Advancements in optically detected nuclear magnetic resonance applied to nanoscopic GaAs AlGaAs heterostructures

Abstract

This thesis presents advancements in the spectroscopic method of optically detected nuclear magnetic resonance (ODNMR) applied to semiconductor heterostructures to investigate nanoscopic structural details in these systems. Experiments are carried out on type I GaAs/AlGaAs quantum wells of various thicknesses, investigating new aspects about the behaviour of the coupled electron-nuclear spin system. Characterization and interpretation of the specific features of the ODNMR spectra are carried out, providing spectroscopic applications for studying low-dimensional heterostructure systems. Utilizing the sensitivity and selectivity of ODNMR, structural information is revealed about GaAs/AlGaAs interfaces and quantum wells.A high degree of nuclear spin polarization is achieved by optically pumping he coupled electron-nuclear spin system. The hyperfine coupling constant is derived for quantum wells of various thicknesses by calculating the electronic wavefunction within a quantum well and values for the magnetic fields of electronic and nuclear origin are estimated.The ODNMR technique is used to investigate different relaxation processes governing the optical pumping such as the field dependent optical pumping relaxation time, the 'dark' relaxation time and the hyperfine relaxation time. It is shown that delocalized optically oriented electrons do not contribute to polarizing the nuclear spin system. It is assumed that trapping of excitons at interface defects plays the key role in amplifying the hyperfine interaction. A model is presented, to approximate the effect on the hyperfine relaxation time and it is found that the relaxation time is decreased by one to two orders of magnitude compared to unbound excitons.The ODNMR resonance lines show large splitting which are attributed to quadrupole interactions of the nuclei with electric field gradients caused by the neighbouring atoms. The reduction of the cubic symmetry of GaAs is attributed to interfacial effects at the GaAs/AlGaAs barrier, to strain within the quantum well applied externally by the mounting of the sample, and by a homogenous electric field across the sample. The spectra also show significant broadening of the satellite lines, which arises from internal strain created by lattices mismatch of the heterostructure and by monolayer splitting (interface roughness), which creates an additional shear strain component.