Messages

46

General Questions and Answers / Re: Calculation using STD method cannot get converged

« on: September 7, 2018, 18:07 »

Thanks for your reply! For non-STD device, do you mean the non-interacting case? And for lack of screening in the electrode region, can I solve it by increase the electrode length? But by how much? How do I decide?
More importantly, why the Huckel method cannot be applied to STD simulations? Is there any other available method other than the DFT method? (which could be very computationally expensive for this device) For example, like the  ATK-SE Slater-Koster?

Best regards!

47

General Questions and Answers / Re: Calculation using STD method cannot get converged

« on: September 6, 2018, 02:04 »

I suspect that the width mismatch of the GNR at left/right electrodes may be one possible reason that cause difficulty in convergence (the reason for using a wider GNR for electrodes is that AGNR of such width is nearly metallic, while a narrower one in the central region is semi-conductive).
So I did the noninteracting (without considering e-ph interaction) calculation for the same device iv_scan_300k_pristine.py. And  I found out even for noninteracting calculation the convergence is still hard. The negative bias half all failed to converge and the calculated current at the negative region shows remarkable oscillation, which I believe is wrongly calculated..


I don't know if my suspect is reasonable or not. But that is what I thought. Any comment or advice is appreciated!

48

General Questions and Answers / Calculation using STD method cannot get converged

« on: September 5, 2018, 23:57 »

Dear Sir,
I am studying the the inelastic current of a graphene nanoribbon Based TFET device using the STD method.

 I have read through the case studyhttps://docs.quantumwise.com/casestudies/std_transport/std_transport.html on Si device and trying to do the same thing on a GNR based device. So basically I  first computed the dynamic matrix of my device (GNR_PN_dynmat.py), then I computed the corresponding std configuration of my device (std-300k.py), next I started a loop to calculate the DeviceConfiguration at different bias voltage(300K_iv_scf.py), finally I calculated the transmission spectrum at these bias voltage (300K_transmission.py). The procedure is same as shown in the case study. The only difference is that I used a different forcefield potential Tersoff_CH_2010, since the materials is carbon rather than Silicon. And I used the Extended Huckel calculator rather than the DFT-LCAO calculator to calculated the DeviceConfiguration and transmission spectrum.
But the hard part is that the DeviceConfiguration calculation is very difficult to get converged.  Actually most of them did not converged at the max step. As a result the calculated current is oscillating wildly, making it hard to interpret:

The result is very unreasonable to me, and I don't know if it is solely due to the unconverged calculation.
So, can any one help me with:
(1)  why is the STD calculation so hard to get converged, and is there any specific advice for increase the convergence chance for my device calculation?
(2) If the calculation converged at all bias points will the oscillation diminishes, and the current curve looks more reasonable?

49

General Questions and Answers / Re: Effect of electron-phonon scattering in electron transport process

« on: June 9, 2018, 05:12 »

Dear Petr Khomyakov and Wang Jin,
You can also check this paper IEEE Transactions on Electron Devices 65.6 (2018): 2654-2659. It treats a GNR transistor with EPC considered, using self-consistent NEGF via TB Hamiltonian. The materials system and method might be different (or might possibly be similar)  from that in  IEEE Trans. Electron Devices 2013, 60, 4133 but it gives very similar phonon effect on the ON state and OFF state current.  I wonder if the calculation method rather than the materials system dominate the results here?

50

General Questions and Answers / Re: Effect of electron-phonon scattering in electron transport process

« on: May 25, 2018, 06:03 »

Thanks for the reply! That clarifies a lot!

51

General Questions and Answers / Re: Effect of electron-phonon scattering in electron transport process

« on: May 17, 2018, 19:15 »

I am actually mentioning the difference between the following two figures. In the left figure, the top branch is in log scale and the bottom branch is in linear scale.
 

Ref: Left PhysRevB.80.155430
         Right Phys. Rev. B 96, 161404(R)

52

General Questions and Answers / Effect of electron-phonon scattering in electron transport process

« on: May 17, 2018, 18:26 »

Hello experts,
I am studying the e-ph interaction effect in graphene nanoribbon transistor device, and I have read through some Quantumwise case study page https://docs.quantumwise.com/tutorials/inelastic_current_in_si_pn_junction/inelastic_current_in_si_pn_junction.html
as well as the STD  case study for Si p-n junction in paper Phys. Rev. B 96, 161404(R). I got the idea that phonon will have the positive effect that increases the off-state current by several magnitudes (the phonon-assisted tunneling), while unchanging the on state current.
However other papers speak otherwise. In paper  PhysRevB.80.155430, they studied Si nanowire and their simulation shows an opposite result that the off-state current is rather unchanged, but the on-state current is reduced because of the phonon scattering. Some other paper also indicates so. For example in a paper named " Role of phonon scattering in graphene nanoribbon transistors: Nonequilibrium Green’s function method with real space approach" They also give a similar result for GNR transistor.

I just wonder why does the STD method give an opposite result than others? Is there anything that I misunderstand here?

Thanks!