Study Of The Humidity Sensing Characteristics Of Porous Silicon

The main purpose of this thesis was to investigate the the humidity sensing characterization of Porous silicon. And we manged to use different methods to enhance the performance of the porous silicon sensor. Also we put forward the related solutions for the problems of our sensor.First, we fabricated porous silicon with uniform surface through electrochemical etching process. The substrate silicon we used is P (100) type, with low doping concentration (p=8¹5Ω·cm).The electrolyte used was a mixture of HF and ethanol, with volume ratio of 1:1. The parameters for the PS formation were as below: current density of 30mA·cm^-2 for 10 minutes. The morphology of the porous silicon was characterized by the SEM and AFM.Second, according to the large specific surface area and the strong activity, porous silicon was used for the humidity sensing device. Two kinds of methods, resistance and impedance, were used to evaluate the sensor performance. The parameters related to relative humidity, like sensitivity and response time were measured. And the frequency trait of the humidity sensing element was tested simply.In order to regenerate the porous silicon on to improve the bad desorption of the porous silicon humidity sensor, the positive temperature coefficient (PTC) element was introduced. PTC was bonded to the backside of the porous silicon sensor as heating element.By this way,we can heat the whole device up to 230℃ in a short time. And the regeneration of the porous silcin sensor was resolved. The advantages of this method were that we can control the temperature through the heating voltage automatically and the extra temperature sensor was not needed. By this technique, the fast desorption results had been achieved, which can totally desorb the PS samples within ～3minutes, with the regeneration/reproducible rate >99%.This technique proposed was simple, low-cost and effective method.At last, in order to improve the humidity sensing characteristic of the porous silicon, the ZnO/PS composite structure was introduced. The sol-gel precursor of Zinc Oxide was applied to the porous silicon substrates and annealed at 450℃. By this technique, it was possible to obtain the Zinc Oxide nanoparticles on the porous silicon substrates. Our study indicated that the modification of porous silicon by sol-gel Zinc Oxide increased the sensitivity and shortened the response time to the relative humidity. The other parameters, such as the concentration of zinc oxide precursors, which can affect the sensing performance, were also discussed.Meanwhile, according to the sensing characterization of the ZnO/PS composite structure and the temperature which can be kept constant automatically of the PTC, we made the gas sensing element with high temperature. With the PTC heating element, we heated the whole device up to high temperature (>200℃). At this temperature, we can measure the sensing behavior of the active gas excluding of the vapour.