Theoretical Study on the Electronic Structure of Si-Ge Copolymers

The electronic structures of polysilane, polygermane, and their copolymers have been calculated by the first principle local density functional method. Polysilane and polygermane with trans-planar skeleton have direct band gaps of 3.89 and 3.31 eV, respectively. This direct-type band structure is conserved independently of the skeleton forms and the copolymerization. The ordered regular and/or block Si-Ge copolymerization introduces the zone-folding image in the copolymer band structures. Si_mGe_n ordered copolymers have the potential to be the ID superlattice high polymers. For Si-Ge copolymers having over five blocks, the band-edge electronic structure can be approximately estimated by using the effective mass theory, and a picture of a 1D-QW wire model can be imaged. Typical characteristics in the superlattice, the energy gaps, and optical transition profiles are theoretically discussed.