Phonons In GaAs Research Articles

Optical phonons in quantum-wire structures

The optical vibrational modes in GaAs/AlAs structures grown on a (311)A-oriented GaAs surface are investigated. It is found that the line corresponding to the fundamental TO vibrational mode localized in a GaAs quantum wire is split into two lines with different directions of the polarization vector. The dispersion of the TO phonons of GaAs in the (311) direction is determined from the IR spectra of periodic structures.

Local stress measurements in laterally oxidized GaAs/AlxGa1−xAs heterostructures by micro-Raman spectroscopy

Lattice deformation induced in surface GaAs layers by the selective lateral oxidation of buried AlxGa1−xAs layers was determined from the energy shift of the GaAs phonon line in Raman spectra excited with a focused laser beam. The procedure included a correction for the laser beam induced heating effects. The surface GaAs layer was found under small tensile stress in the oxidized regions of our samples with respect to the unoxidized regions. The deformation is 8×10−4 and is the same, within experimental error, for heterostructures incorporating pure AlAs layers or Al0.98Ga0.02As. Strong differences in Raman efficiency, as well as in photoluminescence efficiency, were observed for different samples, which are discussed in terms of the GaAs/oxide interface nonradiative recombination efficiency.

Magnetospectroscopy of Bound Phonons in High Purity GaAs

Far infrared magnetophotoconductivity performed on high purity GaAs reveals the existence of fine structures in the resonant magnetopolaron regions. The fine structures are attributed to the presence of bound phonons due to multiphonon processes. We demonstrate that the magnetopolaron energy spectrum consists of bound phonon branches and magnetopolaron branches. Our results also indicate that different phonons are bound to a single impurity, and that the bound phonon in Si-doped GaAs is a quasilocalized mode.

Observation of hot electron relaxation in GaAs/AlGaAs multiple quantum wells by excitation spectroscopy

Hot electron relaxation in GaAs/AlGaAs multiple quantum well (MQW) structure was studied with the use of photoluminescence excitation (PLE) spectroscopy. Oscillation due to the emission of confined longitudinal optical (LO) GaAs phonons, by photoexcited electrons were observed in the (6 K) excitation spectra. The period of the oscillation is different from that observed in bulk GaAs. The calculation from a four-band Kane model, describing the mixing of heavy- and light-hole bands at wave vector away from k=0, was used to interpret the difference of oscillation features between the GaAs/AlGaAs MQW structure and bulk GaAs. The recorded PLE spectrum and calculated results show that photoexcited electrons can directly cascade downwards to the exciton energy state by LO phonon emissions.

Chemisorption of chlorine on GaAs(100) surfaces studied by high-resolution electron energy-loss spectroscopy

The adsorption of chlorine on GaAs(100)-c(8 × 2) surfaces was investigated by high-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS) and low-energy electron diffraction (LEED). Chlorine molecules were released from an electrochemical AgAgCl-graphite cell. For the clean c(8 × 2)-reconstructed surface, besides the well-known Fuchs-Kliewer phonon of GaAs at 35.8 meV, two further surface phonons are observed at 7.9 and 11.5 meV. They can be characterized by comparison with the volume phonon dispersion curves of GaAs in the high-symmetry directions of the Brillouin zone. After chlorine exposure, the XPS measurements show an etching process which removes gallium atoms from the surface. With HREELS three adsorbate-induced loss features at 18, 43 and 46 meV are found. Comparing the loss energies with vibrational energies of AsClBr 2, GaCl and GaCl 2 suggests the attribution of the loss features to the GaCl valence vibration (46 meV) and the GaCl 2 scissor mode (18 meV) for small chlorine exposures, and to the AsCl stretching vibration (43 meV) in the high exposure regime when etching occurs.

Coherent TO phonon relaxation in GaAs and InP

Relaxation of coherent TO phonons is investigated in bulk GaAs and InP using an infrared time-resolved coherent anti-Stokes Raman scattering technique. Measurements were performed as a function of the crystal temperature in the range 10--300 K permitting determination of the dominant TO phonon relaxation channels. The experimental results are consistent with TO phonon decay into a TA and a LA phonon in GaAs while in InP decay into two identical energy LA phonons dominates at low temperatures.

Probing optical-phonon propagation in GaAs/AlxGa1-xAs quantum-well samples via their nonequilibrium population.

We develop a theoretical model to analyze a nonequilibrium optical- (LO-) phonon population by Raman scattering in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum wells. With the assumption of bulklike hot-electron relaxation, the effect of LO-phonon confinement on a nonequilibrium optical-phonon population (NOP) is isolated. Our analysis shows that the decrease in spatial extent or coherence length of LO phonons is reflected by a decrease in NOP. This is because the contributions from large q wave vectors with small occupation numbers dominate as the spatial extent decreases. Our method is applied to explain picosecond Raman-scattering experiments on GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As. The increasing NOP with decreasing x is interpreted as the result of an increase in the coherence length of LO phonons, since for smaller x, the ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As barrier is no longer effective in localizing GaAs LO phonons within the well. Using this model, we also deduce coherence length of LO phonons as a function of x. Our results show that for values of x between 0.2 and 0.4, the GaAs LO phonon in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum wells changes from a bulklike propagating mode to one localized within the wells. \textcopyright{} 1996 The American Physical Society.

Light scattering in p-type GaAs:Ge

We present in this work the results of a Raman spectroscopy study on the plasmon–phonon coupling in Ge-doped p-type gallium arsenide. A series of polarized Raman scattering experiments were carried out on epitaxial films grown by liquid-phase epitaxy on (100) GaAs substrates at 20, 100, and 300 K. The films were p type with free hole densities varying in the range of 5×1017–1×1020 cm−3. Under the scattering configurations employed, the longitudinal optical (LO) mode is forbidden for crossed polarization while the transverse optical (TO) mode is forbidden for both parallel and crossed polarizations. However, all the polarized Raman spectra showed two peaks with frequencies close to the TO and LO phonons of semi-insulating GaAs. The appearance of such forbidden modes was accounted for with a theoretical model which considers phonon–plasmon coupled modes with wave vectors much larger than those given by the regular q≊0 wave vector transferred by photons. Ionized acceptor impurities provide such additional wave vector transfer through elastic scattering of the photoexcited electrons and holes. It is demonstrated that the experimental values for position and linewidth of the peaks are well described by the theoretical calculations when Fröhlich-type and deformation potential mechanisms are considered as means of interaction.

The temperature dependence of the LO(T) and TO(T) phonons in GaAs and InP

AbstractThe LO and TO phonon frequencies and dampings have been measured by near‐infrared Raman spectroscopy both in bulk GaAs and InP. The phonon damping of the LO phonon in InP is much smaller than the LO phonon damping in GaAs. The experimentally obtained temperature dependences were compared with a theory including cubic anharmonic processes and the thermal expansion of the lattice. It is shown that the phonon damping is mainly affected by decay processes into acoustic phonons.

Time-resolved observation of coherent phonons by the Franz-Keldysh effect.

We report on the time-resolved observation of coherent optical phonons in GaAs by the Franz-Keldysh effect. This effect leads to a modulation of the optical interband transitions with a nonlinear dependence on the macroscopic electric field associated with the coherent phonons. We find that the nonlinear electro-optic effect forms a far more efficient detection mechanism for coherent phonons than the linear electro-optic effect near the band gap. \textcopyright{} 1996 The American Physical Society.

Resonant Raman scattering from phonons in GaAs/(GaAs)m(AlAs)n quantum wire structures

Raman spectroscopy has been used to measure phonons in GaAs v-groove quantum wire structures containing (001) and (111) GaAs/AlAs superlattice barrier regions. Resonance enhancement permits the identification of modes in different regions of the structure, and the measured phonon frequencies provide structural information which shows clear evidence of GaAs migration during growth from (001) surfaces into the grooves. Confined and interface phonons with large in-(111) plane wavevectors are observed.

Heating of two-dimensional electron gas and LO phonons in δ-doped GaAs by far-infrared radiation

A change in the conductivity of the two-dimensional electron gas in δ-doped GaAs samples due to pulsed far-infrared laser irradiation has been detected. It is shown that the observed positive photoconductivity is caused by heating of 2D electrons. The energy loss of hot electrons is accomplished by scattering with nonequilibrium longitudinal optical (LO) phonons. An expression for the effective emission frequency of LO phonons coupled with electrons and the thermal bath of acoustic phonons is derived and the dependence of the emission frequency on the lattice temperature is experimentally obtained in the range 77–300 K.

Effect of gate voltage on hot-electron and hot phonon interaction and transport in a submicrometer transistor

This paper studies the effects of gate voltage on heat generation and transport in a metal–semiconductor field effect transistor made of gallium arsenide (GaAs) with a gate length of 0.2 μm. Based on the interactions between electrons, optical phonons, and acoustic phonons in GaAs, a self-consistent model consisting of hydrodynamic equations for electrons and phonons is developed. Concurrent study of the electrical and thermal behavior of the device shows that under a source-to-drain bias at 3 V and zero gate bias, the maximum electron temperature rise in this device is higher than 1000 K whereas the lattice temperature rise is of the order of 10 K, thereby exhibiting nonequilibrium characteristics. As the gate voltage is decreased from 0 to −2 V the maximum electron temperature increases due to generation of higher electric fields whereas the maximum lattice temperature reduces due to lower power dissipation. The nonequilibrium hot-electron effect can reduce the drain current by 15% and must be included in the analysis. More importantly, it is found that the electron temperature rise is nearly independent of the thermal package conductance whereas the lattice temperature rise depends strongly on it. In addition, an increase of lattice temperature by 100 K can reduce the drain current by 25%.

Nonequilibrium dynamics of hot carriers and hot phonons in CdSe and GaAs.

Nonequilibrium dynamics of hot carriers and hot phonons in CdSe and GaAs.

Electron-electron interactions, coupled plasmon-phonon modes, and mobility in n-type GaAs.

This paper investigates the mobility of electrons scattering from the coupled system of electrons and longitudinal-optical (LO) phonons in n-type GaAs. The Boltzmann equation is solved exactly for low electric fields by an iterative method, including electron-electron and electron--LO-phonon scattering dynamically screened in the random-phase approximation (RPA). The LO-phonon self-energy is treated in the plasmon-pole approximation. Scattering from ionized impurities screened in static RPA is calculated with phase-shift cross sections and scattering from RPA screened deformation potential and piezoelectric acoustic phonons is included in the elastic approximation. The results show that dynamic screening and plasmon-phonon coupling significantly modify inelastic scattering at low temperatures and densities. The effect on mobility is obscured by ionized impurity scattering in conventionally doped material, but should be important in modulation doped structures. For uncompensated bulk n-type GaAs, the RPA phase-shift model for electron-impurity scattering gives lower drift mobilities than the standard Thomas-Fermi or Born calculations, which are high compared to experiment. Electron-electron scattering lowers the mobility further, giving improved agreement with experiment, though discrepancies persist at high donor concentrations (n>${10}^{18}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$). When impurities are ignored, inelastic scattering from the coupled electron-phonon system is the strongest scattering mechanism at 77 K for moderate doping. This result differs from the standard model, neglecting mode coupling and electron-electron scattering, which has the acoustic modes dominant in this regime.

Electric-field-induced Raman scattering in GaAs: Franz-Keldysh oscillations.

We have studied the influence of strong electric fields on the Raman scattering intensity from LO phonons in GaAs (100) at room temperature using laser excitation energies above the fundamental ${\mathit{E}}_{0}$ gap. Striking oscillations are found in the scattering intensity for configurations where either the deformation potential or Fr\"ohlich electron-phonon interaction contribute. The oscillations in the deformation-potential-mediated scattering intensity can be related to Franz-Keldysh oscillations derived from the ${\mathit{E}}_{0}$ gap, whereas a more complicated mechanism has to be invoked for processes where Fr\"ohlich interaction is responsible.

Raman scattering of the optical vibrational modes in (GaAs)n(AlAs)n superlattices grown on (311)A and (311)B surfaces.

Raman spectra of confined optical phonons in (GaAs${)}_{\mathit{n}}$(AlAs${)}_{\mathit{n}}$ superlattices grown by molecular-beam epitaxy on (311)A, (311)B, and (100) surfaces have been measured in order to study the corrugation of (311) interfaces. Careful analysis of the experimental data reveals no evidence of such a corrugation. The dispersion of optical phonons in both GaAs and AlAs layers has been obtained and was found to be in good agreement with recent published data.

Anisotropy of zone-centre optical phonons in (GaAs)n/(AlAs)m superlattices

We present a study of the anisotropy of zone-centre optical phonons in (GaAs)n/(AlAs)m superlattices by Fourier-transform infrared spectroscopy. We show that the reflection spectra of semiconductor superlattices taken at off-normal incidence of the light contain information about both confined phonons propagating normal to the layers and interface phonons propagating in the plane of the layers. The frequencies of confined and interface modes obtained from reflection spectra are compared with those calculated by dielectric continuum theory.

Barrier height versus confinement efficiency for the optical phonons in GaAs/AlxGa1-xAs heterostructures.

The applicability of the infinite-barrier scheme to the study of the confinement mechanism for the optical phonons in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As heterostructures is critically addressed. In the framework of a bond-charge-model calculation, we demonstrate that the line shape of ${\mathrm{LO}}_{1}$ GaAs-like displacement patterns differs from a sinelike function and depends on the Al content of the ternary barrier, preventing the extension of the infinite-barrier scheme to ternary systems. Finally, the analysis of Raman intensities is indicated as a suitable probe of the confinement efficiency because of its sensitivity to the displacement pattern line shape.

Reflectance study of the oscillator strength of excitons in semiconductor quantum wells.

The oscillator strength of excitons is investigated systematically in lattice-matched GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As, strained-well ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/GaAs, and strained-well GaAs/${\mathrm{GaAs}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{P}}_{\mathit{x}}$ quantum wells (QW's) using Fourier-transform reflectance spectroscopy. The oscillator strength in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As QW's increases as the well width is reduced, in agreement with the existing theory, while those in ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/GaAs and GaAs/${\mathrm{GaAs}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{P}}_{\mathit{x}}$ QW's show a maximum at a certain well width. The well-width dependence of the band nonparabolicity and the overlap of the electron and hole wave functions is responsible for this behavior. The oscillator strengths in ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/GaAs and GaAs/${\mathrm{GaAs}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{P}}_{\mathit{x}}$ QW's are differently dependent on the alloy composition; this is also explained in terms of the band nonparabolicity and the wave-function overlap. The temperature dependence of the oscillator strength in these structures is well described by a modified Debye-Waller expression with an averaged phonon mode. The exciton-phonon interaction, deduced from the temperature dependence of the oscillator strength of the zero-phonon line, is mainly determined by confined phonons in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As QW's, and by quasi-three-dimensional phonons in ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/GaAs QW's.