IV curves from analyzer

[esp]

ok thank you i just want to make sure i understand correctly ..

i have another question . my understanding is that when making changes to atoms, such as i am doing with selective doping locations, .. you need to optimize the geometry in order to get a realistic structure .. my understanding is that although you can specify atoms in any position, that that is not realistic unless you let the optimizer relax the forces, etc .. in order to make a "Stable" configuration, for lack of a better word ... is this a correct understanding, and is it safe to say optimization has to be done always when making precise atomic level changes to structures like nanoribbons?  


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i found this statement in the tutorial here: http://quantumwise.com/documents/tutorials/latest/ATKTutorialDevice/index.html/chap.relax.html

"Applying a finite bias will cause the position of atoms to shift slightly, so the geometry should in principle also be optimized under bias. However, the optimization under bias can be very time consuming and usually only has little effect on the transmission and is thus often omitted."

Now, i am concerned about the "usually" statement ... do you think it is possible the results i am seeing for current versus Vgs that i posted earlier could be beacause i am not optimizing after doping the electrodes?

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and one more question ...

what i am doing is creating p-i-n tfet GNR structures, doped with N and B in the electrodes ... i am running bandstructure and transmission calcs ... now:
(1) should i optimize geometry after replaceing carbon atoms with N and B? 
(2) when should I optimize? (before bandstructure calc? before trans calc?  before any calc? ) I want to reduce time but be accurate
(3) as noted above, i see that it seems i would need to re-optimize under each bias ... does this include different gate voltages, or just Vds bias?

ed

thank you

[Anders Blom]

ok thank you i just want to make sure i understand correctly ..

i have another question . my understanding is that when making changes to atoms, such as i am doing with selective doping locations, .. you need to optimize the geometry in order to get a realistic structure .. my understanding is that although you can specify atoms in any position, that that is not realistic unless you let the optimizer relax the forces, etc .. in order to make a "Stable" configuration, for lack of a better word ... is this a correct understanding, and is it safe to say optimization has to be done always when making precise atomic level changes to structures like nanoribbons?  

Usually, yes (with a wink to your question below).

i found this statement in the tutorial here: http://quantumwise.com/documents/tutorials/latest/ATKTutorialDevice/index.html/chap.relax.html

"Applying a finite bias will cause the position of atoms to shift slightly, so the geometry should in principle also be optimized under bias. However, the optimization under bias can be very time consuming and usually only has little effect on the transmission and is thus often omitted."

Now, i am concerned about the "usually" statement ... do you think it is possible the results i am seeing for current versus Vgs that i posted earlier could be beacause i am not optimizing after doping the electrodes?

No, I don't, because you are looking at an effect which is a function of Vgs, not as a function of the inserted doping. Optimizing might change the overall curve, slope/offset etc, but not its internal properties. At least I wouldn't expect that in such a system as yours.

and one more question ...

what i am doing is creating p-i-n tfet GNR structures, doped with N and B in the electrodes ... i am running bandstructure and transmission calcs ... now:
(1) should i optimize geometry after replaceing carbon atoms with N and B? 
(2) when should I optimize? (before bandstructure calc? before trans calc?  before any calc? ) I want to reduce time but be accurate
(3) as noted above, i see that it seems i would need to re-optimize under each bias ... does this include different gate voltages, or just Vds bias?

1. I think that would be wise, but you can probably do this "locally", i.e. check how the B-N bond compares to a C-C bond when embedded in a graphene matrix; probably the difference is small, and the primary effect on the current will be the fact that you now have a dopant present, rather than the small structural details.
2. The optimization is sort of step #1 always, after creating the starting guess geometry. Once you have it you don't need to reoptimize unless you change atoms (as for bias, see 3).
3. This is rarely necessary, the effects of this are usually very small, and typically only relevant if that's the specific effect you are investigating.

The burden of evidence is however always on you

[esp]

thank you very much

[esp]

what about running self consistently?  is it needed for optimization calculations or can i turn it off when running the optimization (and still get accurate results)? 

[esp]

How do I know if the optimization did anything that is useful?  in other words, for example, I ran optimization on a structure, it took 6 hours on a $12k computer with 24 cores, ... then i look at it and it looks exactly the same ... should i compare the coordinates?  What else does optimization do?

[Nordland]

Optimization only does one thing. Move atoms (and possible changes the cells if you enabled it).
When the atoms move the Hamiltonian of the system changes, and hence almost everything derived from the Hamiltonian might change.
So in very short - optimization can change everything.

You can compare coordinates but small displacement might seem insignificant, but they can also be significant.

So the way I usually go about it for devices, is like this:

• Do a calculation of 0.5 Volt bias and calculate the Transmission Spectrum
• Do a optimization at 0.5 Volt bias, and calculate the Transmission Spectrum
• Compare the results to one another (perhaps also a zero bias Transmission Spectrum), and if they are significant different (which they rarely are), I know that this systems strongly depends of the exact coordinates of the atoms, and hence I need to optimize the structure for each bias

[esp]

ok thank you very much

[Anders Blom]

Regarding your question of self-consistency, it's not just a simple choice you can make. Rather, there are some semi-empirical methods where the parameters themselves are derived to fit in a self-consistent or non-self-consistent calculation scheme, and using them inversely can give unpredictable results. So one must actually know which is the case for the method you plan to use.

DFT is inherently self-consistent only, so the non-self-consistent results are dangerous. They can be used for quick-and-dirty calculations, or (at least with some care) to check for convergence in e.g. k-points. In ATK, non-self-consistent calculations for devices has a special meaning of running only the equivalent bulk (the central region) self-consistently and using this as the result for the device. This is a crude approximation, but again it can be used for some early checks - or, to run an optimization of the device geometry (i.e. by optimizing the central region, which typically is a good approximation).

We should make some of these things clearer in the future, so it doesn't appear a completely free choice.