Polarization Anisotropy in Electroreflectance Spectrum of Porous Silicon
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Polarization Anisotropy in Electroreflectance Spectrum of Porous Silicon
Toshihiko Toyama, Yasuharu Nakai, Koji Moriguchi, Hiroaki Okamoto Department of Physical Science, Graduate School of Engineering Science, Osaka University Toyonaka, Osaka 560-8531, Japan ABSTRACT Linearly polarized electroreflectance (ER) measurements have been firstly performed on PSi with near-normal incident (NNI) and glancing-angle incident (GAI) light. In the NNI geometry, no significant polarization anisotropy is observed. Meanwhile, in GAI geometry, a clear polarization anisotropy is observed. The s-polarized ER spectrum consists of the ER features corresponding to those found in the NNI geometry, while in the p-polarized ER spectrum, the two features found in s-polarized spectrum in infrared-visible region are not found, while a new feature is observed at ~2.1 eV. The mechanism of the unusual polarization anisotropy is discussed in association with structural anisotropy of PSi.
INTRODUCTION The study of polarization dependent optical properties of semiconductors gives important information about their electronic band structure [1]. In particular, with respect to low-dimensional systems based on cubic semiconductors, the measurement of polarization anisotropy is powerful for distinguishing low-dimensional optical processes from bulk-related processes. The bulk of a cubic semiconductor is nominally isotropic, so that the observed optical anisotropy should be related to the lower symmetry of the semiconductor low-dimensional systems such as surface and nanostructures. Recently it has been reported that porous Si (PSi) [2] made from bulk Si being a typical cubic semiconductor shows optical anisotropy in reflectance [3–4] and photoluminescence (PL) spectra [5–8]. Electroreflectance (ER) spectroscopy is a highly sensitive and highly resolved spectroscopy [9], so that polarized ER spectroscopy has been applied for the study on optical anisotropy of low-dimensional systems such as surface [10] and quantum wells [11] based on semiconductors other than Si. On the other hand, there has been no report on optical anisotropy taken by the polarized ER spectroscopy. We have reported results on the optical properties of PSi measured by ER spectroscopy with unpolarized probe light [12–14]. We have found some ER features in infrared-visible region arising from optical transitions in confined electron-hole (e-h) pairs in Si nanocrystals incorporated in PSi [12]. Hence, the ER features are insensitive for surface oxidation without reduction in the crystal size [13]. Also already found by ER spectroscopy are alternative ER features in ultraviolet region corresponding to E1(E0') critical points still retaining in electronic structure of the Si nanocrystals [12] and its behavior against boron-doping concentrations [14]. In this article, we report the optical anisotropy of PSi taken by polarized ER spectroscopy with a use of two optical geometries for probe light. We present a clear polarization anisotropy in ER spectra taken with glancing-angle inci
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