I. Multiple-pulse radio-frequency gradient nuclear magnetic resonance imaging of solids ; II. Optical nuclear magnetic resonance analysis of epitaxial gallium arsenide structures

Citation

Marohn, John Aaron (1996) I. Multiple-pulse radio-frequency gradient nuclear magnetic resonance imaging of solids ; II. Optical nuclear magnetic resonance analysis of epitaxial gallium arsenide structures. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/PDK5-7T44. https://resolver.caltech.edu/CaltechTHESIS:02242011-083555306

Abstract

This dissertation details two techniques for materials analysis by nuclear magnetic resonance. The first is a general strategy for recording spin density maps from solids through improved nuclear magnetic resonance imaging. The second involves ultrasensitive methods for detecting nuclear magnetic resonance optically and is applicable to semiconductors at low temperature. Conventional liquids magnetic resonance imaging (MRJ) protocols fail in solids, where rapid local-field dephasing of nuclear magnetization precludes the frequency encoding of spatial information with conventional magnetic field gradients. In our approach, a multiple-pulse line-narrowing sequence is delivered with a solenoid coil to prolong a solid's effective transverse relaxation time. A radiofrequency gradient coil, delivering resonant pulses whose amplitude varies across the sample, is driven in concert with the line-narrowing coil to encode spatial information. The practical implementation of this protocol demanded the construction of an active Q-spoiling circuit to negate coupling of the two isoresonant coils. Two-dimensional Fourier-zeugmatographic images of hexamethylbenzene have been obtained that exhibit 300 µm x 300 µm planar resolution. This imaging protocol is one of the highest sensitivity methods for imaging solids by NMR (the other involves line narrowing and pulsed DC gradients). Extraordinary increases in detection sensitivity are required for NMR to study epitaxial semiconductor devices. Optical pumping is one route to such increased sensitivity. Here, a transfer of angular momentum from polarized light to electrons (via selection rules), and electrons to nuclei (through hyperfine couplings), can result in > 10 % nuclear spin polarization in less than 5 seconds at 2 K. A total sensitivity gain of 10^5 follows by detecting this large polarization optically, through the inverse process, allowing collection of the NMR spectra for several GaAs-based epitaxial devices. Previous workers observed these spectra to be either power-broadened at the rf levels required to induce optical response, or distorted due to the presence of photocarriers during optical detection. An innovation of the Weitekamp group was to time-sequence and separately optimize the periods of optical pumping, NMR evolution, and optical detection. Although time sequencing in principle allows the collection of multiple-pulse high-resolution NMR spectra, it appeared inadequate when applied to a semiconductors heterojunction. In conventional NMR, the entire dipole-allowed spectrum may be collected following a single pulse. In time-sequenced optical NMR however, the desired interferogram must be built up pointwise by repetitively incrementing an evolution time. Although sensitive, this experiment is time consuming and sensitive to drift. A new optical detection protocol has been developed which removes these problems and allows NMR spectra to be collected optically in real tillle. In this experiment, a circularly polarized reference nuclear hyperfine field is introduced during the precession of a signal field. The observed luminescence polarization is sensitive to the instantaneous vector sum of the fields, producing Larmor beats. With the reference magnetization in equilibrium through the use of either continuous irradiation or a pulsed spin-lock, the oscillation of luminescence polarization at the Larmor beat frequency is able to record the spectrum of the signal nucleus alone. A spectrometer has been constructed for implementing both time-sequenced and Larmer-beat optical detection of NMR. In order to implement rotation studies in a way compatible with optical detection at 2K, variable-angle Helmholtz coils have been added to the apparatus so that the direction of the static field can be varied. The results of preliminary rotation studies put a surprisingly low upper bound on the electric fields present at the most rapidly polarizable sites in a AlGaAs/GaAs heterojunction. This can be understood in terms of a model where these sites are neutral donors at locations where the built-in interfacial electric field has fallen off.

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