High field transport properties in a GaN/AlGaN heterostructure

The drift velocity, electron temperature, electron energy and momentum loss rates of a two-dimensional electron gas are calculated in a GaN/AlGaN heterojunction (HJ) at high electric fields employing the energy and momentum balance technique, assuming the drifted Fermi–Dirac (F–D) distribution function for electrons. Besides the conventional scattering mechanisms, roughness induced new scattering mechanisms such as misfit piezoelectric and misfit deformation potential scatterings are considered in momentum relaxation. Energy loss rates due to acoustic phonons and polar optical phonon scattering with hot phonon effect are considered. The calculated drift velocity, electron temperature and energy loss rate are compared with the experimental data and a good agreement is obtained. The hot phonon effect is found to reduce the drift velocity, energy and momentum loss rates, whereas it enhances the electron temperature. Also the effect of using drifted F–D distribution, due to high carrier density in GaN/AlGaN HJs, contrary to the drifted Maxwellian distribution function used in the earlier calculations, is brought out.     

Phonon drag effect in three- and two-dimensional electron systems

The nonequilibrium phonon flow drags the electrons, and depending upon experimental conditions manifests itself in the acoustoelectric current, acoustomagnetic field or acoustoelectric field. The results of these phenomena in Sn, Al, Ga, Ag measured with SQUID technique are discussed. In the two-dimensional (2D) case the phonon drag is studied on the interface of bicrystals and on the cleavage (111) surface of Ge and on the inversion layer on (111) (100) planes of Si. In all these cases the phonon drag is about two orders of magnitude larger than in metals with the same charge density. This is due to the drag of surface electrons by nonequilibrium phonon of the whole specimen. The Kohn resonance of phonons with Fermi surface and topological transitions on Fermi surface of 2D electrons produced sharp singularities of phonon drag effect in 2D cases.     

Phonon focusing and electron–phonon drag in semiconductor crystals with degenerate charge-carrier statistics

We study the effect of anisotropy in elastic properties on the electron–phonon drag and thermoelectric phenomena in gapless semiconductors with degenerate charge-carrier statistics. It is shown that phonon focusing leads to a number of new effects in the drag thermopower at low temperatures, when diffusive phonon scattering from the boundaries is the predominant relaxation mechanism. We analyze the effect of phonon focusing on the dependences of the thermoelectromotive force (thermopower) in HgSe:Fe crystals on geometric parameters and the heat-flow directions relative to the crystal axes in the Knudsen regime of the phonon gas flow. The crystallographic directions that ensure the maximum and minimum values of the thermopower are determined and the role of quasi-longitudinal and quasi-transverse phonons in the drag thermopower in HgSe:Fe crystals at low temperatures is analyzed. It is shown that the main contribution to the drag thermopower comes from slow quasi-transverse phonons in the directions of focusing in long samples.     

Magnetic field studies of the frictional drag between coupled two-dimensional electronic systems – Coulomb versus phonon coupling

The coupling between systems of two spatially separated two-dimensional (2D) electron gases and between systems of a separated 2D electron gas and a 2D hole gas is studied as a function of magnetic field . The small barrier (30 nm) separated coupled electron gases showed a transition from a phonon dominated interaction at to a Coulomb dominated one at quantising fields. For large barriers (190 nm) phonons have been found to be the dominant coupling mechanism both at zero and finite fields. However, for all barriers investigated we could observe novel screening effects manifested in a suppression of the coupling at half-filled Landau levels. For the coupled electron–hole gases we have investigated samples with large barriers ( ) so that the coupling is both in zero and finite fields dominated by phonon mediated processes. The enhanced screening effects could not be observed in those samples possibly due to the less pronounced quantisation of the hole gas.     

Nonohmic behavior of semiconductors in a quantizing magnetic field

We have investigated the nonohmic resistivity of a nondegenerate semiconductor in quantizing magnetic fields for the case where acoustic phonons are the dominant scattering mechanism. The type of I-V characteristics found depends upon which of three mechanism are dominant. The three mechanisms are due to collisional broadening, inelasticities due to the finite phonon energy and phonon drag. When collistion broadening is important, the nonlinearities in the current voltage characteristic arise only from electron heating, while when the inelasticities are dominant, there is also an intrinsic nonlinearity in the characteristic. Finally, when phonon drag is dominant, high frequency acoustoelectric amplification will occur when the Hall velocity exceeds the sound velocity, i.e. V_H > S.For the case where inelasticities dominate, a region of negative differential resistance is obtained that should persist even when there is considerable optical phonon scattering.     

Phonon drag resistivity of potassium

The phonon drag resistivity for potassium is calculated by solving the Boltzman equations for both the electrons and phonons as opposed to the conventional method of Ziman where the phonon equation is not considered. By an application of the Schwartz inequality we can show that the drag resistivity in the present formalism is larger than that obtained by the conventional method. We substantiate this result by numerical calculation for potassium at very low temperatures, using a realistic phonon spectrum obtained from inelastic neutron scattering data. Parts of this paper were presented as partial fulfilment for a Ph.D. degree at Cornell University. The work was partially supported by CSIR funding.     

Thermoelectric Coefficients of Silicon MOSFETS in Quantizing Magnetic Field

The phonon drag and electron diffusion contribution to the tensor M which determines 3 the heat flux U = M·E is calculated for a silicon MOSFETS in a perpendicular magnetic field B. We used nearly the same theoretical formalism as Ref [6], but improvements are made in several respects. First of all the dielectric function of Fermi-Thomas approximation which has been proved to result in overscreening of the interaction is replaced by rigorous Lindhard-type dielectric function to take account of the screening between electrons and phonons. Secondly the contributions of localized electrons are separated from those of the free state electrons which are the only part that contributes to both conductivity tensor and magnetothermopower tensor. The calculated M_yx and S_xx reveal magneto-oscillationsoriginating Gom oscillations in the density of states at the Fermi level. At T = 5.02 K, our new results show that the diffusion components of thermopower are negligibly small compared with those due to phonon drag. All the theoretical values of M_yx, S_xx and S_yx are in accordance with the experimental data better than previous theoretical results.     

Magnetophonon resonance in a GaAs quantum well with AlAs/GaAs superlattice barriers at high filling factors

The magnetotransport of a high-mobility 2D electron gas in single GaAs quantum wells with AlAs/GaAs superlattice barriers is studied at high filling factors. For the selectively doped structures under study in the temperature range from 10 to 25 K, magnetoresistance oscillations periodic in the inverse magnetic field are observed with their frequency being proportional to the Fermi wave vector of the 2D electron gas. The experimental results are explained by the interaction of the 2D electron gas with leaky interface acoustic phonons.     

A deterministic solver for the transport of the AlGaN/GaN 2D electron gas including hot-phonon and degeneracy effects

The transport of the two-dimensional electron gas formed at an AlGaN/GaN heterostructure in the presence of strain polarization fields is investigated. For this purpose, we develop a deterministic multigroup model to the Boltzmann transport equations. The envelope wave functions for the confined electrons are calculated using a self-consistent Poisson–Schrödinger solver. The electron gas degeneracy and hot phonons are included in our transport equations. Numerical results are given for the dependence of macroscopic quantities on the electric field strength and on time and for the electron and phonon distribution functions. We compare our results to those of Monte Carlo simulations and with experiments.     

Calculation of the dark current in a quantum well infrared photodetector

Dark currents in a biased quantum well fabricated using Al_0.27Ga_0.73As/GaAs heterojunctions are calculated at two different temperatures including thermionic field emission currents arising from the electron scattering with phonons and plasmons. In the electron–phonon scattering process several modes due to heterojunctions such as the confined, half-space and interface longitudinal optic phonons are taken into account. It is found that the confined phonon scattering process results in maximum currents compared to those obtained in the half-space and interface scattering modes. However, the magnitude of the currents that resulted from the electron–plasmon scattering process is found to be higher than that found from the electron scattering with confined phonons. Comparison of the calculated dark currents with experiments shows that the thermionic emission currents due to phonon and plasmon assisted processes are essential to get better agreement with experiments than the previously employed bulk phonon scattering process.     

Phonons in 3C-, 4H-, and 6H-SiC

Silicon carbide epilayers of cubic (3C) and hexagonal (4H and 6H) polytypes were investigated by Auger electron spectroscopy, high-resolution electron energy-loss spectroscopy and Raman spectroscopy to determine the excitation energies of the optical Fuchs-Kliewer surface phonons and their relation to bulk phonon frequencies. The surfaces were treated in a buffered hydrofluoric acid solution. Loss structures attributed to excitation of Fuchs-Kliewer phonons were clearly resolved. Their energies were found at 115.9 ± 1 meV irrespective of the SiC polytype. The experimental data agree with values calculated from the experimental bulk phonon frequencies and tabulated dielectric constants.     

Atomistic simulation of the motion of dislocations in metals under phonon drag conditions

The mobility of dislocations in the over-barrier motion in different metals (Al, Cu, Fe, Mo) has been investigated using the molecular dynamics method. The phonon drag coefficients have been calculated as a function of the pressure and temperature. The results obtained are in good agreement with the experimental data and theoretical estimates. For face-centered cubic metals, the main mechanism of dislocation drag is the phonon scattering. For body-centered cubic metals, the contribution of the radiation friction becomes significant at room temperature. It has been found that there is a correlation between the temperature dependences of the phonon drag coefficient and the lattice constant. The dependences of the phonon drag coefficient on the pressure have been calculated. In contrast to the other metals, iron is characterized by a sharp increase in the phonon drag coefficient with an increase in the pressure at low temperatures due to the α-∈ phase transition.     

Phonon density of states in nanocrystalline<Superscript>57</Superscript>Fe

The Born-von Karman model is used to calculate phonon density of states (DOS) of nanocrystalline bcc Fe. It is found that there is an anisotropic stiffening in the interatomic force constants and hence there is shrinking in the nearest-neighbour distances in the nanophase. This leads to additional vibrational modes above the bulk phonons near the bottom of the phonon band. It is found that the high energy phonon modes of nanophase Fe are the surface modes. The calculated phonon DOS closely agree with the experimental data except a peak at 37 meV. The calculated phonon dispersion relations are also compared with those of the bulk phonons and anomalous behaviour is discussed in detail. The specific heat in nanophase enhances as compared to bulk phase at low temperatures and the calculated Debye temperature Θ_D agrees with the experimental results. It is predicted that the nanocrystalline Fe may consist of about 14 GPa pressure     

Zur Quantentheorie der Transportprozesse im Magnetfeld. III

In the presence of a transversal magnetic field the electric thermoelectric and thermal transportcoefficients are calculated, taking into account phonon drag. The calculations are based on the model of free electrons, phonons, and impurity scatterers. Starting from Kubo's formulae, and using truncation technique for Green's functions an integral equation (generalized transport equation) is developed. It is solved in the case of strong magnetic field. If phonon drag is neglected Titeica type formulae hold for all transportcoefficients. Phonon drag reduces the values in the sense that the electron-phonon-relaxation time is replaced by the sum of the electron-phonon- and phonon-scatterer-relaxation time.     

Transverse electron focusing as a way of studying phonon kinetics. Turbulence of phonon flow

Novel techniques have been created for studying phonon kinetics. A supersonic conduction electron flux is used to produce a phonon flux. Transverse electron focusing (TEF) is applied for checking the electron gas affected by phonons. A turbulent regime of phonon flow-phonon flux step broadening-reveals itself as step broadening of non-equilibrium electron flux.     

Energy loss rate of two-dimensional electron gas in GaInAs/AlInAs, InSb/AlInSb and GaSb/AlGaAsSb heterostructures

Energy loss rates of two-dimensional electron gas in GaInAs/AlInAs, InSb/AlInSb and GaSb/AlGaAsSb heterostructures are theoretically investigated over a wide range of temperature based on the electron–one-phonon and electron–two-phonon interactions. Calculations are presented for electron acoustic one-phonon interaction via deformation potential and piezoelectric coupling and electron–LO phonon interaction with hot phonon effect. In addition, energy loss rate due to electron-two-zone edge transverse acoustic (TA) phonons is also presented. A very good agreement is obtained between the calculations and experimental data in GaInAs/AlInAs structure with the inclusion of electron–two-zone edge TA phonon interaction. In all these three structures energy loss is dominated by (i) acoustic one-phonon scattering at low temperatures, (ii) two-TA zone edge phonons at intermediate temperatures and (iii) LO phonons at high temperatures. It is observed that, hot phonon effect reduces the energy loss rate considerably in these structures.     

Dispersive anisotropic ballistic phonon propagation in [100] GaAs

We report experimental observations of the phonon focusing pattern in [100] GaAs using low temperature electron beam scanning for phonon generation. The typical dispersive effects for high-frequency phonons expected from the calculations by Tamura have clearly been observed using PbIn tunnel junctions for phonon detection. The quantitative comparison of our experimental results with the frequency dependent calculations by Tamura allowed to determine the dominant phonon frequencies contributing to the detector signal in our different experiments. Above the temperature of the -point the dominant phonon frequencies appear to be shifted considerably to lower values, which could be explained by a heating effect in the liquid-He layer adjacent to the tunnel junction detector. By comparing the observed magnitude of the detector signal with different theoretical treatments of the detector response, we have found satisfactory agreement for a model where the perturbation due to the high-frequency phonons is restricted to the base electrode of the detector reached first by the phonons following their passage through the crystal.     

Phonon-pulses propagation and phonon focusing in hexagonal-crystal rods in the boundary-scattering regime

The influence of the sample surface on the propagation of ballistic phonons in cylindrical samples of hexagonal crystals is studied. Our approach is based on the solution of the Boltzmann-Peierls equation with an external phonon source. The phonon irradiation of a detector face is calculated for⁴He and Zn crystals. It is shown how the strong phonon focusing, occurring in the slow modes of these solids, affects on the shape of energy flux falling upon the detector area. For an appropriately chosen lengthto-radius sample ratio, phonons reflected from the sample surface dominate in the detected signal.     

Contribution of the electron-phonon interaction to the broadening of cyclotron resonance line of a two-dimensional electron gas in multiple quantum well structures

Summary The theory of cyclotron resonance (CR) lineshape of a two-dimensional electron gas (2 DEG) due to the electron-phonon interaction in multiple-quantum-well structures (MQWS) is investigated. The contribution of the deformation potential acoustic and piezoelectric phonon scattering to the broadening of the cyclotron resonance spectra (CRSB) of such a system is calculated fro GaAs/AlAs. The piezoelectric phonon scattering contribution to the linewidth is smaller as compared to the deformation potential acoustic phonon scattering but is significantly comparable. The magnetic-field dependence of CRSB due to the deformation potential acoustic and piezoelectric phonons isB ^1/2 andB ^1/4, respectively, and the frequency shift Δ _N,p =0 for both interactions in the elastic-scattering approximation. Observed numerical values of the CRSB indicate that at low temperatures acoustic and piezoelectric phonons are dominant scatterers and interact strongly with 2 DEG in MQWS where the impurity scattering is suppressed due to the modulation doping. To speed up publication, the author of this paper has agreed to not receive the proofs for correction.     

Normal processes of phonon-phonon scattering and the drag thermopower in germanium crystals with isotopic disorder

A strong dependence of the thermopower of germanium crystals on the isotopic composition is experimentally found. The theory of phonon drag of electrons in semiconductors with nondegenerate statistics of current carriers is developed, which takes into account the special features of the relaxation of phonon momentum in the normal processes of phonon-phonon scattering. The effect of the drift motion of phonons on the drag thermopower in germanium crystals of different isotopic compositions is analyzed for two options of relaxation of phonon momentum in the normal processes of phonon scattering. The phonon relaxation times determined from the data on the thermal conductivity of germanium are used in calculating the thermopower. The importance of the inelasticity of electron-phonon scattering in the drag thermopower in semiconductors is analyzed. A qualitative explanation of the isotope effect in the drag thermopower is provided. It is demonstrated that this effect is associated with the drift motion of phonons, which turns out to be very sensitive to isotopic disorder in germanium crystals.