A Si1-xGex/Si1-yCy short-period superlattice which functions as a single SiGeC layer is formed by alternately growing Si1-xGex layers (0 < x < 1) and Si1-yCy layers (0 < y < 1) each having a thickness corresponding to several atomic layers which is small enough to prevent discrete quantization levels from being generated. This provides a SiGeC mixed crystal which is free from Ge-C bonds and has good crystalline quality and thermal stability. The Si1-xGex/Si1-yCy short-period superlattice is fabricated by a method in which Si1-xGex layers and Si1-yCy layers are epitaxially grown alternately, or a method in which a Si/Si1-xGex short-period superlattice is first formed and then C ions are implanted into the superlattice followed by annealing for allowing implanted C ions to migrate to Si layers.