Structural and Optoelectronic Properties of Rare Earth Doped Silicon Photonic Materials

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Abstract

The thesis presented here entitled “Structural and Optoelectronic Properties of Rare Earth Doped Silicon Photonic Materials” for the degree of Doctor of Philosophy is submitted to the University of Manchester by Hang Li in December 2012.The original work presented in this thesis concentrates on the origin of the luminescence and the possible radiative loss mechanisms in rare earth (RE) and silicon nanocrystals (Si-NCs) co-doped SiO2. The optoelectronic properties of these materials were studied by employing laser or Xe lamp correlated photoluminescence (PL) and time-resolved PL spectroscopy and the structural and compositional characterisation was carried out using transmission electron microscopy (TEM) and scanning TEM (STEM).The pressure dependence of the band-gap of Si-NCs is strongly correlated to the co-doped erbium (Er) concentration. A conventional diamond anvil cell (DAC) was used for applying the hydrostatic pressure. A strong quenching and a non-linear red-shift of the PL of Si-NCs were obtained with increasing pressure, which was attributed to the quantum confinement effect. The rate of the red-shift as a function of pressure (pressure coefficient) increases with increasing Er concentration. We propose that this is the result of a reduction in the surface tension of Si-NCs when Er ions gather at their surface.Er is present as the trivalent oxide (Er2O3) in silicon rich silicon oxide (SRSO). Large Er2O3 clusters are formed when silicon excess is low as silicon is considered as a competitor for oxygen. Under the indirect excitation, the PL of Er3+ at 1.54 μm is highly dependent on the sensitization by Si-NCs. The decay lifetime of this emission contains a slow component of about 10 ms and a fast component of the order of μs. We found that the fast component became considerably faster with increasing Er2O3 cluster size. This is an indication of strong Er3+ ion-ion interactions in large clusters, which give rise to the non-radiative recombination of excitons.A novel fabrication of RE doped SiO2 was developed by implanting RE ions into silicon wafer directly and followed by the thermal oxidation and rapid thermal annealing (RTA) in contrast to the conventional procedure, in which the ion implantation was carried out after the oxidation of silicon wafer. TEM images showed that RE ions were distributed close to the surface of SiO2 film via the novel method but via the conventional method they were located at certain depth below the surface. Ce3+ doped SiO2 prepared via both methods has a broad blue PL ranging from around 380 to 470 nm attributed to the 5d-4f transitions and Eu3+ has a red PL with several narrow bands in the range of 570 to 700 nm attributed to the intra-4f transitions. Concentration quenching is one of the limiting factors for the conventional fabrication whilst it is successfully minimised by the novel method. A new phase of Ce silicate (Ce2Si2O7) may grow after the high temperature RTA via the novel method and leads to a remarkable enhancement of the Ce3+ luminescence.

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