[QE-users] Shifting of Fermi level by applying bi-axial strain

[Lorenzo Paulatto]

On 05/07/18 11:52, Rajneesh Chaurasiya wrote:
> In basic semiconductor, if the Fermi level is close to conduction band 
> then its called n types semiconductor while if the Fermi level is close 
> to the valance band is called p type semiconductor.

It the Fermi energy is *in* the conduction band, because of electrons 
doping, it is a m-type semiconductor. It the Fermi energy is in the 
valence band, because of hole doping, it is p-type.

If there is no doping, the Fermi energy is in the gap: the semiconductor 
is to all effects an insulator.

> In most the software like Wien2k Fermi level is located close to the 
> valance band means all states are filled upto that level and also remain 
> same by applying the strain. but in espresso, its switch to valance band 
> maxima and conduction band minima by according to applied strain. i 
> think, its not acceptable. please correct if i am wrong.

I'm not sure I understand what's your complaint. The zero of the energy 
bands is somewhat arbitrary. If you have a precisely-defined Fermi 
energy, like in a metal where the position of Ef is strictly constrained 
by the number of electrons, than you can use it to align different 
calculations. If your Fermi energy is not so precisely defined, like in 
a insulator where you can put it anywhere in the gap, then you should 
use something else to align the bands: the maximum of the valence band 
is a possible choice.

Using a different type of smearing function (m-v, m-p, fd) could make 
the Fermi energy more stable. But the main point is that if you have a 
gap, it is usually better to treat your system as an insulator 
(occupations="fixed") and use the highest occupied level instead of the 
Fermi energy.

hth

> 
> 
> On Thu, Jul 5, 2018 at 11:47 AM, Rajneesh Chaurasiya 
> <rajnano2012 at gmail.com <mailto:rajnano2012 at gmail.com>> wrote:
> 
>     Dear All,
> 
>     I have computed the electronic band structure under the compression
>     and tensile strain. Change in the conduction band minima and valance
>     band maxima are slightly shifted with strain which i have also
>     verified from the WIEN2K but when i add or subtract the Fermi
>     energy, then i observed in some cases, Fermi level close to
>     conduction and some cases the Fermi energy close to valance band.
>     since by applying the strain the types of semiconductor behavior (n
>     type or p type) does not change. then why the Fermi level is shifted
>     close to valance band maxima or conduction band minima. such types
>     of behavior are not observed in the WIEN2k. why the Fermi level is
>     shifted according to strain?
> 
>     -- 
>     Thanks & Regards
>     Rajneesh Chaurasiya
>     Research Scholar
>     IIT Jodhpur, India
>     Mob. No. +91-9584499697
>                    +91-7610950803
> 
> 
> 
> 
> -- 
> Thanks & Regards
> Rajneesh Chaurasiya
> Research Scholar
> IIT Jodhpur, India
> Mob. No. +91-9584499697
>                +91-7610950803
> 
> 
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-- 
Lorenzo Paulatto - Paris