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 f ilms allows for the development of innovative devices, but the accurate con trol of both size and size distribution of Ge quantum dots in these matrice s still constitutes a challenging step. In this paper, both the structure a nd morphology of amorphous carbon-germanium films (a-Ge1-xCx), deposited by d.c. magnetron sputtering onto silicon substrates and annealed in vacuum a t temperatures up to 550 degrees C, are investigated by Raman spectroscopy and scanning electron microscopy. The main features of Raman spectra obtain ed 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, I-D/I-G, 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. Cre-Ge optic modes merge up into a well-s haped peak at 300 cm(-1) in the germanium-richest sample, and underwent ann ealing treatment at 550 degrees C, thus indicating the precipitation of cry stalline Ge. Scanning electron microscopy analysis shows an apparently unif orm nucleation of Ge crystallites at the sample surface. Microprobe Raman s cattering results suggest the formation of a nearly homogeneous distributio n of Ge nanocrystals in germanium-rich films annealed at 550 degrees C. (C) 1999 Elsevier Science S.A. All rights reserved.