Simulating Metal-Semiconductor Junction using SILVACO TCAD

[RICuser]

Hi,

Sorry if it is too specific. I could not find a more specific group in the site. Would really appreciate if someone who has come across this already can find me a clue.

I stumbled upon a strange result while playing with SILVACO TCAD, to simulate a metal semiconductor junction.

I made a Metal-GaN structure (1D, nearly, along horizontal direction: x,) [GaN thickness along x was 6.4 micron (just to keep consistency with an experimental result) and the electrode was 0.4 micron on either side of the sample. The dimension along the vertical downward direction: Y, was 100 microns.]

GaN was doped to a level of 1E14

There was no work function specified at electrode: manual calls this an ohmic contact
Expected consequences:
i) No charge transfer: The semiconductor and the metal electrode remain charge-neutral: no band bending.
ii) Voltage should be zero in both of themDuring simulation I had to use "solve init" command, which forced the electrodes on either side to assume a voltage of zero volts. That should not change the situation, since the voltage should have been already zero in absence of any charge transfer.

After simulation of the zero volt situation I found that there was a constant potential inside GaN (about +1.428 V) with respect to the metal electrode.

The doubt is: If there are no net charges in semiconductor & electrode, why should there be any potential in either of them?

Manual [version release date: Feb 2012] says this voltage is the actual electrostatic potential as in Poisson’s equation [‘ψ ’ in page 102, referring to eqn. 3.1 in Page 96: ∇. (ε ∇ψ) = - ρ]. I was wondering where is this potential coming from?

The magnitude of the potential happens to be the same as the Fermi energy shift [divided by the electronic charge] due to doping in GaN. Is it possible that the doping-induced change in Fermi level is assumed to be due to an increase in voltage? In that case, when we require the true electrostatic potential, should we subtract the doping induced Fermi level shift from the simulated value of the potential?

Please let me know if you have come across this. I am attaching the image of the str file and the potential cut-line [horizontally through the middle of the sample, 50 micron deep, from the top].

If you know of an existing thread on this, please do let me know.

Thanks in advance

 

[Achy47]

I have come across similar results in my own simulations of metal-semiconductor junctions using SILVACO TCAD. From my understanding, the constant potential inside the semiconductor is due to the built-in potential created by the metal-semiconductor interface. This built-in potential is a result of the difference in work functions between the metal and the semiconductor, which creates a depletion region near the interface.

In your simulation, you have specified an ohmic contact at the metal electrode, which means that there is no barrier for charge carriers to move between the metal and the semiconductor. However, there is still a built-in potential present due to the difference in work functions.

I would suggest trying to specify a Schottky contact at the metal electrode instead, which would create a barrier for charge carriers and eliminate the built-in potential. This should result in a flat potential profile throughout the semiconductor.

I hope this helps and let me know if you have any further questions. Best of luck with your simulations!