Nanofabrication and its effect on electrical and optical properties of gallium nitride
Thesis DisciplineElectrical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
GaN and related materials keep drawing attention because of their successful application in light emitter diodes (LEDs) and laser diodes (LDs) and their high potential application in high power and high temperature electric devices. Lots of efforts have been devoted to dry etching of GaN due to ongoing device size minimization and chemical inertness of the material. The first goal of this research is to develop a procedure for fabricating GaN patterns in the nanometer scale. To achieve this goal, the dose of electron beam lithography and plasma parameters have been varied. The effort has concentrated on obtaining the fine patterns with steep sidewall and smooth etched surface for the device applications. Scanning electron microscopy has been used to inspect the pattern profile and atomic force microscopy has been employed to monitor the surface roughness. Nano-wires and dots have been fabricated in gallium nitride using electron beam lithography and reactive ion etching (RIE). The developed nanofabrication procedure has been successfully implemented in nano-dots fabrication in Si/Siâ‚“Ny superlattice. High density plasma (HDP) has been recommended for etching GaN, because of high reactive species and ion density, predominately controllable ion energy and easier scale-up for production (inductively coupled plasma (ICP)). Thus, study ICP for etching GaN has become the second goal. A Langmuir probe has been used to quantify the amount of ion bombardment during plasma etching by measuring the ion flux, while optical emission spectroscopy has been employed to monitor atomic F radical. ICP etching of GaN is highly ion induced. The etch condition has been optimised, resulting in an almost vertical pattern profile and a etch rate of 67nm/min. This is about 5 times more of that obtained in RIE by using the same gas composition (SFâ‚†+Nâ‚‚). To achieve higher etch rate, ICP chlorine plasma etching of GaN has been investigated. The fastest etch rate of 314nm/min has been obtained with good vertical nature of pattern profile. However dry etch induced damage may affect material properties and deteriorate device quality. Thus, the study of dry etch-induced damage effects at the microscopic scale and optimisation of the etch conditions to obtain fast etch rate, smooth surface morphology, anisotropic profile and minimal etch damage is the final goal of this project. We achieved this goal by correlating the plasma characteristics to etch behaviour, diodes properties to photoluminescence (PL) properties for studying the top surface damage, and quantum device conductivity for the sidewall damage. Our data show that SFâ‚† + Nâ‚‚ ICP plasma with high ion energy improves diodes properties with reasonable etch rate, almost vertical profile and smooth surface morphology. However, it reduces PL D0X intensity dramatically. While, 25%Clâ‚‚ + 75%Ar ICP plasma produces a fast etch rate and does not reduce the PL intensity significantly. However, it deteriorates the diodes properties. These results suggest that a SFâ‚† + Nâ‚‚ (1:1) plasma with high ion energy should be appropriate for GaN transistor gate recessing, subjecting to mobility investigation. A 75% Ar + 25% Clâ‚‚ plasma is suitable for the fabrication of optical devices, such as light emitting diodes and laser diodes. The preliminary result of quantum wire devices shows that sidewall depletion in quantum wire structures is about 65 nm at the given SFâ‚† + Nâ‚‚ plasma conditions. This gives an indication that length scale of the depth of defects introduced by etching is about 52 nm in our GaN. However, the defects may propagate deeper.