Optical spectroscopy of donor bound excitons and spin relaxation of donor electrons in isotopically enriched silicon

The spin of electrons bound to neutral phosphorus donors in isotopically enriched silicon is a promising candidate for future quantum information processing. In this thesis, the intriguing properties of the associated optical transition, i.e., the donor bound exciton (D0X) transition are investigated by means of high precision laser absorption spectroscopy. The ultra-narrow spectral linewidth of the D0X transition allows for individual optical addressability of the electron spin and the phosphorus nuclear spin which is used to unambiguously quantify the microscopic origin of the enhanced donor electron spin lattice relaxation rate caused by optical excitation. For this purpose, the transient decay of the donor electron polarization is studied via a time-resolved pump-probe absorption spectroscopy technique where a significant shortening of the polarization decay with increasing laser excitation is observed. The theoretical analysis of the complete optically driven donor system shows that this shortening is caused by the creation of free electrons via the ubiquitous D0X Auger recombination. It is shown that, in addition to electron-phonon interaction, the hot Auger electrons relax their excess energy via inelastic collisions with donors and promote the donor electron from the ground state to a spin-mixed excited state giving rise to an Orbach-type spin relaxation mechanism which sets a fundamental limit to the spin relaxation and spin coherence time of optically driven donor systems. Furthermore, the ultra-narrow linewidth of the D0X transition enables the test of fundamental semiconductor physics such as the low temperature behavior of the silicon bandgap and the extraction of material parameters like the Landé g-factors