Indirect Excitons In Coupled Quantum Wells Research Articles

Direct and indirect excitons in semiconductor coupled quantum wells in an applied electric field

An accurate calculation of the exciton ground and excited states in AlGaAs and InGaAs coupled quantum wells (CQWs) in an external electric field is presented. An efficient and straightforward algorithm of solving the Schrodinger equation in real space has been developed and exciton binding energies, oscillator strengths, lifetimes, and absorption spectra are calculated for applied electric fields up to 100 kV/cm. It is found that in symmetric 8-4-8 nm GaAs/Al(0.33)Ga(0.67)As CQW structure, the ground state of the system switches from direct to indirect exciton at approximately 5 kV/cm with dramatic changes of its binding energy and oscillator strength while the bright excited direct-exciton state remains almost unaffected. It is shown that the excitonic lifetime is dominated either by the radiative recombination or by tunneling processes at small/large values of the electric field, respectively. The calculated lifetime of the exciton ground state as a function of the bias voltage is in a quantitative agreement with low-temperature photoluminescence measurements. We have also made freely available a numerical code for calculation of the optical properties of direct and indirect excitons in CQWs in an electric field.

The Crystallization of Indirect Excitons in Coupled QuantumWells

The region of the existence of the crystal phase in two-layersystem of spatially separated electrons (e) and holes (h) in coupledquantum wells at different densities n, temperatures T and interlayer separations D is considered. Foranalysis of the crystal phase the variational calculation of theenergy of the ground state of the system with the account of thepairing between quasiparticles with nonzero momenta is used. Regionsin (T, n, D)-space where electron (or hole)charge density waves (CDW) exist in each layer is obtained. Theseregions contain smaller (T, n, D)-regionswhere crystallization of indirect excitons (or pairs with spatiallyseparated e and h) takes place (this crystallization corresponds tothe existence of electron and hole CDWin-phase).