Nanofabrication and characterization of high density nanostructures and QDs using ni annealing and anodic porous alumina methods

Abstract

My thesis presents a study on the fabrication of high density nano-structures. These structures have a number of useful applications, for example, nano-LEDs to increase the light extraction and to reduce stress and dislocations from bulk III-nitride material.

III-nitride epitaxial material grown on foreign substrates are known to have large dislocation densities and internal stress, which is caused by tensile and compressive strains due to the lattice and thermal mismatch of the foreign substrate.

Before experimentation, I designed and built the anodisation equipment used for both anodisation and polishing. I have also designed and constructed the worktube, wafer carrier and loading chamber for the Carbolite furnace.

Experimentation commences with Chapter 3 to produce porous anodic alumina (PAA). The study includes optimisation of the processes, followed by producing PAA from bulk Al foils and transferring the pattern onto GaN templates. The basic techniques were developed further by depositing high purity Al directly onto specific substrates. For this latter fabrication, I successfully achieved creating thin PAA membanes on the III-nitride material. The method included a fabrication technique of depositing Ni within the membranes to create a Ni nano-dot mask, which was more uniform than that created by annealing.

An alternative method was explored in Chapter 4, to create nano-columns by annealing thin Ni layers deposited on specific substrates. This technique involves less steps than the PAA method. Hence, it is less hazardous with the use of chemicals, less time consuming and more cost effective. Extensive work was conducted to understand the stages of formation of the Ni dots. Time, temperature, N2 ambient, the thickness of the material layers and the type of substrate were studied to achieve high density and well formed nano-dots, suitable for an etch mask to create nano-columns both for GaN on sapphire and nitride layers on Si.

In Chapter 5, I examine the conditions and effects of plasma etching to create the required nano-structures (nano-columns and nano-holes), in terms of vertical sidewalls and specific depth. Selectivity between the III-nitride material and etching mask (dielectric layer) and the influence of the aspect ratio were analysed to determine the most suitable recipes.

The final chapter studies the optical properties of nano-structures. GaN nano-columns were investigated using RT PL and temperature dependent PL and strain relaxation was indicated from the red shift compared with as-grown GaN. In addition, spectrum analysis of the nano-structures, such as the nano-LED, tends to show greater intensity due to the improvement of light extraction. Moreover, cracking is not visible for the MOCVD overgrowth on nano-columns, which would suggest strain reduction due to the inclusion of this type of nano-structure.

Date of Award	1 Sept 2009
Original language	English
Awarding Institution	University of Bath
Supervisor	Wang Wang (Supervisor)