Fourier transform infrared spectroscopy and scanning tunneling spectroscopy of porous silicon in the presence of methanol

Porous silicon samples were obtained from p+- and n-type silicon wafers. Gas sensing measurements showed that the electrical conductivity of porous Si on p+- and n-type wafers increases strongly and decreases weakly in the presence of methanol gas, respectively. Scanning tunneling spectroscopy (STS) indicates that the adsorption of methanol on the surface of n-porous silicon decreases the average density of states especially in the band gap. Fourier transform infrared (FTIR) spectroscopy reveals that after methanol exposure partial surface oxidation occurs which produces electron traps as well as methanol adsorption on the porous surfaces. These observations imply that the number of free-carriers in p+-type (n-type) porous silicon increases (decreases) in methanol environment. These results explain the change in electrical conductivity of porous Si in the presence of methanol.