Ultrasound treatment (UST) applied at room temperature enhances electroluminescent intensity (maximum at 600 nm) and optical output in thin-film light-emitting diodes with hydrogenated amorphous-carbon as an active layer. This positive UST effect is attributed to a reduction of the diode series resistance caused by a change of the interface and contact resistances. The UST effect is saturated with increase of the ultrasound amplitude. (C) 1999 American Institute of Physics. [S0003-6951(99)02232-9].