optical Properties of Evaporated InSb Films ROY F . POTTER AND GEORGE G. KRETSCHMAR

U. S. Naval Ordnance Laboratory, Corona, California (Received November 8, 1960)

THE preparation of metallic and dielectric films by vacuum-evaporation techniques has reached a high state of develop­

ment (see Holland1). Most dielectric evaporation has been for the purpose of preparing such optical devices as beam-splitters, anti-reflection coatings, and interference filters; all of these devices depend upon materials having suitable refractive indices and transmission characteristics over selected wavelength regions. Many semiconductors in bulk form have infrared optical properties which are desirable for optimizing optical-device performance. Germanium, silicon, tellurium, selenium, and lead telluride are among the semiconductors which have been successfully vacuum evaporated as layers having optical properties usefully similar to those of the bulk material.

Although the intermetallic semiconductor, indium antiraonide, has a relatively high refractive index (~4.0) and small absorption beyond 7 μ, it has not had application for optical devices because it has proven difficult to produce under the conditions of vacuum

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evaporation. Several such attempts have been reported in the literature2 in which the principal difficulty encountered was with the disparity between the vapor pressures of the constituents, indium and antimony. Recently, however, Gunther reported on a three-temperature technique for preparing InSb layers in vacuum by evaporating the constituents onto a substrate whose tempera­ture is closely controlled.3

Based upon this technique, layers of InSb were prepared on substrates and examined for transmittance and refractive index at wavelengths between 2 and 15 μ.

The method of preparation depends upon the fact that the compound has a vapor pressure intermediate to those of its constituents. At a too-high substrate temperature the compound will decompose, leaving only the lower-vapor-pressure component (for example, indium) adhering to the substrate. The substrate must be held at an intermediate temperature below the decom-

FiG. 1. Vacuum evaporator apparatus for preparing films of InSb.

position temperature of indium antimonide, but above the condensation temperature of the higher-vapor-pressure element (for example, antimony). The indium and antimony evaporators must be held at temperatures that will give adequate quantities of the vapors. The three heaters are adjusted until films of indium antimonide are produced.

The temperature of the substrate is monitored by means of an iron-constantan thermocouple which is welded to the nickel-sheet, substrate holder. It was found by experiment that a temperature

of ~330°C is required to produce crystalline films of indium antimonide on a glass or cadmium-sulfide substrate.

Figure 1 is a cross section through the two-component evapora­tion equipment. In this arrangement, all the electrical connections and supports are hung from a single brass cover plate which makes the vacuum seal to a piece of Corning glass pipe by means of an O ring. Removal of this plate gives ready access to the heaters, the supports, and the evaporators. The evaporator ovens should be aligned to allow the constituent atoms to strike the surface at equal angles of incidence, and as close to the normal as possible. The metal vapor ovens are made of Vycor glass with molybdenum heaters. The evaporators and the substrate are shielded by means of nickel sheet as shown in the drawing.

Two substrate materials were used, namely, microscope cover slips of glass and polished slabs (2-3 mm thick) of single-crystal cadmium sulfide.4 This latter material, whose infrared properties have been described recently by Francis and Carlson,5 has a refractive index between 2.4 and 2.2 and very little absorption at wavelengths between 2 and 14 μ. One additional property, how­ever, makes this material very attractive for the present purpose; it has a coefficient of expansion5 similar enough to that of InSb that the film and substrate can be heated or cooled over a very wide temperature range without destroying the film.

Although the films are polycrystalline, they definitely consist of indium-antimonide crystals as can be seen from the x-ray diffraction pattern of Fig. 2. The crystallographic planes corre­sponding to the peaks appearing on the chart of a GE XRD5 diffractometer are also shown. The x irradiation was CuKα1. All lines are accounted for except for one appearing at a diffraction angle 20 = 30°. Not only do the diffraction lines appear at the proper angles for indium antimonide, they are also extremely sharp, providing evidence that the compound film has the density of the bulk material.

The film masses for several disk-shaped samples prepared on glass cover slips were determined from very careful weighings of the substrate, before and after evaporation, on a microbalance. From the sample diameter, its mass, and, the density of InSb one can make a determination of the film thickness. These are shown in Table I. These samples then could be used to set the values of refractive indices at the shorter wavelengths based upon inter­ference data as discussed in the next section.

All optical measurements were made with a Perkin-Elmer model 21 spectrophotometer equipped with NaCl prisms. The resolution was better than 100 in the wavelength region 2-15 μ. The PE refiectometer attachment 021 for the model 21 was also used for relative reflectance measurements. No compensating windows were used in the balance beam, hence the transmittance as shown in Figs. 3 and 4 are total for the film plus substrate.

From the positions of the maxima and minima of the trans­mittance and reflectance taken at small angles of incidence one can determine the values of 2n(λ)d as a function of λ from the following well-known relationships7:

FIG. 2. Trace of x-ray diffractometer pattern for InSb film No. 9.

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where d=thickness (microns); λ=wavelength in vacuum (microns); n=refractive index; and m=integer.

Formula (1) assumes that the imaginary part of the complex refractive index is negligible, an approximation in the present case which improves for wavelengths λ>2.0μ.

The determination of thickness for Figs. 3 and 4 was based on a value of 4.0 for the index of refraction at a wavelength of 4:μ, as determined from the measurements made for Table I. It is estimated that this gives the thickness with an error of ± 2 % .

Dispersion due to the presence of free carriers is very evident in all films of InSb prepared to date. Figures 5 and 6 show this

FIG. 5. Variation of refractive index as a function of wavelengtlh for several films.

F IG. 3. Transmittance of film No. 3 of thiclcness 1.4 μ.

FIG. 6. Variation of refractive index of film No. 9 (see Fig. 4) as a function of wavelength. FIG. 4. Transmittance of film No. 9 of thickness 2.7 μ.

TABLE I. Assignment of refractive-index values based on film-weight determination.

variation for two sets of films. The large amount of dispersion observed is not unexpected, inasmuch as indium antimonide has a very small electron effective mass. Film 9 of Fig. 6 has approxi­mately 1018 excess negative charge carriers, as determined from Hall-effect measurements. From the refractive-index curve of Fig. 6 one determines the effective mass as being 0.027 that of the free-electron mass based on the following expression8:

where N=density of charge carriers; m* = effective mass; and ε = dielectric constant in absence of free carrier effects (given a value of 16).

We have confirmed that the three-temperature method of Gunther permits the production of crystalline InSb films in a vacuum, having properties with very definite possibilities for the manufacture of optical components based on evaporated-film technology. However, one must work with materials having similar expansion coefficients; for example, in the present instance use was made of crystals of cadmium sulfide. Components made of such materials will permit use over a wide temperature range.

Excess absorption is observed in the films and is being studied to determine, if possible, the source. It does not appear to be attributable to free-carrier absorption alone and may be caused in part by the structure of the evaporated films. Efforts are being made to decrease the excess carrier density as well, however, in order to increase the refractive-index value at the longer wave­lengths.

Acknowledgments are due J. Kupecz, G. Hydro, and R. Bates for assistance during the course of experiments and to A. Clawson for providing the Hall-effect measurements.

1 L. Holland, Vacuum Deposition of Thin Films (John Wiley & Sons, Inc., New York, 1956).

2 I. Dietrich and K. Lark-Horovitz, Eleventh Quarterly Report, Purdue Research Foundation, Dept. of the Army, 3-98-13-022, p. 18 (1954) (unpublished) and other reports in this series; R. F. Potter and P. R. Bradshaw, Proceedings of Materials Research in the Navy ONR 2, 727 (1959) (unpublished). The films reported were prepared by flashing a finely ground powder of the compound InSb; I. K. Konozenko and R. M. Khaikina, J. Tech. Phys. (U.S.S.R.) 3. 738 (1958); G. A. Kurov, ibid. 2, 2022 (1957); G. A. Kurov and 2. G. Pinsker, ibid. 3, 26 (1958); V. A. Presnov and V. F. Synorov, ibid. 2, 104 (1957); G. A. Kurov, Soviet Phys. Solid State 1, 151 (1959).

3 K. G. Gunther, Z. Naturforsch. 13, 1081 (1958); W. Hanlein and K. G. Gunther, Advances in Vacuum Science and Technology 2, 727 (1960).

4 Eagle Picher Company, Miami, Oklahoma. 5 A. B. Francis and A. I. Carlson, J. Opt. Soc. Am. 50, 118 (1960). 6 Ohio Semiconductor Corporation, Columbus, Ohio, SRG-grade indium. 7 See, e.g., O. S. Heavens, Optical Properties of Thin Solid Films (Butter-

worths Scientific Publications, Ltd., London, 1955). 8 T. S. Moss, S. D. Smith, and T. D. F. Hawkins, Proc. Phys. Soc.

(London) B70, 776 (1957).

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