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Title: Optical characterisation of silicon nanoclusters embedded in SiO2 and SiOxNy matrices
Author: Diamare, D.
ISNI:       0000 0004 5358 6930
Awarding Body: University College London (University of London)
Current Institution: University College London (University of London)
Date of Award: 2014
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The original work presented in this thesis concentrates on the origin of the visible and near-IR luminescence from silicon nanocrystals (Si-NCs) embedded in two different matrices, namely SiO2 and SiOxNy, and prepared by three different chemical vapour deposition (CVD) techniques and ion implantation. The optical properties of these materials were studied by time-integrated photoluminescence, time correlated single photon counting (TCSPC) and time-resolved PL spectroscopy (TRPL) while the structural and compositional characterisation was carried out using ellipsometry, Fourier Transform Infrared spectroscopy (FTIR), Rutherford backscattering spectroscopy (RBS) and X-ray photoelectron spectroscopy (XPS). As opposed to the near-IR emission, well investigated and commonly associated with phonon-assisted excitonic recombination, the visible emission from Si-NCs in SiO2 is still subject to debate, with the majority of studies pointing to a defect-related origin. Recent works suggested that it is possible to achieve no-phonon direct-like transitions in Si nanostructures and thus extract a more efficient emission in the visible range. These works have sparked new interest in the high-energy emission from Si-NCs and motivated that part of the present research that aims at exploring the possibility of radiative recombinations in a process that does not involve phonons. In this work, the analysis of the PL decay curves evidenced the coexistence of multiple lifetimes, with components mainly grouped around two regions in the microsecond and picosecond domain. The decrease in PL intensity with annealing is correlated with the shortening of PL lifetime, characteristic of a class of non-radiative defects. Although my time resolved PL study shows the presence of ultra-fast lifetime components that could play a role in the populations of energy levels at the  point, there is no evidence in my samples for no-phonon recombinations via direct channels. Ion implanted samples of Si-NCs embedded in SiO2 were investigated by means of PL and TRPL at different photon fluxes. No significant dependence of the PL dynamics on emission energy was observed, indicating that decay lifetimes are not linked to different discrete sizes in the NCs size. I proposed a model of diffusion of excitation between neighbouring nanocrystals, with the saturation of acceptor nanocrystals providing a switching-off mechanism of the excitation transfer. The evolution of the PL emission with increasing silicon excess and annealing temperatures was found to be in agreement with a diffusion limited, Ostwald ripening process. For lower temperature treatments, a factor of 5 PL enhancement was observed and attributed to a thermal-activated carrier “recovery mechanism”, i.e. de-trapping of carriers from localized states within the band-gap to the states of Si-NCs. I propose that the PL contribution at around 2 eV observed in the same low temperature regime (10-100 K) arises from Si-NC-sensitized luminescent defects. At higher temperatures a monotonic quenching of the PL peak emission was observed and attributed to the enhancement of non-radiative recombination from defect states. Finally the Si-NC PL intensity in SiOxNy films was studied and found to be strongly dependent on the annealing conditions. RBS and XPS measurements showed that the composition of the thin films is significantly affected by oxygen contamination. Surprisingly, only the PL spectra of the as-deposited samples are well correlated with the evolving Si-NC size according to the quantum confinement (QC) model in which thin films containing larger clusters emit at lower energy. The formation of cracks after annealing the samples at temperatures from 400 °C to 1150 °C for 1 hour in forming gas, results in the suppression of the emission in the near-IR and in the arising of a defect-related emission peaking at higher energy.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available