Messages

16

General Questions and Answers / About the charge option

« on: April 8, 2014, 16:01 »

Hello everyone,

I want to clarify myself about the meaning of the charge option for electrodes (ATK version 12.8.2).
It is clear that the Fermi level gets shifted when additional charge is added.

To estimate the influence of the electrode charge in a device configuration, I looked at one single (7,0) CNT (bulk configuration) and varied the charge (periodic boundary conditions along the CNT, Neumann boundary conditions perpendicular to the CNT (as this is needed for the device configuration)).
Afterwards, I subtracted the Fermi level from the resulting band structures (to see the influence on the band structure and the Fermi level separately).
 
The result is attached.
You can see the energy values at the Gamma point versus different amounts of additional charge. The red curve is the Fermi level, the black curves correspond to the band structure. The inset shows the band structure of the (7,0) CNT for 0.0 additional charge.

The movement of the Fermi level relative to the bands is clear. However, there is also a shift of the whole band structure in addition, which I don't understand yet (shouldn't the black curves be constant?).

Can someone explain this behavior?
I would also be interested in further information on how exactly the additional charge is added to the system.

Thanks in advance!

17

General Questions and Answers / Re: CNTFET using doped electrodes

« on: March 25, 2014, 18:59 »

I want to illustrate what ziand said in the previous post.
I have therefore attached an image, where you can see transfer characteristics for various cell sizes (the size of the cell can be found in the legend).
Neumann boundaries were used perpendicular to the device to obtain these data. I have also attached one structure as an example.
Even for a cell of 166 Angstrom, the transfer characteristic is further shifted towards smaller gate voltages.

This effect may be related with the doping of the electrodes (which was set to 0.5 using the charge keyword in ATK 12.8.2) and resulting electrostatics. However, I would not expect such a drastic shift.

Any ideas what could be the problem?
Should I maybe also adjust the value for the charge keyword when increasing the cell extend?
(since an increasing doping leads to a shift towards smaller gate voltages during our simulations, too)

18

General Questions and Answers / Re: CNTFET using doped electrodes

« on: March 10, 2014, 19:15 »

Thank you for your reply. This definitely helps.

Can you give a short explanation, why multipole boundaries are not suitable for devices (only perpendicular to the device of course)?

19

General Questions and Answers / Re: CNTFET using doped electrodes

« on: March 10, 2014, 17:32 »

No answer here?

20

General Questions and Answers / CNTFET using doped electrodes

« on: February 24, 2014, 14:26 »

Hello everyone,

I'm trying to simulate a cylindrical gated CNTFET using ATK 12.8.2. The system is based on a (7,0) CNT, where I dope the electrodes using the "charge" keyword (each electrode has two unit cells of (7,0) CNT - 56 atoms - and the charge was set to "-0.5").
The gate is a spatial region and no dielectric region is inserted.
Extended Hueckel is the chosen method.

I have a question about the most suitable choice for the boundary conditions:
According to the tutorials and previous discussion (http://quantumwise.com/forum/index.php?topic=1597.0#.UwtBlduVvRY), Neumann boundaries are most suitable for studying the gate influence.
However, multipole boundaries are suggested for charged molecules.
I'm now wondering if I could use multipole boundaries for the transistor as well (as I have some kind of charged molecules as electrodes).

When I create a script using VNL to simulate the CNTFET, I cannot access the Multipole option. But when I directly edit the python script and change "NeumannBoundaryCondition" to "MultipoleBoundaryCondition", the job starts and ends successfully. Is there a reason why Multipole boundaries should not be used for a device calculation (e.g. a CNTFET)?