Submit Manuscript | http://medcraveonline.com Introduction The optical transmittance and refectance 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 flms 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 refected 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 flms 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. It is not possible to determine the exact number of the free carriers in a particular thin flm sample. However, certain estimates can be made by applying the Drude model on the optical spectra. Such an analysis can provide us information that how the density of free carriers may affect the properties of a material. In this report, we have collated refectance and transmittance optical spectra on the DyN thin flms fabricated at variable conditions. The flms prepared at different dysprosium and nitrogen ion fux rates have different values of the free carrier density which clearly also affects the optical energy gap. Information about the crystal structure and lattice constant was obtained using X–ray diffraction (XRD). Experimental details The DyN thin flms were prepared by thermally evaporating the Dy metal in the presence of ionized nitrogen gas as described in details. 1–3 The samples with the different values of N 2 /Dy fux ratio were prepared. Films were deposited on the sapphire substrates. Since the compounds of rare earth metals oxidize in the atmosphere, samples were protected with an additional capping layer of MgF 2 , which is transparent to the photon energy range of our interest. Fourier transform infrared spectrometer was used to obtain transmission and refection spectra from the multilayer samples. The DA8 model of BOMEM was used to make measurement in infrared region whereas in the visible regions, a conventional visible–UV spectrometer was used. A reference flm was required to make the refection measurements. A gold flm was used in the infra–red region whereas a quartz wedge was in the visible region as the comparison standard for refectance measurements. Refectance measurements were made for the light incident on both the flm and the substrate surfaces, but since the transmittance is unaffected by the direction that light traverses through the sample it was taken from one side alone. The light is partially refected and transmitted at the every interface of the multilayer sample due to discontinuous refractive indices. These multiply transmitted and refected rays then interfere to form a complex interference pattern. This results in the loss light which otherwise would signal the absorption edge. A commercial software, TFCalc, 5 was used to analyze the optical spectra obtained from the multilayer and to extract the optical constants of the DyN. The software makes use of the characteristic matrix method and the data were analyzed as three layers: two flms (cap and DyN) and substrate. Results and discussion Figure 1 shows the XRD scan of a DyN flm. The strongest peak is from the sapphire whereas the peak labelled as (111) and a rather weak (222) peak are contributed by the cubic structure of DyN, therefore the flm is strongly textured in the <111> direction. The lattice constant of the flms is approximately 4.970±0.003Å, slightly larger than the previously reported value 6,7 of 4.895Å. The average crystallite size is about 10 nm. In this work, the refection/transmission spectra and their sum, R+T, of the two flms are being reported, one is the near stoichiometric while the other is affected by nitrogen vacancies. Figure 2 shows the R–T spectra from a nearly 300nm DyN thin flm, prepared at the highest N 2 /Dy fux ratio. In the region below 1.0eV, the absorptance is almost zero showing a very low free carrier density as expected of a semiconductor and also signaling that no interband transitions are present here. The interference fringes in both the refection and transmission spectra are apparent in this region. Above 1.2eV the transmitted light falls gradually due to interband transitions and continues to fall till 5.0eV where it is less than 1%. Phys Astron Int J. 2018;2(4):256‒258. 256 © 2018 Azeem. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and build upon your work non-commercially. Free carrier density effects on DyN optical spectra Volume 2 Issue 4 - 2018 Muhammad Azeem 1,2 1 Department of Applied Physics and Astronomy, University of Sharjah, United Arab Emirates 2 MacDiarmid Institute for Advanced Materials and Nanotechnology, School of Chemical and Physical Sciences, Victoria University, New Zealand Correspondence: Muhammad Azeem, Department of Applied Physics and Astronomy, University of Sharjah, PO Box 27272, Sharjah 27272, United Arab Emirates, Tel 0097-1563-9500-17, Fax +97165050352, Email mazeem@sharjah.ac.ae Received: June 14, 2018 | Published: July 02, 2018 Abstract The optical energy gap is a parameter of fundamental importance in semiconductors. However, concentration of free carriers may conceal the true absorption edge. In this paper, we show that the density of the free carriers can affect the onset of the direct absorption. The optical reflectance and transmittance spectra were obtained in the photon energy range of 0.5–5.0 eV thin films of the nitride compound of Dy metal. The films with the greater number of carriers exhibit a greater blue shift in the absorption edge. Keywords: semiconductors, spintronics, thin films, optical absorption Physics & Astronomy International Journal Research Article Open Access