Multilayer porous silicon structures for optical devices

Porous silicon (PS) has a great potential in optical applications due to its tuneable refractive index. In particular, multilayer structures consisting on PS layers with different refractive indexes can be used as interference filters. Due to the characteristics of PS and its production process, many types of interence filters can be produced: Bragg reflectors, monochromatic filters for light emitting devices (which can also be based on PS), microcavities for biosensing applications, Fabry-Pérot resonators, photonic crystals, etc.

In the present work the optical properties of porous silicon single layers and multilayer structures have been studied. PS is formed by the electrochemical etch of monocrystalline silicon wafers. Since PS can be modelled as an homogeneous mixture of silicon and air (and, eventually, silicon dioxide), its refractive index varies depending on the air content of the porous structure, according to the effective medium theories (EMTs). The electrochemical process gives a perfect control over the porosity –and hence, over the index of refraction- and thickness of the porous silicon layer, since both parameters are highly sensitive to the formation conditions, mainly current density and etching time. In addition, when the current density applied is periodically varied, a multilayer structure is formed. By simply adjusting the refractive index and thickness of each individual layer we can obtain a stack of porous silicon layers with the desired optical properties, resulting in an interference filter of predetermined band width.

The optical characterization has been carried out by spectrophotometric and spectroscopic ellipsommetry measurements. In addition, compositional analysis has been performed by means of Ion Beam Analysis (IBA) techniques, with special interest devoted to oxygen and hydrogen, in order to correlate the optical parameters and the chemical composition. Finally, multilayers structures have been produced and their operation has been checked.