Research On Gas Adsorption Dynamics Of Porous Silicon Device Surface

The porous silicon surface is widely used in the sensor field, which is due ro the adsorption specifity of gas on the nanometer porous silion surface. Using the theoretical and experimental methods, the adsorbing process of gas on the porous silicon surface is analyzed. The physical and chemical kinetic process of the CH4, CO on device surface is obtained, which plays an important role on the devise of nano-silicon sensors.For designing the gas sensors, we need to consider the different adsorbing capacity depends on the different Si surface and the adsorption sites are related to the stability of gas on surface. We mainly investigate the silicon surface appearance and the physical adsorption and chemical adsorption process of the gas on Si surface. Using the gas chromatography and Autosorb-i Q-C, we analyze the gas physical adsorption on the different number Si(311) surface, the methane adsorbing capacity changes with the different pressures when the temperature is constant and the methane chemical adsorbing on the Si(311) surface in the different temperatures. Additionally, the material studio software which is based on the DFT is used to analyze the physical and chemical adsorption of CH4 and CO molecules on different Si surface and different adsorption sites in theoretical sides.The physical adsorption experiment results show that the porous silicon surface distribute so many mesoporous that the weak physical adsorption of CH4 on Si surface. However, the gas physical adsorbing capacity increased with the the silicon number change. The increased Si number improves the weak physical adsorption of methane on Si surface. Then the chemical adsorption data illustrate the Si(311) surface can not interact with methane at 60-80℃, and the chemical adsorption effect of CH4 and Si surface occur at 100℃. The adsorption models theoretically show anisotropy of CH4 adsorbing on Si surface, and adsorption sites, coverage and distance have large impact on Si surface, but the effects of adsorded molecule direction are small. When CH4 and CO physical adsorbed on the Si(311) surface, Si surface is more sensitive to methane. The chemical adsorption displays Si-C strong bonding effect lead to the methane decompose to methyl and H atom, which bonds with silicon surface atoms, respectively. The electron cloud reconstructs, and with the electron transfer, the PDOS molecular orbit electron peaks delocalization are strong and appear new band group, which show the chemical bonds formed.The gas absorbing sensitivity of porous silicon surface is strong, because the mesoporous structure and the large porosity. So the Si(311) surface can be used as the substrate to detect gas. The conclusion can provide the experimental and theoretical instruction for porous silicon gas sensor design.