Materials Characterization using THz Ellipsometry
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1163-K08-04
Materials Characterization using THz Ellipsometry T. Hofmann1, C. M. Herzinger2, J. A. Woollam2, and M. Schubert1 1 University of Nebraska-Lincoln, Lincoln, NE, USA. 2 J.A. Woollam Co., Lincoln, NE, USA. ABSTRACT We employ spectroscopic ellipsometry in the terahertz (0.2 to 1.5 THz) and the midinfrared (9 to 50 THz) spectral range for the non-contact, non-destructive optical determination of the free-charge-carrier properties of low-doped Silicon bulk and thin film structures. We find that carrier concentrations as low as 1015 cm-3 in thin films can be unambiguously determined. We envision ellipsometry in the THz spectral range for future non-contact, non-destructive monitoring and control applications. INTRODUCTION The absorption of electromagnetic radiation by free-charge-carriers in doped semiconductors has been widely studied in the mid and far infrared spectral ranges [1]. For small doping concentrations the plasma frequency of silicon and other semiconductors falls in the terahertz spectral range which has been used for the optical determination of electrical properties of semiconductors in the last two decades [2 - 8]. However, most of the experiments which have been carried out so far were simple (un)polarized transmission measurements [9 - 2]. This imposes limitations especially for layered structures where low-doped layers are deposited on highly-doped substrates which are opaque for incident THz radiation [3]. Thus, the non-contact optical determination of electrical properties of the individual constituents of semiconductor layer systems with low doping concentrations remains very challenging. Here we report on the THz and infrared ellipsometric determination of free-charge-carrier properties in low doped Silicon layer structures. Ellipsometry is the preeminent technology used for precise determination of optical properties of layered systems and has been employed in the spectral range from the far-infrared to the VUV [10]. In the mid- and far-infrared spectral range ellipsometry has been used successfully for the contact-less and nondestructive determination of free-charge-carrier concentration and mobility and the spatial distribution thereof in complex semiconductor layered structures [11, 12]. In the THz spectral range, however, ellipsometry is still in its infancy and experimental reports are scare [13 - 16]. For isotropic media, ellipsometry determines the ratio ρ of the complex-valued Fresnel reflection coefficients rp and rs for light polarized parallel p and perpendicular s to the plane of incidence, respectively. Commonly, ρ is presented in terms of the ellipsometric angles Ψ and Δ ⁄ where ρ tan [17]. In general, stratified layer analysis of experimentally determined Ψ- and Δ- spectra are required in order to determine the complex valued dielectric function of the sample constituents.
EXPERIMENT The custom-made THz ellipsometer used for the measurements presented here is operating in a polarizer-sample-rotating-analyzer scheme [10]. A backward wave oscillator (BWO, Microtech Inst
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