Photoluminescence and resonant Raman spectra of silicon films produced by size-selected cluster beam deposition

Silicon clusters and nanocrystals have been generated by CO${}_2$-laser-induced decomposition of SiH${}_4$ in a flow reactor. By introducing a conical nozzle into the reaction zone, the clusters are extracted into a molecular-beam machine and analyzed with a time-of-flight mass spectrometer (TOFMS). Since the clusters have a size-dependent velocity, a mechanical velocity selector is used to further narrow their size distribution and to select a specific mean size. Employing this technique, silicon clusters with different preselected mean sizes have been deposited at low energy on various substrates. Photoluminescence (PL) and resonant Raman spectra of the resulting films are presented. The crystallite sizes deduced from the Raman spectra confirm the TOFMS measurements. The PL spectra are shifted with decreasing cluster size to smaller wavelengths. Our results agree very well with theoretical predictions for silicon quantum dots.