undeformed (L1model of conduction band) and deformed under uniaxial pressureP=3 GPа (∆1model of conduction band) single crystals n-Ge are obtained. Resistivity for undeformed single crystals n-Ge is changed according to the law ρ~T1.66. Resistivity for uniaxially deformed single crystals n-Ge is changed as ρ~T1.53. The given dependencies show that for L1 model of the conduction band in contrast to ∆1model one must equally with intravalley scattering of electrons on acoustic phonons take into account scattering of electrons on optical and intervalley phonons. Therefore reduction of the magnitude piezoresistance n-Ge with the increasing temperature is associated with “turning-off” at the expense of the inversion of (L1-Δ1) type of absolute minimum under uniaxial pressure P>2.7 GPа mechanism for scattering of electrons on intervalley and optical phonons. Comparison of results of theoretical calculations with relevant experimental data shows that peculiarities of piezoresistance n-Ge under uniaxial pressures  1.6<P<2.7 GPа for (L1-Δ1) model of conduction band n-Ge can be described only taking into account nonequivalent intervalley scattering of electrons between the minima L1 and Δ1.

Valley-antisymmetric potential in graphene under dynamical deformation

When graphene is deformed in a dynamical manner, a time-dependent potential is induced for the electrons. The potential is antisymmetric with respect to valleys and has far-reaching effects. In this paper we find some straightforward applications to Raman spectroscopy: a valley-antisymmetric potential broadens the Raman $D$ band but does not affect the $2D$ band, which is already observed by recent experiments. The space derivative of the valley-antisymmetric potential gives a force field that accelerates intervalley phonons, while it corresponds to the longitudinal component of the previously discussed pseudoelectric field acting on the electrons. Effects of a pseudoelectric field on the electron is quite difficult to observe due to the valley-antisymmetric coupling constant, on the other hand, such obstacle is absent for intervalley phonons with ${A}_{1g}$ symmetry that constitute the $D$ and $2D$ bands.

Optimization of uniaxial stress for high electron mobility on biaxially-strained n-MOSFETs

The uniaxial stress effect for high electron mobility on biaxially-strained n-MOSFET is investigated by using a one-dimensional self-consistent Schrödinger–Poisson solver. The electron mobility model includes Coulomb, intravalley phonon, intervalley phonon, and surface roughness scattering. We have found that the uniaxial stress effect on biaxially-strained n-MOSFET is significantly different from the uniaxial stress effect on unstrained Si n-MOSFET. It is well known that longitudinal and transverse tensile uniaxial stresses are advantageous for strain-induced high electron mobility. However, we found that the uniaxial strain condition for electron mobility enhancement is changed when it is applied to the biaxially-strained n-MOSFET. To optimize the combined effect of uniaxial and biaxial strain, the longitudinal tensile and transverse compressive uniaxial stresses are advantageous and vertical stress is not helpful for biaxially-strained n-MOSFET.

A hydrodynamical model for covalent semiconductors with a generalized energy dispersion relation

We present the first macroscopical model for charge transport in compound semiconductors to make use of analytic ellipsoidal approximations for the energy dispersion relationships in the neighbours of the lowest minima of the conduction bands. The model considers the main scattering mechanisms charges undergo in polar semiconductors, that is the acoustic, polar optical, intervalley non-polar optical phonon interactions and the ionized impurity scattering. Simulations are shown for the cases of bulk 4H and 6H-SiC.

Monte Carlo simulation of spin relaxation of conduction electrons in silicon

Recently, electrical injection of spin polarization in n-type and p-type silicon up to room-temperature has been experimentally carried out. Despite of these promising experimental results, a comprehensive theoretical framework concerning the influence of transport conditions on the spin depolarization process in silicon structures, in a wide range of values of temperature, doping concentration and amplitude of external fields, is still in a developing stage. In this contribution we use a semiclassical multiparticle Monte Carlo approach to simulate the electron transport and spin dynamics in lightly doped n-type Si crystals and numerically calculate the spin lifetimes of drifting electrons. Spin flipping is taken into account through the Elliot-Yafet mechanism, which is dominant in group IV materials. We discuss the influence of different intravalley and intervalley phonon interactions in the spin relaxation process during the spin transport. Our findings are in good agreement with those obtained by using different theoretical approaches. Moreover, our Monte Carlo predictions, in ranges of temperature and field amplitude yet unexplored, can guide future experimental studies towards a more effective design of room-temperature silicon based spintronic-devices.

Monte Carlo simulation of spin relaxation of conduction electrons in silicon

Recently, electrical injection of spin polarization in n-type and p-type silicon up to room-temperature has been experimentally carried out. Despite of these promising experimental results, a comprehensive theoretical framework concerning the influence of transport conditions on the spin depolarization process in silicon structures, in a wide range of values of temperature, doping concentration and amplitude of external fields, is still in a developing stage. In this contribution we use a semiclassical multiparticle Monte Carlo approach to simulate the electron transport and spin dynamics in lightly doped n-type Si crystals and numerically calculate the spin lifetimes of drifting electrons. Spin flipping is taken into account through the Elliot-Yafet mechanism, which is dominant in group IV materials. We discuss the influence of different intravalley and intervalley phonon interactions in the spin relaxation process during the spin transport. Our findings are in good agreement with those obtained by using different theoretical approaches. Moreover, our Monte Carlo predictions, in ranges of temperature and field amplitude yet unexplored, can guide future experimental studies towards a more effective design of room-temperature silicon based spintronic-devices.

Inter valley phonon scattering mechanism in strained Si/(101)Si1−xGex

Inter valley scattering has a great impact on carrier mobility of strained Si materials, so based on Fermi's golden rule and the theory of Boltzmann collision term approximation, inter valley phonon scattering mechanism of electrons in nano scale strained Si (101) materials is established under the influence of both energy and stress. It shows that inter valley phonon f2, f3, g3 scattering rates decrease markedly in nano scale strained Si (101) materials with increasing stress. The quantized models can provide valuable references to the understanding of strained Si materials and the research on electron carrier mobility.

Time-resolved electroluminescence of bulk Ge at room temperature

Time-resolved spectroscopy of bulk Ge at room temperature allows to differentiate direct-gap electroluminescence (EL) from indirect-gap EL. The results indicate that there exists strong coupling between the eight-fold indirect L-valleys and the zone-center Γ-valley due to intervalley phonon scattering at room temperature in the absence of built-in strain. Pulse modulation of EL at 10-MHz is consistent with the relevance of the direct-gap transition.

Decay and frequency shift of both intervalley and intravalley phonons in graphene: Dirac-cone migration

By considering analytical expressions for the self-energies of intervalley and intravalley phonons in graphene, we describe the behavior of D, 2D, and D$'$ Raman bands with changes in doping ($\mu$) and light excitation energy ($E_L$). Comparing the self-energy with the observed $\mu$ dependence of the 2D bandwidth, we estimate the wavevector $q$ of the constituent intervalley phonon at $\hbar vq\simeq E_L/1.6$ ($v$ is electron's Fermi velocity) and conclude that the self-energy makes a major contribution (60%) to the dispersive behavior of the D and 2D bands. The estimation of $q$ is based on an image of shifted Dirac cones in which the resonance decay of a phonon satisfying $q > \omega/v$ ($\omega$ is the phonon frequency) into an electron-hole pair is suppressed when $\mu < (vq-\omega)/2$. We highlight the fact that the decay of an intervalley (and intravalley longitudinal optical) phonon with $q=\omega/v$ is strongly suppressed by electron-phonon coupling at an arbitrary $\mu$. This feature is in contrast to the divergent behavior of an intravalley transverse optical phonon, which bears a close similarity to the polarization function relevant to plasmons.

Electron spin relaxation in rippled graphene with low mobilities

We investigate spin relaxation in rippled graphene where curvature induces a Zeeman-like spin-orbit coupling with opposite effective magnetic fields along the graphene plane in K and K′ valleys. The joint effect of this Zeeman-like spin-orbit coupling and the intervalley electron-optical phonon scattering opens a spin relaxation channel, which manifests itself in low-mobility samples with the electron mean free path being smaller than the ripple size. Due to this spin relaxation channel, with the increase of temperature, the relaxation time for spins perpendicular to the effective magnetic field first decreases and then increases, with a minimum of several hundred picoseconds around room temperature. However, the spin relaxation along the effective magnetic field is determined by the curvature-induced Rashba-type spin-orbit coupling, leading to a temperature-insensitive spin relaxation time of the order of microseconds. Therefore, the in-plane spin relaxation in low-mobility rippled graphene is anisotropic. Nevertheless, in the presence of a small perpendicular magnetic field, as usually applied in the Hanle spin precession measurement, the anisotropy of spin relaxation is strongly suppressed. The spin relaxation channel revealed in this work contributes to spin relaxation around room temperature in rippled graphene with low mobility.

Study of THz-wave-induced photoluminescence quenching in GaAs and CdTe

A novel model of ultrafast interaction between THz pulse and carriers is built to study the THz-wave-induced quenching of femtosecond-laser-excited photoluminescence in CdTe and GaAs. Photoluminescence quenching is due to the nonequilibrium intervalley phonons induced by the THz field and subsequent decrease of the recombination efficiency of the electron–hole pairs. And the PLQ versus laser intensity experimental result agrees with the analysis.

Two-electron dephasing in single Si and GaAs quantum dots

We study the dephasing of two-electron states in a single quantum dot in both GaAs and Si. We investigate dephasing induced by electron-phonon coupling and by charge noise analytically for pure orbital excitations in GaAs and Si, as well as for pure valley excitations in Si. In GaAs, polar optical phonons give rise to the most important contribution, leading to a typical dephasing rate of $\ensuremath{\sim}$5.9 GHz. For Si, intervalley optical phonons lead to a typical dephasing rate of $\ensuremath{\sim}$140 kHz for orbital excitations and $\ensuremath{\sim}$1.1 MHz for valley excitations. For harmonic, disorder-free quantum dots, charge noise is highly suppressed for both orbital and valley excitations, since neither has an appreciable dipole moment to couple to electric field variations from charge fluctuators. However, both anharmonicity and disorder break the symmetry of the system, which can lead to increased dipole moments and therefore faster dephasing rates.

Electron transport in wurtzite InN

Using ensemble Monte Carlo simulation technique, we have calculated the transport properties of InN such as the drift velocity, the drift mobility, the average electron, energy relaxation times and momentum relaxation times at high electric field. The scattering mechanisms included scattering mechanisms are polar optical phonon, ionized impurity, acoustic phonon and intervalley phonon. It is found that the maximum peak velocity only occurs when the electric field is increased to a value above a certain critical field. This critical field is strongly dependent on InN parameters. The steady-state transport parameters are in fair agreement with other recent calculations.

A Hydrodynamic Model for Covalent Semiconductors with Applications to GaN and SiC

In this paper we present a hydrodynamical model which, in principle, is able to describe charge transport in a generic compound semiconductor. The model makes use of an analytic approximation for the conduction bands. Energy dispersion relationships in the neighbors (valleys) of the lowest minima are, in fact, taken to be spherical, nonparabolic. The model considers the main scattering mechanisms in polar semiconductors, that is the acoustic, polar optical, intervalley non-polar optical phonon interactions and the ionized impurity scattering. Simulations are shown for the cases of bulk GaN and SiC.

Model of Electron Scattering of Strained Si /Si1-xGex (100)

Based on Fermi's golden rule and the theory of Boltzmann collision term approximation, taking into account all the scattering mechanisms contributed by ionized impurity, acoustic phonon and intervalley phonon, the model of total scattering rate of strained Si/(100) Si1-xGex is established. Simulating of the scattering models with Matlab software, it was found that the total scattering rate of electron in strained Si/(100) Si1-xGex decreases obviously with the increasing stress when Ge fraction x is less than 0.2 and the values continue to show a constant tendency, and that the total electron scattering rate of strained Si/(100) Si1-xGex decreases about 57% at most comparison with one of unstrained Si. The result can provide valuable references to the research of electron mobility of strained Si materials and the design of NMOS devices.