  Spin splitting of conduction electron states has been analyzed for all
possible point symmetries of SiGe quantum well structures. A particular
attention is paid to removal of spin degeneracy caused by the rotoinversion
asymmetry of a (001) heterointerface between two diamond-lattice materials. The
asymmetry is shown to result in spin splitting of both Rashba and Dresselhaus
types in symmetrical SiGe quantum wells. Consequences of the spin splitting on
spin relaxation are discussed.
