General Questions

[gM]

Dear QuantumWise team,

I've been exploring ATK and have come up with a few general questions:

1. As I understand, the electron concentration can only be calculated with the DFT package.  If I'm using the semi-emprical Huckel model I cannot determine the concentration.  What is the reason for that? 

2. I've looked in the manual, and tutorials, but I still have trouble understanding the meaning of ElectronDifferenceDensity. Could someone please elaborate on that or refer me to a detailed explanation? Could I extract electron concentration from this?

3. In the "ballistic coherent tunnelling current" regime, an electron accelerates indefinitely in the central region (due to a finite voltage bias between the two terminals) until it reaches an electrode where it is absorbed or is scattered in the central region if there are deformities from periodicity, there is no drift or diffusion currents...true or false? In this way, does it matter what material type one is using if its perfectly periodic?

4. When the transmission coefficient is calculated, the user has the option to define the energy range.  Later, the product of the transmission coefficient and the fermi function is integrated over that range to give an electric current.  If one computes for larger energy range, you would get a larger current and if one computes for narrower range, smaller current.  How can I determine the correct range that will give accurate current?

5. How can I plot the electric field strength in the XY plane in the scattering region?

6. Can the physical temperature of a device configuration be adjusted? How?

Kind Regards,
gM

[zh]

1. Please check the background of "semi-emprical" Huckel method:
http://www.quantumwise.com/documents/manuals/latest/ReferenceManual/index.html/chap.atkse.html
The charge density can be calculated by both DFT and semi-emprical Huckel model.

2. For the definition of "ElectronDifferenceDensity", please refer to the following paper:
J. M. Soler, E. Artacho, J. D. Gale, A. García, J. Junquera, P. Ordejón, and D. Sánchez-Portal, J. Phys. Condens. Matter 14, 2745 (2002).
On page 2753 of that paper, the electron difference density \delta \rho is clearly explained.

3. For the understanding of ballistic coherent tunnelling current, please refer to a tutorial on the following link:
http://nanotube.msu.edu/nt05/abstracts/NT05tutor-Nygard.pdf
The drift or diffusion  currents cannot be taken into account in the ballistic coherent tunnelling regime.  

4. In the integration of transmission coefficient for the calculation of current, the energy range depends on the applied voltage.

5. You may check the voltage-drop.

6. It is hard to do a real simulation for the physics temperature of a device.  The temperature in the Fermi-Dirac distribution is meaningful to reflect the temperature of electrons.

[gM]

Thank you zh for your thorough and meaningful response.  It cleared up a lot of issues.

I still have trouble with points 4. and 5.

4. Indeed, the applied voltage shifts the fermi level up/down and the energy range needs to be adjusted.  My questions is what general guideline should I follow in order to adjust the energy range appropriately for a given bias?

5. As per this post: http://quantumwise.com/forum/index.php?topic=21.0
The voltage drop is calculated by:
voltage_drop = calculateEffectivePotential(finite_bias) - calculateEffectivePotential(zero_bias)
With the electrode constraint 'DensityMatrix'.
Is it possible to find the effective potential if my self-consistent calculations were done using the semi-empirical Huckel method?

I also have one more point to add to this discussion:
7. Even though the workfunction of the gate was discussed in a previous post: http://quantumwise.com/forum/index.php?topic=976.0, I still don't understand what work function is used in the metallic region to carry out the self-consistent calculation.  Is it 0 eV? Perhaps, an example would help clarify:
Suppose I have a:
channel workfunction of 4eV.
I'd like to specify a gate workfunction of 5eV (which I can't pre-simulation).
Simulation running at various gate voltage range [-2,2] V.
   a)If I can't specify a work function pre-simulation, how do I know that what is happening at -2V is not in reality happening at -20V?
   b)In this example, what must I shift my x-axis by?

Thanks again,
gM

[zh]

For 4:
It is difficult to give a general guideline for the range of applied bias voltage, because it depends on the case under study.

For 5.
The manual of ATK has already mentioned that "Calculation of the effective potential is not supported for the HuckelCalculator."
http://quantumwise.com/documents/manuals/latest/ReferenceManual/index.html/ref.effectivepotential.html

[gM]

And with regards to 7.?