I am using chemical vapor deposition(CVD) to synthesize WSe2. Now I still cannot get WSe2 of high quality, so I'm thinking about the influence of the substrate, and the surface atom should be crucial(whether it is O or Al). So is there any method that can be used to confirm the surface atoms of the sapphire substrate?
you mean you want to check the chemical composition of your substrate surface ?
If you want to check the elemental composition of the material, it is necessary to use EDAX (Energy-dispersive analysis of x-rays) and PIXE (Particle induced X-ray emission) methods.
EDX is a good technique for this kind of analysis, but maybe could be intersting you use EELS (Electron Energy Loss Spectroscopy) that can be performed in some Transmission Electron Microscopes. Then you could have images, chemical composition and structural arrangement Information of your samples.
XPS is very useful in such circumstances, and you can get quantitative population of the elements on the surface with their oxidation states !!
Hi Yanan,
The classic paper on this topic is here. You may track the recent development by following the citations to this paper (>100). Basically, LEED and EELS are essential as given by Fabio and Sridhar.
But I do not think you need to do this kind characterization—too complex for you. Very likely, surface contamination might be the cause. You may have a look at my page for research papers and review on critical cleaning of sapphire substrates.
J Toofan, P.R Watson, The termination of the α-Al2O3 (0001) surface: a LEED crystallography determination, Surface Science, Volume 401, Issue 2, 1 April 1998, Pages 162-172
http://dx.doi.org/10.1016/S0039-6028(97)01031-5
Abstract: We have performed a tensor low-energy electron diffraction (TLEED) determination of 10 diffracted beams at off-normal incidence from the (0001) surface of α-Al2O3 produced by oxygen plasma cleaning/annealing cycles. Structures with single domain O- or Al-terminated surface layers give a poor agreement (Pendry R-factor=0.42). Such domains are related by steps in the (11-2̄0) direction that produce an extra plane of symmetry in the LEED pattern. Allowing a mixture of domains due to steps produces a good agreement. An optimized mixture of 2:1 O/Al-terminated domains lowers the R-factor to 0.26 with an inner potential of 19.2 eV. At the R-factor minimum, we find that in both domains, the outermost atomic layer relaxes outward, with some twisting and relief of buckling in deeper layers. An outward relaxation of the top layer is in disagreement with theoretical predictions. However, such predictions are for vacuum-cleaved surfaces that are not realistic descriptions of the surface used in our experiment.
Hope this helps.
Best,
Yang
Hi, I think (0001) alpha Al2O3 will always be terminated by O if you work in air. There is a good article in Science here: http://www.sciencemag.org/content/282/5387/265.full which also describes the surface reconstruction.
If you heat very high (1300deg) you will get flat terraces and might want to check the polarity, which you can do by either in-situ XPS or AES in UHV, or by making a cross-section (rather difficult!) and determine the polarity by electron diffraction. Chemical etching may work as well, like for GaN, but I could not find any reference for this so you would need to try it out yourself with two samples whose different terminations you know.
Are you sure that your sapphire substrate surface is not contaminated? If you are not sure and need to check, you could use surface reflectance IR spectroscopy to look for organic contaminants.
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