Slow decay dynamics of visible luminescence in porous silicon: Hopping of carriers confined on a shell region in nanometer-size Si crystallites

We have studied the decay dynamics of visible photoluminescence (PL) from nanometer-sized Si crystallites fabricated by electrochemical etching of single-crystalline Si and laser breakdown of SiH₄ gas. In the two types of Si crystallites, the slow-decay behavior of red PL in the time range 10^-6--10^-2 s is characterized by a stretched exponential function and the PL decay time scarcely depends on the size and the surface structure. The temperature dependence of the PL decay rate is identical with that of variable-range hopping of carriers in two-dimensional systems. It is concluded that the slow-decay PL is caused by the hopping-limited recombination in a quasi-two-dimensional interface region between the crystalline Si core and the surface layer.