Vertical ordering of amorphous Ge nanoclusters in multilayer $a$-Ge/$a$-Si:H heterostructures

A multilayer heteronanostructure consisting of three pairs of amorphous silicon and amorphous germanium ($a$-Ge/$a$-Si:H) layers grown on a silicon substrate by low-frequency plasma-chemical deposition at temperature 225$^\circ$C was investigated. From the analysis of the Raman spectra, the phase composition of the silicon and germanium layers was determined, which showed that the layers are completely amorphous. The transmittance electron microscopy images show vertically ordered amorphous Ge nanoclusters initiated by local inhomogeneities in the first germanium layer, the lateral dimensions of which increase from the lower to the upper layer.