Gas Sensing Properties Of Metal Oxides Modified Porous Silicon

Porous silicon is a room temperature micro-nanoscale gas-sensing material.Composite metal oxide semiconductors is studied to improve sensitivity and selectivity in gas-detection.In this paper,structure of metal oxides modification on porous silicon is proposed to enhance sensing properties of porous silicon.WO₃ is prepared onto fabricated porous silicon as representative metal oxide semiconductor to form WO₃modified porous silicon.The composite structures are investigated by SEM,XRD and EDS,and samples'gas-sensing characteristics are texted and analyzed.N-type porous silicon with pore size of 20-40nm was fabricated by double-cell electrochemical etching,and the etching rate is 0.8-1?m/min.Porous silicon is layered when etching time exceeded 2min.Based on the N-type single-layer porous silicon,2methods were studied to form WO₃ modification on top of porous silicon.Through thermal oxidation,the sputtered W thin film has grown to WO₃ nanowires,and the optimum heating temperature is 700?.Samples with different sputtering time exhibited P-type inversion sensing performance,sensitivity to NO₂ was enhanced.1min sputtered sample exhibited the highest sensitivity and best selectivity to NO₂.Through annealing,spin-coated tungstic acid sol was decomposed into WO₃nanoparticles,which were coated uniformly on top of porous silicon,with size of 90-130nm.WO₃ content was controlled by spin-coating times.Samples exhibited different inversion characteristics.N-P conversion was observed through increasing WO₃content,inversion to different gases corresponded different WO₃ content.Meanwhile,1-time coated sample exhibited N-P conversion properties through increasing NO₂concentration.Enhancement of sensitivity by WO₃ modification was observed and the enhancement first rose and then fell through increasing WO₃ contend,also the best enhancement to different gases corresponded different WO₃ content.Samples with different coated times exhibited different selectivity:1 time coated sample exhibited best NO₂ selectivity,and 3 times coated sample exhibited best ethanol selectivity.Influence of WO₃ modification on porous silicon in gas sensing characteristics is studied and is inferred as effect of the inversion layer and depletion layer formed by WO₃/porous silicon heterojunction.