Role of defects on the growth mechanism and on the structural, optical, electrical and electronic properties of GaN nanostructures

Abstract

The fundamental and technological importance of the group III-nitride materials can not be overstated, due to their exciting structural and bandstructure properties. These materials have been used widely in applications such as optoelectronics, high-frequency and high-power electronics, photovoltaics and sensing, to name a few. Because of thermodynamic constraints, it is not easy or cheap to produce large scale good quality single crystals of AlN, GaN and InN, and due to unavailability of native substrates one is forced to grow them on foreign substrates. The mismatch of the lattice parameters and thermal expansion coefficients between the substrate and the overlayer leads to the generation of point and extended defects and residual strain in the films. Nanostructuring III-nitride films can help alleviate some of these issues and can also be used to enhance and tailor the properties of the material because of the novel properties that arise due to the low-dimensionality. The systematic work in this thesis helps us understand the enhanced luminescence and superior transport properties of GaN nanowall network along with observation of room temperature ferromagnetic properties by correlating these with experimentally derived electronic band-structure, and quantum mechanical effects. Due to its unique morphology, polarisation induced spontaneous formation of 2DEG is possible and it determines many of the transport properties. To further understand how the unique structure of nanowall network influences its properties, comparative studies with low-dimensional nanorods, and flat or porous thin films have been carried out. In this thesis, we carry out comparative studies of GaN thin films and one and two-dimensional nanostructures to understand the effect of defects and low-dimensionality on the properties of the material. The work has been divided into seven chapters described as follows.