Study On Biosensing Properties Of Porous Silicon Composite Structure

Porous silicon (PS) is a new type silicon-based material developed in recent years, which has different properties compared with the crystalline silicon materials. Porous silicon can emite luminescence efficiently across the whole range from the near infrared, through the visible region, to the near UV region. This characteristic makes it possible to fabricate light-emitting devices and solve the key problem of the optoelectronic integrated circuit (OEIC), opening up the bright future for the very large integrated circuit (VLIC).Bulk silicon, with indirect band gap of 1.12 eV, doesn't emit visible light at room temperature. With a broad and likely direct band gap, porous silicon has a different band structure from that of the bulk silicon. Thus the porous silicon can emit visible light at room temperature. Several methods commonly used in preparing porous silicon have been reviewed. Qualitative description has been presented for the prevalent mechanisms proposed to explain the bright PL and EL. Some aspects of porous silicon sensors are discussed in this paper, including their fabrication methods and sensitivity characteristics, with focus on the principle of gas sensors, humidity sensors and biosensors. The main contentet are as following:1.Oxidation of porous silicon by nitric acidN type silicon wafers, with (111) orientation and 15-20 ?.cm resistivity, were used as substrates. The as-prepared porous silicon was formed by etching n type silicon photo-electrochemically. The visible photoluminescence ( PL) of the porous silicon affected by the conditions of nitric acid oxidation. The photoluminescent intensuty of the porous silicon increase with the immersing time and nitric acid concentration. The orange-red photoluminescence of the porous silicon would disappear if the nitric acid concentration is up to 67%-69%. The photoluminescent intensity is directly proportional to the bonds of Si-O and Si-O-Si, which were detected by Fourier transform infrared spectra (FTIR). The enhancement of photoluminescent intensity of the porous silicon might attributed to the decrease of the surface state, which are non-radiation recombination centres, caused by hanging bonds on the as-prepared porous silicon due to the formation of Si/SiOx (x≤2) interface. 2. Study of porous silicon treated by silver nitrate solutionThe visible photoluminescence ( PL) of the porous silicon formed by etching n type silicon photo-electrochemically was affected by immersing in silver nitrate solution. Self-assembled silver nano-islands on the surface of immersed PS has been obsevered by scanning electron microscopy (SEM). Measurement results of Fourier transform infrared (FTIR) spectroscopy and the x-ray photoelectron spectroscopy(XPS) showed that PL enhancement of the PS might be attributed to Si-O structure, which was formed by the interaction between the rare silver islands and silicon and played effective luminescence centers. And the PL intensity decayed more dramatically with the increasing concentration of the silver islands which are non-radiative carrier-recombination centers.3. Sensitive action to urea by the porous silicon compositesThe as prepared PS was stabilized by illumination oxidation in hydrogen chloride solution. Aluminum was evaporated onto the PS to form longitudinal structure, Al/PS/Si/Al and transversal structure Al-PS-Al.Urea was absorbed into the PS nanostructure in different concentrations to form Al/PS-urea/Si/Al or Al/PS-urea/Al. Electrical characteristics of the Al/PS-urea/Si/Al or Al-PS-urea-Al was investigated by measuring lgI ~V . It has been found that the lgI decreases with the absorbed urea content and there is a linear relation in the range of 1μg ml-1 ~ 1mg ml-1 urea solution used to immerse the PS. Accordingly, PS-based the two structures were potential to develope biomolecules sensors.4. Measurement ascorbic acid by passive porous siliconThe porous silicon etched by anodic polarization was exchanged between the positive and negative electrode F- and H+ ions were transfered to anode and cathode respectively, by Columb force from the electric filed. The corrosion of the F- ions to the silicon wafer was been reduced and promoted combination between the active Si atoms and H+ ions to form stable Si-H bonds, which reduce the hangling bonds on the surface and passivate the porous silicon.The passivated porous silicon, Pt plate and saturated calomel electrode(SCE) was used working electrode, counter electrode and reference electrode respectively. PH is decreasing with the ascorbic acid concentration increase, but not linear relationship. The current-voltage measurements showed that the current increased with the voltage increase, The ascorbic acid current is line with PH at a given voltage. The results showed that the porous silicon might be used to detect the ascorbic acid concentration,which provided a reference to biosensor based porous silicon.