Band-tail thermalization of photoexcited carriers in doped and undoped a-Si1-xCx:H alloys is studied using the picosecond photoinduced-absorption technique. With carrier densities of about 1020 cm-3 most of the charged impurity states as well as native defects are saturated and all photoinduced-absorption decays can be attributed to the band-tail width of the material. We find that carbon alloying and doping both increase the band-tail width by increasing the network disorder and by introducing new states for deeper thermalization. The optical absorption cross sections eff obtained for a given excitation energy decrease in high-band-gap materials and highly doped samples. © 1989 The American Physical Society.