Investigation of optical anisotropy of refractive-index-profiled porous silicon employing generalized ellipsometry
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Investigation of optical anisotropy of refractive-index-profiled porous silicon employing generalized ellipsometry S. Zangooie,a) R. Jansson, and H. Arwin Laboratory of Applied Optics, Department of Physics and Measurement Technology, Linko¨ping University, SE-581 83 Linko¨ping, Sweden (Received 3 May 1999; accepted 2 August 1999)
Porosity depth profiles in porous silicon were realized by time modulation of the applied current density during electrochemical etching of crystalline silicon. The samples were investigated by variable angle spectroscopic ellipsometry. Using a basic optical model based on isotropy assumptions and the Bruggeman effective medium approximation, deviations from an ideal profile in terms of an interface roughness between the silicon substrate and the porous silicon layer and a compositional gradient normal to the surface were revealed. Furthermore, optical anisotropy of the sample was investigated by generalized ellipsometry. The anisotropy was found to be uniaxial with the optic axis tilted from surface normal by about 25°. The material was also found to exhibit positive birefringence.
I. INTRODUCTION 1
Since the first reports on room-temperature photoand electroluminescence2 of porous silicon (PS), extensive attention has been paid to this material, which is commonly fabricated electrochemically in ethanoic solutions of hydrofluoric acid (HF).3 The main concerns so far have been to fully understand the microstructural and optical properties of this complex material and to find further applications within widespread disciplines such as optoelectronics and sensor technologies. For example, optical devices such as Bragg reflectors1–8 Fabry–Pe´rot filters,9–13 and waveguides14 made of PS are in sight. Furthermore, gas sensitivity and biosensitivity based on the microstructural properties of PS have previously been reported.9,15–17 Of special importance for these applications are the simplicity of PS fabrication and the possibility of varying the properties of the material in a simple manner by changing the etching parameters. In general, thickness, volume porosity, pore size distribution, and internal surface area as well as luminescence properties of PS are determined by etching parameters like anodization time, HF concentration, current density, sample illumination, and properties related to the silicon substrate such as doping type, doping level, and crystal orientation.3 Thus, by a proper choice of these parameters, it is possible to manufacture PS layers with desired properties in terms of, e.g., a volume porosity depth profile. This approach is used for fabrication of complex struca)
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 14, No. 11, Nov 1999
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tures such as different types of optical filters. In such cases, the applied current density is time modulated in a manner that would provide stacking of periodic porous layers of high and low ind
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