Abstract: We studied the photoluminescence spectra of silicon and phosphorus co-implanted silica thin films on (100) silicon substrates as a function of isothermal annealing time. The rapid phase segregation, formation and growth dynamics of intrinsic silicon nanocrystals is observed, in the first 600s of rapid thermal processing, using dark field mode X-TEM. For short anneal times, when the nanocrystal size distribution exhibits a relatively small mean diameter, formation in the presence of phosphorus yields an increase in the luminescence intensity and a blue shift in the emission peak compared with intrinsic nanocrystals. As the mean size increases with annealing time, this enhancement rapidly diminishes and the peak energy shifts further to the red than the intrinsic nanocrystals.
These results indicate the existence of competing pathways for the donor electron, which depends strongly on the nanocrystal size. In samples containing a large density of relatively small nanocrystals, the tendency of phosphorus to accumulate at the nanocrystal-oxide interface means that ionization results in a passivation of dangling bond (Pb-centre) type defects, through a charge compensation mechanism. As the size distribution evolves with isothermal annealing, the density of large nanocrystals increases at the expense of smaller nanocrystals, through an Ostwald ripening mechanism, and the majority of phosphorus atoms occupy substitutional lattice sites within the nanocrystals. As a consequence of the smaller band-gap, ionization of phosphorus donors at these sites increases the free carrier concentration and opens up an efficient, non-radiative de-excitation route for photo-generated electrons via Auger recombination. This effect is exacerbated by an enhanced diffusion in phosphorus doped glasses, which accelerates silicon nanocrystal growth.
Abstract: Erbium doped silicon-rich silica offers broadband and very efficient excitation of erbium photo-luminescence (PL) due to a sensitization effect attributed to silicon nanocrystals (Si-nc), which grow during thermal treatment. PL decay lifetime measurements of sensitised Er3+ ions are usually reported to be stretched or multiexponential, very different to those that are directly excited, which usually show a single exponential decay component. In this paper, we report on SiO2 thin films doped with Si-nc’s and erbium. Time resolved PL measurements reveal two distinct 1.54um Er decay components; a fast microsecond component and a relatively long lifetime component (10ms). We also study the structural properties of these samples through TEM measurements and reveal the formation of Er clusters. We propose that these Er clusters are responsible for the fast us decay component and we develop rate equation models that reproduce the experimental transient observations and can explain some of the reported transient behaviour in previously published literature.
Abstract: Studies of the mechanical properties of single-walled carbon nanotubes are hindered by the availability only of ensembles of tubes with a range of diameters. Tunable Raman excitation spectroscopy picks out identifiable tubes. Under high pressure, the radial breathing mode shows a strong environmental effect shown here to be largely independent of the nature of the environment . For the G-mode, the pressure coefficient varies with diameter consistent with the thick-wall tube model. However, results show an unexpectedly strong environmental effect on the pressure coefficients. Reappraisal of data for graphene and graphite gives the G-mode Grueuneisen parameter gamma = 1.34 and the shear deformation parameter beta = 1.34.
Abstract: The charge confinement in silicon nanocrystals (Si-nc) over the temperature range 35–300 K was investigated by complex impedance spectroscopy (CIS). A charge response found in the ‘dark’ (i.e., no laser pumping) CIS spectrum indicated a frequency shift with temperature, which was well correlated with the temperature dependent photoluminescence (PL) intensity. From equivalent circuit analyses of this frequency shift, we were able to determine the charge ‘trapping-detrapping’ mechanism giving rise to the luminescence. We find that the luminescence decay transient, expressed as a stretched exponential function, can be mathematically converted to a CIS spectrum.
Abstract: Raman excitation and emission spectra for the radial breathing mode (RBM) are reported, together with a preliminary analysis. From the position of the peaks on the two-dimensional plot of excitation resonance energy against Raman shift, the chiral indices (m, n) for each peak are identified. Peaks shift from their positions in air when different pressure media are added – water, hexane, sulphuric acid – and when the
nanotubes are unbundled in water with surfactant and sonication. The shift is about 2–3 cm−1 in RBM frequency, but unexpectedly large in resonance energy, being spread over up to 100 meV for a given peak. This contrasts with the effect of pressure. The shift of the peaks of semiconducting nanotubes in water under pressure is orthogonal to the shift from air to water. This permits the separation of the effects of the
pressure medium and the pressure, and will enable the true pressure coefficients of the RBM and the other Raman peaks for each (m, n) to be established unambiguously.
Abstract: Time-resolved photoluminescence measurements of silicon nanocrystals formed by ion implantation of silicon into silicon dioxide reveal multi-exponential luminescence decays. Three discrete time components are apparent in the rise and decay data, which we associate with different populations of nanocrystals. The values of decay time are remarkably constant with emission energy, but the relative contributions of the three components vary strongly across the luminescence band. In keeping with the quantum confinement model for luminescence, we assign emission at high energies to small nanocrystals, and that at low energies to large nanocrystals. By deconvolving the decay data over the full emission band it is possible to study the migration of excitation from smaller (luminescence donor) to larger (luminescence acceptor) nanocrystals. We propose a model of diffusion of excitation between neighbouring nanocrystals, with long lifetime emission being from the largest nanocrystal in the local neighbourhood. Our data also allow us to study the saturation of acceptor nanocrystals, effectively switching off excitation transfer, and Auger recombination in non-interacting nanocrystals.
Abstract: We validate for the first time the phenomenological phonon confinement model (PCM) of H. Richter, Z. P. Wang, and L. Ley [Solid State Commun. 39, 625 (1981)] for silicon nanostructures on the sub-3 nm length scale. By invoking a PCM that incorporates the measured size distribution, as determined from cross-sectional transmission electron microscopy (X-TEM) images, we are able to accurately replicate the measured Raman line shape, which gives physical meaning to its evolution with high temperature annealing and removes the uncertainty in determining the confining length scale. The ability of our model to explain the presence of a background scattering spectrum implies the existence of a secondary population of extremely small (sub-nm), amorphous silicon nanoclusters which are not visible in the X-TEM images. Furthermore, the inclusion of an additional fitting parameter, which takes into account the observed peak shift, can be explained by a size-dependent interfacial stress that is minimized by the nanocluster/crystal growth. From this we obtain incidental, yet accurate estimates for the silicon surface energy and a Tolman length, d = 0.15 +/- 0.1nm using the Laplace-Young relation.
Abstract: Tunable Raman excitation spectroscopy with 1.55 eV to 1.77 eV laser energies was used to map the second van Hove singularities of semiconducting single-walled carbon nanotube ropes on a glass slide in air and immersed in water. The optical transitions are assigned to different (n, m) tubes using a correlation of the diameter and radial breathing mode (RBM) in the Raman spectrum. The resonance energies are blue-shifted when the tubes are immersed in water and the shift depends on the chiral angle. In addition, the radial breathing mode frequencies are also blue-shifted upon immersion in water.
Abstract: We present a study of silicon (Si) and erbium (Er) coimplanted silica (SiO2) in which we observe, by combining high resolution scanning transmission electron microscopy and selective electron energy loss spectroscopy (EELS), a high spatial correlation between silicon nanocrystals (Si-NCs), Er, and oxygen (O) after a single high temperature (1100 °C) anneal. The observation of a spatial overlap of the EELS chemical maps of dark field (DF) images at the Er N4,5, Si L2,3, and O K edges is concomitant with an intense room temperature infrared luminescence around 1534 nm. We suggest that these observations correspond to Er–O complexes within an amorphous silicon (a-Si) shell at the Si-NC/SiO2 interface. The presence of a crystalline phase at the Si-NC center, verified by high resolution electron micrographs and DF diffraction contrast images and the low solubility of Er in crystalline Si (c-Si) would tend to suggest a preferential Er agglomeration toward the Si-NC/SiO2 interface during formation, particularly when high concentrations of both Si and Er are obtained in a narrow region of the SiO2 after coimplantation. The absence of narrow Stark related features in the Er emission spectrum at low temperature and an inhomogeneous broadening with increasing temperature, which are characteristic of Er confined by an amorphous, rather than a crystalline host further support these hypotheses. After comparing the luminescence to that from a SiO2:Er control sample prepared in exactly the same manner but without Si-NCs, we find that, despite the observed spatial correlation, only a small fraction ( ∼ 7%) of the Er are sensitized by the Si-NCs. We ascribe this low fraction to a combination of low sensitizer (Si-NC) density and Auger-type losses arising principally from Er ion-ion interactions.
Abstract: We describe the impact of non-radiative de-excitation mechanisms on the optical emission from silicon nanocrystals formed in SiO2. Auger excitation via free carriers deliberately introduced through phosphorus ion implantation, shows a
monotonic increase with increasing phosphorus concentration which can be modelled adequately using a simple statistical approach. We also report a reduction in nanocrystal luminescence intensity with increasing exposure to UV radiation and
suggest this phenomenon results from the introduction of non-radiative defects in the Si/SiO2 network. The effect of UV radiation varies significantly depending on the sample preparation.
Abstract: Co-doping of SiO2 with Si and Er to achieve silica fibre amplifiers has resulted in encouraging levels of light emission, much above those of Er-only doped SiO2. However, different fabrication methods, i.e., co-implantion and sequential implantation of Er and Si, has led to several factors difference in light levels. This paper looks into the reasons for these differences by establishing structure and local stoichiometry of the created entities via analytical transmission electron microscopy. In both cases Si-nanocrystals (NCs) have formed in the SiO2 matrix. In the former case Er-ions are co-located with /integrated within the NCs, in the latter case NCs and Er are separate. By assessing the NCs’ internal and interfacial structure with the surrounding material, we attempt to identify chemical/structural Er-phases/defects and their effect on the sensitising efficiency in the Er:Si-NCs system; high resolution phase contrast and high angle dark field imaging as well as nano-scale spatially resolved electron energy core loss- and plasmon-spectroscopy carried out in an aberration corrected dedicated STEM lend valuable support to these studies.
Abstract: Er-doped SiO2 and Si nano-crystals (NCs) embedded in a SiO2 matrix were produced by ion beam implantation of Si (100) substrates. After annealing Er ions agglomerate in different positions with different compositional properties in samples with and without Si implants. HAADF and EELS show that in the sample with Si implants the Si and Er distribution is identical and within a band of ~110nm width ~75nm below theSiO2 surface whereas in the sample with no excess Si, Er forms on average much larger, amorphous aggregates, presumably an Er-oxide, in the SiO2 matrix with tendency to move towards the surface of the SiO2 layer.
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