Interwell excitons in GaAs superlattices

The formation of spatially indirect excitons in superlattices with narrow minibands is investigated experimentally. The interwell exciton is similar to the first Wannier–Stark localized exciton of an electrically biased superlattice. However, in the present case the localization is mediated by the Coulomb interaction of the electron and the hole without external fields.     

Dynamical conductivity of type-I semiconductor superlattices

We calculated longitudinal and transverse macroscopic as well as microscopic dynamical conductivity for a modulation-doped type-I superlattice. Our computed long wavelength macroscopic conductivity significantly differs from Drude conductivity in the low-frequency regime (microwave and infrared radiations). Macroscopic conductivity shows oscillatory behaviour along the direction of growth of the superlattice. Propagation of transverse electromagnetic waves in a superlattice has been studied for all possible values of frequency and wavevector. It is found that microscopic transverse conductivity exhibits poles along both real and imaginary axes of frequency. Depending on the values of wavevector components, along and perpendicular to the direction of the superlattice, both poles can lie on real or imaginary axes of frequency. We also find that there can be more than one penetration depth for a superlattice and one of them decreases with frequency for frequencies below the microwave regime.     

Spectrum of two-dimensional excitons in heterojunction superlattices

The Keldysh model of a two-dimensionally confined exciton in a heterojunction superlattice, which corresponds to a maximal deviation from the isotropic semiconductor, is analyzed and solved numerically. The results suggest that the high angular momentum provides a reliable diagnostic of the dimensionality of the exciton, even when details of the potential are not known.     

Optical properties of excitons in semiconductor superlattices and microcavities

The optical response of Mott–Wannier excitons is investigated in semiconductor superlattices and microcavities. p-Polarized light is considered to calculate the reflectivity R_pand dispersion relation of the collective normal modes in superlattices accounting for extrinsic Morse potential wells, andR_p in microcavities. Results of R_pexhibit well-defined peaks of the exciton bound states in the Morse potentials for both transverse and longitudinal modes. Comparisons ofR_p with experimental reflectivity data of light for semiconductor microcavities exhibit good qualitative agreement as well as Rabi splitting.     

Resonant optical orientation and alignment of excitons in superlattices

Exciton states in short-period GaAs/AlGaAs superlattices have been studied by optical orientation and optical dipole-moment alignment methods. The effect of magnetic field in the Faraday and Voigt geometries on the degree of linear and circular polarization of photoluminescence have been studied under resonant and nonresonant excitation. The constants of electron-hole exchange interaction in the exciton have been determined. Fiz. Tverd. Tela (St. Petersburg) 40, 2229–2235 (December 1998)     

Carbon acceptor luminescence in type-I GaAs/AlAs ultrathin-layer superlattices

The luminescence associated with residual carbon acceptors in type-I (direct-gap) ultrathin-layer superlattices (UTLS) with well and barrier widths of 22.7<L _z<25.2 Å and 11.5<L _b<14.0 Å, respectively, is composed two lines reflecting the on-center and on-edge state of the impurities. In these narrow wells the on-center acceptor binding energy increases to 60 meV in agreement with theoretical calculations for GaAs single quantum wells using a valence band offset of 500 meV. While the binding energy of the on-center state does not vary significantly within the studied L _z and L _b range, the on-edge state shows a strong dependence on the very narrow barrier width. This increase of the acceptor binding energy makes the energy position of the impurity-related luminescence in UTLS very sensitive to the actual barrier height. Investigation of the impurity-related luminescence thus provides a versatile tool to determine the band offset ratio at the heterojunction.     

Interwell excitons in GaAs multiple quantum wells and superlattices

Il Nuovo Cimento D - The optical properties of multiple quantum wells in the transition from isolated wells to a superlattice are investigated theoretically and experimentally. For superlattices...     

Variational tight-binding theory of excitons in compositionally modified semiconductor superlattices

We present results for the binding energy of an exciton formed when an electron–hole pair is photoexcited within a single, compositionally modified layer of a semiconductor superlattice, for example by adding a small percentage of In atoms to a single GaAs layer of a GaAs/AlGaAs system. Such a system could serve as the basis for spatially-selective photoexcitation, a process whereby a laser pulse would create electron–heavy-hole pairs exclusively in the modified layer. We first derive an effective one-dimensional (1D) Hamiltonian for an electron, by averaging the 3D electron–hole Hamiltonian using a one-parameter trial wavefunction, which is dependent on the in-plane relative coordinates, as well as a normalized Wannier orbital for a single hole. The exciton binding energy is then obtained by computing the lowest bound-state energy of the effective 1D electron Hamiltonian in the nearest-neighbor tight-binding approximation. As a demonstration of the effectiveness of our approach, we find that for periodic superlattices our results for the exciton binding energy are in very good agreement both with experiment and the results of other theoretical calculations.     

The effects of photoexcitation on excitons in semiconductor doping superlattices

Calculations of the properties of excitons in doping superlattices have been made as a function of doping density using a variational approach. Interesting new features are obtained when the exciton energy becomes comparable to the superlattice potential energy. These results are compared to recent experimental data on GaAs doping superlattices.     

Exchange interactions of excitons localized at opposite interfaces in type-II GaAs/AlAs superlattices: Optically detected magnetic resonance and level anticrossing

Two types of excitons, localized at opposite interfaces and characterized by different magnitudes of the exchange interactions at the same radiation energies, are simultaneously in type-II GaAs/AlAs superlattices. It is shown that the additional long-wavelength luminescence line in superlattices grown with growth interruptions after the GaAs layers is due to the recombination of an exciton localized at an inverted interface in regions where the quantum-well width is increased by one monolayer. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 10, 701–706 (25 November 1996)     

Photoreflectance study of folded above-barrier states in (InAs)1/(GaAs)m strained-layer superlattices

We have performed a photoreflectance study of the above-barrier states in (InAs)₁/(GaAs)_m strained-layer superlattices (m=10 and 30 monolayers) grown on a (001) GaAs substrate by molecular-beam epitaxy. We have clearly observed the optical transitions associated with the above-barrier states at the Γ and π (mini-Brillouin-zone edge) points. The layer-thickness dependence of the transition energies is explained by the zone-folding effect on the above-barrier states based on a simple Kronig-Penney analysis taking account of the lattice-misfit strain effects.     

Exciton-exciton collisions and conversion of interwell excitons in GaAs/AlGaAs superlattices

The ratio of the densities of intra-and interwell excitons in a symmetric system of coupled quantum wells — a superlattice based on a GaAs/AlGaAs heterostructure — is investigated over a wide range of optical excitation power densities. Conversion of interwell excitons into intrawell excitons as a result of exciton-exciton collisions is observed at high exciton densities. Direct evidence for such a conversion mechanism is the square-root dependence of the interwell exciton density on the optical excitation level. The decrease in the lifetime of interwell excitons with increasing excitation density, as measured directly by time-resolved spectroscopy methods, confirms the explanation proposed for the effect. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 8, 623–628 (25 April 1997)