engleski

X-ray study of Ge nanocrystals formation in Ge:SiO2/SiO2 multilayers

A structure which consists of nanometer sized Ge particles embedded in insulator can find applications in opto-electronic devices or charge storage devices. Since physical properties and usefulness of these structures depend strongly on the particle size distribution and the particles' location with respect to each other, a key element is to achieve the control of the size, shape and arrangement of Ge nanocrystals. Here, we present the study of the multilayer germanosilicate (Ge:SiO2) films which have been grown at room temperature, by magnetron sputtering with Ar as a working gas. Each Ge:SiO2 (molar ratio: 60:40) layer is separated by a pure SiO2 layer with equal thickness (6 nm). The samples were then annealed for 1h at different temperatures (up to 900 °C) which produces Ge nanocrystals. X-ray reflectivity (XRR), combined with grazing incidence high/small angle X-ray diffraction/scattering (GIWAXD & GISAXS) techniques were used to globally characterise the multilayer structure as well as the morphology and structure of inlayer inhomogenities. XRR spectra exhibited superlattice reflections which position, width and intensity vary with annealing temperature, indicating that the period of the multilayer stack decrease for the samples heated at 600 °C and above (by ~1nm for 800 °C), and that the ration of layers thickness in one bilayer and the interfacial roughness are not constant with annealing temperature. GIWAXD was performed at several angles of incidence very close to critical angle. In that way, it was possible to detect Ge nanocrystal fraction and to estimate average crystallite size ; the analysis of GIXRD data revealed that the formation of Ge nanocrystals start at 600°C and that there is no change in nanocrystal size from 650 to 900 °C. The analysis of the 2D GISAXS pattern has shown that Ge nanocrystals are present in the annealed samples. From the 1D traces it is possible to determine size distribution of Ge nanocrystals and also their relative mean distance.

Germanium; nanocrystals; multilayers; XRD

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