Exciton dephasing and biexciton binding in CdSe/ZnSe islands

The dephasing of excitons and the formation of biexcitons in self-organized CdSe/ZnSe islands grown by molecular-beam epitaxy is investigated using spectrally resolved four-wave mixing. A distribution of exciton-exciton scattering efficiencies and dephasing times in the range of 0.5–10 ps are observed. This indicates the presence of differently localized exciton states at comparable transition energies. Polarization-dependent measurements identify the formation of biexcitons with a biexciton binding energy of more than four times the bulk value. With decreasing exciton energy, the binding energy slightly increases from 21.5 to 23 meV, while its broadening decreases from 5.5 to 3 meV. This is attributed to a strong three-dimensional confinement with improving shape uniformity for decreasing exciton energy

Excitons, biexcitons, and phonons in ultrathin CdSe/ZnSe quantum structures

The optical properties of CdSe nanostructures grown by migration-enhanced epitaxy of CdSe on ZnSe are studied by time-, energy-, and temperature-dependent photoluminescence and excitation spectroscopy, as well as by polarization-dependent four-wave mixing and two-photon absorption experiments. The nanostructures consist of a coherently strained Zn1−xCdxSe/ZnSe quantum well with embedded islands of higher Cd content with sizes of a few nanometer due to strain-induced CdSe accumulation. The local increase in CdSe concentration results in a strong localization of the excitonic wave function, in an increase in radiative lifetime, and a decrease of the dephasing rate. Local LO-phonon modes caused by the strong modulation of the Cd concentration profile are found in phonon-assisted relaxation processes. Confined biexcitons with large binding energies between 20 and 24 meV are observed, indicating the important role of biexcitons even at room temperature

Localization-enhanced biexciton binding in semiconductors

The influence of excitonic localization on the binding energy of biexcitons is investigated for quasi-three-dimensional and quasi-two-dimensional AlxGa1−xAs structures. An increase of the biexciton binding energy is observed for localization energies comparable to or larger than the free biexciton binding energy. A simple analytical model for localization in the weak confinement regime ascribes the increase to a quenching of the additional kinetic energy of the exciton-exciton motion in the biexciton

Phonon interaction of single excitons and biexcitons

The exciton-phonon coupling of a single zero-dimensional exciton is studied in epitaxially grown CdSe/Zn1−xCdxSe quantum dot structures by analyzing its linewidth and phonon-assisted recombination. The single excitons do not couple to only a single phonon mode, but to a distribution of phonons, here consisting of the zone-center ZnSe longitudinal optic (LO) phonon and of mixed modes with an energy statistics centered between the ZnSe and CdSe LO phonon. Both exciton and corresponding biexciton reveal LO-phonon replica intensities of ∼3% of the respective zero-phonon emission. These similar LO-phonon coupling strengths are evidence of a reduced polarity of the exciton states within the biexciton as the result of exciton-exciton correlation

Temperature-dependent line widths of single excitons and biexcitons

The coupling of single zero-dimensional excitons and biexcitons to acoustic and optical phonons is studied in epitaxially grown Zn1−xCdxSe/ZnSe quantum structures. From the temperature-dependent line width of the exciton photoluminescence (PL) an enhanced coupling to acoustic phonons is found which we explain by the quantum confinement. The LO-phonon coupling is reduced compared to CdSe bulk material. The temperature dependence of the line width of the biexciton PL is similar, indicating a similar coupling strength of the exciton and the exciton–biexciton transition to the crystal lattice

Huge binding energy of localized biexcitons in CdS/ZnS quantum structures

Biexcitons localized in ultrathin CdS/ZnS single quantum wells with fluctuating well thicknesses are investigated by two-photon absorption and single-exciton spectroscopy. For the binding energy of the confined biexciton, a value of ΔEB=38 meV is determined. Comparing ΔEB with the CdS bulk exciton binding energy ERyd=28 meV, the largest known ratio ΔEB/ERyd=1.4 is found. Additionally, a strong enhancement of exchange splitting effects is observed resulting in a linearly polarized exciton doublet and a single, linearly polarized biexciton line