Overview

Porous silicon (pSi) layers are prepared by the electrochemical dissolution of crystalline silicon wafers in hydrofluoric acid. The etching generates a uniform layer of porous silicon: the thickness and porosity, P, of any layer is controlled by the current density, the etch duration, and etchant composition. The porosity of a layer, the fraction of silicon removed during etching, can be easily varied between 40% and 85% during etching of heavily doped (p+) silicon wafers. Critical drying of ultra-high porosity layers has produced porosities up to 95%.

As the pores in mesoporous silicon are on a length scale of tens of nanometers, an effective index approximation can be used for light at visible and longer wavelengths to find the refractive index of any layer. Thus, the etching process defines the index of a layer and this gives control over the optical modes of the layers and so the reflectivity of the whole multilayer device.

Useful effective medium approximations for the dielectric function of pSi. The symbols have the following meanings: ε, dielectric constant of Si; εM, dielectric constant of host material (normally: air); εeff, calculated effective dielectric constant, Porosity, P (table after: Porous Silicon by Z. Gaburro, N. Daldosso and L. Pavesi)

An important property of multilayer mirrors and microcavities (a thin layer sandwiched between two multilayer ‘Bragg’ mirrors) is the ability to enhance the intensity of the fields stacked in the layers of the structures.