As we can see from my calculations, the transmission spectra is unsymmetrical.
In general all transmission spectra is unsymmetrical, it is only in very special cases that is symmetrical.
The only case I have ever seen, is in a nearest neighbour tightbinding approximation for carbon nanotubes, which simple overlap model. The transmission spectrum can only be expected to be symmetrical, if the band structure is symmetrical around the fermi level, and the system is homogenious and has no spatial variation in structure. Since your system has a spatial varation with ZnO in the middle,
it is the most natural thing, that the transmission spectrum is unsymmetrical.
Look at this simple system in the manual
(link).
You can look at transmission spectrum which is perfect and unsymmetrical.
If I choose -6 to 6 as the X axis, in the bias window, there will be lots of values of zero. I don't know the reason, and I also can not decide the results is right or not!
The transmission spectrum is not a bias window. The transmission spectrum is plot of the probability T of a electron with a certain energy E in the electrode being transmitted through the scattering region.
So if the transmission has a value 0.325 at the E=-3.0 eV, it means that a incident electron with an energy for -3.0 eV relative to the fermi level, have a probability of 0.325 of being transmitted through the scattering region.
If the value is zero in the transmission spectrum for a given energy (fx E= -7 eV) it means one of the two following things:
1) Either there is no electrons ( no allowed eigenstates ) in the electrode with this energy relative to the fermi level.
2) There is electrons in the electrode with this energy relative to fermi level, but the electrons at this energy has no chance for being transmitted through the device, therefore there is a transmission probability of zero.
Following up the note about the bias window - it is important to understand that the transmission spectrum is only related to the bias/current, that you can get the current from integrating the transmission spectrum from -V/2 to V/2 if you have bias of V taking into account the fermi distribution of the two electrodes
Therefore if you have a bias of 6 Volt, then you must integrate the transmission spectrum from -3 eV to 3 eV weight the fermi function. In order to be correct, the calculation must then be done at 6.0 V bias as well.
In your case the transmission spectrum could indicate that the the current will grow linearly with the bias until a certain thresshold, where it would enter a maximum current until the bias becomes very high.
Therefore
