[DFTB-Plus-User] R: Fermi Energy

Alessandro Pecchia pecchia at ing.uniroma2.it
Wed May 15 07:44:55 CEST 2013


I guess you are comparing Ef from contact calc with Ef that makes the device neutral in the open calc.
In principles the two shoud converge to the same value.
Mind however that a sufficient k-sampling might be necessary when you perform contact calculations. In this respect be carful to set the right supercell folding indeces. The first index refers to the supercell vector in the transport direction. See the SiNW example.
As a second point, the Poisson grid needs to be tight enough and be careful to define a sufficiently large PL since small errors may rise at the contact/device interfaces.

If you apply a small gate potential you should be able to appreciate small charge accumulations on the tube.



Xiaobao <xiaobaojsyz at 163.com> ha scritto: Dear dftb+ users:

I am currently working on carbon nanotube using the nonequilibrium Green's function part of the code . First, I have some fundamental questions about setting up the Fermi energy in a transport calculation. Currently, I take the fermi energy calculated from the source/drain calculation step. However, when I perform a self-consistent calculation on a longer tube (source+device + drain region) to determine the fermi energy, its value changes slightly. Which approach is more appropriate for obtaining the fermi energy?
The second question is relating to the first one. The transport direction in my calculation is set to be along the tube axis. I am interested in obtaining accumulate charge density under gate bias. Please be mindful that the bias, is applied along the gate direction, not the transport direction. The applied voltage along the transport is set to zero. My preliminary results didn't show any correlation between the charge accumulation and the applied gated bias. I am looking for the charge change in the order of 10^-4 to 10^-5 in electron unit. Are there anything else I need to be careful about, despite the Fermi energy setting?
Best wishes,

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