Structural properties of Si nanoclusters produced by thermal annealing of SiO x films
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Structural properties of Si nanoclusters produced by thermal annealing of SiOx films Simona Boninelli1, Fabio Iacona2, Corrado Bongiorno2, Corrado Spinella2, and Francesco Priolo1 1 INFM-MATIS and Dipartimento di Fisica e Astronomia dell’Università di Catania, via Santa Sofia 64, 95123 Catania, Italy 2 CNR-IMM, Sezione di Catania, Stradale Primosole 50, 95121 Catania, Italy ABSTRACT The structural properties of Si nanoclusters embedded in SiO2, produced by high temperature annealing of SiOx films, have been investigated by energy filtered transmission electron microscopy. The presence of amorphous nanostructures, not detectable by using dark field transmission electron microscopy, has been demonstrated. By taking into account also this contribution, a quantitative description of the evolution of the samples upon thermal annealing has been accomplished. In particular, the nanocluster mean radius and the density of amorphous and crystalline clusters have been determined as a function of the annealing temperature.
INTRODUCTION Si nanocrystals (nc) embedded in a SiO2 matrix are currently attracting a great interest as a candidate system to solve the physical inability of bulk Si to act as an efficient light emitter. Indeed, the band gap of Si nc is enlarged with respect to the bulk material due to quantum confinement effects, and an intense visible photoluminescence at room temperature is obtained. Recently, the interest towards this material is greatly increased due to the observation of light amplification in Si nanostructures [1-4], as well as to the demonstration of the feasibility of efficient light emitting devices based on Si nc [5-9]. Indeed, both of these points open the route towards the development of a Si-based optoelectronics. A key point for a full understanding of the optical properties of this system is the availability of a clear picture of its structural properties and their evolution upon thermal annealing. In this work, the main stages of the thermal evolution of SiOx films (separation of the Si and SiO2 phases, formation of amorphous Si clusters and their transition to the crystalline phase) have been investigated in details by energy filtered transmission electron microscopy (EFTEM). The presence of a relevant contribution of amorphous Si nanostructures, not detectable by using the conventional dark field TEM (DFTEM) technique, has been demonstrated. By taking into account also this contribution, the mean size and the density of the Si clusters have been determined as a function of the annealing temperature.
EXPERIMENTAL SiOx thin films with a total Si concentration of 46 at.% have been prepared by using a parallel plate plasma enhanced chemical vapor deposition system. The source gases used are high purity (99.99% or higher) SiH4 and N2O. Further details about deposition processes can be found elsewhere [10]. After deposition, the SiOx films have been annealed for 1 h in ultra-pure N2 at temperatures ranging between 900 and 1250 °C to induce the separation between the Si and the SiO2 ph
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