Hot Electron Gas Research Articles

Transmitted acoustic phonon drag between parallel two-dimensional electron gases: Monte Carlo simulation

We present Monte Carlo (MC) simulation of transmitted acoustic phonon (TAP) drag between barrier-separated two-dimensional (2D) electron gases in the AlGaAs/GaAs system. Non-equilibrium acoustic phonons emitted by the hot 2D electron gas in the biased GaAs channel travel across the sample. These phonons are partially absorbed in an unbiased 2D channel where they induce a drag current. Simulation includes 2D electron-non-equilibrium acoustic phonon interaction for both deformation-potential and piezoelectric coupling. Non-equilibrium phonon distribution is calculated numerically. TAP drag is simulated at 4.2 K in a multiple quantum well containing equivalent high-mobility 2D electron gases. Drift velocities around 1000 m s-1 are found in the drag channel (2D gas without outer field), when it is driven by TAP drag from a large number (10-50) of 2D electron gases subjected to electric field of 1000 V m-1 TAP drag is mainly due to the deformation-potential coupling.

LO-phonon emission by hot electrons in one-dimensional semiconductor quantum wires.

We provide a theoretical study of hot-electron energy loss via LO-phonon emission in GaAs quantum wires. Calculations are done for a model in which the hot-electron gas is described by a temperature T which is much higher than the lattice temperature. We take into account several relevant physical mechanisms such as quantum degeneracy, dynamic screening, quantum confinement, and hot-phonon bottleneck effect. We include the effects of quantum-wire phonon confinement using two prevailing boundary conditions, namely, the so-called electrostatic and the mechanical macroscopic models. The energy-loss rate calculated for the bulk LO-phonon emission is comparable to, but somewhat higher than, that due to slab (electrostatic) phonon emission, whereas the guided (mechanical) modes produce more than an order of magnitude slower energy relaxation than that due to the slab (electrostatic) or the bulk modes. Our results show that in the experimentally interesting electron-temperature range of 50--300 K, the hot-phonon bottleneck is the single most important quantitative effect provided the hot-phonon lifetime in GaAs quantum wires is comparable to that (\ensuremath{\sim}5--7 ps) in the bulk GaAs. The numerical magnitudes of the quantum-wire hot-electron energy-loss rate due to LO-phonon emission are comparable to those in quantum-well structures under comparable conditions.

Coulomb lifetime and electronic distribution function in a drifting two-dimensional electron gas.

The nonequilibrium distribution function of a two-dimensional electron gas (2DEG) drifting under a high electric field is calculated within the Lei-Ting approach of high-field transport. The separation of the center-of-mass motion from the relative motion of the electrons leads to anisotropic interactions with impurities and phonons that disturb the relative electron gas from a thermodynamic equilibrium state. The correcting amount to the equilibrium momentum distribution function turns out to be the product of the impurity and phonon interactions scattering rate by the particle Coulomb lifetime. This last one is expressed from the full spectral density functions so that its validity extends beyond the quasiparticle theory frame. The importance of a self-consistent calculation is first illustrated by a study of the zero-temperature many-body properties of the Coulomb gas: the plasmaron is shown to be an actual excitation; the threshold for plasmon emission is softened, and undamped Fermi-level particles are found to have a spectral weight nearly equal to unity. Then, within the drifting hot-electron gas, the Doppler shift of the LO-phonon frequencies is shown to enhance the plasmon--LO-phonon coupling. Since the electron--LO-phonon scattering rate is similar to the Coulomb damping rate that characterizes the electron-electron interaction strength, the linear handling of the electron--LO-phonon interaction is questionable for a 2DEG drifting under a high electric field.

Electron—Electron Scattering in a Hot Electron Gas Interacting with Optical Phonons

AbstractThe electron—electron scattering influence on the hot electron gas is analyzed. The strongest influence is observed in the slightly heated electron gas (in the case of so‐called warm electrons) at the lattice temperature T ∼ T0/5 (T0 being the characteristic temperature of the optical phonon). The interparticle collisions are shown to increase the electron energy loss via optical phonons and to decrease by the same amount the energy loss conditioned by acoustic phonon scattering.

Sputtered netural and molecular ion mass spectrometries in the characterization of multilayer samples

In this paper we report some results obtained in the analysis of multilayer samples employed in VLSI and ULSI microelectronic technology by means of hot electron gas SNMS (sputtered neutral mass spectrometry) and MCs+ SIMS. A direct comparison of molecular ion SIMS with a post-ionization technique pushed to its limits is presented. A discussion in terms of dynamic range, optimum sensitivity and depth resolution is given.

Effects of a hot two-dimensional electron gas on optical properties of modulation-doped GaAs/AlGaAs heterostructures

The effects of a hot two-dimensional electron gas (2 DEG) on optical properties of modulation-doped GaAs/AlGaAs heterostructures have been investigated by photoluminescence (PL) and optically detected cyclotron resonance (ODCR) techniques. A decrease of the PL intensity from shallow excitons and an enhancement of the interband recombination between the 2 DEG (confined in a notch potential near the heterointerface) and photo-excited holes are observed, when the cyclotron resonance (CR) condition for electrons is fulfilled. The mechanisms responsible for these observations are discussed in terms of a hot 2 DEG heated in the microwave (MW) field. The angular dependence of the ODCR peak proves the 2D character of the corresponding hot electrons. In this way the authors identify these PL emissions as arising from the same GaAs active layer.

Changeover from acoustic to optic mode phonon emission by a hot two-dimensional electron gas in the gallium arsenide/aluminium gallium arsenide heterojunction

The authors have used heat pulse techniques to study the energy relaxation of a hot two-dimensional electron gas (2 DEG) in a GaAs/AlGaAs heterojunction. The 2 DEG was heated by applying short ( approximately=100 ns) electrical pulses to the drain-source contacts of the device. The electrons lost energy by emitting phonons which were detected by a CdS bolometer on the opposite side of the GaAs substrate. A change in the nature of the phonon signal occurring at an excitation level of about 5 pW per electron indicated a change in the phonon emission process. The corresponding electron temperature, Te, at which optic phonon emission is expected to become the dominant energy relaxation process was estimated to be about 60 K. At powers well below the change-over, the authors found that the energy loss rate per electron, Pe, due to acoustic phonon emission is proportional to Te3. At much higher powers, Pe varies as exp(-h(cross) omega LO/kTe), where h(cross) omega LO is the longitudinal optic phonon energy. They obtained a value of 3.3 ps for the electron-optic phonon scattering time, which is consistent with the range of values found in the literature.

Electronic distribution function under high field transport in modulation-doped GaAs/GaAlAs multi-quantum wells

The non-equilibrium distribution function of a two-dimensional electron gas in a static high electric field is calculated within the Lei-Ting approach of high-field electronic transport. The separation of the centre of mass motion from the relative motion of the electrons leads to anisotropic interactions with impurities and phonons which displace the relative electron gas state from thermodynamic equilibrium. The calculated distribution function is highly anisotropic with a 'cold' part in the direction of the drift velocity and a 'hot' part in the opposite direction. An effective temperature is introduced by averaging the electronic distribution function in all the directions of the reciprocal space. This effective temperature is compared with the experimental temperatures measured by photoluminescence, during the steady state high electric field transport experiments. A finite LO-phonon lifetime as low as 1 ps is shown to give good agreement between theory and experiment for the energy relaxation rate of a two-dimensional hot electron gas.

Hot-electron relaxation in semiconductor quantum wires: Bulk-LO-phonon emission.

We calculate, within the electron-temperature model, the rate of energy loss due to bulk-LO-phonon emission from a hot-electron gas to a cold lattice in quasi-one-dimensional GaAs quantum-wire structures. Our theory includes the known important physical mechanisms, such as degeneracy, dynamical screening, quantum confinement, and the hot-phonon bottleneck effect. In the experimentally interesting electron-temperature range of 50--200 K, we find the hot-phonon effect to be quantitatively the most significant physical mechanism determining hot-electron energy relaxation. The typical intrasubband relaxation time is of the order of a picosecond, quite comparable to that found in two-dimensional quantum-well structures.

Processes of Hot Electrons and Lattice Relaxation in Ion Implantation

AbstractThe relaxation processes of a hot electron gas formed under bombardment with particles of energy in the MeV and GeV range are described by using molecular dynamics simulation methods based on a classic representation. The calculations lead to a noticeable lattice disorder whose features and mechanisms of formation differ from the ones of a Coulomb explosion or of a thermal spike.

Far infrared and phonon emission from a hot two-dimensional electron gas in a silicon mosfet at 4K.

The weak FIR intensity from a heated 2DEG in a Si MOSFET at 4K has been used to determine the electron temperature as a function of power input and sheet density. The resulting values of heat transfer between the electrons and phonons are believed to be significantly more accurate than previous measurements.

High-energy gamma radiation from black holes with slow friction accretion

This paper studies the production of high-energy gamma radiation (E-gamma about 100 MeV) in the vicinity of a black hole through the collisions of accreting protons on the stable orbits. A spherical accretion with a small rate is considered as a fully computable example. If the accretion rate is about 0.001 of the critical rate, protons stream down to the black hole along the stable orbits, slowly braking due to the friction in hot electron gas. This motion is referred to as friction accretion. The collisions of relativistic protons on the stable orbits give rise to gamma radiation through the production and decay of neutral pions. It is claimed that the considered mechanism can provide the detectable flux from both galactic and extragalactic sources. 34 refs.

Many-body theory of energy relaxation in an excited-electron gas via optical-phonon emission.

We consider relaxation of a hot-electron gas by emission of LO phonons in quasiequilibrium when both the electrons and the lattice are in separate internal equilibria, and the electron distribution function can be described by an effective temperature. We find that, in addition to dynamical screening and the hot-phonon effect, the many-body renormalization of the LO phonons also plays a crucial role, both qualitatively and quantitatively, in the power-loss process at low electron temperatures. The density of states of a LO phonon coupled to an electron gas has three branches: (i) the bare-phonon-like branch near the bare-phonon energy, (ii) the plasmonlike branch near the plasmon energy, and (iii) the low-energy quasiparticle-excitation-like branch in the quasiparticle-excitation region. We show that even though the oscillator strengths of the plasmonlike and the quasiparticle-excitation-like branches are extremely small, they dominate the power-loss process at low enough electron temperatures. This produces an enhancement of the power loss at low electron temperatures by many orders of magnitude relative to the power loss to bare unrenormalized LO phonons. We believe that the hot-electron relaxation experiments are a very suitable way of studying the novel quasiparticle-like LO phonons, which are unlikely to be observed directly because of their small oscillator strength. We provide a detailed quantitative theory for hot-electron relaxation in both two- and three-dimensional systems based on this many-body approach and find quantitative agreement with existing experimental results in the 20--200-K electron-temperature regime.

Monte Carlo study of hot-electron transport in an InGaAs/InAlAs single heterostructure

Monte Carlo simulation of two-dimensional hot-electron gas in an InGaAs/InAlAs single heterostructure was carried out in order to clarify high-field drift velocity and drift velocity overshoot at 77 K. The electronic states of the two-dimensional electron gas are calculated self-consistently and are used to calculate the scattering probabilities. The importance of the conduction band nonparabolicity is shown by using the energy-band structures calculated by the k*p perturbation method for In/sub 0.53/Ga/sub 0.47/As and In/sub 0.52/Al/sub 0.48/As. In the present calculations alloy disorder scattering and two modes of LO phonons in InGaAs layers are taken into account in addition to acoustic deformation potential scattering, screened ionized impurity scattering, nonpolar optical phonon scattering, and intervalley phonon scattering. It is found that alloy disorder scattering reduces the drift velocity and that the maximum drift velocity and negative differential mobility depend on the energy separation between the Gamma and L valleys, indicating that the intervalley scattering plays an important role in hot-carrier transport in InGaAs/InAlAs single heterostructures.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Effect of phonon self-energy correction on hot-electron relaxation in two-dimensional semiconductor systems.

We study the phenomenon of hot-carrier relaxation via LO-phonon emission in two-dimensional semiconductor quantum wells. Calculations are done for a model in which the hot-electron gas is described by a temperature T, which is higher than the lattice temperature. In addition to including the usual dynamical screening and hot-phonon effect, we also include a phonon self-energy correction due to its coupling with the electron gas in the plasmon-pole approximation. This leads to a large enhancement of the power loss at low temperatures for low-density samples. The numerical results are in good agreement with the available experimental results.

Phonon emission by a hot two-dimensional electron gas in a quantizing magnetic field

A detailed analysis is made of the acoustic phonon emission spectrum of a heated 2DEG in a (0 0 1) n-Si inversion layer, when a quantizing magnetic field B is applied perpendicular to the layer. Particular attention is given to the case of high magnetic fields when only a few Landau levels are occupied. The emitted power is then quasimonochromatic at the cyclotron frequency and the angular distribution shows a sharp maximum at a propagation angle ϑ m to the field, given by sin ϑ m ∼ m ∗v s/( 1 2 eB h ̷ ) 1 2 , where m ∗ is the effective mass and v s the sound velocity. Both longitudinal and transverse modes are considered and the transition to zero field is discussed.

Theory of the spectral acoustic phonon emission intensity of hot 2D electrons in quantized n-inversion layers

The spectral acoustic phonon emission intensity of the hot quasi two-dimensional electron gas (2DEG) in quantized n-Si (GaAs) inversion layers is calculated as a function of the phonon angular frequency w at different values of the carrier temperature Te and density Ns. In the long wave length limit (ℏw ⪡ kBTe) the emission intensity increases ∝ ws(ws+1) for bulk- (surface-) modes where s = 3 for the unscreened acoustic deformation potential coupling. At w ≈ vj2kF (vj: sound velocity of the phonon mode j, kF: radius of the Fermi-circle) the emission intensity reaches a maximum whose position is shifted to higher w-values if Ns increases. For given values of Ns, Te, T (lattice temperature) and ϑ (emission angle) the emission intensity maximum of the n-GaAs inversion layer is found to be about one order of magnitude smaller than the intensity maximum of the n-Si inversion layer.

Nonequilibrium electron-phonon scattering in semiconductor heterojunctions.

We calculate the energy-loss rate of a quasi-two-dimensional (2D) hot-electron gas at a semiconductor heterojunction due to the emission and reabsorption of nonequilibrium optical phonons. A kinetic equation for the nonequilibrium lattice excitations (hot phonons) in the spatially inhomogeneous field created by the quasi-2D electrons is obtained. The equation is solved by a transformation that introduces the occupation number for wave packets of phonons that are ``localized'' in a spatial region near the electron layer. Our result does not contain the spurious dependence on the size of the sample that results when the nonequilibrium phonons are represented in terms of decoupled 3D plane waves. We apply our results to electrons in a GaAs-${\mathrm{Ga}}_{\mathrm{x}}$${\mathrm{Al}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As heterojunction. We find that the reabsorption of the emitted hot phonons reduces considerably the electron cooling rate if the optical-phonon lifetime ${\ensuremath{\tau}}_{\mathrm{op}\ensuremath{\ge}}$5 psec. Furthermore, the electron-energy relaxation rate, 1/\ensuremath{\tau}, changes from a weakly decreasing function of the electron temperature, ${T}_{e}$, when ${\ensuremath{\tau}}_{\mathrm{op}}$=0 to a increasing function of ${T}_{e}$, as ${\ensuremath{\tau}}_{\mathrm{op}}$ increases. This result compares favorably with recent experimental data.

Theory of the hot free carrier intraband-absorption coefficient of n-inversion layers

The absorption coefficient K of a quasi two dimensional (2D) hot free electron gas is calculated for the first time as a function of the lattice temperature T, the photon angular frequency w, the carrier density N s as well as the electron temperature T e when the carriers are scattered by ionized impurities, acoustic phonons and polar optical phonons. Analytical expressions are derived in the limiting cases of non-degeneracy and degeneracy of the electron system. In the quantum limit ħw/k BT e ≳ 1 where the interaction between the electron and the photon is inelastic K sensitively depends on the limiting scattering mechanism showing that the electron motion is completely controlled by the photon field. In the classical limit ħw/k BT e ⪡ 1 the absorption decreases proportional to w 1 independent of the limiting scattering mechanism in agreement with the experimental data deduced from far-infrared absorptivity measurements on GaAs heterolayers.

Two-dimensional electron transport and hot electron effects in InP/n-AlInAs heterostructures

Abstract We report two-dimensional electron gas (2DEG) transport in InP/n-AlInAs Heterostructures. This is the first demonstration of forming 2DEG in an InP with semiconductor heterojunctions. Temperature and electron density dependences of the 2DEG mobility are presented. This 2DEG system can be utilized for high speed and high power FETs. Hot electron gas transport in an InP is simulated with including electron-electron scattering and screening by high density electrons.