How does the interatomic spacing of alloys differ from that of pure crystals?

[Helena Wells]

In pure crystals the interatomic distance is well defined :in a carbon crystal lattice it is the distance between 2 carbons.

However if we have an alloy(Silicon-Germanium) how does the spacing between atom work?

If we have SiGe(50-50 alloy) how can we find the interatomic distance?

 

[Useful nucleus]

Alloys can be ordered or random. IF the alloy is ordered , then their will be a set of well defined bond lengths (Si-Si, Ge-Ge, and Si-Ge). IF the alloy is random, then we will have a "spectrum" of bond lengths.

 

[Helena Wells]

Alloys can be ordered or random. IF the alloy is ordered , then their will be a set of well defined bond lengths (Si-Si, Ge-Ge, and Si-Ge). IF the alloy is random, then we will have a "spectrum" of bond lengths.

Don't the interatomic distances depend on the lattice costant , which varies linearly with the composition of the 2?If it is as you say how can we draw a band diagram for an alloy?

 

[Useful nucleus]

For a random alloy, the lattice parameter reported is an average over all unit cells. Each unit cell can have a slightly different lattice parameter than other unit cells. Thus, we have an average lattice constant but a distrubition of bond lengths.

The band diagram is a different story. If you have a dilue limit doping (say less than 1%), then the band diagram is the same for the host (99%) modified by dopant (or impurity) levels. Once the doping moves into the conecetrated alloy regime, the levels merge to form a continuous band of their own modifying the host original band diagram. Because of the formation of the new continuous bands, the collective band gap of the alloy can change dramatically (e.g. closing the gap and changing from a semiconductor to metal).

 

[Helena Wells]

For a random alloy, the lattice parameter reported is an average over all unit cells. Each unit cell can have a slightly different lattice parameter than other unit cells. Thus, we have an average lattice constant but a distrubition of bond lengths.

The band diagram is a different story. If you have a dilue limit doping (say less than 1%), then the band diagram is the same for the host (99%) modified by dopant (or impurity) levels. Once the doping moves into the conecetrated alloy regime, the levels merge to form a continuous band of their own modifying the host original band diagram. Because of the formation of the new continuous bands, the collective band gap of the alloy can change dramatically (e.g. closing the gap and changing from a semiconductor to metal).

Yeah i thought the same a P type semiconductor can be considered an 'alloy' and the band diagram of is just the band diagram of the semiconductor with the only addition being the acceptor level. About higher concentration(50-50) if I understand correctly are you saying we can have '2' valence bands and by 2 I mean the valence band before the 'doping' and a valence band formed due to the injected atoms and they form a single energy band?

 

[Useful nucleus]

You are right the band diagram before doping is different that the band diagram after doping. Together both elements will form new valence and conduction bands.