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A study of the effect of phosphorus doping on silicon nanocrystal formation

Papachristodoulou, Nicolas

[Thesis]. Manchester, UK: The University of Manchester; 2014.

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Abstract

The original work presented in this thesis describes work done using simulations and experimental techniques for the investigation of the growth stages of silicon nanocrystals (Si-NCs) and the effect of phosphorus doping on the optical properties and size of Si-NCs. The model used to investigate the growth of Si-NCs and the effect of phosphorus on the size of Si-NCs is a simple Molecular Dynamics model employing a Monte Carlo (MC) method. The model was run on a desktop PC and the results were analyzed in Mathematica. The model uses a temperature dependent MC step that includes an activation energy, which is the barrier, needed to overcome for diffusion to occur. The model contains equations which govern the energetics of the atoms. When the atoms are allowed to diffuse, the equations give the probability of a jump of an atom from one lattice site to a random neighbouring site. The experimental work presented in this thesis employs photoluminescence (PL), X-TEM (cross-sectional TEM) and time resolved photoluminescence to investigate the structural properties of intrinsic Si-NCs and the optical properties of intrinsic and phosphorus doped Si-NCs. The samples were prepared by ion implantation and rapid thermal annealing using isothermal annealing.The results presented in Chapter 2 show that a simple model as the one developed in this work can describe quite well the main stages of size evolution of Si-NCs. These stages are the accretion, the growth and the Ostwald Ripening. The activation energy of diffusion is determined by an Arrhenius relationship between the MC step time and the temperature. The number of isolated Silicon atoms can be then used to calculate the solubility of silicon in silicon dioxide. The value found is in reasonable agreement with previous experimental results. When phosphorus is introduced in the model it is found that the size of Si-NCs increases for all values of the binding energy ratio. This agrees with previous experimental results. Furthermore, it is found that the fraction of isolated silicon atoms increases with increasing annealing time in the presence of phosphorus.The structural properties of intrinsic Si-NCs, investigated by X-TEM, reveal that there is a growth and Ostwald Ripening stage in the size evolution of Si-NCs. The optical properties show a PL enhancement of 50% in the case of phosphorus doped Si-NCs compared with intrinsic Si-NCs. Also it is found that the shift in peak energy is greater for phosphorus Si-NCs due to accelerated growth of Si-NCs. The time resolved PL experiments reveal that the PL enhancement is due to suppression of the dangling bond defects at the surface of Si-NCs. The fraction of phosphorus atoms that contains ionized substitutional donors increases for long anneal time. The PL Intensity quenching is due to the opening of an efficient, non-radiative, recombination path due to Auger recombination. The transition from defect passivation to free electron generation implies a threshold size which below that phosphorus passivates dangling bond defects at the surface of Si-NCs. The estimation found is in agreement with previous experimental results.