Thesis presented January 27, 2015

Abstract:
This work reports on the design, epitaxial growth, and the structural, and optical characterization of two types of nanostructures, namely AlGaN/AlN Stranski-Krastanov quantum dots (SK-QD) and AlGaN/AlN nanodisks (NDs) on GaN nanowires (NWs). These nanostructures were grown using plasma-assisted molecular beam epitaxy (PA-MBE) and were conceived to be the active media of electron-pumped ultraviolet (EPUV) emitters for water purification, operating in mid-ultraviolet range. The peak emission wavelength of three-dimensional SK-QD can be tuned in mid-ultraviolet range while keeping high internal quantum efficiency (IQE > 35%) by modifying the Al composition and the QD geometry. The efficient carrier confinement was confirmed by the stability of the photoluminescence intensity and decay time with temperature. The optimal deposited amount of AlGaN in AlGaN/AlN QDs which grants maximum luminescence at room temperature was determined by finding a compromise between the designs providing maximum IQE and maximum QD density. The effect of the variation of the QD height/base-diameter ratio on the interband and intraband optical properties was explored by fitting the experimental data with three-dimensional calculations of the band diagram and quantum levels. Regarding AlGaN/AlN NDs on GaN NWs, the Al-Ga intermixing at Al(Ga)N/GaN interfaces and the alloy inhomogeneity in AlGaN/AlN NDs are attributed to the strain relaxation process. This interpretation was proved by correlation of experimental data with three-dimensional strain distribution calculations performed on structures that imitate the real growth sequence. Despite the challenge of inhomogeneity, the emission wavelength of AlGaN/AlN NDs can be tuned in mid-ultraviolet range while preserving high IQE by adjusting the ND thickness and Al content. A prototype of EPUV emitter was fabricated using the AlGaN/AlN SK-QDs active region with proposed optimal design of active region thickness, AlN barrier thickness, and amount of AlGaN in each QD layer. For this first device, SiC was used as a substrate to prevent problems associated to charge or heat evacuation. A water purification test by such prototype EPUV emitter was carried out by irradiating E-coli bacteria, showing that all the specimens were successfully purified at the predicted ultraviolet dose.

Keywords:
Molecular beam epitaxy, Nanowires, Quantum dots, III-nitrides

On-line thesis.