Effect of porosity on the performance of surface modified porous silicon hydrogen sensors

Porous silicon (PS) samples of different porosities were prepared from a single crystalline p-type silicon (1 0 0) wafer of resistivity 2–5 Ω cm by electrochemical anodization using a mixture of HF and ethanol in different ratios as the electrolyte solutions. The PS surfaces were then chemically modified by dipping in a low concentration acidic aqueous solution of PdCl2 (0.01 M) for 5 s and were characterized by EDAX line scan analysis. It was observed that the density of the dispersed Pd increased with increasing porosity. The Pd–Ag/PS/Si/Al Metal-Insulator-Semiconductor (MIS) structure was studied for hydrogen sensing at 27 °C in a nitrogen atmosphere, using both the unmodified and Pd modified porous silicon. While for the unmodified surface an increase in the response with increasing porosity was followed by saturation, the Pd modified PS showed an optimum response (84%) with the minimum response and recovery time (8 s and 207 s, respectively) at 55% porosity. The decrease in the sensor response for modified samples with further increase in porosity was possibly due to increased Pd loading, causing a decrease in the effective surface area of the dispersed Pd islands for gas adsorption.