Excitation Of Er3 Research Articles

The features of electro-optical memory effect for 1.54μm electroluminescence of an Er doped Si diode

Abstract The features of electro-optical memory effect (“stored” electroluminescence) for 1.54 μm electroluminescence of an Er-doped Si diode have been investigated. It is shown, that excitation mechanism for the stored electroluminescence is impact excitation of Er3+ ions by the electrons, released from deep traps. High excitation efficiency for stored electroluminescence has been demonstrated (500 times higher compared to forward bias electroluminescence). Optical excitation of “stored” electroluminescence has been demonstrated with band-to-band illumination instead of forward bias injection pulse.

Band-to-band and direct optical excitation of Er in silicon: Comparison of kinetics, temperature dependence of erbium PL

Photoluminescence excitation spectra of erbium ions in the epitaxial Si:Er/Si structures have been measured in a wide range of excitation wavelengths ( λ ex =800–1580 nm) including the range of direct optical excitation of Er ( λ ex =1460–1580 nm). The temperature quenching of erbium PL have been compared for the regimes of direct optical excitation and excitation by photo-generated carriers in silicon matrix. It has been demonstrated that in case of direct optical excitation of Er ions the temperature quenching of Er PL was much weaker than under band-to-band pumping and the major processes of erbium non-radiative deexcitation such as Auger-deexcitation with equilibrium free carriers and “back-transfer” are substantially suppressed.

Concentration dependence of the Er3+ visible and infrared luminescence in Y2−xErxO3 thin films on Si

Y 2 − x Er x O 3 thin films, with x varying between 0 and 0.72, have been successfully grown on crystalline silicon (c-Si) substrates by radio-frequency magnetron cosputtering of Y2O3 and Er2O3 targets. As-deposited films are polycrystalline, showing the body-centered cubic structure of Y2O3, and show only a slight lattice parameter contraction when x is increased, owing to the insertion of Er ions. All the films exhibit intense Er-related optical emission at room temperature both in the visible and infrared regions. By studying the optical properties for different excitation conditions and for different Er contents, all the mechanisms (i.e., cross relaxations, up-conversions, and energy transfers to impurities) responsible for the photoluminescence (PL) emission have been identified, and the existence of two different well-defined Er concentration regimes has been demonstrated. In the low concentration regime (x up to 0.05, Er-doped regime), the visible PL emission reaches its highest intensity, owing to the influence of up-conversions, thus giving the possibility of using Y2−xErxO3 films as an up-converting layer in the rear of silicon solar cells. However, most of the excited Er ions populate the first two excited levels I411/2 and I413/2, and above a certain excitation flux a population inversion condition between the former and the latter is achieved, opening the route for the realization of amplifiers at 2.75 μm. Instead, in the high concentration regime (Er-compound regime), an increase in the nonradiative decay rates is observed, owing to the occurrence of cross relaxations or energy transfers to impurities. As a consequence, the PL emission at 1.54 μm becomes the most intense, thus determining possible applications for Y2−xErxO3 as an infrared emitting material.

On the effects of thermal treatment on the composition, structure, morphology, and optical properties of hydrogenated amorphous silicon-oxycarbide

The composition, structure, morphology, and optical characteristics of hydrogenated amorphous silicon-oxycarbide (a-SiCxOyHz) materials were investigated as a function of experimental processing conditions and post-deposition thermal treatment. Thermal chemical vapor deposition (TCVD) was applied to the growth of three different types of a-SiCxOyHz films, namely, SiC-like (SiC1.08O0.07H0.21), Si-C-O (SiC0.50O1.20H0.22), and SiO2-like (SiC0.20O1.70H0.24). The resulting films were subsequently annealed at temperatures ranging from 500 °C to 1100 °C for 1 h in an argon atmosphere. The composition, structure, and morphology of as-deposited and post-annealed films were characterized by Fourier transform infrared spectroscopy (FTIR), x-ray photoelectron spectroscopy (XPS), Rutherford backscattering spectroscopy (RBS), nuclear-reaction analysis (NRA), and scanning electron microscopy. Corresponding optical properties were assessed by spectroscopic ultraviolet-visible ellipsometry (UV-VIS-SE). These studies led to the identification of an optimized process window for the growth of Er doped silicon oxycarbide (SiC0.5O1.0:Er) thin film with strong room-temperature photoluminescence emission measured around 1540 nm within a broad (460 nm to 600 nm) wavelength band. Associated modeling studies showed that the effective cross section for Er excitation in the SiC0.5O1.0:Er matrix was approximately four orders of magnitude larger than its analog for direct optical excitation of Er ions.

Energy transfer and 1.54 μm emission in amorphous silicon nitride films

Er-doped amorphous silicon nitride films with various Si concentrations (Er:SiNx) were fabricated by reactive magnetron cosputtering followed by thermal annealing. The effects of Si concentrations and annealing temperatures were investigated in relation to Er emission and excitation processes. Efficient excitation of Er ions was demonstrated within a broad energy spectrum and attributed to disorder-induced localized transitions in amorphous Er:SiNx. A systematic optimization of the 1.54 μm emission was performed and a fundamental trade-off was discovered between Er excitation and emission efficiency due to excess Si incorporation. These results provide an alternative approach for the engineering of sensitized Si-based light sources and lasers.

Towards an optimum coupling between Er ions and Si-based sensitizers for integrated active photonics

Series of Er-doped Si-rich silicon oxide layers were studied with the aim of optimizing the coupling between Er ions and the Si-based sensitizers. The layers were grown at substrate temperature between 400 and 600°C by the cosputtering of three confocal targets: Si, SiO2, and Er2O3. The influence of Si excess (5–15at.%) and annealing temperature (500–1100°C) was examined for two concentrations of Er ions (3.5×1020 and ∼1021cm−3). We report the first observation of significant Er photoluminescence (PL) from as-grown samples excited by a nonresonant 476nm line, with a lifetime in the range of 1.3–4ms. This suggests the occurrence of an indirect excitation of Er through Si-based entities formed during the deposition. A notable improvement was observed for both Er PL intensity and lifetime after annealing at 600°C. This temperature is lower than that required for phase separation, suggesting the formation of “atomic scale” sensitizers (Si agglomerates, for example) considered in recent work. For high Er doping (∼1021cm−3), an optimum Er PL was obtained for the sample grown at 500°C, annealed at 600°C, and containing ∼13at.% of Si excess. This high PL should correspond to an optimum fraction of coupled Er for this series, which was roughly estimated to about 11% of the total Er content for an excitation photon flux of few 1019phcm−2s−1. For the moderately Er-doped series (3.5×1020cm−3) grown at 500°C, the optimum Er PL was found for the samples containing about 9at.% silicon and annealed in the 600–900°C range. The time decay reached a value as high as 9ms for low Si excess (<6at.%) and 6–7.5ms for high values of Si excess. The fraction of Er ions coupled to sensitizers was similarly estimated for the best sample of this series and found to be as high as 22% of the total Er content.

VUV spectroscopy of wide bandgap materials

In this paper we will present VUV spectroscopy experiments performed at the Superlumi station of Hasylab, DESY, Hamburg, on samples of BaF 2 crystals activated with Ce and BaF 2, (Ba,La)F 2 crystals activated with Er. The results of these experiments include time resolved luminescence and luminescence excitation spectra obtained under wavelength selective VUV and UV excitation by pulsed synchrotron radiation. We will reveal the information provided by the VUV/UV excitation spectra of the Ce 3+ 5d → 4f as well as Er 3+ 4f n −15d → 4f n and 4f n → 4f n emissions on energy transfer mechanisms from the fluoride host to the rare earth ion. We will demonstrate that the fast energy transfer channels involve bound excitons while the generation of free electrons and holes leads to slower processes dependant on hole and/or electron trapping. We will demonstrate that differences between the excitation spectra of the 5d → 4f emission in Ce and 4f 105d → 4f 11 emission in Er activated BaF 2 are generated by the coupling of the 4f → 5d transition to the 4f 10 core of the Er 3+ ion. We will also identify the additional band, absent for Ce, which is due to the exchange split high spin (HS) state of the 4f 105d configuration responsible for the slow decay of the excited Er 3+ ions in BaF 2 and (Ba,La)F 2. Finally we will provide evidence and explain why the dominant VUV 4f 105d → 4f 11 Er 3+ emission in BaF 2 is spin-forbidden and slow while in the mixed (Ba,La)F 2 crystals it is spin-allowed and fast.

Er-based materials for Si microphotonics

We have investigated the role of the Si excess on the photoluminescence properties of Er-doped Si nanoclusters (Si nc). We demonstrate that the Si excess has two competing roles: when agglomerated to form Si nc it enhances the Er excitation efficiency but it also introduces new non-radiative decay channels. When Er is excited through an energy transfer from Si nc, the beneficial effect on the enhanced excitation efficiency prevails and the Er emission increases with increasing Si content. The luminescence quenching processes limiting quantum efficiency in Er-doped Si nc light emitting devices are investigated and identified. It is found that carrier injection, while needed to excite Er ions through electron–hole recombination, at the same time produces an efficient non-radiative Auger de-excitation with trapped carriers. The maximum excited Er fraction in this system is only of the order of percent. In order to increase the concentration of excited Er ions, a different approach based on Er silicate thin films has been explored. Under proper annealing conditions, an efficient luminescence at 1535 nm is found and all the Er ions in the material are optically active. The possibility to efficiently excite Er ions also through electron–hole mediated processes is demonstrated in nanometer-scale Er–Si–O/Si multilayers.

Effect of hydroxyl groups on Er 3+-doped bismuth-borate glass and fiber

A series of Er 3+-doped Bi 2O 3–B 2O 3–GeO 2–Na 2O glasses with different OH − group contents were prepared and the interactions between Er 3+ ions and OH − groups were investigated. The observed increase of the fluorescence intensity and decaying variation from an evident non-exponential to a nearly exponential behavior of the Er 3+: 4I 13/2 level with the oxygen gas bubbling time was related to the reduction of the OH − group contents evidenced by infrared (IR) absorption spectra, which demonstrated that the OH − groups were dominant quenching centers of the excited Er 3+. Meanwhile, by reducing the OH − group contents in the fiber cores with the same glass composition, the simulated gain characteristics improved dramatically and a 12.4 dB gain at 1560 nm was achieved in a 20 cm fiber with 150 mW pumping power at 980 nm.

Direct evidence of trap-mediated excitation in GaN:Er3+ with a two-color experiment

We report a direct confirmation of the generally proposed rare-earth excitation model in GaN. The intermediate step involving the formation of excitons bound to rare-earth ions prior to the rare-earth excitation is demonstrated. Using a two-color experiment, we show that the erbium photoluminescence decay following a pulsed laser excitation is quenched when an additional continuous-wave (cw) laser is applied. As the two beams excite the sample, the cw beam perturbs the intermediate step of the Er excitation mechanism either by photoionization of Er-related trapped carriers or by inducing an Auger effect between the trapped carriers and free carriers.

On relation between the 1.5 μm Er-related emission and 9 μm vibrational modes of oxygen in silicon

We demonstrate that excitation of Er 3+ ions embedded in crystalline silicon is effectively quenched when vibrational modes of interstitial oxygen are activated. In a two-color pump–probe experiment with a free-electron laser, we show that the 1.5 μm Er-related photoluminescence diminishes as the probing beam scans through the relevant 9 μm absorption band. Making use of time resolution of the experimental set-up, we investigate temporal aspects of this effect.

New approaches for enhancing light emission from Er-based materials and devices

In this work, we present some approaches recently developed for enhancing light emission from Er-based materials and devices. We have investigated the luminescence quenching processes limiting quantum efficiency in light-emitting devices based on Si nanoclusters (Si nc) or Er-doped Si nc. It is found that carrier injection, while needed to excite Si nc or Er ions through electron–hole recombination, at the same time produces an efficient non-radiative Auger de-excitation with trapped carriers. A strong light confinement and enhancement of Er emission at 1.54 μm in planar silicon-on-insulator waveguides containing a thin layer (slot) of SiO 2 with Er-doped Si nc at the center of the Si core has been obtained. By measuring the guided photoluminescence from the cleaved edge of the sample, we have observed a more than fivefold enhancement of emission for the transverse magnetic mode over the transverse electric one at room temperature. Slot waveguides have also been integrated with a photonic crystal (PhC), consisting of a triangular lattice of holes. An enhancement by more than two orders of magnitude of the Er near-normal emission is observed when the transition is in resonance with an appropriate mode of the PhC slab. Finally, in order to increase the concentration of excitable Er ions, a completely different approach, based on Er disilicate thin films, has been explored. Under proper annealing conditions crystalline and chemically stable Er 2Si 2O 7 films are obtained; these films exhibit a strong luminescence at 1.54 μm owing to the efficient reduction of the defect density.

Phase separation growth of Er 2SiO 5 thin film in Si-rich ErSiO preform

ErSiO crystalline compounds such as erbium silicates (Er 2Si 2O 7 and Er 2SiO 5) and Er 2O 3 have been attracting attention as the new light source material for silicon photonics. In our previous reports, several results indicated that the ErSiO crystalline compound was non-stoichiometry (Er:Si:O=1:2:2.4) and showed carrier mediated excitation. In our later study, however, it has been found the ErSiO crystalline compound contains Er 2SiO 5 (PDF#52-1809) crystalline matrix. ErSiO compounds made from the sol–gel solution of Er:Si=1:2 and Er:Si=2:1 show that the X-ray diffraction (XRD) peak patterns are in good agreement with the Er 2SiO 5. Rutherford back scattering (RBS) and secondary ion mass spectrometry (SIMS) results for the ErSiO compounds also show phase separation of Er 2SiO 5 and SiO 2. SiO 2-rich Er 2O 3–SiO 2 systems usually show phase separation of Er 2Si 2O 7 and SiO 2 but this system behaves in a different manner. Furthermore, bulk Er 2SiO 5 emits light only by direct excitation of Er. From PL excitation (PLE) measurements, the indirect excitation emission observed in the Si-rich non-stoichiometric ErSiO compound is considered due to Er 2SiO 5 crystallites embedded in SiO x , which can be formed by the phase separation.

Features of erbium nonradiative deexcitation and electroluminescence temperature quenching in sublimation MBE-grown Si:Er/Si diode structures

Temperature dependencies of electroluminescence intensity and decay kinetics from n-Si:Er:O/p-Si diodes, grown by sublimation molecular-beam epitaxy, have been studied. Radiative lifetime of excited Er 3+ ion (1.1 ms) and activation energy of the nonradiative deexcitation process (70 meV) have been measured. Contribution of nonradiative deexcitation of erbium ions to the temperature quenching of electroluminescence has been determined. It is revealed that considerable part of luminescence temperature quenching is due to the decrease in erbium excitation efficiency with an increase in temperature.

Excitation of Er3+ ions in SiO2 with Si nanocrystals

Probabilities of excitation of erbium ions via Coulomb interaction with carriers localized in silicon nanocrystals embedded in SiO2, in recombination and intraband relaxation of these carriers, have been calculated.

Excitable Er fraction and quenching phenomena in Er-dopedSiO2layers containing Si nanoclusters

This paper investigates the interaction between Si nanoclusters Si-nc and Er in SiO2, reports on the optical characterization and modeling of this system, and attempts to clarify its effectiveness as a gain material for optical waveguide amplifiers at 1.54 m. Silicon-rich silicon oxide layers with an Er content of 4 – 6 10 20 at./ cm 3 were deposited by reactive magnetron sputtering. The films with Si excess of 6 – 7 at. %, and postannealed at 900 ° C showed the best Er 3+ photoluminescence PL intensity and lifetime, and were used for the study. The annealing duration was varied up to 60 min to engineer the size and density of Si-nc and optimize Si-nc and Er coupling. PL investigations under resonant 488 nm and nonresonant 476 nm pumping show that an Er effective excitation cross section is similar to that of Si-nc 10 �17 –1 0 �16 cm 2 at low pumping flux 10 16 –1 0 17 cm �2 s �1 , while it drops at high flux 10 18 cm �2 s �1 . We found a maximum fraction of excited Er of about 2% of the total Er content. This is far from the 50% needed for optical transparency and achievement of population inversion and gain. Detrimental phenomena that cause depletion of Er inversion, such as cooperative up conversion, excited-stated absorption, and Auger deexcitations are modeled, and their impact in lowering the amount of excitable Er is found to be relatively small. Instead, Auger-type short-range energy transfer from Si-nc to Er is found, with a characteristic interaction length of 0.4 nm. Based on such results, numerical and analytical Er as a quasi-two-level system coupled rate equations have been developed to determine the optimum conditions for Er inversion. The modeling predicts that interaction is quenched for high photon flux and that only a small fraction of Er 0.2–2 % is excitable through Si-nc. Hence, the low density of sensitizers Si-nc and the short range of the interaction are the explanation of the low fraction of Er coupled. Efficient ways to improve Er-doped Si-nc thin films for the realization of practical optical amplifiers are also discussed.

Erbium Doped Materials for a Si-Based Microphotonics

We have investigated the role of the Si excess on the photoluminescence properties of Er doped substoichiometric SiO x layers. We demonstrate that the Si excess has two competing roles: when agglomerated to form Si nanoclusters (Si-nc) it enhances the Er excitation efficiency but it also introduces new non-radiative decay channels. When Er is excited through an energy transfer from Si-nc, the beneficial effect on the enhanced excitation efficiency prevails and the Er emission increases with increasing Si content. Nevertheless the maximum excited Er fraction is only of the order of percent. In order to increase the concentration of excited Er ions, a different approach based on Er silicate thin film has been explored. Under proper annealing conditions, an efficient luminescence at 1535 nm is found and all of the Er ions in the material is optically active. The possibility to efficiently excite Er ions also through electron-hole mediated processes is demonstrated in nanometer-scale Er-Si-O/Si multilayers. These data are presented and discussed.

Mid-infrared spectroscopy of the Er-related donor state in Si/Si:Er 3+ nanolayers

We present experimental evidence on the donor level related to optical properties of the Er 3+ ion in crystalline silicon. Using two-color spectroscopy with a free-electron laser we provide a direct link between the identified level in the bandgap and the optical properties of Er 3+. The investigation is performed in sublimation MBE-grown Si/Si:Er multinanolayer structure, which allows us to take advantage of the preferential formation of a single Er-related center. Quenching of the Er-related 1.5 μm photoluminescence, due to ionization of the donor state with energy E D ≈ 225 meV, is demonstrated. A microscopic model of the PL quenching mechanism as Auger type energy transfer between excited Er 3+ ions and free carriers optically ionized from the Er-related donor states is put forward.

Donor-State-Enabling Er-Related Luminescence in Silicon: Direct Identification and Resonant Excitation

We conclusively establish a direct link between formation of an Er-related donor gap state and the 1.5 microm emission of Er in Si. The experiment is performed on Si/Si:Er nanolayers where a single type of Er optical center dominates. We show that the Er emission can be resonantly induced by direct pumping into the bound exciton state of the identified donor. Using two-color spectroscopy with a free-electron laser we determine the ionization energy of the donor-state-enabling Er excitation as E(D) approximately 218 meV. We demonstrate quenching of the Er-related emission upon ionization of the donor.

The effect of hole confinement on photoluminescence from Er in SiGe/Si quantum wells

We have investigated the photoluminescence (PL) decay from Er implanted, or grown, into strained SiGe/Si multiple quantum well (MQW) structures grown by molecular beam epitaxy, and compared it with that of Er implanted into crystalline Si. The Er concentration in all MQW structures was of the order of 4 × 10 18 cm −3. Prior to the erbium implant, PL originating from confined states in the strained MQWs confirmed that this was an optically active MQW sample. The implanted MQW sample was re-grown at 550 °C for 5 h and X-ray diffraction measurements after re-growth showed that a high degree of strain had been retained. The Er emission at 1.54 μm was more intense from all MQW structures than from the implanted Si, but the PL decay time was not significantly different, suggesting that the excitation efficiency is improved when Er is in a quantum well structure and close to a long lived hole population, but the recombination efficiency is largely unaffected. When the excitation laser power density was increased by 10 times, the PL decay from the Er in the MQW structures was largely unaffected, but the PL decay time from Er in Si reduced; this is discussed in terms of non-radiative competition at end-of-range damage. It is proposed that the presence of a long-lived hole population increases the potential for efficient excitation of Er in the MQW structures.