Excitons In Semiconductor Quantum Wells Research Articles

Quantum beats of light hole and heavy hole excitons in quantum wells

We report the first observation of quantum beats due to the interference of the polarization decay of heavy hole and light hole excitons in semiconductor quantum wells.

Temperature-dependent exciton linewidths in semiconductor quantum wells.

The temperature-dependent linewidth of excitons in semiconductor quantum wells due to the interaction of the exciton with LO phonons is studied with use of perturbation theory for the exciton-phonon interaction and by assuming an infinite-barrier-quantum-well model. The interaction is taken to be of the Fr\"ohlich form, and the scattering of the exciton to both bound and scattering states of the interacting electron--heavy-hole system has been taken into account. The dependence of the linewidth on the quantum-well width and on the choice of the heavy-hole mass is discussed, and comparison is made with available experimental data. The effects of the confinement of the optical phonons on the exciton linewidth are also studied and are found not to alter substantively the results for the dependence of the linewidth on the quantum-well width.

Interference Effects on Optical Spectra of Mahan Exciton in Semiconductor Quantum Wells

Optical absorption spectra of modulation-doped n-type semiconductor quantum-well structures are studied theoretically. It is shown that effective Coulomb interaction of a photo-excited hole with the sea of electrons in n=1 conduction-subband via 2D hydrogenic exciton states associated with the unoccupied higher subbands yields an appearance of new subband interference structure on the continuous tail of the Mahan exciton. Numerical results with use of material parameters for GaAs exhibit clearly such effects on the calculated absorption spectra.

Strain-induced lateral confinement of excitons in semiconductor quantum wells

Strain-induced lateral confinement of excitons in semiconductor quantum wells

Strain-induced lateral confinement of excitons in semiconductor quantum wells

We 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.

Binding energies of excitons in semiconductor quantum wells: Quantum subband interference effect.

The set of eigenequations for determining the energy levels and wave functions of Wannier excitons in a semiconductor quantum well is derived by use of Green's-function theory. These equations form a matrix labeled by the quantum-well subband index of the exciton. The coupling between excitons of different subband indices is included for the first time through the nondiagonal elements of the electron-hole Coulomb interaction. We found that this quantum interference effect represents a significant modification to calculated values of exciton binding energies.

Phonon-assisted optical absorption by excitons in semiconductor quantum wells.

Phonon-assisted optical absorption by excitons in semiconductor quantum wells.

Binding energies of wannier excitons in semiconductor quantum wells: Numerical integration of the in-plane radial equation

We solve the ground state and binding energy of a Wannier exciton in a semiconductor quantum well using a new numerical method. By choosing fixed envelope functions for the electron and hole coordinates perpendicular to the quantum well interface, the two-body exciton problem is reduced into a radial eigenequation involving the electron-hole relative motion in an effective inplane Coulomb potential. This radial equation is then integrated numerically. The exciton binding energy is determined from the eigenvalue of this equation. The numerical results are compared with those from using the existing variational theories.

Excitons in semiconductor quantum wells with a small band offset: Exact numerical solution to the envelope function

We use an accurate method to solve the ground state and binding energy of a Wannier exciton in a semiconductor quantum well with a small valence (or conduction) band offset. By choosing fixed envelope function in the conduction (or valence) band quantum well and taking the relative part of the exciton wavefunction in the commonly assumed variational form, a one-dimensional second-order differential equation for the envelope function in the valence (or conduction) band is derived. The exciton intergap energy is then deduced from the eigenvalue of this equation. This approach includes the effect of the additional confinement on the carrier in the valence (or conduction) band quantum well by the electron-hole Coulomb interaction, and has the advantage of being numerically exact in determining the envelope function comparing to the generalized variational approach developed earlier. The application of this method to II–VI compound semiconductor quantum wells is discussed.

Wannier excitons in semiconductor quantum wells with small valence-band offsets: A generalized variational approach.

The ground-state and binding energy of a Wannier exciton in a quantum well with a small valence-band offset are calculated by generalizing the variational approach normally used to study excitons in GaAs/${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As quantum wells. The central issue is to properly include the additional confinement of the hole caused by the electron-hole Coulomb interaction in the direction perpendicular to the quantum-well interface. In addition to the relative motion part of the exciton wave function, the envelope function of the hole in that direction is also determined variationally. The application of this method to II-VI--compound semiconductor quantum wells studied in recent experiments is discussed.

Resonant-Raman scattering from quasi-localized excitons in semiconductor quantum wells

Large polarization-dependent resonance enhancement is observed in first and second order Raman scattering in multiple quantum well GaAs (Al xGa 1−x)As and GaAs AlAs semiconductor heterostructures. Resonance enhancement is much stronger in the forbidden polarization than in the allowed and shows double resonance behaviour at incoming and outgoing photon energies. The resonance is stronger at incoming photon energies for one-phonon scattering and at outgoing photon energies for two-phonon scattering.

Temperature dependent exciton linewidths in quantum wells

Calculations have been made of the temperature dependent linewidths of excitons in semiconductor quantum wells due to coupling to LO phonons. The contributions to the linewidth from exciton scattering to both bound and interacting continuum states have been included. The dependences of the linewidth on the quantum well width and on the choice of heavy hole mass is discussed, and comparison is made with available experimental data.

Electric-field-induced dissociation of excitons in semiconductor quantum wells.

We have calculated the effect of a constant electric field on the energy position of the exciton ground state in semiconductor single-quantum wells. We discuss only the case of quantum wells where electron and hole wave functions are mostly localized within the same layer at zero electric field (e.g., GaAs-${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Al}}_{\mathrm{x}}$As structures). In such quantum wells an electric field applied parallel to the growth axis polarizes free-electron and -hole wave functions in opposite directions and therefore weakens the excitonic binding. Our theoretical results are in qualitative agreement with recent absorption data.

Raman Scattering Resonant with Quasi-Two-Dimensional Excitons in Semiconductor Quantum Wells

Raman scattering resonant with quasi-two-dimensional excitons in GaAs-$({\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x})\mathrm{As}$ heterostructures shows enhancement profiles in agreement with exciton structure measured in other optical spectra. The profiles have characteristic asymmetries in the incoming and outgoing resonances that are specific to the exciton-optical-phonon interaction in quantum wells. Direct evidence of the degree of exciton confinement appears in the Raman spectra.