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The density of states in a-Six C1-x:H and a-Cy Sn1-y:H (F) semiconducting materials has been investigated by phototheTmal deflection spectroscopy (PDS) and their photoconductive properties have been related to the gap states distribution parameters, the Urbach energy Eo and the localized states density gm. We found that an Eo of 200 meV is the highest value above which no photoconductivity can be detected. This threshold corresponds to a density of gap states of about 1019 eV−1 cm−3 for optical gaps ranging between 1.1 and 2.5eV. Fourier spectral analysis of the long photoresponse decay time observed in our fluorinated a-Cy, Sn1-y samples shows the existence of well defined trapping levels and randomly distributed defect states.

Absorption, excitation and emission spectra of F centers in KCN were measured between 4.5 K and 175 K. Due to the cubic to orthorhombic structural phase transition at 168 K, the F band splits into three components. Their properties in the antiferroelectric phase (T<83 K) are explained by an internal Stark effect due to the local electric field. The single emission band has a decay time of 21.5 neec at 4.5K.