Probing Optical Transitions in Porous Silicon by Reflectance Spectroscopy in the Near Infrared, Visible and UV
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Abstract Reflectance spectroscopy has been used to obtain the dielectric function of the solid phase of porous silicon. The method is based on a fit of a parameterized dielectric function model to measured spectra. A crucial step in the procedure is the 'dielectric averaging' of the microscopic dielectric function of the pore wall material to the macroscopic effective dielectric function which governs the optical properties. Results are given for heavily and moderately p-doped samples of various porosities. For the latter large differences to bulk silicon have been found. The obtained dielectric functions are compared to the results of band structure calculations taken from literature.
Introduction The extraordinary luminescence properties of porous silicon have caused an intense discussion about those features of the electronic system in low dimensional silicon structures that are responsible for the enhanced light emission. What is still missing is a convincing agreement between experimentally observed optical properties and theoretical predictions to a large extent this is due to the complex microtopology whose irregularities prevent detailed calculations but rather require a statistical treatment. Theoretical work resulting in the calculation of radiative and nonradiative electron-hole recombination rates is - due to the complexity of the problem - restricted to the consideration of individual (more or less spherical) microcrystallites or single 'quantum wires' in most cases [1][2][3]. By summing up all possible dipole-allowed transitions in the electronic band structure a dielectric function (including all size dependent effects) can be obtained for the material that makes up the microstructure. In homogeneous materials the dielectric function is the appropriate quantity to link band structure calculations to measured optical properties [4]. Nevertheless, in the case of mesoscopic systems like porous silicon the situation is more complicated: optical experiments do not deal with isolated silicon particles but rather probe the response of a large particle array to oscillating electric fields. Whereas long-range interactions between electron and hole wavefunctions from different microcrystallites can be neglected (i.e. quantum mechanical calculations can be carried out successfully considering single particles only) the mutual dielectric interaction of the polarizable silicon particles must be taken into account properly. This can be done using appropriate effective medium theories which provide the desired connection between the polarizability of the microscopic constituents and the macroscopic response to external electric fields that is probed in optical experiments [5][6]. In this article we show how the dielectric function assigned to the silicon skeleton in porous silicon can be obtained from experimental optical reflectance spectra in the near 435 Mat. Res. Soc. Symp. Proc. Vol. 358 0 1995 Materials Research Society
infrared, visible and ultraviolet spectral range by applying a reasonable effective med
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