Stephan Frohnhoff

Porous silicon as a substrate material for potentiometric biosensors

For the first time porous silicon has been investigated for the purpose of application as a substrate material for potentiometric biosensors operating in aqueous solutions. Porous silicon was prepared from differently doped silicon substrates by a standard anodic etching process. After oxidation, penicillinase, an enzyme sensitive to penicillin, was bound to the porous structure by physical adsorption. To characterize the electrochemical properties of the so build up penicillin biosensor, capacitance - voltage (C - V) measurements were performed on these field-effect structures.

A non-destructive study of the microscopic structure of porous Si

Abstract The microscopic structure of porous Si films formed on different p-type doped substrates has been investigated by Raman spectroscopy. From a detailed line shape analysis of the Raman phonon peak, nanocrystal size distributions are obtained. These distribution functions depend on different sample parameters like the porosity and the doping level of the substrate, but are also influenced by the current density during the formation process. On thick samples, a change in the microscopic structure with depth is observed. This is due to a further chemical thinning close to the surface and to the limitation of the diffusion of reactive species from the deeper lying regions through the pores.

Investigation of different oxidation processes for porous Si by XPS

Abstract The oxidation behaviour of porous Si films formed on p-type doped substrates has been investigated. On freshly prepared porous Si no oxygen is found. During the native oxidation suboxides are formed. For low p-doped samples SiO 2 is formed after 1000 h, whereas on highly doped samples only Si 2 O is found. Part of the difference between low and high doped substrates is probably due to different orientations of the inner surfaces. It seems that the orientation of the inner surfaces of porous Si films formed on p + -doped substrates is more preferentially (111) than it is the case for the lower doped samples. For the anodic oxidation process also suboxide formation is found which results in the formation of SiO 2 .

Fabrication and properties of multilayer porous silicon filters

Porous silicon multilayer systems formed by different techniques were investigated. Type I layer systems are fabricated by changing the current density during the anodic etch process. Type II samples are formed with a constant current density but using a substrate with alternating doping levels. The superlattice structure is clearly visible in transmission electron microscope pictures. The quality of the interfaces depends on the formation technique. Porous silicon multilayer systems exhibit sharp peaks in the reflectance spectrum and can be used as filters. The line narrowing of the broad photoluminescence band of porous silicon by a Fabry- Perot filter structure is demonstrated.