Epitaxial growth, optical properties and structural studies of GaN nanorods and related heterostructures

Abstract

Group three-nitride nanorods have attracted substantial research focus thanks to their direct bandgap, full solar spectrum coverage, and wide optoelectronic applications. GaN nanorod growth has been achieved by several growth methods, and among them, self-assembled nanorod growth by metalorganic chemical vapour deposition (MOCVD) is an optimal and time-saving method due to its catalyst-free and minimal substrate preparation requirements. This approach has been applied in devices that do not demand precise positionings, such as single nanorod light-emitting diodes (LEDs) and flexible nanorod array LEDs. This dissertation presents a detailed growth study of GaN self-assembled nanorods on sapphire substrates by MOCVD. Both the pre-growth and growth parameters have been systemically studied. Due to their wurtzite crystal structure, how to control their polarity has been a critical issue in the nanomaterial growth and nano-device fabrication. In this thesis, we show and discuss how the polarity and optical properties are affected by nitridation, one of the pre-growth conditions. With appropriate nitridation, the nanorods can be grown with uniform hexagonal morphology and N-polar, resulting in homogenous luminescence with a strong near-band edge emission. This thesis also studies the effect of growth parameters such as silane co-injection, growth temperature and growth time on the morphology and density of the nanorods. A rosette-shaped cathodoluminescence (CL) pattern is found in GaN nanodisks and nanorods. This unique pattern forms at a very early stage of nanorod growth and consists of yellow luminescence (YL) and non-luminous regions. To explore its origin, CL, electron microscopy and nanoscale secondary ion mass spectrometry studies are conducted. These studies found optical resonance modes and polarity inversion do not contribute to this phenomenon. Higher concentration of carbon and nitrogen clusters are found at the pattern area, which indicates pattern could be related to facet preferential distribution of defects related to excess carbon/nitrogen. This study adds the knowledge of defect-related emission in GaN nanorod, which is essential for future optoelectronic applications. Finally, this dissertation presents InxGaN1-x/GaN multi-quantum well (MQW) core-shell structures and the formation of InxGa1-xN quantum dots (QDs). Through high-resolution CL and transmission electron microscopy (TEM) studies, the strong QW emission is found only at the tip area and indium segregation is observed at the nanorod sidewall MQW area. The InxGa1-xN MQW emission shows high sensitivity to minor changes in trimethylindium source flow and quantum barrier growth temperature. The MQW emission shifts to longer wavelengths due to increasing indium source supply and decreasing quantum barrier growth temperature as a result of higher indium incorporation. This study extends our knowledge of growth & optical properties of InxGaN1-x/GaN MQW and QDs, which also demonstrates the potential of application in LEDs in the future.