Gas Transport and Response in Porous Silicon Sensors
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Gas Transport and Response in Porous Silicon Sensors Serdar Ozdemir1 and James L. Gole1,2 1 School of Physics, Georgia Institute of Technology, 837 State St Atlanta, GA, 30332 USA. 2 School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA, 30332 USA. ABSTRACT Nanopore covered microporous silicon conductometric gas sensors have been produced via electrochemical etching and standard microfabrication techniques. Reversible and sensitive gas sensors working at room temperature have been fabricated. Sensing of NH3, NOx and PH3 at or below the ppm level have been achieved. The porous silicon (PS) surface has been modified using selective coatings including electroless tin, gold, nickel and copper solutions to increase the response to NOx, NH3, and PH3 respectively. The diffusion of the analyte species has been investigated in the nanopore and micropore regimes by numerical analysis. Comparing the response time of the hybrid porous sensor surface with numerical diffusion calculations on the pores, it has been observed that Knudsen diffusion time scales dominate the sensor response. A transduction model is proposed based on nanopore limited gas diffusion and the experimental response and recovery data. INTRODUCTION Porous silicon (PS) has drawn considerable attention since the discovery of apparent quantum effects in its UV induced visible light emission [1, 2]. The large surface area intrinsic to PS and the activity of the porous silicon layer to changes in the surrounding environment suggest PS as a gas sensor candidate among a variety of additional applications. The gas sensing properties of PS has been studied widely and different sensor designs and operation principles have been proposed [3]. Humidity, organic solvents, CO, NOx, NH3, O2, H2, HCl, SO2, H2S and PH3 have been detected using PS gas sensors at or below ppm levels. Porous silicon gas sensors exhibit important characteristics. They can be operated over a broad range of environmental temperature, pressure, and humidity fluctuations as it is possible to eliminate response variations due to such enviromental factors by operating in a gas pulsing mode [4]. Although a conductometric PS gas sensor is sensitive to a variety of gases, considerable research on cross selectivity/gas mixture detection is still required in order that quantitative multiple gas sensing can be accomplished. This requirement suggests that as simple a matrix of distinct sensor responses as possible be obtained with limited requirements for the modification of sensitivity. With this goal, we have focused on the application of simple nanostructured deposits on a hybrid PS interface to significantly change the interface sensitivity. The application of nanostructured metals, metal oxides, and nanoparticle catalytic coatings has been found to considerably promote the enhancement of the PS interface sensitivity. In addition to proposing a response mechanism which leads to an identifiable trend in PH3 sensitivities, we provide simulations of our data using a 1D di
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