eV Emission Band Research Articles

Luminescence and excited state dynamics in Bi3+-doped LiLaP4O12 phosphates

Photo- and X-ray-excited luminescence characteristics of Bi-doped LiLaP4O12 phosphates with different bismuth contents (from 1 to 25 at% in the melt) are investigated in the 4.2–300K temperature range and compared with the characteristics of the undoped LiLaP4O12 phosphate. The broad 2.95eV emission band of LiLaP4O12:Bi excited around 5.4eV is found to arise from the bismuth dopant. Relatively large FWHM and Stokes shift of the emission band and especially the data on the low-temperature decay kinetics of the 2.95eV emission and its temperature dependence, indicating a very small spin-orbit splitting energy of the corresponding excited state, allow the conclusion that this emission arises from the radiative decay of the triplet state of an exciton localized around a Bi3+ ion. No spectral bands are observed, arising from the electron transitions between the energy levels of Bi3+ ions. Phenomenological model is proposed for the description of the excited state dynamics of the Bi3+-related localized exciton in LiLaP4O12:Bi and the parameters of the triplet localized exciton state are determined.Keywords: Photoluminescence; Time-resolved spectroscopy; Excited states; Bi3+ centers; LiLaP4O12:Bi powders

On the Origin of the 4.7 eV Absorption and 2.8 eV Emission Bands in Bulk AlN Substrates

AlGaN alloys are the building blocks for deep UV optoelectronics and high-power devices. It has been demonstrated that the highest crystalline quality AlGaN films with high Al content are obtained on AlN single crystal substrates. The resulting mismatch between AlGaN and AlN causes a compressive strain within the AlGaN layers, which varies with composition. Nevertheless, pseudomorphic AlGaN films with Al content higher than 50%, and dislocation densities lower than 104 cm-2 have been achieved, sustaining compressive stresses with thicknesses exceeding 3 µm. Such results demonstrate the advantages of using AlN substrates for this technology, and at such some have been realized on several deep-UV optoelectronics applications. Kinoshita et al. have demonstrated UV LEDs emitting at 265 nm with output powers exceeding 80 mW. UV LEDs grown on these native substrates have higher reliabilities and higher output powers. In addition, optically pumped lasers emitting at wavelengths between 230 nm and 280 nm that display cavity modes and single polarized-state emission with low lasing thresholds have been developed. Nevertheless, there are several limitations related to the performance and further improvement of the active regions as related to their quantum efficiencies and the polarization of their emission is desired. These limitations will be classified in two main categories: (1) identification and control of point defects, and (2) efficient doping. One of the main limitations based on point defects is the one related to the UV absorption band present at 265 nm in AlN. This relatively broad absorption band limits the use of the substrate within the deep UV range and several complicated fabrication procedures were devised to overcome this limitation. In this work, the origin of this absorption band will be discussed, along with other related optical features and the use of co-doping as a way of removing this undesired absorption. Experimental and theoretical results that explore the optical properties of native and impurity point defects as well as point-defect complexes in AlN with respect to the 265 nm absorption band will be presented. Theoretical results, based on DFT, are compared to photoluminescence and absorption measurements of AlN samples grown by physical vapor transport, metal-organic chemical vapor deposition, and hydride vapor-phase epitaxy . Our results demonstrate that there are multiple sources of a deep-UV absorption between 4.5 and 5.0 eV. The presence of the isolated carbon substitutional CN - produces an absorption band centered at 4.7 eV and emission at 3.9 eV, thus its reduction leads to the reduction of the absorption band. In addition, one mechanism that contributes to the apparent UV transparency when the material is co-doped with Si is discussed. These schemes produced transparent AlN wafers suitable for deep UV LED production. The other limitation relates to obtaining technologically useful n-type conductivities in high Al-content AlGaN within the low and high doping regimes. Besides dislocation density reduction, identification and elimination of compensating defects are necessary to achieve this goal. AlGaN films with high Al mole fractions (0.65<x<1) were grown by MOCVD on single crystalline AlN and sapphire. It is found that with an increase in Al content, there was both a decrease in the free carrier concentration and mobility. The reduction in free carrier concentration for x>0.8 was primarily caused by an increase in activation energy. This is due to the formation of a Si DX-center, thus inspiring the search for alternative dopants. Preliminary data on Ge doping of AlGaN will be presented to assess its effectiveness as a donor throughout the whole composition regime. In addition, for samples grown with different Si doping levels it is observed that up to a critical Si concentration, the carrier concentration was always proportional to the Si source flow. Any further increase led to a free carrier concentration reduction. PL spectra exhibited deep defect-related peaks corresponding to transitions between Si and deep acceptor states. For Si concentrations less than an Al-content dependent critical concentration, a higher energy peak dominates the spectra. This peak was related to a (VAl-complex)2- based on DFT calculations. For Si concentrations greater than the critical concentration, a (VAl)3- related lower energy peak dominates the spectra. The energy levels associated with these defects increased with Al composition where the energy increase was the same as the increase in band gap. Acknowledgement: Partial financial support from NSF (DMR-1312582 and ECCS-1508854), DARPA (W911QX-10-C-0027), ARPA-E (DE-AR0000299), ARO (W911NF14C0008) is greatly appreciated. All PVT AlN wafers used for homoepitaxial growth were supplied by HexaTech, Inc., Morrisville, NC.

Ionoluminescence study of Zn− and O− implanted ZnO crystals: An additional perspective

An investigation into the role of native point defects on the optical properties of ZnO using ion implantation, rapid thermal annealing (RTA) and ionoluminescence (IL) is presented. Low-energy (60keV) fixed-fluence (8×1015cm−2) Zn− and O− implantation has been used to directly introduce native point defects into ZnO single crystals. It is shown that annealing of implanted samples in Ar at T=1000°C for 2min amplifies the deep band emission (DBE) peak centered around 2.4eV while at the same time revealing subtle differences not clearly resolved in similar implanted samples treated under prolonged annealing. Particularly, a relative shift in the DBE peak maxima of the O and Zn doped samples subjected to RTA is observed. Gaussian decomposition of the IL spectra show distinct enhancements of the red (1.62eV) and yellow (2.15eV) emission bands in the O− implanted sample and the green (2.36eV) emission band in the Zn− implanted sample. Based on these results, and recent density functional theory (DFT) calculations, we have proposed a possible energy level scheme for some common ZnO native point defects.

Synthesis and enhanced effect of vanadium on structural and optical properties of zinc oxide

In this work, we have prepared undoped and vanadium doped ZnO nanopowders by sol–gel method. The treated powder at 500 °C has an average particle size of 25 nm. Pellets have been prepared at sintered temperature of 900 °C in atmospheric furnace. The characterization of the obtained pellets has been made by the X-ray diffraction, the UV–visible spectrophotometer, and cathodoluminescence (CL). The XRD analysis shows that ZnO has hexagonal wurtzite structure with a preferential direction along (101). It has also revealed that grain size and lattice parameters have increased after vanadium doping. The optical band gap has been found to increase from 3.15 to 3.27 eV as the V content varied from 0 to 10 %. CL spectra have shown a broad yellow emission at 2.2 eV and UV emission band around 3.16 eV. Doping with vanadium has enhanced the CL intensity of near band emission.

Origin and significance of the yellow cathodoluminescence (CL) of quartz

The origin of yellow cathodoluminescence (CL) in quartz has been investigated by a combination of CL microscopy and spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, and spatially resolved trace-element analysis by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). The study shows that the appearance of a ~570 nm (2.17 eV) emission band can be attributed to high oxygen deficiency and local structural disorder in quartz. A proposed luminescence center model implies self-trapped exciton (STE) emission from localized amorphized regions in quartz. Although the high-intensity emission at 570 nm is in general consistent with high concentrations of E ′1 defects detected by EPR spectroscopy, CL studies with different electron beam parameters and annealing experiments up to 600 °C show a temperature and irradiation dependence of the luminescence related defects excluding the role of E ′1 centers as direct luminescence activators for the 570 nm emission. The evaluation of geochemical data shows that quartz with yellow CL occurs in low-temperature hydrothermal environment (mostly <250 °C) and is related to fast crystallization in an environment with oxygen deficiency.

Optical and luminescence characterization of LiBaAlF6 single crystals

Far ultraviolet optical, luminescence excitation and pulse cathodoluminescence spectroscopy methods were used to determine the electronic structure properties of undoped single crystals of LiBaAlF6 (LBAF). The spectrum of the refraction index in the ultraviolet–visible range was recorded. The photoluminescence excitation spectroscopy was carried out for both the excitonic (4.3eV) and fast vacuum ultraviolet (6.6eV) emission bands of LBAF single crystals at 10K. Modeling of the photoluminescence excitation spectra for excitonic emission was performed to characterize the low-energy tail of the host absorption of LBAF single crystals. The pulse cathodoluminescence spectroscopy method was used to characterize the fast vacuum ultraviolet (6.6eV) emission band and proof its origination from the radiative core-valence transitions F− 2p→ Ba2+ 5p.

Effect of the chemical impurities on the luminescence emission of natural apatites

This paper reports on both cathodoluminescence (CL) and blue thermoluminescence (TL) emission of well-characterized natural Spanish and Brazilian apatites [Ca5(PO4)3(OH, F, Cl)]. Chemical analyses performed by means of Electron Microprobe Analysis (EMPA) have shown the presence of trace elements that can induce CL bands. In this sense, the apatites shown emission bands peaked at 3.26, 2.86, 2.62, 2.14, 2.02 and 1.94eV are respectively linked to substitutional Ce3+, Tb3+, Dy3+, Pr3+, Sm3+ and Mn2+ in structural Ca2+ positions. The 3.18eV emission band can be associated with intrinsic electron defects on oxygen of the phosphate group (PO4)3−. The presence of (UO2)2+ gives rise to an emission at 2.14eV. All the studied aliquots exhibit one single UV–blue TL peak that modifies the position from one sample to another (370, 256 and 268°C) probably due to (i) the variation in the crystallinity index (from 0.88 to 1.34) and (ii) successive chemical processes such as oxidation, dehydration, dehydroxylation, and fluorine ions losses due to the thermal readout.

Cu Doping of CdTe Layers in Polycrystalline CdTe Thin-Film Solar Cells for Top Cells of Multijunction Solar Cells

The Cu doping of CdTe layers in CdTe solar cells without using a Cu-doped carbon electrode was investigated in order to develop transparent back contacts for multijunction solar cells. Cell performance was drastically improved by heat treatment after coating with Cu-doped diethylene glycol monobutyl ether (DEGBE) as compared with the cell without Cu doping. Capacitance–voltage (C–V) characteristics revealed that the acceptor concentration increased with increasing Cu concentration in DEGBE. These results suggest that Cu atoms are incorporated into the CdTe layer as acceptors by the novel Cu-doping method. Furthermore, in photoluminescence (PL) spectra of CdTe layers, the intensity of the 1.47 eV emission band was decreased by the heat treatment after coating with Cu-doped DEGBE, which suggests that the defect density is decreased by Cu doping.

Comparative study of the luminescence properties of macro- and nanocrystalline MgO using synchrotron radiation

MgO nano-powder with an average crystallite size of nanoparticles ranging 10–15nm was synthesized using the extractive-pyrolytic method and was studied by room temperature VUV spectroscopy under synchrotron radiation excitation. Comparative analysis of their luminescent properties with that of macrocrystalline powder analogues and an MgO single crystal, grown by the arc-fusion method, has been performed under excitation by pulsed VUV synchrotron radiation. Special attention was paid to VUV spectral range, which is not reachable with commonly used lamp and laser sources. A considerable blue shift of about ∼0.3eV in the excitation spectra of 2.95eV emission band, was revealed in nanocrystalline MgO samples.

Spectral and kinetic features of thermoluminescence in hexagonal boron nitride powder after UV-irradiation

Abstract Thermally stimulated luminescence (TL) from UV-irradiated h-BN powder synthesized using carbamide technique was studied. Three TL peaks at Tmax = 380, 500 and 600 K during linear heating with 2 K/s rate in RT – 773 K temperature range were observed. It was found that the 2.90 and 3.25 eV emission bands, which were related with recombination centers on the basis of VN and BO−-complexes, dominate in TL spectra of h-BN. Experimental TL glow curves were analyzed in terms of general order kinetics and energy parameters of responsible capture levels were estimated. It was shown, considering the independent data on the luminescent properties of hexagonal boron nitride in different structural states, that TL peaks at 380 K and 600 K were due to traps based on the one-boron and the three-boron centers with thermal depth EA = 0.7 and 1.0 eV, respectively. The possible origin of the trap with EA = 1.6 eV, responsible for the TL peak at 500 K, is discussed.

Photoluminescence and excited state structure of Bi3+-related centers in Lu2SiO5:Bi single crystalline films

Single crystalline films of Lu2SiO5:Bi, prepared by the liquid phase epitaxy method from the melt-solution based on Bi2O3 flux, are studied at 4.2–350K by the time-resolved spectroscopy methods under excitation in the 2.4–6.2eV energy range. An intense dominant ultraviolet (≈3.5eV) luminescence of Lu2SiO5:Bi is shown to arise from the radiative decay of the metastable and radiative minima of the triplet relaxed excited state (RES) of Bi3+ centers which are related to the 3P0 and 3P1 levels of a free Bi3+ ion, respectively. At T<50K, the radiative transitions from the metastable minima mainly take place. Thermally stimulated non-radiative transitions between the metastable and radiative minima of the triplet RES appear at T>50K in the temperature evolution of the emission spectrum and decay kinetics. The excitation bands of the ultraviolet emission, located at 4.2eV, 5.03eV, and 5.95eV, are assigned to the 1S0→3P1, 1S0→3P2, and 1S0→1P1 transitions of a free Bi3+ ion, respectively. The phenomenological model is proposed to describe the excited-state dynamics of Bi3+ centers in Lu2SiO5:Bi. Characteristic parameters of the triplet RES, in particular the energy separation between the excited states and the rates of the radiative and non-radiative transitions from these states, are determined. Much weaker ≈3.3eV emission is ascribed to the Bi3+ ions located in the Lu2 lattice sites. Weak broad ≈2.2eV and ≈2.3eV emission bands are assumed to arise from the Bi3+-related localized excitons.

Photoluminescence properties of Al‐rich AlXGa1‐XN grown on AlN/sapphire template by MOCVD

AbstractOptical properties of the un‐doped and the Si‐doped AlGaN epilayers have been studied by using the deep ultraviolet photoluminescence (PL) spectrum. The samples were grown under the same conditions on a HT‐AlN template by low pressure metal‐organic chemical vapour deposition (LP‐MOCVD). The Al composition and the relaxation status were detected to be 0.654 and 42% for the un‐doped AlGaN, 0.657 and 63% for the Si‐doped AlGaN by use the high resolution XRD. The temperature dependent PL spectra of both AlGaN epilayers reveal a typical energy broadband around 2.85 eV involving the deep level impurity transitions and the different peak shift behaviour of localized excitons around 4.97 eV with temperature increasing from 13 K to 120 K. The luminescence intensity of the excitons in both samples indicated a thermal quenching process with two activation energies. The two activation energies Eact1 and Eact2 were assigned to the thermal barrier resulting from the strong composition fluctuation in AlGaN and the binding energy of the excitons, respectively. The different PL oscillation properties of two samples at the 2.85 eV emission band indicate the Si‐doped AlGaN epilayer has significant interference of interface and surface. The significant oscillations were attributed to less centres of nucleation in the Si‐doped AlGaN, as has been supported by atomic force microscopy (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Characterization of freestanding semi-insulating Fe-doped GaN by photoluminescence and electromodulation spectroscopy

A detailed optical characterization of a freestanding wurtzite semi-insulating Fe-doped GaN (SI-GaN:Fe) grown by hydride-vapor-phase-epitaxy was carried out by photoluminescence (PL) and contactless electroreflectance (CER) at 10 and 300 K, respectively, and photoreflectance at 300 K. Low-temperature PL spectrum of the Ga-face consisted of the Fe3+ impurity characteristic series of IR peaks with a sharp zero-phonon line (ZPL) at 1.299 eV, yellow and blue broad emission bands, and near-band-edge (NBE) emission in the ultraviolet region. The narrow linewidth of 135 μeV of the ZPL and the measured energy difference of ∼70 meV between the ZPL and E2 (high) phonon replica, which is sensitive to the lattice strain, shows good crystal quality and a strain-free incorporation of iron. The obtained transition energies of A, B, and C excitonic features in the CER spectra and the n = 2 excited states (2s) of the A and B excitons enable the estimation of the exciton binding energies. In addition to the free excitonic recombination, the PL spectrum of the Ga-face exhibited clear donor and acceptor related features in the NBE region, while the N-face exhibited a broad emission band related to the free-to-bound recombination only. The differences were explained by the presence of impurity-induced band-tail states in the N-face SI-GaN:Fe due to an increased impurity density and the incorporation of large volume vacancy-type defects.

&lt;I&gt;In-Situ&lt;/I&gt; Cathodoluminescence Spectroscopy of Silicon Oxide Thin Film Under Uniaxial Tensile Loading

In this paper, in-situ cathodoluminescence (CL) stress analysis of a silicon oxide (SiO(x)) thin film prepared by wet thermal oxidation is described. The specially-developed uniaxial tensile loading jig was used to apply tensile displacement to the SiO(x) film specimen. CL spectra of the specimen during tensile loading were obtained, and the peak position of around 1.85 eV emission band was monitored for tensile stress analysis. The peak position gradually shifted towards higher/lower energy side when tensile displacement increased/decreased. The tensile stress-to-emission energy ratio of 6.21-8.97 x 10(2) GPa/eV was estimated on the basis of linear elastic theory, which demonstrated that CL is able to provide information on stress induced in the film. Scanning electron microscopy (SEM) revealed that the fracture of SiO(x) and SCS laminated structure occurred at the vicinity of SiO(x) film surface.

Hydrogen passivation effect on the yellow–green emission band and bound exciton in n - ZnO

Photoluminescence (PL) measurements performed on as-grown, hydrogenated, and annealed n -type ZnO bulk samples investigated the origins of their yellow (2.10 eV) and green (2.43 eV) emission bands. After hydrogenation, the defect-related peak at 2.10 eV was no longer present in the room temperature PL spectrum, the peak intensity at 2.43 eV was unchanged, and the intensity of the emission peak at 3.27 eV increased significantly. These results indicate that yellow band emission is due to oxygen vacancies, as the emission peak at 2.10 eV disappears when hydrogen atoms passivate these vacancies. The emission peak at 2.43 eV originates from complexes between oxygen vacancies and other crystal defects. We discuss the shallow donor impurities arising due to these hydrogen atoms in the ZnO bulk sample.

Study of the stacking faults in a-plane GaN on r-plane sapphire grown by metal–organic chemical vapor deposition

Optically specular a-plane GaN was grown on r-sapphire substrate by metal–organic chemical vapor deposition (MOCVD). Surface morphology and crystal structure anisotropic behavior related to defects with a specific distribution were characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM). Cathodoluminescence (CL) images and depth-profiling spectra show Basel stacking faults (BSFs) related emission at 3.42eV, yellow band emission at 2.25 and 3.00eV emission bands of the a-plane GaN. From the results of CL and transmission electron microscopy (TEM), the origin of the blue emission band was attributed to donor–acceptor pair (DAP) emission correlated with prismatic stacking faults (PSFs).

Photoreflectance and photoluminescence study of annealing effects on GaAsBi layers grown by metalorganic vapor phase epitaxy

The optical properties of GaAsBi layers grown by atmospheric pressure metalorganic vapor phase epitaxy on p-type GaAs substrates and annealed at different temperatures are investigated by photoreflectance and photoluminescence spectroscopies. Photoreflectance spectra show no significant shift in the band gap energy of GaAs0.965Bi0.035 with annealing temperature except 600 °C for which the band gap energy reaches a minimum value corresponding to a red shift of 60 meV. The low temperature photoluminescence spectra of GaAsBi layers show a broad band centered at ∼1.36 eV. The temperature dependence of the 1.36 eV emission band in addition to the increasing intensity of this band with bismuth flow suggests that it is originated from Bi clusters or from complex defects probably located at the surface/interface of the GaAsBi epilayer.

GaN/PMMA nanocomposite: synthesis and optical properties

High crystalline quality wurtzite-GaN nanoparticles, with diameters of about 11 nm, well dispersed in a poly-methyl methacrylate polymer matrix, were synthesized using a routine two-step route and were structurally and optically characterized. The as-prepared composite showed three broad overloading photoluminescence emission peaks with centers at 3.45, 3.29, and 3.10 eV. The 3.45 eV emission band is assigned to the recombination of the free exciton of GaN particles. The other two emission peaks possibly originate from the interfacial structure between the nanoparticles and polymer matrix or aggregation. The Fourier transform infrared (FTIR) spectrum further confirms the interfacial interaction that can be described as R-(C=O)-O-(GaN particle).

Intense interface luminescence in type II narrow-gap InAs-based heterostructures at room temperature

Positive and negative luminescence was observed in a type II broken-gap p-InAs/p-GaAsSb heterostructure in the mid-infrared spectral range 3–5 μm at room temperature. Interface-related radiative recombination was provided by Mn acceptor states on InAs surface. I–V characteristics behavior was discussed using the tunneling-assisted current transport mechanism through surface states. Redistribution between the interband (hv1=0.36eV) and interface (hv2=0.31eV) emission bands in electroluminescent spectra at reverse bias was found in dependence on Fermi level position pinning by surface states at the type II broken-gap heterointerface.

Probing the excitonic emission of ZnO nanoparticles using UV–VUV excitations

This study deals with the influence of the excitation (UV-lamp, UV-laser and VUV synchrotron radiation) on the 3.31 eV band of ZnO microcrystals and of variously treated nanoparticles. The nanoparticles are synthesized in ultra high vacuum condition and their stoichiometry and crystallinity can be controlled. This provides an efficient way to probe the influence of these factors on the excitonic emission. The energy and intensity of the excitation have a strong influence on the excitonic luminescence and particularly on the 3.31 eV emission band. The result of these experiments are used to probe the origins of this band which is found to be not linked to any surface phenomena. Indeed, the only way to fully explain our results is to consider that the 3.31 eV band involve the superposition of two emissions features: the first due to acceptor defects and the other originates form the LO phononic repliqua of the free exciton.