Potential distribution across a Metal-Semiconductor juntion

[sudipto_sspl]

Hi,

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 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 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 them


During 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 [‘psi ’ in page 102, referring to eqn. 3.1 in Page 96: div (epsilon*grad (psi)) =-row]. I was wondering where is this potential coming from?

The magnitude of the potential is also the same as the Fermi energy shift due to doping in GaN. Is it possible that the doping-induced change in Fermi level is interpreted as 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? Is it calculating the doping effect on the Fermi energy at all?

Please let me know if you have come across this. I am attaching the image of the str file and the potential cutline [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

 

[dick_freebird]

According to my limited understanding, there is indeed a built
in piezoelectric effect and a reverse biased (or zero biased)
region with no bleed mechanisms expressed, might develop
an internal potential. But this is more about the fine and
non-heterogeneous structure, not a bulk slab pinned between
two other ideal slabs.

I would expect, gut level, a 1E14 doped region to have
Schottky contact characteristics. That you define it to be
ohmic, I'd think might cause some "cognitive dissonance".
Though maybe TCAD is OK with solving unrealistic meshes,
I'd be disinclined to trust.

 

[erikl]

I would expect, gut level, a 1E14 doped region to have Schottky contact characteristics.

That's what I'd think also. And for both sides, symmetrical. Then, with the whole structure, he'd probably get this potential distribution?

     |########|
-----|########|-----
     |########|
  M      GaN     M
 0V     +1.4V    0V