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