The role of the size of amorphous silicon quantum dots in the Er luminescence at 1.54 mum was investigated. As the dot size was increased, the Er luminescence intensity was decreased and the temperature quenching was also fast because of the small band gap resulting in the decrease of electron-hole pair energy. Accordingly, the critical dot size, needed to take advantage of the positive effect on Er luminescence, is considered to be about 2.0 nm, below which a small dot is very effective in the efficient luminescence of Er. (C) 2004 Elsevier B.V. All rights reserved.