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Concept# Quantum wire

Résumé

In mesoscopic physics, a quantum wire is an electrically conducting wire in which quantum effects influence the transport properties. Usually such effects appear in the dimension of nanometers, so they are also referred to as nanowires.
Quantum effects
If the diameter of a wire is sufficiently small, electrons will experience quantum confinement in the transverse direction. As a result, their transverse energy will be limited to a series of discrete values. One consequence of this quantization is that the classical formula for calculating the electrical resistance of a wire,
: R = \rho \frac{l}{A},
is not valid for quantum wires (where \rho is the material's resistivity, l is the length, and A is the cross-sectional area of the wire).
Instead, an exact calculation of the transverse energies of the confined electrons has to be performed to calculate a wire's resistance. Following from the quantization of electron energy,

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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.

Semiconductor quantum wires (QWRs) and quantum dots (QDs) are nanoscale heterostructures, which form fascinating low-dimensional systems for fundamental studies of quantum-mechanical effects and are attractive candidates for integration into optoelectronic semiconductor devices. With plenty of already established technologies for their fabrication, extensive research efforts are concentrated nowadays on achieving precise control of their position, geometry and heterostructure potential, which provides a tool for tailoring their electron transport and light emission properties. Tuning of the quantum confining potential in a given system offers the possibility of realization of complex nanostructure configurations. In particular, it allows to design and control transfer paths within the surrounding potential landscape, energy relaxation, capture and radiative recombination of charge carriers generated through a non-resonant optical excitation of the QWR-QD structure. This Thesis is devoted to investigation of these processes in tailored-potential GaAs/Alx Ga1−x As QWR-QD structures, which are formed along the vertical axis of pyramidal recesses etched in a (111)B GaAs substrate and regrown with AlGaAs. The reproducibility and controllability characteristic to this system make it particularly suitable for realization of mixed-dimensionality and intermediate-dimensionality 1D-0D structures. An important feature exploited here is the fact that a QWR structure with well-controlled lateral confinement and tunable length can be obtained by a growth of an AlxGa1−xAs/AlyGa1−yAs heterostructure (x < y). The system offers also the possibility of precise bandgap engineering by modification of the chemical composition x during the growth: as shown before, single or multiple QDs can be embedded in the QWR by creating sharp heterointerfaces along the growth direction. In this work, a continuous axial modulation of the QWR potential was realized, which yields novel 1D and quasi-1D systems. The work was started with systematic investigation of carrier dynamics in a QD-QWR system, emphasizing the effect of reduced barrier dimensionality on the efficiency of carrier trapping into the QD. By combining temperature-dependent and time-resolved spectroscopy with theoretical modeling, it was shown that the carrier distribution in the QD and its QWR barriers is determined by different temperature-dependent processes: phonon-assisted capture, limited at low temperatures by diffusion in the 1D barriers (defined by the fraction of the exciton population, which is delocalized) and thermal activation from the QD to the QWR (equivalent to thermalization of carrier population over the available higher-energy states). A V-shaped potential modulation was then used to fabricate QD structures with intermediate (quasi-1D) dimensionality. It was demonstrated that such a potential profile induces additional energy quantization in the vertical direction, which gives rise to multiple confined electron and hole states with dimensionality evolving gradually from 1D to 0D with decreasing energy. The V-shaped potential profile allows also efficient trapping of exciton population at a relatively large cross section and its localization at the center of the structure, where a fully-confined QD state is formed. Pronounced bunching photon statistics and strong temporal correlations between photons emitted from different energy levels were observed, which indicates sequential photon emission from the multiple discrete energy levels. Comprehensive description of exciton dynamics in this multi-level system was provided by a Master-equations model for microstates, which was employed to explain the peculiar features observed both in time-resolved and photon correlation spectroscopy. The axial modulation of the QWR potential was also employed for the realization of QWRs with a linearly graded potential, which introduces a "quasielectric" field driving both electrons and holes in the same direction. Weak linear potential modulation was shown to induce efficient relaxation via the quasi-continuum of excited states, leading to progressive shift of both electron and hole density towards the potential minimum, which resembles the effect of exciton drift. Finally, a graded-QWR structure was realized with two trapping states, localized at the opposite ends of the potential gradient, which were used as probes for directed carrier transport in the graded potential and allowed evaluation of the ambipolar drift mobility in 1D. In general, this work shows that axial modulation of a QWR potential provides a convenient tool for defining the regions of the QWR where the excitons are localized, allowing in this way control over localization effects in disordered 1D systems and efficient trapping into connected QD states.