Porous Silicon Multilayer Structures Research Articles

Cross-Correlation of Optical Microcavity Biosensor Response with Immobilized Enzyme Activity. Insights into Biosensor Sensitivity

Porous silicon multilayer structures have remarkable optical and morphological properties that can be exploited for biosensing. In particular, a high internal surface area (>100 m(2)/cm(3)) and a linear response profile to changes in the dielectric environment enable fabrication of sensitive devices and a straightforward quantitation of the optical response. These essential operating characteristics are illustrated for p+ mesoporous silicon (pore diameter 15-20 nm) optical microcavities. A series of devices were prepared to permit the immobilization of glutathione-S-transferase ( approximately 50 kDa) within the porous matrix. Enzyme activity was exploited as an indirect means to quantitate the amount of protein immobilized. Activity was positively correlated with the optical sensor response. However, at high enzyme load the activity becomes nonlinear while the microcavity response remains linear. These data were used to determine the transduction limit (minimum amount of protein required to transduce an optical response), which is reported as areal mass sensitivity ranging between 50 and 250 pg/mm(2). This value is considered in context with the dynamic range of the bulk sensitivity, defined as the magnitude of the wavelength shift per refractive index unit, which was measured as a function of microcavity design parameters. This work has uncovered key parameters that can be tuned to improve the detection limit of this sensor modality. Because of the ever increasing number of emerging new biosensor technologies, defining sensor detection limits has become an ambiguous topic and a need exists to standardize measurements and sensitivity units. For chip-based devices, it seems appropriate to report sensitivity in terms of the minimum number of grams of bound target per surface area.

Development of interference filters for porous silicon based photoluminescent devices

Porous silicon's photoluminescent and electroluminescent properties make it a very interesting material for the development of optoelectronic devices. To obtain high quality devices it is mandatory to narrowen the porous silicon luminescent spectrum, tipically about 100 nm wide. This can be obtained by forming a porous silicon multilayer structure on the luminescent layer that acts as an interference filter. Thus, the emission spectrum can be narrowed as much as wished by simply designing the filter in the appropriate way. In this article, the optical properties of porous silicon layers are studied for future filter design.

Porous silicon multilayers for sensing by tuneable IR-transmission filtering

The suitability of porous silicon multilayer (PSM) structures in sensing applications has been investigated. Bragg-filters as well as Fabry–Perot interference structures were prepared using controlled anodisation of silicon in an HF electrolyte by a tight control of structural parameters. The relationship between process parameters (current density and etching time) and final structural parameters (porosity and layers thickness) was established earlier by means of spectroscopic ellipsometry and optical modelling. Transmission filtering in the NIR range as well as reflective type operation in wet ambient was demonstrated in the visible range. On the example of coupled resonator structures we proved the slightly increasing porosity developed in the multilayer during subsequent anodisation steps. The shifting of the resonant peak positions by incorporated liquid in the pores can be monitored in situ enabling the dynamic, spatial analysis of pore filling or drying.

Optimisation of porous silicon based passive optical elements by means of spectroscopic ellipsometry

Recently, growing interest in the development of passive optical elements based upon electrochemically etched porous silicon multilayer (PSM) stacks raised the need for efficient control of the preparation. Optimisation of PSM structures experimentally by try and error is, however, troublesome and time consuming. In the present study we demonstrate that spectroscopic ellipsometry (SE) with appropriate optical models can provide the missing feedback for the process engineer for exact design of structural parameters, thereby reducing the efforts for an optimisation cycle. The capabilities of the suggested methodology were experimentally verified on Bragg reflectors and Fabry-Perot interference filters.

Fabrication of Flexible One-Dimensional Porous Silicon Photonic Band-Gap Structures

Photonic crystals are periodic dielectric structures that have a photonic band gap to control the propagation of light in a certain wavelength range. This property offers a means to manipulate photons in the same way as electrons can be controlled in an atomic lattice. Porous silicon is an ideal candidate fo r the fabrication of photonic crystals because of the availability of a variety of silicon micromachining techniques. One-dimensional photonic crystals with customized parameters can be economically fabricated using porous silicon multilayer structures with periodically modulated porosity. Despite the structural non-homogeneities, porous silicon fabricated on a p-type Si substrate has optical properties similar to a dielectric material with a single effective refractive index. The exact value of the refractive index for each layer depends on its porosity. An engineered porosity can be obtained by changing the etching currents during the anodization process. This results in a modulation of the refractive index. A stack of alternating layers with high and low porosity produces a distributed Bragg reflector (DBR). Various designs incorporating multilayer porous silicon structures with an optical Fabry-Perot resonator and coupled microcavities are under development and can serve as an optical filter. Prototypes of such free-standing structures with 21–200 stacked layers to be used as DBRs, Fabry-Perot resonators or coupled microcavities are being fabricated. These structures are coated with polystyrenesulfonate on their backsides to increase mechanical strength and at the same time maintain flexibility. In this work, reflectance spectra of these porous silicon multilayers with and without polymer on the backside were measured. Simulations of the multilayer one-dimensional photonic crystals were performed to predic t the reflectance spectrum and optimize their structures before the fabrication and to compare to experimental data.

Porous silicon multilayer structures: A photonic band gap analysis

A photonic model for freshly anodized porous silicon multilayer structures is presented. The photonic structures are composed of alternating high and low dielectric function porous silicon layers. The model takes into account the presence of silicon dioxide and its lattice expansion in the porous structure. We work with oxidized structures and our results fit completely the experimentally measured optical shift.

One-Dimensional Porous-Silicon Photonic Band-Gap Structures with Tunable Reflection and Dispersion

The method of fabrication of porous-silicon multilayer structures with periodically modulated porosity was used to obtain one-dimensional photonic band-gap structures with tunable reflection and dispersion. We present the results of transfer-matrix calculations of reflection and dispersion properties of such photonic band-gap structures illustrating the idea of fabricating one-dimensional porous-silicon photonic band-gap broadband reflectors and elements to control parameters of short light pulses. Reflection spectra of the fabricated porous-silicon multilayers indicate that the reflection and dispersion properties of 1D porous-silicon PBG structures can be precisely controlled by varying the parameters of the technological process.

Light Emission from Highly Reflective Porous Silicon Multilayer Structures

Porous silicon photoluminescence and electroluminescence can be controlled by periodically modulating the material porosity to form high quality multilayer stacks and microcavities. Important issues not yet fully addressed are (a) the precise role played by this microstructuring, given that the luminescence is distributed throughout the entire structure and that the low porosity layers are highly absorbing at short wavelengths, and (b) whether the quality of such microcavities could be sufficient to support lasing. Using both experimental and theoretical techniques, the emission and reflection properties of different porous silicon single and multilayer structures have been investigated in order to understand further and exploit the nature of light propagation within them.

Tunable, narrow, and directional luminescence from porous silicon light emitting devices

Controlled room-temperature photoluminescence (PL) and electroluminescence (EL) from porous silicon (PSi) multilayer structures are achieved. The basic structure is composed of two PSi multilayer mirrors sandwiching a highly luminescent PSi film. This creates an active microcavity resonator, in which a significant PL and EL line narrowing is observed. EL from a microcavity resonator is shown to have a high angular concentration of the output emission (30° around the normal axis). A tunable and narrow EL device, from an all-PSi microcavity resonator is presented, which offers the possibility for high spectral purity, flat-panel displays.

Porous Silicon Multilayer Mirrors and Microcavity Resonators for Optoelectronic Applications

AbstractPorous silicon multilayer structures are easily manufactured using a periodic current density square pulse during the electrochemical dissolution process. The difference in porosity profile, corresponding to a variation in current density, is attributed to a difference in refractive index. Manipulating the difference in refractive index, high quality optical filters can be made with a maximum reflectivity peak ˜ 100%. The next logical step to further exploit these optical mirrors is to incorporate them into an LED device. The benefit of adding a multilayer mirror below a luminescent film of porous silicon is to significantly reduce the amount of light loss to the silicon substrate and increase the light output. However, oxidation is required to stabilize the as-anodized porous silicon film. This disrupts the overall index profile of the multilayer stack, causing the peak reflectance to blue shift. This phenomenon must be quantified and accounted before device implementation. We present a detailed study on the effects of oxidation temperature, gas environment, and annealing time of porous silicon multilayer structures in a device configuration.