“…The tin oxide thin films are grown by reactive sputtering from a SnO 2 target under a 10:90 oxygen-argon mixture. A detailed description of the complete procedure for the preparation of the sensors has been reported elsewhere [13,14]. Deposition conditions have been fixed during the sputtering process (independently of the target used) and are as follows: substrate holder temperature 250 • C, plasma pressure 0.5 Pa, acceleration voltage 500 V, radio frequency power 100 W. Some of the sensors have been doped with different amounts of Cr and In, by changing the deposition time during the sputtering process.…”