Spatial and quantum confinement in crystalline and amorphous porous silicon

Similar to crystalline silicon, device-grade amorphous silicon (a-Si:H) has been made microporous by anodisation in ethanoic HF: in both cases, starting from a non-luminescent material, we obtain an increase of photoluminescence (PL). The difficulty with a-Si:H is that the resistivity of the material is much larger than that of the electrolyte, this difference induces a new instability which limits the thickness of a porous layer that can be obtained. For crystalline porous silicon, by decreasing the size of the structure a blue shift (1.5 to 2.3 eV) is observed until, eventually, the whole porous material has been etched out. The same experiment performed on amorphous porous a-Si:H shows a decrease of the PL intensity with the loss of material, but without any blue shift. We interpret the large blue shift observed in the crystal as the quantum confinement of the wave function, as the nanosize structures become thinner. In a-Si:H, the situation is quite different. Because of the disordered nature of the material, the wave function is strongly localised, in a range (a few atomic distances) much smaller than the nanosize structure and, thus, does not produce any quantum effect, which explains why no shift is observed.