ApplOpt88b

ApplOpt88b - Determination of (nk) for absorbing thin films...

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Determination of (nk) for absorbing thin films using reflectance measurements J. M. Siqueiros, Luis E. Regalado and R. Machorro We propose a method for determination of the complex refractive index of absorbing materials either in bulk or film geometry by measuring its reflectivity when coated with a well-characterized transparent dielectric at two specific optical thicknesses: ndj = XO/4 and njdj = X 0 /2. The complex refractive index of the sample n = (n, k) is calculated for the monitoring wavelength X0. The selected optical thicknesses of the coating allow the calculation of its geometrical thickness, therefore the variation of h with wavelength in the region where the reflectivity is measured can be determined. 1. Introduction There are many methods for determination of the optical constants of thin films; several reviews were published before computers were developed and acces- sible to everybody.1- 3 Since then, many other meth- ods have been proposed 4 -1 2 using the advantages of accurate commercial apparatus for both, measure- ments and calculations. However, a comparison of several techniques and methods used in different lab- oratories for characterizing the same samples1 3 shows considerable discrepancies among their results. We present here our method for determining the complex refractive index of thin metal film samples. It is based on reflectance measurements that can be obtained in a conventional spectrophotometer or with a monitoring attachment in the evaporation chamber; the convenience of this method is the small effect that the experimental errors have on the determination of the thickness and (n,k) of the dielectric and that it provides comparable results with those obtained by more elaborate techniques. A discussion is given for an absorbing thin film char- acterized in the visible spectral region and a compari- son is made with results reported in the literature. Luis Regalado is with Universidad de Sonora, Centro de Investiga- cion en Fisica, Apdo. Postal H-88, Hermosillo, Sonora, Mexico; the other authors are with Universidad Nacional Autonoma de Mexico, Instituto de Fisica, Lab. Ensenada, Apdo. Postal 2681, Ensenada, B.C., Mexico. Received 23 March 1988. 0003-6935/88-204260-05$02.00/0. © 1988 Optical Society of America. 11. Reflectance Data Since we wish to determine a complex refractive index we must know at least two relevant experimental values; our choice is the reflectance (R), because it allows the study of opaque films as well as bulk materi- als, measured at two different conditions, which are defined as follows. One part of the sample is coated with a transparent dielectric of optical thickness h, defined as the film thickness times the refractive index, equal to X 0 /4, and the other part with the same dielectric but optical thickness of Xo/2 for a given wavelength X0, as shown in Fig. 1. In practice, this can be done in a single run of the evaporating plant, overcoating the absorbing layer with the selected dielectric of real index n, until the
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This note was uploaded on 03/15/2011 for the course FISICA 101 taught by Professor Chavez during the Spring '11 term at ASU.

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ApplOpt88b - Determination of (nk) for absorbing thin films...

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