Gas sensor applications of porous Si layers J. Mizsei ⁎ Budapest University of Technology and Economics, Department of Electron Devices H-1521 Budapest, Hungary Available online 20 March 2007 Abstract The porous silicone (PSi) is a relatively new and promising semiconductor material with special physical and chemical properties which somewhat differ from the properties of single crystal Si. Some of these properties are valuable in the field of gas sensor technology, but a lot of questions arise in connection with its application. Do we really need porous semiconductor material for proper gas sensing function? How can electrical properties of the PSi layer be measured if the electrical contacting is problematic? Is it possible to activate the PSi with catalytic noble metal layers or particles? What about the Fermi-level pinning in the PSi layer? The main target of this article is to seek answers to questions listed above and to give a short, but still comprehensive review of the application of the PSi layers on the field of the gas sensor technology, with special care on electrical output signal giving sensors. © 2007 Elsevier B.V. All rights reserved. Keywords: Porous silicon; Gas sensor; Interface states; Fermi-level pinning 1. Introduction Anodic oxidisation and etching of the c-silicon surface results in a porous Si (PSi) layer. In the PSi some part of mono- crystalline silicon is etched away by electrolytic process in HF content solution. The rest of the silicon layer is still crystalline, but contains a lot of pores, therefore, many strange physical phenomena can be observed in connection with this material. The technology and different applications of the porous silicone (PSi) have been discussed in detail in the literature [1,2]. Some recent review articles and tutorials help to understand importance of this peculiar material among nanosensors [3]. The fabrication methods and many physical properties are discussed in [4] and [5]. However, in spite of several successful experiments there are some problems on the field of gas sensor applications, especially in the case of electrical readout: the PSi layer contains huge number of volume traps and interface states, resulting in Fermi-level pinning in the layer and at the PSi–Si interface. This may block the electrical response (gas sensitivity) of the PSi–Si structure. The scientific literature mirrors these problems. The expression “porous silicon gas sensor” results in 208 items on internet search engine (Scopus), while the word “resistance” occurs only 8 times among them. The discussion of the resistor type PSi chemical sensors is completely missing from a recent review [3]. 2. Semiconductor gas sensor materials There is a strong connection between the surface and the bulk in the semiconductors. That is why these materials are used as gas sensors too, in addition to many other applications. Practical form of appearances of the semiconductor gas sensors are different: thin film with comparable thickness of space charge layer to the total layer thickness (simple, or with doped surface [6,7]), thick film with comparable thickness of space charge layer to the grain size [7], nanocrystals, where the particle size is smaller than the space charge layer, thus the total volume of the nanoparticle is controlled by the adsorption at the grain boundary [8], macroscopic Schottky barrier [9,10], MOS with Pd gate [11,12], or with some other gas sensitive material on the gate (see Fig. 1). In spite of these structural differences, the experimental values of surface and interface potentials are in the same range. For example, the maximum values of the H 2 adsorption induced potential changes are about 0.5–0.6 V on the Pd surface [12] and Pd–SnO 2 interface [13,14]. Thus the measurement or calculation of the surface and interface potentials enables a comparison between the effects of the adsorption on many different semiconductor gas sensor structures, and reflects the similarity of operation: charge Thin Solid Films 515 (2007) 8310 – 8315 www.elsevier.com/locate/tsf ⁎ Tel.: +36 361 463 2715; fax: +36 361 463 2973. E-mail address: mizsei@eet.bme.hu. 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.03.033