Messages

241

General Questions and Answers / Re: the scf of iv-curve in some bias did not convengency, some did converge

« on: August 31, 2016, 10:33 »

Hi,

it is quite difficult to say how to modify the convergence parameters without seeing the system and the *.py input script.
Could you attach it here or send it via a private mail?

Thanks,
Daniele

242

General Questions and Answers / Re: Question about Modeling metal–semiconductor contacts: The Ag–Si interface

« on: August 30, 2016, 16:39 »

Yes, check also the note in the tutorial:
http://docs.quantumwise.com/casestudies/ag_si_interface/ag_si_interface.html

243

General Questions and Answers / Re: Question about Modeling metal–semiconductor contacts: The Ag–Si interface

« on: August 30, 2016, 16:02 »

r_pot is the value of the potential obtained from the HartreeDifferencePotential whereas CBmin is the value of the conduction band minimum obtained from the DeviceDensityOfStates.

Ideally the two should match, but in practice there is always some small uncertainty, since one has to decide the threshold under which the DDOS is equal to zero. In the script the threshold is set to 0.01 eV^-1:

# Threshold under which the DOS is considered zero (Band gap region)
thr = 0.01

Regards,
Daniele.

244

General Questions and Answers / Re: the electrics difference potential （ EDP） of MGGA results are trustfuL or GGA ?

« on: August 30, 2016, 09:57 »

Hi,

the MGGA results can not be trusted for slab systems, or whatever system in which you have a vanishing electronic density.

In fact, the exchange term in the MGGA functional depends on the inverse of the electronic density, and diverges at low electronic densities. This might lead to very non-physical results. See Eq. (1) in PRL 102 226401 (2009).

Regards,
Daniele.

245

General Questions and Answers / Re: Opening a band gap in silicene sheet with an electric field

« on: August 29, 2016, 09:27 »

Hi,

I just performed the calculation of the 2x2 silicene with a 20 V applied electric field, following the recipe in the tutorial, and I indeed observe the band-gap opening.

Also, please refer to the new documentation page for the tutorials:
http://docs.quantumwise.com/tutorials/opening_a_band_gap/opening_a_band_gap.html?highlight=silicene

Regards,
Daniele.

246

General Questions and Answers / Re: Question about Modeling metal–semiconductor contacts: The Ag–Si interface

« on: August 29, 2016, 08:51 »

Hello,

r_pot gives the value of the averaged potential at the rightmost side of the interface, e.g., at Z = 400,200,80 Ang of Fig.6 in PRB 93 155302 (2016).

In fact, few lines later in the script you can see that the Schottky barrier at the interface is evaluated as:
schottky = max(av_pot-r_pot)

Regards,
Daniele.

247

General Questions and Answers / Re: iv-curve calculation stoped with error in atk2016

« on: August 25, 2016, 09:32 »

Hello,

it is a little difficult to say what's gone wrong, from the information you provided. It would be great if you could provide the python script used for these calculations.

Regards,
Daniele.

248

General Questions and Answers / Re: Box on top of any S.C. surface

« on: July 13, 2016, 13:03 »

Dear Dipankar,

it would be great if you can specify:
1) which configuration you are talking about (DeviceConfiguration, BulkConfiguration?)
2) which potential drop you are interested in (between the source and drain electrodes? between the gate and the device?)

Daniele

249

General Questions and Answers / Re: Box on top of any S.C. surface

« on: July 13, 2016, 10:34 »

If you do finite V_G - V_G=0, you will obtain the difference between the two potentials. Then you can do a planar average to see the potential drop between the gate and the 2D layer.

250

General Questions and Answers / Re: Box on top of any S.C. surface

« on: July 11, 2016, 13:06 »

Dear Dipankar,

the grid that you specify using the keyword "density_mesh_cutoff" (see https://www.quantumwise.com/documents/manuals/latest/ReferenceManual/index.html/ref.numericalaccuracyparameters.html) is a discrete 3D grid in real space. The potential is represented on this grid. A spatial region (for example, a metallic gate) modifies this potential in a certain region of space, but in order for this to work the spatial region MUST include the points of the grid.

Take a simple 1D example: let's say that we have a C vector of 10 Angstrom, and a grid of 1000 points along C. Hence, we will have a grid spacing of 100/1000 = 0.1 Angstrom, which means having a grid point every 0.1 Angstrom. If the thickness of the spatial region is, for example, 0.001 Angstrom, it can happen that NONE of the grid point is included in the spatial region. The minimum thickness, in this case, would be 0.1 Angstrom.

Daniele. 

251

General Questions and Answers / Re: Use of a user specified norm conserving pseudopotentials in the calculations

« on: July 11, 2016, 10:03 »

Dear Guangping,

yes, ATK can read *.pdf format.

Daniele

252

General Questions and Answers / Re: Box on top of any S.C. surface

« on: July 11, 2016, 09:55 »

Dear Dipahar,

1) No, in principle, there is no strict requirement about the gap. One important point, however, is that the spatial region should be thick enough to include at least 1 point of your real space grid. You will get an warning if this requirement is not complied.

2) To find the (electrostatic) potential drop, you have to calculate the HartreeDifferencePotential (or the ElectrostaticDifferencePotential, if you are using a version prior to ATK 2016) for the case V_bias==0 V and V_bias=/0. Then you have to subtract the two, and that will give you the potential drop across the semiconductor.

253

General Questions and Answers / Re: How to identify the bandstructure of a specific material

« on: May 30, 2016, 08:38 »

http://www.quantumwise.com/documents/manuals/latest/ReferenceManual/index.html/ref.projectionlist.html

254

General Questions and Answers / Re: Change bond lengths and atoms in half of the system

« on: May 27, 2016, 08:38 »

Dear James,

in VNL, you can do it in the way you mentioned by using the Periodic Table tool to change the atom type, and the Coordinate Tools -> Transform by expression in the Builder to change the atomic distances.

Alternatively, you can also use the interfface builder (got to Builder -> Interface) to create the interface starting from the bulk structures of the two materials. See also http://docs.quantumwise.com/tutorials/ag_au_interface.html?highlight=interface

Regrads,
Daniele

255

General Questions and Answers / Re: asymmetric molecular junctions

« on: May 20, 2016, 12:55 »

Dear Sweta,

there is absolutely no problem in doing heterogeneous systems such as asymmetric junctions, where "heterogeneous" means that the right and left electrodes are not the same.

Here you can find two articles with calculation on heterogeneous systems:
http://journals.aps.org/prb/abstract/10.1103/PhysRevB.88.075317
http://journals.aps.org/prb/abstract/10.1103/PhysRevB.93.155302