Sensors and Actuators B 103 (2004) 190–199 H 2 , CO and high vacuum regeneration of ozone poisoned pseudo-Schottky Pd–InP based gas sensor L. Mazet ∗ , C. Varenne, A. Pauly, J. Brunet, J.P. Germain LASMEA, UMR 6602 du CNRS, Université Blaise Pascal, Clermont-Ferrand II, 24 Avenue des Landais, 63177 Aubière Cedex, France Available online 28 May 2004 Abstract This article deals with pseudo-Schottky diode Pd–InP gas sensor. Catalytic activity of such palladium layer permits to measure very low concentrations of two powerful oxidizing gases, nitrogen dioxide (NO 2 ) and ozone (O 3 ), constituting main urban atmospheric pollution. After many O 3 submissions, degradation of gas sensor characteristics (response time, recovery time and sensitivity) appears, probably due to ozone oxidation of palladium metallization. Different treatments (H 2 reduction, CO reduction and high vacuum) were then tested to avoid degradation of sensor parameters. Initial response and recovery times of this sensor can be recovered by H 2 reduction and high vacuum treatment of metallization layer after exposure to ozone. Moreover, results emphasize that these treatments improve sensor sensitivity. CO reduction will permit to have reproducible measurements of low NO 2 and O 3 concentrations. © 2004 Elsevier B.V. All rights reserved. Keywords: Gas sensor; Schottky diode; Regeneration 1. Introduction In industrialized countries, air quality monitoring has become a priority since atmospheric pollutants have been identified for their noxious effects on human health and vegetation. Most of them, carbon monoxide (CO), sulfur dioxide (SO 2 ), nitrogen oxides (NO and NO 2 ) and volatile organic compounds (VOCs), come from combustion pro- cesses; only ozone (O 3 ), a secondary pollutant, is produced by action of UV radiations on NO 2 and VOCs. They mainly cause respiratory and lung irritations. Among all theses pollutants, it is now of great interest to measure the two powerful oxidizing gases mainly present in urban atmo- sphere: nitrogen dioxide (NO 2 ) and ozone (O 3 ). As existing monitoring techniques need bulky and expensive apparatus, it is very difficult for air quality control networks to achieve a dense mapping of pollution distribution. To extend the number of monitoring locations, low costs, reliable and selective semiconductor gas sensors are one alternative. Different materials can be used for this application. Molec- ular semiconductors like phthalocyanines [1] show a high sensitivity towards oxidizing species but they suffer from too long response times and lake of selectivity. Metal ox- ide semi-conductors have been investigated too. Tin oxide ∗ Corresponding author. E-mail address: mazet@lasmea.univ-bpclermont.fr (L. Mazet). SnO 2 [2], indium oxide In 2 O 3 [3,4], tungsten trioxide WO 3 [5,6], NiO [7] have been used for the detection of NO 2 (0.2–10 ppm) or O 3 (10–130 ppb). Such devices offer good response times and sensitivities, but, unfortunately they are not intrinsically selective (detection of CH 4 , NH 3 , CO, NO, etc.) and often work at high temperature (200–500 ◦ C). The original device studied in this paper is pseudo- Schottky diode grown on p-type indium phosphide substrate. This structure, operating at low temperature (100 ◦ C), is very interesting because after submission to ozone concen- trations, it becomes sensitive and highly selective towards ozone specie [8]. Using such monocristalline based struc- ture allows to avoid the reproducibility problems occurring with gas sensors made with polycrystalline or molecular materials. Moreover, this structure is entirely compatible with microelectronic technologies. 2. Experimental Schottky contact metallization is realized with a noble metal (Pd). Structures are thus made by successive evapo- rations of metallic thin layers on InP-p bulk substrate with a contact scheme consisting in Pd/Ge/Pd/InP-p. Anneal- ing process at 320 ◦ C make Ge to diffuse at the InP/metal interface, creating so a thin n-type InP layer which in- creases the initial Schottky barrier (0.8 eV for Pd/InP-p) to 0.9eV. The current across the polarized structure depends 0925-4005/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2004.04.051