Carriers In Quantum Wells Research Articles

Carrier distribution, spontaneous emission and gain engineering in lasers with nonidentical quantum wells

Novel quantum-well lasers with a 100-/spl Aring/ GaAs quantum-well (QW) (/spl lambda//spl sim/840 nm) and a higher energy 40-/spl Aring/ AlGaInAs QW (/spl lambda//spl sim/810 nm) in a graded Al/sub x/Ga/sub 1-x/As separate confinement heterostructure (SCH) with the bandgap increasing from the p- to the n-side are characterized. A sizeable variation in the QW carrier densities can be achieved as a function of well composition and placement due to the built-in electric field that forces carriers toward the p-side of the SCH and the different densities of states and carrier capture rates of the QW's. Transversely emitted spontaneous emission (SE) is measured through a windowed contact to determine the relative contribution from each QW to the total SE. Information from these measurements is incorporated into a detailed device model to determine the carrier density and evaluate the gain characteristics in a new way. Since the nonidentical wells emit different photon energies, it is shown for the first time that the carrier density can be determined at a specific location within the SCH of a semiconductor laser. By changing the placement of dissimilar QW's and the grading of the SCH, it is found that the gain spectrum can be substantially engineered.

Calculations of the Absorption Coefficient of a Weak Electromagnetic Wave by Free Carriers in Quantum Wells by the Kubo-Mori Method

Analytic expressions for the absorption coefficient of a weak Electromagnetic Wave (EMW) by free carriers for the case of electron-optical phonon scattering in quantum wells are calculated by the Kubo-Mori method in two cases: –The absence of a magnetic field. –The presence of a magnetic field applied perpendicular to its barriers. A different dependence of the absorption coefficient on the temperature T of system, the electromagnetic wave frequency ω, the cyclotron frequency Ω (when a magnetic field is present), and characteristic parameters of a quantum well in comparison with normal semiconductors is obtained. The analytic expressions are numerically evaluated, plotted and discussed for a specific quantum well AlAs/GaAs/AlAs.

Impact ionization of excitons by hot carriers in quantum wells

Dissociation of free excitons into electrons and holes, due to collisions between the excitons and charge carriers accelerated by an in-plane electric field in a multiple quantum well structure, is considered. We found that at normally used 2D excitonic concentrations the impact ionization process is rather strong and dominates over the exciton formation process at electric fields of the order of a few hundred V .

Numerical evaluation of energy loss rate for hot carriers in quantum wells

Using the Fröhlich potential associated with realistic optical phonon modes in quantum well systems, the energy loss rates of hot electrons, holes, and electron–hole pairs are calculated, with special emphasis on the effects of carrier density, hot phonon population, quantum well width, and phonon dispersion on the hot-carrier relaxation process in quasi-two-dimensional systems.

Auger recombination in strained quantum wells

A nonthreshold mechanism for Auger recombination of nonequilibrium carriers in quantum wells with strained layers is investigated theoretically. It is shown that the dependence of the Auger recombination rate on the magnitude of the strain and the height of the heterobarriers for electrons and holes can be analyzed only by calculating the overlap integrals between initial and final particle states microscopically. In quantum wells with strained layers the presence of strain affects qualitatively and quantitatively the electron-hole overlap integral. The dependence of the Auger recombination rate on the quantum well parameters, the magnitude of the stress, and temperature are analyzed for heterostructures based on InGaAsP/InP and InGaAlAs/InP.

TRANSIENT DECAY OF PHOTOEXCITED CARRIERS AND PHOTOLUMINESCENCE EFFICIENCY IN QUANTUM WELLS

We present a phenomenological model for the recombination of photoexcited carriers in quantum wells,which takes into account of the nonradiative recombination and the screening effect of free carriers in the wells.The results show,that the decay time of photoluminescence is strongly related to the sample quality,doping level,as well as the excitation intensity.

Efficient exciton energy transfer between widely separated quantum wells at low temperatures.

Energy transfer between quantum wells is of fundamental interest and also contributes to the dynamical response of devices based on multiple quantum wells. We report the observation of efficient energy transfer at low temperatures between unequal GaAs quantum wells separated by a thick (10--30 nm) ${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As barrier. The transfer efficiency is about 30% for transfer from the narrow well to the wide well (Stokes transfer), about ${10}^{\mathrm{\ensuremath{-}}2}$% for the anti-Stokes transfer, and nearly independent of temperature (2--80 K) and barrier thickness. Tunneling, thermal excitation, and impurity-related transitions cannot explain these observations. We present a calculation for transfer efficiency using F\"orster-type dipole-dipole interaction between excitons and between excitons and free carriers in quantum wells, and show that this dipole-dipole transfer process can reproduce the observed temperature dependence and the magnitudes of the transfer efficiency. This process has not been considered previously for energy transfer between quantum wells. \textcopyright{} 1996 The American Physical Society.

Carrier profile for In0.35Ga0.65As/GaAs multiquantum well lasers from capacitance–voltage measurements

The carrier profile for MBE grown In0.35Ga0.65As/GaAs multiquantum well laser structures with nominally undoped and beryllium-doped active regions was determined by using the capacitance–voltage (C–V) technique at room temperature. A simple theoretical model was used to extract the impurity concentration and the quantum-well carrier density from the experimental profiles. We obtained a high carrier concentration in nominally undoped devices caused by a strong growth temperature dependent Be diffusion from the p-cladding layer, and no difference between doped samples with different nominal dopant location.

Dynamical equilibrium between excitons and free carriers in quantum wells

We have investigated the exciton and free carrier populations dynamics in quantum wells by low temperature time-resolved photoluminescence spectroscopy. When the excitation energy is set above the free carrier quantum well gap, excitons are formed from random binding of electrons and holes. On the basis of this bimolecular formation process, we show that a 2D mass action law, describing the coexistence of free carriers and excitons, explains the time evolution of the exciton photoluminescence linewidth. This demonstrates the existence of a thermodynamic equilibrium between excitons and free carriers at each time delay following the excitation. The consequence is a very short exciton formation time (≲ 10 ps) in the density range investigated (10 9 – 10 10 cm −2), implying a bimolecular formation coefficient γ higher than 14 cm 2s −1.

Capture and emission of carriers in semiconductor quantum wells

The surface capture velocity is introduced as a new parameter which should be used to describe the capture and emission processes of carriers in quantum wells. A calculation of the surface velocity is presented for deformation and polarization types of carrier interaction with optical phonons. The dependence of the capture velocity on the free carrier kinetic energy and/or quantum well parameters is investigated. Application of the theory is illustrated with the capture of holes in Si/Si0.8Ge0.2/Si quantum wells.

Ultrafast relaxation of photoexcited carriers in quantum wells and superlattices

The dynamics of photoexcited carriers in GaAs quantum wells and superlattices has been measured with femtosecond resolution using luminescence as well as pump-probe experiments. The electron capture mechanism shows well defined resonances with a time as short as 500 fs at room temperature for a well thickness of 60 AA. This gives strong evidence for the importance of the quantum mechanical, LO phonon-assisted capture mechanism predicted by theoretical calculations. In type-II superlattices, we show that the transfer of electrons from the GaAs layers to the AlAs layer is mediated by zone edge phonons (both LO and TA) in a way similar to intervalley scattering. The observed times range from 140 fs up to 30 ps depending on the thickness of the GaAs layer.

Hot-phonon reabsorption by free carriers in quantum wells

The influence of a quasi-equilibrium electron-hole plasma on the population lifetime of non-equilibrium longitudinal optic (LO) phonons is studied in GaAs quantum wells using a multiple-beam Raman scattering technique to separately control the phonon generation and free-carrier injection processes. The lifetime of non-equilibrium, small-wavevector LO phonons is found to decrease from 4.5 ps to 1.9 ps as the density of the cold plasma is increased to approximately 3*1011cm-2 by adjusting the intensity of an infrared laser tuned close to the lowest subband gap of the quantum wells.

Free carrier effect on the refractive index change in quantum-well structures

Dielectric response of spatially inhomogeneous free carriers in quantum wells is studied by random phase approximation (RPA) for a high frequency (near infrared) light field. A general form for susceptibilities is derived for a guided optical mode. The analysis shows that a conventional treatment (Drude model) for the TE mode provides a good approximation. The susceptibility for the TM mode is almost the same as that for the TE mode, in spite of quantization of carrier motion, as long as the photon energy is much larger than the intersubband transition energy. The free carrier component of the refractive index change in quantum-well waveguides is thus isotropic near the band gap. Carrier confinement will not reduce the free carrier component in the linewidth enhancement factor around the lasing wavelength.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Experimental studies of proton-implanted GaAs-AlGaAs multiple-quantum-well modulators for low-photocurrent applications

We describe the first attempts to control photocurrent, and thus power dissipation, in surface-normal multiple-quantum-well (MQW) modulators. We have made detailed experimental studies of proton-implanted p-i-n GaAs-Al/sub x/Ga/sub 1-x/As MQW modulators having barrier layers of x=0.3, 0.45, and 1.0. Structures were implanted to levels of 1/spl times/10/sup 12/ cm/sup -2/, 1/spl times/10/sup 13/ cm/sup -2/, and 1/spl times/10/sup 14/ cm/sup -2/. Photocurrent progressively decreased with increasing implant-dose and barrier mole fraction (x). Exciton linewidths showed a strong voltage and implant dose dependence, demonstrating a tradeoff between photocurrent and modulation performance. We obtained our best results with x=1.0 barriers. For example, 1/spl times/10/sup 13/ cm/sup -2/-implanted asymmetric Fabry-Perot modulators were realized in which the optical performance was similar to that of unimplanted devices. The photocurrent responsivity was, however, only 0.007 A/W at 12.5 V bias. We report measurements of carrier lifetime in these materials that show the reduction in photocurrent arises from a reduction in lifetime due to implant-induced damage. In addition, the reduced lifetime decreases the optically-excited quantum-well carrier population, leading to an increase in cw saturation intensity. Specifically, 1/spl times/10/sup 13/ cm/sup -2/-implanted devices with x=1.0 have a saturation intensity of roughly 45 kW/cm/sup 2/, while unimplanted devices have 3.5 kW/cm/sup 2/. Asymmetric self electro-optic effect devices (A-SEED's) are demonstrated, and power dissipation issues associated with the use of low-photocurrent modulators in integrated systems are discussed. >

Inter- and intra-subband relaxation of hot carriers in quantum wells probed by time-resolved Raman spectroscopy

Anti-Stokes Raman scattering has been used to investigate hot carrier dynamics in GaAs/AlGaAs quantum wells and has revealed new information on the physics of the electron-phonon interaction. Measurements of inter-subband relaxation have for the first time revealed sub-picosecond scattering rates. Also, it has been demonstrated that intra-subband relaxation in narrow quantum wells is dominated by the emission of interface modes. Furthermore, the lifetime of LO phonons in quantum wells is found to be smaller than in bulk GaAs.

On the thermoelectric power in quantum well of A 3 II B 2 V semiconductors in the presence of a classically large magnetic field

An attempt is made to study the thermoelectric power of the carriers in quantum well of A 3 II B 2 V semiconductors, taking Cd3As2 quantum well as an example. It is found, on the basis of newly derived 2DE−k s dispersion relation by including various types of anisotropies in the energy spectrum that the thermoelectric power decreases with increasing electron concentration and decreasing film thickness respectively. In addition, the corresponding well-known results for bulk specimens of isotropic parabolic energy bands are also obtained from the expressions derived.

Excitons and deexcitons in a neutral two-dimensional magnetoplasma with a strong population inversion.

Magnetoabsorption and emission by charge carriers in quantum wells under the conditions of a strong population inversion is investigated. The magnetoplasma is assumed to be neutral, the filling factor to be integer for both electrons and holes, and the temperature T=0. It is shown that under these conditions there exist two types of elementary excitations: excitons and deexcitons. The latter consist of a Fermi hole in the conduction band and an electron on an empty level in the valence band. The quasimomentum may be ascribed to deexcitons just as for excitons but the energy of the deexcitons is negative. It is shown that both excitons and deexcitons are stable near the direct Burstein edge (stability path), and a relation between their energies is found. Existence of deexcitons ensures the presence of narrow bands in the emission spectra due to final-state Coulomb interactions.

Photoluminescence from GaAs/AlGaAs quantum wells with InAs monomolecular planes grown by flow-rate modulation epitaxy

GaAs/AlGaAs quantum well (QW) structures with InAs monomolecular planes are fabricated by flow-rate modulation epitaxy. The InAs planes cause an energy shift of photoluminescence (PL) toward the lower energy side, and improve the PL efficiency. This suggests that the InAs planes efficiently localize the carriers in QWs. The PL photon energies of QWs with two InAs planes are lower than those of QWs with one plane, and increase with distance between the two InAs planes. Experimental data are in good agreement with a finite square well model calculation using a valence band discontinuity between strained InAs and GaAs of 40% of the band gap difference, as determined by photoluminescence excitation measurement.

Spin conservation of photocreated carriers in quantum wells in high magnetic fields: A new spectroscopic tool.

The spin relaxation of photocreated carriers in GaAs/GaAlAs quantum wells in high magnetic fields is investigated by luminescence experiments under selective optical excitation. The complete quantization of the two-dimensional energy structure in high magnetic fields implies a simultaneous exchange of energy and momentum for spin relaxation and makes spin-conserving relaxation processes, both in thermalization and recombination, much faster than spin-flip processes. This results in a new spectroscopic tool to identify magnetoexcitonic states with the same electronic spin orientation.

Picosecond imaging of photoexcited carriers in quantum wells: Anomalous lateral confinement at high densities.

We have measured the in-plane motion of photoexcited carriers in semiconductor quantum wells and have discovered several surprising results. The effective diffusivity of the carriers at densities below $n=2\ifmmode\times\else\texttimes\fi{}{10}^{11}$ ${\mathrm{cm}}^{\ensuremath{-}2}$ is found to depend upon the excitation level, possibly indicating defect-limited diffusion or phonon-wind effects. Above this density the spatial profiles exhibit two distinct components with widely differing diffusivities. We postulate that the slowly diffusing component represents carriers which are "thermally confined" to a phonon hot spot.