Optical and photo properties of PSi sensors

The features of optical absorption and emission in PSi have been discussed by Alexanyan et al. (2003). An allowance for the finite nature of the potential well depth when calculating the shift of the absorption edge of the quantum wire, as well as the probability of transition between states in 

2 (a)Transmission spectra of several 20 pm thick freestanding PSi films (5-7 Qcm p-type Si substrate) of various porosities (P) and a 20 pm thick c-Si (crystalline silicon) film, (b) Absorption spectra for Si nanocrystals with sizes from 4.3 nm to 2.4 nm deduced from the transmission spectra of (a).The absorption of c-Si is shown for comparison.The saturation beyond 3 eV is an artifact of the measurement procedure. After von Behren et al. (1998) with the permission of the authors. 

It has been shown that the refractive index of a material is proportional to its porosity (Pavesi, 2004). Therefore, complex structures can be realized using PSi etched layers. Moreover, the refractive index of PSi can be changed 

In multiparametric PSi optical microcavities, developed by Baratto et al. (2002), three independent quantities can be measured the electrical conductance, the PL intensity and the wavelength of the optical resonance. The change of these parameters as a function of NO2 (0.5-5 ppm), ethanol (300-15 000 ppm) and relative humidity (0-100%) has been investigated. 

A new method for using photo-electromotive force (Photo-EMF) in the detection of gas and for controlling sensitivity is proposed (Vashpanov et al., 2011). Photo-EMF on the heterojunction between the PSi thin layer and the crystalline silicon wafer depends on the concentration of ammonia in the measurement chamber. A light-transparent contact to the porous Si was formed. Photo-EMF sensitivity corresponding to the ammonia concentration in the range from 10 ppm to 1000 ppm can be controlled by the intensity of the source of illumination. 

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