Group IV semiconductor spintronics

Thesis (Ph. D.)--University of Rochester. Dept. of Electrical and Computer Engineering, 2013.Group IV semiconductors are natural material choices for spintronic devices. Space-inversion symmetry precludes spin relaxation by the Dyakonov-Perel mechanism. Hyperfine interactions are suppressed due to the natural abundance of zero-spin nuclear isotopes. As a result, the intrinsic spin lifetime is relatively long (~10 ns at room temperature in nondegenerate n-type silicon). Combined with the dominant role of Si and Ge in the semiconductor industry, there is a wide interest in recent experiments with Si and Ge spintronics. The conservation of angular momentum during the radiation-matter interaction allows one to study the spin of charge carriers from the state of light polarization. Si was the first material studied in optical orientation experiment. However, a parameter-free method of determining the spin polarization of electrons in Si and Ge was missing for decades. This missing link is established in this dissertation with a comprehensive theory, which provides concise selection rules of spin-dependent optical transitions. These indirect absorption edge transitions involve the interaction with phonons, whose symmetries are crucial in elucidating recent spin injection experiments in Si and Ge. The theory also studies the effect of strain. Strain lifts the band edges degeneracies and controls the mixing between holes with different angular momenta, and could be used to regulate and validate the relation between measured circular polarization degrees and electron spin polarizations. The availability of transparent spin-dependent theories in direct gap semiconductors has spurred the field of spintronics. Prior to our work, however, the study of spin-orbit properties in indirect bandgap Si requires elaborate numerical methods. To establish a lucid and compact theory, we derive a spin-dependent Hamiltonian that captures the symmetry of the zone edge X-point states in Si. We present analytical expressions of the spin-dependent states and of spin relaxation due to electron-phonon interactions, showing excellent agreement with experimental results. In addition, we investigate the intrinsic spin relaxation of conduction electrons in Ge. Spin-flip matrix elements are derived for a general spin orientation that quanties the anisotropy of spin relaxation. Spin lifetimes from integrations of these matrix elements show excellent agreement with independent elaborate numerical calculation