Temperature dependence of absorption edge in p-type porous silicon

The observed blue shift of both the PL and the absorption edge in porous silicon is generally understood within the framework of quantum confinement of carriers in small crystallites where reduction of the density of states in the vicinity of c-Si band edge as well as increased oscillator strength, lead to higher direct radiation rates. Temperature dependence of transitions in semiconductors may provide additional information to clarify the nature of the recombination mechanism in porous silicon. In this work, we have studied the temperature dependence of the absorption edge of free standing porous films in the photon range of 3.1-1.35 eV and in the temperature range of 8.5-300 K. We find a smoothly varying spectral dependence of transmission on photon energy without any sharp features down to 8.5 K. The absorption edge for all samples studied show a gradual blue shift with decreasing temperature. Comparison with c-Si shows that red luminescing porous Si preserves the indirect nature of the c-Si band gap