G. Franzò

Light amplification in silicon nanocrystals by pump and probe transmission measurements

Journal of Applied Physics, 2004

Net optical gain in silicon nanocrystals (Si-nc) has been measured by pump and probe transmission... more Net optical gain in silicon nanocrystals (Si-nc) has been measured by pump and probe transmission experiments. Si-nc active layers have been produced by plasma enhanced chemical vapor deposition on transparent quartz substrates. Continuous and pulsed pump and probe transmission measurements have shown clear evidences of net probe amplification with fast (nanosecond) response time. Transfer matrix and rate equations modeling of the nonlinear signal transmission allowed to extract the gain spectra for different pump intensities yielding good qualitative agreement with the experimental data. Gain cross sections per nanocrystals of the order of 3 ϫ 10 −16 cm 2 have been deduced. The physical origin of the optical amplification is interpreted within a four level recombination model describing the dynamics of strongly localized excitons at the Si-nc/ SiO 2 interface.

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Evidence of energy transfer in an aluminosilicate glass codoped with Si nanoaggregates and Er[sup 3+] ions

Journal of Applied Physics, 2004

... for annealing treatments which are well below the threshold for the formation of , as ... and... more

Electrical and optical characterization of Er-implanted Si: The role of impurities and defects

by C. Spinella and G. Franzò

Journal of Applied Physics, 1993

Abstract The electrical and optical properties of Er‐implanted Si are shown to be critically depe... more

Oxidation induced precipitation in Al implanted epitaxial silicon

Journal of Applied Physics, 2000

The behavior of Al implanted in silicon has been investigated during thermal oxidation. It has be... more The behavior of Al implanted in silicon has been investigated during thermal oxidation. It has been found that precipitation of Al into Al-O-defect complexes depends on the implant energy, i.e., on the distance of the dopant from the surface. It occurs at 650 keV, but it does not at 2.0 MeV or higher energies. This phenomenon has been explained taking into account the diffusivity of self-interstitials introduced during oxidation, the oxygen present in the Si, the Al concentration, and the annealing out of defects.

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Optical and structural properties of Er[sub 2]O[sub 3] films grown by magnetron sputtering

Journal of Applied Physics, 2006

The structural properties and the room temperature luminescence of Er 2 O 3 thin films deposited ... more The structural properties and the room temperature luminescence of Er 2 O 3 thin films deposited by magnetron sputtering have been studied. In spite of the well-known high reactivity of rare earth oxides towards silicon, films characterized by good morphological properties have been obtained by using a SiO 2 interlayer between the film and the silicon substrate. The evolution of the properties of the Er 2 O 3 films due to thermal annealing processes in oxygen ambient performed at temperatures in the range of 800-1200°C has been investigated in detail. The existence of well defined annealing conditions ͑rapid treatments at a temperature of 1100°C or higher͒ allowing to avoid the occurrence of extensive chemical reactions with the oxidized substrate has been demonstrated; under these conditions, the thermal process has a beneficial effect on both structural and optical properties of the film, and an increase of the photoluminescence ͑PL͒ intensity by about a factor of 40 with respect to the as-deposited material has been observed. The enhanced efficiency of the photon emission process has been correlated with the longer lifetime of the PL signal. Finally, the conditions leading to a reaction of Er 2 O 3 with the substrate have been also identified, and evidences about the formation of silicate-like phases have been collected.

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Microstructural evolution of SiO[sub x] films and its effect on the luminescence of Si nanoclusters

Journal of Applied Physics, 2008

In this paper we demonstrate that the structural and optical properties of Si nanoclusters (Si nc... more In this paper we demonstrate that the structural and optical properties of Si nanoclusters (Si ncs) formed by thermal annealing of SiOx films prepared by plasma enhanced chemical vapor deposition (PECVD) and magnetron sputtering are very different. In fact, at a fixed Si excess and annealing temperature, photoluminescence (PL) spectra of sputtered samples are redshifted with respect to PECVD samples,

Nonlinear optical properties of silicon nanoclusters/nanocrystals doped SiO[sub 2] films: Annealing temperature dependence

by Luca Tartara and G. Franzò

Journal of Applied Physics, 2010

Comprehensive studies on nonlinear refractive indices ͑n 2 ͒ of SiO 2 films containing Si nanocry... more Comprehensive studies on nonlinear refractive indices ͑n 2 ͒ of SiO 2 films containing Si nanocrystals and/or nanoclusters ͑SiO 2 : Si-ncs͒ are performed. The comparison of the nonlinear refractive indices with the electron spin resonance signals reveals that defect states play a major role in the large n 2 when the annealing temperature is low, i.e., when Si nanoclusters exist in films. On the other hand, when Si nanocrystals are grown by high-temperature annealing, the contribution of defect states becomes small and that of the quantized electronic states of Si nanocrystals becomes large. The present results demonstrate that both the defect states and the quantized electronic states should be taken into account to explain the origin of large n 2 of SiO 2 : Si-ncs and to optimize the structure to maximize n 2 .

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Light absorption in silicon quantum dots embedded in silica

Journal of Applied Physics, 2009

The photon absorption in Si quantum dots ͑QDs͒ embedded in SiO 2 has been systematically investig... more The photon absorption in Si quantum dots ͑QDs͒ embedded in SiO 2 has been systematically investigated by varying several parameters of the QD synthesis. Plasma-enhanced chemical vapor deposition ͑PECVD͒ or magnetron cosputtering ͑MS͒ have been used to deposit, upon quartz substrates, single layer, or multilayer structures of Si-rich-SiO 2 ͑SRO͒ with different Si content ͑43-46 at. %͒. SRO samples have been annealed for 1 h in the 450-1250°C range and characterized by optical absorption measurements, photoluminescence analysis, Rutherford backscattering spectrometry and x-ray Photoelectron Spectroscopy. After annealing up to 900°C SRO films grown by MS show a higher absorption coefficient and a lower optical bandgap ͑ϳ2.0 eV͒ in comparison with that of PECVD samples, due to the lower density of Si-Si bonds and to the presence of nitrogen in PECVD materials. By increasing the Si content a reduction in the optical bandgap has been recorded, pointing out the role of Si-Si bonds density in the absorption process in small amorphous Si QDs. Both the photon absorption probability and energy threshold in amorphous Si QDs are higher than in bulk amorphous Si, evidencing a quantum confinement effect. For temperatures higher than 900°C both the materials show an increase in the optical bandgap due to the amorphous-crystalline transition of the Si QDs. Fixed the SRO stoichiometry, no difference in the optical bandgap trend of multilayer or single layer structures is evidenced. These data can be profitably used to better implement Si QDs for future PV technologies.

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New strategies to improve the luminescence efficiency of Eu ions embedded in Si-based matrices

Journal of Applied Physics, 2013

The comparison of the performances of SiO 2 and SiOC layers as host matrices for optically active... more The comparison of the performances of SiO 2 and SiOC layers as host matrices for optically active Eu ions is presented. A SiO 2 matrix allows to observe light emission from both Eu 2þ and Eu 3þ ions, owing to a proper tuning of the thermal annealing process used for the optical activation of the rare earth. However, the photoluminescence efficiency of both ions remains relatively low and quite far from the requirements for technological applications, mainly due to the extensive formation of Eu-containing precipitates. A detailed study by transmission electron microscopy allowed us to analyze and elucidate the clustering process and to find suitable strategies for minimizing it. We found that the substitution of SiO 2 matrix with a SiOC film allows to obtain a very bright light emission centered at about 440 nm from Eu 2þ ions. In fact, SiOC is able to efficiently promote the Eu 3þ ! Eu 2þ reduction; furthermore, Eu ions are characterized by an enhanced mobility and solubility in this matrix, and as a consequence, Eu precipitation is strongly reduced. Since SiOC is a material fully compatible with standard Si technology, Eu-doped SiOC layers can be considered a highly interesting candidate for photonic applications. V C 2013 American Institute of Physics. [http://dx.

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Probe of the Si nanoclusters to Er[sup 3+] energy transfer dynamics by double-pulse excitation

by Francesco Enrichi, Mauro Falconieri, and G. Franzò

Applied Physics Letters, 2005

In this letter the dynamics of the energy transfer ͑ET͒ process from amorphous silicon nanocluste... more In this letter the dynamics of the energy transfer ͑ET͒ process from amorphous silicon nanoclusters ͑Si-nc͒ to Er 3+ ions is investigated in co-doped silica glasses. Using a double pulse setup the recovery time of the sensitizing effect is monitored and direct evidence is found for the occurrence of a fast ͑few hundreds ns͒ ET process in a sample containing large ͑2 nm͒ aggregates, while a slower transfer time of few microseconds is revealed in a sample containing very small ͑Ͻ1 nm͒ aggregates.

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Light absorption enhancement in closely packed Ge quantum dots

Applied Physics Letters, 2013

ABSTRACT Multilayers of Ge quantum dots (QDs, 3 nm in diameter) embedded in SiO2, separated by Si... more ABSTRACT Multilayers of Ge quantum dots (QDs, 3 nm in diameter) embedded in SiO2, separated by SiO2 barrier layer (3, 9, or 20 nm thick), have been synthesized by sputter deposition and characterized by transmission electron microscopy and light absorption spectroscopy. Quantum confinement affects the optical bandgap energy (1.9 eV for QDs, 0.8 eV for bulk Ge); moreover, the absorption probability greatly depends on the QD-QD distance. A strong electronic coupling among Ge QDs is evidenced, with a significant increase of the light absorption efficiency when the QD-QD distance is reduced. These data unveil promising aspects for light harvesting with nanostructures.

Dynamics of stimulated emission in silicon nanocrystals

Applied Physics Letters, 2003

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Sensitizing properties of amorphous Si clusters on the 1.54-μm luminescence of Er in Si-rich SiO[sub 2]

Applied Physics Letters, 2003

... The Si nanocrystals are excited by the impinging laser source, and they transfer the energy t... more

High efficiency and fast modulation of Er-doped light emitting Si diodes

Applied Physics Letters, 1996

We demonstrate that the electrical excitation of Er ions incorporated within the depletion layer ... more

Electron paramagnetic resonance of erbium doped silicon

Applied Physics Letters, 1996

Electron paramagnetic resonance measurements have been made on samples of float zone silicon, imp... more Electron paramagnetic resonance measurements have been made on samples of float zone silicon, implanted with 10 15 Er/cm 2 . One sample was coimplanted with oxygen to give an impurity concentration of 10 20 O/cm 3 and 10 19 Er/cm 3 . In this coimplanted sample, sharp lines are observed which are identified as arising from a single spin 1/2 Er 3ϩ center having a g tensor exhibiting monoclinic C 1h symmetry. The principal g values and tilt angle are g 1 ϭ0.80, g 2 ϭ5.45, g 3 ϭ12.60, and ϭ2.6°. In the absence of O, the sharp lines are not observed. No Er 3ϩ cubic centers were detected in either sample. Possible structures for the center are discussed.

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Electroluminescence at 1.54 μm in Er-doped Si nanocluster-based devices

Applied Physics Letters, 2002

... Time resolved EL measurements at 1.54 μm have been done to gain information on the excitation... more

Excitation and de-excitation properties of silicon quantum dots under electrical pumping

Applied Physics Letters, 2002

In this work, the stationary and time-resolved electroluminescence (EL) properties of Si quantum ... more

Room-temperature electroluminescence from Er-doped crystalline Si

Applied Physics Letters, 1994

We have obtained room-temperature electroluminescence (EL) at -1.54 pm from Er and 0 co-doped cry... more We have obtained room-temperature electroluminescence (EL) at -1.54 pm from Er and 0 co-doped crystalline p-n Si diodes fabricated by ion implantation, under both forward and reverse bias conditions. Under forward bias, the EL intensity decreases by a factor of -15 on going from 110 to 300 K, where a weak peak is still visible. In contrast, we report the first sharp luminescence peak obtained under reverse bias conditions in the breakdown regime. In this case the EL intensity decreases only by a factor of 4 on going from 110 to 300 K and the room-temperature yield is more than one order of magnitude higher than under forward bias. The data suggest that Er excitation occurs through electron-hole mediated processes under forward bias and through impact excitation under reverse bias.

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Evolution of the local environment around Er upon thermal annealing in Er and O co-implanted Si

by Antonio Terrasi and G. Franzò

Applied Physics Letters, 1997

We present extended x-ray absorption fine structure (EXAFS) analyses of the Er edge in Er-doped (... more

Temperature dependence and quenching processes of the intra-4f luminescence of Er in crystalline Si

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