Early Stages of Growth of Ge Quantum Dots
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P. TOGNDNfa, M. PATRIN3a, A. STELLAa, D. TONOVAa, p.
CI-EYSSACb, R. KOFMANb
C. E. BOTIrANf, M. GEMMi1 aIStituto Nazionale per la Fisica della Materia, Dipartimento di Fisica , UniversitA
di Pavia, Via Bassi 6, 27100 Pavia, Italy bLaboratoire de Physique de la Mati~re Condens6e, URA 190, Universit6 de Nice Sophia Antipolis, Nice Cedex, France c~stituto Nazionale per la Fisica della Materia, Dipartimento di Ingegneria Nucleare, Politecnico di Milano, Via Ponzio 34/3, 20133 Milano, Italy dCNR-LAMEL, Via P. Gobetti 101, 40129 Bologna; INFM - Dip. di Fisica, UniversitA di Bologna, Viale Berti Pichat 6, 40127 Bologna, Italy. ABSTRACT The present work is concentrated on the investigation of the initial stages of growth of Ge nanoparticles embedded in an amorphous A1203 matrix on suitable substrates. The growth technique is based on self-organization processes related to the balance of the interface energies involved. Combined data from complementary optical techniques (absorption, Raman scattering, spectroscopic ellipsometry) give evidence of a behaviour which can be ascribed to the existence of a wetting layer, not detectable by conventional transmission microscopy. INTRODUCTION In the self-organisation process leading to the formation of quantum dots it is crucial to study the early stages of growth and to investigate on the possible existence of a critical thickness marking the transition from a two-dimensional wetting layer to a distribution of nanoparticles spatially confined in the three dimensions. This has been recently the object of interest in the case of Ir-V quantum dots originated by stress driven heteroepitaxy. In this case, the transition is from a 2D layer to an ensemble of nanoparticles sitting on a wetting layer (Stransky-Krastanov growth) [1]. In contrast to that, the Volmer-Weber mode describes a full coalescence of nanoparticles without the presence of an additional wetting layer; this has been observed in surface tension-driven processes, based on the non-wetting character of the material constituting the nanoparticles with respect to the substrate and on a balance among the interface energies, leading to relatively high contact angles cc (e.g. Sn-A120 3 and Pb-AI20 3 where (o>120°) [2]. In the case of Ge, the contact angle is = 1060, so that the initial formation of a wetting layer may be favoured. In other words, due to the less pronounced non-wetting character of Ge relative to A120 3 the adhesion forces start playing a role and compete with the cohesion forces. As specified in more detail in the next section, Ge and A120 3 present a good coupling in the non-wetting (partial wetting) configuration. In addition, it should be stressed that Ge is an indirect-gap semiconductor which allows to compare the behavior of El and E 2 structures versus size and presents interesting linear and non-linear optical properties above the gap.
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Mat. Res. Soc. Symp. Proc. Vol. 571 ©2000 Materials Research Society
EXPERIMENTAL The samples investigated have been grown using an evaporation-condensation techniq
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