Raman spectroscopy and scanning electron microscopy investigation of annealed amorphous carbon-germanium films deposited by d.c. magnetron sputtering

The precipitation of germanium nanocrystals in amorphous carbon–germanium films allows for the development of innovative devices, but the accurate control of both size and size distribution of Ge quantum dots in these matrices still constitutes a challenging step. In this paper, both the structure and morphology of amorphous carbon–germanium films (a-Ge1−xCx), deposited by d.c. magnetron sputtering onto silicon substrates and annealed in vacuum at temperatures up to 550 °C, are investigated by Raman spectroscopy and scanning electron microscopy. The main features of Raman spectra obtained from carbon-rich films (x>0.43) are the D and G bands, characteristic of graphitic carbon films. The ratio between the intensities of the bands, ID/IG, increases with the annealing temperature, suggesting a progressive increase of the graphitic domains within the films. Raman spectra obtained in the low frequency region from both as-deposited and annealed germanium-rich films (x<0.43) show broad bands associated with transverse acoustic and transverse optic Ge–Ge modes. Ge–Ge optic modes merge up into a well-shaped peak at 300 cm−1 in the germanium-richest sample, and underwent annealing treatment at 550 °C, thus indicating the precipitation of crystalline Ge. Scanning electron microscopy analysis shows an apparently uniform nucleation of Ge crystallites at the sample surface. Microprobe Raman scattering results suggest the formation of a nearly homogeneous distribution of Ge nanocrystals in germanium-rich films annealed at 550 °C.