Abstract : "Optical Properties of AlGaN Nanostructures"

Group III nitride semiconductor materials are very important because of their applications in short wavelength and high frequency optoelectronic devices such as blue to UV and deep–UV light emitting diodes and laser diodes. The studies in the thesis include the synthesis and characterization of 1D AlGaN nanostructures using the chemical vapor deposition technique via catalytic assisted vapour–liquid–solid (VLS) as well as self–catalytic vapor–solid growth mechanism. The optical properties of AlGaN nanostructures are investigated using near– and far–field characterization techniques. The major highlight of the thesis is the study of light–matter interactions of nanostructures in the sub–diffraction limit. In the case of sub–diffraction limited nanostructures, the confined light is used to improve the resolution and hence the focusing/imaging of the objects. The present work demonstrates the spectroscopic imaging as well as polarized Raman study for precise crystallographic orientation of sub–diffraction limited AlGaN nanowires with the proximity induced optical confinement and the plasmonic assisted confinement of light using near–field scanning optical microscopy (NSOM).

In addition, the quantum efficiency of optoelectronic devices depends on the recombination rate which can be affected by the polar and non–polar orientations of the crystallites. Therefore, the effect of crystalline orientation on vibrational and emission properties of hexagonal shaped AlGaN nanostructures is investigated. The observations revealed that the non–polar m–plane oriented crystallites are suitable for the better optoelectronic device performance.

The piezoelectric properties of group III–nitrides are finding potential applications in the areas of nano–sensors, nano–actuators and other nano–mechanical devices. The crystal orientation dependent piezo–response force microscopic (PFM) studies were also carried out to confirm the existence of internal electric field as well as the observation of piezoelectric domains of AlGaN single crystals.