Gas sensitive porous silicon devices: responses to organic vapors E. Galeazzo * , H.E.M. Peres, G. Santos, N. Peixoto, F.J. Ramirez-Fernandez Sensores Integrados e Microssistemas, SIM Group, Laborato ´rio de Microeletro ˆnica, Escola Polite ´cnica, Universidade de Sa ˜o Paulo (USP), Av. Prof. Luciano Gualberto, Trav. 3, 158, CEP 05508-900, Sao Paulo, SP, Brazil Abstract 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 (2pf) 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. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Porous silicon; Gas sensor; Organic vapors; Impedance; Acetone; Ethanol 1. Introduction Porous silicon (PS) is obtained conventionally by anodi- zation of silicon substrates. Crystallites of silicon formed by this means can present diameters varying from units of nanometers to tens of micrometers, depending on formation parameters (current density, electrolyte concentration, etch- ing time, and substrate type). This characteristic, that is, the possibility of porosity control, makes PS suitable for several applications on gas sensing [1–5]. Its large internal surface area and high chemical reactivity with the environment further enhance the interest on sensing applications. Proper- ties, such as photoluminescence (PL) and electrical con- ductivity, change when molecules are adsorbed to its surface [6,7], in such a way that these parameters may be monitored and related to physical reactions, to environmental condi- tions, or even to air quality, which makes porous silicon a promising material for a wide range of applications ranging from gas sensing to chemical detection in very small con- centrations [8]. Despite PS sensitivity to humidity and high concentration of organic vapor, problems such as stability, reversibility, and selectivity must be investigated, if it is to be applied as a commercially useful device. Some of these problems have been addressed before by our group [9] and other researchers [2], and we believe most of them are solvable by tuning anodization conditions, adapting masking alternatives, and proposing suitable models for PS formation and reactivity with the environment. In this paper, we specifically address this latter issue, and discuss possible applications by showing experimental results that confirm model predictions. Stabilization of PS photoluminescent properties and mini- mization of electrical drift in the actual device are two important steps towards the fabrication of devices for char- acterization in reactive environments. Several processing sequences for PS surface passivation after anodization have been suggested in the past, in an attempt to deal with both issues. For example, Parkhrutik et al. analyzed the electrical drift of PS devices and attributed it to residual electrolyte inside the porous layers after anodization [10]. Taking into account previous reports as that one, the main objective of this work is to analyze the electrical behavior of PS devices during and after tests in environments with polar molecules, and to propose a model which relates the electrical response Sensors and Actuators B 93 (2003) 384–390 * Corresponding author. E-mail address: bete@lme.usp.br (E. Galeazzo). URL: http://sim.lme.usp.br 0925-4005/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0925-4005(03)00200-4