Laser spectroscopy of semiconductor quantum wells: optical pumping and optically detected nuclear magnetic resonance

Abstract

This thesis describes the experimental results of optical pumping studies made in some Type 1 GaAs/Al_0.3Ga_0.7  As single quantum wells (QWs), ranging in widths from 50 to 200Å. Optical pumping uses circularly polarised light to photocreate spin orientated carriers in the QWs. The polarised carriers, principally the electrons, transfer angular momentum to the nuclear spins of the lattice through the Fermi contact hyperfine interaction. The orientated nuclei act back on the electrons through the same interaction. Studies of the nuclear and carrier spin systems may be effected by monitoring the circular polarisation of the recombination luminescence.
The Hanle Effect of the electron orientation within the QWs is measured as a function of well width, and of the non-photoexcited hole concentration. Correlation of this data with carrier recombination time measurements produces the magnitude of the carrier Landé factor. Allowing the nuclear field to influence the Hanle curves determines the sign of these g - factors, whence the free conduction electron value is observed to change sign at 55Å and become positive for thinner wells. This result is compared to the bulk g -values of GaAs (-0.44) and Al_0.3Ga_0.7As (+0.40), and modelled using k . p theory to estimate the contributions to g* from the well and barrier materials.
Nuclear orientation within the wells is observed by the optical detection of nuclear magnetic resonance (ODNMR), via the application of a radio frequency field (RF) to the orientated nuclear spins. ODNMR of ~ 10¹¹ nuclear spins is measured in almost all the single QWs. The signals are studied as a function of various experimental influences, such as well width, p-type carrier concentration, RF magnitude, optical pumping time, and RF sweep rate. Observation is made of extrinsic lattice strain and spin diffusion of the nuclear magnetization, and results suggest increased nuclear relaxation rates within the barrier. n-type wells show no ODNMR, indicating that incomplete hole relaxation is present within the wells. The very wide and undisplaced Hanle curves observed in this type of well support this view.
Nuclear orientation is not observed in some GaAs/In_xGa_1-xAs QW systems. This null result is discussed in terms of the experimental factors important for its successful observation in the GaAs/Al_0.3Ga_0.7As systems.

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