Contribution of the Nanocrystallites and Their Interfaces to the Optical Response of Porous Silicon Layers

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ABSTRACT We discuss the dependence of the dielectric function on the nanocrystalline size of porous silicon layers. The layers were grown by a standard electrochemical process and characterized by spectroscopic ellipsometry. By a lineshape analysis values of the critical point energies and broadening parameters of the Interband critical points were derived. In order to obtain further information about the nanocrystallites, the preparation conditions were varied (HF concentration, NH4F was added) or the layers were further treated by etching and arsenic deposition. The lineshape analysis values indicate that the layers consist mainly of nanocrystallites, or more accurately, that the electrons are confined to regions of a few nanometers in size. Furthermore, there is strong indication that some preparation conditions may leave these nanocrystallites heavily strained. 1. INTRODUCTION The quantitative analysis of the optical spectra of porous silicon layers obtained transmission, reflection or ellipsometric spectroscopy remains a challenge. Complications include inhomogeneities in the layers, aging, and in most cases the unknown topology of the layers. Inhomogeneities lead to elastic light scattering and may cause depolarization of polarized light. While elastic light scattering affects mainly the intensity of the transmitted or reflected light, depolarization introduces inaccuracies for optical techniques which use polarized light such as ellipsometry. The porous silicon layers consist of pores and a silicon skeleton. The optical response is given by the dielectric functions of the pores, the silicon skeleton, and the geometric arrangement or topology of the layers. The thickness of the layer must be taken into consideration because absorption in these layers is small in the visible range. Any inhomogeneities in the layer thickness introduces additional complications in the analysis. In order to estimate the contribution of the nanocrystallites to the effective dielectric function of porous silicon layers we modified the layers by varying the preparation parameters and post preparation treatments. First, we varied the HF concentration, which is expected to change the nanocrystallite sizes but have less effect on the interfaces. In further experiments NH4F was added, which is known to flatten Si(111) surfaces [1]. This is of interest since etching Si(001) surfaces with highly concentrated HF for extended periods of time is known to create Si(1 11) facets. These facets may thus be flattened by introducing NH4 F to the electrolyte or 429 Mat. Res. Soc. Symp. Proc. Vol. 358 01995 Materials Research Society

by posttopreparation treatments layers. We also silicon have exposed layers arsenic in UHV. Arsenicofisthe known to passivate surfaces the and porous create a bulk- like termination [2,3].

2. EXPERIMENTAL Data were obtained with a spectroscopic ellipsometer of the rotating analyser type [4] with a useful spectral range of 1.8 to 5.5eV. The information depth varies strongly in this spectral range and is lowest around