Can someone explain to me why is it that when measuring UPS (Ultraviolet photoelectron spectroscopy) in a semiconductor, the onset binding energy is considered the Fermi Level and not the Valence Band? The way I see it, we can only extract electrons from the valence band and not from the Fermi Level (given that the Fermi Level is just a theoretical level), so the onset should be the Valence Band. I am finding it a bit confusing that both Valence Band and Fermi Level can be extracted from UPS. As an example, a link to a paper is provided.
Thank you in advance.
http://pubs.rsc.org/is/content/articlehtml/2015/cp/c5cp00321k
A good answer can be found in a short article online:
https://www2.warwick.ac.uk/fac/sci/physics/current/postgraduate/regs/mpags/ex5/techniques/electronic/ups/
Hi Joao,
Theoretically, the Fermi energy is the electrochemical potential defined at 0K, and it corresponds to the top of the valence band in a semiconductor. At finite temperatures, however, semiconductors have a different value of electrochemical potential. In photoelectron spectroscopy, the electrochemical potential is many times referred as Fermi edge, which makes sense for metallic materials, where this value is almost temperature-independent.
Therefore, if you think this level as the electrochemical potential, you will see that makes sense to start the scale of binding energy there, as a measure of how bound is the electron.
Best regards,
Carlos
Measuring E_F on a sample which does not have a continuous occupied electronic DOS at/around E_F can be rather unreliable as it is a relative measurement to a known material just described in previous posts (a metal). In a bandgap material, you can shift E_F around in the bandgap up to ca. 3 k_BT away from any band edge (exp[-3] ≈ 0.05 = 5 % of DOS at band edge) before a change in detected DOS (UPS spectrum) becomes notable. You can check this in UPS by measuring p-Si and n-Si. As long as the doping is not heavily degenerate (with E_F being very near or entering the respective electronic DOS), the UPS spectra under the same characterization conditions look identical. The reason for this ‘leeway’ within the bandgap is simply given by the missing allowed states which could be excited near E_F.
If above explanation is not clear, then figure this:
Si at T = 300 K has a bandgap of 1.12 eV. If you have intrinsic Si, its E_F would be at midgap^^, the energy difference from E_F to the CB edge is then only about half the bandgap. This looks like you only need to supply the energy difference from E_F to the CB edge to induce carrier generation (e.g. by photon absorption we can detect). However, the bandgap measured for Si at room temperature is still 1.12 eV because no electronic DOS exist within the bandgap apart from some defects which are way too small to be detected by UPS (you can use DLTS for this). While E_F defines Fermion statistics, it is a required but not a sufficient condition for changing the energy threshold of electrons to leave the solid (ionization energy). You need the occupied DOS at or very near E_F.
Hence, an exact measurement of E_F requires a continuous electronic DOS around it to see the cut-off in the DOS. It follows that such a measurement is possible only with metals, semi-metals, or heavily doped (heavily degenerate) bandgap materials. Usually the bandgap of materials where such degeneracy is feasible is not very big (semiconductors).
^^: more precisely, midgap energy minus ca. 0.5 * k_B T since the ln of the ratio of effective DOS in VB vs. CB is ln[1.02/2.08] ≈ -1; for details, see p. 34 in MOS (Metal Oxide Semiconductor) Physics and Technology by Nicollian & Bruce (1982), Wiley & Sons, ISBN 0-471-08500-6.
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