An investigation of efficient room temperature luminescence from silicon which contains dislocations

This thesis presents an investigation of the phenomenon of efficient, room temperature luminescence from dislocation-engineered (DE) silicon. Previous work had demonstrated that the introduction of near-surface dislocation loops to a silicon substrate by boron ion implantation and high temperature annealing produced efficient electroluminescence at room temperature. However, the mechanism by which high efficiency luminescence is produced was not understood. A wide matrix of specimens containing dislocations was fabricated by a variety of methods, including ion implantation, and their luminescence efficiencies were correlated to their physical properties. Transmission electron microscopy was used to characterise the defect structures created by ion implantation. In the majority of specimens a band of dislocation loops in close proximity to the surface was observed. The dislocation loops were shown to be consistent with a mixture of Frank and perfect dislocation loops, the relative proportions of which were dependent upon processing conditions. The thermal evolution of the dislocation loop size distribution was investigated. For the first time, a size distribution displaying a double peak was observed. The size distribution was shown to be consistent with the Gaussian distribution of two defect populations of different mean diameter. The thermal evolution of the size distribution was investigated in silicon implanted samples. A flux of self-interstitials from Frank dislocation loops to perfect dislocation loops was deduced. The evolution of the dislocation loop sizes was found to be consistent with Ostwald ripening. Cathodoluminescence (CL) was used to investigate the luminescent properties of silicon at room temperature for the first time. A new CL system was installed for this work, initially the CL system was characterised and a routine to ensure a high degree of reproducibility was formed. The luminescence mechanism of DE-silicon was shown to be the same as in unprocessed silicon wafers; TO phonon-assisted recombination. The mechanism of enhanced luminescence from DE-silicon was unambiguously shown to be due to the gettering of electrically active impurities from the specimen bulk. A reduction in the bulk transition metal impurity concentration of up to 35 times was inferred. In samples which were implanted with boron the degree of gettering was found to show a logarithmic dependence on the dislocation density. Using a crosssectional mapping technique, implanted samples were shown to contain a lower concentration of transition metal impurities throughout the entire wafer in comparison to as-received, unprocessed specimens. Furthermore, the impurity concentration was found to be lowest in close proximity to the band of dislocation loops. The dislocation loops were found to act as non-radiative recombination centres and their strength was strongly influenced by the local carrier concentration. The high doping levels of samples implanted with boron were found to minimise the non-radiative recombination action of the dislocations. Low temperature annealing was used to improve the luminescence efficiency of DE-silicon further.

Keywords: dislocations
Czochralski
cathodoluminescence
luminescence
Silicon
TEM
defects
Advisor: Wilshaw, P