Phonons In GaAs Research Articles

Determination of ionic and pure electronic contributions to the electro-optic coefficient of cadmium telluride and gallium arsenide single crystals

High electro-optic figure of merit n 0 3 r 41 and the absence of natural birefringence make semi-insulating gallium arsenide (GaAs) and cadmium telluride (CdTe) attractive materials for the fabrication of electro-optical devices. In this context we characterized both acoustic and optical phonon contributions to the electro-optic coefficient of CdTe and GaAs. Accurate measurements of n 0 3 r 41 as a function of modulation frequency and temperature were carried out on CdTe:In and GaAs crystal at 1.5 μm. For CdTe, to the best of our knowledge, this is the first time that a contribution to the electro-optic coefficient due to optical phonons has been determined. A positive value for the ionic contribution, due to optical phonons in GaAs, is obtained, in disagreement with that previously inferred from Raman scattering efficiency measurements. The pure electronic contribution was then isolated, and the second-order non-linear optical coefficient was derived. The latter was compared to previously reported data from non-linear wavelength conversion measurements.

Dispersion of the long-wavelength phonons in Ge, Si, GaAs, quartz, and sapphire

We have used the picosecond ultrasonic technique to measure the dispersion of long-wavelength longitudinal acoustic phonons in Ge, Si, GaAs, z-cut quartz, and sapphire. In these experiments, a subpicosecond light pulse was used to generate a strain pulse at one surface of the sample. After propagation through the sample, the shape of this strain pulse was modified because of the phonon dispersion. From this change in shape, the magnitude of the dispersion could be determined. The results are compared with various lattice dynamical models. We also report on measurements of the temperature dependence of the sound velocity and the acoustic attenuation.

Magneto infrared absorption in high electron density GaAs quantum wells.

Magneto infrared absorption measurements have been performed in a highly doped GaAs quantum well which has been lifted off and bonded to a silicon substrate, in order to study the resonant polaron interaction. It is found that the pinning of the cyclotron energy occurs at an energy close to that of the transverse optical phonon of GaAs. This unexpected result is explained by a model taking into account the full dielectric constant of the quantum well.

COMPUTER EXPERIMENTS ON PROPAGATION AND ELASTIC SCATTERING OF BEAMS OF DOWN - CONVERTING PHONONS

We present the program MCAnScat which simulates propagation of beams of down-converting phonons in anisotropic cubic and isotropic media containing point mass defects. It is assumed that the excitation the phonon subsystem is low-level. The program produces time-of-flight spectrograms and phonon energy and quasi-momentum focusing patterns for samples having the form of a rectangular parallelepiped, as well as for cylindric and spherical specimens. Wave vectors of beam phonons belong to body angles ranging from 4π to zero. The axes of corresponding cones are arbitrarily oriented. When down-conversion processes are excluded the outcomes of experiments on initial spatially homogeneous states and on diffusive propagation of phonons are compared with exact results obtained for isotropic and cubic media. As an example of application of MCAnScat we study ballistic and diffusive propagation of beams of phonons in GaAs. The obtained results are in excellent agreement with theoretical and experimental findings.

Optical phonon dynamics of GaAs studied with time-resolved terahertz spectroscopy

We investigate the reflection near the reststrahlen band of the optical phonon in bulk GaAs in the time domain, using time-resolved terahertz spectroscopy. We find that the dynamics of the reflection measured for GaAs differs strongly from the reflection dynamics that would be expected for a TO phonon with a frequency-independent dephasing time.

Coupled bloch-phonon oscillations in semiconductor superlattices

We investigate coherent Bloch oscillations in GaAs/AlxGa1-xAs superlattices with electronic miniband widths larger than the optical phonon energy. In these superlattices the Bloch frequency can be tuned into resonance with the optical phonon. Close to resonance a direct coupling of Bloch oscillations to LO phonons is observed which gives rise to the coherent excitation of LO phonons. The density necessary for driving coherent LO phonons via Bloch oscillations is about 2 orders of magnitude smaller than the density necessary to drive coherent LO phonons in bulk GaAs. The experimental observations are confirmed by the theoretical description of this phenomenon [A.W. Ghosh et al., Phys. Rev. Lett. 85, 1084 (2000)].

Arsenic Oxide Microcrystals in Anodically Processed GaAs Electrochemical Growth, Spectroscopy, and Morphology

Anodic etching of GaAs wafers in concentrated HCl produces many microcrystals (1–50 μm in size) on the GaAs substrate. The etching process and the nature of the luminescence from these GaAs surfaces have been explored in detail. The etch pits are initiated at point defect sites, not extended defects. High HCl concentration, high current density, as well as long etching times facilitate microcrystal growth. Polarization‐dependent Raman spectroscopy identified the microcrystals and accessed symmetry changes of the GaAs surface induced by the electrochemical processing. Raman signals from quantum‐size effect confined phonons in GaAs were not observed. Far‐field photoluminescence of such samples shows a strong emission band centered around 540 nm. Spatially resolved spectroscopy and imaging (cathodoluminescence and near‐field scanning optical microscopy) unambiguously indicate that the 540 nm emission comes from many weak emitters . A few strong emitters originating from amorphous impurity inclusions are also identified in samples prepared in a macor cell. © 2000 The Electrochemical Society. All rights reserved.

Normal incidence Raman scattering enhancement in double-cavity microresonators

We report on the observation of huge enhancements of the Raman scattering efficiency at quasi-backscattering due to double optical resonances in a microcavity. The structure, based on a double-cavity configuration separated by a central mirror, was specifically designed and grown by molecular beam epitaxy to achieve a large splitting, close to the energy of the LO phonon of GaAs, between the two confined photon modes.

Dynamics of internal electric and phonon fields in n-GaAs pumped with ultrashort pulses

We investigate the ultrafast dynamics of an electron-hole plasma coupled to phonons in bulk GaAs excited with femtosecond laser pulses. Our approach is based on balance equations directly derived from the Boltzmann equation within the relaxation-time approximation. Poisson's equation together with a phenomenological driven-harmonic-oscillator equation supplements our description by accounting for time-dependent electric and vibrational fields. Our calculated internal fields show oscillations at frequencies characteristic of those of coupled plasmon-phonon modes. Our results are consistent with recent experimental data.

Anharmonic effects in the phonons of III–V semiconductors: first principles calculations

We present an ab initio calculation of the real part of the phonon self energy for longitudinal and transverse optical Raman phonons in GaAs, AlAs, GaP and InP. The calculated temperature dependences of transverse and longitudinal modes at the Γ-point are in good agreement with available experimental data. This calculation permits us to analyze the different contributions to the real part of the phonon self-energy. In particular we are able to reproduce the observed asymmetry of the Raman line of the transverse optical mode in GaP. Furthermore, we explain why the frequency of this mode changes less with temperature than that of the transverse mode in the other materials investigated. The pressure dependence of the real and the imaginary parts of the phonon self-energy are also discussed.

Optical phonons in nanosize GaAs and AlAs clusters in an InAs matrix

A Raman scattering method is used to investigate structures containing nanosize GaAs and AlAs clusters, which were grown by molecular-beam epitaxy on InAs substrates by the mechanism of self-organized growth under mechanical stress. A large shift of the phonon lines of GaAs and AlAs clusters with respect to the phonon frequencies in the bulk materials (36 and 24 cm−1 for GaAs LO and TO phonons and 55 and 28 cm−1 for AlAs LO and TO phonons, respectively) is observed in the spectra. This fact is explained by the presence of strong mechanical stresses in the GaAs and AlAs clusters. A comparison of the experimental data with the computed strain dependences of the phonon frequencies shows that the GaAs and AlAs clusters are pseudomorphic, i.e., they do not contain dislocations, which lead to relaxation of the mechanical stresses. In the interval between the InAs TO and LO phonon frequencies, the Raman scattering spectra contain features associated with interfacial phonons. The position of these features also attests to the formation of three-dimensional GaAs and AlAs islands and are described well by a continuum dielectric model.

Grating-coupler excited interface phonons in GaAs/AlAs superlattices

The optical phonon modes of short-period GaAs/AlAs superlattices were investigated by means of far-infrared reflection spectroscopy using the grating coupler technique. Grating couplers provide a wavevector transfer parallel to the layers, enabling us to study the in-plane dispersion of the phonon modes in the superlattice structure. The fundamental longitudinal and transverse confined phonons of GaAs and AlAs were detected. For an increasing in-plane wavevector transfer the superlattice phonon modes show dispersion due to interface effects.

Stimulated decay of nonselectively pumped optical phonons in GaAs

We have considered the processes determining the steady-state population of longitudinal optical ~LO! phonons being intensively produced in GaAs by electrons optically excited or accelerated in strong external electric field. The emitted LO phonons pass their energy to acoustic phonon systems through anharmonic decay into longitudinal acoustic ~LA! phonons. The latter emerge as almost monoenergetic and concentrated in a narrow layer in the reciprocal space. Had the occupation numbers of k states within this layer grown to unity, the decay process would have become stimulated, effectively desolating the built-in LO states and giving rise to various nonlinear phenomena. We show that this is actually the case by tracing LA phonon kinetics and calculating the number of phonons in each k state available for feedback. The results obtained substantially clarify the physical picture of energy transfer processes mediating heat removal in GaAs. @S0163-1829~99!02131-1#

A many-body projection approach to quantum-limit magneto-optical transition of two-dimensional electrons confined in a parabolic well

We apply the theory of magneto-optical transitions, introduced earlier by utilizing a many-body projection technique, on a system of electrons scattered by piezoelectric phonons in GaAs; the temperature dependence of the cyclotron resonance linewidth is investigated. It is shown that the width in the quantum limit increases with the temperature.PACS Nos.: 70.00--79.90

Diameter of As clusters in LT-GaAs by Raman spectroscopy

Raman scattering measurements on low temperature GaAs layers are presented. Phonons in both GaAs and As are studied. The transition from diluted As in the GaAs matrix to the small As clusters formed after annealing is analyzed. This is performed by observing the reduction of the GaAs bound charge, i.e., the longitudinal optical GaAs frequency downshift, to the appearance of the crystalline As vibrational mode. From the phonon shifts of crystalline As, the diameter and the strain are derived, accordingly to transmission electron microscopy measurements.

Raman spectroscopy of GaAs and InGaAs under pressure

Strained layers of InGaAs on GaAs have been studied by Raman scattering under high pressure. More accurate values are given for the frequencies of the LO and TO phonons in GaAs and their pressure dependence. The frequency and pressure dependence of the LO GaAs-like phonon in 20% InGaAs is also reported. It is shown that the Raman shift at the phase transition at high pressure is a valuable diagnostic, allowing comparisons of different experiments. The phase transition pressure of the ternary alloy is consistent with linear interpolation between the binary compound values.

Optical properties of the (3 1 1) oriented GaAs/AlAs superlattices and quantum wire-like structures

Optical vibrational modes in the (3 1 1)-oriented GaAs/AlAs superlattices and structures with quantum wires were investigated by the FTIR spectroscopy. A splitting of the fundamental TO vibrational mode localized in the GaAs quantum wire into two ones with a different polarization was observed. The dispersion of the transverse optical GaAs phonons in the (3 1 1) direction was obtained.

Localized optical phonons in GaAs/AlAs superlattices grown on (311)A and (311)B surfaces

A study is reported of optical vibrational modes in [311]-grown GaAs/AlAs superlattices. An analysis of the TO and LO localized modes observed in IR reflectance spectra showed that the difference between the TO and LO mode frequencies in superlattices grown on (311)A and (311)B surfaces is due to the different localization lengths of these modes. The dispersion of transverse optical phonons in GaAs derived from IR reflectance spectra is in a good agreement with Raman scattering data.

Γ to Xzelectron transfer times in type-II GaAs/AlAs superlattices due to emission of confined and interface phonons

We calculate the Γ→ Xzelectron transfer times due to the emission of confined and interface LO phonons in type-II GaAs–AlAs and AlGaAs–AlAs superlattices. A dielectric continuum model is employed to describe the electron–phonon interaction, and the electron envelope wavefunctions are obtained from a Kronig–Penney model. The calculated transfer times are in good agreement with available experimental results. We have used two different sets of AlAs X-valley effective masses obtained from different experiments and we show that the transfer times calculated with the heavier masses are in closer agreement with the measured data.

Electron self-energy in quantum dots

The self-energy of an electron in a quantum dot with two electronic energy levels is calculated in the self-consistent Tamm-Dancoff approximation. The electron is coupled to dispersionless bulk modes of optical phonons of GaAs. The joint spectral density, characterizing the relaxation rate of the electron between the excited and the ground state, is presented. The relation of the results to the issue of the so-called phonon bottleneck, to the width of the spectral lines of the optical transitions, and to the hot-electron relaxation in small quantum dots, is discussed.