Deformation Potential Acoustic Phonons Research Articles

Electrical conductivity and Hall effect in n-type CdS

ABSTRACT The experimental data of the temperature dependence of the electrical conductivity σ(T) and the Hall coefficient RH (T) on single crystal samples of n-type CdS reported in literature are reviewed and have been critically analysed including hopping conduction in an impurity band. It is shown that the experimental data of σ(T) and RH (T) reported in almost previous studies can be simultaneously fitted using a common set of the values of material parameters for calculating free-electron conduction, i.e. the conduction and the density-of-state effective mass of electrons, the acoustic-phonon deformation potential, the Fröhlich coupling constant and the piezoelectric coupling coefficient, whereas wide ranges of values for these material parameters had been adopted in different studies. The correct assignment of impurity conduction mechanisms in n-type CdS single crystals is presented. In addition, reasonable explanations are given for unresolved enigmas regarding impurity pairing, unexpected large Hall mobility at low temperatures, and the effect of electric field on hopping Hall mobility.

Tunneling transport in open nitride resonant tunneling structures taking into account the acoustic phonons: An variational approach

Quantum-mechanical theory of tunneling transport through open nanostructures based on nitride semiconductors, taking into account the contribution of acoustic phonons, is proposed. A variational method taking into account the component of Hamiltonian due to the deformation potential of acoustic phonons at different temperatures is developed. On the basis of the developed theory the transmission coefficient of a two-barrier AlN/GaN resonant tunneling nanostructure, its electronic conductivity and tunneling current are calculated taking into account the impact of acoustic phonons. It was found that with the increase of temperature values the frequencies of quantum transitions between electronic quasistationary states increase, an increase in temperature from cryogenic to room values results in a shift of the tunneling current peak and a decrease in the maximum value of dynamic conductivity in almost seven times.

Noise performance of avalanche transit-time devices in the presence of acoustic phonons

Through this paper, the effects of acoustic phonons on the noise performance of avalanche transit-time devices have been investigated and reported. For this study, a double-drift-region silicon-based impact avalanche transit-time diode has been considered at operating frequencies of 94 GHz, 140 GHz and 220 GHz. To analyze the acoustic phonon effects on noise performance, the interactions of charge carriers with acoustic deformation potential and piezoelectric acoustic phonons have been considered in addition to all possible types of scattering events. These effects have been analyzed through a numerical expression for the ionization rate of charge carriers and incorporated in the noise analysis. The noise performance is evaluated in terms of noise spectral density (NSD) and noise measure (NM). The results show that due to acoustic phonons, values of NSD and NM significantly increase.

Spin qubits: spin relaxation in coupled quantum dots

The spin-flip scattering mechanism in coupled self-assembled quantum dots made with InAs/GaAs with the use of realistic material parameters is theoretically and numerically investigated. The electron wave functions within the coupled system have been calculated by the 8-band strain dependent theory. The phonon coupling to electrons is described by deformation potential and piezoelectric acoustic phonons. First order perturbation theory has been employed to evaluate the spin relaxation rates and spin-flip time T1. The numerical results show that parameters like the interdot distance and the applied static magnetic field are of crucial importance in spin-flip mechanism. The spin relaxation time has been also studied by varying the lattice temperature and by showing the differences between the quantum computing operation temperature ( K) and large temperatures.

Hot electron transport in wurtzite-GaN: effects of temperature and doping concentration

The hot electron transport in wurtzite phase gallium nitride (Wz-GaN) has been studied in this paper. An analytical expression of electron drift velocity under the condition of impact ionization has been developed by considering all major scattering mechanisms such as deformation potential acoustic phonon scattering, piezoelectric acoustic phonon scattering, optical phonon scattering, electron-electron scattering and ionizing scattering. Numerical calculations show that electron drift velocity in Wz-GaN saturates at 1.44 × 105 m/s at room temperature for the electron concentration of 1022 m−3. The effects of temperature and doping concentration on the hot electron drift velocity in Wz-GaN have also been studied. Results show that the saturation electron drift velocity varies from 1.91 × 105–0.77 × 105 m/s for the change in temperature within the range of 10–1000 K, for the electron concentration of 1022 m−3; whereas the same varies from 1.44 × 105–0.91 × 105 m/s at 300 K for the variation in the electron concentration within the range of 1022–1025 m−3. The numerically calculated results have been compared with the Monte Carlo simulated results and experimental data reported earlier, and those are found to be in good agreement.

An analysis of phonon emission as controlled by the combined interaction with the acoustic and piezoelectric phonons in a degenerate III–V compound semiconductor using an approximated Fermi–Dirac distribution at low lattice temperatures

Compound semiconductors being piezoelectric in nature, the intrinsic thermal vibration of the lattice atoms at any temperature gives rise to an additional potential field that perturbs the periodic potential field of the atoms. This is over and above the intrinsic deformation acoustic potential field which is always produced in every material. The scattering of the electrons through the piezoelectric perturbing potential is important in all compound semiconductors, particularly at the low lattice temperatures. Thus, the electrical transport in such materials is principally controlled by the combined interaction of the electrons with the deformation potential acoustic and piezoelectric phonons at low lattice temperatures. The study here, deals with the problem of phonon growth characteristics, considering the combined scattering of the non-equilibrium electrons in compound semiconductors, at low lattice temperatures. Beside degeneracy, other low temperature features, like the inelasticity of the electron–phonon collisions, and the full form of the phonon distribution have been duly considered. The distribution function of the degenerate ensemble of carriers, as given by the heated Fermi–Dirac function, has been approximated by a simplified, well-tested model. The model which has been proposed earlier, makes it much easier to carry out analytically the integrations without usual oversimplified approximations.

Prediction of Large Enhancement of Electron Mobility in Direct Gap Ge1−xSnx Alloy

Group IV semiconductor‐based alloy Ge1−xSnx can be grown on Si platform and a crossover from indirect (L valleys) to direct gap (Γ valley) occurs for x > 0.08. The direct gap alloy shows promise for use in high‐speed electronic and photonic devices. An earlier study predicted that the electron mobility in the alloy would increase by a few orders of magnitude above the value for pure Ge. In that work, however, electrons were assumed to occupy only the low effective mass Γ valley and only the deformation potential acoustic phonon and alloy scatterings were considered. In the present work, we give a more realistic estimate of the values of electron mobility in direct gap GeSn alloy, considering the Γ and L valleys and the phonon (deformation potential acoustic, optical and intervalley), alloy, and impurity scatterings. The effect of strain, both tensile and compressive, on the mobility is also studied. It is found that for higher alloy composition and suitable strain, mobility values ∼2 × 105 cm−2/V · s, more than 50 times higher than the value in pure Ge (3900 cm2/V · s) may be achieved. This is attributed to increased separation between Γ and L valleys and consequent reduction in intervalley scattering.

SA-phonon-assisted cyclotron resonance via two-photon process in graphene on GaAs substrate

The phonon-assisted cyclotron resonance (PACR) effect in a single-layer graphene on a GaAs substrate are theoretically studied via one and two-photon absorption processes. The result is limited by the combined interaction of the extrinsic potential of piezoelectric surface acoustic phonons of the substrate (PA) and of the intrinsic deformation potential of acoustic phonons of graphene (DA). The two-photon absorption process gives a significant contribution to magneto-optical absorption coefficient compared to one-photon process. It is shown that in the regime of low magnetic fields and temperatures the PA phonons dominate the magneto-optical absorption coefficient and half-width at half-maximum (HWHM). This finding would be useful to examine in subsequent research about HWHM in graphene. The present results have also demonstrated that the optical properties of graphene on a GaAs substrate can be controlled via extrinsic piezoelectric coupling.

Effects of increased acoustic phonon deformation potential and surface roughness scattering on quasi-ballistic transport in ultrascaled Si-MOSFETs

It is a common view that ballistic transport is enhanced by channel length scaling because of a decreased scattering number. On the other hand, the acoustic phonon (AP) scattering rate is higher in silicon-on-insulator (SOI) MOSFETs than in bulk Si-MOSFETs; moreover, surface roughness (SR) scattering caused by spatial fluctuation of quantized subbands emerges in extremely scaled SOI channels. Therefore, the influences of these scattering mechanisms on ballistic transport in ultrathin-body Si-MOSFETs are examined in this paper using a Monte Carlo simulation technique. First of all, the effect of increased AP scattering rate on the drain current and ballistic efficiency is found to be negligible. Furthermore, contrary to the common view, ballistic transport in double-gate MOSFETs is shown to be degraded when the channel length decreases to less than 10 nm, mainly owing to SR scattering intensified by the spatial fluctuation of quantized subbands. The gate and drain bias voltage dependencies of ballistic efficiency are also discussed.

Spin Relaxation of an Impurity Polaron in a Parabolic Quantum Dot

We have studied theoretically the spin relaxation of an impurity polaron, which arises from the electron interactions with the longitudinal optical phonon between the sublevel Zeeman splitting of the ground-state, by employing a variational method for a parabolic quantum dot (QD). In fact, this process occurs by the absorption of a deformation potential acoustic phonon. With Rashba spin-orbit coupling, the expression of the spin relaxation rate of an impurity polaron as functions of the radius of QD, the Lande factor parameter, the magnetic field adjusting length has been derived. Results of the numerical calculation show that the spin relaxation rate decreases with increasing the radius of QD and enlarges with increasing the magnetic field adjusting length when the magnetic field adjusting length \(R_{b} <0.7r_{0}\). In addition, we find that the spin relaxation rate is an increasing function of the Lande factor parameter.

Anharmonic effects in the thermoelectric properties of PbTe

In this work, we investigate the crystal anharmonic effects in the thermoelectric properties of n-type PbTe. The lattice thermal transport coefficient is computed by employing an isotropic continuum model for the dispersion relation for acoustic as well as optical phonon branches, an isotropic continuum model for crystal anharmonicity, and the single-mode relaxation time scheme. The electronic components of the transport coefficients in a wide temperature range are calculated using the isotropic-nearly-free-electron model, interaction of electrons with deformation potential of acoustic phonons, and the effect of the band non-parabolicity. It is found that the transverse optical branches play a major role in determining the phonon conductivity and the thermoelectric figure of merit of this material.

A Comparative Study on the Influence of Elastic Scattering Mechanisms on the Electron Mobility in Wurtzite and Zincblende Gallium Nitride

The electron mobility of zincblende (ZB) and wurtzite (WZ) structures of n-type GaN is investigated as a function of both temperature and donor concentration. Numerical calculations of the mobility are carried out in a temperature range from 10 K up to 400 K and donor doping concentrations from 10 19 m -3 to 10 27 m -3 . Elastic types of scattering mechanisms are considered. These types include ionized impurity scattering, neutral impurity scattering, deformation potential acoustic phonons scattering and piezoelectric potential scattering. The electron drift mobility versus temperature shows peak behavior for both zincblende and wurtzite GaN structures. Below approximately 120/140 K (for ZB and WZ GaN respectively), the mobility increases when the temperature is increased. In this temperature range, the ionized impurity scattering is assumed to be the dominant scattering mechanism. Above 120/140 K, the mobility is lowered down by raising the temperature. In this regime, the lattice scattering is considered to be the dominant mechanism. The variation of the electron drift mobility with donor concentration at room temperature shows a continuous decrease with the increase of impurity doping concentration. This is probably due to Coulomb scattering present particularly at low temperatures.

Surface polar optical phonon scattering of carriers in graphene on various substrates

The surface polar optical phonon scattering of carriers in graphene on various substrates is thoroughly studied using Rode's iteration method. The dependence of the scattering rate on carrier energy, temperature, and Fermi energy is investigated. The experimental data in the literature on the temperature dependence of the resistivity can be fitted using only one free fitting parameter. The fitting results show that the coupling strengths of surface optical phonons are much stronger, by at least a factor of 3.5, than the theoretical prediction. The strong coupling also suggests that the deformation potential of acoustic phonons might have been overestimated in the literature.

Polaron spin relaxation in a two-dimensional quantum dot

We theoretically investigate the spin relaxation of polaron, which arises from the electron interactions with the longitudinal optical phonon between the sublevel Zeeman splitting of the polaron ground state by using the vibrational and the unitary transformation methods in a two-dimensional quantum dot, where the Rashba spin-orbital coupling is taken into account. In fact, this process occurs by means of the absorption (or emitting) of a deformation potential acoustic phonon (or a piezoelectric one). The relaxation rate dependences of the magnetic, the quantum dot radius, the lander parameter, the Rashba spin-orbital coupling parameter are studied under the conditions of the strong coupling limitation and the weak one.

Shubnikov-de Haas-like oscillatory pattern of dc conductivity in a 2-dimensional indium gallium arsenide

The conductivity formula derived by using the KC reduction identity and a state-dependent projection technique is applied to a two-dimensional system of electrons interacting with deformation potential acoustic phonons. The system is in the presence of a perpendicular magnetic field and of the Rashba spin-orbit interaction (RSOI). The dc conductivity is shown to oscillate with the magnetic field above 0.6T near the Zeeman factor g=-0.8 and the RSOI strength α0=0.5×10-11eV·m. The pattern of oscillations is shown to be susceptible to change even by small change in g and α0, while the deformation potential constant, which characterizes the electron-phonon interaction, plays no role in changing the pattern at the temperature T=70 K.

Theory of single electron spin relaxation in Si/SiGe lateral coupled quantum dots

We investigate the spin relaxation induced by acoustic phonons in the presence of spin-orbit interactions in single electron Si/SiGe lateral coupled quantum dots. The relaxation rates are computed numerically in single and double quantum dots, in in-plane and perpendicular magnetic fields. The deformation potential of acoustic phonons is taken into account for both transverse and longitudinal polarizations, and their contributions to the total relaxation rate are discussed with respect to the dilatation and shear potential constants. We find that in single dots the spin relaxation rate scales approximately with the seventh power of the magnetic field, in line with a recent experiment. In double dots the relaxation rate is much more sensitive to the dot spectrum structure, as it is often dominated by a spin hot spot. The anisotropy of the spin-orbit interactions gives rise to easy passages, special directions of the magnetic field for which the relaxation is strongly suppressed. Quantitatively, the spin relaxation rates in Si are typically two orders of magnitude smaller than in GaAs due to the absence of the piezoelectric phonon potential and generally weaker spin-orbit interactions.

Electron mobility in modulation-doped AlSb/InAs quantum wells

We present a theoretical study of electron mobility in modulation-doped AlSb/InAs quantum wells. The theory also accounts for the nonparabolicity effect. All major scattering mechanisms, including scattering by deformation potential and piezoelectric acoustic phonons, polar optical phonons, ionized impurities, and interface roughness, have been included in our calculations. The low field transport properties of the two dimensional electron gas (2DEG) in the AlSb/InAs quantum wells are studied as a function of temperature, quantum well width, and spacer width and strategies for optimizing the 2DEG mobility are discussed. Depending on the quantum well parameters, the high-mobility limit in this quantum well structure may be determined by either ionized impurity scattering or interface-roughness scattering. The calculated 2DEG mobilities are in very good agreement with the reported experimental data for modulation-doped AlSb/InAs quantum wells.

Energy loss in degenerate semiconductors due to inelastic interaction with acoustic and piezoelectric phonons at low lattice temperatures

The energy loss rate of an electron in a degenerate semiconductor because of inelastic interaction with deformation potential and piezoelectric acoustic phonons is calculated in the case when the lattice temperature is low, so that the approximations of the well-known traditional theory are not valid. Compared to the traditional results and those for non-degenerate semiconductors, the theory here reveals a more complex and altogether different dependence of the loss rate on the carrier energy and the lattice temperature. The numerical results obtained here for Si and GaAs show how significantly the degeneracy level, the true phonon distribution or the inelasticity of the interaction affects the loss characteristics at low temperatures.

Concentration dependence of energy relaxation time in AlGaAs/GaAs heterojunctions: direct measurements

We present measurements of the energy relaxation time, τε, of electrons in a single heterojunction in a quasi-equilibrium state using microwave time-resolved spectroscopy at 4.2 K. We find the relaxation time has a power-law dependence on the carrier density of the two-dimensional electron gas, τε∝nγs with γ = 0.40 ± 0.02 for values of the carrier density, ns, from 1.6 × 1011 to 6.6 × 1011cm−2. The results are in good agreement with predictions taking into account the scattering of the carriers by both piezoelectric and deformation potential acoustic phonons. We compare these results with indirect measurements of the energy relaxation time from energy loss measurements involving Joule heating of the electron gas.

Surface polar phonon dominated electron transport in graphene

The effects of surface polar phonons on the electronic transport properties of monolayer graphene are studied by using a Monte Carlo simulation. Specifically, the low-field electron mobility and saturation velocity are examined for different substrates (SiC, SiO2, and HfO2) in comparison to the intrinsic case. While the results show that the low-field mobility can be substantially reduced by the introduction of surface polar phonon scattering, corresponding degradation of the saturation velocity is not observed for all three substrates at room temperature. It is also found that surface polar phonons can influence graphene’s electrical resistivity even at low temperature, leading potentially to inaccurate estimation of the acoustic phonon deformation potential constant.