Ge NCs have attracted considerable attention because of their potential applications in nonvolatile memory and integrated optoelectronics. A number of groups have already proposed integrate flash memories based on Ge NCs embedded SiO₂ matrix.
In this work Ge NCs embedded in alumina thin films were deposited over Si (111) substrates using RF co-sputtering technique. Annealing was performed in order to improve the cristallinity of the Ge phase in the films and achieve control over the NCs size [1]. Ge NCs of suitable size and good crystalline quality were obtained by different techniques, including X-ray diffraction, Raman spectroscopy and high resolution transmission electron microscopy. The NCs size was estimated from Raman spectra using Fauchet and Campbell model [2] and from X-ray diffraction using Debye-Scherrer equation. A good agreement between the three techniques for the size estimation was observed for smallest NCs. However, for larger NCs (> 20 nm) there is a discrepancy between XRD and Raman results. For this sample the TO Raman peak presents a blue shift relative Ge bulk and X-ray peaks are shifted for higher angles relative to the Ge diamond structure. A possible explanation for this shift is the compressive stress exerted on Ge NCs by the Al₂O₃ matrix [3].  Further detailed studies are in course in order to clarify both confinement and stress effects.
References:
[1] P. Caldelas et al, in press in Journal Nanoscience and Nanotechnology
[2] I. H. Campbell, F. M. Fauchet, Solid State Commun. 58, (1986) 739
[3] I. D. Sharp et al, Journal. Applied Physics 100, (2006) 114317, and references there in.

Acknowledgements
This work has been partially supported by FCT-Fundação para a Ciência e a Tecnologia-Portugal, and   by the "Programa Operacional Ciência e Inovação 2010-POCI 2010" co-financed by FEDER through the Projects POCTI/FIS/56930/2004 and PTDC/FIS/70194/2006. SRCP thanks FCT for the financial support (grant SFRH/BD/29657/2006).

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