Molecular beam epitaxy growth of aluminum gallium nitride/gallium nitride quantum wells and investigation of excitonic and intersubband transitions

This dissertation addresses the epitaxial growth, structural and optoelectronic properties, and novel device applications of AlxGa 1-xN/GaN multiple quantum wells. The investigated structures were grown both heteroepitaxially and homoepitaxially by plasma-assisted molecular beam epitaxy. Structural properties were determined by high resolution x-ray diffraction, scanning electron microscopy and atomic force microscopy. Excitonic recombination and absorption was investigated by a combination of photoluminescence, lateral photocurrent spectroscopy and electroabsorption spectroscopy.; It was found that the excitonic properties were strongly affected by structural disorder due to the inherent randomness of the epitaxial processes. The distribution of excitons among the disorder-induced band-tail states was determined to crossover from a thermal quasi-equilibrium distribution at high temperatures to a nonthermal distribution at low temperatures. The inhomogeneous broadening parameter of the excitonic density of states and the average exciton localization energy were determined systematically as a function of well width. Photoluminescence spectra were observed to be Stokes shifted with respect to optical absorption. The AlGaN/GaN multiple quantum wells were modeled numerically using a single band effective mass approximation, taking polarization effects into account, and an estimation of the interface roughness was calculated.; These quantum wells were then used to address two novel technological applications. The first of these was an ultraviolet electroabsorption modulator. In this device, an enhancement of the exciton-photon coupling was achieved by applying a bias across the active region such that the polarization-induced electric fields were partially cancelled, thus increasing the wave function overlap between electrons and holes. A change in the absorption coefficient of 4 x 104 cm-1 was obtained, which was around one order of magnitude greater than similar devices based on III-arsenide and III-phosphide materials. The second application was the growth of AlN/GaN quantum wells for near infrared optoelectronic devices based on intersubband transitions. The effect on the intersubband absorption spectra of varying the main multiple quantum well design parameters was determined. Despite the well-known difficulty in doping bulk AlN, intersubband absorption was measured for the case in which the AlN barrier layers were doped, a result attributed to the efficient field-ionization of donors due to polarization effects.