Optical Sensors based on Porous Silicon Multilayers: a Prototype
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R.Arens-Fischer*,
S.Hilbrich*,
D.Scheyen*,
M.G.Berger**,
M.Kriiger**,
M.Th6nissen**, *I. Phys. Inst., Aachen University of Technology (RWTH), D-52056 Aachen, Germany **Institut for Schicht- und Ionentechnik, Forschungszentrum Julich, D-52425 Jilich, Germany
ABSTRACT Porous silicon multilayers are easily produced by a variation of the current density during the electrochemical etching process. The obtained porosity profile in depth is equivalent to a refractive index profile. Preparing appropriate multilayer systems one can design optical filters with sharp reflectivity peaks from the infrared through the whole visible spectral range (dielectric mirrors). Stability in time is achieved applying a suitable oxidation process. In many cases of optical process and material control the required spectral decomposition of the probing light can be done with sufficient accuracy by a set of porous silicon optical filters whose spectral characteristics can be adapted easily to the given problem. In this work we present the prototype of an optical sensor that is a low-cost alternative to 'real' (and expensive) spectrometers. Application examples in the infrared and visible are given to demonstrate the sensor performance. INTRODUCTION Porous silicon has been investigated mainly with respect to its extraordinary luminescence
properties which have been discovered a few years ago [1]. Meanwhile it is clear that fast optoelectronic devices will be difficult to make using porous silicon. This is due to the fact that typical decay times of porous silicon luminescence are in the gs range. On the other hand, display applications of porous silicon layers are still under discussion. The easy fabrication of complex porous layer stacks (such as porosity sample filter wheel superlattices) by current variation during the electrochemical etching process [2] favours the application of porous silicon in optical thin film systems. This is because the produced porosity profiles are also refractive index profiles which can be used to design quite freely the optical properties of such a system. In this work we apply porous silicon Fig. 1: Basic scheme of the optical sensor. A layer stacks as optical reflectance reflection design has been realized since - up to filters. They are used as selective now -no transmission filters can be produced elements in optical sensors following easily. 637
Mat. Res. Soc. Symp. Proc. Vol. 452 ©1997 Materials Research Society
the scheme shown in fig.1. A wide variety of filter characteristics, i.e. the frequencydependence of the reflectivity, can be achieved. The most intuitive case is used in this work: so-called Bragg filters are applied which exhibit a large reflectivity in a narrow frequency range only and a low reflectivity everywhere else. These dielectric mirrors can be tuned from the mid infrared spectral range up to the near UV by proper choice of porosity and thickness of the individual sublayers. Of course, the frequency position and the width of these band pass filters must be adapted to the given
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