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Topics - weixiang

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1
Dear QuantumATK experts,
I would like to know if there exists a physical limit for the transistor On/Off ratio in terms of the channel material energy band gap.
For example, Si has a band gap of 1.14 eV and it may have some kind of limits on the maximum On/Off ratio in the form of exp(Eg/KT)?
I am not sure about this relation exists or not because I don't have a strong background in physics, but I would like to justify my result from the calculation.

Thanks!

2
Hi,
I would like to do an electron transport analysis using the transmission eigenstates tool in VNL.
I have the following two questions regarding the isosurface plot option for the eigenstates.
(1) For choosing the isovalue, should I choose the isovalue = the eigenvalue that I am interested in? e.g. I have a eigenvalue = 0.5, I should choose the isovalue = 0.5?
I checked this tutorial page and it seems to say so. https://docs.quantumatk.com/manual/Types/TransmissionEigenstate/TransmissionEigenstate.html#transmissioneigenstate-c
(2) The phase of the eigenstates is expressed using a color legend, what does the phase of the eigenstates mean? Does it have anything to do with the transmission contribution?  How to interpret it physically?

Any help is appreciated! Thank you in advance.


3
Dear QuantumATK expert,
I would like to calculate the total inelastic transmission for a device at positive 0.2V bias. Should I sum up the inelasticTransmissionAsymmetricPositiveBias and  inelasticTransmissionSymmetricPositiveBias because my device is at a positive biased state, and thus neglect for negative transmission? Or should I sum up all the 4 transmissions? (neg_sym, neg_asym, pos_sym, pos_asym)

What is the meaning for negative transmission generated at a positive biased state? i.e. the options in the plot settings of Inelastic Transmission Spectrum Analyzer of VNL.

Thank you!

4
Dear ATK experts,
I am studying the e-ph interaction in a two-probe device. I am wondering if it is possible to calculate the local inelastic current and plot them pictorially,  just like the TransmissionPathway did.
From my understanding, the transmission pathways split the transmission coefficient into local bond contributions Tij and plot them using the volume-weighted arrow on the atomic bonds, which is pretty nice and helps a lot in carrier transport analyzing. But it seems like it only calculates from the regular transmission coefficient. i.e. the transmission without considering e-ph interaction.
So my question is, in ATK, is there a way to calculate the local inelastic current which takes into account the e-ph interaction effect?

Thanks!

5
Hi,
I am calculating the InelasticTransmissionSpectrum for a defective GNR device which requires DynamicalMatrix and HamiltonianDerivative calculated first.
I am wondering if the  Dynamical matrix and Hamiltonian Derivative calculation are independent of the spatial region? i.e. For the same device configuration with different Gate voltage on the metallic region, do I need to calculate the  Dynamical matrix and Hamiltonian Derivative differently?   Or I can use the  Dynamical matrix and Hamiltonian Derivative calculated at one gate voltage for all other gate voltage.
I am studying the transfer characteristics of the device, so I need to calculate the inelastic current of the device at different gate voltage.

Thank you!

6
Hi,
Is it possible to use the InelasticTransmissionSpectrum module in ATK to account for e-ph interaction and calculate the inelastic current for a defective device? e.g. a GNR tunneling transistor where the GNR  has edge/vacancy defects.
There is already a QuantumWise tutorial of calculating TransmissionSpectrum for a GNR device with Stone-Wales defect. I wonder if the InelasticTransmissionSpectrum also applies to defective structures.
If yes, are the procedures different from calculating the inelastic current of a pristine device?
Thanks!

7
Hi,
Is there a way to calculate the spectral current from the InelasticTransmission spectrum object? e.g. just like the regular method used in normal transmission spectrum TransmissionSpectrum.spectralCurrent()?
I checked the manual, but there is no such method or any related method for InelasticTransmission class.
In the VNL, from the Inelastic Transmission Spectrum Analyzer I can see the spectral current, but I don't know how to export the data.

Thank you in advance for any help!

8
Hello,
From the Inelastic Transmission Spectrum Analyzer I got the I-V curve (attached image.png) for 0-0.2V bias voltage and it can be noted that at bias = 0.2V the sum current (elastic current, inelastic current as well) is of ~10-22 A = 10-13 nA
However, I also generate its corresponding spectral current at 0.2V.(attached image-1.png), it can be approximated from the area under the curve that the current is ~10-14  A= 10-5 nA.
These two currents are supposed to be same with each other, but now they are 8 order of magnitude different.
Why is the case? Is this a bug in ATK?

Please check my attached scripts, I wonder if there is anything wrong in my calculation procedure.

Thank you! Any help is appreciated!

9
Dear Quantumwise experts,
I am calculating the inelastic transmission spectrum of a GNR p-n junction (see attached python file for the configuration) using the InelasticTransmissionSpectrum method in ATK.
First I calculated the Transmission spectrum and PLDOS without the e-ph interaction, which gives a current I = -8.3168 e-5 nA.  As shown  In the attached image that from the spectral current and Projected Local Density of States it can be noticed the major current contribution is from the tunneling near the Fermi level. So I am expecting the sum current (elastic+inelastic) from the InelasticTransmissionSpectrum is several magnitudes larger than that from the TransmissionSpectrum as is shown in the  case study of https://docs.quantumwise.com/tutorials/inelastic_current_in_si_pn_junction/inelastic_current_in_si_pn_junction.html
I just wonder that is it possible to get a reasonable current by just taking one or a few representative energy points in the InelasticTransmissionSpectrum method parameter setup, e.g.  0 eV or (-0.05, 0, 0.05)*eV ?
Since the spectral current at these energies is largest.
I have the InelasticTransmissionSpectrum sum current for  0 eV which is -1.38e-4 nA (one magnitude larger than the non-interacting case) and for (-0.05, 0, 0.05)*eV which is -5.84e-5 nA. (slightly smaller than the non-interacting case)
They are quite different, but which one is more accurate?  Or maybe neither?

Thank you!
Happy New Year!

10
Hi,
Every time I try to open the VNL on my laptop it stuck and show the "not responding"  for a while, sometimes 10 mins, 20mins, and then it resumes normal to use.
This issue bothers a lot. Is there a way to solve this?

11
Hi,
I have calculated the transfer characteristics for a GNR TFET device considering the electron-phonon interaction using the STD method at different temperatures, as shown in the left image of the attached file.
I also calculated the transfer characteristics for its corresponding pristine configuration ( no e-ph interaction) at different temperatures, as shown in the right image of the attached file.
In the pristine I_V curves, the current minimum point at all temperatures are all aligned at about 1.2 V.
However, in the STD I_V curves, there are not aligned: the Imin point shift to the left as temperature increases.
I was using the Slater-Koster method with the DFTB Hotbit basis to calculate for both the STD and pristine configurations.
I just wonder why the Imin points in the STD I_V curves do not align. Is this a physical phenomenon or is this something to do with the STD method?

Thanks!

12
Dear Quantumwise Experts,
In calculating the Inelastic transmission spectrum, there is a speed-up method called "Phonon Energy Intervals" that instead of calculating explicitly all the 3N phonon modes of the device, it groups the 3N modes into M energy intervals and we will only need to calculate these M new effective phonon modes. So that it is much faster.
My question is to choose E0 and E1, how to ensure the energy interval [E0,E1] span that of the phonon modes of the calculated dynamical matrix.  Is there a way to check the relevant information from the dynamical matrix object?

13
Dear Quantumwise experts,
I am trying to under the  Inelastic Transmission Spectrum Analyzing result in this tutorial page https://docs.quantumwise.com/tutorials/inelastic_current_in_si_pn_junction/inelastic_current_in_si_pn_junction.html.
(1)Particularly,  in the following left figure, it shows " that the phonon that contributes the most to the current is that with index 66, and energy ℏω=63.01meV."
Since the analysis is conducted at the Γ point, does this means that this phonon mode corresponds to the red-circled point in the right figure of the phonon band structure of Si? Which is roughly of the same energy.

(2) In addition, in the phonon band structure, I can see that at the Γpoint there are only two possible energy for phonons to occupy, either 0eV or ~63meV. However, in the lower window of the left figure, there are many blue points represent phonon occupied energy ranges almost continuously from 0eV to 70meV.  This is contradictory of my understanding of the phonon band structure. Can you explain it a little on this?
Many thanks!


14
Dear ATK experts,
I would like to specify or identify the effect of the phonons on the current of a  GNR TFET device at different temperatures.
Is it reasonable to compare the current calculated from a STD configuration with the current calculated from its corresponding pristine configuration at the same temperature? I.e. the I_STD denote the current with both electron ( the Fermi-Dirac distribution)  and the phonon effect, and the I_pristine denote the current with only the electron effect.
And by the way, can I say that the I_pristine = I_noninteracting? as shown in this figure from this paper Phys. Rev. B 96, 161404(R)


Thanks!

15
General Questions and Answers / The STD configuration at 0K
« on: September 27, 2018, 21:27 »
Dear ATK experts,
I Have generated a Special Thermal Displacement (STD) configuration for a GNR geometry at 0K. From the common sense of physics, I know that there is no phonon at the absolute zero temperature.  So the STD configuration which use atomic displacement to account for the lattice phonon effect should be a perfect geometry at 0K, i.e. no phonon to account for at 0K. However, it is not as what I expected a perfect configuration of GNR. Instead, I can find some small out-of-plane displacement of the atoms.  The top image shows a perfect geometry of a GNR device and the bottom image shows an STD configuration of the same device at 0K. You can easily notice the difference.
 

So why there are still some atomic displacements even at 0K? Is my understanding of phonon mode wrong here that there does exist some phonon mode at 0K? Or Possibly that I calculate the STD configuration wrongly?

Any help is appreciated!

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