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We show a vapor controlled one-dimensional photonic crystal built with a silicon-based dielectric mesoporous material, in which the refractive index can be continuously tuned by flowing organic vapors through the pores. Exposure of the crystal to vapors induces an inhomogeneous change in the refractive index through the depth and results in a tilted photonic band structure. We drive the photonic crystal to the optical Zener breakdown condition, introducing an enhanced transmission channel through the crystal. This phenomenon closely resembles the electronic Zener breakdown in a reverse-biased p–n junction. The effect is reversible and opens new perspectives for physics in photonic crystals and novel device concepts based on sensing, switching and processing of light. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

After electrochemical etching, we have made a study of the effects generated on p+-type porous silicon layers when they are left in presence of the electrolyte for different post-etching times. Using an interferometric technique, we have monitored the change of its porosity during the post-etch process due to a chemical dissolution mechanism. These data are complemented with a study of photoluminescence and transmission electron microscopy measurements for different post-etching times. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The electrical injection in porous silicon fabricated with heavily doped p-type silicon is very sensitive to NO2. The known effect is an injection increase associated to NO2. We show experimentally a strong correlation between two structural properties and the sensitivity of electrical injection to NO2. The first property is the microstructure, i.e. the pore morphology at nm scale. A structure with straight, elongated pores shows large sensitivity, as opposed to a branching structure. The second property is the layer thickness, which determines the sign of the effect of NO2. If the thickness is sufficiently low – of the order of few µm – the injection in presence of NO2 decreases, instead of increasing. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We discuss the optical properties of one-dimensional complex dielectric systems, in particular the time-resolved transmission through thick porous silicon quasiperiodic multi-layers. Both in numerical calculations and experiments we find dramatic distortion effects, i.e. pulse stretching and coherent beatings, when band-edge states are resonantly excited. Numerical simulations and experiments are in good agreement. We argue that porous silicon can be used conveniently for the fabrication of one-dimensional complex photonic structures.

We have fabricated a multiparametric gas sensor based on porous silicon. The sensing parameters are the electrical conductivity of a single porous Si layer and the resonance wavelength of a porous silicon microcavity, both fabricated on the same substrate. The electrical conductivity allows detection of 50 ppb of nitrogen dioxide in dry air. Although relative humidity also affects the electrical conductivity, the contributions due to nitrogen dioxide and to humidity can be distinguished by simultaneously monitoring the cavity resonance wavelength and the current. We also demonstrate a three-contact configuration which further increases the sensitivity of the electrical response of the sensor.

The optical and electrical properties of porous silicon microcavities are strongly dependent on the environment. For highly luminescent samples both the luminescence intensity and the peak position are affected by organic substances, which also strongly modify the electrical conductivity, giving the possibility to obtain a multi-parametric optical/electrical sensor. The peak position depends on the refractive index of the organic compound, whereas the luminescence intensity depends on its low frequency dielectric constant. Electrical properties depend on the dipole moment of the molecules. This allows discriminating between different organic substances.

The fabrication in a standard CMOS line of a light emitting diode (LED) based on silicon/porous silicon heterojunction is discussed. To fabricate the LED in a CMOS line, the porous silicon formation must be performed either as the last or as an intermediate step. The former option requires a masking layer to protect the metallization level of the CMOS devices from the electrochemical solution for the porous silicon formation, whereas the latter forces an interruption in the process. Experimental tests on several materials, routinely used in CMOS processes, show that no standard mask is suitable to fully protect the CMOS from the electrochemical etch. Hence porous silicon formation should be performed as an intermediate step.