Dependence of spin lifetime on spin injection orientation in strained silicon films

The electron spin properties of semiconductors are of huge interest because of their potential for future spin-driven microelectronic devices. Modern charge-based electronics is dominated by silicon, and understanding the details of spin propagation in silicon structures is key for novel spin-based device applications. The peculiarities of the subband structure and details of the spin propagation in surface layers and thin silicon films in the presence of the spin-orbit interaction is under research. We investigate the influence of the spin injection direction on the spin relaxation. Beginning with the four-component wave functions, we show that the surface roughness induced spin intersubband relaxation matrix elements get reduced for an in-plane spin injection compared to perpendicular-plane spin injection, henceforth the corresponding spin relaxation rate (time) is diminished (enhanced). In order to explain this observation we point out that at the spin relaxation hot spots the perpendicular-plane spin injection results in a maximal spin randomization at any in-plane momentum, which increases the spin relaxation rate and decreases the spin lifetime as compared to an in-plane spin injection.