The values are required for the simulations of InGaN/GaN based devices designed over sapphire substrate.
on the web you can find many transmittance graphs and you can just invert the relation between the transmittance and the absorbance to obtain the absorption coefficient. Or you can have a look on page 131 of this article
http://archives.njit.edu/vol01/etd/1990s/1998/njit-etd1998-007/njit-etd1998-007.pdf
Use this. http://refractiveindex.info/?shelf=main&book=Al2O3&page=Hagemann
Could it be that you overestimate the importance of precise knowledge of this coefficient? In many cases, you can simply accept that it is zero - pure sapphire is transparent, especially for dimensions of a substrate. Absorption can be significant only if you have impurities but then the values from the literature are useless, you have to experimentally measure the very substrates you want to use in your design. If you would consider converting transmittance into absorbance, make sure you don't forget to take the reflectance into account, which depends on the wavelength (with the refractive index) and angle of incidence. My advice - zero absorption, do not make things unnecessary complicated.
Thank you all and especially Roman for the very useful discussion.
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