Electrochemical stabilization of porous silicon multilayers for sensing various chemical compounds

Porous silicon rugate filters are fabricated and investigated for their ability to sense chemical species. The durability of the filter is tested by allowing the structure to undergo many cycles of adsorption and desorption of vapor-phase ethanol molecules. The characteristic reflectivity peak of the structure exhibits a relative blueshift of 2.7% after 86 adsorption/desorption cycles. The observed shift is ascribed to the formation of silicon dioxide, which has a lower refractive index than that of silicon. In order to stabilize the structure against oxidation expected from cycling and environmental exposure, the filter is subjected to electrochemical oxidation in an aqueous sulfuric acid electrolyte. The treatment dramatically improves stability of the sensor; a relative blueshift of < 0.4% is observed after 100 adsorption/desorption cycles for this sensor. The sensitivity of the sensor is also affected by electrochemical oxidation: the response to saturated ethanol in air changes from Delta lambda=100 nm to Delta lambda=70 nm, respectively. Theoretical calculations using the Bruggeman effective medium approximation and the characteristic matrix method indicate that up to 15% (by volume) of silicon is transformed to silicon dioxide by the electrochemical oxidation procedure. This volume ratio is close to that estimated from Auger electron spectroscopy measurements.