What makes semiconductor indirect or direct?

[scivet]

I'm wondering what determines Si indirect semiconductor?
Si is cubic diamond structure. So it's optically and electrically isotropic (1st&2nd rank tensor properties). And why is GaAs direct semiconducto even though its crystal structer and chemical bonding is so simliar to Si?
Isn't it crystal structure-related?
Is there any cubic structure materials with indirect band gap?

Thanks in advance.

 

[0xDEADBEEF]

Well I guess you know what direct and indirect means, but I'll repeat it nonetheless. Indirect semiconductor means that the highest energy of the valence band is at a different point in k-space than the lowest point of the conduction band, making it a bad photon absorber/emitter. The group theory which is dependent on the lattice structure determines only where bands can split and where they can cross, not if they do or how they bend.
From my gut feeling direct semiconductors should happen fairly often because we just need a slightly broken symmetry to get a nice "avoided crossing" with the peaks opposing each other.

 

[Dr Transport]

The chemical species is a large contributer to where a semiconductor is direct or indirect. The X-Nitride's are predominately indirect, as well as the x-Phosphide's. Even Silicon is an indirect semiconductor. A possible reason is the mass of the compound.

Just a guess...