Optical spectroscopy study of carriers recombination in quantum wires
We report the optical properties of high quality V-groove GaAs/Al0.3Ga0.7.As quantum wires (QWRs) with different thicknesses of the GaAs layer. The systematic investigation of photoluminescence (PL) and photoluminescence excitation (PLE) spectra as a function of the wire size combined with model calculations allows us to study in detail the effect of two-dimensional quantum confinement on valence band mixing and polarization anisotropy. We show that the polarization anisotropy of PL spectra critically depends on localization effects thus making any polarization analysis based on extended states insufficient at low temperature. The influence of exciton localization and surface corrugation on PLE spectra is also clarified; the observed large polarization anisotropy is unambiguously related to the one-dimensional (1D) character of our QWRs. Comparison of the experimental results obtained for all three QWR samples with theoretical predictions is clearly compatible with a strong suppression of the 1D band-edge singularity in PLE spectra. The temperature dependence of PL and PLE spectra for QWRs of different thicknesses is investigated. We are able to measure photoluminescence up to room temperature due to high quantum efficiency. Additionally, large subband spacings and small inhomogeneous broadenings allow us to perform PLE measurements up to high temperature (100 – 200 K, depending on the wire size). A thermally-enhanced transfer of carriers from the side-QWs into the QWRs is evidenced and attributed to increasing 2D exciton mobilities with increasing temperature and to the combination of classical activation and tunneling through the potential barrier due to the "necking" in the side quantum Wells (QWs) next to the QWRs. A strong impact of disorder on the PL (PLE) peak position and on the line shape is clearly shown. The evolution of the PL and PLE peak positions with increasing temperature together with a line shape analysis of PL spectra indicates the dominance of excitonic effects over the whole temperature range. The enhancement of the lowest-energy PLE peak with respect to higher-energy transitions with increasing temperature is explained in terms of an effective mobility edge. The variation of the full width at half maximum of PL and PLE spectra with increasing temperature is attributed to the combined effects of structural disorder and exciton-phonon scattering. We measure PL spectra of our high-quality QWRs through holes which vary from 2.0 μm clown to 0.2 μm in width (micro-PL experiments). For sufficiently thin QWRs it is possible to observe directly the localization of 1D excitons in local potential minima. PL spectra detected through the smallest apertures (0.2 μm) are attributed to recombination of excitons in isolated quantum dot (QD)-like potentials wells. Homogeneously broadened lines can be resolved. The correlation between structural disorder and micro-PL features is studied. We determine the most realistic wire morphology as inferred from studios on planar and non-planar A1GaAs and GaAs layers by transmission electron and atomic force microscopy. A localization potential due to fluctuations of the wire cross-section along the wire axis is estimated and used to evaluate the impact of disorder on the optical properties. The effect of exciton-exciton interactions on the PL properties of localized excitons is evidenced. A shift to high energy of the exciton resonance at exciton densities much below the Mott density is attributed to the predominantly repulsive interaction between excitons due to the Pauli exclusion principle. The formation of multi-excitonic states in the dot-like structure is also evidenced by the evolution of the finely structured PL spectrum with increasing excitation density. We also investigate the diffusion of 1D excitons as a function of temperature (5 < T < 80 K) and wire size. The longitudinal motion of the excitons can be well described by isothermal diffusion. A strong dependence of the diffusivity on temperature and wire size is observed. The increase of the exciton diffusivity with increasing temperature is attributed to the preponderant role of scattering by the interface roughness. The impact of the 1D nature of the excitons on the diffusion process is shown to appear at temperatures (corresponding to a clear delocalization of the carriers (T > 60 K): In this temperature range, larger diffusivities are found for thinner wires. This result, in striking contrast with similar investigations in 2D systems, is explained by the strong suppression of elastic scattering in 1D structures due to the reduced phase space and by the wire-size dependent contribution of intersubband scattering. Finally, a decrease of the diffusivity with increasing excitation power density is explained in terms of exciton-exciton interactions.