TEMPERATURE-DEPENDENCE OF ABSORPTION-EDGE IN P-TYPE POROUS SILICON

The observed blue shift of both the PL and the absorption edge in poro us silicon is generally understood within the framework of quantum con finement of carriers in small crystallites where reduction of the dens ity of states in the vicinity of c-Si band edge as well as increased o scillator strength, lead to higher direct radiation rates. Temperature dependence of transitions in semiconductors may provide additional in formation to clarify the nature of the recombination mechanism in poro us silicon. In this work, we have studied the temperature dependence o f the absorption edge of free standing porous films in the photon rang e 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 t he indirect nature of the c-Si band gap.