Gas Sensing Properties of Ge-As-S Thin Films

The sensitivity of thin Ge-As-S films to ammonia was investigated by the quartz crystal microbalance (QCM) using 16MHz AT-cut quartz resonators Films with appropriate composition are consecutively deposited on each side of the QCM by vacuum thermal evaporation with a rate of 50 angstrom/s. The previously synthesized Ge-As-S glass was used as source of evaporation The sorption properties of the as-deposited films as well as irradiated ones were investigated. The films were exposed to a HBO Hg lamp through an lit cut-off filter, for 45 min on each of the QCMs sides. The applied power density of irradiation was 0.12 W/cm(2) The sorption properties of Ge-As-S films were investigated in NH3 concentration interval from 10 ppm to 10000 ppm in a specially designed set-up for measurement of QCM mass-loading in dynamic regime It has been determined that the sorption properties to NH3 are only observed in the irradiated Ge-As-S films. A considerable response to NH3 was detected at concentrations over 250 ppm, where the measured frequency change (Delta F) of QCM is 15 Hz, while at 10000 ppm Delta F reaches value of 173 Hz Based on the measured frequency-time characteristics (FTCs) of QCM the processes of sorption were studied. The sorption and desorption velocity and absorbed NH3 mass for each of the investigated concentrations were calculated The obtained results show that thin Ge-As-S films could be used as gas sensor elements for detection of NH3 concentrations higher than 250 ppm