Date on Master's Thesis/Doctoral Dissertation

5-2019

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemical Engineering

Degree Program

Chemical Engineering, PhD

Committee Chair

Sunkara, Mahendra

Committee Member

Sumanasekera, Gamini

Committee Member

Starr, Thomas

Committee Member

Willing, Gerold

Committee Member

Alphenaar, Bruce

Author's Keywords

Pulsed plasma; nitrogen radicals; nitrogen concentration; dissolution; recombination; growth

Abstract

Next generation semiconductor materials such as Gallium Nitride (GaN) and Silicon Carbide (SiC) are rapidly replacing Silicon (Si) for high power and high frequency applications due to Si’s inherent limitations. Despite the advantages of GaN over SiC, adoption of GaN has been hindered due to the lack of a cost-effective bulk production technique. Thus, the inability to precure native substrates requires GaN-based architectures to be heteroepitaxially synthesized on non-native substrates, such as sapphire and even SiC. This research seeks to develop a cost effective and scalable method to produce low defect, bulk GaN encouraging the adoption of GaN based devices which ultimately will allow reduction of energy losses in the grid. Specifically, two conceptual methods for the growth of bulk GaN will be explored. The first one is a film-based method, which is based on self-oriented growth of GaN films on a melt gallium layer. In this process, a highly oriented GaN film, made by exposing Ga to plasma-activated nitrogen atoms, is epitaxially thickened into a millimeter-thick GaN film via metalorganic chemical vapor deposition (MOCVD) or halide vapor phase epitaxy (HVPE). The second proposed concept is a crystal-based method. In this process, single crystalline GaN crystals are obtained by nucleation of GaN out of a Ga melt when exposed to nitrogen plasma. Later, those crystals are enlarged via plasma-assisted liquid phase epitaxy. As a first step to develop a plasma-assisted liquid phase epitaxy technique the wetting properties of Ga, as well as the interaction between molten Ga and plasma were studied. It was found that both an increment in temperature and the addition of other elements can improve the wettability of Ga by reducing the surface tension of the molten metal. However, these variables were not as effective as the dissolution of nitrogen radicals into the melt. Absorption/desorption experiments indicated a rapid adsorption/dissolution of the gas into the molten metal when gallium was exposed to plasma. The overall interaction between Ga and plasmas is composed of five processes: (1) surface adsorption, (2) diffusion into the bulk, (3) recombination inside the bulk, (4) surface recombination, and (5) desorption of species from the bulk. The concentration of radicals inside the metal is determined by the rate in which each process is completed. The self-oriented growth of GaN crystals on molten Ga was found to be dependent of the ability of Ga to spread. XRD characterization showed that flat GaN films only presented reflections of the (0002) and (0004) planes of the hexagonal GaN, whereas the non-flattened GaN films showed the presence of all the characteristic planes of the wurtzite GaN. On the other hand, micron-sized wurtzite and zincblende GaN crystals were obtained by exposing a mixture of Ga and LiCl to nitrogen plasma. It is believed that GaN crystals crystalized from a Li-Ga-N melt that was formed by the interaction between Li3N, Li and Ga. In a typical plasma nitridation experiment, spontaneous nucleation of GaN out of molten Ga leads to the formation of a thick GaN crust on top of the surface of the metal. Similarly, the regrowth experiments using GaN seeds or GaN-on-sapphire substrates failed because of such spontaneous nucleation. The formation of the GaN crust can be explained by the spinodal decomposition mechanism when the concentration of nitrogen inside the Ga reaches a limit. To control the concentration of nitrogen inside the melt, a unique concept of pulsed plasma was introduced. Results showed that pulsed plasma-assisted liquid phase epitaxy allowed a delay in spontaneous nucleation while promoting the growth of additional layers on the pre-existing seeds. A mass transport model was developed to discuss the effect of bulk recombination, diffusion, and pulsing in the concentration of nitrogen into the molten Ga. Results indicated that in the pulsing experiments both the recombination of radicals in the bulk and the diffusion of species into the metal are favored compared to the dissolution of radicals. As a result, the concentration of nitrogen at the surface of the metal is decreased, while the concentration of nitrogen at the surface of the substrate is increased. The results presented in this work provide insights into low-pressure, metastable crystal growth processes that include both nitrogen dissolution and crystallization of GaN out of a Ga melt. Specifically, the roles of plasma and alkali or semi alkali metals in the dissolution of nitrogen into Ga and the understanding of the mechanism in which GaN crystals nucleate are elucidated. Furthermore, the results obtained in this work could be extended to several other compound semiconductor systems that decompose before melting and are unsuitable for traditional crystal growth techniques.

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