QuantumATK Forum

QuantumATK => General Questions and Answers => Topic started by: Peraz on September 27, 2016, 22:16

Title: Boundary conditions for transport simulation
Post by: Peraz on September 27, 2016, 22:16

Why in Quantumwise  Dirichlet boundary conditions is used for the potential at the source/
drain contacts when solving the Poisson equation for a nanowire?

Does not Diriclet boundry condition limit the potential in the source/drain from following the distribution imposed by quantum mechanics^[1]?

Should not one use Neumann boundary conditions for the potential at the source/drain contacts  to maintain charge neutrality?


[1] A. R. Brown, A. Martinez, M. Bescond and A. Asenov, "Nanowire MOSFET variability: a 3D density gradient versus NEGF approach", Silicon Nanoelectronics Workshop, 10–11 June, Kyoto, Japan, pp. 127–128 (2007)

Title: Re: Boundary conditions for transport simulation
Post by: Daniele Stradi on September 28, 2016, 09:24

Dear Peraz,

both conditions are valid, but at the moment we use Dirichlet because the SCF convergence is easier, as the electrodes do not enter in the SCF loop. Using Dirichlet has the only additional constrain that you have to ensure that the electric field is zero at the boundary with the electrode.

We are working on an implementation of the Neumann boudary conditions, but it is not straightforward because the electrodes have to be treated self-consistently within DFT.

Notice also that it should be possible to select all and only the valid options in the Script generator in VNL.

Regards,
Daniele.