No description available.

A transparent conductive electrode (TCE) is an important component in many of our modern optoelectronic devices like photovoltaics, light emitting diodes and touch screens. These devices require good current injection and spreading as well as a high transparency. In this thesis we explore the use of graphene as an alternative to the current widely used indium tin oxide (ITO) as TCE in gallium nitride (GaN) based light emitting diodes (LEDs). Monolayer crystalline graphene can be produced on copper foils using chemical vapor deposition (CVD), where metals (especially copper) has a catalysing effect on the formation of graphene. However, transfer of graphene from copper foils is not suitable for an industrial scale and it results in a poor contact with the target substrate. We investigate the possibility of directly integrating graphene on GaN-based LEDs by using plasma-enhanced chemical vapor deposition (PECVD). We try to obtain the optimal conditions under these catalyst-free circumstances and propose a recipe adapted for the setup that we used. We will also study ideas of using a metal (we tried copper and nickel) to assist the direct growth that could help to increase the fraction of sp2 carbon bonds and reduce the sheet resistance. The metals are evaporated onto our samples either before or after we grow a carbon film to either assist the growth or rearrange the carbon respectively. The focus was not on trying to optimize the conditions for one metal treatment but rather to briefly explore multiple methods to find a suitable path for further studies. The direct grown pristine carbon films shows indications from Raman measurements of being nanocrystalline graphene with a sheet resistance ranging from about 20-50 kΩ/sq having a transmittance of approximately 96 % at 550 nm. A transmittance at this level is closely related to the value of an ideal monolayer graphene, which indicates that our carbon films could be close to one atom in thickness while being visually homogeneous and complete in coverage. Due to the use of a temperature close to the melting point of copper we struggled to keep the assisting copper from evaporating too fast or staying homogeneous after the treatment. Nickel has a higher melting temperature, but it appears as if this metal might be diffusing into the GaN substrate which changes the properties of both the GaN and carbon film. Even though the metal treatments that we tested did not provide any noticeable improvements, there is need for further investigations to obtain suitable treatment conditions. We suggest that the treatments involving copper are a more promising path to pursue as nickel seem to cause unavoidable intermixing problems.

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

No description available.

Wide bandgap semiconductors have entered into Naval radar use and will eventually replace vacuum tube and conventional solid-state amplifiers for all modern military radar and communications applications. Gallium Nitride (GaN) High Electron Mobility Transistors (HEMTs) are on the leading edge of wide bandgap technology and have the performance characteristics to dominate in high power â high bandwidth applications. The Defense Advanced Research Projects Agency (DARPA), Office of Naval Research (ONR) and Missile Defense Agency (MDA) are all sponsoring research projects to apply wide bandgap technology. This thesis studies the effects of changing the substrate material of an existing GaN HEMT from sapphire to diamond through the use of commercially available Silvaco software for modeling and simulation. The unparalleled thermal properties of diamond are expected to dramatically decrease device temperatures and increase component lifetimes and reliability.

Thesis (Ph. D.)--University of Hong Kong, 2002. / Includes bibliographical references (leaves 68-71).

published_or_final_version / Physics / Master / Master of Philosophy

Optical properties of graphene/GaN hybrid structure were investigated by using a variety of optical spectroscopy techniques including low-temperature photoluminescence (PL) spectroscopy, time-resolved PL (TRPL) spectroscopy, confocal scanning micro-Raman spectroscopy. Single-layer graphene grown by chemical vapor deposition was transferred to GaN epilayer surface, which is verified by the Raman spectrum with a sharp characteristic peak at ~2690 cm-1and a homogeneous Raman image. Three main band-edge emissions including the free exciton A transition (denoted as FXA), the donor bound exciton transition (denoted as DX) and the third peak (denoted as Ix) were well resolved in the PL spectra of the hybrid structure as well as the as-grown GaN epilayer at low temperatures. Interestingly, the FXA transition and Ix line of the GaN epilayer were found to be dramatically altered by the top graphene layer while the DX is almost unaffected. The intensity of Ix line substantially drops after the transfer of graphene layer on GaN, indicating surface defect nature of the Ix line. More interestingly, an unpredictable dip structure develops in the FXA peak when the temperature is beyond 50 K. Similar spectral structure change also occurred in the emission of free exciton B (referred as FXB)with higher transition energy .A free exciton dissociation and electron transfer model was proposed to explain the “dip effect”. More supporting evidence to the model was found in the time-resolved PL spectra of the hybrid structure and the control sample. The results showed the significant influence of graphene monolayer on the fundamental optical properties of GaN. / published_or_final_version / Physics / Master / Master of Philosophy

No description available.