Formation and Properties of Porous Si Superlattices
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spectroscopy. Type I superlattices were formed by periodically changing the current density during the etch process. The quality of the interface between layers of different porosity depends on the quality of the silicon substrate. Optical components such as Bragg reflectors or Fabry-Perot filters were designed using the optical data of single porous layers. A good long term stability of the layers is achieved by using thermal oxidation steps. Type II superlattices were formed on substrates with layers of alternating doping level. The more complex layer structure of these superlattices is explained by the selectivity of the etch process on the doping level. INTRODUCTION Porous silicon (PS) is formed by an anodic etch process in concentrated HF solutions [1, 2]. The microstructure and hence the optical properties of the porous layers depends on the doping level and doping type of the silicon substrate, the HF concentration, the applied current density and any possible illumination parameters. The thickness of the porous layers depends on the etching time and allows the formation of layers with well defined thicknesses. For p-doped substrates etched in the dark the etching process is mainly restricted to a region next to the substrate at the bottom of the pores [3]. Therefore, a change in the current density during the etch process will not influence the microstructure of the layer formed up to this time. Periodical changes in the current density will result in the formation of a superlattice consisting of layers with different porosity. We have for the first time demonstrated the feasibility of this simple technique by TEM images and reflectance spectra taken from these so called type I porosity superlattices [4, 5, 6]. In this paper we investigate the influence of the substrate quality on the layer structure of porosity superlattices. While it is also possible to form superlattices on n-doped substrates [7], this paper will only deal with PS-SL formed on p-doped silicon substrates. Besides offering a deeper insight into the formation process of porous silicon these superlattices exhibit interesting optical properties which can be useful for various applications in the field of microoptics such as mirrors, filters or waveguides. In order to allow a proper design of e.g. the filter characteristics, the dependence of the dielectric function and the etch rate on the current density must be known. After a superlattice - or more generally speaking a multilayer system - has been formed the aging behaviour of the porous layers becomes important. Therefore, the long term stability of the characteristics of untreated and thermally oxidized filter structures is investigated. 327 Mat. Res. Soc. Symp. Proc. Vol. 358 01995 Materials Research Society
Another approach to form porosity superlattices is the use of a substrate which consists of layers with different doping levels [6]. Due to the dependence of the microstructure on the doping level, anodizing with a constant current density will also lead to the formation of a super
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