Sensitive, Selective, & Tunable Porous Silicon Gas Sensor
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Sensitive, Selective, & Tunable Porous Silicon Gas Sensor Stephen Lewis(1), James Gole(1), and Peter J. Hesketh(2) 1. School of Physics, Georgia Institute of Technology, Atlanta, GA 30332 2. School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332 ABSTRACT Hybrid porous silicon, consisting of a microporous framework whose walls are covered with a thin nanoporous layer, proves to be an inexpensive and robust platform for fabrication of rapid, reversible, and sensitive semiconductor sensors. An extremely high surface area provides a mechanism for the detection of ppm and ppb levels of a range of gases including ammonia, NOx, and CO. A general method of coating electroless metal onto the surface provides a basis for nanostructure induced selectivity between these gases. Further, the introduction of FFT analysis to the rapidly reversible and linear response of the porous silicon gas sensor allows the gas response to be acquired and filtered on a drifting baseline. INTRODUCTION We extend the concept of a rapid, reversible, and sensitive porous silicon (PS) gas sensor [1], based upon a uniquely formed highly efficient electrical contact to a nanopore covered microporous array. We introduce selectivity and develop a fast Fourier transform technique for sensing in the presence of a drifting baseline. Utilizing a combination of photoluminescence induced electroless metallization and electron beam deposition as a means to obtaining a highly efficient electrical contact we subsequently demonstrate the detection of HC1, NH3, CO and NOx at the ppm (calibrated) and ppb level. The problem of spreading resistance (200k Ω –M Ω) is overcome as low resistance contacts which can approach 20Ω are made to the PS structure [1,2]. The response of the device, which can operate with limited selectivity at a bias voltage of 1-10 mV, is rapid and reversible. With an electroless gold treatment, introducing gold nanostructures to the PS framework, we find that the impedance response of the device to ammonia increases by 2.5 times, while the CO and NOx responses are virtually unchanged. With an electroless tin coating, to produce a nanoscale tin oxide structure, the room temperature responses to NH3, NOx, and CO are all amplified. With this combination, we can introduce a significant degree of selectivity. By introducing an FFT analysis to the rapidly reversible, linearly responding, PS gas sensor, the gas response can now be acquired and filtered on a drifting baseline, further increasing sensitivity. These sensors should be compared with porous silicon based sensors with resistances in excess of 200kΩ and a 2V bias voltage and a slower response [3], SnO2 sensors operating at 300-500°C [4], and other similar gas sensors operating at 2-5V [5]. FABRICATION OF THE POROUS SILICON SENSOR The fabrication of a porous silicon sensor is performed via an electrochemical etch on the polished surface of a p-type (100) silicon wafer with 1-20 Ω-cm resistivity. To isolate the porous silicon etch from other parts of the wafe
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