Evidence and evaluation of the Bychkov-Rashba effect in SiGe/Si/SiGe quantum wells

From spin resonance of two-dimensional (2D) conduction electrons in a modulation doped SiGe/Si/SiGe quantum well structure we find a 2D anisotropy of both the line broadening (dephasing time) and the g factor. Both can be explained consistently employing the Bychkov-Rashba (BR) term ${\mathcal{H}}_{\mathrm{BR}}=\alpha (\mathbf{k}\times \sigma )\cdot {\mathbf{e}}_z,$ which turns out here to be the dominant coupling between electron orbital motion and spin. We obtain a BR parameter of $\alpha =0.55\mathrm{}\times {10}^{-12}\mathrm{eV}\mathrm{}\mathrm{cm}$—three orders of magnitude smaller than in quantum well structures based on III-V semiconductors, consistent with the much smaller spin-orbit coupling in Si.