Defect-related Luminescence Research Articles

Plasma assisted molecular beam epitaxial growth of GaN with low growth rates and their properties

A systematic investigation on PA-MBE grown GaN with low growth rates (less than 0.2 µm/h) has been conducted in a wide growth temperature range, in order to guide future growth of sophisticated fine structures for quantum device applications. Similar to usual growths with higher growth rates, three growth regions have been revealed, namely, Ga droplets, slightly Ga-rich and N-rich 3D growth regions. The slightly Ga-rich region is preferred, in which GaN epilayers demonstrate optimal crystalline quality, which has been demonstrated by streaky RHEED patterns, atomic smooth surface morphology, and very low defect related yellow and blue luminescence bands. The growth temperature is a critical parameter to obtain high quality materials and the optimal growth temperature window (~ 700–760 °C) has been identified. The growth rate shows a strong dependence on growth temperatures in the optimal temperature window, and attention must be paid when growing fine structures at a low growth rate. Mg and Si doped GaN were also studied, and both p- and n-type materials were obtained.

Transformation of free-standing ZnO nanorods upon Er doping

Erbium doped zinc oxide (ZnO:Er) powder was synthesized by hydrothermal growth. The morphology changed from undoped nanorods to microrods. The effect of Er doping on luminescence properties of ZnO:Er was observed. Furthermore, annealing in air at elevated temperatures up to 700 °C results in additional modification of defect states and related luminescence properties. In particular, exciton-related UV emission band and the defect-related red emission band appear in all annealed samples. Er3+infrared (IR) photoemission, the typical4I13/2→4I15/2transition, not observed in as grown samples with low Er concentration, occurs at room temperature in all ZnO:Er samples after annealing in air at elevated temperatures. Oppositely, the electron paramagnetic resonance (EPR) signal of Er3+is getting weaker upon the annealing at elevated temperatures, disappearing completely at about 500 °C. The intensity of theg = 1.96 EPR spectrum, typical for ZnO structures, is getting decreased and almost disappears after the annealing at 500 °C but increasing again after the annealing at 600 °C and 700 °C. X-ray photoelectron spectroscopy and IR Raman spectroscopy suggested that annealing in air causes diffusion of Zn from the surface to the bulk and oxidation of Er to Er3+in Er2O3and Er(OH)xforms.

Role of native and impurity defects in optical absorption and luminescence of Li2MoO4 scintillation crystals

The Li2MoO4 crystals have been considered as perspective cryogenic scintillation materials to be used in elementary particle physics experiments related to dark matter search and properties of neutrino. Since the crystals may be used as cryogenic scintillators, the native vacancies, uncontrolled impurities and other defects play the key role in worsening of their scintillation characteristics. The paper presents complex experimental and computational studies of the electronic structure and optical properties of Li2MoO4 crystals. The studies are aimed to explain peculiarities of the optical (luminescence) properties of Li2MoO4 using the electronic structure results obtained for a wide range of point defects and their combinations. For investigations, several samples of Li2MoO4 with different defectiveness were grown using Czohralski method and characterized by SEM and ICP-OES technique. Luminescence emission spectra of the samples under X-ray and laser excitations, optical and IR absorption spectra, TSL characteristics of the samples were recorded and analyzed. Ability to incorporate some gases was examined by measuring absorption spectra after one-year storage of the samples at ambient conditions. The electronic structures of several kinds of point defects (natural vacancies, iso- and aliovalent cationic and anionic substitutions, interstitial ions and molecular groups) in Li2MoO4 were calculated using the DFT-based plane-wave pseudopotential method. Geometries of lattice relaxations around defects, partial densities of states and energies of defect formation were calculated and analyzed together with experimental results. Performed analysis allowed making several conclusions regarding the origin of spectral components of intrinsic and defect-related luminescence, formation of defect-related bands in the optical and IR absorption spectra, dependence of TSL characteristics on sample defectiveness. It was found that isovalent cationic substitutions on lithium positions are presented in substantial concentrations in the Li2MoO4 samples and such defects form shallow traps which can affect scintillation characteristics in cryogenic experiments. Substantial effect of gases inclusions on the optical characteristics of crystals can be related to the peculiarity of the crystal lattice structure - the presence of empty "channels", which can incorporate molecular objects like H2O, -CH3 or -OH.

Wafer-Scale Epitaxial Growth of Unidirectional WS2 Monolayers on Sapphire.

Realization of wafer-scale single-crystal films of transition metal dichalcogenides (TMDs) such as WS2 requires epitaxial growth and coalescence of oriented domains to form a continuous monolayer. The domains must be oriented in the same crystallographic direction on the substrate to inhibit the formation of inversion domain boundaries (IDBs), which are a common feature of layered chalcogenides. Here we demonstrate fully coalesced unidirectional WS2 monolayers on 2 in. diameter c-plane sapphire by metalorganic chemical vapor deposition using a multistep growth process to achieve epitaxial WS2 monolayers with low in-plane rotational twist (0.09°). Transmission electron microscopy analysis reveals that the WS2 monolayers are largely free of IDBs but instead have translational boundaries that arise when WS2 domains with slightly offset lattices merge together. By regulating the monolayer growth rate, the density of translational boundaries and bilayer coverage were significantly reduced. The unidirectional orientation of domains is attributed to the presence of steps on the sapphire surface coupled with growth conditions that promote surface diffusion, lateral domain growth, and coalescence while preserving the aligned domain structure. The transferred WS2 monolayers show neutral and charged exciton emission at 80 K with negligible defect-related luminescence. Back-gated WS2 field effect transistors exhibited an ION/OFF of ∼107 and mobility of 16 cm2/(V s). The results demonstrate the potential of achieving wafer-scale TMD monolayers free of inversion domains with properties approaching those of exfoliated flakes.

Unraveling the Defect-Related Luminescence in a Eu2+-Doped Chlorosilicate Phosphor.

Defects play non-negligible roles in many luminescent processes, where the significant and remarkable influence in the phosphor performance in various ways is observed. A full and clear perception of defects would be beneficial for the further development of the luminescence mechanism and design of phosphors. In this study, the defect-related luminescence in a chlorosilicate phosphor Ca2SiO3Cl2: Eu2+ was deeply studied. Except for the green emission originating from the d-f transition of the Eu2+ ion, a blue emission band with unique sensitivity to temperature and excitation energy was confirmed to be induced by the defects. The defects related to anion vacancies in the material acted as electron traps and luminescence centers simultaneously, which were well-identified by luminescence spectra and theory calculation analysis. This study uncovered the peculiar behavior and action mechanism of defects in the luminescence process and demonstrates a new path to understanding the origin of the luminescence center.

Studies of Be, Si doping regulated GaAs nanowires for phase transition and optical properties

GaAs-based semiconductor doping technology, in which impurity energy levels are introduced into the band gap, can give rise to a decisive effect on its electrical and optical properties. When GaAs material is reduced to one-dimensional nanoscale, due to the increase of specific surface area, wurtzite- zinc blende coexisting structure is prone to appearing. GaAs nanowire doping can not only adjust its electro-optical properties, but also have a significant regulatory effect on its structural phase transition. The effects of beryllium (Be) and silicon (Si) doping on crystal structure and optical properties of gallium arsenide (GaAs) nanowires (NWs) are studied in this paper. Primitive, Si-doped and Be-doped GaAs NWs are grown on Si(111) substrates by molecular beam epitaxy in virtue of the self-catalyzed growth mechanism. The Raman spectra of primitive, Si-doped and Be-doped GaAs NWs are measured. The E2 mode peak unique to the WZ structure of primitive GaAs NWs is found in the Raman spectrum, and the E2 mode peak in the Raman spectrum of Si-doped GaAs NWs weakens or even disappears. Moreover, The E2 mode peak is not found in the Raman spectrum of Be-doped GaAs NWs. Furthermore, the structural changes of GaAs NWs are observed more intuitively by high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED). The PL spectra show that the wurtzite (WZ)-zinc blende (ZB) mixed phase II-type luminescence exists in primitive GaAs NWs, then the luminescence disappears due to Si or Be doping and turns into impurity defect related luminescence.

Optical and structural properties of Mn/Cu doped ZnS nanostructures

Pure ZnS, Mn-doped ZnS (Zn0.97Mn0.03S) and Mn, Cu co-doped ZnS (Zn0.95Mn0.03Cu0.02S) nanostructures have been synthesized by co-precipitation method. The effects of Mn/Cu-doping on the structural and optical characteristics on Mn/Cu doped ZnS was investigated. X-ray diffraction studies confirmed the cubic structure of ZnS even Mn/Cu were added into ZnS. The substitution of Cu into Zn0.97Mn0.03S nanostructure reduced the crystallite size and lattice parameters, and also induced the defect related luminescence centres. Energy dispersive X-ray spectra confirmed the phase purity and efficient insertion of Cu and Mn into ZnS nanostructure. Energy gap of ZnS (3.51 eV) was sharply decreased to 3.28 eV by Mn addition through s-p exchange interaction between s-p electrons of ZnS and d electrons of Mn2+ in Zn0.97Mn0.03S. The observed enhanced energy gap in Mn/Cu dual doped ZnS (3.94 eV) compared with undoped ZnS was discussed by both Burstein–Moss effect quantum size effect. The present energy gap tailoring by Mn/Cu doping in ZnS host is useful for opto-electronic device applications.

State-of-the-art and prospects for intense red radiation from core\u2013shell InGaN/GaN nanorods

Core–shell nanorods (NRs) with InGaN/GaN quantum wells (QWs) are promising for monolithic white light-emitting diodes and multi-color displays. Such applications, however, are still a challenge because intensity of the red band is too weak compared with blue and green. To clarify this problem, we measured photoluminescence of different NRs, depending on power and temperature, as well as with time resolution. These studies have shown that dominant emission bands come from nonpolar and semipolar QWs, while a broad yellow-red band arises mainly from defects in the GaN core. An emission from polar QWs located at the NR tip is indistinguishable against the background of defect-related luminescence. Our calculations of electromagnetic field distribution inside the NRs show a low density of photon states at the tip, which additionally suppresses the radiation of polar QWs. We propose placing polar QWs inside a cylindrical part of the core, where the density of photon states is higher and the well area is much larger. Such a hybrid design, in which the excess of blue radiation from shell QWs is converted to red radiation in core wells, can help solve the urgent problem of red light for many applications of NRs.

Defect-related luminescence properties of hydroxyapatite nanobelts

In this work, we study the generation of visible and near-infrared (NIR) luminescence associated with the formation of point defects in hydroxyapatite (HAp) nanobelts. Three different HAp samples were synthesized by a modified hydrothermal method at growth times of 24, 30, and 36 h. Transmission electron microscopy measurements revealed the presence of stacking faults in the nanobelts grown during shorter times. X-ray diffraction and Raman spectroscopy measurements confirmed the high crystalline quality of the samples, showing a hexagonal calcium deficient (Ca-D) HAp phase, supported by the Ca/P ratio of 1.2 and 1.3 revealed by X-ray photoelectron spectroscopy and energy dispersive spectroscopy measurements, respectively. Electron paramagnetic resonance, photoluminescence, and cathodoluminescence measurements demonstrated the presence of hydroxyl ion vacancies (VOH−), calcium vacancies (VCa), and oxygen vacancies (VO) in HAp nanobelts generates a strong luminescence in the visible and NIR ranges.

Coalescence, crystallographic orientation and luminescence of ZnO nanowires grown on Si(001) by chemical vapour transport

We analyse the morphological, structural and luminescence properties of self-assembled ZnO nanowires grown by chemical vapour transport on Si(001). The examination of nanowire ensembles by scanning electron microscopy reveals that a non-negligible fraction of nanowires merge together forming coalesced aggregates during growth. We show that the coalescence degree can be unambiguously quantified by a statistical analysis of the cross-sectional shape of the nanowires. The examination of the structural properties by x-ray diffraction evidences that the nanowires crystallize in the wurtzite phase, elongate along the c-axis, and are randomly oriented in plane. The luminescence of the ZnO nanowires, investigated by photoluminescence and cathodoluminescence spectroscopy, is characterized by two bands, the near-band-edge emission and the characteristic defect-related green luminescence of ZnO. The cross-correlation of scanning electron micrographs and monochromatic cathodoluminescence intensity maps reveals that: (i) coalescence joints act as a source of non-radiative recombination, and (ii) the luminescence of ZnO nanowires is inhomogeneously distributed at the single nanowire level. Specifically, the near-band-edge emission arises from the nanowire cores, while the defect-related green luminescence originates from the volume close to the nanowire sidewalls. Two-dimensional simulations of the optical guided modes supported by ZnO nanowires allow us to exclude waveguiding effects as the underlying reason for the luminescence inhomogeneities. We thus attribute this observation to the formation of a core–shell structure in which the shell is characterized by a high concentration of green-emitting radiative point defects as compared to the core.

Testing performance of Pr3+-doped KLuP2O7 upon UV-, synchrotron X-ray and cathode-ray excitation

Abstract The luminescent characteristics of the double phosphate KLuP2O7 doped with Pr3+ ions, a promising optical material for scintillator applications, were investigated using various experimental spectroscopic techniques. KLuP2O7:Pr3+ shows Pr3+ emission associated with interconfigurational 4f15d1→4f2 transitions located in the UV range (250–320 nm), intraconfigurational 4f2→4f2 transitions (weak lines in the visible spectral range) and defect-related luminescence. The light output of X-ray or cathode ray excited Pr3+ 4f15d1→4f2 luminescence is found to be nearly independent of temperature in the range of 90–400 K. The output of pulsed cathode luminescence shows thermal quenching above 450 K characterized by an activation energy of about 0.5 eV. Upon excitation with pulsed cathode rays or high-frequency (~8 MHz) X-ray synchrotron radiation, the luminescence decay kinetics of Pr3+ 4f15d1→4f2 transitions are dominated by a decay component with lifetime of about 20 ns. An additional slower decay component with lifetime of 75 ns was observed upon cathode ray excitation and was shown to result from delayed host-to-impurity energy transfer. The latter, in turn, originates from re-trapping of charge carriers at defect-related traps whose presence was demonstrated by thermoluminescence measurements. Parameters of the trapping centers were estimated. Peculiarities of host-to-impurity energy transfer are analysed and discussed.

Excitation Density Dependent Photoluminescence Studies on Homo-Epitaxial GaN Nanowall Networks Grown by Laser Assisted Molecular Beam Epitaxy.

The optical properties of laser-assisted molecular beam epitaxy grown homo-epitaxialGaN nanowall networks (NWNs) were investigated using power dependent photoluminescence (PL) spectroscopy and compared with homo-epitaxial GaN thin film. The pore size and tip width of GaN NWN sample is ˜120-180 nm and 10-15 nm, respectively. The ultraviolet-visible spectroscopy study shows that the GaN NWNs have low optical light reflection and minimum Fabry-Perot cavity effect than GaN film. The room temperature PL spectroscopy reveals that the GaN NWNs possesses enhanced band gap of 3.51 eV with blue shift of 90 meV than the GaN film (3.42 eV). The excitation density dependent PL spectroscopy measurements reveal that the GaN NWNs nanowall and near band emission (NBE) peak position and its linewidth invariant. The intensity of NBE peak for GaN film and nanowalls varies linearly whereas NBE to defect related yellow luminescence peak intensity ratio shows a non-linear variation on the excitation density. The excitation density in PL measurements plays a key role when the sample quality compared on the basis of PL data.

Defect Related Emission in Calcium Hydroxide: The Controversial Band at 780 cm−1

Calcium hydroxide, a crystal involved in the cycle of calcination and carbonation of calcium oxide, finds several applications from cultural heritage to the dentistry branch or to the construction industry. When excited at 1064 nm, Raman spectra of calcium hydroxide show a broad composite band peaked at about 780 cm−1, corresponding to 1170 nm. Since it is not observed with visible excitation, the origin of this band is debated, being assigned to some pre-existent luminescent impurities or some structural defect of the lime formed after the synthesis of the material. To shed light on the formation of this band, we synthetised the lime paste starting from pure calcite powders. The obtained fresh Ca(OH)2 samples did not show any band in the investigated range, irrespective of the laser excitation applied. A detailed analysis of the excitation and emission spectra in the near infrared region did not show the 1170 nm band, supporting the hypothesis of a post-synthesis origin. Thus, we carried out thermal treatments at different temperatures (90–500 °C) and under different environments (in air or under nitrogen flux) on synthesised fresh Ca(OH)2 powders. We also investigated the time evolution of the samples, monitoring the Raman spectra over 90 days after a specific treatment. The collected data support the hypothesis of a defect-related luminescence centre, whose formation depends on the temperature and environment of the treatment, which appears as a preferential site for the carbonation process of the calcium hydroxide. These results can be useful in the field of Cultural Heritage for dating purposes, and to determine the conservation state of Ca(OH)2 containing relics to prevent the possible activation of degradation processes.

Structural and luminescence imaging and characterisation of semiconductors in the scanning electron microscope

The scanning electron microscopy techniques of electron backscatter diffraction (EBSD), electron channelling contrast imaging (ECCI) and cathodoluminescence (CL) hyperspectral imaging provide complementary information on the structural and luminescence properties of materials rapidly and non-destructively, with a spatial resolution of tens of nanometres. EBSD provides crystal orientation, crystal phase and strain analysis, whilst ECCI is used to determine the planar distribution of extended defects over a large area of a given sample. CL reveals the influence of crystal structure, composition and strain on intrinsic luminescence and/or reveals defect-related luminescence. Dark features are also observed in CL images where carrier recombination at defects is non-radiative. The combination of these techniques is a powerful approach to clarifying the role of crystallography and extended defects on a material’s light emission properties. Here we describe the EBSD, ECCI and CL techniques and illustrate their use for investigating the structural and light emitting properties of UV-emitting nitride semiconductor structures. We discuss our investigations of the type, density and distribution of defects in GaN, AlN and AlGaN thin films and also discuss the determination of the polarity of GaN nanowires.

Рентгенолюминесценция нитевидных микроструктур ZnO

The morphology, optical and luminescent properties of an ensemble of ZnO whisker microcrystals on sapphire substrate obtained by gas transport synthesis from zinc and oxygen vapors by the vapor – liquid – crystal mechanism are studied. The ensemble is formed by uniaxial ZnO microcrystallites of two morphologies: a combination of a hexagonal prism and single crystal microrods. The absorption edge of the ensemble of whisker microstructures is located in the 385-395 nm region. The total transmittance of the sample in the visible and near-infrared regions is around 10–20% with a layer thickness around 15–18 μm. The X-ray luminescence spectrum is represented by two bands: narrow peak of excitonic emission with a maximum at 388.3 nm and wide peak of defect-related luminescence in the region of 430–600 nm. The decay time constant for excitonic luminescence is around 1.1 ns (not accounted for the width of the excitation pulse), which was obtained for the first time in undoped ZnO microstructures.

Low temperature photoluminescence study of GaAs defect states**Project supported by the National Natural Science Foundation of China (Grant No. 21972103), the National Key Research and Development Program of China (Grant No. 2016YFB040183), and Research and Development Program of Shanxi Province, China (Grant No. 201703D111026).

Low temperature (77 K) photoluminescence measurements have been performed on different GaAs substrates to evaluate the GaAs crystal quality. Several defect-related luminescence peaks have been observed, including 1.452 eV, 1.476 eV, 1.326 eV peaks deriving from 78 meV GaAs antisite defects, and 1.372 eV, 1.289 eV peaks resulting from As vacancy related defects. Changes in photoluminescence emission intensity and emission energy as a function of temperature and excitation power lead to the identification of the defect states. The luminescence mechanisms of the defect states were studied by photoluminescence spectroscopy and the growth quality of GaAs crystal was evaluated.

Structural properties and defect related luminescence of Yb-doped NiO sol-gel thin films

Abstract In this work, ytterbium doped nickel oxide thin films were successfully synthesized and deposited on glass substrates using the sol-gel method combined with the spin-coating technique. The structural, morphological and optical properties of the films were investigated through XRD, AFM, UV–Vis and PL spectroscopy. Doping NiO with low Yb3+ ions content (1%, 2% and 3%) highly affected the lattice periodicity of the cubic nickel oxide structure through possible substitution of Ni atoms by higher sized Yb atoms, increasing the unit cell volume of the films and causing additional strains in the host lattice. The average crystallite size of the films varies between 4.34 nm and 8.25 nm. Morphological studies indicate a significant decrease in the surface roughness of the films from 2.5 nm to 0.4 nm with increasing Yb content. The as synthesized thin films are relatively transparent especially in the IR region. Room temperature PL spectra confirmed the presence of point defects such as nickel vacancies ( V N i ) and oxygen interstitials ( O i ). The effect of trivalent Yb3+ ions doping was investigated using Gaussian distribution. The variation of the position and intensity of sub-gap luminescence centers suggests the formation of secondary phases inside of NiO lattices contributing to the quenching of luminescence.

A Study of the Photoelectrochemical Etching of n-GaN in H3PO4 and KOH Electrolytes

We investigated the photoelectrochemical etching of n-GaN in H3PO4 and KOH as a function of electrolyte concentration, potential and light intensity. Etch rates measured by stylus profilometry were compared with coulometric and amperometric values. In both electrolytes, etch rates increased with concentration, reaching a maximum at 3.0 mol dm−3 and decreasing at higher concentrations. The increase in etch rate with concentration of either H3PO4 or KOH reflects the amphoteric nature of gallium and the decrease above 3.0 mol dm−3 is attributed to common-ion effects. Profilometric etch rates were lower than coulometric and amperometric etch rates reflecting formation of a surface film. SEM and profilometry demonstrated that thick surface films are formed at lower concentrations. Etch rates increased linearly with light intensity indicating a carrier-limited etching regime: a quantum efficiency of 57.6% was obtained. At light intensities greater than ∼35 mW cm−2 the etch rates showed evidence of saturation. AFM and SEM images of the etched GaN surfaces showed a distinctive ridge-trench structure with a hexagonal appearance. Photoluminescence spectra of the etched GaN show a significant increase in the defect-related yellow luminescence peak suggesting correlation to the formation of the ridge structures, which may represent dislocations terminating at the surface.

Energy band gaps and excited states in Si QD/SiOx/Ry Oz (R = Si, Al, Zr) suboxide superlattices

X-ray and optical spectroscopies were applied in order to study the band structure and electronic excitations of the SiOx/Ry Oz (R = Si, Al, Zr) suboxide superlattices. The complementary x-ray emission and absorption measurements allow for the band gap values for the SiOx layers to be established, which are found to have almost no dependency on the cation type R. It is determined that, after annealing, the stoichiometric factor x remains near 1.8 in all the systems under study, implying that the silicon quantum dot synthesis reaction is not fully completed. It is shown that the SiOx/Al2O3 multilayer contains octahedral structural motifs (SiO6) usually found in stishovite, whereas SiOx/SiO2 and SiOx/ZrO2 demonstrate an electronic structure similar to conventional silica. The intrinsic electronic excited states are examined by means of synchrotron-excited photoluminescence spectroscopy. Low-energy UV-excited luminescence of SiOx layers is found to have the same spectrum in all of the studied structures, while VUV-excited spectra strongly depend on the cation R. In these measurements, manifestations of ‘slow’ exciton-mediated and ‘fast’ defect-related luminescence are distinguished using nanosecond time resolution. It is shown that both mobile and bounded excitons appear in the suboxide layer under 6.2 eV and 5.8 eV irradiation and then relax radiatively through the triplet-singlet transition of the neighbouring oxygen-deficient centers. The complete picture of the optical excitation and relaxation processes in these materials is illustrated in a general diagram depicting electronic states.

Impurity- and defect-related luminescence of ZnSe:Fe at Low Temperatures

Polycrystalline zinc selenide is doped with an iron impurity to a concentration of ~1019 cm−3 in a zinc vapor atmosphere by the thermal diffusion method, and the distribution profile of optically active iron Fe2+ is obtained. The effect of the Fe2+ concentration on impurity-and defect-related ZnSe luminescence spectra at low temperatures is determined. In the range of 1.28—1.31 eV, a luminescence line with a decay time of ~0.341 ms is detected, with its intensity increasing in regions with Fe2+ concentration close to the maximum one. The line is attributed to the luminescence center arising due to the Fe2+ interaction with a background impurity or defect in ZnSe.