NMR of dilute sites in GaAs

Citation

Hwang, Jack Yanchai (1994) NMR of dilute sites in GaAs. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/0C96-JQ24. https://resolver.caltech.edu/CaltechETD:etd-12072007-093730

Abstract

The presence of a point defect in the zincblende lattice of GaAs breaks the tetrahedral symmetry and induces a range of quadrupole splittings into the NMR spectra of the surrounding Ga and As sites (all spin 3/2). Two-dimensional NMR nutation experiments are used to observe the resulting resonances in the case of an isovalent dopant. Differences in the nutation frequency of resonances near the unperturbed Zeeman frequency allow separation of the contributions from sites with large and small electric field gradients. Rotation studies allow the determination of field gradient tensor components in the crystal frame and provide clues to the assignment to specific sites. The inability to observe the NMR of the indium dopant at a concentration of 1 x 10^20/cm3 highlights the insensitivity of conventional NMR to the study of defects found at the concentrations present in device quality materials or to the low number of sites present at a single-crystal surface or interface.

Optical nuclear polarization (ONP) and optical detection (OD) of nuclear magnetic resonance (NMR) in III-V semiconductors have proven to be effective methods for increasing the sensitivity of NMR over that of conventional methods. The combination of ONP and OD into one optical NMR experiment has led to sensitivity enhancements of at least 10^5 in several GaAs-based materials. The quasi steady-state optically detected NMR (ODNMR) method used in all of these studies has resulted in NMR lineshapes which were either distorted by the high rf fields needed to see signals or broadened by the presence of spin-polarized electrons during NMR. The previous introduction of time-sequenced optical NMR (TSONMR) by the Weitekamp group, in which ONP, NMR, and OD occur in successive periods and are thus separately optimizable, has allowed the full implementation of multiple-pulse NMR. Several such methods are demonstrated on sub-ppm donor sites in epitaxial p-GaAs. These results suggest that TSONMR may contribute to defect identification at sensitivities of technological relevance.

In conventional rf-detected Fourier transform (FT) NMR, the entire dipole- allowed spectrum is obtained from a single free induction decay. In contrast, FT time- sequenced optical NMR methods have necessarily been pointwise methods in which a period of coherent NMR evolution is incremented on successive repetitions to map out the desired interferogram. This method has the disadvantage, relative to ordinary NMR, that even simple experiments are time-consuming and susceptible to signal drift, giving rise to t1 noise. This restriction has now been lifted by introducing a circularly-polarized transverse rf reference field so that the magnitude of the vector sum field (reference plus nuclear field of interest) is modulated at the difference frequencies of its Fourier components. The circular polarization of the luminescence is thus likewise modulated, due to the Hanle effect. This method allows real-time collection of NMR transients. To simultaneously optimize sensitivity and linearity, it is desirable to have a reference field which is larger than the signal field and comparable to the total longitudinal field. This is achieved by tuning the frequency of the applied reference field into resonance with one isotope so that the spin-locked nuclear hyperfine field, which can be orders of magnitude larger than practical applied rf fields, becomes the effective reference field. The circularly polarized luminescence signals at the difference frequencies are demodulated by a heterodyne rf spectrometer and digitized.

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