NEGF query

[ams_nanolab]

Dear Sir,

I'm looking to undertake NEGF simulations of some direct and indirect band gap nano sheets.

As I understand an effective mass hamiltonian at K point is used to carry out NEGF studies in most such systems such as monolayer MoS2 (direct band gap), but if we go to multilayer MoS2 then the VB maxima and CB minima no longer are at K point.

Kindly advise what difference in the approach is to be applied when we are treating indirect band gap materials under the NEGF formalism.

[Anders Blom]

This isn't only related to NEGF, but also affects the electronic structure part of the calculation, and is handled as normal by performing the correct k-point sampling, both for the self-consistent calculation and the transmission spectrum computation.

[ams_nanolab]

Is it possible to carry out nanoscale MoS2 MOSFET simulation (Transmission spectra and more importantly Id-Vg, Ig-Vg) with ATK?
As far as I could gather from the tutorials GNR FETs are well covered by ATK but what about nano sheets of larger dimensions say 10nmx10nm, could ATK successfully carry out MOSFET performance of MoS2 transistors having such dimensions?

Also could you kindly add a tutorial on building a nano MOSFET from scratch...using the Custom Builder for GNR FETs is okay but for other materials like MoS2, WS2 etc. we need to build the device step by step, a tutorial in this sense would be helpful.

[Anders Blom]

It's pushing the boundaries a bit, but if you have plenty of memory in the computer and a bit of patience, there is no formal upper limit to the number of atoms you can handle. In 12.8 we are btw introducing some new tools for estimating the memory usage, and also tips on how to reduce it. This can be rather crucial for such large systems. For a preview, see http://quantumwise.com/documents/tutorials/ATK-12.8/MemoryUsage/ (requires 12.8!)

[ams_nanolab]

Also could you kindly add a tutorial on building a nano MOSFET from scratch...using the Custom Builder for GNR FETs is okay but for other materials like MoS2, WS2 etc. we need to build the device step by step, a tutorial in this sense would be helpful.

[ams_nanolab]

I have also seen some papers that use the DFTB approach coupled with NEGF to solve problems faster... do you think such an approach could help in such nano MOSFET.....??

[Anders Blom]

DFTB is definitely faster than DFT and generally allows you to treat larger systems. Of course, as always with parameterized models one must be more careful not to step outside the domain for which the parameters are valid, and of course it will not work at all if you desire to include other elements not part of the parameter set.