Theory of optical properties of quantum wires in porous silicon

We present theoretical studies of the electronic and optical properties of free-standing Si quantum wires which exist in porous Si. We use a second-neighbor empirical tight-binding model which includes d orbitals and spin-orbit interaction. The excitonic effects are included within the effective-mass approximation. We found that for narrow quantum wires with widths around 8 Å, the averaged exciton oscillator strength is comparable to that of bulk GaAs. However, the average exciton oscillator strength decreases dramatically (faster than 1/${\mathit{L}}^5$) as the quantum-wire width L increases. The radiative lifetimes of excitons in quantum wires are estimated and we find that the lifetime of the shortest-lived exciton ranges from 57 ns to 170 μs for wire widths from 7.7 to 31 Å. We have also calculated the absorption spectra and found strong polarization dependence.