Dissertation Abstract

Gallium-69, 71 and nitrogen-14 nuclear magnetic resonance of amorphous gallium nitride

Publication Number:  AAT3282439
Author:  Schuster, David Michael, Ph.D.
School:  Washington University in St. Louis
Date:  2007
Pages:  216
Subject:  Nuclear Medicine, Magnetic Resonance Imaging

This work reports results of nuclear magnetic resonance experiments on amorphous gallium nitride (a-GaN) made via room-temperature-MBE deposition on sapphire and aluminum-foil substrates. The amorphous gallium nitride made in this way has been compared with powdered-crystal GaN (px-GaN). The temperature dependence of spin-lattice relaxation time below room temperature was found to be similar, which indicates that the Debye temperature for a-GaN and crystal gallium nitride (x-GaN) may be similar. However, the actual spin-lattice relaxation times are much shorter in a-GaN.

Several important distinctions between the materials are also clear. Amorphous gallium nitride has a very broad NMR lineshape, which has been determined to have been broadened by the second-order-quadrupole interaction and further inhomogenously broadened by chemical shifts. This work has also demonstrated that a-GaN's NMR signal results from the central transition of the I = 3/2 gallium nuclei. Using this information, estimates for the electric field gradients present in a-GaN have been obtained. A large distribution of EFG's has been inferred, including a significant fraction of sites experiencing EFG's much greater than those of single-crystal, hexagonal GaN.

The spin-lattice relaxation times of a-GaN are two orders of magnitude shorter than those of x-GaN, and between one-fortieth and one-two-hundredth as long as T 1 's for px-GaN. This is likely due to the significance of local disorder modes (modeled as two-level systems) in the amorphous network, which provide more relaxation pathways than those available to the single-crystal sample. The spin-spin relaxation times of a-GaN have been found to be about 2/3 as long as those of px-GaN, possibly indicating that the disorder modes in a-GaN also play a role in the decay of transverse magnetization.

An abundance of local disorder modes could explain the temperature dependence of the a-GaN lineshape and also its uniquely fast relaxation. These disorder modes may result from significant threefold coordination, which is predicted theoretically. The presence of threefold coordination can also explain the large range of EFG's observed in a-GaN.

[Schuster is an ARRT registrant and an ASRT member.]

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