In this paper, we examine a number of factors concerning the relaxation of hot photoexcited electron-hole plasmas in semiconductors. Analytical solutions are utilized to probe the influence of the light-holes on the longer time behavior. The role of the electron-hole interaction and dynamic, self-consistent screening is discussed. Then, the scattering to the satellite L and X valleys is examined in the absence of the inter-carrier interactions. Ensemble Monte Carlo calculations used in this latter approach indicate that the time constant for relaxation of the central valley electrons due to inter-valley scattering cannot be faster than 80-100 fs.

The paper presents a Monte Carlo study of ultrafast phenomena in polar semiconductors. The main focus is given to the analysis of cooling of carriers following subpicosecond laser excitations in GaAs and InP. Excellent agreement is found with time-resolved photoluminescence data. A strong non-equilibrium LO phonon population is found, which at high densities and low temperatures slows down the cooling of the photoexcited carriers. The role of upper valleys in the cooling process is crucial and explains the different behavior found in GaAs versus InP. A discussion of experimentally determined effective temperatures is given.

The role of electron-hole interaction in the ultrafast relaxation of hot photoexcited carriers in GaAs and how it depends on the carrier density and energy of the laser pulse is discussed. The intervalley transfer of carriers photoexcited by a 2.04 laser pulse and their response to 500V/cm field is examined for two excitation levels of 5x10¹⁶ cm^-3 and 1018 cm^-3. It is found that the transfer rates are not affected by e-h or electron-electron interaction at low excitation levels. At high excitation levels the e-h interaction accelerates the return rates to the central valley and provides an important energy loss channel for the electrons. In response to a 500V/cm field, the electrons exhibit very small velocities during the first two picoseconds. At times beyond 4 ps, the velocities are smaller for higher electron densities.

Energy relaxation of hot carriers generated and probed by means of ultrafast laser spectroscopy occurs at very short times. In particular, relevant phenomena seem to occur already in a time interval of the order of 50-100 femtoseconds. Classical transport theory, based on Boltzmann equation, is not adequate for the description of the physical processes that are taking place on this time scale. In the present paper a new Monte Carlo technique, recently developed for the solution of the Liouville equation for quantum transport, is presented and applied to the above problem. The method allows the evaluation of the electronic density matrix as a function of time without any assumptions about the strength and the duration of the phonon collisions and without any arbitrary separation between electron and phonon variables. Results obtained with a simplified model of GaAs show that for times less than or of the order of 0.1 ps energy non-conserving transitions do play a significant role, and the energy relaxation predicted by quantum theory is considerably weaker than that obtained with classical transport theory.

Simulations of electron dynamics in semiconductors during the first half picosecond after optical excitation by a 2-eV laser are reported. Emphasis is given to a comparison between the effects of intervalley phonon scattering, electron-electron scattering and electron-polar optical phonon interactions. The advantages of Monte Carlo simulations for the visualization of the complex processes is stressed.

The transport of highly excited carriers is governed by their energy loss to phonons. There is a special interest in a thorough understanding of this phenomenon in semiconductors, as it can be of help in determining the characteristics of ultrasmall, high field devices. Recently developed steady-state electric-field-induced heating techniques provide a very adequate tool to study hot electrons since the electronic measurement of the power innut to the electron system, together with the optical determination of the electron temperature, gives a direct determination of the nower loss (P) of hot electrons as a function of temperature (T) for a fixed electron density (n).

A novel formalism for treating Bloch electron dynamics and quantum transport in inhomogeneous electric fields of arbitrary strength and time dependence is presented. In this formalism, the electric field is described through the use of the vector potential. This choice of gauge leads to a natural set of basis functions for describing Bloch electron dynamics; in addition, a basis set of localized, electric field-dependent Wannier functions are established and utilized to derive a quantum "Boltzmann equation" which includes explicit band-mixing transients such as effective mass dressing and Zener tunneling. The applications of this formalism to quantum transport in spatially localized inhomogeneous electric fields such as occur in problems involving tunneling through "band-enginereed" tunneling barriers and impurity scattering are discussed.

A general analysis of transient and steady-state velocity fluctuations, diffusion and noise is presented for electrons in GaAs-AlGaAs quantum wells under high-field conditions. Electrons are confined in a square well representing the effective ID potential that arises from the band offset between GaAs and AlGaAs. The analysis of velocity fluctuations is carried out by means of the autocorrelation function, which is directly evaluated from an Ensemble Monte Carlo simulation of the 2D electron gas. In order to compare 2D results with 3D results an Ensemble Monte Carlo program for bulk GaAs has also been used with a physical model and input parameters consistent with the 2D case. From the analysis of the results and the comparison between 2D and 3D data we can study how the noise features are modified in quantum wells both in transient and stationary conditions, and what is the role of interband scattering, a new noise source not present in bulk structures. The effect of initial conditions on the transient of the system is analysed, and the weight of the different sources of noise (thermal, convective, intervalley, intersubband) on the total results is shown for the stationary case.

We report a slow rise of luminescence in GaAs following photoexcitation by 0.3 ps dye laser near 6000 Å. We show that both the near bandgap luminescence (at 1.44 eV) and the integrated luminescence take nearly 10 ps to reach their maximum value. We show that the primary cause of this slow rise is the slow return of electrons from the L to the r valley. By fitting our data with an ensemble Monte Carlo calculation, we determine the T-L deformation potential to be (6.5±1.5) x108 eV/cm. We show that the electrons returning to the I' valley act as a source of heating for the photoexcited plasma. We further show the importance of electron-electron scattering and inadequacy of a simple phonon cascade model, even at a density as low as 5x 10 ¹⁶cm^-3.

Pump-probe spectroscopy is a common method in the study of ultrafast relaxation processes. In the study of the relaxation of hot electrons in semiconductors, there are two possible approaches using this method, each of whici yields different data about the relaxation processes. The first approach probes the relaxation of a population of hot electrons, which has previously been created by photoexcitation, by measuring the saturated absorption of a probe pulse which has a longer or shorter wavelength than the exciting pulse. As the wavelength of the probe is tuned, a picture of how the electrons move down the central valley is obtained.1,2 Some of the electrons, however, may not remain in the central valley as they lose energy but could be scattered to the satellite valleys, at least for short times. Since carriers in the satellite valleys are not connected by a direct optical transition to the valence band at typical photon energies, even an experiment which uses a tunable probe wavelength will not be able to probe all the possible states into which the electrons may scatter. The second approach is complementary to the first. Rather than probe the passage of electrons through states of lower energy than the initial state, the way in which they leave the initial states is determined by measuring the saturated absorption of a probe pulse of the same wavelength as the exciting pulse. A complete picture of the electron relaxation requires the results of both approaches. We have chosen to pursue the second scheme. A point that is often overlooked is that rough numbers for the scattering rates involved already exist, and if time-resolved spectroscopy is to add anything new it must be quantitative. This means, of course, that the data must have a high signal-to-noise ratio. One advantage of the second approach is that the high repetition rate of the pump laser may be put to good use in reducing noise in the data. Because the pump and probe pulses have the same wavelength, they may be generated directly from the output of the laser, and consequently the repetition rate of the experiment is equal to that of the repetition rate of the laser pulses, or about 100MHz. This leads to an increased signal-to-noise ratio as compared with the tunable pump-probe method, in which the pump and probe repetition rates are generally about 10 kHz.

We report the investigation of excited carrier scattering, energy relaxation, and intervalley scattering in GaAs and AlGaAs. Pump and continuum probe absorption saturation measurements provide evidence for femtosecond transient nonthermal carrier distributions and permit a measurement of carrier cooling processes. Measurements performed using a tunable femotsecond laser allow an investigation of intervalley scattering.

We report novel experimental and theoretical results of our investigation of hot carrier dynamics in disordered materials. Femtosecond optical spectroscopy and cw photoluminescence appear to be very promising techniques for the study of carrier relaxation in the extended states and carrier trapping in the weakly localized states. In this paper, we focus on undoped a-Si:H.

We present a theoretical study of optical phonon build-up associated with the ultrafast cooling of highly photo-excited electron-hole plasmas in polar semiconductors. Confining ourselves to the study of high carrier concentrations and subpicosecond laser excitation of bulk GaAs and GaAlAs-GaAs single quantum well structures, we find that rather long optical phonon lifetimes are primarily responsible for the experimental observation of reduced carrier cooling rates. The importance of the carrier-carrier interaction, screening, carrier confinement, and slab modes is discussed.

We have previously reported on the dynamics of coherent longitudinal optical phonons in compound semiconductors and on their interaction with two-component electron-hole plasma. We now compare the results of these investigations with preliminary data on the interaction of the coherent optical phonons with a one-component plasma. We find marked differences in the overall interaction.

Time-resolved Raman scattering has been employed to investigate phonon-phonon interactions in GaAs-AlxGal_xAs multiple quantum well structures. By separately monitoring the growth as well as decay of the non-equilibrium LO phonons created by the relaxing carriers inside each type of layers, information about the hot carrier dynamics such as the population relaxation time of the LO phonons has been obtained. Our experimental results have shown that (1) phonon zone-folding plays little role in the determination of the population relaxation time of GaAs, GaAs-like and AlAs-like LO phonons; (2) hot-phonon effect should also be important in the AlxGal_xAs layers of GaAs-AlxGai_xAs multiple quantum well structures; (3) the average population relaxation time of the LO phonons which are active in hot-phonon effect in GaAs quantum wells is determined to be 8±1ps at T=10K.

The dephasing time of the 1086 cm^-1 mode in calcite has been measured as a function of temperature using a new innovative single shot streak camera technique that measures the phonon dephasing rate in real time. Phonon-phonon interaction models have been proposed and tested to explain the temperature dependence of the experimentally measured dephasing time. It is found that multiphonon splitting and scattering processes involving at least four phonons substantially contribute to the dephasing of the 1086 cm^-1 mode in calcite.

Picosecond Raman scattering is a useful probe of non-equilibrium photo-excited carrier and phonon dynamics in group III-V and group IV semiconductors. In group III-V materials, time-resolved Raman scattering is used to explicitly demonstrate the strong long-range coupling of non-equilibrium free carriers with longitudinal optic phonon modes. This effect is exploited to provide a probe of the non-equilibrium free carrier density. In group IV materials, the absence of long range plasma-lattice interactions enables the generation and observation of non-equilibrium optical phonon modes in the presence of photo-excited plasma densities as high as 1 x 1019 cm^-3. The non-equilibrium phonons in Ge and Gel_xSix alloys exhibit significantly different properties than those observed previously by others in GaAs and Ga1_xA1xAs.

The relaxation rate, 1/τ , of a nonequilibrium population of zone center TO phonons in Ge is obtained by measuring the decay of the intensity of spontaneous anti-Stokes Raman scattering from two picosecond laser pulses as the pulse separation is increased., The relaxation rate 1/τ is the difference between the rate, 1/T1, of the breakup of optical phonons into two or more phonons and the inverse processes which replenish the phonon population. The lifetime contributions to the Raman linewidth, pv is ▵νℓ=1/27πTi. A comparison of the temperature dependence of Δνℓ with Av measured by Menendez and Cardona shows that the line is homogeneously broadened and is the sum of a lifetime and pure dephasing contributions due to scattering of the phonon k vector by isotopic disorder, Δν = Δνℓ +Δvd. The ability to resolve various contributions to the Raman linewidth can clarify the nature of phonon interactions.

The technique of picosecond time resolved Raman scattering in semiconductors is reviewed. With the technique, details of the initial relaxation of optically injected hot carriers in polar semiconductors can be directly studied. Experiments on intrinsic and doped GaAs measure carrier-phonon interaction times, the wavevector dependence of the hot phonon distribution, and the influence of holes on the longitudinal optical phonon lifetime. Studies in AlxGa1-As probe the influence of alloy disorder on the generation of hot phonons. In addition, because there are two optic phonon modes in AlGa1-xAs, we can experimentally measure the effect of ionicity (the frequency difference between longitudinal and transverse optical phonons) on these processes.

A novel laser system and detection scheme is described which has been developed to investigate the transient dynamics of photoexcited electrons at material surfaces and interfaces with photoemission. The excited carrier population on the surface of GaAs (110) and the related Cr/GaAs (110) surface has been studied with 1-2 picosecond time resolution. Studies reveal a rapid rise and fall of the photexcited carrier population at the clean semiconductor surface within 15 picoseconds of excitation. For times greater than 15 picoseconds the carrier density decays slowly. Studies of the photoexcited surface after deposition of small numbers of Cr atoms reveal a remarkable decrease in the carrier density observed at the surface for a coverage as low as .006 monolayer.

ÅOptical second-harmonic studies show that the electronic structure in the top 75 - 130 A of a crystalline Si surface loses cubic order only 150 fsec after the Si is excited by an intense 100 fsec optical pulse. This suggests that atomic disorder can be induced directly by electronic excitation, before the material becomes vibrationally excited. In contrast, the electronic properties of the equilibrium molten phase are not obtained for several hundreds of fsec.

A pump-and-probe technique was used to investigate the shock wave effects on the photoluminescence spectra from GaSe and GaAs semiconductors. Shock waves were generated by focusing intense picosecond laser pulses of the pump beam at 1.064 μm onto an aluminum foil attached to the sample. Under laser driven shock loading, a 24 nm spectral red-shift of the spontaneous emission peak which corresponds to 14 kbar shock pressure was detected in GaSe. Significant line broadening is attributed to the shock-wave-induced collision mechanism. The observed larger red-shift of 36 nm and the intensity decrease of the stimulated emission were explained by the shock-wave-induced band gap shrinkage through the gain reduction mechanism based on exciton-exciton scattering process. In GaAs, the photoluminescence peak was observed to blue-shift and split into two components, corresponding to the transitions from the r6 conduction band to the valence heavy-hole- and light-hole-subbands due to symmetry breaking by the uniaxial shock compression along the [001] direction.

We present a new, purely optical technique for time-of-flight experiments in the picosecond time range using a-Si:H/a-SiNx:H multilayers. We demonstrate the presence of vibrational surface modes in a-Ge:H/a-Si:H multilayers by measuring the acoustic response in real time.

The photoluminescence spectra of CdSxSe1_x doped glasses were found to be strongly dependent on the pumping laser intensity. Two spectral features corresponding to two different recombination mechanisms can be identified. The dynamic behavior can be qualitatively explained by the competition between (A) tunneling-mediated recombination of deeply trapped charges and (B) direct geminate recombination of excitons and nongeminate radiative recombination of free and shallowly trapped carriers and (C) nonradiative recombination. Large (> 30 nm) blueshifts of both peaks were also observed as the laser intensity increased. Additionally, it was found that both the fluorescence spectrum and the recombination lifetime of these microcrystallites could be modified by laser irradiation. Recombination times faster than 10 ps can be achieved in these materials.

Electron-hole recombination processes in quasi-zero dimensional semiconductor microcrystallites embedded in glasses are examined by time-resolved photoluminescence (PL) and degenerate four wave mixing experiments. These two types of experiments provide evidence that quantum confinement effects play important roles in the understanding of ultrafast processes in the microcrystallites in glasses.

Femtosecond nonlinear detection techniques for weak signals are discussed. New methods for the characterization of femtosecond signals, in amplitude and phase, are presented.

Time-resolved photoluminescence is used to study the influence of enhanced intervalley scattering and band mixing on the dynamics of dense electron-hole plasmas created by intense picosecond excitation of Alx Ga1_x As samples with Al concentrations near the direct-to-indirect band gap crossover value. Long-wave-vector scattering in these materials is enhanced by the near-degeneracy of the direct and indirect conduction-band valleys and by the alloy disorder. The relaxation of strict translational symmetry caused by this strong scattering allows the observation of indirect spontaneous emission that is comparable in strength to the direct emission, of zero-phonon indirect transitions, and of stimulated emission from the indirect band gap. Moreover, the strong mixing of the states in the direct and indirect conduction band valleys arising from this scattering causes enhanced band-gap renormalization in nominally direct-gap materials.

The description of the dynamics of nonequilibrium phonons in heterolayers, where the carriers are quasi-two-dimensional (2D), while the equilibrium lattice excitations are, to a first approximation, those of the bulk, require a new theoretical approach. We have derived a kinetic equation for the phonon one-body density matrix, and then transform it to a tractable set of equations by introducing a "phonon wavepacket" representation. Our result removes the spurious dependence on the size of the sample that results when the nonequilibrium phonons are represented in terms of decoupled 3D plane waves. Using this approach we have calculated the relaxation and the transport of carriers in both steady state and time dependent processes. The results show the importanc6 of considering the hot phonon effect, which explains the deviation between the recent experiments and previous theoretical calculations.

Angular and linear momentum of carriers in semiconductors can be oriented 1,2 using polarized light . Over past twenty years, spin alignment and relaxation in bulk samples have been studied by applying various techniques^3-7. The degree of carrier spin polarization depends on the optical matrix elements, selection rules, the symmetry of ^8-12 bands, crystal structure, excitations, relaxation mechanisms and dimensionality. The search for highly spin polarized source for high energy and atomic physics is still in progress. Optical pumping offers the most direct, convenient, and simple method. One of the ways to study spin alignment is to measure the degree of circular polarized photoluminescence. A great deal of work has been done on bulk semiconductors in particular GaAs. Since the energy states and phonon modes in quantum well are altered, the degree of spin polarization and spin relaxation processes in quantum wells should be different and may offer a new highly polarized spin source. At k=0, the valence band degeneracy in a quantum well is lifted and may give rise to higher spin polarization. In this paper, we report for the first time on the degree of spin alignment and spin relaxation of carriers at different energies in quantum wells using time resolved picosecond photoluminescence spectroscopy.

Picosecond photoluminescence experiments at low temperature (6K) have been employed to study the trapping dynamics of photoexcited carriers in GaAs/AlGaAs single quantum wells for different shapes of the AlxGai_xAs confinement layers. We have obtained the following results by analyzing the spectral and temporal distribution of the photoluminescence after picosecond pulse excitation: Trapping efficiency is ==, 40% for a standard ungraded cladding layer (A10.3G1.7As with constant band gap and 5nm thick wells) but increases to ,-, 60% and 100% for samp es with a spatially parabolic or linear band gap profile of the confinement layers, respectively. Trapping times are appreciably shorter than the luminescence risetime which is between 60ps to 100ps. Thus carrier trapping does not impose severe limitations on the modulation speed of single quantum well devices up to frequencies in the order of 10GHz. Similar results are obtained for a well with a width of 1.2nm. Inhomogeneities in the carrier trapping mechanism due to well width fluctuations are not observed in our samples. In the second part we describe the photoluminescence properties of GaAs/A1,Gai_x As quantum wells (x=0.3) under the influence of electric fields perpendicular to the layers. We observe a drastic red shift and a concomitant strong increase of the electron-hole recombination lifetime for well widths > lOnm due to the quantum-confined Stark effect. At high fields (50-100kV/cm) field ionization due to tunneling leads to a decrease of both the photoluminescence yield and decay time, in accordance with a simple WKB theory

The cooling of hot carriers is systematically investigated in undoped, n-doped, and p-doped GaAs/A1GaAs quantum wells of different well widths by time-resolved luminescence spectroscopy. The energy-loss of electrons by scattering with optical phonons is highly reduced at all excitation densities compared to a simple theory of the interactions; for holes, the energy-loss comes close to theory at low excitation density. The energy-loss rates of the carriers are, however, within error independent of dimensionality and well width. The reduction of the energy-loss-rate by optical phonon scattering cannot be explained by screening or degeneracy. It is qualitatively consistent with a hot phonon effect. The energy-loss-rate due to deformation potential scattering with acoustical phonons increases with decreasing well width.

Experimental results of quasi steady state and time-resolved luminescence spectroscopy are reported from highly excited CdTe quantum wells and discussed in terms of the exciton and electron-hole plasma (EHP) phases. Steady state spectra shows the presence of three principal regimes with increasing excitation intensity: exciton gas in the collision dominated regime, exciton-EHP phase transition, and the plasma dominated regime with bandgap renormalization. The phase transition regime has been further investigated by time-resolved spectroscopy which indicates a degree of coexistence for the dense exciton and EHP phases shortly following the excitation by a short laser pulse.

Time correlated single-photon-counting is a high-sensitivity high-resolution spectroscopic method for picosecond photoluminescence which we apply to two important II-VI semiconductors. In crystalline bulk Cd1-xZnxTe we measure lifetimes of impurity-bound excitons and relate them to theory. In Cd1-xMnx-CdTe superlattices we measure lifetimes of confined excitons decreasing with well thickness, which we relate to two-dimensional behavior, and find lifetimes possibly associated with subband energies.

The energy relaxation of photoexcited hot electrons and holes in quantum well structures has been studied extensively using time resolved photoluminescence techniques. The energy loss rates (ELR) for both types of carrier have been systematically measured using undoped and modulation doped structures for a wide range of well widths. It is now well established that the ELR is reduced in low dimensional structures, especially for intense photoexcitation, and this effect has been explained, at least in part, by invoking the presence of nonequilibrium phonons which are generated in the relaxation process. Our measurements show that the ELR is not a strong function of well width for either low or high excitation densities, although in the former case the electron rate is substantially lower than that for holes. For intermediate excitation densities we find a substantial increase in the ELR both in GaAs and GaInAs structures for decreasing well width. Theoretical calculations of the ELR of electrons and holes have been made using a model in which carriers confined to a single subband interact with bulk optical phonons. In wide wells the wavevector to which the carriers couple lies within the plane of confinement. For narrower wells there is an increased coupling to out of plane modes due to the relaxation of momentum conservation. Under these conditions the carriers then couple to a larger number of phonon modes which reduces the nonequilibrium phonon population.

Steady state hot luminescence measurements can be used to determine hot carrier distributions and relaxation rates for extremely fast processes with low injected carrier densities. Luminescence gives a direct measure of the distribution of hot carriers in a semiconductor. We discuss experiments which probe the distribution of electrons high in the conduction band. These measurements have been used in GaAs to determine the LO phonon emission time by a hot electron(100 fs), the F to L intervalley scattering rate (250 fs for an electron initially about 70 meV above the bottom of the L-valleys), and the r to X intervalley scattering rate (30 fs for an electron about 85 meV above the bottom of the X-valleys). Recent experiments which measure the relaxation of hot electrons in the presence of a high density of cold electrons in bulk GaAs and GaAs quantum wells are presented.

We study the transport of the photoexcited quasi-2D electron-hole (e-h) plasma in a p-doped semiconductor quantum well, where electrons are a minority. Using the drifted temperature model for both electrons and holes and introducing a coordinate transformation to the center-of-mass system, separately, for electrons and holes, we obtain a set of coupled equations for the drift velocities and the temperatures of electrons and holes. We show that negative absolute mobility for minority electrons occurs at low temperature and under a weak electric field due to the electron-hole drag. In a strong electric field and at room bath temperature, our results show that the electrons are heated much more than the holes. The electron mobility is smaller in the presence of the hole plasma than in the absence of holes. These results are in agreement with experiments.

Several semiconductor materials were studied using femtosecond pump-probe spectroscopy. Oscillatory structures are observed in the differential transmission spectra of the probe pulse at the very early times when the probe pulse precedes the pump pulse. These oscillations are around the exciton frequency when pumping is either above or below the exciton, and in the vicinity of the pump frequency when the pump is tuned inside the semiconductor band. The observed data are discussed in terms of semiclassical theory.