Tight-binding electronic state calculations of silicon nanostructures with local disorders: Origin of the 'F' band luminescence from porous silicon

In order to clarify the mechanism of the so-called 'F' band luminescence from porous silicon (Si), we introduce a new structural model for Si nanocrystal. Our model consists of very small Si clusters (< 1 nm) with disordered surfaces. We assert that our model is a realistic model for samples which give the 'F' band luminescence. Our assertion is based on the fact that the 'F' band luminescence is observed after thermal oxidization of the so-called 'S' band samples, which leads to Si clusters with characteristic sizes of I nm or smaller. with inhomogeneous pressures acting on their surfaces. In such a situation, very small Si clusters with disordered surfaces are constructed. Electronic state calculations based on the tight-binding approximation are performed for our model structures, and the radiative recombination time is calculated. From the results of our calculations. we find that for our model, (1) luminescence energy ranges from 1.8 eV to 3.2 eV, (2) radiative recombination time ranges from 10(-9)s to 10(-7) s, and (3) radiative recombination time is independent of luminescence energy. These features are consistent with the experimental results of the 'F' band luminescence. which cannot be obtained from calculations based on a simple quantum confinement hypothesis.