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

摘要

This dissertation details two techniques for materials analysis by nuclear magnetic resonance.udThe first is a general strategy for recording spin density maps from solids through improved nuclearudmagnetic resonance imaging. The second involves ultrasensitive methods for detecting nuclear magneticudresonance optically and is applicable to semiconductors at low temperature.udConventional liquids magnetic resonance imaging (MRJ) protocols fail in solids, where rapidudlocal-field dephasing of nuclear magnetization precludes the frequency encoding of spatial informationudwith conventional magnetic field gradients. In our approach, a multiple-pulse line-narrowing sequence isuddelivered with a solenoid coil to prolong a solid's effective transverse relaxation time. A radiofrequencyudgradient coil, delivering resonant pulses whose amplitude varies across the sample, is driven in concertudwith the line-narrowing coil to encode spatial information. The practical implementation of this protocoluddemanded the construction of an active Q-spoiling circuit to negate coupling of the two isoresonant coils.udTwo-dimensional Fourier-zeugmatographic images of hexamethylbenzene have been obtained that exhibitud300 µm x 300 µm planar resolution. This imaging protocol is one of the highest sensitivity methods forudimaging solids by NMR (the other involves line narrowing and pulsed DC gradients).udExtraordinary increases in detection sensitivity are required for NMR to study epitaxialudsemiconductor devices. Optical pumping is one route to such increased sensitivity. Here, a transfer ofudangular momentum from polarized light to electrons (via selection rules), and electrons to nuclei (throughudhyperfine couplings), can result in > 10 % nuclear spin polarization in less than 5 seconds at 2 K. A totaludsensitivity gain of 10^5 follows by detecting this large polarization optically, through the inverse process,udallowing collection of the NMR spectra for several GaAs-based epitaxial devices. Previous workersudobserved these spectra to be either power-broadened at the rf levels required to induce optical response, oruddistorted due to the presence of photocarriers during optical detection. An innovation of the Weitekampudgroup was to time-sequence and separately optimize the periods of optical pumping, NMR evolution, andudoptical detection. Although time sequencing in principle allows the collection of multiple-pulse high-resolutionudNMR spectra, it appeared inadequate when applied to a semiconductors heterojunction.udIn conventional NMR, the entire dipole-allowed spectrum may be collected following a singleudpulse. In time-sequenced optical NMR however, the desired interferogram must be built up pointwise byudrepetitively incrementing an evolution time. Although sensitive, this experiment is time consuming andudsensitive to drift. A new optical detection protocol has been developed which removes these problems andudallows NMR spectra to be collected optically in real tillle. In this experiment, a circularly polarizedudreference nuclear hyperfine field is introduced during the precession of a signal field. The observedudluminescence polarization is sensitive to the instantaneous vector sum of the fields, producing Larmorudbeats. With the reference magnetization in equilibrium through the use of either continuous irradiation oruda pulsed spin-lock, the oscillation of luminescence polarization at the Larmor beat frequency is able toudrecord the spectrum of the signal nucleus alone.udA spectrometer has been constructed for implementing both time-sequenced and Larmer-beatudoptical detection of NMR. In order to implement rotation studies in a way compatible with opticaluddetection at 2K, variable-angle Helmholtz coils have been added to the apparatus so that the direction ofudthe static field can be varied. The results of preliminary rotation studies put a surprisingly low upperudbound on the electric fields present at the most rapidly polarizable sites in a AlGaAs/GaAs heterojunction.udThis can be understood in terms of a model where these sites are neutral donors at locations where theudbuilt-in interfacial electric field has fallen off.