My background is chemistry and now encounter question in semiconductor physics.
I use Polyethylene oxide (PEO) to modify heavily doped silicon. Based on UPS data, I find that the the work function (WF) of silicon continuously decreases while the the valence band edge remains unchanged if I increase the PEO thickness (of course thin enough and below certain value). This data means that the ionization energy decreases also.
The mechanism that PEO decreases WF is due to the formation of surface dipole.
Now comes the questions. the decrease ionization energy contradict that the ionization energy is the intrinsic property of material and should remains unchanged.
I highly appreciate your valuable help.
Interesting work. You can explore the attached article. It should be helpful.
If you have doped the material, then it is no more the intrinsic material and the WF of the new material maybe different from intrinsic one.
A surface dipole will change the electron affinity and by this also the workfunction. Think of the dipole as an accelerating or decelerating potential in addition to the surface barrierfor the electron leaving the surface. Even a layer of Xe may introduce a dipole at the surface changing the workfunction. The work Funktion is not simply the chemical potential µ it has additional components as the Galvani potential discussed in electrochemistry.
so if we assume that the bandgap remains unchange, the changed electron affinity leads to consequent change in ionization. Is this right?
the Si substrate should be still intrinsic. The mechanism for reduction of WF is due to the surface dipole.
To your comment regarding the band gap: You are right if no band bending occures. Band bening changes the position of the Fermi level in the gap by accepting or donating charges e.g. bonding action of the adlayer to the substrate. This changes also the work function.
I think the presence of surface dipoles will affect the ionisation energy and it will further depend on the allignment of dipole moments whether it is perpendicular or parallel to the surface.
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