Excited Carrier Density Research Articles

Ultrafast nonlinear response of high density carriers at silicon surface detected by simultaneous measurements of transient reflecting first and second order diffractions

The simultaneous detections of transient reflectivity (TR), transient reflecting first and second order diffraction signals, at a silicon surface revealed that each signal reflected different physical processes of carrier dynamics under a high pump power of 5 mJ/cm2. It was shown that the second order diffraction could detect a refractive index change which was not linearly dependent on the excited carrier density, and it was suggested that the nonlinearity was caused by many-body interactions among carriers at the band-edge states. The dynamics observed with the second order diffraction corresponded to the recombination of the band-edge carriers. Analysis of the first and second order diffractions in combination with the recently developed spectroscopic detection provided selective information on the ultrafast carrier and heat dynamics for a silicon surface, that is, carrier-phonon scattering, recombination of carriers, heat generation, and diffusion. Additionally, it was shown the TR might allow observation of mixed physical processes detected by the first and second order diffractions and it was suggested that deducing exact physical processes only from the TR signal, especially under high pump power conditions, was difficult.

Separation of screened Coulomb interaction and phase-space filling in exciton bleaching of multiple quantum wells

Excitonic nonlinearity in multiple quantum wells due to many-body interactions consisting of phase-space filling and screened Coulomb interaction is investigated. In particular, we show that the effects of the screened Coulomb interaction and the phase-space filling on the exciton bleaching can be separated for highly excited carrier densities by femtosecond pump pulses. We also find that the phase-space filling effects on the excitonic bleaching are about three times larger than the effects of screened Coulomb interaction and gradually decreases with increasing carrier density within the Mott density.

Temperature and carrier density dependence of Auger recombination in a 3.4 µm InAs/GaSb/AlSb type-II laser device

We report on the temperature and carrier density dependence of non-radiative recombination processes in an InAs/GaSb/InAs type-II W-laser emitting at 3.4 μm. The measurements were performed with a sub-picosecond photoluminescence upconversion set-up in a temperature range between 10 K and 300 K and with initial excited carrier densities in the range between 2.96 × 1018 cm−3 and 4.44 × 1019 cm−3. The excellent growth quality of the device is indicated by a Shockley–Read–Hall coefficient of 2.2 × 108 s−1 at 10 K and 1.1 × 108 s−1 at 300 K. The cubic Auger recombination (AR) coefficient decreases in a characteristic manner with increasing initial excited carrier density. From a convergence equation, we obtained a cubic AR coefficient C03 of 1.2 × 10−28 cm6 s−1 for low carrier densities at 200 K. For low temperatures, due to degenerate carrier population of valence and conduction bands, a sublinear increase of the reciprocal lifetime versus carrier density is measured. With rising temperature the sublinear increase becomes linear and at 300 K a quadratic AR coefficient C02 of 1.73 × 10−11 cm3 s−1 was determined.

Nano-scale Schottky contacts: ultrafast drift-diffusion dynamics studied in the optical near field

We present direct measurements of the ultrafast carrier dynamics around buried nano-scale Schottky contacts with high spatial and time resolution, performed with a novel femtosecond near-field scanning optical microscope. It is shown that high optically excited carrier densities screen the built-in field around a Schottky contact and allow for efficient transport of electrons towards the contact, followed by trapping of electrons into the metal. The experimental results are modeled by a self-consistent treatment of the drift-diffusion equation for the carriers and Poisson's equation for the built-in electric field.

Carrier and field dynamics around nanoscale Schottky contacts investigated by ultrafast near-field optics

We present femtosecond-resolved optical near-field pump-probe measurements of spatiotemporal carrier dynamics around a single nanoscale tungsten (W) disk embedded in GaAs. In these samples, Schottky contacts are formed at the W/GaAs interface. The experimental results are modeled by a selfconsistent treatment of the drift-diffusion equation for the carriers and Poisson's equation for the built-in electric field. At lower optically excited carrier densities, we observe that the built-in field suppresses electron transport towards and trapping into the metal particles. In this regime, an accumulation of carriers is seen at the edge of the depletion region of the Schottky contacts. The calculation reveals that the formation of a self-induced dynamic potential well is the origin of this result. In the high-density regime, efficient carrier transport towards and trapping into the W nanoparticle take place, resulting from the screening of the built-in field. These results allow us to describe measurements of the carrier dynamics in annealed low-temperature grown GaAs and demonstrate that the coupling of the carrier and field dynamics can substantially affect carrier trapping in metal-semiconductor composite materials.

Ultrafast Carrier Thermalization in Hydrogenated Amorphous Silicon

AbstractWe present detailed studies of the initial relaxation processes of photoexcited carriers in hydrogenated amorphous silicon. We have carried out time-resolved measurements of the photoexcited carrier response in HWCVD a-Si:H thin films using a wavelength-resolved femtosecond pump-probe technique, in which an intense 35-fs pump pulse excites carriers in the sample and a time-delayed probe pulse measures the resulting change in optical properties as a function of time delay following the pump pulse. Measurements of the transient optical absorbance were carried out as a function of the density of excited carriers, sample temperature, and probe wavelength. These studies indicate fast carrier thermalization via phonon emission on a ∼ 150 fs time scale and rapid phonon equilibration on a ∼ 230 fs time scale.

Carrier dynamics around nano-scale Schottky contacts: a femtosecond near-field study

We report spatially and time-resolved measurements of ultrafast carrier dynamics around buried nano-scale Schottky contacts, performed with a novel femtosecond near-field scanning optical microscope. The experimental results are modeled by a self-consistent treatment of the drift–diffusion equation for the carriers and Poisson’s equation for the built-in electric field. We show that the built-in field suppresses electron transport towards and trapping into the metal particles at lower optically excited carrier densities. In contrast, efficient electron trapping into the metal occurs at higher electron densities, which screen the built-in field, allowing for efficient transport of electrons towards the Schottky contact.

A picosecond time-resolved photoluminescence microscope with detection at wavelengths greater than 1500 nm

We describe a picosecond resolution time-resolved photoluminescence microscope with high detection sensitivity at wavelengths extending beyond 1500 nm. The instrument performs time-correlated single photon counting using an InGaAs/InP single photon avalanche diode as a detector, and provides temporal resolution of less than 300 ps (full width at half maximum) and spatial resolution down to 4 μm at a sample temperature between 4 and 300 K. Analysis of noise characteristics indicates the ability to measure the excess carrier lifetimes of semiconductor devices with excited carrier densities of less than 1014 cm−3.

Formation Process of High-Field Domain in Superlattices Observed by Photoluminescence Spectra Branch

We report photoluminescence Stark-shift behaviors under various optical excitation intensity for a GaAs/AlAs superlattice. The study clarifies the formation process of the high-field domain by analyzing photoluminescence (PL) spectra. We observed changes in splitting feature of the PL spectra, by varying excitation carrier density near resonance voltages between the Γ states. This clarifies how the inner electric field in superlattices varies by increasing carrier density.

Detection of terahertz radiation from longitudinal optical phonon–plasmon coupling modes in InSb film using an ultrabroadband photoconductive antenna

Terahertz radiation from longitudinal optical (LO) phonon–plasmon coupling modes in InSb films is observed using an ultrafast photoconductive antenna detector. We demonstrate a response frequency of up to 7 THz for a low-temperature-grown GaAs-based photoconductive antenna gated with 25 fs laser pulses. It is found that the emission frequencies of the coupling modes are dependent only on the residual carrier density, not on the excitation carrier density. It is also found that the LO phonon–plasmon oscillations in semiconductors can serve as an efficient THz source.

Time-dependent screening of the carrier-phonon and carrier-carrier interactions in nonequilibrium systems

For a nonequilibrium electron system in semiconductors the time-dependent screened effective interaction due to longitudinal optical phonon and Coulomb scattering is derived. The polarization is described in terms of a time-dependent random-phase approximation. The nonequilibrium electron propagators are expressed in terms of the single-time density matrix by means of the genaralized Kadanoff-Baym approximation. We illustrate the theory for a femtosecond-pulse-excited semiconductor by calculating the resulting two-time-dependent interaction. Using an incomplete Fourier transformation we study the buildup of the phonon-plasmon double resonance in time as a function of the excited carrier density.

Ultrafast electron and lattice dynamics in semiconductors at high excited carrier densities

We directly measure ultrafast changes in the dielectric function of GaAs over the spectral range from the near-IR to the near-UV caused by intense 70-fs laser excitation. The dielectric function reveals the nature of the ultrafast phase transformations generated in the material, including a semiconductor-to-metal transition for the strongest excitations. Although the electron and lattice dynamics are complex when large carrier densities are excited — between 1 and 20% of the valence electrons — the dominant processes and their timescales can be deduced.

Auger recombination dynamics of InxGa1-xSb

The alloy In1-xGaxSb has been identified as potentially an important component in mid-infrared laser diodes which use band structure engineering of quantum structures based on the narrow-gap III-V material InSb. A pump-probe measurement has been made of carrier recombination in bulk In1-xGaxSb, for a range of alloy compositions. Over the range of excited carrier densities (5 × 1016-3 × 1017 cm-3) and at the temperatures (30-300 K) studied experimentally, contributions to the recombination from Auger, Shockley-Read-Hall and radiative mechanisms were calculated using an analytic approximation, with carrier degeneracy included. Excellent agreement with experiment was obtained over the alloy range x = 0.0-0.2 (corresponding to a room-temperature energy gap variation from 0.175 eV to 0.215 eV). Numerically the room-temperature Auger coefficient, C, decreased from the value 1.17 × 1026 cm6 s-1 at x = 0 (i.e. InSb) to 0.98 × 1026 cm6 s-1 at x = 0.2. The fact that C decreases with energy gap increase, in good agreement with theoretical predictions, is important for strained layer quantum well device applications.

Optically induced electric-field domains by bound-to-continuum transitions inn-type multiple quantum wells

We report on the experimental evidence of electric-field domain formation induced by the intersubband photocurrent in $n$-type $\mathrm{GaAs}/{\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ multiple quantum wells. The domain structure manifests itself by a plateaulike regime in the voltage dependence of the total current under infrared illumination. Domain formation is caused by a negative differential field dependence of the photoexcited carrier mean free path. The domain structure does not exist in the dark since the increase of the thermally excited carrier density in the continuum overrides the decrease of the mean free path.

Influence of indentation-forming conditions on the estimation of the photomechanical effect

The anisotropy, spectral dependence, and time dependence of relaxation of the dimensions of microhardness indentations in silicon are investigated. It is shown that the time and spectral dependences correlate with the density of excited carriers in the (defective and elastically deformed) region in which they are excited, and the anisotropy is determined by various types of deformations of the chemical bonds for different positions of the indenter.

Auger Recombination Dynamics in Highly Excited HgCdTe

We present quantitative experimental and theoretical results of Auger recombination in highly excited Hg0.795Cd0.205Te. The first direct measurement of carrier density dependence of the recombination processes has been made on a picosecond timescale, with the pump–probe technique using a free-electron laser. Over the excited carrier density range (5×1016 to 3×1017 cm–3) and at temperatures from 50 to 295 K studied experimentally, contributions from Auger, Shockley-Read-Hall and radiative recombination mechanisms were calculated. The Auger recombination rates were evaluated using a compact analytic form, with carrier degeneracy included, which has been shown to agree closely with more accurate calculations. Excellent agreement was obtained, with Auger-1 dominant at all temperatures, and significantly for T > 225 K when the sample is intrinsic, the Auger-7 contribution was found to be important.

Tunneling Transfer and Energy Relaxation Rate of Photo-Excited Carriers in Coupled Quantum Wells

Tunneling transfer rate between coupled quantum wells/dots and the energy relaxation rate are studied as a function of the density of excited carriers. Both are expected to be determined by the emission rate of LO/LA phonons. It is found that if the carrier density is reduced, the tunneling escape time decreases but the energy relaxation time increases. This suggests that the emission of LO phonons produces non-equilibrium phonons which reduce the effective LO phonon emission rate, but carrier-carrier interaction enhances the scattering rate due to LA phonons. By reducing the size of the tunneling structure, i.e., in a quasi-zero-dimensional structure, the LA phonon emission rate is also reduced.

Role of nonequilibrium dynamical screening in carrier thermalization

We investigate the role played by nonequilibrium dynamical screening in the thermalization of carriers in a simplified two-component two-band model of a semiconductor. The main feature of our approach is the theoretically sound treatment of collisions. We abandon Fermi's Golden rule in favor of a nonequilibrium field theoretic formalism as the former is applicable only in the long-time regime. We also introduce the concept of nonequilibrium dynamical screening. The dephasing of excitonic quantum beats as a result of carrier-carrier scattering is brought out. At low densities it is found that the dephasing times due to carrier-carrier scattering is in picoseconds and not femtoseconds, in agreement with experiments. The polarization dephasing rates are computed as a function of the excited carrier density and it is found that the dephasing rate for carrier-carrier scattering is proportional to the carrier density at ultralow densities. The scaling relation is sublinear at higher densities, which enables a comparison with experiment.

Superradiant emission from Bloch oscillations in semiconductor superlattices.

The superradiant character of submillimeter-wave emission from optically excited electronic Bloch oscillations in a GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As superlattice is investigated by time-resolved terahertz emission spectroscopy. The intensity of the radiation and its decay time are studied as a function of the density of excited charge carriers. From the measured emission efficiency, the spatial amplitude of the charge oscillations is estimated. \textcopyright{} 1996 The American Physical Society.

Suppression of Auger recombination in arsenic-rich InAs1−xSbx strained layer superlattices

Room-temperature pump–probe transmission experiments have been performed on an arsenic-rich InAs/InAs1−xSbx strained layer superlattice (SLS) above the fundamental absorption edge near 10 μm, using a ps far-infrared free-electron laser. Measurements show complete bleaching at the excitation frequency, with recovery times which are found to be strongly dependent on the pump photon energy. At high excited carrier densities, corresponding to high photon energy and interband absorption coefficient, the recombination is dominated by Auger processes. A direct comparison with identical measurements on epilayers of InSb, of comparable room-temperature band gap, shows that the Auger processes have been substantially suppressed in the superlattice case as a result of both the quantum confinement and strain splittings in the SLS structure. In the nondegenerate regime, where the Auger lifetime scales as τ−1aug=C1N2e, a value of C1 some 100 times smaller is obtained for the SLS structure. The results have been interpreted in terms of an 8×8 k⋅p SLS energy band calculation, including the full dispersion for both k in plane and k parallel to the growth direction. This is the strongest example of room-temperature Auger suppression observed to date for these long-wavelength SLS alloy compositions and implies that these SLS materials may be attractive for applications as room-temperature mid-IR diode lasers.