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

Yuranan Hanlumyuang yhanlumyuang at uh.edu
Thu Mar 28 16:43:42 CET 2013


   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
        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
In the mean time, any comments will be greatly appreciated.

Yuranan Hanlumyuang
University of Houston
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