Combined Excitation Emission Spectroscopy Research Articles

An efficiently excited Eu3+ luminescent site formed in Eu,O-codoped GaN

For the development of III-nitride-semiconductor-based monolithic micro-light-emitting diode (LED) displays, Eu,O-codoped GaN (GaN:Eu,O) is a promising material candidate for the red LEDs. The luminescence efficiency of Eu-related emission strongly depends on the local atomic structure of Eu ions. Our previous research has revealed that post-growth thermal annealing is an effective method for reconfiguring luminescent sites, leading to a significant increase in light output. We observed the preferential formation of a site with a peak at ∼2.004 eV by the annealing process. In this study, we demonstrate that it is a previously unidentified independent site (OMVPE-X) using combined excitation–emission spectroscopy and time-resolved photoluminescence measurements. In addition, we perform excitation power-dependent photoluminescence measurements and show that this OMVPE-X site dominates the emission at a low excitation power region despite its small relative abundance, suggesting a high excitation efficiency. Most importantly, applying our annealing technique to an LED exhibits a reasonably increased electroluminescence intensity associated with OMVPE-X, confirming that this site has a high excitation efficiency also under current injection. These results demonstrate the importance of OMVPE-X as a notable luminescent site for brighter and more efficient GaN:Eu,O-based LEDs.

Red Electroluminescence from Light Emitting Diodes Based on Eu-Doped ZnO Embedded in p-GaN/Al2O3/n-ZnO Heterostructures

Heterojunction p-GaN/n-ZnO light emitting diode (LED) structure using Eu-doped ZnO (ZnO:Eu) as an active component is demonstrated in order to realize low-cost and environmentally-friendly red LEDs with sharp linewidth and temperature stability against surrounding environment including operating temperature and injection current. Chemically stable Al2O3 is inserted as an electron blocking layer between p-GaN and ZnO:Eu/n-ZnO in order to facilitate the injection of carriers into the ZnO:Eu active layer. Al2O3 insertion with a moderate thickness (∼10 nm) facilitates the carrier recombination at the ZnO layer with comparatively low resistivity. Device characteristics of the p-GaN/Al2O3/ZnO:Eu/n-ZnO LED structures show red luminescence under current injection with reversed bias voltage originated from Eu3+ ions in the ZnO host. Detailed optical characteristics of the ZnO:Eu layer in the LED structures utilizing the combined excitation emission spectroscopy measurement enable the identification of the luminescence centers contributing to Eu luminescence under both indirect excitation and collisional excitation. The luminescence center contributing to Eu luminescence under indirect excitation via the ZnO host is different from that under collisional excitation, which would pave the way to understand the Eu luminescence mechanism in ZnO:Eu, and hence realize high-brightness LED structures based on rare-Earth doped ZnO as an active component.

Detection of In segregation in InGaN by using Eu as a probe

Eu ions were doped into InGaN by organometallic vapor phase epitaxy. The influence of In composition on the optical properties of Eu-doped InGaN (InGaN:Eu) was investigated using photoluminescence (PL), and combined excitation-emission spectroscopy (CEES). The PL spectra of InGaN:Eu showed a new series of emission lines, which became dominant with increasing In content. CEES measurements confirmed that these emission lines originated from a new Eu luminescent center, with In as a second nearest neighbor. Thus, it is possible to distinguish the typical Eu-N related luminescent sites from the new Eu-N-In related luminescent site. Based upon our findings, we propose that Eu3+ ions doped in InGaN could be used as a microscopic probe for the detection of In segregation.

Resonant oscillation language of a futuristic nano-machine-module: eliminating cancer cells & Alzheimer aβ plaques.

Nano-machine-module is designed and synthesized as a futuristic drug (PCMS) for cancer and Alzheimers by doping 2 Nile Red molecules in the cavity of a 5(th) generation PAM AM dendrimer P, and attaching 32 molecular rotors M, 4 pH sensors S on its surface. Molecular rotors and sensors enable the dendritic box surface to target specific sites, minimizing termination of healthy cells, e.g. cancer cells, nuclei acids (DNA) & spirals of Abeta Amyloid are disintegrated. Combined Excitation Emission Spectroscopy (CEES) shows directed energy transfer along M↔C↔S, this energy transmission path is itself an oscillation, and we image live resonant oscillation of the PCMS and the target molecular system. PCMS engages into resonant oscillations with spiral molecular structures. PCMS is designed to sense microsatellite instability & spirals with resonance frequencies in the kHz range. PCM is toxic, but the toxicity disappears as S is added to derive PCMS. PCMS does not even affect the dynamic instability of microtubule, a basic operator of living cells.

Effect of thermal annealing on luminescence properties of Eu,Mg-codoped GaN grown by organometallic vapor phase epitaxy

The effects of thermal annealing on Eu,Mg-codoped GaN (GaN:Eu,Mg) grown by organometallic vapor phase epitaxy were investigated. After annealing in nitrogen ambient, Eu-Mg related photoluminescence emission was quenched to 13% without a change in the spectral shape. The quenched emission recovered to 65% of the original intensity after a subsequent annealing in ammonia ambient. Combined excitation emission spectroscopy and time-resolved photoluminescence results revealed that the quenching behavior is attributed to a nonradiative process induced by unpassivated Mg acceptors in the relaxation of excited 4f electrons of Eu ions.

Observation and partly characterisation of silver nanoparticles in X-ray irradiated radiophotoluminescent phosphate glass

The direct observation and characterisation of silver nanoparticles or clusters embedded in a Ag +-activated phosphate glass was demonstrated. Correlations between the isolated Ag + concentration and X-ray-induced silver species were systematically examined using optical measurements. Evidence of spherical silver nanoparticles or clusters with sizes ranging from 5 to 30 nm was obtained from the lattice spacings and from energy dispersive X-ray spectroscopy combined with transmission electron microscopy. Three-dimensional contour plots using combined excitation–emission spectroscopy were also recorded to allow identification of the complicated spectroscopic features of the sample.

Crystal-field analysis and Zeeman splittings of energy levels of Nd3+ (4f3) in GaN

The crystal-field splitting and Zeeman splitting of energy levels of Nd3+ (4f3) doped into semi-conducting GaN (3.2 eV) grown in the hexagonal (huntite) phase by plasma-assisted molecular beam epitaxy have been modeled using a parameterized Hamiltonian defined to operate within the complete 4f3 electronic configuration of Nd3+ substituted for Ga3+ in the lattice. Zeeman splittings were obtained by applying magnetic fields up to 6.6 T with the fields parallel and perpendicular to the crystallographic c-axis. The experimental energy (Stark) levels were obtained from a recent spectroscopic study on the same samples, where the combined excitation emission spectroscopy (CEES) identified the majority of Nd3+ ions as replacing Ga3+ in sites of C3v symmetry. The manifolds of Nd3+ (4f3)2S+1LJ modeled for the crystal-field splitting include the ground state, 4I9/2, and excited states 4I11/2, 4I13/2, 4F3/2, 4F5/2, 2H9/2, 4F7/2, 4S3/2, 4G5/2, and 4G7/2. The energies of 41 experimental Stark levels from these manifolds were modeled through the use of a Monte Carlo method in which independent crystal-field parameters (CFP) were given random starting values and optimized using standard least-squares fitting between calculated and experimental Stark levels. Irreducible representations (irreps) and crystal field quantum numbers (μ) were assigned to the energy level states of the 4I9/2 and 4F3/2 multiplet manifolds based on an analysis of the Zeeman data. This allows determination of which of the competing local minima should be considered to be the physically significant minimum. Using standard least-squares fitting between calculated and experimental Stark levels for Nd3+ in C3v symmetry, we obtain a final standard deviation of 7.01 cm−1 (rms = 5.48 cm−1).

Near-infrared photoluminescence properties of neodymium in in situ doped AlN grown using plasma-assisted molecular beam epitaxy

We present luminescence spectroscopy measurements of in situ Nd doped AlN grown by plasma-assisted molecular beam epitaxy. A Nd concentration as high as 0.08 at. % is incorporated into the host material. The Nd incorporation efficiency within the AlN matrix is found to be highly sensitive to the Al flux but independent of the substrate temperature (between 800 °C to 950 °C). Photoluminescence, photoluminescence excitation, and combined excitation-emission spectroscopy (CEES) spectra are used to identify the Stark sublevels of the following manifolds: 4I9/2, 4I11/2, 4I13/2, 4F3/2, 4F5/2, 2H9/2, 4F7/2, 4S3/2, 4G5/2, and 4G7/2. A main Nd incorporation site and two minority sites are identified using CEES measurements.

Optical Investigation of Sm doped ZrO2

A samarium doped polycrystalline ZrO2 bulk sample was investigated for its phase composition as well as optical properties within the temperature range 6–300 K. From micro-Raman measurements the existence of tetragonal as well as monoclinic phase was confirmed. The emission spectrum of Sm3+ reveals drastic changes under excitation near the region of fundamental absorption. Combined excitation-emission spectroscopy (CEES) measurements reveal three different sites for the dopant ions to be present in the sample. Nature and excitation scheme of the three sites are discussed.

Site and sample dependent electron–phonon coupling of Eu ions in epitaxial-grown GaN layers

The incorporation of Eu ions into GaN has been studied using combined excitation–emission spectroscopy for samples that were in situ doped during organometallic vapor-phase epitaxy (OMVPE) growth. The obtained fingerprints of characteristic emission spectra are subsequently used to determine the coupling of the Eu ions to the crystal lattice. We find a majority site, which exhibits coupling to bulk-like phonon modes as well as a localized phonon mode. For this site, we also find that the zero-phonon line of the 7F 0 to 5D 0 transition, which is forbidden, is much weaker than the phonon-coupled excitation transitions. The ratio of zero-phonon to phonon-coupled transition strength depends on the crystalline quality of the layer. These observations are consistent with the assignment of the majority site to an unperturbed Eu ion on Ga position. We find that the relative abundance of the majority site is strongly underestimated whenever the zero-phonon 7F 0 to 5D 0 excitation transition is used as a measure.

Optical and magneto-optical properties of erbium doped InGaN and GaN epilayers

Using combined excitation–emission spectroscopy we have studied the erbium incorporation into GaN and InGaN for in situ doped MOCVD-grown layers and compared them to samples grown by MBE. A smaller up-conversion efficiency for the main site is observed compared to minority sites in the same sample as well as versus all sites from MBE grown samples. Furthermore, we show that the 1.54 μm emission efficiency is reduced with increasing In-content both under excitation above the bandgap in the UV as well as under resonant excitation at around 980 nm. This indicates that non-radiative decay channels for the Er ion are the largest contributing factor for this behavior. Finally, the Zeeman splitting of the excitation and emission lines of Er:GaN under application of magnetic fields up to 6.6 T with B|| c-axis is shown.

Advanced optical spectroscopy of luminescent color centers in lithium fluoride thin films

Lithium fluoride, LiF, is a well-known dosimeter material. In thin films the direct use of optical absorption spectra to individuate the stable formation of different kinds of point defects is often precluded by the presence of interference fringes due to the refractive index difference between film and substrate. Photoluminescence measurements are more sensitive. Recently a very effective investigation method is developing: it is often called combined excitation–emission spectroscopy (CEES) in the literature. In this work we present the basic characteristics of this technique and the first results of the investigation of polycrystalline LiF films grown by thermal evaporation on fused silica substrates and gamma irradiated at several doses up to 106 Gy in air.

Excitation pathways and efficiency of Eu ions in GaN by site-selective spectroscopy

Using combined excitation emission spectroscopy, we performed a comparative study of europium ions in GaN in samples that have been in situ doped during interrupted growth epitaxy (IGE) or conventional molecular beam epitaxy (MBE) as well as samples that were grown using organometallic vapor phase epitaxy (OMVPE) and subsequently ion implanted with Eu ions. Through site-selective resonant excitation, we are able to unambiguously assign all major observed transitions to a combination of different incorporation sites and electron–phonon coupled transitions. We identified at least nine different incorporation sites of Eu ions in GaN and studied how these sites behave under different excitation conditions and how their relative number is modified by different growth and doping conditions. The coupling to phonons has also been studied for a series of AlxGa1−xN samples with x=0…1. We find that a main site most resembling an unperturbed Eu ion on Ga site is always dominant, while the minority sites are changing substantially in relative numbers and can occur in some samples fairly close in emission intensity to the main site. In terms of the excitation pathway after the creation of electron-hole pairs, we found three types of centers: (1) sites that are dominantly excited through shallow defect traps; (2) sites that are excited through a deep defect trap; (3) sites that cannot be excited at all including the majority of the main sites. We interpret this finding to indicate that the ion in this environment is not very efficient in trapping excitation and that the indirect excitation involving other traps depends on the ion/trap distance. Many of the main sites are far away from these traps and cannot be excited through this channel at all. The efficiency of excitation is highest for the deep traps, indicating that it would be desirable to enrich the respective site, as has been done with some success in the IGE grown samples.

Crystal-field split levels of Nd3+ ions in GaN measured by luminescence spectroscopy

We present the Stark energy sublevels of Nd3+ ions in GaN grown by plasma-assisted molecular beam epitaxy as determined by luminescence spectra. We correlate the photoluminescence spectra with transitions from the F43/2 excited state to the I49/2, I411/2, and I413/2 multiplets of the Nd3+ ion for above and below bandgap excitation, with the strongest emission occurring at 1.12 eV (1106 nm). We determine a splitting of the F43/2 excited state to be 4.1 meV. From photoluminescence excitation spectra, we also identify the Stark sublevels of the upper states F45/2, H29/2, F47/2, S43/2, G27/2, and G45/2. Photoluminescence excitation spectra reveal an optimal excitation energy of 1.48 eV (836 nm). Site-selective spectroscopy studies using combined excitation-emission spectroscopy with confocal microscopy indicate enhanced substantial doping at the Ga site.

Spatially Resolved Site Selective Optical Spectroscopy on Nd Doped GaN Epitaxial Layers

AbstractDue to its favorable electronic and thermal properties GaN has been considered as a rare-earth host material for solid state amplifier and laser applications. To this end, we performed spatially resolved combined excitation emission spectroscopy (CEES) on Nd ions which were in-situ-doped into GaN epitaxial films grown by plasma assisted molecular beam epitaxy (PA-MBE) on c-plane sapphire substrate. For a wide range of concentration (up to 8at%) we find in the emission a dominant incorporation site, which can be identified with good certainty as a substitutional ‘Ga’ site. Energy levels and electron-phonon coupling to a localized mode can be identified. For the majority site, confocal spectral imaging under selective excitation show changes in emission intensity, excitation and emission wavelength on a submicron length scale suggesting spatial inhomogeneities in terms of Nd3+ ion concentration.

GaN Doped with Neodymium by Plasma-Assisted Molecular Beam Epitaxy for Potential Lasing Applications

AbstractWe provide an investigation of in situ doping of GaN with the RE element Nd by plasma assisted-molecular beam epitaxy (PA-MBE). GaN epilayers are grown on c-plane sapphire and free standing GaN substrates and the Nd doping is controlled by an effusion cell. The ideal growth conditions for Nd incorporation maintaining crystal quality in GaN were investigated. The optical absorption characteristics indicate that the GaN:Nd epilayer remains transparent at the Nd emission wavelength of interest. For the highest Nd effusion cell temperatures, Rutherford backscattering and secondary ion mass spectrometry data indicate ˜5 at. % in epilayers grown on c-plane sapphire. X-ray diffraction found no evidence of phase segregation up to ˜1 at. % Nd. The highest luminescence intensities correspond to a doping range 0.05-1 at. %, with the strongest emission occurring at 1.12 eV (1107 nm). We also present the Stark energy sublevels of Nd3+ ions in GaN as determined by luminescence spectra. Photoluminescence excitation spectra reveal an optimal excitation energy of 1.48 eV (836 nm). We correlate the photoluminescence spectra with transitions from the 4F3/2 excited state to the 4I9/2, 4I11/2, and 4I13/2 multiplets of the Nd3+ ion for above (325nm) and below (836nm) bandgap excitation. Spectral correlation of the Nd emission multiplets in addition to site-selective spectroscopy studies using combined excitation-emission spectroscopy with confocal microscopy indicate enhanced substantial doping at the Ga site compared to other techniques (ion implantation and co-sputtering).

Site‐specific excitation of Eu ions in GaN

AbstractWe performed cathodoluminescence (CL) studies on Eu‐doped GaN layers that were grown using the Interrupted Growth Epitaxy (IGE) method. We compare these results with our site‐selective combined excitation‐emission spectroscopy (CEES) studies in which numerous Eu3+ incorporation sites can be identified unambiguously. We find that for high beam currents the emission intensity saturates in a way that can only be accounted for by a coexistence of at least two different excitation processes. The ratio in excitation efficiency between these two processes is about 5. This conclusion is supported by the CL spectra which show less pronounced saturation for the majority substitutional incorporation site compared to the defect trap‐related (most likely interstitial) site. We further find that only a small fixed portion of the majority site can be excited through electron bombardment. We explain this behavior with a model for the excitation process in which defects are involved as an intermediate trap of excitation. Energy transfer can occur only to those substitutional Eu ions that are in close proximity to the trap. In the case that the Eu ion defect constitutes the trap itself, the spectra are clearly distinguishable and the transfer rates are highest. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Laser spectroscopy of Eu3+ centers in CaF2:Eu–CdF2 superlattices

Combined excitation–emission spectroscopy was carried out to study Eu 3+ centers in CaF 2 :Eu–CdF 2 superlattices. Two distinctive centers were observed and assigned as remote (R) and interface (I). The energy level of the R center appeared to be close to the cubic center in a bulk crystal and mainly consists of Eu 3+ ions located away from the SL boundaries. The relative intensity of the interface centers was very sensitive to the layer thickness and became more prominent in thinner layers indicating it is associated with Eu 3+ ions in layers next to the SL boundary. A splitting of the 5 D 1 manifold indicates the interface center has axial symmetry, which is suggested to come from a charge-compensating electron trapped in the conduction band of CdF 2 layer.

Site‐selective studies of erbium ion defects in thermally grown silicon oxides

AbstractUsing the site‐selective technique of combined excitation emission spectroscopy (CEES), we have studied a variety of Er‐doped silicon oxide layers and silicon‐rich oxide (SRO) layers which contain silicon nanocrystals. With this technique we identi.ed Er cluster defect sites which are created during thermal annealing and which dominate at high Er concentrations. We investigate the role that Si nanocrystals play in the relative abundance of these cluster sites. In attempts to reduce this clustering effect, we observed mixed results by modifying the growth procedure such that Er is already present in the silicon during oxide growth. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Identification of defect‐trap‐related europium sites in gallium nitride

AbstractWe performed site‐selective combined excitation‐emission spectroscopy (CEES) studies on Eu‐doped GaN layers grown using Interrupted Growth Epitaxy (IGE). We identified numerous Eu3+ incorporation sites, which exhibit different relative emission intensities as the growth conditions are varied. We found defecttrap related Eu sites that can be excited over a wide spectral range and that dominate the photoluminescence spectra for above‐bandgap excitation. Spectra obtained through above‐bandgap excitation, which simulates EL operation, are identical to the emission spectra of these defect‐trap related sites, indicating that these particular Eu ions would be the major contributors to electroluminescent (EL) emission in these samples. In resonant excitation, however, the emission from these sites is rather weak suggesting that these are minority sites. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)