The effects of the point-group symmetry in silicon nanostructures on radiative recombination time

In order to clarify the origin and mechanism of the efficient photoluminescence emitted from low-dimensional nano-silicon, we introduce a new structural model for silicon wires that has lower symmetry in the spatial point group when compared to the previously-studied models with higher symmetry such as N x N and N x M wires. Naturally, our 'low-symmetry' model is more realistic for practical light-emitting nano-silicon. We calculate, in the tight-binding scheme, the electronic states, the energy gap, the oscillator strength, the complex dielectric constant and the radiative recombination time tau We show that our 'lower-symmetry structures yield the oscillator strength greater than that of 'high-symmetry' structures. This result comes from the fact that, for structures with high point-group symmetry, some pairs of terms in the transition probability to determine the oscillator strength cancel each other out. The radiative recombination time tau evaluated from our'low-symmetry' model beautifully agrees with experimental data of the so called 'S' (slow) band in the log tau vs. E (emission energy) plane, which the previously-studied 'high-symmetry' models were unable to achieve.