About charged electrodes

[BigRabbit_91]

Hi QuantumWise,

I used ATK 12.2.2. to calculate a device with two identical electrodes, but I made the two electrodes charged different as follows.

left_electrode_calculator = HuckelCalculator(
    numerical_accuracy_parameters=left_electrode_numerical_accuracy_parameters,
    iteration_control_parameters=left_electrode_iteration_control_parameters,
    poisson_solver=left_electrode_poisson_solver,
    charge=0.03
    )

right_electrode_calculator = HuckelCalculator(
    numerical_accuracy_parameters=right_electrode_numerical_accuracy_parameters,
    iteration_control_parameters=right_electrode_iteration_control_parameters,
    poisson_solver=right_electrode_poisson_solver,
    charge= -0.03
    )

I wish to realize p/n type doping for left/right electrode. 
However, when finished the calculation of the left electrode, the calculation log said :'Reuse the left electrode for the right electrode'. And the I-V curve turned out to be the same under forward bias and reverse bias.  I also used DFT and change the value of charge, but it still only calculated the left one and reused it for the right one. Was there something wrong in my script?  Thanks for your answer!

[Anders Blom]

Thank you very much for reporting this, I think it's something we need to fix for 12.8. For now, you can work around it buy explicitly doing an update of the electrodes. Make the following changes to your script: 1. After the definition on the electrode calculators, update them manually:

Code: python

left_electrode.setCalculator(left_electrode_calculator)
left_electrode.update()

right_electrode.setCalculator(right_electrode_calculator)
right_electrode.update()

2. For safety, remove the lines

Code: python

electrode_calculators=
[left_electrode_calculator, right_electrode_calculator],

from the definition of the device calculator. Run the script. The charged electrodes is brand new feature, so we are very grateful for any feedback users can provide on using it.

[BigRabbit_91]

Thanks. The problem is solved.

[leslie]

What is the unit of the parameter "charge"?  Does 0.03 mean 0.03 electron or 0.03 eV...

[Anders Blom]

In ATK we are very careful to have proper units. So if it were eV, you would have given it as 0.1*eV. Thus, it's a pure number, indeed as you write the number of electrons.

[leslie]

Hi Anders,

Thanks. Is it number of electron per unit-cell of the electrode material? Or per any cell, say supercell? For example, if I use three formula unit of the heusler compound Co2FeSi as the electrode image, how should I set the parameter charge if I would like to simulate the case of Co2FeSi0.97Al0.03? Should it be -0.03? Or -0.03 multiplied by 3 as that in VASP ?

Best,
leslie

[Anders Blom]

For devices it's per electrode unit cell. Also note that -1 means one electron extra, so it's not "in units of electrons" but "in units of electron charge", and thus in light of my previous statement about units we should probably have made it "charge=-1*e".

In your case it should be -0.09 (provided Al donates electrons and not holes).

[leslie]

For devices it's per electrode unit cell. Also note that -1 means one electron extra, so it's not "in units of electrons" but "in units of electron charge", and thus in light of my previous statement about units we should probably have made it "charge=-1*e".

In your case it should be -0.09 (provided Al donates electrons and not holes).

Hi Anders,

Actually one formula unit of Co2FeSi0.97Al0.03 has 0.03 less electron than that of Co2FeSi, so in order to simulate Co2FeSi0.97Al0.03, we should subtract 0.03 electron/unit cell from Co2FeSi. And if the parameter "charge" means electron charge, it should be set as positive (-(-0.03)/unit cell * 3 unit cell= 0.09) in my case containing three formula unit of Co2FeSi0.97Al0.03 and the corresponding Fermi level would shift downwards. Am I right?

Another question is about the surface layer (in principle we must also include at least one unit cell of the electrode material in the scattering region as surface layer). The problem is that we cannot add electron to the scattering region which is an open system. So the device structure we really have is Co2FeSi0.97Al0.03/Co2FeSi/Spacer/Co2FeSi/Co2FeSi0.97Al0.03 instead of Co2FeSi0.97Al0.03/Spacer/Co2FeSi0.97Al0.03.... Is it correct?

Best,
leslie

[Anders Blom]

All correct.