The optical transmittance and reflectance measurements are popular techniques to experimentally determine the optical energy gap. Recently, the onset of absorption has been determined with great degree of certainty in various rare earth nitride thin films by using this technique.1–3 Further, this technique has also been used successfully to resolve the spin split bands at point X which has an excellent agreement with the band structure calculations.4 However, quite often the true absorption edge is masked by the interference fringes formed by the multiply reflected and transmitted rays from the sample as well as due to the artefacts in the spectra due to surface roughness. The most prominent parameter affecting the absorption, however, is the number of free carriers in a sample. Particularly for the case of thin films of monochalcogenides and monopnictides with rare earth or transition metals, the density of carriers can adversely affect the position of absorption edge on the photon energy axis.
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