Sensors & Transducers Journal, Vol. 113, Issue 2, February 2010, pp. 1-17
Although the electrochemical anodization is commonly used in the fabrication of PS, several other fabrication methods have been introduced. Stain etching method is one of them. The stain films are produced by immersion of Si substrate in HF solutions without any electrical bias [89-91]. This method is even simpler than the previously presented anodization. However, the control of the porosity, layer thickness and pore size of PS is quite limited. In addition, in the stain etching of Si microparticles, it is quite difficult to control the porosification of particles. Incomplete porosification of Si particles might cause problems in drug delivery applications. Porous silicon fabricated by stain etching method shows low photoluminescence efficiency than the electrochemically etched one.
Nanoporous silicon consists of a complicated network of silicon threads of 2-5 nm thickness with an internal surface area-to-volume ratio of around 500 m2/m3. Thus PS can absorb large amounts of foreign molecules onto its surface eventually changing the effective refractive index of the semiconductor porous material. Due to the quantum confinement effect strong luminescence at room temperature is observed from the tiny pores. This photoluminescence (PL) intensity changes when PS is exposed to various chemicals and biological samples and the final photoluminescence efficiency depends on the dipole moment of the molecules attached to the pores. Similarly the effective dielectric constant and the conductivity of PS layer changes when the pores are filled with some other molecule. This property helps in developing electrical and optical PS biosensors adsorbing foreign materials on its surface.
Fig. 3. SEM images of PS for different current densities and n-type doping densities [Source: Ref 107].
It has been shown that porous silicon can be used as a base material for passive or active optical devices like Fabry-Perot interferometers, Bragg filters and optical microcavities , because of its lower effective refractive index than that of bulk silicon. It can also be used as an antireflection coating for silicon solar cells. A wide range of refractive index varying from 1.25 to 3, allows this material for