Study of germanium diffusion and nanoclustering in Si-based host matrices for optoelectronic applications such as photovoltaic cells

Student thesis: Doc typesDoctor of Sciences


The development of modern optoelectronic devices undoubtedly involves the study of the properties of materials at the nanoscale. In this context, the association of germanium and silicon nanocrystals, or quantum dots, incorporated in a dielectric film, offers new possibilities thanks to manifold attractive optoelectronic properties. This is particularly the case in the field of photovoltaics, which will be taken as an example throughout this manuscript. Quantum confinement, multiple exciton generation (MEG) or tunable bandgap are all properties associated with quantum dots that can make it possible to exceed the theoretical conversion limit of Shockley-Queisser calculated for a single-junction cell (~33%) (1), to achieve theoretical efficiencies up to 66% in the most ideal case (2).
Among the armada of experimental techniques available to form semiconductor quantum dots, we opted in this work for ion implantation followed by thermal treatment in an inert atmosphere. The implantations were mainly carried out using the ALTAÏS particles accelerator available at LARN laboratory. This technique enables a great flexibility in the formation of nanocrystals via experimental parameters such as the energy or fluence of the ion beam.
This purely experimental thesis aims to propose solutions to precisely control the formation of germanium nanocrystals (location, size, distribution) in SiO2/Si films. The results presented in this thesis are based on the characterization of thin films via the combination of several analysis techniques, ranging from ion beam analysis to optical spectroscopies, including electronic microscopy. In particular, it has been shown that it was possible to completely annihilate the diffusion of germanium, occurring during post-implantation annealing, by judiciously generating a local excess of silicon by co-implantation, the particular affinity of germanium with silicon playing a preponderant role in the trapping effects highlighted in this thesis. It has been demonstrated that this singular relation between Ge and Si could allow controlling the position and the size distribution of the Ge nanocrystals (Ge-ncs) through the dielectric layer. For photovoltaic applications, the idea is to form a size gradient, ranging from 0.6 to 4-5 nm, to optimize absorption over almost the entire solar spectrum. The mechanisms responsible for the diffusion of germanium have also been brought to light, with special attention to the involvement of oxygen in the redistribution of germanium atoms during thermal treatments.
The possibility of forming Si1-xGex crystalline alloys by Ge implantation in silicon substrates has also been investigated. As expected from results obtained in SiO2 films and due to the miscibility of Ge in silicon, it is shown that these implantations do not give rise to the formation of Ge nanocrystals in c-Si, nevertheless with a glimmer of hope when the sample is heated during the implantation.
Date of Award28 Sept 2021
Original languageEnglish
Awarding Institution
  • University of Namur
SponsorsUniversity of Namur
SupervisorGuy Terwagne (Supervisor), Robert Sporken (President), Olivier Deparis (Jury), Ian Vickridge (Jury), François Schiettekatte (Jury) & Denis Flandre (Jury)

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