Synthesis and Characterisation of Sputtered thin films for renewable energy purposes
Appears in the following Collection
- Fysisk institutt 
AbstractIn the past years, a large body of work has been dedicated to semiconductor quantum dots embedded in thin films of oxide and nitride, as the tailorable electronic and optical properties of these nanostructures make them desirable for various optoelectronic applications. The properties of these low-dimensional semiconductor systems are directly related to the atomic arrangement, distribution and size of the quantum dots, the structure of the surrounding matrices and the distance between the quantum dots. The quantum confinement of carriers in the quantum dots, their interaction, and local states in the matrices are the dominating factors determining the material properties.
The present work focuses on studying the atomic structures of these materials, and how their optical and electronic properties vary as a function of size and structure. Two material systems were especially synthesized as part of this work; Si and Ge quantum dots embedded in Si3N4 and SiO2 matrices, respectively. Direct evidence of quantum confinement effects on plasmon energies, interband and conduction band structures as a function of the size and morphologies of the aforementioned Si and Ge quantum dots are provided by exploiting the high spatial and energy resolution of state of the art Scanning Transmission Electron Microscopy (STEM). The discrepancies between earlier theoretical and experimental works on the behavior of confined plasmons and interband transitions are discussed in terms of crystal imperfections, interface states and the chemistry of the surrounding matrix of each system. A comparison between the confinement strength in the system of Si and Ge quantum dots was also brought into discussion. To date, the existing experimental works found in literatures cannot unambiguously identify the correlation between chemical bonding and optical properties of the embedded quantum dots, due to the complexity of these systems. Moreover, the theoretical approaches to this matter are more than often contradictory. The combination of complementary techniques in the present work has drawn a consistent picture of bonding structures and resulting optical properties of Si quantum dots embedded in silicon nitride, and simultaneously highlighted the important impact of oxygen incorporated into the films. A model of gap states and electronic transitions for the silicon nitride films is proposed, which explains adequately the role of QC excitons vs defects in the visible luminescence of silicon nitride films with and without Si quantum dots. These results provide important information for understanding the quantum dots’ physical properties and the possibility for improvements of optoelectronic applications, like solar cells.
List of papers. Paper IV is removed from the thesis due to copyright restrictions.
Paper I P. D. Nguyen, D. M. Kepaptsoglou, Q. M. Ramasse and A. Olsen Direct observation of quantum confinement of Si nanocrystals in Si-rich nitrides Physical Review B 85, 085315 (2012) doi:10.1103/PhysRevB.85.085315 Copyright 2012 American Physical Society
Paper II P. D. Nguyen, D. M. Kepaptsoglou, Q. M. Ramasse, M. F. Sunding, L. O. Vestland, T. G. Finstad and A. Olsen Impact of oxygen bonding on the atomic structure and photoluminescence properties of Si-rich silicon nitride thin films Journal of Applied Physics 112, 073514 (2012) doi:10.1063/1.4756998 Copyright 2012 American Institute of Physics
Paper III P. D. Nguyen, D. M. Kepaptsoglou, R. Erni, Q. M. Ramasse, and A. Olsen Quantum confinement of volume plasmons and interband transitions in germanium nanocrystals Physical Review B 86, 245316 (2012) doi:10.1103/PhysRevB.86.245316 Copyright 2012 American Physical Society
Paper IV P. D. Nguyen, A. E. Gunnæs, M. F. Sunding, and A. Olsen Heterogen nucleation and growth of CrSi2 in sputtered thin films of silicon and silicon nitride Submitted to Journal of Materials Research (November 2012)