Study On Porous Silicon Based Composite Film Gas Sensors

Porous silicon （PS） is a new type gas sensor which operated at room temperature.As it has large specific surface and can be easily produced, it has been widely used inthe fields of gas detecting and environmental protection. In this paper, macro-PS,WO₃/macro-PS, meso-PS and WO₃/meso-PS were prepared. The gas sensingproperties of macro-PS, WO₃/macro-PS, meso-PS and WO₃/meso-PS werethoroughly analyzed and studied.Ordered macro-PS was formed by electrochemical anodisation ofmonocrystalline p-type silicon wafer in a double-tank cell. The effect of etching timeon the structure, porosity, electrical characteristics and gas sensing properties ofmacro-PS were investigated. It is shown the depth and the porosity of the macro-PSget larger with that the increase of etching time. The electrical behavior exhibits thatthere is ohmic contact of between the macro-PS and Pt contact. The macro-PS withthe etching time of5min shows best gas sensing properties. The sensitivity of themacro-PS with the etching time of5min to50ppm NH₃is4.2. It has goodresponse-recovery characteristic. The response/recover time is51s/242s.WO₃thin films were deposited on macro-PSsubstrate by the facing targets directcurrent reactive magnetron sputtering method. The morphology, crystallization,valence of the W and gas sensing properties of the WO₃/macro-PS were characterized,respectively. It can be concluded by the SEM that the WO₃nano-particles form asemi-continuous thin film on the surface of macro-PS layer. The sensitivity of theWO₃/macro-PS gas sensor upon exposure to1,10,25and50ppm NO₂are1.40,5.01,9.01and14.4, respectively. The WO₃/macro-PS gas sensor has higher sensitivity thanthat of the Macro-PS gas sensor in each NO₂gas concentration.Meso-PS was formed by electrochemical anodisation of monocrystalline p+-typesilicon wafer in a double-tank cell. The effect of etching current density on thestructure, porosity, electrical characteristics and gas sensing properties of meso-PSwere investigated. It can be concluded that the depth, porosity and resistance value ofthe meso-PS get larger with the increase of etching time. The meso-PS with theetching current density of80mA/cm²shows best gas sensing properties. It was shownthat meso-PS had much higher sensitivity and good selectivity to NO₂, whilemacro-PS showed a good response–recovery characteristic and good selectivity to NH₃. The stability of gas sensing properties of the meso-PS by different storagemethod was also studied. It can be concluded that the meso-PS stored in the vacuumhas better stability.Then, WO₃thin films were deposited on meso-PS layers using dc reactivemagnetron sputtering method. The effects of sputtering time, working pressure andAr/O2ratio on the microstructure and gas sensing properties of WO₃/meso-PS weresystemically analyzed by orthogonal trial experiment method. The optimal depositionparameters for WO₃thin films based on meso-PSsubstrate were obtained. It can beconcluded that the gas sensing properties of WO₃/meso-PS to NO₂and NH₃werebetter than of those of PS. The sensitivity of the WO₃/meso-PS to1ppm NO₂was4.22and that to50ppm NH₃was18.3. The response time of WO₃/meso-PS was shorterthan that of meso-PS.