Exciton localization and diffusion in low-dimensional nanostructures formed on non-planar substrates

Abstract

Quantum wells (QWs), quantum wires (QWRs) and quantum dots (QDs) are semiconductor heterostructures with nanoscopic dimensions. At this length scale, their properties are governed by quantum mechanics. The interest in these nanostructures is motivated by applications in the domain of electronics and opto-electronics. QWs are widely used in diodes and lasers. QDs now attract much attention because of the ability to precisely tailor their interaction with light. With carrier dimensionality in between, QWRs have been less considered, partly because of their higher sensitivity to disorder and the difficulties in fabricating homogeneous structures approaching ideal electronic and photonic 1D systems. The common technique for fabricating semiconductor quantum nanostructures is epitaxial growth. Metalorganic vapor phase epitaxy (MOVPE) is one of the major implementations of the epitaxial process. One of the exciting aspects of this technique is that one can make use of self-organization mechanisms to control the growth front of a crystalline surface. In this thesis, we make use of these phenomena to improve the homogeneity of GaAs/AlxGa1-xAs QWs and QWRs. In the case of QWs, we investigated the growth of GaAs/AlxGa1-xAs structures on vicinal substrates. Discontinuities of the atomic planes are known to occur on the surface of such structures. We found that the growth mode is very sensitive to the initial miscut angle. It allowed us to create very different configurations of the disorder at the interfaces of QWs. In particular, we determined conditions for which the optical characteristics of QWs grown by MOVPE reached unprecedented quality. Moreover, by using a selective etching technique, we were able to correlate the narrow optical linewidth observed to a step-flow morphology of the hetero-interfaces. In that case, terraces are smooth over the length scale determined by the exciton radius. The GaAs/AlxGa1-xAs QWRs that we studied are formed at the center of V-grooves patterned onto the substrate. A lateral self-ordering mechanism creates a quasi one-dimensional (1D) region with a crescent-shaped cross-section. These structures suffer from strong fluctuations of the interfaces along the QWR axis. We investigated the possibility of reducing the effects of this disorder by decreasing the potential height of the confining barriers. By properly adjusting the growth conditions to grow AlxGa1-xAs barriers with low Al concentration, we obtained a strongly reduced spectral linewidth of the emission. When the probed area is of the order of one micron, the spectral line of the QWR decomposes into two main components, the origin of which are discussed. To improve the homogeneity of V-groove QWRs while retaining a strong confinement, we also investigated the effect of growing these structures on vicinal patterned substrates. We evidenced a strong modification of the relative growth rates of the facets forming the QWRs and a narrower spectral linewidth for QWRs grown on substrates with a large miscut angle. This narrower linewidth offers new possibilities for the study of 1D physics. Finally, we also addressed the question of exciton diffusion in QWRs. This subject has been at the center of important theoretical efforts, but experimental data have been scarce so far. We present a systematic study of the exciton diffusion as a function of the lattice temperature. Using a time-of-flight technique, we have evidenced an activation of the diffusion at intermediate temperatures ( 50 K). We present data suggesting that excitons are localized below this temperature and that the diffusion is determined by interface roughness. In summary, this thesis presents important improvements of the properties of QWs and QWRs grown by MOVPE. Record low spectral linewidths of the emission are obtained for both types of structures. Yet, diffusion measurements in QWRs indicate that the interface roughness is still the dominant factor in limiting the exciton mobility at low temperatures. The growth mechanisms that we evidenced offer new routes for further improvement of the homogeneity of V-groove QWRs.