[DFTB-Plus-User] Different Fermi energy calculated by DFTB+NEGF and only DFTB+

[Gabriele Penazzi]

On 05/08/2014 05:06 PM, Enlong Liu wrote:
> Dear all,
>
> I am doing transport calculation to boron nanotubes (BNT). But I find 
> out that there is a huge difference in fermi energy in different cases.
>
> In the transport calculation, I used DFTB+NEGF to calculate the 
> contact firstly, which is part of BNT at one end and the result is 
> -10eV approximately. But if I just calculate the DOS or band structure 
> to the same BNT part, the fermi energy is just -4eV. It is interesting 
> since I think all other parameters are the same, only the calculation 
> situation is different, one is just DFTB+ for DOS, the other includes 
> NEGF to calculate contact properties before doing transport calculation.
>
> I tried some other system like Si or Ag. There is the same problem, 
> fermi energy difference in common calculation and transport 
> calculation. Could any one help me with that?

Dear Enlong,

we need to be a bit careful here as there is a couple of issues.

First: is the system a semiconductor or a metal or a degenerate 
semiconductor?
In the first case, as long as the Fermi level is in the gap, at zero 
temperature there's no difference between quantitative differences in 
the numerical value.

Second: be careful with the k point sampling, sometimes this can look 
confusing. Check the manual to verify that your k-points are consistent 
between dos and band structure calculation.

Third: compare consistently. I am not sure whether it makes difference 
or not, but to be safe just compare quantities you get with the same 
version of the code as the Fermi level, being an energy, is defined up 
to a given reference.

Fourth: if all the above is verified, in some specific cases there could 
be still unknown issues. My experience is that in non-periodic systems 
everything works well. In periodic systems there could be more delicate 
cases due to the different boundary conditions in the poisson. 
Nevertheless the huge difference (6 eV) makes me believe that you are in 
one of the previous cases. It is a good idea that you check your 
equilibrium calculation and verify that at the interface between contact 
and device region discontinuities in the potential are not large, 
because that is often a signature of a wrong fermi level.

Regards,
Gabriele


>
> Thanks a lot!
>
> Best regards,
> Enlong
>
> -- 
> Faculty of Engineering at K.U. Leuven
> BIOTECH at TU Dresden
> Email:liuenlong20 at gmail.com <mailto:liuenlong20 at gmail.com>; 
> enlong.liu at student.kuleuven.be 
> <mailto:enlong.liu at student.kuleuven.be>; 
> enlong.liu at biotech.tu-dresden.de <mailto:enlong.liu at biotech.tu-dresden.de>
> Mobile Phone: +4917666191322
> Mailing Address: Zi. 0108R, Budapester Straße 24, 01069, Dresden, Germany
>
>
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-- 
Dr. Gabriele Penazzi
BCCMS - University of Bremen

http://www.bccms.uni-bremen.de/
http://sites.google.com/site/gabrielepenazzi/

phone: +49 (0) 421 218 62337
fax: +49 (0) 421 218 62770

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