Carriers In Semiconductor Quantum Wells Research Articles

Realizing Optical Persistent Spin Helix and Stern-Gerlach Deflection in an Anisotropic Liquid Crystal Microcavity.

Spin-orbit interactions which couple the spin of a particle with its momentum degrees of freedom lie at the center of spintronic applications. Of special interest in semiconductor physics are Rashba and Dresselhaus spin-orbit coupling. When equal in strength, the Rashba and Dresselhaus fields result in SU(2) spin rotation symmetry and emergence of the persistent spin helix only investigated for charge carriers in semiconductor quantum wells. Recently, a synthetic Rashba-Dresselhaus Hamiltonian was shown to describe cavity photons confined in a microcavity filled with optically anisotropic liquid crystal. In this Letter, we present a purely optical realization of two types of spin patterns corresponding to the persistent spin helix and the Stern-Gerlach experiment in such a cavity. We show how the symmetry of the Hamiltonian results in spatial oscillations of the spin orientation of photons traveling in the plane of the cavity.

Effect of Coulomb interaction with barrier carriers in the waveguide region on multiple quantum well laser inversion threshold

A mechanism of nonradiative recombination of nonequilibrium carriers in semiconductor quantum wells is suggested and discussed. For a studied Auger recombination process the energy of localized electron-hole pair is transferred to barrier carriers due to Coulomb interaction. It is shown that the Auger coefficient is a weak function of temperature and nonmonotonically depends on the quantum well width.

Auger Recombination in Quantum Well Laser with Participation of Electrons in Waveguide Region

Abstract A new mechanism of nonradiative recombination of nonequilibrium carriers in semiconductor quantum wells is suggested and discussed. For a studied Auger recombination process the energy of localized electron-hole pair is transferred to barrier carriers due to Coulomb interaction. The analysis of the rate and the coefficient of this process is carried out. It is shown, that there exists two processes of thresholdless and quasithreshold types, and thresholdless one is dominant. The coefficient of studied process is a non-monotonous function of quantum well width having maximum in region of narrow quantum wells. Comparison of this process with CHCC process shows that these two processes of nonradiative recombination are competing in narrow quantum wells, but prevail at different quantum well widths.

Polaron mass of charge carriers in semiconductor quantum wells

A theory of the interaction of charge carriers with optical phonons in a quantum well is developed with consideration for interface optical phonons. The dependence of the polaron effective mass on the quantum-well dimensions and dielectric characteristics of barriers is analyzed in detail. It is shown that, in narrow quantum wells, a quasi-two-dimensional polaron can be formed. In this case, however, the interaction parameters are defined by the charge-carrier effective mass in the quantum well and by the frequencies of interface optical phonons. If barriers are made of a nonpolar material, the polaron effective mass depends on the quantum-well width. As the quantum-well width is increased, a new mechanism of enhancement of the electron–phonon interaction develops. The mechanism is implemented, if the optical phonon energy is equal to the energy of one of the electronic transitions. This condition yields an unsteady dependence of the polaron effective mass on the quantum-well width.

Spin-sensitive intersubband dynamics of optically generated carriers in semiconductor quantum wells

A theoretical analysis of the intersubband dynamics in undoped quantum wells is presented where spin-oriented carriers are initially generated by circularly polarized interband excitations. Subsequent resonant intersubband excitations induce Rabi rotations between the subbands resulting in a fast periodic modulation of the spin orientation in each subband. It is investigated whether pure spin modulations can be performed where the charge density of the subbands is kept constant when initially equal occupations of the two lowest conduction subbands with opposite spin orientations have been prepared. While this is the expected behavior for a three-band model of noninteracting particles it turns out that when the Coulomb interaction is taken into account the spin modulations are typically accompanied by corresponding modulations of the subband occupations. It is demonstrated that under realistic conditions it should nevertheless be feasible to realize a pure spin rotation in a given subband provided the intersubband excitations are sufficiently short or the carrier density is sufficiently low. Successive spin rotations are shown to decrease the degree of spin polarization even when spin-relaxation processes are neglected.

Spin-orbit-assisted electron-phonon interaction and the magnetophonon resonance in semiconductor quantum wells

We introduce a spin-orbit assisted electron-phonon (e-ph) coupling mechanism for carriers in semiconductor quantum wells and predict qualitatively the form and shape of anticrossings between the cyclotron resonance (CR) and optical (LO and TO) phonons. Since this e-ph interaction involves the electron spin it makes the predicted TO-CR anticrossing dependent on the filling factor and therefore on the spin polarization of the two-dimensional electron gas in the quantum well.

Relaxation properties of the quantum kinetics of carrier–LO-phonon interaction in quantum wells and quantum dots

The time evolution of optically excited carriers in semiconductor quantum wells and quantum dots is analyzed for their interaction with LO phonons. Both the full two-time Green's function formalism and the one-time approximation provided by the generalized Kadanoff-Baym ansatz are considered, in order to compare their description of relaxation processes. It is shown that the two-time quantum kinetics leads to thermalization in all the examined cases, which is not the case for the one-time approach in the intermediate-coupling regime, even though it provides convergence to a steady state. The thermalization criterion used is the Kubo-Martin-Schwinger condition.

Spatially selective laser cooling of carriers in semiconductor quantum wells

A successive four-step model is proposed for spatially selective laser cooling of carriers in undoped semiconductor quantum wells. The four physical steps include the following processes: (1) cold electrons with nearly zero kinetic energy are initially excited across a bandgap in a coherent and resonant way by using a weak laser field; (2) the induced cold carriers in two different bands are heated via inelastic phonon scattering to higher-energy states above their chemical potentials; (3) the resulting hot electrons and holes radiatively recombine to release photons, thus extracting more power from the quantum well than that acquired during the weak pump process; and (4) hot phonons in two surrounding hot barrier regions thermally diffuse into the central cool quantum well, thereby cooling the entire lattice with time. Based on this model, a thermal-diffusion equation for phonons including source terms from the carrier-phonon inelastic scattering and the thermal radiation received by the lattice from the surrounding environment is derived to study the evolution of the lattice temperature. At the same time, an energy-balance equation is applied to adiabatically find the spatial dependence of the carrier temperature for a given lattice temperature at each moment. There are two interesting findings in thismore » paper. First, a V-shape feature in the carrier temperature is predicted by numerical calculations, which becomes apparent only for initial lattice temperature above 150 K. Second, a thermal-drag of the carrier temperature is found as a result of the strong carrier-phonon scattering. The difference between the lattice and carrier temperatures resulting from the thermal-drag effect is larger in the barrier regions than in the well region.« less

Rate of intersubband transitions of charge carriers in semiconductor quantum wells as a result of their Coulomb interaction

The intersubband scattering of charge carriers in semiconductor quantum wells as a result of their Coulomb interaction has been theoretically investigated. Analytical expressions for the rate of intersubband transitions in the process of electron—electron and electron—hole collisions have been derived in the Born approximation. The theoretical and experimental data on the photoluminescence decay time, obtained for the case of a nondegenerate distribution of charge carriers, were in qualitative agreement.

A near-room-temperature all-optical polarization switch based on the excitation of spin-polarized “virtual” carriers in quantum wells

Near-room-temperature operation of an all-optical polarization switch based on the virtual excitation of spin-polarized carriers in semiconductor quantum wells is demonstrated. The device is shown to exhibit a pulse-width-limited switching time, a contrast ratio of >18dB, an optical bandwidth of ∼3THz, and an energy throughput of >0.1% using a thin (40 wells) GaAs∕AlGaAs sample. The results of differential transmission measurements are used to identify the dominant switching mechanisms and to monitor the spin and temporal dynamics of the carriers excited during the operation of the switch.

Polarization state dynamics induced by the virtual excitation of spin-polarized carriers in semiconductor quantum wells

By using a circularly polarized pulse that is tuned well below resonance, we create a virtual population of spin-polarized coherent excitons in a GaAs/AlGaAs multiple quantum well, and we study its effects on the polarization state of a weak signal pulse using a combination of time-resolved and spectrally resolved ellipsometric techniques. From these data the polarization state contributions to an all-optical polarization switch, which exhibits a ∼415 fs switching time and large contrast ratios, are identified and the performance of the switch is demonstrated.

Recombination kinetics of excess carriers in semiconductor quantum wells

A theoretical investigation of radiative and nonradiative recombination of excess carriers in a semiconductor quantum well (QW), which takes into account the exciton screening effect, is presented. The analytical formula of photoluminescence (PL) lifetime and efficiency of a QW are obtained. Calculations for the case of GaAs/AlGaAs quantum wells show that the variation of PL lifetime and efficiency with temperature is strongly related to material characteristics as well as the detailed growth techniques. When the ratio of the radiative recombination rate to the nonradiative recombination rate is less than unity, there is no characteristic temperature. In the regime of low excitation intensity, the increase of the majority carrier density is helpful to enhance the formation of the electron-hole pairs into excitons. Under higher excitations, the exciton population factor and the PL efficiency increases with increasing excitation intensity. The exciton screening effect depends, in addition to excitation intensity, upon background carrier density.

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.

Tunneling and relaxation of photogenerated carriers in semiconductor quantum wells.

The competition between tunneling (vertical transport) and relaxation (exciton formation) of photogenerated carriers in GaAs-${\mathrm{Al}}_{0.29}$${\mathrm{Ga}}_{0.71}$As multiple quantum wells in an electric field has been studied by means of nonlinear luminescence and photocurrent measurements and time-correlated single-photon counting. The nonlinearity is brought about by an exciton formed by bimolecular processes. The nonlinear luminescence and photocurrent signals are qualitatively explained in terms of the competition between tunneling and relaxation. Furthermore, the heavy-exciton luminescence shows two exponential decays when the electric field is 10--29 kV/cm. The slow component is ascribed to excitons formed by the bimolecular processes as a result of multiple successive processes of tunneling, relaxation, and exciton dissociation. Under the resonant electric field ${F}_{r}$=29 kV/cm, where electron sequential resonant tunneling occurs, the slow component diminishes because of the increase of the electron tunneling rate.

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.

Strain-Induced Lateral Confinement of Carriers in Semiconductor Quantum Wells

ABSTRACTWe describe here an effort to provide lateral confinement of carriers within a continuous InGaAs quantum well by creating a pattern of strain in the well. A compressed InGaAsP layer overlying the quantum well and the InP barrier was patterned into submicron stressor wires by etching to within approximately 20 nm of the InP barrier. The relaxation of the compression at the edges of the quaternary stressors resulted in dilation of the quantum well material under their centers, thus lowering the band gap of the material, providing confinement for both electrons and holes there. We observed a red shift of the quantum well luminescence of 7 meV for 400 nm wide wires, evidence for the strain-induced lateral confinement. This is the first observation of a red-shifted band gap in submicron strain-confining structures.