Excitation Of Erbium Ions Research Articles

Photoluminescence excitation spectroscopy of erbium in epitaxially grown Si:Er structures

Abstract Excitation of erbium photoluminescence in Si:Er epitaxial structures has been studied within a broad pump wavelength range ( λ ex = 780–1500 nm). In all the investigated structures considerable signal of the 1.5 μm erbium photoluminescence has been observed at pump photon energies well below the silicon band-gap value ( λ ex > 1060 nm) where seemingly no exciton generation occurs. Possible mechanism of erbium ion excitation in silicon without participation of excitons is discussed.

Er3+ photoluminescence excitation spectra in erbium-doped epitaxial silicon structures

Excitation spectra of erbium photoluminescence (λ=1540 nm) in Si: Er epitaxial structures were studied within a broad pump wavelength range (λ=780–1500 nm). Erbium photoluminescence was observed to occur at pump energies substantially less than the silicon band-gap width. Possible mechanisms of erbium ion excitation in this pump radiation energy region are discussed.

Excitation mechanism of erbium photoluminescence in bulk silicon and silicon nanostructures

We present a short review of the theoretical and experimental results concerning the problem of excitation mechanism of erbium photoluminescence in silicon and silicon nanostructures. The excitation process consists of two stages, the first being absorption of radiation by bulk silicon matrix or nanocrystals while the second is the Auger excitation of erbium ions by recombining electron–hole pairs. The large values of Auger excitation cross-section under optical pumping in semiconductor matrices are due to large values of band-to-band absorption coefficient of bulk silicon or silicon nanocrystals exceeding by several orders of magnitude the absorption coefficient of erbium in dielectric SiO 2 matrix. The specific features of Auger process in silicon nanocrystals when excitation of erbium ions is produced by quantum-confined electron–hole pairs are discussed.

Auger deexcitation ofEr3+ions in crystalline Si optically induced by midinfrared illumination

We report on the de-excitation of ${\mathrm{Er}}^{3+}$ ions in crystalline silicon, induced by midinfrared radiation from a free electron laser. The effect is interpreted as an Auger energy transfer between excited erbium ions and free holes in the valence band. These are liberated from shallow traps by the powerful mid infrared laser beam. The traps are dynamically populated during the initial band-to-band excitation. The efficiency of the proposed de-excitation mechanism depends on the number of traps occupied at the moment when the free electron laser pulse is applied. Therefore the quenching effect is sensitive to the total number of acceptor traps present in the sample and the excitation density of the pump pulse. A competition between this de-excitation process and the previously reported mid infrared-induced Er photoluminescence enhancement is investigated. The optically induced Auger process, as revealed in this study, complements the description of energy transfer processes in the Si:Er system under optical pumping.

Increasing the efficiency of erbium-based sources using silicon quantum dots.

Silicon nanoclusters exhibit novel and interesting optical and electrical properties that are not observed in bulk silicon. Moreover, it has been discovered that there exists a strong coupling between nanoclusters and rare-earth ions that results in efficient energy exchange between the two species. This paper presents a review of recent work at University College London in this area, in which we have studied the optical properties of silicon nanoclusters in silica with rare-earth co-dopants and have developed a model for the excitation of erbium ions in erbium-doped silicon nanocrystals via coupling from optically generated excitons confined within the silicon nanoclusters. The model provides a phenomenological description of the exchange mechanism and allows us to evaluate an effective absorption cross-section for erbium that is up to four orders of magnitude higher than the corresponding value in stoichiometric silica. This paper also discusses the origin of the 1.6 eV emission band associated with the silicon nanoclusters and determines absorption cross-sections and excitonic lifetimes for nanoclusters in silica.

Silicon/silicon suboxide heterostructures grown by molecular beam epitaxy

Thin silicon suboxide (SiO x ) layers were fabricated in situ by molecular beam epitaxy. The layers were grown by introducing molecular oxygen up to a background pressure of 2×10 −6 mbar, while maintaining a silicon flux of 0.1 Å s −1. The layer thickness was varied between 2 and 200 nm. Ellipsometry and SIMS measurements indicate an oxygen content x between 1 and 1.6. The suboxide layers were overgrown with polycrystalline silicon. Transmission electron microscopy (TEM) measurements reveal a uniform amorphous suboxide layer with a sharp interface on the substrate side and some roughness on the surface side. Current–voltage characteristics of transport through such a layer show current blocking behavior up to ≈1.5 V for 10 nm layer thickness at low temperatures. At higher temperatures, the current at fixed voltage increases with an activation energy of ≈120 meV. It is demonstrated that the suboxide potential barrier can be used to enhance impact excitation of erbium ions in silicon by increasing the number of hot electrons with energies >0.8 eV.

Luminescence from erbium-doped silicon nanocrystals in silica: Excitation mechanisms

We develop a model for the excitation of erbium ions in erbium-doped silicon nanocrystals via coupling from confined excitons generated within the silicon nanoclusters. The model provides a phenomenological picture of the exchange mechanism and allows us to evaluate an effective absorption cross section for erbium of up to 7.3×10−17 cm2: four orders of magnitude higher than in stoichiometric silica. We address the origin of the 1.6 eV emission band associated with the silicon nanoclusters and determine absorption cross sections and excitonic lifetimes for nanoclusters in silica which are of the order of 1.02×10−16 cm2 and 20–100 μs, respectively.

Flashlamp pumping of erbium‐doped silicon nanoclusters

AbstractWe report recent results showing broad‐band excitation of erbium ions implanted into thin films of silica containing silicon nanoclusters. Indirect excitation of the rare‐earth ions is mediated by the nanoclusters, which are either grown in during plasma‐enhanced chemical vapour deposition of the films, or are formed by implantation of thermally grown SiO2 layers with Si+ ions. We demonstrate efficient flashlamp pumping of the erbium 1535 µm photoluminescence band and discuss the device implications of this material. Copyright © 2001 John Wiley & Sons, Ltd.

Broad-band and flashlamp pumping of 1.53 μm emission from erbium-doped silicon nanocrystals

We report recent results showing broad-band excitation of erbium ions implanted into thin films of silica containing silicon nanocrystals. Evidence for the existence of nanocrystals is presented in the form of HR-TEM images of crystalline regions of the thin films. Indirect excitation of the rare-earth ions is mediated by the nanocrystals, which are either grown in during plasma enhanced CVD of the films, or are formed by implantation of thermally grown SiO 2 layers with Si + ions. We demonstrate efficient flashlamp pumping of the erbium 1.535 μm PL band and discuss the device implications of this material.

Mechanism of electroluminescence in the amorphous silicon-based erbium-doped structures

We have studied electroluminescence (EL) in amorphous silicon-based erbium-doped structures at reverse bias in the temperature range 77–300 K. The intensity of electroluminescence at the wavelength of 1.54 μm corresponding to a radiative transition 4I 13/2→ 4I 15/2 in the internal 4f-shell of the erbium ion Er 3+ is low at 77 K but sharply increases starting from 220 K and exhibits a maximum near the room temperature. Theoretical analysis and comparison with the experiment have shown that the excitation of erbium ions occurs by an Auger process, which involves the capture of conduction electrons by neutral dangling bonds (D 0) defects located close to erbium ions. It is demonstrated that the stationary concentration of free electrons in the conduction band is kept by a reverse process being multiphonon tunnel ionization of erbium-induced donors and D −-centers by the applied electric field.

Photoluminescence of erbium-doped silicon: Excitation power and temperature dependence

Photoluminescence measurements have been made on float-zone and Czochralski-grown silicon samples which were doped with erbium by ion implantation. The characteristic luminescence spectra in the wavelength range between 1.5 and 1.6 μm were observed. Differences in the multiple line structure of the spectra indicated that the active luminescent centers have different symmetries and atomic structure. The dependence of the photoluminescence intensity on the laser excitation power and on the temperature was measured. Results are discussed on the basis of a physical model which includes the formation of free excitons, the binding of excitons to erbium ions, the excitation of 4f inner-shell electrons of the erbium ions, and their subsequent decay by light emission. To obtain a quantitative agreement between model analysis and experimental data the consideration of Auger processes by which erbium-bound excitons and erbium ions in excited state can decay by dissipating energy to conduction band electrons appears to be required. From the temperature dependence two activation energies are derived which are associated with the exciton binding energies and with an energy transfer process from excited erbium ions back to erbium-bound excitons, respectively. A good quantitative agreement can be obtained for suitable values of the model parameters. The luminescent properties of the samples of the different types of crystalline silicon are remarkably similar.

The photoluminescence mechanism of erbium in silicon: intensity dependence on excitation power and temperature

The photoluminescence intensity of erbium in silicon was measured as a function of laser excitation power and temperature. Results of these measurements are described on the basis of a physical model which includes the formation of free excitons, the binding of excitons to erbium ions, excitation of 4f-shell electrons of erbium ions and decay of excited erbium ions by light emission. An Auger energy transfer to free carriers by both erbium-bound excitons and excited erbium ions must be included in the model in order to obtain a quantitative agreement with experiment. From the temperature dependence two activation energies are derived, which are associated with the binding of excitons to erbium centers and with an energy transfer process from excited erbium ions back to erbium-bound excitons, respectively. The luminescence properties of the different types of Er-doped crystalline silicon are remarkably similar.

Mechanism of excitation of erbium electroluminescence in amorphous silicon

We have studied electroluminescence (EL) in the amorphous silicon-based erbium-doped structures in the temperature range 77–300K. The EL intensity at the wavelength of 1.54μm corresponding to a radiative transition in the internal 4f-shell of the Er3+ ion is low at 77K but sharply increases starting from 220K and exhibits a maximum near the room temperature. Measurements of the resistance of the electroluminescent structure as a function of temperature performed in parallel with the measurements of the EL intensity demonstrated a correlation in behavior of these two quantities: a pronounced decrease of the resistance occurs at the same temperature where the EL intensity starts to rise. Our results can be explained by the excitation of erbium ions via an Auger process which involves the capture of conduction electrons by neutral dangling bonds (D0) defects located close to erbium ions and thermally activated tunnel emission of electrons from deep donors to the conduction band that keeps the stationary current through the structure. A theoretical model proposed explains consistently all of our experimental data.

Effective Auger excitation of erbium luminescence by hot electrons in silicon

In an electroluminescent structure based on erbium-doped crystalline silicon we have found and studied a new mechanism of excitation of erbium ions involving Auger recombination of electrons from the upper subband of the conduction band with holes from the valence band. The new excitation mechanism is weak at low temperatures, but it is resonantly enhanced at 160K, when the energy distance of the edge of the upper subband of the conduction band from the valence band edge coincides with the energy of the second excited state of the erbium ion due to temperature shrinking of the silicon energy gap.

Efficient Auger-excitation of erbium electroluminescence in reversely-biased silicon structures

In p–n junctions based on c-Si:Er, we have observed strongly efficient excitation of erbium electroluminescence at 1.54 μm. Excitation of erbium ions is accompanied by strong recombination of free carriers indicating a participation of an Auger mechanism. A possible excitation mechanism is proposed which is the Auger recombination of electrons occupying the upper subband of the conduction band with free holes in the valence band, whereas the energy of the recombination process is transferred by Coulomb interaction to 4f electrons of an erbium ion transmitting it to the second excited state I411/2 (excitation energy 1.26 eV). The observed three-level excitation of erbium ions is promising for the development of a Si:Er laser.

Mechanisms of excitation and thermal quenching of erbium-ion luminescence in crystalline and amorphous silicon

A short review is presented of the erbium-ion excitation mechanisms in crystalline and amorphous silicon and of the processes governing thermal quenching of erbium luminescence in these materials, which draws both from the studies carried out by the present authors and from available literature data.

New efficient mechanism of excitation of electroluminescence from erbium ions in crystalline silicon

In p–n junctions based on c-Si : Er we have realized highly efficient excitation of erbium electroluminescence at 1.54 μm with an efficiency close to unity. A possible mechanism is Auger recombination of electrons occupying the upper subband of the conduction band with free holes in the valence band whereas the energy of the recombination process is transferred by Coulomb interaction to 4f-electrons of an erbium ion transmitting it to the second excited state 4 I 11/2 (excitation energy 1.26 eV). The observed three-level excitation of erbium ions is promising for development of a Si : Er laser.

Room-temperature electroluminescence from erbium-doped amorphous hydrogenated silicon

We have studied electroluminescence (EL) in the amorphous silicon-based erbium-doped structures under reverse bias in the temperature range 77–300 K. The intensity of electroluminescence at the wavelength of 1.54 μm exhibits a maximum near the room temperature. The excitation of erbium ions occurs by an Auger process which involves the capture of conduction electrons by neutral dangling bonds (D 0-centers) located close to erbium ions. The stationary current through the structure is kept by a reverse process of thermally activated tunnel emission of electrons from negatively charged dangling-bond defects (D −-centers) to the conduction band of the amorphous matrix. A theoretical model proposed explains consistently all of our experimental data.

Defect-related Auger excitation of erbium ions in amorphous silicon

A transition probability for defect-related Auger excitation (DRAE) of rare-earth ion inserted into amorphous matrix is calculated. The result is applied to excitation of an erbium ion in amorphous silicon occurring via capture of an electron by the dangling bond (D) defect. We have demonstrated high efficiency of the DRAE process which ensures photo- and electroluminescence of erbium ions in amorphous silicon matrix. It is shown that the temperature quenching of erbium luminescence in amorphous silicon is controlled by competition of the DRAE and the multiphonon nonradiative transitions.

Room-temperature electroluminescence of Er-doped hydrogenated amorphous silicon

We have observed room-temperature erbium-ion electroluminescence in erbium-doped amorphous silicon. Electrical conduction through the structure is controlled by thermally activated ionization of deep D − defects in an electric field and the reverse process of capture of mobile electrons by D 0 states. Defect-related Auger excitation (DRAE) is responsible for excitation of erbium ions located close to dangling-bond defects. Our experimental data are consistent with the mechanisms proposed.