Phonons In Semiconductor Superlattices Research Articles

Coherant acoustic excitation of nanostructures probed with asynchronous optical sampling

We report the high-sensitivity detection of coherent acoustic excitation in semiconductor heterostructures and metallic nanostructures by using high-speed asynchronous optical sampling. Asynchronous optical sampling is based on two tunable femtosecond Ti:sapphire lasers with slightly different repetition rates close to 1 GHz. This new technique provides the performance of an all-optical oscilloscope for coherent excitations in a pump-probe set-up without any mechanically moving part. A time delay of 1 ns is scanned with a frequency of 10 kHz and a time resolution of 100 fs. Investigations on coherent zone-folded phonons in semiconductor superlattices and nanoscale metallic structures are discussed. For the latter the influence of the substrate on the damping of acoustic excitations is investigated in detail.

Coherent phonons in semiconductor superlattice under DC electrical bias

Resonant pumping of superlattices SLs by femtosecond laser pulses results in the generation of coherent phonons with frequency centred approximately on s/dSL, where s is the speed of longitudinal sound and dSL is the SL period. The phonons can be detected by measuring the changes in reflectance of time-delayed probe pulses. To date, measurements have been made on nominally undoped SLs, and phonon frequencies are typically in the range 100 GHz - 1 THz. The motivation for these studies is that such hypersound could be used for acoustic probing of nanostructures. Here we describe the generation and detection of coherent phonons in a doped and electrically-biased SL. The studied SL consisted of 50×(6 nm GaAs and 4 nm AlAs), uniformly doped with Silicon to density 2×1022 m−3. Pump-probe measurements were made at λ∼770 nm on an optical MESA at T=12K. At zero bias, we observed a similar phonon spectrum as previously observed in comparable but undoped SLs, with a mode at ̃ 450GHz. Under applied bias this mode increased in amplitude, and the decay time was also increased. We discuss these observations in terms of the effects of the bias on the coherent phonon generation and detection process and also the possibility of coherent phonon amplification occurring in the structure.

Decoupling of optical generation and detection of acoustic phonons in semiconductor superlattices

We present a study of the generation and detection of terahertz acoustic phonons in GaAs/AlAs superlattices through femtosecond pump and probe optical pulses, where the spectral contributions of the generation and detection processes are decoupled and identified. The processes are spatially separated in different superlattices grown with an intermediate 1-µm-thick GaAs layer. Pumping and probing occur on opposite superlattices. A thickness gradient present on one of the superlattices allows us to identify the features in the spectra resulting either from the generation or from the detection. A Raman scattering characterization of the sample is also presented.

Generation of Acoustic Phonons in Semiconductor Superlattice in the Case of an Intraband Absorption of Electromagnetic Wave

We obtain the damping coefficient of an acoustic wave for the case of intraband multiphoton absorption of an electromagnetic wave in a superlattice. The ranges in which the acoustic damping coefficient reverses sign are determined for the sound-propagation directions which are transverse and parallel to the superlattice axis. Numerical summation of the series for the acoustic-wave gain is performed for typical parameters of the superlattice. The gain is estimated numerically. It is noted that multiphoton absorption affects the acoustic-wave gain in the superlattice if the field value is much smaller than that in a standard semiconductor.

Interaction of Electrons with Polar Optical Phonons in Semiconductor Superlattices

On the basis of the microscopic theory of lattice dynamics, simulation of the electric potentials created by optical phonons in semiconductor superlattices is performed. It is shown that the spatial distribution of the amplitudes of electric potentials differs from that in a dielectric continuum predicted by the conventional macroscopic model without dispersion. A modified macroscopic continuum theory is proposed that takes into account the dispersion of short-range interatomic forces and allows one to obtain analytical expressions for the potentials of electron-phonon interaction.

Study of coherent folded acoustic phonons in semiconductor superlattices by pump-probe technique

We report on the time-resolved study of coherent folded longitudinal acoustic phonons propagating in GaAs/AlAs superlattices using a two-color pump-probe technique with violet pump and near-IR probe pulses. From comparing the frequencies of the observed coherent phonons with the dispersion relations, we find that the wavevectors of the propagating coherent folded acoustic phonons depend on the wavevector of the probe light: q=2 k probe.

Coherent Zone-Folded Longitudinal Acoustic Phonons in Semiconductor Superlattices: Excitation and Detection

Coherent zone-folded acoustic phonons are excited in GaAsyAlAs superlattices by femtosecond laser pulses via resonant impulsive stimulated Raman scattering in both forward and backward scattering directions. The relative amplitudes of three distinct modes of first and second backfolded order match well with scattering intensities calculated within an elastic continuum model. The detection of the coherent acoustic modes is based on the modulation of the interband transitions via the acoustic deformation potential and exhibits a strong enhancement at interband transitions. [S0031-9007(98)08288-X]

Anisotropy of optical phonons in semiconductor superlattices: Raman scattering experiments

Experiments on Raman scattering in the “forward” geometry, permitting observation of anisotropy of the optical phonons, are performed on specially prepared short-period GaAs/AlAs superlattice structures with the substrates removed and the surfaces covered with an antireflective layer. The experimental data agree well with the computational results obtained for the angular dispersion of optical phonons in superlattices on the basis of a modified continuum model.

Coherent control of acoustic phonons in semiconductor superlattices

Coherent acoustic phonons are generated in GaAs/AlAs superlattices by excitation with femtosecond laser pulses. Several modes of the acoustic phonon spectrum are observed, in agreement with the effect of zone folding in the mini-Brillouin zone of the superlattice. By applying successive pump pulses we are able to silence the first back-folded mode near q=0, while selectively enhancing the coherent amplitude of higher order backfolded modes. This increase in the spectroscopic sensitivity opens the way to detailed time resolved studies of higher order acoustic modes in superlattices.

Study of phonons in semiconductor superlattices by Raman scattering spectroscopy and microscopic model calculation

Raman spectroscopy is used to study phonons in a series of thin (AlAs) m /(GaAs) n superlattices (SLs) grown by molecular beam epitaxy (MBE). The influence of buffer layer type on the interface roughness of heterostructures is carefully evaluated. The accuracy of optical phonons and the degree of peak sharpness of GaAs-like confined modes are examined via off-resonance Raman spectroscopy. Theoretical calculations of phonons in thin (AlAs) m /(GaAs) n superlattices (i.e. samples with m, n ≤ 12) are reported for various directions of propagation by using a rigid-ion model. Optical phonons acquire significant dispersive character when the wavevector q forming an angle θ with the growth axis of the superlattice is changed from θ = 0 to π/2, i.e. from [001] to [100]. The frequency gaps in the angular dispersions due to mode anti-crossing behavior observed recently by Zunke et al. using micro-Raman spectroscopy and studied by a continuum model are found to be in reasonably good agreement with our lattice dynamical model calculations.

Interface broadening and Raman scattering in Si1-xGex/Si superlattices.

The dispersion curves and scattering efficiency of folded longitudinal acoustic phonons in semiconductor superlattices with broadend interfaces have been calculated by solving the elastic wave and diffusion equations simultaneously. The Raman spectra were measured from ${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$/Si superlattices with different interface widths and were analyzed in detail. The energy gap near the edge of the Brillouin zone is observed to shrink with interface width, in quantitative agreement with the theoretical calculation. Although the scattering intensity is a complicated function of superlattice structure, phonon branch index, and its wave vector, the change in intensity with interface width is similar to that predicted by a simpler model. The diffusion coefficient as a function of temperature and the activation energy for interdiffusion are derived by comparing the measured Raman intensities with the calculated results. \textcopyright{} 1996 The American Physical Society.

Phonons in semiconductor superlattices studied by in-plane Raman scattering

Abstract In-plane Raman scattering, via cleaved or polished sample edges, has been applied to GaAs/AlAs superlattices for investigating the angular anisotropy predicted for optical phonons in polar heterostructures. For both extreme short-period and long-period superlattices, the angular dispersion can be described within a simple dielectric continuum model, whereas this model fails for intermediate layer thicknesses. For intermediate thicknesses, mode mixing and effects of bulk phonon dispersion produce frequency gaps in the angular dispersion. For non-polar Si/Ge superlattices, in-plane Raman scattering enables a comprehensive investigation of interface roughness. The experimental features for the mechanical interface mode and the Si-like confined modes are well explained by microscopic calculations assuming two intermixed alloy monolayers at each interface.

The electron-optical phonon interaction in semiconductor microstructures

The description of the interaction of electrons and optical phonons in semiconductor superlattices and quantum well structures requires analytical expressions for atomic displacements and macroscopic electrostatic potentials accompanying the excitation of polar phonons. Recent discussion of this problem has revealed that the conventional macroscopic dielectric continuum theory contains uncertainties giving rise to controversial results even in the long-wavelength limit. The authors demonstrate that the uncertainties can be lifted if the complete branch dispersion of the underlying bulk phonons and boundary conditions derived from microscopic equations of motion are taken into the continuum theory. In the limit of vanishing dispersion the resulting envelopes confirm the predictions of Huang and Zhu as well as of Bechstedt and Gerecke within the unified model.

Phonons in semiconductor superlattices

We present theoretical studies of phonon modes in semiconductor superlattices in a rigid-ion model. We examine the dispersion curves for phonons propagating in any directions, taking into account the long-range Coulomb interaction. We introduce an alternative approach which avoids direct computation of the Coulomb interaction in superlattices. Our calculations indicate that several superlattice optical phonon branches can have different frequencies when → k approaches zero from different directions. Such anisotropic splitting was observed experimentally and was interpreted in a macroscopic model involving layers of different dielectric constants. The predictions of our microscopic model are in qualitative agreement with those of the macroscopic model.

Calculations of phonon spectra in III–V and SiGe superlattices: A tool for structural characterization

This paper reviews some of the basic concepts of phonons in semiconductor superlattices with emphasis on the aspects which are of relevance for characterization.

Phonons in Semiconductor Superlattices

This paper presents a review of the novel features of the vibrational properties of the semiconductor superlattices, as studied mostly by Raman scattering. The concepts of folding and confinement applicable to acoustic and optic phonons and the existence df interface phonocs are brought out. The role of Raman scattering in characterising the structural quality of the superlattices is mentioned.

Proposal for the direct electromagnetic generation of coherent terahertz acoustic phonons in semiconductor superlattices at the University of California, Santa Barbara far-infrared free-electron-laser facility

We discuss the possibility of the generation of coherent packets of monochromatic acoustic waves in the far-infrared frequency range by electromagnetic radiation. The University of California, Santa Barbara free-electron laser could be used to excite resonantly transverse acoustic phonons in periodic space-charge layers formed in modulation-doped GaAs–Ga1−xAlxAs superlattice structures. The conversion efficiency for the process (acoustic power/incident electromagnetic power) is estimated to be of the order of 10−3 at 0.25 THz.

Surface confined phonons in semiconductor superlattices

Surface confined phonons in semiconductor superlattices

Surface confined phonons in semiconductor superlattices

We show that the phonon spectra of finite GaAs/AIAs(001) superlattices along their growth direction display a new type of confined surface modes which split off the superlattice confined optical branches. Their frequency location and decay inside the superlattice is investigated by means of a model calculation and compared with the behaviour of surface states of semi-infinite GaAs.

Resonant quasiconfined optical phonons in semiconductor superlattices.

We point out that resonant phonon modes with quasiconfined behavior may arise in semiconductor superlattices in the continuum frequency range, i.e., where both constituents have allowed bulk frequencies with real wave vector. With reference to the cases of Si/Ge and InAs/GaSb (001) superlattices, we show that such modes appear close to the edge of overlapping optical frequencies, with displacement patterns and Raman strengths comparable to those of true confined modes. However their degree of confinement and their actual number and frequency location are found to be more sensitive to the adjacent layer and to the details of the interfaces. Their study can therefore yield additional structural information on the interface region with respect to the study of true confined modes.