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I have a physics question on "surface + adsorbates" DOS calculation and I would appreciate if someone could comment on it:

My DOS calculation for such a system (modeled with 3 bilayers of bulk material + the adsorbates) gives me a band gap that is larger than the band gap of the bulk material itself. Is this reasonable? Can the adsorbates change the band gap of the material so much? I am quite confused given that in my system I have 3 bilayers of the bulk material. Can someone please respond. Thank you.

The widening of band gap is probably due to quantum confinement.

hi Tlchan,
Thank you for replying.
Can you please elaborate a bit more on your comment though...
I thought that the idea of quantum confinement refers to the increase of the band gap with the size decrease of a nanostructure. I don't know how to understand q. confinement when it comes to adsorbates.
I would appreciate your help.

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In general, a semiconductor slab with 3 double layers is thin enough to see the effect of quantum confinement. Typically, the studies of quantum confinement are done with hydrogen or hydrogen-like "adsorbates" (usually called passivation) on the surface. Without knowing the details of your project and what you have done, I cannot tell what is exactly happening in your calculations.

hi Tlchan,
Thanks for replying. I appreciate it.
Here are the details of my calculations: I have modeled the (0001) surface of GaN with three double layers, keeping the atoms in the bottom 4 single layers fixed and I am putting various adsorbated (hydrogen like adsorbates, as you have guessed correctly). For some structures, my gap increases and exceeds the band gap of bulk GaN. Does it mean that my slab is too thin? Or is there some interesting physics behind it? Would increasing the number of layers decrease the band gap for these structures? (While I can definitely try enlarging my system, I would like to know what to expect.)
I would be grateful if you could help me in understanding what is going on and if the observed phenomenon is just due to the modeling constraints I imposed on the system. Thank you in advance.

I think GaN(0001) surfaces are polar, i.e. one side consists of Ga, but the other is N. Hydrogenation of such systems may have complications compared to simpler systems like Si. I am not an expert in GaN, maybe other people can be more helpful. You can do a literature search for this system and see how the hydrogenation can be done properly.

hi tlchan,
thanks for replying. Yes, the surface is polar. Atoms in the bottom layer were passivated with H and I am fairly confident I have done it correctly. On the top surface (the one I am interested in) I have four different H-like species. The resulting structure leads to a band gap that is larger than the bulk InN band gap. Is this result valid? Or does it mean I don't have enough (bulk) layers?
Thanks.