Defect-mediated nanostructures and luminescence centres in silicon
Date
2011
Authors
Charnvanichborikarn, Supakit
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Research over the past decade has indicated that ion implantation provides an attractive way of forming nanocrystals in solids that exhibit some exciting new material properties. However, the size and size distribution of these nanoparticles are both very difficult parameters to control, especially when a subsequent thermal anneal is needed to form well-defined precipitates. This usually leads to somewhat uncontrolled growth by the Ostwald ripening effect. In addition, ion implantation is known to create structural damage to crystals (especially semiconductors such as silicon), which evolves into many types of defects depending on the annealing regime and degree of damage. Some of these defects in silicon are optically active and can be detected by photoluminescence (PL), while others are inactive and may act as non-radiative recombination centres. The first part of this thesis concentrates on the encapsulation of Au nanoparticles in silicon and SiO2 using a combination of ion implantation, high temperature annealing, and wet oxidation. Structural analysis was undertaken by Rutherford backscattering spectrometry and channeling (RBS-C) in conjunction with transmission electron microscopy (TEM). The final structure of Au embedded at a precise depth in SiO{u2082} was achieved by wet oxidising the top silicon layer of a silicon-on-insulator (SOl) wafer containing Au precipitates. Several interesting phenomena including the segregation of Au precipitates at the oxidising interface, Au-enhanced oxidation, and preferential reprecipitation of Au on the Si-SiO{u2082} interface after dissolution are observed. The role of excess silicon-interstitials, which are injected into the underlying silicon during the oxidation process and mechanisms have been proposed to explain the results. In the second part, Si implantation to a range of fluences and subsequent annealing under various conditions were used to form different types of interstitial-based defects in crystalline silicon. Results show that low dose implantation (10{u00B9}{u2070} to 10{u00B9}{u00B3} cm{u207B}{u00B2}) and a low thermal budget annealing process (up to 525{u00B0}C for 2 minutes) are favourable for the observation of luminescence from small interstitial-related defect clusters (principally as the W-line at 1218 nm and the X-line at 1193 nm). Higher fluences create higher damage levels that lead to a formation of extended {311} defects and dislocations by annealing at higher temperatures, as can be identified by the R-line luminescence (at 1375 nm), as well as D-bands (around 1428 nm and 1530 nm), respectively. The W-line, in particular, is so sharp and bright that it was exploited to realise a sub-bandgap silicon light emitting diode (LED). The quantum efficiency of the fabricated LEDs is, however, poor especially when additional boron is introduced to create a p{u207A} layer that serves the purpose of making a good electrical contact at low temperature. An extended study on the effect of boron is consequently carried out in the last experimental chapter, where the competing formation between optically active silicon-interstitial defects and boron-interstitial clusters (BICs) is argued for this luminescence intensity loss. The results presented in this thesis have provided a better understanding of defect-defect and defect-impurity interactions and may inspire future research on silicon-based applications.
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