URBACH EDGES IN LIGHT-EMITTING POROUS SILICON AND RELATED MATERIALS

At room temperature. both the visible photoluminescence excitation spe ctra and the non-radiative recombination excitation spectra deduced fr om photothermal deflection spectroscopy on free-standing p(-) porous s ilicon layers art shown to yield an exponential edge encompassing the emission region. The width of the corresponding absorption tail is in the 150-300 meV range. These results an compared with those obtained o n hydrogenated amorphous SiON disordered semiconductors with an optica l gap around 2.8 eV where the weaker but visible emission peak lies be low the exponential absorption tail also observed in these materials. Finally, the coincidence of the slopes of these exponential edges with both the activation energy of the lifetime and its dependence on the emission energy above room temperature is discussed.