Electrical band-gap energy of porous silicon and the band offsets at the porous-silicon/crystalline-silicon heterojunction measured versus sample temperature

Photocurrent measurements have been carried out on a series of samples consisting of porous silicon on top of crystalline silicon, in the temperature range 10–300 K. From the experimental data set, the electrical band-gap energy of porous silicon is deduced to be $(1.80\mathrm{}\pm 0.01)\mathrm{eV},$ independent of sample temperature. In contrast to this, some temperature variations are observed for the band offsets, reflecting qualitatively the temperature dependence of the fundamental band-gap energy of crystalline silicon. However, whereas the latter decreases monotonically for increasing temperature, a maximum is observed at around 125 K for the conduction-band offset together with a corresponding minimum for the valence-band offset. The results are discussed with the conclusion that for the samples studied here, the electrical band gap in porous silicon is of a molecular nature and cannot be related to quantum-confinement properties of nanocrystals of elemental silicon.