Nonequilibrium LO Phonons Research Articles

Evolution Equations for the Nonequilibrium LO-Phonon Dynamics in Laser Excited Polar Semiconductors

Starting from an extensire model Hamiltonian a system of evolution equations is derived by means of a time-dependent projection operator technique which describes the nonequilibriurn dynamics of the LO-phonon system in a laser excited polar semiconductor. The carrier system is described in a hydrodynamic stage, whereas the phonon system is treated in an a thermal stage. Third-order anharmonic interactions between optical and acoustic phonons as well as between the acoustic phonons are contained in the theory

Confined-to-propagating transition of LO phonons in GaAs/AlxGa1-xAs superlattices observed by picosecond Raman scattering.

We report the first observation of a confined-to-propagating transition of LO phonons in GaAs/Al{sub {ital x}}Ga{sub 1{minus}{ital x}}As superlattices (SL's). Picosecond Raman scattering was used to investigate the generation rate of nonequilibrium GaAs LO phonons in two series of GaAs/Al{sub {ital x}}Ga{sub 1{minus}{ital x}}As SL's with constant GaAs layer widths ({ital L}{sub {ital z}}) and {ital x}, while the Al{sub {ital x}}Ga{sub 1{minus}{ital x}}As layer widths ({ital L}{sub {ital b}}'s) varied. In both series we have observed a sudden increase in the generation rate of the Raman-active hot phonons as {ital L}{sub {ital b}} decreased over a narrow range. We demonstrate that this abrupt increase is due to the sharp transition from the so-called confined to propagating LO phonons. From our results we have determined the GaAs/LO phonon penetration depth into the Al{sub {ital x}}Ga{sub 1{minus}{ital x}}As layers.

Time-resolved Raman scattering of nonequilibrium LO phonons in GaAs quantum wells.

Time-resolved resonant Raman spectroscopy has been used to study the properties of nonequilibrium GaAs LO phonons generated as a result of the cascade of photoexcited carriers in GaAs quantum wells. The average population relaxation time of these LO phonons, which are responsible for hot-phonon effects in GaAs quantum wells, is directly measured to be 8\ifmmode\pm\else\textpm\fi{}1 ps at $T\ensuremath{\simeq}10$ K. These experimental results should help determine quantitatively and accurately the role hot phonons play in the hot-carrier dynamics of multiple-quantum-well structures.

Subpicosecond Dynamics of the Nonequilibrium Electron–Phonon System in GaAs

AbstractThe relaxation of hot photoexcited electrons and the ultrashort laser pulse transmission under nonequilibrium LO phonon conditions in GaAs are studied by the Monte Carlo technique. It is shown that the electron relaxation from photoexcited levels consists of two stages. The first stage is controlled by the nonequilibrium phonons, and the second one – by intervalley processes. The laser pulse transmission is governed by the first stage.

Localization and wave-vector conservation for optical phonons in AlxGa1-xAs and thin layers of GaAs.

The dependence on the scattering wave vector of a nonequilibrium LO phonon distribution generated by hot-electron relaxation is measured in thick GaAs and ${\mathrm{Al}}_{0.11}$${\mathrm{Ga}}_{0.89}$As layers, and a 500-\AA{} GaAs layer using picosecond Raman scattering in back and forward scattering geometries. The absence of $q=0$ nonequilibrium phonons in the thick samples, and their presence in the 500-\AA{} sample, demonstrates that Raman-active LO phonons in ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ have well-defined wave vectors and are not localized by alloy disorder.

Secondary emission studies of hot carrier relaxation in polar semiconductors

The details of the initial relaxation of optically injected hot carriers in polar semiconductors are studied by Raman scattering from nonequilibrium LO phonons and anti-Stokes hot luminescence. Experiments on intrinsic and doped GaAs reveal carrier-phonon and carrier-carrier interaction times, the wavevector dependence of the hot phonon distribution, and the influence of holes on the LO phonon lifetime. Studies in Al xGa 1−xAs probe the influence of alloy disorder on the generation of hot phonons. In addition, because there are two optic phonon modes in Al xGa 1−xAs, we can experimentally measure the effect of ionicity (the frequency difference between LO and TO phonons) on the generation rate for the hot phonons.

Picosecond Raman scattering from non-equilibrium LO and TO phonons in germanium

The temporal evolution of non-equilibrium LO and TO phonon populations in photo-excited Ge was monitored at 77K and 300K using a picosecond Raman scattering technique. In addition to demonstrating that significant non-equilibrium populations are produced in non-polar semiconductors, the results also indicate that carrier-TO phonon interactions are of similar strength to carrier-LO phonon interactions. The phonon lifetimes and non-equilibrium populations are discussed and compared with data obtained by others in GaAs.

Sub-picosecond Raman spectroscopy and dynamics of nonequilibrium electrons and phonons in semiconductors

Recent progress made in the use of time-resolved pump-probe Raman scattering technique to study the dynamics of photo-excited nonequilibrium carriers and LO-phonons in III–V semiconductors up to the sub-picosecond time scale will be discussed. It will be shown how this technique has allowed direct time-domain measurements of electron-LO-phonon scattering times for “hot” carriers and lifetimes for “hot” LO-phonons in semiconductors like GaAs. The presentation will include new experimental results of Kash, Jha and Tsang on picosecond Raman studies of the Frohlich interaction in alloys like AlxGa1−xAs and InxGa1−xAs. The present theoretical understanding of the dynamics of the highly excited carriers, dominated by strong LO-phonon emission, will be examined along with the discussion of a recent calculation of transient electrical conductivity of such hot carriers, showing extremely interesting oscillations with respect to the pump laser frequency, on the scale of the long wavelength LO-phonon frequency, and a highly nonlinear behaviour as a function of time.

Monte Carlo study of nonequilibrium phonon effects in GaAs

A many-particle Monte Carlo technique for nonequilibrium phonon problems is proposed. The laser induced nonequilibrium LO phonon population in GaAs is calculated. The intervalley coupling constants of GaAs are estimated from the comparison of Monte Carlo calculation and available experimental results.

Population relaxation time of nonequilibrium LO phonons and electron-phonon interactions in GaAs-AlxGa1-xAs multiple-quantum-well structures.

Population relaxation time of nonequilibrium LO phonons and electron-phonon interactions in GaAs-AlxGa1-xAs multiple-quantum-well structures.

Model calculation of the laser-semiconductor interaction in subpicosecond regime

We develop a model calculation to investigate the interaction of a subpicosecond laser pulse with a direct-gap semiconductor. In this problem, carrier distributions are extremely far from equilibrium and nonequilibrium LO phonons are generated at the center of the Brillouin zone. We solve numerically a system of three transport equations, namely one for the conduction band, one for the valence band and one for the LO phonons. Contrary to all previous work, we do not assume that carrier distributions are nondegenerate so that we can investigate the following problems: (a) conditions of internal thermalization in degenerate nonequilibrium plasma (thermalization time, influence of screening on the energy-loss rate, formation of nonequilibrium LO phonons) and (b) absorption saturation and transmission of the pumplaser pulse.

Sub-picosecond time-resolved Raman spectroscopy of LO phonons in GaAs.

Time-dependent spontaneous Raman spectra of optically pumped GaAs have been measured with subpicosecond time resolution. When the optically injected carrier density is less than ${10}^{17}$ ${\mathrm{cm}}^{\ensuremath{-}3}$, we temporally resolve the growth of the optically induced nonequilibrium LO-phonon population and deduce an average electron-phonon scattering time of about 165 fs. For higher injected carrier densities, substantial time-dependent changes are observed in the spectra which result from screening of the LO phonons by the free carriers and the relaxation of the free carriers to the band edges.

Raman Scattering from Nonequilibrium LO Phonons with Picosecond Resolution

A novel technique is described for time-resolved Raman scattering for studying the dynamics of nonequilibrium excitations on a picosecond time scale. The generation and the decay of nonthermal LO phonons in GaAs is measured, and ${\ensuremath{\tau}}^{\ensuremath{'}}=7\ifmmode\pm\else\textpm\fi{}1$ ps is obtained for the relaxation time of the phonon population at 77 K.

Very high non-thermal equilibrium population of optical phonons in GaAs

Extremely high equivalent temperatures of optical phonons were obtained in GaAs without detectable increase in the lattice temperature. The improvement upon previous experiment was due to the use of a pulsed Nd:Yag laser which was doubled by the GaAs sample itself and absorbed in its bulk. The non-equilibrium LO-phonon population attained before crystal breakdown was ∼ 27 times larger than the thermal equilibrium population at the room temperature (290K). The equivalent temperature observed for LO-phonons was ∼3700K.