[DFTB-Plus-User] DFTB+NEGF: the Oxide radius

[Gabriele Penazzi]

Dear Yuranan,

that part has not been tested. If you can send your input file
(structure included) or a sample one in list I can give a look and see
if I spot a bug, as the input file looks fine to me.

Gabriele



On 03/28/2013 04:43 PM, Yuranan Hanlumyuang wrote:
> Hi
>
>    I am exploring the cylindrical gate option in the DFTB+NEGF code.
> This implementation is
> found in the poisson.f90 source code.  To set up a cylindrical gate, I
> assigned  the following parameters
>
>     Gate = Cylindrical{
>         GateLength[Angstrom] = 10.0
>         GateRadius[Angstrom]= 11.0
>         Kappa=3.9
>         InsulatorLength[Angstrom]=20.0
>         InsulatorRadius[Angstrom]=7.0
>         GatePotential [eV] = 0.0001
>     }
>
>    However, after a number of checks,  the inner gate radius and inner
> oxide radius
> always overlap, i.e. no oxide exists in the computation. I am
> wondering if this is a bug in
> the code, or I have skipped some steps.   The following is an example
> of the main output.
> The gate radius clearly coincides with the oxide radius...
>
> init Poisson ...
>  Atomic density tolerance:    1.0000000000000001E-005
>  Resulting atomic density cutoff:    7.5048399163437232      a.u.
>  Input PoissonBox=   25.0000   25.0000   25.0000  A
>  PoissAcc=   9.9999999999999995E-008
>  Bulk Boundary Potential:    No
>  Atomic cutoff radius=   3.9713905411161612      A
>  Gate: Cylindrical
>  Gate bias=   1.0000000000000000E-004 V
>  Gate length=   10.000000000000000      A
>  Oxide length=   20.000000000000000      A
>  Inner gate radius=   11.000000000000000     
> A                         <======== Problem here!
>  Inner oxide radius=   11.000000000000000      A                      
> <======== Problem here!
>  Dielectric constant of gate insulator=   3.8999999999999999    
>  Smoothing of eps_r=   5.4802053668788773    
>  Local Boundary Conditions= Squared
>  dR_cont( 1)=   2.000A
>  Local Boundary Conditions= Squared
>  dR_cont( 2)=   2.000A
>
>    There is a following line in the code parcheck.F90:
>
>      if(Rmin_ins.lt.Rmin_gate) Rmin_ins=Rmin_gate
>
>
>
>    As another check, I plotted the voltage drop across cross-section
> of the cylindrical gate. I observed
> no change in the slopes of the voltage as spatial distance ran across
> the supposed interface between
> the oxide and vacuum region. To me, this indicates that the oxide has
> never been taken into account when the poisson
> equation is solved. ...
>
>   If someone would like to test my input file, I would be happy to
> post it here.
> In the mean time, any comments will be greatly appreciated.
>
> Best,
> Yuranan Hanlumyuang
> University of Houston
>
>
>
>
>
>
>
>
>
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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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