Excitation Of Er Ions Research Articles

Visible and infrared photoluminescence of Er-doped AlN films: Excitation of Er ions via efficient nonradiative energy transfer from the host level

Aluminum nitride (AlN) is a multifunctional semiconductor. AlN doped with rare earth ions has broad application prospects in monochromatic lighting, color display, laser medium, medical treatment, etc. In this paper, an Er3+-doped AlN film has been prepared by radio frequency magnetron sputtering and characterized by surface morphology, chemical composition and photoluminescence (PL), focusing on the emission mechanism of Er3+ in the AlN host. The films show PL in a wide range of the visible (540, 560 and 668 nm) and near-infrared (816, 869, 985 and 1534 nm) ranges. It has been found a certain energy transfer bridge between Er3+ and AlN defect-related optical transitions. At 532 nm excitation, VAl-ON vacancy-oxygen complex in the AlN host efficiently absorbs photons, then, nonradiative energy transfer (NRET) occurs from the defect levels to the levels of Er3+ since they are in resonance. Since there is a resonance of a donor emission level and an acceptor absorption level, Förster resonance energy transfer (FRET) occurs with the transfer efficiency about 51%. This results in a very strong enhancement of excitation of the 4f related PL. On the contrary, optical excitation at other energies results in a low intensity of the Er3+ PL. As a matter of facts, a strong absorption of the AlN host and the FRET-enabling resonance of donor-acceptor levels allow for a great enhancement of the PL efficiency of Er3+ ions. The results show that the impurity defect state of AlN has an important influence on the Er3+ luminescence efficiency.

Efficient carrier-envelope offset frequency stabilization through gain modulation via stimulated emission

A novel scheme for intracavity control of the carrier-envelope offset (CEO) frequency of a 100 MHz mode-locked Er:Yb:glass diode-pumped solid-state laser (DPSSL) based on the modulation of the laser gain via stimulated emission of the excited Er(3+) ions is demonstrated. This method allows us to bypass the ytterbium system few-kHz low-pass filter in the f(CEO) stabilization loop and thus to push the phase lock bandwidth up to a limit close to the relaxation oscillations frequency of the erbium system. A phase lock bandwidth above 70 kHz has been achieved with the fully stabilized laser, leading to an integrated phase noise [1 Hz-1 MHz] of 120 mrad.

Photophysics of resonantly and non-resonantly excited erbium doped Ge nanowires

We have fabricated Er doped germanium nanowires of different diameters by pulsed laser deposition and chemical methods. Er induced photoluminescence emission due to the intra-4f 4I13/2→4I15/2 transition of Er energy levels at 1.53 µm has been achieved at room temperature using both resonant (980 nm) and non-resonant (325 nm) excitation of Er ions. The observed 1.53 µm photoluminescence signal upon non-resonant 325 nm excitation is attributed to the Ge related oxygen deficiency centers surrounding the Ge core. For direct excitation, the infrared photoluminescence characteristics have been studied as a function of Er concentration, photon flux, and diameter of the nanowires. The Er related emission signal is found to be enhanced with increase in Er concentration, pump flux of 980 nm, and the nanowire diameter. The time resolved characteristics of the Er induced emission peak have been studied as a function of the pump flux as well as the diameter of the Ge nanowires.

Enhanced photoluminescence response of Er 3+–Si nanoparticle codoped Al 2O 3 films by controlled synthesis in the nanoscale and thermal processing

Nanostructured Er 3+–Si nanoparticles (NPs) codoped Al 2O 3 films were synthesized by a one step laser based deposition process which allows to form the Si NPs in situ at room temperature, and to control their size and separation with the Er ions in the nanoscale. Two different thermal annealing treatments are studied in order to optimize the photoluminescence (PL) emission: rapid thermal annealing (RTA) at 900 °C during 2 min, and conventional furnace step annealing at different temperatures up to 750 °C for 1 h. After RTA process the films show an important enhancement on the photoluminescence lifetime values which is related to a reduction of the non-radiative decay channels. Nevertheless, the Si NPs to Er ions energy transfer is strongly reduced. In contrast after conventional furnace annealing up to 700 °C, although there is only a moderate increase of the photoluminescence lifetime values, the excitation of Er ions through Si NPs is still active and as a consequence a large enhancement of the photoluminescence intensity with respect to the Er-only doped film is achieved. These different behaviours are most likely related to structural and chemical changes in the Er environment upon the different annealing processes.

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.

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.

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.

Synthesis, morphology and spectroscopy of cubic Y 3NbO 7:Er

Synthesis of Y 3NbO 7:Er powders with the aid of Li 2SO 4 flux is reported and spectroscopic properties of the resultant powders are presented. The dopant content varied in the range of 0.1–15 at%. The materials crystallized in the fluorite-type cubic structure in which all the metal ions—Y, Nb, and Er—randomly occupy the same site offered by the host lattice and the O-vacancy is also randomly distributed within the metal surrounding. Transmission electron microscopy images revealed that the agglomeration of particles is very low and the sizes of the grains are around 500 nm. Selected area electron diffraction patterns proved that each grain is monocrystalline. Absorption, excitation, and emission spectra are characterized by relatively broad structures related to the Er 3+ ion. The broadening results from some inhomogeneity of the activator ion surroundings related to the specific structure of the host lattice. When the Er content is only 0.1% both photoluminescence and up-converted emission are dominated by a green luminescent band around 550 nm. However, the efficiency of up-conversion is very low . With increasing concentration of the dopant, a red band located around 665 nm appears and becomes systematically stronger. In up-converted emission, the intensity of the red band surpasses the green one when the Er concentration exceeds 5%. For low concentrations, the up-conversion occurs through a sequential absorption of two infrared (IR) (980 nm) photons from the excitation beam by Er 3+ ion through excited-state absorption mechanism. For higher concentrations, the energy transfer between two neighboring excited Er ions plays dominant role. Surprisingly, the mechanism of up-converted low-intensity luminescence from 2H 11/2 state seems to diverge from the mechanism characteristic for the 4S 3/2 level, which conclusion comes from different slopes of the double-log relationships.

Maximum fraction of Er 3+ ions optically pumped through Si nanoclusters

By quantifying the amount of excited Er ions and considering the interaction distance between Si nanoclusters (Si-ncs) and the nearby Er ions, we were able to determine the maximum excitable Er concentration, when indirectly pumped through the Si-ncs. Our study was carried out on silica layers codoped with Si clusters and Er ions, grown by reactive magnetron sputtering. Si-rich silica layers were formed with different amounts of Si excess and doped to a wide range of Er concentrations. Thermal treatments were performed at 900 °C for different durations to maximize the Er emission at 1540 nm. The saturation limit of Er ions optically excitable through the Si-ncs was found about 3% of the total concentration. Simulations were carried out in order to explain this low value. The modelization takes into account the spatial distribution of the Er population, the measured size and density of Si clusters and the short-range character of the interaction between Er and Si-ncs. The small excitable fraction thus obtained appears as the ultimate intrinsic limitation to attain very high optical efficiency in such structures.

On the photoluminescence from Si nanocrystals in Er-doped silica by a double-pulse technique

The mechanism of photoluminescence (PL) from Si nanocrystals (Si-nc’s) in Er-doped silica is investigated by a double-pulse technique. It is shown that the decay time and the spectral intensity of PL emission from Si-nc’s do not change when Si-nc’s are reexcited by a delayed pulse in the presence of still excited Er ions. Results are compatible with a strong quenching of photoexcited Si-nc’s through trap states and/or Auger-like interaction between Si-nc’s and Er ions, both in the excited states, while PL emission originates from excitonic recombination in Si-nc’s that do not couple to Er3+.

Evidence of energy transfer from Si agglomerates to Er ions in aluminosilicate glass thin layers

This paper shows evidence of the possible excitation of Er ions in Si-rich aluminosilicate glass layers deposited by magnetron sputtering through an energy transfer from Si nanoagglomerates formed after annealing at 450 °C. The intensity of the Er emission was found unaffected by the change of the excitation line from resonant to nonresonant over a certain range of wavelengths. The time decay dynamics have been also determined using the 476.5-nm-off-resonant excitation. The lifetime values were found to range between 2 and 4.1 ms, while the effective excitation cross section reaches ∼4×10−6cm2, nearly 4 orders of magnitude higher than its counterpart for the direct excitation of Er3+ ions. These results enhance the potential use of aluminosilicate glasses for the fabrication of planar optical amplifiers.

Energy transfer to Er 3+ ions in a‐Si 1− x C x :H alloys: emission at 1.54 mm wavelength

The energy transfer to Er3+ ions in a-Si1−xCx:H films has been studied by absorption, luminescence and magnetic resonance spectroscopy. Erbium is incorporated either by magnetron assisted co-sputtering of Si and Er in a silane plasma or by conventional plasma deposition of a metalorganic compound which yields films of very different defect density and Er environment. The data confirm Er ions as efficient radiative recombination centres in a-Si:H, which are excited by resonant energy transfer from electron hole pairs in the host and disprove that dangling bonds are needed for emission at 1.5 μm wavelength by Erbium ions. We propose a Forster energy transfer, which is based on resonant dipole coupling. Resonance to excited states of Er3+ is achieved as electrons thermalise in tail states and form pairs with localized holes and enables rapid energy transfer to Erbium ions while back transfer is inefficient due to lattice relaxation. Thermalisation and recombination rates in the host material control the excitation of Er ions and can be modified by alloying silicon with carbon. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Rare-earth doped Si nanostructures for Microphotonics

AbstractIn the present paper, we will review our work on rare-earth doped Si nanoclusters. The samples have been obtained by implanting the rare-earth (e.g. Er) in a film containing preformed Si nanocrystals. After the implant, samples have been treated at 900°C for 1h. This annealing temperature is not enough to re-crystallize all of the amorphized Si clusters. However, even if the Si nanoclusters are in the amorphous phase, they can still efficiently transfer the energy to nearby rare-earth ions. We developed a model for the Si nanoclusters-Er system, based on an energy level scheme taking into account the coupling between each Si nanocluster and the neighboring Er ions. By fitting the data, we were able to determine a value of 3×10−15cm3s−1for the Si nanocluster-Er coupling coefficient. Moreover, a strong cooperative up-conversion mechanism between two excited Er ions and characterized by a coefficient of 7×10−17cm3s−1, is shown to be active in the system, demonstrating that more than one Er ion can be excited by the same nanocluster. We show that the overall light emission yield of the Er related luminescence can be enhanced by using higher concentrations of very small nanoaggregates. Eventually, electroluminescent devices based on rare-earth doped Si nanoclusters will be demonstrated.

Er doped Si nanostructures

In the present paper, we will review our recent work on Er-doped Si nanoclusters. We demonstrate that amorphous as well as crystalline Si clusters are efficient sensitizers for the Er 3+ luminescence at 1.54 μm. The samples have been obtained by implanting Er in a film containing pre-formed Si nanocrystals. After the implant, samples have been treated at 900 °C for 1 h, in order to activate Er 3+. It is shown that this annealing temperature is not enough to re-crystallize all of the amorphized Si clusters. However, even if the Si nanoclusters are in the amorphous phase, they can still efficiently transfer the energy to nearby Er ions. We present a model based on an energy level scheme taking into account the coupling between each Si nanocluster and the neighboring Er ions. By fitting the data, we were able to determine a value of 3×10 −15 cm 3 s −1 for the Si nanocluster–Er coupling coefficient. Moreover, a strong cooperative up-conversion mechanism, active between two excited Er ions and characterized by a coefficient of 7×10 −17 cm 3 s −1, will be shown to be active in the system, demonstrating that more than one Er ion can be excited by the same nanocluster. In addition, we found that the overall light emission yield of the Er related luminescence can be enhanced by using higher concentrations of very small nanoaggregates. Finally, room temperature operating electroluminescent devices based on Er-doped Si nanoclusters will be demonstrated.

Coupling and Cooperative Up-conversion Coefficients in Er-doped Si Nanocrystals

AbstractIn the present work, a quantitative understanding of the Er-doped Si nanocrystals interaction is reported. We present a model based on an energy level scheme taking into account the coupling between each Si nanocrystal and the neighboring Er ions. By fitting the steady state and time resolved luminescence signals at both the 1.54 and 0.98 μm Er lines we were able to determine a value of 3×10-15cm3s-1for the coupling coefficient. Moreover, a strong cooperative up-conversion mechanism, active between two excited Er ions and characterized by a coefficient of 7×10-17cm3s-1, will be shown to be active in the system, demonstrating that each Si nanocrystal can actually excite more than one Er ion.

Peculiarities of photoluminescence of erbium in silicon structures prepared by the sublimation molecular-beam epitaxy method

The photoluminescence of semiconducting structures Si: Er: O/Si grown by the molecular-beam epitaxy method is studied. The dependences of Er photoluminescence intensity on the intensity of pumping are measured at the liquid helium temperature. An analysis of the experimental results on the basis of the exciton model of excitation of Er ions in a crystalline silicon matrix reveals the significant role played by an alternative channel of free-exciton trapping (apart from the donor energy levels of erbium-oxygen complexes), as well as that played by the nonradiative channel in the recombination of excitons, bound to erbium donors, without the excitation of erbium. The ratio of the concentration of optically active centers of erbium luminescence to the total concentration of introduced erbium is estimated.

Excitation and non-radiative de-excitation processes in Er-doped Si nanocrystals

Our recent work on the interaction among Er ions and Si nanocrystals (nc) is presented. Silicon nanocrystals embedded within SiO 2 were produced by plasma enhanced chemical vapor deposition followed by high temperature annealing. Erbium ions were then introduced at different concentrations by implantation and a final annealing at 900°C was performed to eliminate the implantation damage. These samples exhibit a room temperature photoluminescence (PL) orders of magnitude higher than both Er in SiO 2 and in crystalline Si. A detailed study of the excitation and de-excitation processes demonstrated that Er is efficiently excited through an exciton recombination in the nc which act as sensitizers for the Er excitation. Indeed, when an Er ion is located close by a nc will become dark transferring preferentially its energy to it. An effective cross-section for Er excitation through this process has been determined to be ∼1×10 −16 cm 2, a value orders of magnitude above that of Er in insulating hosts. Moreover, the de-excitation processes typically present for Er in crystalline Si are absent in this case. We have identified the concentration quenching and the Auger interaction of excited Er ions with excited nc as the main non-radiative de-excitation processes. These data will be presented and their implications discussed.

Correlation between photoluminescence efficiency and density of paramagnetic defects in Er-doped hydrogenated amorphous silicon

Photoluminescence and electron paramagnetic resonance in layers of hydrogenated amorphous silicon doped with erbium and oxygen have been investigated. The intensity of Er-related photoluminescence (1.54 μm) depends non-monotonically on the ratio between erbium and oxygen content varied from 0.1 to 3. The photoluminescence efficiency correlates with the density of spin centers (most likely Si dangling bonds (DBs)) that gives evidence for the role of Si DBs in the electronic excitation of Er-ions.

Electroluminescence studies of Er and SiO co-doped Si layers prepared by molecular beam epitaxy

Er/O co-doped Si light emitting diodes (LEDs) have been fabricated using layer structures prepared by molecular beam epitaxy (MBE). The Er/O doping was realized by sublimation of elemental Er and silicon monoxide simultaneously with Si during MBE growth. Intense Er-related electroluminescence (EL) at 1.54 μm was observed at room temperature from p +-SiGe/i-SiGe-Si/Si:Er/n +-Si LEDs by electron impact excitation under reverse bias. It has been found that the EL intensity was increased with increasing growth temperature of the Si:Er/O layer in the range of 430–575°C. The electrical pumping power dependence of EL intensity has been studied. An excitation cross section value of ∼1×10 −16 cm 2 was estimated based on the experimental data and model fitting. The EL decay behavior under various injection and bias conditions has been studied by time-resolved EL measurements. The overall luminescence decay time is found to strongly depend on the injection parameters. Two types of de-excitation mechanisms due to Auger energy transfer to free carriers introduced by either dopant ionization or carrier injection have been discussed. Both Auger processes play an important role in reduction of the EL intensity when there is a high density of carriers with excited Er ions.