Raman scattering of amorphous germanium clusters and ultrathin films

The near surface vibrational states of amorphous (a-) Ge supported semiconductor clusters are reported from in situ Raman scattering. A combination of multichannel detection and interference enhanced Raman scattering from multilayer structures has provided a means to study the vibrational states of an amorphous solid at coverages less than a monolayer. The deposition of a-Ge by direct current magnetron sputtering onto disordered C trilayer structures in UHV results in a Raman spectrum that is related to the phonon density of states by a smoothly varying coupling parameter. The Raman spectra are observed to exhibit significant changes in the width of the high frequency TO (transverse optic-like) band, as well as a substantial shift of the TO peak position to lower frequencies. These results indicate increased near surface structural disorder with decreasing cluster size. Support for this is provided by hydrogen chemisorption and by ultraviolet photoelectron spectroscopy (UPS) difference spectra that indicate a reduced coordination with decreasing cluster size.