Photoluminescence (PL) and excitation spectra of Bi melt-doped oxide and chalcogenide glasses are very similar, indicating the same Bi center is present. When implanted with Bi, chalcogenide, phosphate and silica glasses, and BaF2 crystals, all display characteristically different PL spectra to when Bi is incorporated by melt-doping. This indicates that ion implantation is able to generate Bi centers which are not present in samples whose dopants are introduced during melting. Bi-related PL bands have been observed in glasses with very similar compositions to those in which carrier-type reversal has been observed, indicating that these phenomena are related to the same Bi centers, which we suggest are interstitial Bi2+ and Bi clusters.
Yanina Fedorenko, Mark Hughes, Julien Colaux, C. Jeynes, Russell Gwilliam, Kevin Homewood, Jin Yao, Dan Hewak, Tae-Hoon Lee, Stephen Elliott, B. Gholipour, Richard Curry
Doping of amorphous chalcogenide films of rather dissimilar bonding type and resistivity, namely, Ga-La-S, GeTe, and Ge-Sb-Te by means of ion implantation of bismuth is considered. To characterize defects induced by ionbeam implantation space-charge-limited conduction and capacitance-voltage characteristics of amorphous chalcogenide/silicon heterojunctions are investigated. It is shown that ion implantation introduces substantial defect densities in the films and their interfaces with silicon. This comes along with a gradual decrease in the resistivity and the thermopower coefficient. It is shown that conductivity in GeTe and Ge-Sb-Te films is consistent with the two-type carrier conduction model. It is anticipated that ion implantation renders electrons to become less localized than holes leading to electron conductivity in certain cases as, for example, in GeTe.
290 fs optical pulses have been demonstrated using a two-section, passively mode-locked InAs quantum dot laser with a proton bombarded absorber, at a repetition rate of 20 GHz, centred around 1220 nm. Optical pulsewidth measurements between 300 K and 20 K, reveal that the pulsewidth decreases by a factor of 29, from 8.4 ps at 250 K to 290 fs at 20 K with a corresponding change in optical bandwidth. Rate equation analysis shows that this is due to the decreased emission rate compared to the recombination rate at 20 K in the random population regime. Random population ensures the dots act as independent oscillators across the entire inhomogeneous distribution thereby allowing access to the full gain spectrum. Proton bombardment of the absorber section shortens the recovery time of the absorber while the dots remain randomly populated.
Silicon photonics shows tremendous potential for the development of the next generation of ultra fast telecommunication,
tera-scale computing, and integrated sensing applications.
One of the challenges that must be addressed when integrating a "photonic layer" onto a silicon microelectronic circuit is
the development of a wafer scale optical testing technique, similar to that employed today in integrated electronics
industrial manufacturing. This represents a critical step for the advancement of silicon photonics to large scale
production technology with reduced costs.
In this work we propose the fabrication and testing of ion implanted gratings in sub micrometer SOI waveguides, which
could be applied to the implementation of optical wafer scale testing strategies.
An extinction ratio of over 25dB has been demonstrated for ion implanted Bragg gratings fabricated by low energy
implants in submicron SOI rib waveguides with lengths up to 1mm. Furthermore, the possibility of employing the
proposed implanted gratings for an optical wafer scale testing scheme is discussed in this work.
We present a combined analysis using cross-sectional transmission electron microscopy (X-TEM) and Raman
spectroscopy to study the early formation dynamics of Si-nanocrystals, formed in SiO2 thin films after Si+ implantation
and rapid thermal processing (RTP). We obtained values for the diffusion coefficient of Si in thermally grown SiO2 and
the activation energy to precipitate formation in the first 100 seconds of high temperature annealing. These values
indicate that the formation of Si-nanocrystals in implanted oxides proceeds much more efficiently than purely via a self
diffusion process. We propose that the nanocrystal formation is assisted by the presence of both oxygen vacancies and
SiO molecular species, presumably generated by the ion irradiation. Microscopy images reveal the ensemble nanocrystal
population to be most accurately represented by a lognormal distribution function with characteristic values for the mean
particle diameter, d and variance, σ. The evolution of the silicon nanocrystals with annealing was also investigated by
measuring the Raman scattering signal associated with the TO phonon mode arising from Si-Si bonds in Si-rich oxides
grown on transparent (Al2O3) substrates. This greatly simplifies the experimental observation of the Raman spectra from
Si-nanocrystals as compared to previous studies of nanocrystals in oxide films on silicon substrates.
Integrated Bragg gratings are an interesting candidate for waveguide coupling, telecommunication applications,
and for the fabrication of integrated photonic sensors. These devices have a high potential for optical integration
and are compatible with CMOS processing techniques if compared to their optical fibre counterpart.
In this work we present design, fabrication, and testing of Germanium ion implanted Bragg gratings in silicon
on insulator (SOI). A periodic refractive index modulation is produced in a 1μm wide SOI rib waveguide by
implanting Germanium ions through an SiO2 hardmask.
The implantation conditions have been analysed by 3D ion implantation modelling and the induced refractive
index change has been investigated on implanted samples by Rutherford Backscattering Spectroscopy (RBS)
and ellipsometry analysis.
An extinction ratio of up to 30dB in transmission, around the 1.55μm wavelength, has been demonstrated for
Germanium implanted gratings on SOI waveguides.
We present the results of an optical investigation of a series of Er doped silicon nanocrystal (Si-NC) samples which were
fabricated via ion implantation into SiO2 on sapphire substrates, followed by a range of rapid thermal processing. The
photoluminescence spectra of the Si-NC emission revealed an increase in luminescence intensity and a red-shift of the
peak wavelength as a function of annealing conditions. We attribute the former effect to the reduction of implantation
induced defects with increasing annealing temperature/duration. Measurements of the rate of decay of
photoluminescence intensity at room temperature show a corresponding increase in the carrier lifetimes which is also an
indication of a reduced contribution from non-radiative centers. The red-shift of the peak Si-NC intensity is ascribed to
an increasing mean Si-NC size as a function of the annealing conditions. Also presented is an estimation of the relative
Er sensitization which reveals that the smallest Si-NC size distribution leads to the greatest sensitization ratio. Further
investigation in the form of excitation spectroscopy was used to show that Er ions are sensitized not only by energy
transfer from the Si-NCs, but also, crucially, from defect states in the SiO2.
Previously we have reported the effects of silicon ion irradiation on free carrier lifetime and propagation loss in silicon
rib waveguides, and simulated net Raman gain based on experimental results. We further extend this work by reporting
the effects of thermally treating a silicon irradiated sample with a higher dose and energy than previously reported,
which produced a poor trade-off between free carrier lifetime and excess optical absorption prior to thermal treatment.
Excess losses greater than 80dB/cm were recorded prior to annealing. After thermal treatment, the sample demonstrated
characteristics of excess loss and free carrier lifetime recorded previously in much lower energy and dose silicon ion
irradiated samples, suggesting that thermally treating samples could enhance the trade-off between free carrier lifetime
and excess loss introduced to the rib waveguides. Raman gain simulations based on the new experimental data are
reported and show an increase in net gain over previously reported data, suggesting that higher dose, shallow silicon ion
implantation is the most efficient way of optimising the trade-off between lifetime reduction and excess optical
absorption in silicon rib waveguides, a proposal in our earlier work. The effects of thermally treating low temperature
oxide clad waveguides with respect to free carrier lifetime are also reported. Results show that thermally treating a low
temperature oxide clad waveguide can vary the intrinsic lifetime. The results of this investigation as well as a discussion
into the possible origin of the lifetime change are given.
Silicon Photonics is a field that has seen rapid growth and dramatic changes in the past 5 years. According to the MIT
Communications Technology Roadmap [1], which aims to establish a common architecture platform across market
sectors with a potential $20B in annual revenue, silicon photonics is among the top ten emerging technologies. This has
in part been a consequence of the recent involvement of large semiconductor companies around the world, particularly in
the USA. Significant investment in the technology has also followed in Japan, Korea, and in the European Union. Low
cost is a key driver, so it is imperative to pursue technologies that are mass-producible.
Therefore, Silicon Photonics continues to progress at a rapid rate. This paper will describe some of the work of the
Silicon Photonics Group at the University of Surrey in the UK. The work is concerned with the sequential development
of a series of components for silicon photonic optical circuits, and some of the components are discussed here. In
particular the paper will present work on optical waveguides, optical filters, modulators, and lifetime modification of
carriers generated by two photon absorption, to improve the performance of Raman amplifiers in silicon.
We investigate the effects of silicon ion irradiation on free carrier lifetime and propagation loss in silicon rib
waveguides, and thus its ability to reduce the density of two-photon-absorption (TPA) generated free carriers, an
undesired effect of the Raman process in crystalline silicon. Our experimental results show that free carrier lifetime can
be reduced significantly by silicon ion implantation. Associated excess optical absorption from the implanted silicon ions
can be kept low if irradiation energy and dose are correctly chosen. Simulations of Raman amplification in silicon rib
waveguides suggest that net gain can be achieved in certain cases without the need for an integrated diode in reverse bias
to sweep out the photo-generated free carriers.
In this paper we report two novel fabrication techniques for silicon photonic circuits and devices. The techniques are
sufficiently flexible to enable waveguides and devices to be developed for telecommunications wavelengths or indeed
other wavelength ranges due to the inherent high resolution of the fabrication tools. Therefore the techniques are
suitable for a wide range of applications. In the paper we discuss the outline fabrication processes, and discuss how they
compare to conventional processing. We compare ease of fabrication, as well as the quality of the devices produced in
preliminary experimental fabrication results. We also discuss preliminary optical results from fabricated waveguide
devices, as measured by conventional means. In these preliminary results we discuss fundamental properties of the
waveguides such as loss and spectral characteristics, as it is these fundamental characteristics that will determine the
viability of the techniques. Issues such as the origins of the loss are discussed in general terms, as resulting fabrication
characteristics such as waveguide surface roughness (and hence loss), or waveguide profile and dimensions may be
traded off against cost of production for some applications. We also propose further work that will help to establish the
potential of the technique for future applications.
In this paper we describe a new approach to the manufacture of Si based optical emitters. Dislocation engineering, using entirely ULSI compatible technology has been shown to be a viable route to the production of efficient LED's operating at room temperature. External quantum efficiencies of ~ 10-4 have been realised for un-packaged devices operating at room temperature. This approach has also been shown to yield great benefits in the control of thermal quenching for other material systems, such as the FeSi2, where room temperature operational devices based at 1.5um have been demonstrated
Efficient silicon-based light emitting diodes have been fabricated using a recently developed approach - dislocation engineering. Crucially this technique uses entirely conventional ULSI processes. The devices were fabricated by conventional low energy boron implantation into silicon substrates followed by high temperature rapid thermal annealing. Strong silicon band edge luminescence was observed. Electroluminescence emissions with an efficiency greater than 2 X 10-4 at rom temperature were measured; the device lifetime was found to be approximately 18 microsecond(s) . The luminescence integrated intensity varied with the device fabrication conditions. In this paper we discuss the influence of processing conditions on the luminescence emissions.
Electroluminescent diodes fabricated on silicon substrates which emit at a wavelength of 1.5 micrometer have been demonstrated. The diodes operate at room temperature and exhibit good I-V characteristics. The diodes use an erbium tris(8-hydroxyquinoline) (ErQ) layer as electron transporting and emitting layer and use N, N'-diphenyl-N, N'-bis(3-methyl)-1,1'-biphenyl-4,4'-diamine (TPD) as the hole transporting layer. Hole injection into the diodes is from a p++ silicon substrate anode and aluminum is used as the cathode electrode. The devices demonstrated start to exhibit electroluminescence at a voltage of approximately 17 V and the electroluminescence intensity rises sub-linearly with the current density through the device. At a drive voltage of 33 V the diodes have an internal efficiency of approximately 0.01%. We have measured the luminescence lifetime for the 1.5 micrometer emission and obtained a value of approximately 200 microsecond(s) . Using this value and estimating the total concentration of erbium present in the diodes we calculate a theoretical maximum optical power generation in these diodes of approximately 100 mW.
Attempts to obtain electroluminescence from silicon-based devices have been largely frustrated by the indirect bandgap of the semiconductor. One approach, described here, is to fabricate a direct bandgap material which is compatible with silicon processing and which can then be excited via standard carrier injection across p-n junctions. We have used ion implantation of iron, typically at an energy of 180 keV and a dose of 1.5 X 1016 cm-2, conditions which are easily achievable in modern commercial implanters, to form precipitates of (beta) -iron disilicide, which has a direct bandgap of 0.8 eV. At 80 K and under forward bias conditions, the devices emit light at 1.5 micrometers with an external quantum efficiency of 5 X 10-3, and emission at room temperature has been observed. The emission lifetime has been placed at shorter than 60 ns, as expected of a direct bandgap material. Results will be presented showing how the electroluminescence properties change with the dose of implanted iron.
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