Does the Theory Objects non-Metallic Electrodes ?

[Hasan Sahin]

Do we have to use metallic electrodes during ATK calculations? Does the theory used by ATK have objection to semiconducting electrodes ? Thanks to shifts in levels due to changing bias, can't we calculate any current throught the molecular device at some voltage values.

[zh]

If you take a look at the book "Quantum transport: atom to transistor" entitled by S. Datta, you will see that the electron flux from source to drain depends on the relative positions of the chemical potentials (i.e., the Fermi level) of source and drain as well as the  energy level of channel.  For example, please see the some pages of this book here:
http://books.google.com.sg/books?id=Yj50EJoS224C&lpg=PP1&dq=Quantum%20transport%3A%20atom%20to%20transistor%20googlebook&pg=PA8#v=onepage&q=&f=false
The methodology implemented in ATK to handle the electron transport does not take care of the conductivity of electrodes.

Thanks to shifts in levels due to changing bias, can't we calculate any current throught the molecular device at some voltage values.

More properly speaking, the obtained current (I) may be zero, in other words, no electron flux from source to drain.  Of course, the methodology in code is still valid.

[Nordland]

Yes, it is perfectly sane and with good agreement to have a semi-conductor electrode instead of metallic one,
however one has to be carefull as the band gap is underestimated by LDA, so therefore the location of peaks etc might be shifted compared to experiments.

[Anders Blom]

From a formal perspective you can certainly use semiconducting electrodes. The question is however, what relevance it has. Unless the electrode is conducting, there will be no injection and hence no current.

In some situations involving semiconducting materials, like gate stacks etc, it's more realistic to have a metallic injection part, then a semiconducting buffer layer, and then perhaps the scattering region.

Another type of system, studied in experiments, might have (heavily) doped semiconductor regions as electrodes, but then again we have conducting electrodes. Still, at the "end" they are probably connected to metal electrodes anyway (connection to external circuitry, for instance).

So, if we are interested in the pure properties of a semiconducting scattering region, it's best to use as transparent and broad band injection as possible, i.e. some metal like gold or even just lithium.

However, thinking about it, perhaps your question is rather, what happens when the bias overcomes the band gap of the semiconductor. That is, if the system is so aligned that the valence band of one electrode overlaps the conduction band of the other. In this case, as Nordland writes, in principle all works out fine - except for the trouble with the band gap in DFT, of course...