Optically detected NMR in semiconductors

Guerrier, Daniel Jason (1997) Optically detected NMR in semiconductors. University of Southampton, Doctoral Thesis.

Abstract

The method of optically detected nuclear magnetic resonance (ODNMR) using an external field, oriented at an oblique angle relative to the incident light direction, is investigated and shown to produce a significant improvement of sensitivity over the longitudinal field ODNMR technique.

Nuclear resonances are presented which are assigned to quadrupole transitions of ⁷⁵As nuclei, perturbed by nearest neighbour Al atoms, in the bulk semiconductor alloy Al_0.32Ga_0.68As. Quadrupole transitions due to ⁷⁵ As nuclei, perturbed by two nearest neighbour Al atoms, are measured for the first time.

A lower limit to the sensitivity of oblique field ODNMR for detection of impurities is determined, by analysis of bulk GaAs doped with ~10^{16 11}B impurity.

Spin polarisation rates of nuclei, oriented by hyperfine interaction with optically excited spin polarised conduction electrons, are measured in GaAs/Al_0.32Ga_0.68As quantum wells for the first time, and the variation of these rates with time of illumination and external field magnitude indicate the existence of a nuclear spin diffusion process, which produces a 2 orders of magnitude enhancement in signal sensitivity over the longitudinal field ODNMR technique.

A new technique for calibration of the strain-quadrupole splitting of nuclear resonances is presented. The quadrupole splitting of ⁷⁵As nuclei is measured for a variety of quantifiable strains applied to GaAs/Al_0.32Ga_0.68As quantum wells and excellent agreement with nuclear acoustic resonance measurements, made in other work, is obtained. This technique will provide the basis for accurate strain determination in non-lattice matched heterostructures.

The first measurements of sign and magnitude of the transverse electron g-factor in three In_0.08Ga_0.92As/GaAs quantum wells of different width are obtained. The magnitude is found from the half width at half height of a Lorentzian depolarisation of conduction electron spin in a transverse magnetic field. The sign is obtained from the shift of Lorentzian maximum from zero field, when an oblique field is applied, which is also the first evidence of nuclear orientation in these quantum wells.

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