Gate and Bias potentials

See also

DFTB-NEGF manual and BAND-NEGF manual

In this tutorial we will show how to include a Gate and Bias potentials in NEGF calculations.

Start up AMSinput
Switch to BAND: ADFPanel → BANDPanel
Select Task → NEGF
Click on MoreBtn to go to the NEGF panel (or click on Model → NEGF)

Download the lead and molecules .xyz files:

Click here to download the lead file Li_lead.xyz
Click here to download the molecule CuAg.xyz

Note

The system studied in this tutorial is just a toy system.

Set up the system:

1. Select the file Li_lead.xyz as Lead
2. Fill the central region with 4 layers of lead material
3. Set the tips as shown in the figure. (Select left two atoms and click the + after Left tip, similarly for the right tip.)
4. Set the right and left lead offsets to -8.5 and 8.5 respectively
5. Select Method → self-consistent
Import the file CuAg.xyz by clicking on File → Import Coordinates…
/scm-uploads/doc.2023/Tutorials/_images/negf_t5_system_set_up.png

To apply a gate potential of 0.1 Volts for the CuAg molecule:

1. Select the CuAg molecule
2. Click on the + next to Potential region
3. Set the gate potential to 0.1 V
/scm-uploads/doc.2023/Tutorials/_images/negf_t5_gate.png

Tip

You can check the regions in the Model → Regions panel. You will see there the right_tip, left_tip and gate_potential regions. Use the checkboxes to see them individually.

We will now apply a bias potential of 0.1 Volt, with a ramp-potential from -3.0 to 3.0 Angstrom (see picture below), do not forget to change the unit to Angstrom, the choices shown by clicking on the unit:

/scm-uploads/doc.2023/Tutorials/_images/negf_t5_biassettings.png

Note

You can only apply a bias potential for the self-consistent and self-consistent + align NEGF methods

1. Set the bias potential Voltage to 0.1
2. Set the bias potential range to -3.0 and 3.0 respectively
/scm-uploads/doc.2023/Tutorials/_images/negf_t5_bias.png

we are now ready to run the calculation:

Click on File → Save
Run the calculation with File → Run
Wait for the calculation to finish

The current (in atomic units) is saved on the binary .rkf file.

Click on SCM → KFBrowser
Activate Expert mode by clicking on File → Expert mode
Click on File → Related Files → band.rkf
Look for the NEGF section
Click on the small arrow next to the NEGF section
The current should be “-0.00289”
/scm-uploads/doc.2023/Tutorials/_images/negf_t5_kfbrowser.png

Tip

You can also extract this information using amsreport: amsreport band.rkf "NEGF%current"

Warning

The Current vs Bias plot you can visualize with AMSspectra is computed from a the transmission function at fixed bias, and it is therefore just an approximation of the real Current vs Bias characteristic. If you want to compute the actual Current vs Bias characteristic you have to run multiple calculations using different bias potentials.