Sapphire substrate is one of the most-used substrate in the CVD synthesis or ALD. Some papers mentioned that the polarity of the substrate has an affect on the growth of the deposited material. So I am wondering what the polarity of the sapphire of c,a,r,m-plane is. Or is there any tool to check the polarity of sapphire?
Sapphire is equivalent to corundum (alpha-Al2O3) and may contain only some "impurities" (color centers) which generates colors which are not reddish and then called ruby. The corundum structure is described by the centrosymmetric space-group type R -3c which excludes any polarity per se for any lattice plane. I assume that the term is simply wrongly used and related to a "plane density" describing the number of atoms within the lattice plane. The bonding character is perhaps also diferent but also the planar symmetry. However, this has nothing to do with polarity which expresses that the property for a plane is different and depends on the side you are looking!
I would recommend you to post some of these papers so that we can have a look what do the authors actually want to say.
Interesting question.
I don't think the "polarity" of the sapphire is controlling the polarity of the material that is deposited. The GaN or ZnO are polar because they are typically wurtzite crystals and if you control the nucleation layer you can control the polarity of the resulting crystal that is grown on top of the sapphire. I think that in some sense that is more control of the surface chemistry as you control the nucleation that is more important. So you can take a c-plane sapphire wafer and grow both polarities of GaN. In fact you can even pattern the sapphire surface in such a way you can pattern the polarity of the resulting material. It is useful to have control rather than domains of different polarities randomly. Usually, of course you would chose to have all the material of one polarity if possible.
I think the main effect of growing on the different planes of the sapphire is that you are controlling the lattice matching. for example on c-axis sapphire you will have c-axis wurtzite crystal of GaN normal to the substrate surface. If you use r-plane sapphire you can grow an a-plane GaN such that the c-axis will be parallel to the substrate surface.
Controlling the polarity of the GaN or ZnO has important device implications, so your question is a very good one.
Sapphire is nonpolar crystal. However, opposite faces of an a-plane sapphire wafer can be distinguished. R-3c symmetry group doesn't contain 2z axis, so there is only one r-plane perpendicular to a-plane. Relative orientation (clockwise or counterclockwise) of c-plane (primary flat for a-plane wafers) and cleavage r-plane indicates a-plane wafer "polarity". See dx.doi.org/10.1107/S1600576718001383
Nevertheless, this fact shouldn't affect the growth issue since the surface atomic nets should be identical due to inversion center.
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