Properties of amorphous carbon-germanium multilayers

Abstract

Amorphous carbon-germanium multilayers have been produced with repeat periods ranging from 2.6nm to 12.5nm. The multilayers have been studied using Raman and infrared spectroscopy, optical absorption and parallel conductivity measurements. The properties of the multilayers change rapidly as the carbon sublayer thickness decreases below =3.0nm. We explain this by suggesting that at small layer thicknesses the carbon does not form a continuous layer but rather forms islands of predominantly sp2 bonded carbon within the a-Ge network. The strong Raman scattering signal from 1000-1700cm-1 is observed to narrow as the carbon sublayer thickness decreases. This is we think due to the change in the connectedness of the islands which affects the ability of phonon modes to propagate through the material. The parallel conductivity of the multilayers is observed to decrease as the carbon sublayer thickness decreases. We are able to fit a 2-d percolation law to this behaviour based on the assumption that the conduction takes place through a percolating path of carbon islands.