1 Optoelectronics, Department of Photonics Engineering, Technical University of Denmark2 Department of Photonics Engineering, Technical University of Denmark3 Nanophotonics, Department of Photonics Engineering, Technical University of Denmark4 Department of Micro- and Nanotechnology, Technical University of Denmark
In this thesis the realization of semiconductor nanostructures in the InAlGaAs material system with molecular beam epitaxy (MBE) is described, as well as the characterization of their optical properties. First, the growth conditions used for different materials and surfaces are given, and the general capabilities of the MBE-systems are demonstrated, with respect to growth of structures with varying thicknesses/alloy compositions, and the synthesis of alloys, so-called digital alloying. In the first main part of the thesis a group of low-dimensional structures are described, the so-called quantum wells, wires and dots. For quantum wells in the InAlGaAs material system, a detailed analysis is presented of the influence of surface segregation during growth, and it is shown how the measured energy levels and linewidths may be calculated with very high precision. Furthermore, the limits of strain in multi quantum wells are deduced, that determine when dislocations will be formed. It is also demonstrated how T-shaped quantum wires with enhanced confinement energies are realized by overgrowing InAlGaAs quantum wells with a GaAs quantum well. Finally, the growth of quantum dots in both InAs, InGaAs and InAlGaAs is decribed, and it is shown how structures with very uniform quantum dots at energies near the visible red part of the spectrum may be realized. The second main part of the thesis deals with the growth of optical microcavities, where the light is strongly interacting with a quantum well and so-called polariton resonances are formed. It is shown how particularly narrow, tunable resonances may be achieved, by an combination of a low cavity energy gradient across the samples and a narrow exciton resonance in a broad quantum well where the density of free carriers is particularly low. Next, the polariton energies and linewidths are measured and analyed as a function of detuning and temperature. It is demonstrated that a coupled-harmonic oscillator model yields a good agreement with the energies, but the analysis of the linewidths requires that the absorption in the quantum well is also taken into account, which is demonstrated in a microcavity with a reduced light-matter interaction. For the polariton with the lowest eigenenergy, it is shown that the probability for scattering on lattice vibrations or free carriers is reduced. Finally, the secondary emission from a microcavity is measured and analysed, where a good qualitative agreement with theories for Rayleigh scattering is found, ant he so-called polariton bottleneck is observed.