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.
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