Raman Spectroscopy of Ge Nanocrystals Grown by Self-Organization Processes
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A. STELLA*, C. E. BOTTANI**, P. CHEYSSAC***, R. KOFMAN***, P. MILANI****, P. TOGNINI*. *Istituto Nazionale per la Fisica della Materia, Dipartimento di Fisica "A. Volta", Universitd di Pavia, Via Bassi 6, 27100 Pavia, Italy **Istituto Nazionale per la Fisica della Materia, Dipartimento di Ingegneria Nucleare, Politecnico di Milano, Via Ponzio 34/3, 20133 Milano, Italy ***Laboratoire de Physique de la Mati~re Condens~e, URA 190, Universit6 de Nice Sophia Antipolis, Nice Cedex, France ****Istituto Nazionale per la Fisica della Materia, Dipartimento di Fisica, Universitd di Milano, Via Celoria 16, 20133 Milano, Italy
ABSTRACT We report Raman spectroscopy measurements on Ge nanocrystals with average radii ranging from about 65 A down to 10 A (with a size dispersion lower than 20 %). Ge has been deposited by UHV evaporation on an amorphous substrate, kept at such a temperature as to produce the Ge nanodroplets nucleation in the liquid phase. A nanocrystalline size dependence of the Raman spectra has been observed and explained in the framework of a phonon confinement model. We have observed the softening of the TO Raman peak predicted by the theory when the dimensions of the particles are decreased. Moreover the observed inhomogeneous broadening of the Raman lines has been correlated with the size distribution of the particles in the samples. Our results provide a characterization of Ge nanoparticles exhibiting a good crystalline nature, down to about 10 A, and in conditions of substantial absence of perturbations of the environment. INTRODUCTION The investigation of the properties of metal and semiconductor clusters embedded in dielectric matrices presents several aspects of high interest both for basic research and technological applications [1],[2]. Due to size reduction down to dimensions of the order of one nanometer, significant deviations from bulk behaviour take place: in particular, quantum confinement gives rise for instance to a large discretization of the electronic levels and to a relevant blueshift of structures and singularities in the density of states [3]. The nanometer size affects rather strongly the thermodynamic properties (size dependence of melting temperature and latent heat of fusion [4]) and produces sizeable nonlinear optical effects [5]. In view of these considerations, it appears to be crucial, for the development of the field, to master the growth techniques for sample preparation in order to tailor, to a large extent, their physical behaviour. High priority must be given to the possibility of varying the size in a wide range, to restrict the size dispersion to a very narrow range and to achieve regular and reproducible shapes. 249
Mat. Res. Soc. Symp. Proc. Vol. 452 01997 Materials Research Society
Several growth techniques have been developed for Ge nanoparticles, each one with advantages and limitations: synthesis in zeolite [6], ultraviolet-assisted oxidation of Si-Ge layers [7], hydrothermal oxidation of SixGet.i,[8], inorganic solution phase synthesis [9], reduction in glassy matr
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