Towards a Microscopic Interpretation of the Dielectric Function of Porous Silicon
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U.Rossow1 ,2 , U. Frotscher 2 , D. E. Aspnesl, W. Richter 2 1North Carolina State Univ., Phys. Dept., Raleigh, NC 27695-8202 2 TU Berlin, Physik, PN6-1, D-10623 Berlin, Germany ABSTRACT The dielectric function of porous silicon layers depends strongly on the electronic properties of the nanostructure of the silicon skeleton. In this paper we discuss the main effects, as determined from spectroscopic ellipsometric measurements of the dielectric function of porous layers formed on p-doped material. Finite-size effects and the high inner surface area of the nanostructure lead to relaxation of kmomentum conservation as defined for infinite crystals and therefore to a broadening of the features in arising from interband critical points. In addition a small threshold energy shift is observed when the percolation of the structure is reduced. However, this shift is too small to explain red photoluminescence as a consequence of a pseudo-direct gap whose energy is blue-shifted by confinement. 1. INTRODUCTION Several publications deal with the measured dielectric function or related properties such as reflectivity, absorptivity or photoconductivity of porous silicon layers. Since depends on the electronic structure its thorough understanding which has not been achieved so far would give important information about the PL mechanism in these layers. In order to separate the different contributions to we modified the preparation conditions (doping concentration, current density, electrolyte composition) and applied post preparation treatments such as oxidation by ozone or H2 0 2 [1-5]. The lineshapes can be classified for layers formed of ptype silicon depending on the doping concentration. For porous layer formed on heavily doped silicon sharp bulk like features appear revealing high interconnectivity within the layer, while for layers formed on lightly doped substrates we find only a single broad feature [1]. In this paper we discuss the present state of the interpretation of and consequences for the electronic properties of the Si skeleton. 2. EXPERIMENTAL Our substrates were lightly (1-10 Ocm) and heavily (0.01 f2cm) p-doped Si of (001) orientation. Aluminum or gold contacts were used on the back side. Electrochemical processing was performed in a standard Teflon cell with constant applied current and with the sample in the dark. The electrolyte consisted of 10ml ethanol and 10ml consisting of x ml 50% HF and (10-x) ml DI H2 0, where the 209
Mat. Res. Soc. Symp. Proc. Vol. 405 01996 Materials Research Society
resistivity of the H2 0 was greater than 17M9.cm) and where x varied from 10 and 3. For no dilution (x=10) the porosity ranged from 65% to 75% and from 30% to 70% for the lightly and heavily doped samples, respectively, depending on the etching conditions. Spectra were taken directly after the layers were prepared. We used a standard rotating-analyzer ellipsometer [6] with a spectral range of 1.8 to 5.5eV. 3. RESULTS AND DISCUSSION For heavily doped material the connectivity is high and consequen
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