Ordinary and Extra-Ordinary Dielectric Function of 4h- and 6H-SiC in the 0.7 to 9.0 eV Photon Energy Range
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ORDINARY AND EXTRA-ORDINARY DIELECTRIC FUNCTION OF 4H- AND 6H-SiC IN THE 0.7 TO 9.0 eV PHOTON ENERGY RANGE O.P.A LINDQUIST,a) K. JÄRRENDAHL, a) H. ARWIN,a) S. PETERS, b) J.-T. ZETTLER, c) C. COBET, c) N. ESSER, c) D.E. ASPNES, d) A. HENRY a) and N.V. EDWARDS a) a) Institutionen för Fysik och Mätteknik, Linköpings Universitet, SE 58183 Linköping, Sweden b) SENTECH Instruments GmbH, Carl-Scheele-Str. 16, 12489 Berlin, Germany c) Institut für Festkörperphysik, Sekr. PN6-1, Technische Universität Berlin,10623 Berlin, Germany d) Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina 27695 ABSTRACT We report ordinary and extra-ordinary dielectric function data of 4H- and 6H-SiC from 0.7 to 9.0 eV. These data, which were obtained by with spectroscopic ellipsometry, are in good qualitative agreement with trends recently reported in ab initio calculations. INTRODUCTION Current knowledge of SiC dielectric functions —important for emerging ab initio bandstructure calculations and as a fundamental parameter for device design— is limited in spectral range and/or restricted to the component perpendicular to the (0001)-direction (the caxis) because of instrumentation limitations and a scarcity of a-plane material, respectively. Since the most thoroughly investigated polytypes, 4H- and 6H-SiC, crystallize in a hexagonal lattice structure and therefore are optically uniaxial, existing measurements [1-4] of the component perpendicular to the (0001)-direction provide only a very limited description of the dielectric response of these materials. We extend the scope of existing measurements by obtaining both ordinary (perpendicular to the (0001)-direction) and extra ordinary (parallel to the (0001)-direction) components of the pseudodielectric functions ( and , respectively) of 4H- and 6H-SiC dielectric functions from 0.7 to 9.0 eV (138 – 1770 nm) using a combination of quartz-optics based and ultraviolet (UV) spectroscopic ellipsometry (SE) on a-plane material [5]. Additionally, a preliminary lineshape analysis of these data yields estimates of the critical point energies of observed optical transitions. While this extends the present knowledge of SiC optical properties by offering a good representation of the actual dielectric function ε, more specifically these results provide the first experimental confirmation of large anisotropies in the UV region of for both polytypes, as theoretically anticipated [6,7]. EXPERIMENTAL DETAILS Complex reflectance ratios ρ were obtained on n-type 4H- and 6H-SiC samples [5] at room temperature with a SENTECH Instruments rotating polarizer ellipsometer enclosed in a N2-purged ambient. The ρ data were converted to = + i spectra using the twophase model [8]. High-energy measurements were facilitated with MgF2 Rochon prisms, UVenhanced Si diode detectors, and a deuterium light source, where a small correction (~1%) was H5.24.1
made to for the increase in source intensity near 8 eV [9]. Two sets of samples were measured for each polytype: one with the [
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