a thick wall implies a low energy barrier for he, which allows h@ to enter the wall and initiate further electrochemical dissolution. This process of the thinning of the walls will continue until the band gap increases to a point where he can no longer enter the wall. The wall becomes depleted of he and the dissolution stops. Applying a higher anodic bias to the substrate will increase the energy of the h@ and therefore decrease the diameter of the remaining walls producing PS with a larger bandgap. Overall the final etching structure may be considered as a "quantum sponge".
The validity of this quantum sponge approach is supported by photoluminescence measurements: If a electron and a hole recombine in PS a photon of a wavelength corresponding to the band gap energy will be emitted. If the thickness of the walls between the pores decreases, the energy of the emitted light increases. ['] This is illustrated by the PS sample shown in Figure 3; the yellow glowing region of the sample was grown under higher bias (or higher current density) than the
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In conclusion, porous silicon is not only a potential candidate for new silicon-based optoelectronic devices but it also enables the study of the interaction of chemical and electronic reactions on a nanometer scale caused by quantum confinement effects.