NO2 monitoring at room temperature by a porous silicon gas sensor

A study on reactivity of p+ porous silicon layers (PSL) to different gas atmosphere has been carried out. Substrate doping was 5–15 mΩ cm and 0.5 Ω cm, porosity ranged from 30 to 75% and the thickness of the porous layers was 20–30 μm. Three different processes to insure good electrical contact are proposed and discussed. PSL were kept at constant bias and current variations due to interaction with different concentrations of NO2 were monitored at constant relative humidity (R.H.). Measurements were performed at room temperature (R.T.) and at atmospheric pressure. Concentrations as low as 1 ppm were tested, but the high sensitivity of the sensor makes possible to test lower values. The recovery time of the sensor is of the order of one minute. Response to interfering gases (methanol, humidity, CO, CH4, NO, NO2) has been examined also. In-situ FTIR spectroscopy in NO2 atmosphere shows a fully reversible free-carrier detrapping in the IR region, confirming the validity of the models proposed in the recent past for electrical conduction in mesoporous silicon.