Optical Components from Mesoporous Silicon

Abstract

Porous silicon with pore sizes below the wavelength of the light in the micro-, meso, or lower macropore region (in what follows always addressed as “mesoporous”) offers the opportunity to “engineer” the refractive index at the visible and the IR spectral range by variations of the porosity of the layer. This property can be utilized in a number of optical components. Note that the luminescence properties of microporous silicon [1] will not be considered here. Reflective type of optical filters, or mirrors based on mesoporous silicon superlattices were proposed first by G. Vincent over a decade ago [2]. In this paper porous silicon super-lattices (roughly two microns thick) were etched on p-doped (100) oriented (8–15) Ωcm wafer; the porous layer was microporous silicon. The porosity and the concomitant refractive index modulations were formed by modulating the applied current density during the electrochemical etching. Independently, a similar approach was demonstrated by another group at the same year [3,4]. In [3] it was shown that in addition to modulating the current density porous silicon superlattices could also be formed by anodization of substrates with layers of different doping concentrations or different compositions at constant current density. Since then a large number of papers was published with respect to (meso)porous silicon filters and mirrors. Porous silicon filters were proposed to be used in color-sensitive photodiodes [5], luminescent devices [4], sensors [6–11]. A detailed review of these activities can be found in [12]. In this chapter we will review filter applications of mesoporous silicon. Particularly, we will focus on mesoporous silicon filters for the mid to far infrared region (light with wavelengths above 3 μm), where the advantages of the mesoporous silicon technology are most promising. We will start with brief description of optical filters in general, will briefly review far IR mesoporous filters and will then address one of the most challenging problems for this technology – environmental instability problem and the ways to mitigate this problem.