  Spin and valley-orbit splittings are calculated in SiGe/Si/SiGe quantum wells
(QWs) by using the tight-binding approach. In accordance with the symmetry
considerations an existence of spin splitting of electronic states in perfect
QWs with an odd number of Si atomic planes is microscopically demonstrated. The
spin splitting oscillates with QW width and these oscillations related to the
inter-valley reflection of an electron wave from the interfaces. It is shown
that the splittings under study can efficiently be described by an extended
envelope-function approach taking into account the spin- and valley-dependent
interface mixing. The obtained results provide a theoretical base to the
experimentally observed electron spin relaxation times in SiGe/Si/SiGe QWs.
