eV Emission Band Research Articles

Luminescence Properties of the Hexagonal Boron Nitride Epilayer

AbstractAs an emerging 2D ultrawide bandgap semiconductor, hexagonal boron nitride (h‐BN) is gaining significant attention for its superior properties and wide applications. Although optical properties of h‐BN have been partially revealed by using h‐BN bulk single crystal, luminescence properties of the h‐BN few‐layer are rare due to its poor crystallinity. In this work, the h‐BN epilayers have been synthesized on sapphire substrates by the submicron‐spacing vapor deposition method. The crystalline quality of the h‐BN epilayer is improved with the increase of the growth temperature, and the full width at half‐maximum of the Raman peak is only ≈13 cm−1. The low temperature cathodoluminescence spectra of h‐BN epilayers exhibit two strong deep ultraviolet (DUV) luminescence peaks at 5.49 and 5.35 eV, as well as a weaker and broader defect‐related emission band at ≈4.0 eV. The DUV emission band is ascribed to the recombination of bound excitons and its phonon replica. Based on the theoretically predicted energy levels, the 4.0 eV emission band is tentatively attributed to the radiative transition from C impurities occupying the B sites to the B vacancies. Moreover, the h‐BN‐based DUV photodetectors exhibit an outstanding performance, making it a promising prospect for optoelectronic applications.

Photoluminescence, electrical and mechanical properties of ultra-long single crystalline Al4O4C nanowires

Ultra-long single-crystalline orthorhombic Al4O4C nanowires (NWs) were synthesized via annealing a mixture of Ta, Al, and graphite powders at 1500 °C for 1 h in a flowing argon atmosphere based on the vapor-liquid-solid (VSL) mechanism. The molten Al-Ta droplets formed by adding Ta resulted in its growth along the [101] direction. Therefore, highly crystalline Al4O4C NWs possessed rough surfaces, a high aspect ratio (∼2200) with diameters ranging from 100 nm to 500 nm and lengths up to hundreds of micrometers (660 μm). They exhibited a decreased bandgap of 2.63 eV and broad emission bands in yellow/near-infrared light region under the excitation of 532 nm, which was originated by Ar doping and surficial defects. Furthermore, the single nanowire electrode devices were fabricated using the FIB-SEM technique to accurately measure the electrical and mechanical properties. Al4O4C NWs possessed the intrinsic resistivity of 202.61 Ω·m, Young's modulus of 93.89 ± 13.34 GPa, and yield strength of 4.08 ± 0.51 GPa, and therefore it is potential for nanodevice applications.

Anisotropic photoluminescence of β-LiGaO2 crystal

Manifestation of the anisotropic properties of β-LiGaO2 crystal revealed in photoluminescence (PL) excitation and emission has been studied. Three exciton bands at 6.043, 6.085 and 5.980 eV, corresponding to three crystallographic axes, have been found in the PL excitation spectrum under the synchrotron irradiation, whose relative contribution depends on crystal orientation. Study of the properties of polarized luminescence 4.43 eV under the excitation with ArF laser (193 nm) with varied intensity in the 10–300 K temperature range has demonstrated that the excitation-induced and temperature-induced band's energy and intensity behaviour depends on the orientation of the crystal in relation to the incident light. The simultaneous coexistence of two radiative processes is proposed for the explanation of the PL properties of β-LiGaO2 crystal: the tunnel recombination of donor-acceptor pairs (DAP) and the recombination of a free electron with an acceptor center (e-A). An assumption is done about the complex nature of the 4.43 eV emission band consisting of subbands corresponding to the DAP radiative recombination in three crystallographic directions, and a subband due to the e-A radiative recombination. Dependence of the luminescence thermal quenching properties on excitation density are discussed.

Effect of reducing and oxidizing atmosphere on photoluminescence of undoped and Eu doped nanostructured CaAl2O4

CaAl2O4 doped with divalent Eu has a photoluminescence emission peak at 2.82 eV (440 nm), which is generally attributed to the parity-allowed 4f65d1 → 4f7 electric dipole transition. Recently a similar emission was found in undoped CaAl2O4 suggesting that at least part of the blue luminescence in Eu doped samples is due to intrinsic defects. In order to determine the contributions of intrinsic and Eu related defects to the blue emission in CaAl2O4, the photoluminescence emission - excitation spectra of undoped and Eu doped CaAl2O4 subjected to a thermal treatment in reducing or oxidizing atmosphere were analyzed. It was found that reduced undoped and Eu doped as well as oxidized Eu doped CaAl2O4 exhibit a well-defined Gaussian shaped emission band centered at 2.82 eV and with a full width at half maximum of 0.335 eV. Because this band was found in undoped reduced CaAl2O4 it should be ascribed to an intrinsic defect that is proposed to be an oxygen vacancy occupied by one unpaired electron (F+ center). No evidence for any emission directly related to the decay of the excited state of Eu2+ was found. The oxidized undoped CaAl2O4 exhibits a broad emission peaked at 3.16 eV, which was fitted by two Gaussian bands centered at 2.80 and 3.17 eV and with full widths at half maximum of 0.87 and 0.49 eV, respectively. The oxidized Eu doped CaAl2O4 exhibits, additionally to 2.82 eV emission band, characteristic narrow emission bands attributed to f – f transitions of the Eu3+ ion. These emissions occur not only due to direct f - f excitation but also due the Eu3+ - O2− charge-transfer transition.

Light emission of self-trapped excitons from ion tracks in silica glass: Interplay between Auger recombination, exciton formation, thermal dissociation, and hopping

The initial luminescence yield of amorphous silica under ion irradiation has been studied at temperatures between 30 and 100 K, using swift ions of different masses and energies (3 MeV H, 3.5 MeV He, 19 MeV Si and 19 MeV Cl). The intensity of the 2.1 eV emission band, ascribed to the intrinsic recombination of self-trapped excitons (STEs), has been found to vary systematically with ion mass, energy and irradiation temperature. A detailed model has been developed to quantitatively describe those variations in terms of the competition between non-radiative Auger recombination, STE formation, STE thermal dissociation, and subsequent STE hopping and capture at non-radiative sinks. The model, which uses a thermal spike approach to describe the effect of swift ion bombardment, is found to quantitatively predict the experimental data without adjustable parameters. It provides new insights into the interactions of carriers in an ion track and the behavior of the luminescence emissions during ion irradiation (ionoluminescence). The model is found to predict the correct temperature dependence of the yield if an activation energy for STE thermal migration of 0.12 eV is assumed, which is in good agreement with values previously reported.

Luminescence of Hf-=SUB=-x-=/SUB=-Zr-=SUB=-1-x-=/SUB=-O-=SUB=-2-=/SUB=- thin films upon excitation with vacuum ultraviolet synchrotron radiation

Using the methods of low-temperature luminescence spectroscopy and vacuum ultraviolet synchrotron radiation (MAX IV synchrotron, Lund, Sweden), charge transfer processes and the nature of charge carrier traps responsible for leakage currents in nanometer dielectric films of hafnium-zirconium-oxygen HfxZr1-xO2 solid solutions on silicon substrate, as well as in films doped with La-ions and annealed in argon atmosphere. The photoluminescence spectra, photoluminescence excitation spectra of the 2.7 eV emission band and previous simulation data within the framework of density functional theory confirm the presence of an oxygen vacancy in the studied films. Based on the study of competing channels for the relaxation of electronic excitations due to the radiative decay of self-trapped excitons (4.35 eV emission band) and the luminescence of defects (2.7 and 3.5 eV emission bands), a conclusion was made about the efficiency of transport and capture of excitons and separate charge carriers in films with varying degrees of defectiveness and films doped with lanthanum. The obtained data confirm the conclusion that oxygen vacancies are traps responsible for leakage currents in the films under study Keywords: Luminescence, synchrotron radiation, defects, energy transfer.

Люминесценция тонких пленок Hf-=SUB=-x-=/SUB=-Zr-=SUB=-1-x-=/SUB=-O-=SUB=-2-=/SUB=- при возбуждении синхротронным излучением вакуумного ультрафиолетового диапазона

Using the methods of low-temperature luminescence spectroscopy and vacuum ultraviolet synchrotron radiation (MAX IV synchrotron, Lund, Sweden), charge transfer processes and the nature of charge carrier traps responsible for leakage currents in nanometer dielectric films of hafnium-zirconium-oxygen solid solutions on silicon substrate, as well as in films doped with La-ions and annealed in argon atmosphere. The photoluminescence spectra, photoluminescence excitation spectra of the 2.7 eV emission band and previous simulation data within the framework of density functional theory confirm the presence of an oxygen vacancy in the studied films. Based on the study of competing channels for the relaxation of electronic excitations due to the radiative decay of self-trapped excitons (4.35 eV emission band) and the luminescence of defects (2.7 and 3.5 eV emission bands), a conclusion was made about the efficiency of transport and capture of excitons and separate charge carriers in films with varying degrees of defectiveness and films doped with lanthanum. The obtained data confirm the conclusion that oxygen vacancies are traps responsible for leakage currents in the films under study.

Reduced nonradiative recombination rates in c-plane Al0.83In0.17N films grown on a nearly lattice-matched GaN substrate by metalorganic vapor phase epitaxy

A record-long room-temperature photoluminescence (PL) lifetime (τPLRT) of approximately 70 ps was obtained for the sub-bandgap 3.7 eV emission band of a 300-nm-thick c-plane Al0.83In0.17N epilayer for the use in cladding layers of an edge laser structure, which were grown by metalorganic vapor phase epitaxy on a low threading dislocation density nearly lattice-matched GaN substrate. The recorded τPLRT value was twice as long as previously reported ones, indicating half concentration of nonradiative recombination centers. Room-temperature spatially resolved cathodoluminescence intensity images for the 3.7 eV band revealed nearly zero carrier diffusion length, which is consistent with the fact that τPLRT of 70 ps is 1/35 of the near-band-edge emission of the GaN substrate (2.4 ns). As the PL decay curves for the 3.7 eV band were sufficiently fitted by the stretched exponential function, the emission likely originates from extended states such as impurities, point defects, and their complexes, as well as localized states of uneven potential profile.

Bright Blue Emitting Cu-Doped Cs2ZnCl4 Colloidal Nanocrystals.

We report here the synthesis of undoped and Cu-doped Cs2ZnCl4 nanocrystals (NCs) in which we could tune the concentration of Cu from 0.7 to 7.5%. Cs2ZnCl4 has a wide band gap (4.8 eV), and its crystal structure is composed of isolated ZnCl4 tetrahedra surrounded by Cs+ cations. According to our electron paramagnetic resonance analysis, in 0.7 and 2.1% Cu-doped NCs the Cu ions were present in the +1 oxidation state only, while in NCs at higher Cu concentrations we could detect Cu(II) ions (isovalently substituting the Zn(II) ions). The undoped Cs2ZnCl4 NCs were non emissive, while the Cu-doped samples had a bright intragap photoluminescence (PL) at ∼2.6 eV mediated by band-edge absorption. Interestingly, the PL quantum yield was maximum (∼55%) for the samples with a low Cu concentration ([Cu] ≤ 2.1%), and it systematically decreased when further increasing the concentration of Cu, reaching 15% for the NCs with the highest doping level ([Cu] = 7.5%). The same (∼2.55 eV) emission band was detected under X-ray excitation. Our density functional theory calculations indicated that the PL emission could be ascribed only to Cu(I) ions: these ions promote the formation of trapped excitons, through which an efficient emission takes place. Overall, these Cu-doped Cs2ZnCl4 NCs, with their high photo- and radio-luminescence emission in the blue spectral region that is free from reabsorption, are particularly suitable for applications in ionizing radiation detection.

Raman and Photoluminescence Study of Al,N‐Codoped ZnO Films Deposited at Oxygen‐Rich Conditions by Magnetron Sputtering

Optical properties of as‐grown high nitrogen‐doped ZnO:Al,N films (with a variation of nitrogen concentration from 2.3 to 4.3 atomic %) are studied by Raman, photoluminescence, and Fourier‐transform infrared spectroscopy (FTIR). The intensity of mode A1LO, peaked at 580 cm−1, increases with increasing nitrogen concentration. The silent mode B1low at 275 cm−1 is clearly observed testifying increased disorder‐activated scattering in ZnO. Photoluminescence spectra reveal near‐band edge emission as well as several defect‐related bands, the intensity of which increases with nitrogen content. The blue band (2.61 eV) can be related to the transition from shallow donor level to deep nitrogen acceptor level. Also, incorporation of nitrogen in ZnO lattice causes appearance of both Zni and Oi defects responsible for violet (3.08 eV) and yellow (2.16 eV) emission band, respectively. At the same time near‐band edge emission of ZnO:Al,N films is not suppressed by simultaneously introduced Al and N impurities. Al compensates distortions in ZnO crystal lattice caused by nitrogen doping and, thus, stabilizes near‐band edge emission.

Photoluminescence and optical transitions in C60 fullerene thin films deposited on glass, silicon and porous silicon

The C60/glass, C60/Si, C60/porous Si/Si as well as porous Si/Si thin film structures for comparison were prepared and studied by photoluminescence spectroscopy at room temperature. Along with a broad-band low-energy emission at 1.774 eV in the photon energy range of the nominaly forbidden optical transitions, the C60 films on glass substrate exhibit high-energy emissions at 2.115 eV and 2. 342 eV in the range of allowed dipole transititions. On the other hand, C60 films on Si or porous Si/Si display only the 1.774 eV emission band. The intensity of this emission in C60/porous Si/Si is 200 times higher than that in C60/Si and at least 20 times exceeds the one for the emission observed on porous Si/Si. Such a dramatic increase in emission intensity is indicative of the relaxation of the selection rules for nominally forbidden optical transitions and suggests strong interaction of C60 molecules with porous silicon walls. Molecular signatures of the C60 films are clearly manifested by the vibronic transitions with 60 meV energy separation (Ag-mode energy) between the irradiative levels, observed on 1.774 eV emission band in C60/porous Si/Si. The origin of the 2.115 and 2.342 eV emission bands is analyzed within a standard band insulator approach, together with the obtained ellipsometric data. The first corresponds to the band-to-band transitions at band gap. The appearance of the second can be related to the splitting of the valence band in the X point of the Brillouin zone of C60 film due to the crystal field. The strength of the crystal field splitting is then ~ 200 meV.

Ultraviolet and violet-blue emission of Zn doped MgO/mg(OH)2 particles prepared by one step spray pyrolysis method

Spherical particles of Zn doped MgO were prepared by one-step spray pyrolysis method. The crystalline nature and particle size of the samples were characterized by X-ray diffraction analysis (XRD). The morphology of samples was studied by scanning electron microscope (SEM) and the presence of Zn in the sample was confirmed by energy dispersive X-ray analysis (EDX). The optical properties of the samples were investigated using photoluminescence spectroscopy (PL) analysis to obtain excitation and emission spectra of the samples. Results indicated that the doped MgO particles exhibited a cubic structure without hexagonal wurtzite structure as the Zn concentrations were increased. Spherical shape and porous particles are found with increasing of doping concentration. The optical band gap of MgO altered with the addition of doping concentration. A considerable redshift of about ~0.08 – 0.13 eV in the excitation spectra of 2.22 eV emission band was revealed in Zn doped MgO samples. It was highlighted that Zn doped MgO prepared by the spray pyrolysis generated emission at UV-Vis wavelength required for many applications.

Ultraviolet and violet-blue emission of Zn doped MgO/mg(OH)2 particles prepared by one step spray pyrolysis method

Spherical particles of Zn doped MgO were prepared by one-step spray pyrolysis method. The crystalline nature and particle size of the samples were characterized by X-ray diffraction analysis (XRD). The morphology of samples was studied by scanning electron microscope (SEM) and the presence of Zn in the sample was confirmed by energy dispersive X-ray analysis (EDX). The optical properties of the samples were investigated using photoluminescence spectroscopy (PL) analysis to obtain excitation and emission spectra of the samples. Results indicated that the doped MgO particles exhibited a cubic structure without hexagonal wurtzite structure as the Zn concentrations were increased. Spherical shape and porous particles are found with increasing of doping concentration. The optical band gap of MgO altered with the addition of doping concentration. A considerable redshift of about ~0.08 – 0.13 eV in the excitation spectra of 2.22 eV emission band was revealed in Zn doped MgO samples. It was highlighted that Zn doped MgO prepared by the spray pyrolysis generated emission at UV-Vis wavelength required for many applications.

Structure of the Room-Temperature 2.5 eV α-Quartz Single-Crystal Pulsed Cathodoluminescence Band

We study the room-temperature (RT) pulsed cathodoluminescence (PCL) spectrum of a high-purity synthetic α-quartz single crystal. The spectrum consists of two wide bands with intensity maxima at 415 and 490 nm (2.99 and 2.53 eV). The band at 490 nm (2.5 eV) is polarized in the XY crystal plane (perpendicular to the third-order symmetry axis) and possesses a structure with three peaks at 480±2, 487±2, and 493±2 nm (2.58±0.01, 2.55±0.01, and 2.52±0.01 eV). The intensities of the peaks at 480±2 and 493±2 nm increased with increase in the irradiation dose up to 45 kGy. Peaks are equidistant at the energetic scale. The energy separation between the peaks Δ = 0.03 ± 0.01 eV is equal in order of magnitude to energies of Li x O y molecular vibrations and to the energy of the optical phonon in α-quartz. We propose an explanation of the experimental data obtained. According to this explanation, the structure observed may be attributed to the amplitude modulation of the quartz 2.5 eV emission band by the crystalline electric fields on frequencies of optical phonons. The nonequilibrium phonons may arise during the electron-beam irradiation.

Luminescence and light guiding properties of Er and Li codoped ZnO nanostructures

Micro- and nanostructures of ZnO co-doped with Er and Li ions have been grown by a catalyst free vapor-solid method under constant N2 flux. Nanowires and nanobelts with lengths of several microns, and thicknesses of a few hundreds of nanometers have been obtained. Scanning Electron Microscopy (SEM), X Ray Microanalysis (EDS) and micro-Raman spectroscopy have been used to characterize the samples. Luminescent emission has been studied by cathodoluminescence (CL). Modifications in the defect structure due to Li incorporation are observed. An increase in the relative intensity of the Er3+ - related 0.8eV (1.54µm) emission band in the co-doped samples has been observed. Micro-photoluminescence (µ-PL) has been used to investigate the optical properties of the structures. The establishment of optical resonant modes and wave guiding along both the longitudinal and transversal axes of the structures have been observed.

Host and defect-related photoluminescence of structurally disordered K3WO3F3 oxyfluoride crystals

Spectra of photoluminescence (PL) in the region of 1.5–5.5 eV, PL excitation spectra (3–22 eV), PL decay kinetics and PL temperature dependence were measured for single crystals and ceramics K3WO3F3 as well as for ceramics K3WO3F3 irradiated by fast electrons. Synchrotron radiation was used for low temperature PL experiments with time resolution. Single crystals are transparent in microwave, visible and near UV range, inter-band transition energy is Eg = 4.3 eV. In K3WO3F3, the wide band luminescence in the region of 2.5 eV with the Stokes shift of 1.5 eV with the microsecond decay kinetics is connected with luminescence of triplet self-trapped excitons (STE). This luminescence is formed by electronic transitions in [WO3F3] octahedron. Different distortion of KWOF crystal lattice is manifested in the change of the Stokes shift of the STE luminescence band. The 3.2 eV emission band in low-temperature PL spectrum with decay times of 1.8 ns and 11 ns corresponds to singlet STE luminescence. A new 2.9 eV emission band is discovered in low-temperature PL spectrum in the samples irradiated by fast electrons (E = 10 MeV, D = 160 kGy). This emission band is excited not through the intracenter mechanism but through the creation of excitons bound on the defects. It is suggested that it is F-like centers of anionic sublattice induced by the mechanism of elastic collision.

Role of oxygen vacancies on light emission mechanisms in SrTiO3 induced by high-energy particles

Light emission under MeV hydrogen and oxygen ions in stoichiometric SrTiO3 are identified at temperatures of 100 K, 170 K and room-temperature. MeV ions predominately deposit their energies to electrons in SrTiO3 with energy densities orders of magnitude higher than from UV or x-ray sources but comparable to femtosecond lasers. The ionoluminescence (IL) spectra can be resolved into three main Gaussian bands at 2.0 eV, 2.5 eV and 2.8 eV, whose relative contributions strongly depend on irradiation temperature, electronic energy loss and irradiation fluence. Two main bands, observed at 2.5 eV and 2.8 eV, are intrinsic and associated with electron–hole recombination in the perfect SrTiO3 lattice. The 2.8 eV band is attributed to recombination of free (conduction) electrons with an in-gap level, possibly related to self-trapped holes. Self-trapped excitons (STEs) are considered suitable candidates for the 2.5 eV emission band, which implies a large energy relaxation in comparison to the intrinsic edge transition. The dynamics of electronic excitation, governs a rapid initial rise of the intensity; whereas, accumulated irradiation damage (competing non-radiative recombination channels) accounts for a subsequent intensity decrease. The previously invoked role of isolated oxygen vacancies for the blue luminescence (2.8 eV) does not appear consistent with the data. An increasing well-resolved band at 2.0 eV dominates at 170 K and below. It has been only previously observed in heavily strained and amorphous SrTiO3, and is, here, attributed to transitions from d(t2g) conduction band levels to d(eg) levels below the gap. In accordance with ab initio theoretical calculations they are associated to trapped electron states in relaxed Ti3+ centers at an oxygen vacancy within distorted TiO6 octahedra. The mechanism of defect evolution monitored during real-time IL experiments is presented. In conclusion, the light emission data confirm that IL is a useful tool to investigate lattice disorder in irradiated SrTiO3.

Temperature dependence of luminescence intensity in ZnMoO4 crystals

Luminescence of ZnMoO4 crystals (of stoichiometric composition, with excess of molybdenum and doped with tungsten) under X-ray irradiation in the temperature interval from 4 to 400 K was studied. The difference in the composition has little influence on the luminescence properties of the samples. An increase of the 1.93eV emission band intensity was observed in the temperature interval from 8K to 90K. Two mechanisms, concurrence with excitonic emission and concurrence with non-radiative recombination channel, were considered to explain the observation. Theoretical formulas were obtained to describe the temperature dependence of luminescence intensity. The experimental data are well described by the developed theoretical model which assumes recombination of free charge carriers with carriers of opposite sign localized on shallow traps. The high temperature quenching of luminescence is well described by the Mott formula.

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

One of the main limitations based on point defects in AlN is related to the UV absorption band present at 265 nm. This relatively broad absorption band limits the use of bulk substrates within the deep UV range and several complicated fabrication procedures have been devised to overcome this limitation. The origin for this absorption band along with corresponding photoluminescence signatures will be reviewed. CN - and VN + were identified as the point defects directly associated with the main absorption at 4.7 eV and the emission at 2.8 eV. The main absorption is due to a transition between the CN - state and conduction band, while the emission arises from a DAP transition between the donor VN + and acceptor CN - through the 4.7 eV excitation channel. New observations based on photoluminescence excitation are presented to further support the assignments previously made.

Photoluminescence fatigue and inhomogeneous line broadening in semi-insulating Tl6SeI4 single crystals

Photoluminescence (PL) properties of semi-insulating Tl6SeI4 have been investigated. A broad emission band centered at 1.63 ± 0.02 eV was observed in all samples. The PL emission band is excitonic in nature and is tentatively attributed to a bound exciton emission. PL fatigue (a reduction in PL intensity under prolonged laser excitation) was always observed. The amount of PL fatigue depended on excitation power and temperature. PL fatigue kinetics are described by a stretched exponential with nominal lifetimes in the 10–265 s range. The recovery of the PL occurred within a few seconds of light cessation. The magnitude of PL fatigue in different samples correlated with inhomogeneous line broadening of the 1.63 eV emission band, such that broader bands exhibited more fatigue. An additional luminescence band centered at 1.78 eV was observed which increased in intensity under prolonged laser irradiation. The fatigue phenomenon is tentatively attributed to two mechanisms—the formation of photo-induced defects and the formation of quasi-stable particles. Both of these mechanisms introduce additional radiative and non-radiative recombination channels that lead to a decrease in the PL intensity under prolonged laser irradiation. Since inhomogeneous line broadening and PL fatigue are related to the concentration of defects or impurities, the measurement of these two parameters is an effective method to screen sample quality.