JOURNAL OF THE OPTICAL SOCIETY OF AMERICA VOLUME 60, NUMBER 4 APRIL 1970

Optical-Phase Properties of Antimony Films N. BΛRΛKΛT*

Faculty of Science, A in Sham University, Cairo, U. A. R. AND

NΛBILA EL-KΛDRY National Institute of Standards, Cairo, U. A. R.

(Received 15 September 1969)

INDEX HEADINGS: Films; Antimony; Interference.

The optical-phase properties of thin metallic films play important roles in interference systems. They also have a marked effect on the characteristics of the fringes, as well as the scope of their applications. Phase data for evaporated metallic films are also used for calculating the optical constants of the material evapo­rated. This note contains measured phase changes for Sb films and optical constants at 5875 A deduced from the measurements.

In previous papers,1,2 the experimental procedures for deter­mining the phase change of light at reflection, air/metal and dielectric/metal, using two-beam fringes formed by a Koster interference comparator were reported. Determination of the phase change in transmission through a few thin films of Ag, using a Rayleigh refractometer, has also been reported.3 No data on the optical-phase properties of thin films of antimony, namely, the change of phase at reflection and in transmission, have appeared in literature. In this note, the phase change at reflection, air/Sb, ZnS/Sb, glass/Sb, and the phase change in transmission through thin films of antimony are measured using the methods reported in Refs. 1,2, and 3. The measured phase changes of light at reflection, air/Sb, glass/Sb for 5875 A, and their variations with thickness over the range from 80 to 500 A are presented. The thicknesses of the antimony films were measured by use of mulitple-beam Fizeau reflection fringes. These films were evaporated from a Mo boat in an oil-diffusion-pump system at pressures of 5X10 -6 torr. The films were deposited on glass optical parallels initially at room temperature, held at a distance of 25 cm from the filament. After evaporation, the coated substrate was left in vacuum until the chamber attained room temperature.

Figures 1 and 2 show the variation of phase change with thick­ness (B 1 ) at reflection air/Sb and (B2) glass/Sb. The behaviors of B1 and B2 show clearly a marked difference between evaporated antimony films of thicknesses less than and greater than 300 Å; the behavior of Sb films of thicknesses greater than 300 A is close

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FIG. 1. The curve shows the variation of the phase change, B1, at reflection air/antimony with thickness of evaporated Sb layers. FIG. 2. The curve shows the variation of the phase change, B2, at

reflection glass/antimony with thickness of layers.

to that of the bulk metal. This is in accordance with conclusions of previous investigators,4 that evaporated antimony films thinner than 300 A behave as semiconductors. The constancy of the values of the phase change at reflection for films thicker than 400 Å enabled us to deduce values for the optical constants of the bulk metal.

Table I gives the experimentally determined values for the optical properties of Sb films. The value of F is 2γ—B1—B2), γ being the change of phase in transmission through the antimony films. Table I shows that the F values for evaporated antimony films are close to — 2π for all thicknesses greater than 150 A. This result has been confirmed by forming multiple-beam Fizeau fringes at reflection using antimony films as the coating of the upper component of the wedge interferometer facing the incident light. The radiance of the multiple-beam reflected fringes was redistributed so that transmission-like fringes appeared at reflec­tion for all thicknesses greater than about 150 A for 5875 A. The percentage error in the measurement of the phase change is within 2%.

The phase quantity F is related to the optical constants of the metal in the form of the thin film. Its variation appears to be related to the state of agregation of the metal of the film.

The optical constants of antimony were measured in the visible region by Shkharevskii and Sharapov.5 They used the polarization method based on multiply reflected light from uniform samples. Their values for n and k are 2.31 and 3.89 at 6000 A. We have used graphical interpolation to deduce n and k from formulas relating the phase change at the dielectric/metal reflection with the optical constants. Experimentally determined values of phase change at the reflections air/Sb, glass/Sb, and ZnS/Sb were found to be 1.9l7r, 0.86π, and 0.787r, respectively, for 5875 A, using Sb layers of thickness greater than 600 Å, i.e., close to the bulk values. From

TABLE I. Optical phase properties of antimony films. The phase quan­tity F=(2γ-B1—B2) is = — 2π for Sb layers of thicknesses greater than 150 A.

the well-known equation tanB = 2μk /(μ2—n2—k2), where B is the change of phase at dielectric/metal reflection, μ is the refractive index of the dielectric, n and k are the optical constants of the metallic layer, and the values of n and k are found to be 3.2±0.1 and 4.6±0.1 for 5875 A. The values reported by Drude6 for massive antimony are 3.04 and 4.94 for 5893 A. With N =3.2—i 4.6, the phase change at reflection for Sb films of thicknesses less than 300 A do not behave in the observed manner. This may be due to a change of the indices associated with electrical properties of Sb.

* Now at Kuwait University, Kuwait. 1 N. Barakat and S. Mokhtar, J. Opt. Soc. Am. S3, 1153 (1963). 2 N. Barakat. S. Mokhtar, and K. Abd El-Hady, J. Opt. .Soc. Am. 54, 213 (1964). 3 K. Ishiguro, J. Phys. Soc. Japan 5, 187 (1950). 4 E. A. Taft and L. Apker, Phys. Rev. 96, 1496 (1954). 5 I. N. Shkliarevskii and A. I. Sharapov, Opt. Spectrosc. 8, 122 (1960V 6 M. Born and E. Wolf, Principles of Optics (Pergamon Press, Inc., London, 1959), p. 618.