Variable Angle Spectroscopic Ellipsometry Studies of HgI 2
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VARIABLE ANGLE SPECTROSCOPIC ELLIPSOMETRY STUDIES OF HgI2 HUADE YAO*AND BLAINE JOHS** *University of Nebraska, Center for Microelectronic and Optical Materials Research, and Department of Electrical Engineering, Lincoln, NE 68588 **J.A. Woollam Co., Lincoln, NE 68502 ABSTRACT Variable angle spectroscopic ellipsometry (VASE) measurements were employed to study the optical properties of HgI 2 . The bulk crystal HgI2 surface was subjected to a 10% KI etching prior to the VASE measurements. SE measurements were performed at room temperature, in air with several different angles of incidence. The uniaxial anisotropic nature of the HgI 2 crystal was treated in the VASE analysis. Anisotropic dielectric functions of single crystal HgI 2 , e 1(wo)and e1 (a), for optical electric field vector oriented parallel and perpendicular to the c axis, respectively, were obtained in the range of -2.0 - 5.0 eV. Surface aging effects of the HgI 2 crystal, after the 10% KI etching, were characterized by VASE. INTRODUCTION Crystalline mercuric iodide (HgI2 ) is a promising candidate for nuclear radiation (i.e., gamma- and x-ray) detectors operated at room temperatures. The optical functions of this crystal, such as dielectric functions (e=e 1+iE2), refractive indices (n and k), etc., have not been determined directly by precise optical measurements. Instead, dielectric functions of HgI 2 were derived from reflectivity spectra under assumptions. 1 In this paper, we report, for the first time, determinations of the anisotropic dielectric functions of HgI 2 by VASE and characterizations of the surface aging effects after chemical etching. THEORY Ellipsometric measurements determine the ratio of complex reflectance rp to rs, where rp and rs are the reflection coefficients of light polarized parallel to (p) or perpendicular to (s) the plane of incidence. The ratio has been traditionally defined as: p = rp/rs = tan(V)eia,
(1)
where the values of tan(O) and A are the amplitude and phase of the complex ratio. The pseudodielectric function of the sample is obtained from the ellipsometrically measured values of p, in a two-phase model (ambient/substrate) regardless of the possible existence of surface overlayers. 2 Since the HgI 2 is a tetragonal crystal, it exhibits uniaxial anisotropy and must be described by two dielectric response functions e,(W) and e,(Wo), for the optical electric field vector oriented parallel and perpendicular to the c axis, respectively. Measurement of the complex ratio, p at a single wavelength and angle of Mat. Res. Soc. Symp. Proc. Vol. 302. ©1993 Materials Research Society
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incidence provides two quantities (1 and A), which can be used to determine two parameters describing the sample, e.g., dielectric function (E=el+ie 2 ) of a bulk sample with no overlayers. Spectroscopic (multiple wavelengths) measurements greatly increase the number of determined parameters of the sample, especially when measurements were made at more than one angle of incidence. Thus, the Variable Angle Spectroscopic Ellipsometry (VASE) ma
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