The temperature dependence of the absorption coefficient of porous silicon

The spectral behavior and the temperature dependence of the absorption coefficient of microporous silicon films are studied in the energy range of 1.2–3.8 eV, between 7 and 450 K. For photon energies above the direct band gap at 3.3 eV, the spectral behavior of the absorption coefficient is similar to that of crystalline silicon, and its absolute value is comparable to that estimated using Bruggeman effective medium approach. For lower energies the absorption coefficient is much reduced with respect to the effective medium model prediction. In this energy region, the absorption depends exponentially on the photon energy. From the temperature dependence of the absorption it is shown that this ‘‘Urbach tail’’-like behavior is not due to absorption in surface states. In the range 300–400 K, the temperature dependence of the absorption coefficient of microporous and crystalline silicon is the same. This is a strong indication that phonon-assisted optical absorption takes place as expected from the indirect nature of crystalline silicon. © 1996 American Institute of Physics. @S0021-8979~96!04122-9#