Optical and Structural Analysis of Three-dimensional GaN and InGaN/GaN Core-Shell Microstructures

Developments of light emitting diodes (LEDs) based on three-dimensional (3D) III-nitride micro-, and nano-structures are being in the spotlight for recent few years. Until now, many studies have been dedicated to the fabrication of these novel structures. However, further advancements of the technology towards more efficient devices demand deeper analysis and characterizations. This will eventually enable us to understand the challenging issues and find the solutions. In spite of the great knowledge on m-plane and c-plane planar LEDs, some of the fundamental properties and behaviors of micro-LEDs are not yet fully investigated. This is largely due to the incompetence of conventional characterization methods in the case of 3D structures. Therefore, it is essential to develop measurement methods to overcome these limitations. In this thesis, in order to analyze GaN micro-rods and InGaN/GaN quantum well (QW) based core-shell LEDs, the opto-electronic and structural properties are correlated. Our state of the art Si-doped n-GaN micro-rods and core-shell micro-LEDs were fabricated using the metal organic vapor phase epitaxy (MOVPE) and selective area growth (SAG) techniques.
In the first part, a direct and accurate analysis of the strain state, the carrier concentration and the mobility along the height of individual GaN:Si micro-rods is introduced. It is based on the spatially-resolved micro-Raman and photoluminescence (PL) spectroscopy. These optical methods offer convenient and direct way to access the opto-electronic properties of the wire-based devices which can not be easily achieved by conventional methods like Hall effect. In Raman spectroscopy, coupled modes of the A1 and E1 longitudinal-optical (LO) phonons, and a collective excitation of free electrons namely LO-phonon-plasmon coupled modes are exploited to evaluate the free carrier density and the mobility in the individual as-grown and dispersed rods.
In the second part, optical properties of InGaN/GaN QW in core-shell structures are addressed. Although the core-shell micro-LEDs have potential advantages over conventional planar LEDs, till now their efficiencies are lower than planar LEDs. To enhance the performance of core-shell micro-LEDs, firstly efficiency loss mechanisms should be well understood. In this work, the effect of different structural parameters like the n-GaN doping condition, indium contents of InxGa1-xN alloy, and the Mg doping of GaN capping shell on the optical properties and internal quantum efficiency (IQE) of core-shell LEDs are studied.