Scapin, Lucia (2016) Diffusione e incorporazione di P in nanocristalli di Si in una matrice di SiO2. [Magistrali biennali]
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Doping of Si nanocrystals (Si NCs) has been the subject of a strong experimental and theoretical debate for more than a decade. A major difficulty in the understanding of dopants incorporation at the nanoscale is related to the fact that theoretical calculations usually refer to thermodynamic equilibrium conditions, whereas, from the experimental point of view, impurity incorporation is commonly performed during NC formation. The research group I’ve worked with have recently developed an experimental methodology in order to decouple the diffusion equilibrium properties to NCs formation phenomena. They managed to introduce the P dopant atoms into the Si NCs embedded in a SiO2 matrix from a spatially separated dopant source. A subsequent thermal annealing induces a P diffusion flux to interact with the already-formed and stable Si NCs, maintaining the system very close to the thermodynamic equilibrium. In this way it is possible to demonstrate that the process of P incorporation into the NCs is thermodynamically favoured and they have deduced the energy barriers regulating the P diffusion process into SiO2 and the trapping and de-trapping of P within the Si NCs. In this thesis it is studied how the size of the NCs influences the doping of nanostructures. Samples similar to those of the previous work have been realized, changing the size of NCs (2nm, 4nm, 6nm and 8nm of diameter) and the modality of formation of the P source using the technique of Monolayer Doping. The samples have been annealed for 4, 9 , 16 hours in order to promote the P diffusion into the NCs. The characterization have been performed by mean of ToF-SIMS measurements in order to obtain P depth profiles which have been quantified using RBS analysis following a specific calibration procedure. Once obtained the P concentration profile in depth, diffusion simulations have been performed using a code employed ad-hoc for this experimental situation. By mean of the simulations one could get the values of diffusivity of P atoms in SiO2 matrix, and the release coefficient of the P within the NCs for every NCs size. The results obtained suggest that the release phenomena is independent by the dimension of the NCs and the bonding energy calculated for each size of NCs is in excellent agreement with the values found previously and with recent theoretical findings. However the P concentration profiles found in this work show a very complex behaviour of the P source that is different from the one of the previous work. Therefore before planning new investigations about Si NCs it is necessary to investigate in deep the role of the source performed by Monolayer Doping. After the understand of the P emission mechanism it will be possible to focus on the doping of Si NCs. Finally a preliminary XANES analysis has been performed in order to determine the reticular position and the electronic properties of P incorporated into NCs. The results obtained show that the signal of the samples can be detected but it is largely covered by an instrumental background. Useful measurement will be obtained only reducing the background through modifying the experimental setup.
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