Physical properties of thin GeO2 films produced by reactive DC magnetron sputtering
Njoroge, W. K.
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We have studied optical and structural properties of GeOx films produced by reactive DC magnetron sputtering of Ge targets in an Ar/O2-mixture. Optical spectroscopy measurements of reflectance and transmittance of the films were employed to determine optical properties. The film structure was determined by X-ray diffraction measurements, while X-ray reflectometry was used to determine the thickness, density, and roughness of the films. The film topography was additionally characterized by atomic force microscopy. For appropriate oxygen flows highly transmitting GeOx films can be grown at rates up to 3 nm/s for power density of 2.6 W/cm2. Optical and structural properties of the films are closely correlated and can be controlled by the oxygen flow. With increasing oxygen flow an increasing growth rate of the oxide films is initially observed. The resulting films above 20 sccm and below 25 sccm O2 are transparent in the visible range, have a negligible roughness and are amorphous. A further rise in oxygen flow leads to a pronounced decrease in growth rate above 25 sccm and a further increase in the bandgap of the germanium oxide up to more than 48000 cm−1 (5.95 eV). This transition in film growth is accompanied by an increase in roughness and a structural transition leading to polycrystalline films of α-GeO2 (quartz-structure). Nevertheless, the film density is hardly affected by the structural change and remains rather constant around 3.65 g/cm3, which is 85% of the density of single crystalline α-GeO2. A model is presented which can account for the change of film properties.