Gas sensitive porous silicon devices: responses to organic vapors

Geometrically scaled PS-based structures were fabricated in order to develop gas sensing devices by exploring porous silicon (PS) electrical characteristics. The electrical behavior of PS devices respond to polar organic vapors (as acetone and ethanol) reversibly in a reproducible way. Devices were fabricated with three different perimeters, maintaining a constant area (5.76 mm(2)) and constant PS porosity (60%) throughout samples, in order to evaluate their electrical impedance depending on the area/perimeter ratio. Electrical impedance was measured from 10 kHz to 10 MHz in acetone, ethanol and vacuum (as reference) environments. The results obtained show the general aspect for impedance variation as expected for disordered materials such as amorphous semiconductors or polymers. Measured impedance is fitted proportionally to (2pif)(s), where f is the excitation frequency. The exponential factor "s" was found to be around -0.55 for ethanol and -0.45 for acetone, whereas in vacuum s equals -0.97, thus providing a method for identifying polar molecules. The parameter "s" for the tested environments is independent of device geometry. (C) 2003 Elsevier Science B.V. All rights reserved.