Andrea Feltrin
In this thesis work, we report scanning near-field optical photoluminescence spectroscopy measurements with at best 200 nm spatial resolution performed on disordered semiconductor V-groove AlGaAs/GaAs quantum wires. In order to interpret the results of these measurements, we develop two different theoretical models. In the first part we study the relationship between disorder and exciton thermalization processes. To do this, we measure the temperature evolution of the excitonic photoluminescence emission lines in near-field spectra at low excitation intensity. The emission peaks in these spectra arise from the radiative recombination of quantum wire excitons localized in potential dips of the confinement potential, due to monolayer width fluctuations along the quantum wire direction. We observe in a non negligible fraction of the measured spectra an increase of the emission line intensities on the low energy side of the spectrum for increasing temperature, which is in contrast with the picture of a thermalized exciton population. In addition to this, the statistical analysis of hundreds of such near-field spectra in terms of autocorrelation functions reveals a shoulder at about 2 meV, which we interpret as a level repulsion feature, related to the energetic and spatial correlation of excitonic states in a disordered system. We develop a theoretical model for the simulation of near-field photoluminescence spectra. In order to do this we model the disordered potential starting from the real interface structure of V-groove quantum wires. The Schr®odinger equation for the exciton is then solved in the center of mass approximation. We calculate the intrinsic radiative recombination rates and the phonon scattering rates, necessary to solve the Boltzmann equation for the exciton population. The statistical analysis of the simulated spectra shows a similar feature in the autocorrelation function. However a good agreement can be obtained only if the diagonal exciton phonon interaction is taken into account, which leads to a modification of the excitonic spectral lineshape and to the appearance of a broad photoluminescence emission background. The temperature dependence of the simulated spectra shows very similar features to the ones observed experimentally. The model for the simulation of steady state near-field photoluminescence spectra is improved in order to simulate time resolved far-field photoluminescence. We study the temperature dependence of the photoluminescence decay time for different disorder configurations and relate it to the exciton population dynamics. We show that at the lowest temperatures the pinning of the decay time at values of several hundreds of picoseconds corresponds to a non thermalized exciton population. On the other hand the square root like temperature dependence, expected in an ideal quantum wire, corresponds to a thermalized exciton population. We also discuss the limits of extracting the intrinsic exciton radiative lifetime from this square like dependence. Taking into account dissociated electron hole states in our model modifies the temperature behaviour, leading to a quasi exponential temperature dependence of the photoluminescence decay time. In the second part, we study the relationship between disorder and biexciton binding energies in quantum wires. We succeed in isolating several extended quantum wire domains, where one single exciton peak dominates the photoluminescence spectrum at low excitation intensity. By increasing the excitation we systematically observe in these large domains the appearance of an additional line in the spectrum, which we attribute to the radiative recombination of the biexciton. The biexcitonic binding energy slightly increases from 2.1 meV for the spatially most extended peaks to 2.3 meV for biexcitons associated to excitons whose localization length is smaller than the experimental resolution. We also observe quantum wire domains with more than one excitonic peak, indicating shorter exciton localization lengths in the order of tens of nanometers. In these domains the appearance of the biexcitonic emission line depends on the number of excitonic peaks observed at low excitation intensity in a near-field photoluminescence spectrum. In spectra containing a few exciton emission lines, biexciton peaks appear associated to each exciton peak. The binding energies show larger variations and range between 2 meV and 2.6 meV. We observe also one biexciton peak appearing on the high energy side of an exciton peak with a negative binding energy of -0.9 meV. On the contrary in spectra containing a larger number of exciton peaks, either only a few additional biexciton peaks appear or no biexcitonic emission lines at all are observed upon increasing the excitation intensity. In order to explain these results we solve the Schr®odinger equation for four one dimensional particles interacting through the Coulomb potential and confined in single particle potential boxes of variable size and barrier height. The results show that the biexciton binding energy first increases as the dip size decreases, reaches a maximum and finally decreases for the smallest sizes as a result of the Coulomb repulsion between the holes in the biexciton. However our model does not account for the disapperance of the biexciton lines, nor for the experimentally measured negative biexciton binding energy.
EPFL2004
