Silicon photonic materials obtained by ion implantation and rapid thermal processing

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Abstract

The original work presented in this thesis describes research into Si-basedluminescent materials, prepared specifically by ion implantation and rapid thermalprocessing of thermal oxide films. An in-depth optical characterisation, employingphotoluminescence (PL) and Raman spectroscopy was complimented withelectron microscopy, revealing the source of efficient room temperature PL asnano-scale silicon inclusions (Si-NCs). The evolution of the Si-NC size and densitywith isothermal and isochronal annealing may be described using classicalthermodynamics according to a diffusion limited, Ostwald ripening process. Valuesfor the coarsening rate and activation energy, extracted from the evolution of theSi-NC size with annealing indicate that the transport of Si atoms and precipitateformation are enhanced in ion implanted films, attributable to the presence ofvacancy and interstitial defects generated during ion irradiation.The PL and Raman spectra are well correlated with the evolving Si-NC size anddensity according to the quantum confinement (QC) model in which samplescontaining larger clusters emit at longer wavelengths. However, the formation ofbound exciton states within the band gap of small clusters (< 2nm), as a result ofspecific surface chemistries, suppresses higher energy emissions. The increase inPL intensity with annealing was exactly correlated with the increase in PL lifetime,characteristic of the removal of non-radiative defects. A dependence of the PLdynamics on emission energy, with higher energies exhibiting shorter lifetimes,further evidences the QC effect. Blue shifted emission at high excitation flux and/orlow temperature is correspondent with the slower PL dynamics and preferentialsaturation at longer wavelengths. Raman spectra were fit using a phononconfinement model, from which Si-NC size distributions were extracted and foundto compare favourably with those obtained from TEM images. Stresses in thefilms, determined from the Raman peak position, were used as an independentmethod for calculating the Si surface energy, which is very close to the literaturevalues.A single, high temperature anneal of Si and erbium (Er) co-doped films revealed apreferential aggregation of Er at the Si-NC formation site, which is of particularimportance for the photo-sensitization of Er PL around 1.5μm. The Er PL wasenhanced in the presence of Si-NCs by several orders of magnitude comparedwith a reference SiO2:Er. Whilst broadband pumping of the Er via Si-NCsevidences a non-resonant energy transfer mechanism with an efficiency whichdepends on the Si-NC size, the process is limited at high excitation flux by acombination of low sensitizer (Si-NC) density and non-radiative losses.Finally the Si-NC PL intensity in phosphorus (P) co-doped films was studied andfound to depend strongly on the annealing conditions and P concentration. Forlower temperature treatments, a factor 2 PL enhancement, relative to an un-dopedreference was obtained, attributed to the passivation of Si-NC surface defects.Higher temperature treatments resulted in the monotonic quenching of the PL withincreasing P concentration, attributed to the introduction of an efficient Augerrecombination channel as a result of the ionization of P-donors inside large Si-NCs. A simple statistical model predicts this behaviour and provides an incidentalestimate of the Si-NC size.

Keyword(s)

Electron Microscopy; Ion implantation; Optical spectroscopy; Rapid thermal processing; Silicon nanocrystals

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