Fabrication and optical characterization of porous silicon heterostructure as matrix for sensing organic solvent via resonance peak and Q factor shift

Abstract

Porous silicon heterostructures with an extensive photonic band gap, featuring a microcavity defect layer, have been successfully fabricated and applied as sensor devices for detecting organic solvent species. These sensors utilize both the resonance peak shift and the inverse Q-factor as sensing parameters. Similar to conventional porous silicon microcavities immersed in various organic solvents, the resonance peak exhibits a linear dependence on the solvent refractive index. However, in the proposed structure, the inverse Q-factor also displays a comparable linear trend with the refractive index, but with greater sensitivity to solvent mixtures. This increased sensitivity arises because the inverse Q-factor deviates from linearity when the prior solvent is not adequately removed from the porous structure before reflectance measurements — a condition not commonly detected by the resonance peak shift. To improve the performance of these sensors, the porous structures were passivated by thermal oxidation. For sensor applications, it is crucial that the contrast in the effective refractive index between the high and low porosity layers of the passivated structures be sufficiently large. Otherwise, when the porous matrix is immersed in solvents, it loses its resonance peak, rendering the structure ineffective for sensor applications.