Excitation Of Rare-earth Ions Research Articles

Optical Properties of Europium-Doped Silicon-Based Thin Films

The integration of a silicon-based light emitter into existing CMOS technology has long been intriguing due to its optoelectronic compatibility with microelectronics [1]. However, the indirect band gap nature of bulk silicon has hindered its effectiveness as a light emitter. In addressing this limitation, rare earth ions have emerged as significantly interesting candidates owing to their unique optical and electronic properties. Rare earth-doped silicon structures have earned special attention as they exhibit sharp light emission in different spectral regions [2] . This notable feature is attributed to the effective excitation of rare earth ions within the host matrix. Efficiently excited rare earth ions can produce visible emissions ranging from infrared to ultraviolet, presenting possibilities in diverse applications such as solid-state lighting, displays, lasers, photovoltaics, and optical communication. The incorporation of rare earth-doped silicon structures presents a promising solution to overcome the challenges posed by silicon's intrinsic properties, thereby paving the way for improved performance across a diverse range of optoelectronic applications. Europium is a attractive rare earth material with two optically active states, Eu2+ and Eu3+, enabling it to generate a diverse range of color emissions extending from blue to red depending on the surrounding matrix [3].In this study, we investigated the optical properties and compositions of europium (Eu)-doped thin films including silicon oxide, silicon oxynitride, and silicon carbonitride. For this purpose, thin films were fabricated by electron cyclotron resonance plasma-enhanced chemical vapor deposition (ECR-PECVD) with in-situ magnetron sputtering on p-type 3" Si (100) substrates. In-situ Eu doping was performed by a radio frequency (RF) magnetron sputtering gun, using a 0.25 in. thick, 2 in. (diameter) 99.9% pure Eu sputtering target. Precursor gases, including silane (diluted in 90% argon), oxygen (diluted in 90% argon), nitrogen (diluted in 90% argon), and ethane were utilized. Annealing was performed on the as-deposited films over a broad temperature range, from 600° to 1100°C, in a nitrogen (N2) environment. Rutherford backscattering spectrometry (RBS) was performed to determine the atomic concentration of the film constituents. Variable angle spectroscopic ellipsometry (VASE) analysis was conducted to investigate the optical properties of the films. Room temperature photoluminescence (PL) experiments were performed using a laser diode excitation source operating at a wavelength of 375nm. Notably, bright visible emission was observed in some of the thin films. Finally, we discuss the influence of the atomic concentration of Eu and the annealing temperature on the emission properties observed in the photoluminescence experiments.[1] F. Azmi, Y. Gao, Z. Khatami, and P. Mascher, “ Tunable emission from Eu:SiOxNy thin films prepared by integrated magnetron sputtering and plasma enhanced chemical vapor deposition ,” J. Vac. Sci. Technol. A, vol. 40, no. 4, p. 043402, 2022, doi: 10.1116/6.0001761.[2] A. Brik et al., “Annealing Effects on Structural Characteristics of Europium Doped Silicon-Rich Silicon Nitride,” Silicon, vol. 14, no. 14, pp. 8417–8425, 2022, doi: 10.1007/s12633-021-01636-w.[3] D. Li, X. Zhang, L. Jin, and D. Yang, “Structure and luminescence evolution of annealed Europium-doped silicon oxides films,” Opt. Express, vol. 18, no. 26, p. 27191, 2010, doi: 10.1364/oe.18.027191.

Strain-mediated ion–ion interaction in rare-earth-doped solids

It was recently shown that the optical excitation of rare-earth ions produces a local change of the host matrix shape, attributed to a change of the rare-earth ion’s electronic orbital geometry. In this work we investigate the consequences of this piezo-orbital backaction and show from a macroscopic model how it yields a disregarded ion–ion interaction mediated by mechanical strain. This interaction scales as , similarly to the other archetypal ion–ion interactions, namely electric and magnetic dipole–dipole interactions. We quantitatively assess and compare the magnitude of these three interactions from the angle of the instantaneous spectral diffusion mechanism, and re-examine the scientific literature in a range of rare-earth doped systems in the light of this generally underestimated contribution.

Adaptive four-level modeling of laser cooling of solids

Laser cooling of rare-earth doped solids has been demonstrated across a wide range of material platforms, inspiring the development of simple phenomenological models such as the four-level model to elucidate the universal properties of laser cooling under various operating conditions. However, these models usually require the input of full absorption spectra that must be provided experimentally or by additional complicated atomic modeling. In this Letter, we propose that a four-level model, when extended to admit effective energy levels adaptive to the pumping photon energy, can accurately predict the cooling efficiency as a function of temperature and pumping frequency using only few inputs such as the absorption coefficient measured at a single frequency and temperature. Our model exploits the quasi-equilibrium properties of the excitation of rare-earth ions for the determination of the effective four energy levels. The model is validated against published experimental results for a span of materials including ytterbium/thulium-doped glass and crystals. With the verified model, we derive explicit expressions for the optimal frequency and the operating bandwidth of pumping lasers. Our model significantly simplifies the modeling process of laser cooling and is expected to stimulate further development of optical refrigeration.

Study of the energy transfer process in rare earth-doped silk fibroin for future application in luminescent compounds

The use of rare earth (RE)-doped materials in photonics; e.g., in solid-state lasers in the UV–vis NIR spectral region, in light emitting devices, and in fibers for optical amplifiers and data storage systems, is well known. Combining the mechanical and optical properties of silk fibroin (SF) with the multifunctionality of rare earth ions could be an interesting strategy to develop new, distinguished photonic devices. For this reason and given that no studies about light emission in RE-doped silk fibroin exist, here we present an innovative approach to develop photonic devices based on SF doped with RE ions, and we employ europium ion as a structural probe in a RE-doped composite SF matrix to obtain systems with better emission parameters. To this end, we prepared self-supported films consisting of RE-doped SF. The Bombyx mori SF bears aromatic amino acids, such as Tyrosine (Tyr) and Tryptophan (Trp), which display fluorescent behavior. These amino acids can function as fluorescent probes of physicochemical properties. In the presence of RE ions, these amino acids can act as sensitizer in energy transfer processes. Our results revealed RE ion emission associated with an antenna effect elicited by the aromatic amino acids. We verified distinct photophysical properties for different RE ions. To understand the process, we determined the triplet state of Trp in SF, which helped to describe the energy process. The energy of the Trp triplet state allowed quite efficient energy transfer from Trp to the Tb3+ ion. In conclusion, this research work improved our understanding of the mechanism of RE ion excitation in SF matrix. The present results will support future work on the development of new photonic systems.

(Invited) Light Management in Efficient Luminescent Si-Based Layers for Photovoltaic Applications

Energy harvesting and saving are the key issues for this century. Many approaches, developments and efforts have been devoted these last years to propose innovative solutions that can offer a way to decrease the CO2production as well as the energy consumption, or to increase the green energy use. Among the different solutions, such as energy storage, energy conversion, fuel cells, the development of luminescent materials with high efficiency in a low cost process is one of the challenges to face. Indeed, luminescent materials are widely used in sensors, scintillators, light emitting diodes as well as photovoltaic applications. In most cases, the photon emission is achieved by means of rare earth ion excitation mechanism that offers a wide range of emission wavelengths ranging from the UV to the infrared. For the Silicon PV industry, increasing Si solar cell efficiency with the aim of always keeping a low cost process is a key issue to continuously decrease the cost of the power generation. This is the condition to remain a major player in the provision of power generation solutions in the forthcoming years. Among the different loss paths in a Si solar cell, the thermalization effect due to the mismatch between the solar spectrum energy range (UV region) and the solar cell band gap energy (1.1 eV of the Si solar cell) can be overcome. To achieve such a goal, frequency conversion layers so-called Down Conversion (DC) layers have been developed in order to convert one UV incident photon into two IR ones that can be absorbed by the Si cell. To reach this objective many systems using a couple of trivalent ions such as Pr-Yb, Tb-Yb have been developed. Unfortunately, their major drawbacks are either a non Si-compatible process due to the nature of the host matrix or a low absorption cross section of the rare earth ions that limits their excitability in the solar spectrum range. This problem can be solved by the development of a Si-PV compatible host matrix containing sensitizers that efficiently excite rare earth ions. This paper details the development a Si-PV compatible host matrix that can either contain sensitizers that efficiently absorb the solar spectrum for exciting rare earth ions and/or favors the incorporation of a high content of rare earth ions without the detrimental clustering effect. We will describe the fabrication and study of Tb:Yb and Ce:Yb doped Si-based thin films deposited by reactive co-sputtering technique. First Tb- or Ce-doped systems have been optimized to get the maximum emission intensity under UV range excitation prior to incorporating the Yb3+ions. Quantum efficiency as high as 200% has been achieved for these systems either in a composite or a multilayer structure. The fabrication parameters have been optimized to improve the coupling rate between the rare earth ions to get the maximum of emitted photon at 980 nm. The use of metallic nanoparticules has been developed to increase the optical path length of pumping photons and thus improve the efficiency of the system. External Quantum Efficiency measurements on a solar cell are carried out and will be presented. This work has been supported the French Research National Agency through the project GENESE (N° ANR-13-BS09-0020-01).

Efficient energy transfer from inserted CdTe quantum dots to YVO4:Eu3+ inverse opals: a novel strategy to improve and expand visible excitation of rare earth ions

Rare earth (RE)-based phosphors demonstrate sharp emission lines, long lifetimes and high luminescence quantum yields; thus, they have been employed in various photoelectric devices, such as light-emitting diodes (LEDs) and solar spectral converters. However, their applications are largely confined by their narrow excitation bands and small absorption cross sections of 4f-4f transitions. In this paper, we demonstrate a novel strategy to improve and expand the visible excitation bands of Eu(3+) ions through the interface energy transfer (ET) from CdTe quantum dots (QDs) to YVO₄:Eu(3+) inverse opal photonic crystals (IOPCs). The significant effects observed in the CdTe QDs/YVO₄:Eu(3+) IOPCs composites were that the excitation of Eu(3+) ions was continuously extended from 450 to 590 nm and that the emission intensity of the (5)D₀-(7)FJ transitions was enhanced ∼20-fold, corresponding to the intrinsic (7)F₁-(5)D₁ excitation at 538 nm. Furthermore, in the IOPC network, the ET efficiency from the QDs to YVO₄:Eu(3+) was greatly improved because of the suppression of energy migration among the CdTe QDs, which gave an optimum ET efficiency as high as 47%. Besides, the modulation of photonic stop bands (PSBs) on the radiative transition rates of the QDs and Eu(3+) ions was studied, which showed that the decay lifetime constants for Eu(3+) ions were independent of PSBs, while those of QDs demonstrated a suppression in the PSBs. Their physical nature was explained theoretically.

Photoluminescent properties of LiSr xBa 1− xPO 4:RE 3+ (RE = Sm 3+, Eu 3+) f–f transition phosphors

Rare-earth ions (Sm 3+ or Eu 3+) doped LiSr x Ba 1− x PO 4 ( x = 0, 0.2, 0.4, 0.6, 0.8, 1.0) f–f transition phosphor powders were prepared by a high temperature solid-state reaction. The resulted phosphors were characterized by X-ray diffraction (XRD) and photoluminescence (PL) spectroscopy. The results of XRD indicate that the phase structure of the sample changes from LiBaPO 4 to LiSrPO 4 when x changes from 0 to 1.0. The excitation spectra indicate that only direct excitation of rare earth ions (Sm 3+ or Eu 3+) can be observed. The doped rare earth ions show their characteristic emission in LiSr x Ba 1− x PO 4, i.e., Eu 3+ 5D 0– 7F J ( J = 0, 1, 2, 3, 4), Sm 3+ 4G 5/2 → 6H J ( J = 5/2, 7/2, 9/2, 11/2), respectively. The dependence of the emission intensities of the LiSr x Ba 1− x PO 4:Sm 3+ and LiSr x Ba 1− x PO 4:Eu 3+ phosphors on the x value and Ln 3+ (Ln 3+ = Sm 3+, Eu 3+) concentration is also investigated.

Non-resonant excitation of rare-earth ions via virtual Auger process

The luminescence of rare-earth ions (REI) is often intensified by defects associated with REIs or excitons bound to these defects. In this paper we show that the presence of such a state opens the possibility of non-resonance optical pumping via the process involving virtual Auger transition. It is the second order perturbation process when an electron arrives in an virtual intermediate state due to the optical transition (the first step) and the Auger transition is the second one. We have calculated the cross-section of such an excitation process when the optical transition is accompanied by creation of the exciton bound to the defect associated with REI and obtained a simple analytical expression for the cross-section. The excess energy of the excitation quanta is taken away by multiphonon emission. The electron–phonon interaction with local phonon vibrations of the bound exciton is assumed to determine the multiphonon process. It is shown that the probability of the process under study exceeds considerably the probability of direct optical 4 f – 4 f absorption even in the case when the energy distance between the excitation quantum energy and the exciton energy is about 0.1 of the exciton energy. The excitation mechanism considered leads to the appearance of a broad unsymmetrical band in the excitation spectrum with the red side much wider and flatter than the blue one.

Optical and Structural Properties of an Eu Implanted Gallium Nitride Quantum Dots/Aluminium Nitride Superlattice

GaN/AIN structures made of GaN quantum dots (QDs) separated by AIN spacer layers, were doped with Europium by ion implantation. Rutherford Backscattering/Channelling measurements showed that Eu is incorporated mainly on near-substitutional cation sites within the superlattice region. Only slight deterioration of the crystal quality and no intermixing of the different layers are observed after implantation and annealing. After thermal annealing, photoluminescence associated with Eu3+ ions was observed. From its behaviour under different photon energy excitation and sample temperature we concluded that the Eu-related emitting centres are located inside the GaN QDs or dispersed in the GaN and AIN buffer or spacer layers. The 624 nm PL line, associated with Eu-doped GaN QDs, shows very low thermal quenching, suggesting recombination of confined carriers through rare-earth ion excitation.

MOS Light Emitting Devices Based on Rare-Earth Ion Implantation

In this article we will give an overview of our work devoted to Si-based light emission which was done in the last years. Si-based light emitters were fabricated by ion implantation of rare earth elements into the oxide layer of a conventional MOS structure. Efficient electroluminescence was obtained for the wavelength range from UV to the visible by using a transparent top electrode made of indium-tin oxide. In the case of Tb-implantation the best devices reach an external quantum efficiency of 16 % which corresponds to a power efficiency in the order of 0.3 %. The properties of the microstructure, the IV characteristics and the electroluminescence spectra were evaluated. The electroluminescence was found to be caused by hot electron impact excitation of rare earth ions, and the electric phenomena of charge transport, luminescence centre excitation, quenching and degradation are explained in detail.

Excitation Pathways of Rare Earth Ions by Energetic Electrons

AbstractUsing cathodoluminescence measurements, we studied the excitation of rare earth ions in LiNbO3and compare it with previously studied Eu ions in GaN. We find that in both hosts and all dopants the most efficient excitation pathways involves a defect traps that capture the energy from the electron-hole pairs created in the host by E-beam irradiation. Even then the excitation efficiency in LiNbO3is very low. Moreover, only a small fraction (<10-4) of the RE ions that are optically active can be excited through this pathway. We explain this behavior by a negligible direct excitation and a strong dependence of the excitation efficiency on the distance between the defect trap and the rare earth ion. The density of the defect traps is small such that many rare earth ions are far away from traps.

Visible emission from Er-doped SnO2 thin films deposited by sol-gel

Emission from Er-doped SnO2 thin film deposited via sol-gel by the dip coating technique is obtained in the range 500-700 nm with peak at 530 nm (green). Electron-hole generation in the tin dioxide matrix is used to promote the rare-earth ion excitation. Evaluation of crystallite dimensions through X-ray diffraction results leads to nanoscopic size, what could play a relevant role in the emission spectra. The electron-hole mechanism is also responsible for the excitation of the transition in the 1540 nm range in powders obtained from the same precursor solution of films. The thin film matrix presents a very useful shape for technological application, since it allows integration in optical devices and the application of electric fields to operate electroluminescent devices.

Analysis of Er3+ incorporation in SnO2 by optical investigation

Er3+ emission in the wide bandgap matrix SnO2 is observed either through a direct Er ion excitation process as well as by an indirect process, through energy transfer in samples codoped with Yb3+ ions. Electron-hole generation in the tin dioxide matrix is also used to promote rare-earth ion excitation. Photoluminescence spectra as function of temperature indicate a slight decrease in the emission intensity with temperature increase, yielding low activation energy, about 3.8meV, since the emission even at room temperature is rather considerable.

Photon and electron excitation of rare-earth-doped amorphous SiN films

Room temperature visible photo and cathodoluminescence have been achieved from amorphous ( a-)SiN films independently doped with Pr 3+, Sm 3+, Dy 3+, Ho 3+, and Er 3+ ions. The films were deposited by co-sputtering a Si target partially covered with metallic rare-earth (RE) in an atmosphere of pure nitrogen. As a consequence of the deposition method and conditions, the films have an amorphous structure and RE contents of 0.5 at.%, as determined by Raman scattering and ion beam analysis, respectively. The luminescence experiments were carried out in films, typically 0.5 μm thick, after excitation with either 488.0 nm photons or 15 keV electrons. All characterizations were accomplished at room temperature and on as-deposited samples. A detailed investigation of the luminescence spectra allowed the identification of all existing optical features and indicates possible RE ion excitation–recombination mechanisms. In addition to the importance of Si in modern (micro-)electronics, the current experimental findings suggest the potential of RE-doped SiN compounds to opto-electronic devices operating in the visible energy range.

GaN quantum dots doped with Eu

Molecular-beam-epitaxy growth of Eu-doped GaN quantum dots embedded in AlN has been achieved. The crucial issue of Eu location has been addressed by extended x-ray absorption fine structure measurements. By comparing the signature of the Eu short-range environment for several samples, it is concluded that Eu is mostly incorporated in GaN dots. Intense cathodoluminescence associated with Eu has been measured, with no GaN bandedge emission, evidence that carrier recombination mostly occurs through rare-earth ion excitation. Persistent photoluminescence of Eu-doped GaN quantum dots as a function of the temperature is suggested to be further confirmation of the recombination of confined carriers through Eu ion excitation.

Structural and Optical Properties of GaN Quantum Dots

ABSTRACTGrowth conditions, structural and optical properties of GaN quantum dots (QDs) grown by plasma-assisted molecular beam epitaxy will be examined. It will be shown that, depending on the Ga/N ratio value and on growth temperature, the growth mode of GaN deposited on AlN can be either of the Stranski-Krastanow or of the Frank-Van der Merwe type. It will be shown that vertical correlation results in a red shift and in a narrowing of the photoluminescence spectra.Growth of Eu-doped GaN quantum dots embedded in AlN will be described. Intense photoluminescence associated with Eu has been measured, with no GaN band-edge emission, as an evidence that carrier recombination mostly occurs through rare earth ion excitation. Persistent photoluminescence of Eu-doped GaN quantum dots as a function of temperature has been put in evidence, as a further confirmation of the recombination of confined carriers through Eu ion excitation.

Role of Coulomb repulsion in 4f orbitals in electrical excitation of rare-earth impurities in semiconductors

The role of the on-site Coulomb repulsion energy in the electrical excitation of rare-earth ions by minority carrier injection in semiconductors is examined. Resonant photoemission measurements of the interfacial electronic energy level alignment in NdF3/silicon heterojunctions show that the large Coulomb repulsion energy of the atomiclike 4f orbitals prevents the direct excitation of the 4f levels by either electron or hole capture.

Dynamical processes involving local modes for rare earth centers in CaF 2

H -, D - and T - ion local modes play a significant role in dynamical processes involving laser excitation of hydrogenic rare-earth ion centers in CaF 2 and SrF 2. The hydrogenic varieties of a particular center are found to have substantially reduced fluorescence lifetimes supportive of models for non-radiative decay to lower energy multiplets through the respective local mode phonons. Likewise, upconversion efficiencies are reduced for the hydrogenic single rare-earth ion centers and are found to vary with inter-center separation consistent with a recent model for single ion upconversion involving a weak coupling between nearby equivalent centers. The efficient channelling of the rare-earth ion excitation into the creation of local mode phonons can induce hydrogenic ion migration to produce re-oriented equivalent centers or new photoproduct centers with different excitation wavelengths.

Multiphonon-assisted anti-Stokes and Stokes fluorescence of triply ionized rare-earth ions

Experimental demonstration of multiphonon-assisted anti-Stokes and Stokes excitation of rare-earth ions (${R}^{3+}$) in different hosts, that is, in a weak-coupling ion-lattice interaction case, is obtained. An exponential behavior of excitation probabilities with respect to energy difference between excitation and electronic line is found, just as for multiphonon-assisted nonradiative decay and energy transfer. Exponential parameters ${\ensuremath{\alpha}}_{\mathrm{AS}}$ and ${\ensuremath{\alpha}}_{\mathrm{S}}$ for anti-Stokes and Stokes excitation are measured and linked to the Miyakawa-Dexter $\ensuremath{\alpha}$ parameter for decay, leading to estimates for Pekar-Huang-Rhys coupling parameter ${S}_{0}$ of about 0.04, in agreement with values obtained from one-phonon spectra or nonradiative-decay measurements. A semiempirical formula is deduced to obtain $\ensuremath{\alpha}$ from only one parameter, an effective phonon energy.

Comments on ``Multiphonon, Raman, and two-photon excitation of rare-earth ions in crystals: photon and phonon noise in 10-μm quantum counters''

An analysis made in a recent publication indicates that two-photon, Raman, and thermal excitation noise contributions in a 10-μm infrared quantum counter (IRQC) are insignificant compared to that due to the background (300 K) radiation. This conclusion is found to be unwarranted because of several errors in the analysis, foremost of which is an overestimation by 9 orders of magnitude of the transition rate Wα′αbir induced by background radiation. The revised calculation presented here demonstrates that the background radiation, far from being the dominant noise source, is at most comparable in magnitude with the other noise mechanisms.