EX SITU DOPING OF SILICON NANOSTRUCTURES

Over the last decades semiconducting nanostructures with reduced dimensionality have attracted considerable scientific interest due to their peculiar properties, arising from the interplay between quantum confinement and surface related effects. Among all the silicon nanostructures, Silicon Nanocrystals (Si NCs) represent a paradigmatic system because the attainable results are in the extreme case of nanoscaling, from bulk to 0D system. Therefore they are extremely useful for the understanding of other silicon systems with reduced dimensionality like nanowires, fins or nanosheet The problems of impurity incorporation and doping of Si NCs is actually very far from being understood and some important issues still need to be clarified from the experimental and theoretical point of view. Actually both p-type (B) and n-type (P, As) impurities have been successfully introduced in very small Si NCs by means of different experimental approaches. Nevertheless, very few data are available about the thermodynamic stability of the impurity atoms in the Si NCs. Usually, doping of Si NCs in a SiO2 matrix was performed by introducing the dopant in the matrix before NCs formation and subsequently inducing dopant incorporation and Si NCs formation simultaneously. This approach indicated that inclusion of electrically active impurities in Si NCs is kinetically possible. However, these results do not provide any information on the energetics of atomic transport at the NCs since the experiments were conducted out of equilibrium condition. In this thesis work, we developed an alternative experimental approach to tackle the fundamental issues od dopant incorporation at thermodynamic equilibrium. The main idea behind our approach is the decoupling of Si NCs synthesis from the dopant incorporation. The incorporation of Phosphorous (P) atoms is promoted in Si NCs after their formation, by delivering a controlled amount of dopant atoms from a spatially separated diffusion source. In this way, the energetics of trapping/detrapping of P in the NCs are measured at equilibrium and modelled as a function of the annealing temperature and time, avoiding kinetic effects due to NCs formation.