Excitation Of Er Ions Research Articles

Guided wave absorption and fluorescence in epitaxial Er:BaTiO 3 on MgO

We report on the observation of ∼1.54 μm guided wave absorption and fluorescence in channel waveguides fabricated in erbium-doped thin-film epitaxial BaTiO 3. Luminescence transient measurements indicate that the relaxation of the excited Er ions is dominantly radiative.

Energy transfer between shallow centers and rare-earth ion cores:Er3+ion in silicon

Two-color optical spectroscopy is used to directly manipulate shallow levels available in Si:Er samples of different parameters. It is observed that the excitation energy provided by a laser operating in the visible can be temporarily stored by trapping photogenerated carriers at shallow centers available in the material. Subsequently, this energy can be transferred to the $4f$-electron core of the ${\mathrm{Er}}^{3+}$ ion in a trap ionization process induced by a mid-infrared pulse from a free-electron laser. In that way, Er-related luminescence at $1.5 \ensuremath{\mu}\mathrm{m}$ can be generated by an infrared pulse applied within several milliseconds after the band-to-band excitation pulse. By scanning the wavelength of the free-electron laser, ionization spectra of the shallow centers participating in the energy transfer are obtained, allowing for their identification. On that basis, the involvement of thermal donors is suggested. The results demonstrate that excitation of Er ions in Si is a multichannel energy transfer process where shallow centers play an important role.

Relaxation of optically excited 4f electrons in Er-doped Ga xIn 1− xP

Luminescence and relaxation properties of optically excited 4f electrons in Er-doped Ga x In 1− x P (0⩽ x⩽1) have been investigated by time-resolved measurements of photoluminescence (PL) due to intra-4f shell transitions in Er 3+ ions at 4.2 K. For x=1, the lifetime of Er-related emission falls in the sub-millisecond range. The lifetime of an emission line at 0.805 eV, which is observed throughout the entire range of x, decreases with increasing Ga content. These effects are explained by relaxation processes of excited Er ions which are perturbed by a crystal field around the ions.

Experimental determination of the upconversion rate in erbium-doped silicate glasses

A new experimental method for direct determination of the upconversion coefficient C(up) in Er-doped glasses is presented. This method is based on measurement of the dependence on pump power of the fluorescence quantum yield at 1.54 microm and of the excited-state absorption. Two methods are used for determination of the population of the excited Er ions. The first is based on the measurement of the variation in stimulated emission and on the variation in ground-state absorption for the (4)I(15/2)<-->(4)I(13/2) transition range. In the second, the excited-state absorption is measured for the (4)I(13/2) -->(4)F(7/2) transition. This method is successfully applied to Er-doped silicate glass fibers. The value of C(up) experimentally obtained is in good agreement with previously published results.

Understanding and control of the erbium non-radiative de-excitation processes in silicon

Erbium doping of silicon has recently emerged as a promising method to tailor the optical properties of Si towards the achievement of a light emission at 1.54 μm. In this paper we will review our recent work on this subject. In particular a detailed investigation of the non-radiative processes, competing with the radiative emission of Er in Si will be presented. Among these processes, an Auger de-excitation with the energy released to free carriers will be demonstrated to be extremely efficient, with an Auger coefficient C A∼4.4×10 −13 cm 3/s. Moreover, at temperatures above 100 K a phonon-assisted back-transfer decay process, characterized by an activation energy of 0.15 eV is seen to set in. This understanding of the physical properties competing with the radiative light emission allowed us to control them and obtain efficient room temperature luminescence. Two examples will be reported. It will be shown that by exciting Er within the depletion region of reverse biased p +– n + Si diodes in the breakdown regime it is possible to avoid Auger quenching and to achieve high efficiency. Moreover, at the switch off of the diode, when the depletion region shrinks, the excited Er ions become suddenly embedded within the neutral heavily doped region of the device. In this region Auger de-excitation with free carriers sets in and quenches rapidly the luminescence. This allows to modulate the light signal at frequencies as high as a few MHz. Furthermore, the introduction of Er within Si nanocrystals is demonstrated to be a promising way to eliminate back-transfer processes by a widening of the bandgap while maintaining the full advantage of the efficient electron-hole mediated excitation present in Si. These data are presented and future perspective discussed.

Erbium implantation in silicon: from materials properties to light emitting devices

Erbium implantation in silicon has recently emerged as a promising method to tailor the optical properties of Si towards the achievement of a light emission at 1.54 μm. In this paper we will review our recent work on this subject. In particular a detailed investigation of the non-radiative processes, competing with the radiative emission of Er in Si will be presented. Among these processes, an Auger de-excitation with the energy released to free carriers will be demonstrated to be extremely efficient, with an Auger coefficient C A~4.4×10 −13 cm 3 s −1. Using the knowledge on the material properties, an efficient Er implanted light emitting diode has been fabricated. It will be shown that by exciting Er within the depletion region of reverse biased p + -n + Si diodes in the breakdown regime it is possible to avoid Auger quenching and to achieve high efficiency. Moreover, at the switch off of the diode, when the depletion region shrinks, the excited Er ions become suddenly embedded within the neutral heavily doped region of the device. In this region Auger de-excitation with free carriers sets in and quenches the luminescence rapidly. This allows modulation of the light emitting devices at frequencies as high as a few MHz. These data will be presented and discussed.

Properties of Er-related emission in in situ doped Si epilayers grown by molecular beam epitaxy

Optical properties of in situ Er-doped Si epilayers grown by molecular beam epitaxy at low temperatures (&amp;lt;450 °C), using oxygen and fluorine as codopants, are studied using photoluminescence (PL) and electroluminescence (EL) spectroscopies. Sharp and intense Er-related PL is observed at low temperatures from the as-grown Si epilayers with Er concentrations up to 5×1019 cm−3. The structure of the dominant optically active Er centers is shown to be dependent of the codopants used and can essentially be modified by postgrowth thermal annealing. The chemical nature of the codopants as well as postgrowth treatments have only a minor effect on the thermal quenching of Er-related emissions. Thermal quenching of the Er-related PL and EL is shown to occur with a similar activation energy suggesting the same quenching mechanism may be involved for both processes. The observed higher EL efficiency at elevated temperatures is tentatively attributed to the higher concentration of excited Er ions under impact ionization conditions (EL).

Photoluminescence of erbium in amorphous hydrogenated phosphorus-doped silicon

The photoluminescence of erbium ions in phosphorus-doped a-Si:H films has been investigated. The observed increase in the Er photoluminescence with increasing defect density in the samples and the correlation of the temperature variation of the Er photoluminescence and defect-associated photoluminescence intensities are explained on the basis of a model of excitation of Er ions as a result of Auger recombination with defect participation.

Mechanism and performance of forward and reverse bias electroluminescence at 1.54 μm from Er-doped Si diodes

We have analyzed the mechanisms and the efficiency of the 1.54 μm electroluminescence from Er-doped crystalline Si. Optical doping of a 0.25 μm deep p+−n+ junction was achieved by multiple Er and O implants which realize a uniform concentration of 1019 Er/cm3 and 1020 O/cm3 from 0.2 to 0.9 μm from the surface. It has been found that, for the same current density passing through the device, the room temperature electroluminescence signal is 2–10 times higher under reverse bias at the diode breakdown than under forward bias. Detailed analyses of the spectrum line shape, temperature, and current density dependencies and modulation performances under both forward and reverse bias allowed us to elucidate the reasons for this difference. In forward bias, in spite of the large effective excitation cross section (&amp;gt;6×10−17 cm2 at 300 K), the efficiency of room temperature electroluminescence is limited by the small number of excitable sites (∼1% of the total Er concentration) and by the efficiency of nonradiative de-excitation processes of the Er ions. Furthermore, since in forward bias Er ions are excited by electron–hole recombination at an Er related level in the Si band gap, the electroluminescence yield is also reduced by competitive carrier recombinations at the residual defects left over after diode processing. On the other hand, under reverse bias, Er ions are excited by hot carrier impact excitation in the thin (∼70 nm) depletion layer. In this case all of the Er atoms in the depletion region are excitable and nonradiative de-excitation processes, such as Auger de-excitation to free electrons, are inhibited. This allows one to achieve an internal quantum efficiency of 1.5×10−4 at 300 K. Moreover, fast modulation of the diode can be achieved. At the diode turn-off, the excited Er ions are embedded in the heavily doped (∼1019/cm3) neutral regions of the diode where Auger-type de-excitation processes produce fast decay of the Er ions thus allowing to achieve modulation frequencies higher than 80 kHz. The major limitations to the achievement of a higher efficiency under reverse bias are the thin excitable region and the limited fraction of hot carriers having enough energy to impact excite the Er ions.

Impact Excitation And Auger Quenching Processes In Er Doped Light Emitting Si Devices

AbstractA detailed investigation of the Auger non radiative de-excitation processes, which compete with the radiative emission of Er in Si, will be presented. This process, in which the energy released by the Er de-excitation is transferred to free carriers, is demonstrated to be extremely efficient and characterized by an Auger coefficient CA˜4.4×l0−13 cm3/s. This Auger process and an efficient incorporation method have been used to improve the performances of Er implanted light emitting diodes. It will be shown that by exciting Er within the depletion region of reverse biased p+-n+ Si diodes in the breakdown regime, it is possible to avoid Auger quenching and to achieve high efficiency. Moreover, at the switch off of the diode, when the depletion region shrinks, the excited Er ions become suddenly embedded within the neutral heavily doped region. In this region Auger de-excitation with free carriers sets in allowing to modulate the light signal at frequencies as high as a few MHz.

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

We demonstrate that the electrical excitation of Er ions incorporated within the depletion layer of a p+−n+ Si diode allows one to simultaneously obtain efficient pumping of rare earth ions and a fast turnoff time of the electroluminescence signal. In fact it is found that during pumping, under reverse bias at the breakdown, a high internal quantum efficiency (10−4) can be achieved since the Er ions are excited with a cross section of 6×10−17 cm2 and exhibit a decay lifetime of 100 μs at room temperature. On the other hand, when the diode is turned off, the electroluminescence signal dies off in less than 10 μs (a limit set by the time response of the adopted detector). These results are explained by observing that fast nonradiative decay of the excited Er ions can occur by Auger transfer of the energy to a free electron or to an electron bound to an Er-related level in the bandgap. These processes are inhibited within the depletion layer and only set in when, at the turnoff, the excited Er ions are suddenly embedded within the heavily doped (∼1019/cm3) neutral region of the diode.

Cooperative upconversion in erbium-implanted soda-lime silicate glass optical waveguides

Soda-lime silicate glass has been locally doped with 0.2-at. % erbium by 3.0- and 5.0-MeV ion implantation. Single-mode fiber-compatible optical waveguides were then fabricated by use of Na+ ↔ K+ ion exchange. Characteristic photoluminescence (PL) of Er3+ centered at 1.54 μm is observed on excitation at 1.48 μm. For low pump intensity the PL decay is nearly single exponential with a lifetime of 7.2 ms. At high intensity it becomes nonexponential as a result of cooperative upconversion, an interaction between excited Er ions. Self-consistent modeling of the PL intensity and decay data yields an upconversion coefficient of 3.2 ± 0.8 × 10−24 m3/s. The effect of upconversion on optical gain is shown and discussed. An extrapolation of measured optical gain shows that 1 dB/cm of net gain is possible in the present Er-implanted soda-lime glass.

Impact excitation of Er-doped GaAs and the rare-earth sites in III-V compound semiconductors

ABSTRACTWe report the first study of impact excitation of Er ions in GaAs. The MOCVDgrown, p+-n structured EL devices were fabricated by growing, at different temperatures, GaAs:Er layers on top of the n+ GaAs substrates. P+ layers were made by Zn diffusion from the top surfaces. When we forward biased these diodes, their EL spectra were similar to their respective PL spectra for each sample but different from each other's. However, when we reverse biased these diodes, EL spectra obtained from all samples are the same, which were different from their PL spectra. These results indicate that the Er center(s) excited by direct impact is different from the Er center(s) excited through electron-hole recombination and subsequent energy transfer. By using RBS channeling, we found that most of the Er ions, in our MOCVD-grown GaAs:Er samples, occupy a displaced tetrahedral interstitial site. From these PL, EL and RBS results, we conclude that only a small amount of Er ions emit luminescence when they are indirectly excited through energy transfer.

Mechanism of the Sensitization Effect of Cu and Na Ions on Er Luminescence in ZnS: Cu, Er, Na Phosphors

In order to investigate the energy transfer process and the mechanism of the sensitization effect of copper and sodium tons on Er luminescence in ZnS: Cu, Er, Na phosphors, emission spectra, time-resolved emission spectra, transient behavior and dependence of intensities of green copper (G-Cu) and Er luminescence have been measured, It is confirmed that the excitation of Er ions is caused by the nonradiative energy transfer from the G-Cu centers to the Er centers. The experimental results show that (i) the copper-ions, which form the G-Cu centers, act as the sensitizor of Er luminescence, (ii) but the sodium ions do not act as the sensitizor, but act as the charge compensators for the trivalent erbium ions.