Porous silicon multilayer structures: From interference filters to light emitting devices to biosensors

The fabrication of high quality porous silicon multilayer structures is made possible by first understanding the unique properties of a single porous silicon layer. It is quite remarkable how an inherently disordered and chemically unstable material, such as porous silicon, can be useful for optical devices. Fortunately, on a visible wavelength scale, porous silicon acts as an effective medium with well-behaved characteristics. This is demonstrated from the fact that porous silicon multilayer mirrors are constructed with high reflectivity near 100% and are tunable from the ultraviolet to the infrared wavelengths. When a luminescent thin layer of porous silicon (active layer) is sandwiched between two highly reflective multilayer mirrors, a microcavity resonator structure is formed. This structure has the advantage of narrowing the luminescence linewidth (FWHM ≤ 20 nm), due to photon confinement. The first light emitting devices based on a porous silicon microcavity resonator have been fabricated, spanning the visible wavelengths from 650 nm to 770 nm), Inserting a thick active layer in the porous silicon microcavity resonators yields a very interesting structure. The photoluminescence spectrum is composed of multiple narrow peaks, with FWHM values as narrow as 3 nm. The number of peaks can be adjusted by changing the thickness of the active layer; as the thickness increases, the number of peaks also increases. This structure is ideal for sensing applications, where any slight shift in the spectrum is detectable. This versatile material with its large surface area and unique optical properties has been utilized for biosensors. By immobilizing DNA on a porous silicon surface, we were able to detect the binding of complementary DNA (cDNA) inside the pores by photoluminescence spectroscopy. The induced red-shift in luminescence peaks is caused by a change in the refractive index of the medium upon molecular recognition of the DNA by its complement. This approach to biosensing is extended to include the detection of viruses.