L. Sorba

Excitonic nonlinearities in wide gap II–VI multiple quantum wells

Abstract We report a comprehensive study of excitonic nonlinearities in Zn 1−x Cd x Se ZnSe multiple quantum wells under optical and electrical excitation. The stability of the heavy-hole exciton is studied as a function of Cd concentration by using pump and probe transmission spectroscopy. The heavy-hole exciton absorption peak is investigated as a function of the intensity of the pump beam until the exciton bleaching is reached. The experimental results are fitted with a theoretical model which includes band-gap renormalization, phase space filling and density dependent lifetime broadening. The correlation of these results with stimulated emission measurements performed also in magnetic field up to 8 T, allowed us to elucidate the role of excitons and free carriers in the lasing process in Zn 1−x Cd x Se ZnSe multiple quantum wells. Electric field induced effects are also studied in order to elucidate the role of quantum confined Stark effect on the optical modulation. Further insight in the nonlinear optical properties of excitons has been obtained by photoluminescence measurements under external bias.

Direct assessment of tunable Schottky barriers by internal photoemission spectroscopy

Al/GaAs(001) junctions in which the Schottky barrier was tuned through fabrication of a pseudomorphic Si interface layer were characterized by internal photoemission spectroscopy. Well-defined photoabsorption onsets corresponding to Schottky barrier heights ranging from 0.3 to 1.1 eV were observed in different devices. Our results point to the possible exploitation of tunable Schottky barriers in metal/semiconductor and metal/semiconductor/metal photon detectors.

Free versus localized exciton recombination in Zn1−xCdxSe/ZnSe multiple quantum wells

Time‐resolved photoluminescence studies of Zn1−xCdxSe/ZnSe multiple quantum wells as a function of photoinjected carrier density show a typical exciton localization dynamics at all injection levels in the high‐x samples (x∼0.2–0.3). On the contrary, the low‐x samples (x∼0.1) exhibit a gradual saturation of the exciton localization process and band‐filling behavior with increasing photoinjection. A rate‐equation model provides a description of the time‐dependent luminescence in all samples and allows a quantitative determination of the concentration of exciton localization centers.

Optical nonlinearities and lasing in II–VI multiple quantum wells

Abstract We investigated optical nonlinearities and stimulated emission in Zn 1− x Cd x Se/ZnSe multiple quantum well structures by using pump and probe transmission and high intensity photoluminescence (PL) measurements with or without magnetic field. We found that the origin of stimulated emission in the investigated samples depends on the exciton stability which can be increased by localization due to compositional and structural parameters. Exciton localization is directly demonstrated by time resolved PL measurements. A complex dynamics between excitons and free-carriers governs the physical mechanisms responsible for stimulated emission. This has been elucidated in detail by a many body theoretical model which has been used to analyze the experimental data based on an equilibrium phase diagram between excitons and free-carriers. Evidence of biexciton recombination is also given, and the stability of such excitonic molecules is investigated in modulation-doped quantum well structures.

Temperature dependent dynamic of excitons in Zn1-xCdxSe/ZnSe multiple quantum wells

The exciton dynamics in Zn1 − xCdxSe/ZnSe multiple quantum wells have been investigated by temperature-dependent time-resolved photoluminescence experiments. A polariton effect (leading to a linearT-dependence of the decay time τPL) and thermal escape of carriers from the quantum well at relatively high temperature (resulting in a decrease of τPLwith the temperature) are observed in samples of different stoichiometry and well.

Recombination mechanisms in photopumped Zn1−xCdxSe/ZnSe multiple quantum well lasers

We investigated exciton stability and the recombination mechanism responsible for lasing in Zn 1-x Cd x Se/ZnSe multiple quantum well structures grown by molecular beam epitaxy. We used photocurrent measurements as a function of sample composition and temperature to gauge exciton stability, and photoluminescence and magnetophotoluminescence spectroscopy to identify the recombination channels associated with stimulated emission. For relatively shallow quantum wells (x∼0.10), the spontaneous emission line shifts diamagnetically with the magnetic field as expected for exciton-related transitions. On the contrary, the stimulated emission exhibits a linear shift with the magnetic field which is consistent with free-carrier recombination from Landau levels