While plotting Electronic band structure for semiconductors, the Fermi level (The upmost occupied level at zero K) should lie in between the valence band and conduction band, but why does it coincide with VBM in VASP calculations? Is there any approximation?
Dear Prabhjot Kaur ,
maybe you used a p-type doped semiconductor. Which can make sense even in a theoretical point of view. If the semiconductor you used was not p-type doped, it could be due to some approximations or even a bug. If not, you should refer to an expert in the implementation of Ab-initio code, especially VAPS for your case.
Regards,
Bayawa
Dear Prabhjot,
to answer your question : why not ?
First, at 0K, the Fermi level (FL) is a step function.
Then, in order to properly set the FL, one defines the total number of electron in the system as a function of the FL (see https://en.wikipedia.org/wiki/Density_of_states).
Therefore, at 0K, the FL can be set anywhere between the CBM and VBM.
However, as soon as T increases, the FL is better defined.
For example, it will go right in the middle of the bandgap for an undoped SM, closer to the CBM (VBM) for a n-doped (p-doped) SM.
So, at 0K, it is only a matter of definition where you put the FL.
Of course, it is better if everyone agrees to a common definition.
Usually, it is at the VBM, CBM or the middle of the gap.
Dear Bayawa Mohamed, I have not doped the intrinsic material. I have taken the pure material and after relaxing it, calculated the band structure along symmetry directions. Apart from this, if it is due to any bug or some wrongly entered tag, that is quite unknown to me.
Dear Samuel Dechamps, Perhaps I have not read the updated definition. To put reference level anywhere in the gap, where is VASP set to put it? What is the criterion?
As I'm not a VASP user, you should look in the VASP documentation.
But as I explained in my answer, it'd make sense to set it at the CBM (conduction band minimum).
It can be anywhere between CBM and the VBM, it is not impotant. Most likely the problem is numerical due to the smearing (ISMEAR) you are using.
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