Silicon Germanium (SiGe) Bandstructure strange behaviour

[Dhirendra]

Hi,

I want to use the SiGe material around 30% Ge. I replaced 3 out of 8 atoms in a 8 atom cubic Si cell. Relaxed and calculated the bandstructure. Strangly the bandgap is a direct bandgap. I did 50% Ge in 8 atom cell (4 Si atoms 4 Ge atoms) bandgap is indirect. Any comments.

[Anders Blom]

You should rather consider a structure with 100 atoms and replace 30. An 8-atom system will be a very crude approximation to a random alloy.

Have a look at http://quantumwise.com/publications/tutorials/item/850-effective-bandstructure-of-random-alloy-ingaas for a more systematic approach to this concept.