Direct Gap In Ordered Silicon Carbon Alloys

We show that alloying silicon with a few percent of carbon can render the band gap direct with strong optical absorption, provided the carbon atoms are ordered. The addition of carbon introduces a significant s character into the conduction band minimum, resulting in a large dipole matrix element. First-principles calculations of the optical absorption in ordered in CxSi1-x alloys for x = 1/54 and 1/32 show a near band edge absorption coefficient about half that of GaAs. (C) 1999 American Institute of Physics. [S0003-6951(99)03845-0].