Deep Trapping Centers Research Articles

Preparation of porous composite coatings to suppress surface charge movement and reduce electrical resistivity

In this work, Cr2O3/TiO2 composite coatings were prepared on α-Al2O3 insulating ceramic matrix surface by screen printing and solid phase sintering methods to yield an average film thickness of about 14.10 μm. Scanning electron microscopy (SEM) revealed changes in the pore structures of the coatings at different TiO2 contents. The atomic force microscopy (AFM) analysis of the coating surface roughness suggested a TiO2 content of 0.025 mol to yield the best surface roughness and pore structure of the coating. The calculation showed an increase in the energy level of the deep trap center on the surface from 1.506 eV to 1.547 eV, reaching a maximum. The surface resistivity of insulating ceramics decreased from 1016 Ω to 1012 Ω. The formation of deep surface traps and the decrease in surface resistivity promoted charge dispersion and suppressed the formation of flashover channels, effectively improving the surface flashover voltage of the ceramics.

Beneficial role of manganese dioxide for reducing surface resistivity to increase surface flashover voltage of vacuum insulating ceramics

Electric vacuum insulated devices generate surface flashover and breakdown under instantaneous high voltage. Flashover voltage can be increased through coating without changing the structure of the ceramic device. Herein, multiphase coatings with different manganese dioxide contents are prepared on α-Al2O3 insulating ceramic surface by screen printing and solid-state sintering methods to yield an average film thickness of about 12.38 μm with good bonding strength. X-ray diffraction shows that the crystal phases of the coating are (Al0.9Cr0.1)2O3 and MnAl2O4. Scanning electron micrographs and atomic force microscopy reveal that greater surface height difference and deeper surface depth are more conducive to increasing the surface roughness and the formation of "pores", and decreasing the surface potential decay rate. When MnO2 content is 0.0069 mol, the deep trap center energy level on the material surface reaches its maximum, increasing from 1.506 eV to 1.550 eV, and the surface resistivity of ceramics decreases from 1016 Ω to 1013 Ω. The formation of deep trap centers on the surface makes it difficult for surface charges to detach, which reduces the charge mobility and surface potential decay rate, and leads to decrease in surface resistivity and increase in surface flashover voltage.

Composition dependent charge retention in amorphous HfxAl1−xOy dielectric layers

Charge trapping properties in dielectric materials serve as a ground principle in data storage devices called NAND flash. High-density nonvolatile storage requires deep trapping centers and localized trapped charge distribution. Current NAND flash technology relies on Si3N4 during the development history, and further development requires a higher dielectric constant material with compatible deep trapping centers and localized trapped charge profiles. In this work, charge trapping properties of HfxAl1−xOy dielectric layers with various compositions are investigated by modeling retention characteristics with trap energy and μτ product, where the former represents the trapping center depth and the latter represents the trapped charge profiles. Comparing the conventional Si3N4, HfxAl1−xOy dielectric layer with a low Hf content may improve long-term charge retention in NAND flash, while a localized trapped charge profile needs improvement to be compatible with Si3N4.

Электризация поверхности кварцевых стекол электронными пучками

To establish the effect of subthreshold defect formation on the charge accumulation in quartz glasses, a comprehensive study of the process of their electrization by electron beams was carried out. Earlier it was shown that the process of radiative electrization of quartz glasses consists of two stages. The short-term stage of charging can be explained by the accumulation of charge on the initial trap centers, and the long-term component can be caused by the generation of deep trap centers capable of capturing electrons. In the studied quartz samples, the trapping centers can be three-coordinated silicon atoms (E'-centers). The presence of two stages of the charging process is confirmed by two different methods for determining the surface potential. Despite the increase in the surface potential during irradiation and the resulting decrease in the energy of the incident electrons, an increase in the intensity of the cathodoluminescent signal is observed. Such an increase in intensity can be caused by an increase in the number of luminescent defects in quartz, a two-coordinated silicon atom or a non-bridging oxygen atom, as well as by charge accumulation at competing nonradiative trap centers.

High energy density and superior charge/discharge efficiency polymer dielectrics enabled by rationally designed dipolar polarization

Although many dielectric polymers exhibit high energy storage density (Ue) with enhanced dipolar polarization at room temperature, the substantially increased electric conduction loss at high applied electric fields and high temperatures remains a great challenge. Here, we report a strategy that high contents of medium-polar ester group and end-group (St) modification are introduced into a biodegradable polymer polylactic acid (PLA) to synergistically reduce the loss and enhance Ue and charge-discharge efficiency (η). The resultant St-modified PLA polymer (PLA-St) exhibits an Ue of 6.5 J/cm3 with an ultra-high η (95.4%), far outperforming the best reported dielectric polymers. It is worth noting that the modified molecular structures can generate deep trap centers and restrict the local dipole motions in the polymer, which are responsible for the reduction of conduction loss and improvements in high-temperature capacitive performance. In addition, the PLA-St polymer shows intrinsically excellent self-healing ability and cyclic stability surviving over 500 000 charge-discharge cycles. This work offers an efficient route to next-generation eco-friendly dielectric polymers with high energy density, low loss, and long-term stability.

Photoluminescence Intensity Enhancement in Tin Halide Perovskites.

The prevalence of background hole doping in tin halide perovskites usually dominates their recombination dynamics. The addition of excess Sn halide source to the precursor solution is the most frequently used approach to reduce the hole doping and reveals photo-carrier dynamics related to defects activity. This study presents an experimental and theoretical investigation on defects under light irradiation in tin halide perovskites by combining measurements of photoluminescence with first principles computational modeling. It finds that tin perovskite thin films prepared with an excess of Sn halide sources exhibit an enhancement of the photoluminescence intensity over time under continuous excitation in inert atmosphere. The authors propose a model in which light irradiation promotes the annihilation of VSn 2- /Sni 2+ Frenkel pairs, reducing the deep carrier trapping centers associated with such defect and increasing the radiative recombination. Importantly, these observations can be traced in the open-circuit voltage dynamics of tin-based halide perovskite solar cells, implying the relevance of controlling the Sn photochemistry to stabilize tin perovskite devices.

A flexible luminescence film with temperature and infrared response based on Eu2+/Dy3+ co-doped Sr2Si5N8 phosphors for optical information storage applications

Deep-trap luminescent materials have attracted great attention for optical information storage applications. However, the flexible luminescence films based on red luminescence materials with temperature and infrared response are scarce. In this study, we have successfully developed various novel flexible red emitting films based on Eu2+/Dy3+ co-doped Sr2Si5N8 phosphors through screen-printing and spin-coating technologies, respectively. Interestingly, the fabricated flexible luminescence films exhibit unique temperature and infrared responsive properties for optical information storage by releasing photons in response to thermal or infrared stimulation. Notably, deep-trap red emitting Eu2+/Dy3+ co-doped Sr2Si5N8 phosphors are crucial for the optical information storage properties of the films. Two emission peaks of Sr2Si5N8:Dy3+ phosphors at 476 nm and 577 nm are observed under excitation at 345 nm, corresponding to the radiative transition occurs from the 4F9/2 level to the 6H13/2 and 6H15/2 levels of Dy3+. When Dy3+ and Eu2+ ions are co-doped in Sr2Si5N8, the energy transfer from Dy3+ to Eu2+ in Sr2Si5N8 matrix is found and the decay time confirms that Dy3+ ions can be acted as deep trap centers to storage photons. For Sr2Si5N8:Eu2+,Dy3+ phosphors and the corresponding flexible luminescence films, the specific patterns (for example apple and note patterns) are firstly recorded under NUV or blue light excitation and then reappear through thermal stimulation or near-infrared photo-stimulation (980 nm laser). This work not only validates the feasibility of Sr2Si5N8:Eu2+,Dy3+ phosphors as deep-trap red emitting luminescence materials, but also suggests the applications of flexible luminescence films for optical information storage.

Optimization of Blue Photorefractive Properties and Exponential Gain of Photorefraction in Sc-Doped Ru:Fe:LiNbO3 Crystals

Sc:Ru:Fe:LiNbO3 crystals were grown from congruent melt by using the Czochralski method. A series of LiNbO3 crystals (Li/Nb = 48.6/51.4) with 0.1 wt% RuO2, 0.06 wt% Fe2O3 and various concentrations of Sc203 were prepared. RF1 and RF4 refers to the samples containing 0 mol% Sc203 and 3 mol% Sc203, respectively. The photorefractive properties of RF4 were measured by Kr+ laser (λ = 476 nm blue light): ηs = 75.7%, τw = 11 s, M/# = 19.52, S = 2.85 cmJ−1, Γ = 31.8 cm−1 and ∆nmax = 6.66 × 10−5. The photorefractive properties of five systems (ηs, M/#, S, Γ and ∆nmax) under 476 nm wavelength from RF1 to RF4 continually increased the response time, while τw was continually shortened. Comparing the photorefractive properties of Sc (1 mol%):Ru (0.1 wt%):Fe (0.06 wt%): LiNbO3 measured by Kr+ laser (λ = 476 nm blue light) with Sc (1 mol%):Fe (0.06 wt%):LiNbO3 measured by He-Ne laser (633 nm red light), ηs increased by a factor of 1.9, Vw (response rate) increased by a factor of 13.9, M/# increased by a factor of 1.8 and S increased by a factor of 32. The ∆nmax improved by a factor of 1.4. A strong blue photorefraction was created by the two-center effect and the remarkable characteristic of being in phase between the two gratings recorded in shallow and deep trap centers. The photorefractive properties (ηS, τw, M/#, S, ∆nmax) were increased with an increase in Sc3+ ion concentration. Damage-resistant dopants such as Sc3+ ions were no longer resistant to damage, but they enhanced the photorefractive properties at the 476 nm wavelength. The experimental results clearly show that Sc-doped two-center Ru:Fe:LiNbO3 crystal is a promising candidate blue photorefraction material for volume holographic storage. Sc-doped LiNbO3 crystal can significantly enhance the blue photorefractive properties according to the experimental parameters. Therefore, the Sc:Ru:Fe:LiNbO3 crystal has better photorefractive properties than the Ru:Fe:LiNbO3 crystal.

Wide Range X‐Ray to Infrared Photon Detection and Energy Storage in LiTaO3:Bi3+,Dy3+ Perovskite

AbstractIt is challenging to obtain a material that can detect photons ranging from X‐rays to infrared light. Such materials have promising use to develop advanced applications like in information storage, anticounterfeiting, or X‐ray imaging. This article reports on such a material; LiTaO3:xBi3+,yDy3+ perovskite phosphor. Experimental spectroscopy, thermoluminescence, and vacuum referred binding energy diagram (VRBE) construction are combined to study the trapping processes of charge carriers. The VRBEs in the ground or excited states of Bi3+ and Bi2+ are discussed. Bi3+ emerges to act as a ≈1.3 eV deep hole capturing center and it may possibly also act as a 0.80 ± 0.5 eV deep electron trapping center. A linear relation between the amount of stored charge carriers and a photochromic phenomenon both induced by X‐rays or 254 nm UV‐light charging appears. The stored charge carriers in LiTaO3:xBi3+,yDy3+ are removed by heating or with an optically stimulated process with 365 nm UV light to 850 nm infrared laser. More than 3 or 40 h of Bi3+ and Dy3+ based white afterglow is measurable in LiTaO3:0.005Bi3+,0.004Dy3+ after exposure to X‐rays or 254 nm UV light. Proof‐of‐concept light detection applications like 2D information storage and anticounterfeiting or X‐ray imaging are demonstrated by using the phtotochromic and white afterglow LiTaO3:xBi3+,yDy3+ phosphors.

Unraveling electron liberation from Bi2+ for designing Bi3+-based afterglow phosphor for anti-counterfeiting and flexible X-ray imaging

It is challenging to rational design persistent luminescence and storage phosphors with high storage capacity of electrons and holes after X-ray charging. Such phosphors have potential applications in anti-counterfeiting and X-ray imaging. Here we have combined vacuum referred binding energy diagram (VRBE) construction, photoluminescence spectroscopy, and thermoluminescence to study the trapping processes of charge carriers in NaYGeO4. In NaYGeO4:0.004Bi3+ and NaYGeO4:0.004Bi3+,0.005Ln3+ (Ln = Tb or Pr), Bi3+ appears to act as a shallow electron trap, while Bi3+ and Ln3+ act as deep hole trapping and recombination centres. We will show how to experimentally determine the VRBE in the Bi2+ 2P1/2 ground state in NaYGeO4 and NaLuGeO4 by thermoluminescence study. The electron trap depth produced by Bi3+ codopant in NaLu1-xYxGeO4:0.003Bi3+,0.008 Tb3+ can be adjusted, by increasing x, resulting in conduction band engineering. By combining Bi3+ as an electron trap and Bi3+ and Tb3+ as the hole traps, excellent X-ray charged afterglow phosphors were developed. The integrated TL intensity of the optimized NaYGeO4:0.004Bi3+ and NaYGeO4:0.003Bi3+,0.008Tb3+ after exposure to X-rays is about 4.5 and 1.1 times higher than that of the state-of-the-art BaFBr(I):Eu2+ storage phosphor. Intense initial Tb3+ 4f → 4f afterglow appears in NaYGeO4:0.003Bi3+,0.008Tb3+ and more than 40 h afterglow is measurable in NaYGeO4:0.004Bi3+ and NaYGeO4:0.003Bi3+, 0.008 Tb3+ after X-ray charging. We will show proof-of-concept anti-counterfeiting and X-ray imaging applications by using the developed afterglow phosphors and CsPbI3 quantum dots. This work not only provides experimental evidence on the VRBE in the Bi2+ 2P1/2 ground state in NaYGeO4, but also shows how to design and develop good afterglow phosphors for anti-counterfeiting and X-ray imaging by deeply studying and controlling the trapping processes of charge carriers in bismuth and/or lanthanides doped inorganic compounds.

The build-up of the laboratory-generated dose-response curve and underestimation of equivalent dose for quartz OSL in the high dose region: A critical modelling study

The application of a regeneration procedure for optically stimulated luminescence (OSL) dating requires that the dose-response curve (DRC) of a natural sample is the same as that of a laboratory-generated one. However, the build-up of the laboratory-generated DRCs of quartz has been widely reported in the literature, i.e., the laboratory-generated DRCs are significantly higher than the natural counterparts in the high dose region (above 150 or 200 Gy). This results in severe underestimation of equivalent dose (D e ) for quartz OSL in the high dose region during the application of a single-aliquot regenerative-dose (SAR) protocol. However, the potential mechanism governing the build-up of the laboratory-generated DRC is still unclear. In this study, we performed a comprehensive investigation of the natural and laboratory-generated OSL signals and DRCs using a kinetic model for quartz. We compared the differences in charge concentrations between natural and laboratory-irradiated aliquots following irradiation and monitored the competition for holes and electrons during preheat and stimulation, for the natural, regenerative, and test dose cycles. In the course of the modelling, we could see the build-up of laboratory-generated DRCs, the underestimation of D e , and a double exponential saturation characteristic of the DRCs. We demonstrated a discrepancy in competition for electrons in the deep electron trap and recombination centres during stimulation between the natural, regenerative, and test dose cycles. The simulation results are directly relevant to quartz OSL D e determination using the SAR protocol and reveal the mechanisms responsible for the experimentally observed different behaviours between natural and laboratory-generated DRCs.

Localized Deep and Shallow Traps of α-Peaks from Thermally Stimulated Current (TSC) Measurement on Thermoplastic Polymers

In this paper, trap levels around the glass transition temperature (Tg) of polymers have been characterized using Thermally Stimulated Current (TSC) technique. Deconvolution on α-peaks of the Tg for PE (-104 °C), plasticized PVC (-35 °C), PMMA (90 °C) and PET (96 °C) were carried out based on the first-order kinetic theory for non-Debye relaxation. Using temperature, T from TSC experimental data, we have successfully separated the α-peaks of the thermoplastic polymers. It is found that the complex curve of α-peaks can composed of four (4) to eight (8) sub peaks. Dominant sub peaks were identified at Tmax = -105 °C, -34 °C, 89 °C and 92 °C for PE, pPVC, PMMA and PET, respectively. These peaks show activation energy, Ea of shallow and deep trap centers ranged from 0.3 eV to 4.6 Ev where they represent the depolarization of localized dipoles and space charges relaxations in the polymers.

CaSiO3:Eu2+, Er3+, BaSiO3:Eu2+, Er3+ and SrSiO3:Eu2+, Er3+ phosphors: molten salt synthesis, optical and thermal studies

In this study, thermal and optical behaviors of CaSiO3:Eu2+, Er3+, BaSiO3:Eu2+, Er3+ and SrSiO3:Eu2+, Er3+ phosphors synthesized by molten salt synthesis technique are investigated. The doped phosphors activated with Eu2+ and Er3+ ions with varying doping concentrations of 0 mol% to 1 mol% were subjected to structural, optical, and thermal analyses. The powder X-ray diffraction analysis confirms the triclinic structure of CaSiO3: Eu2+, Er3+, orthorhombic structure of BaSiO3: Eu2+, Er3+, and monoclinic structure of SrSiO3: Eu2+, Er3+ phosphors with good homogeneity and phase purity. The morphological and topographical traits are studied via field emission scanning electron microscopy and transmission electron microscopy analyses. These analyses indicate the irregular shape and size of particles along with agglomeration that occurred as a result of prolonged heat treatment processes. The thermal behavior studied by thermogravimetric (TG) analysis confirms the thermal stability of these phosphors with less weight loss occurring while heating the phosphors from room temperature to 700 °C. The study of thermoluminescent spectra of ultraviolet (UV) and gamma-irradiated phosphors confirms deep trapping that occurred in the synthesized phosphors along with second-order kinetics. The trap depth analysis indicates the trapping of electrons in deep trap centers and hence high activation energy (~ 3 eV) is required to vacate the trapped electrons. Doping of conjugate rare earth pair Eu2+ and Er3+ in the host lattice leads to formation of more defects and hence enhances the trapping mechanism. The gamma-irradiated phosphors exhibit increased TL intensity, efficient trapping in much deeper trap centers, and improved fading characteristics in comparison to the UV-irradiated phosphors.

A facile strategy for enhanced performance of inverted organic solar cells based on low-temperature solution-processed SnO2 electron transport layer

High-efficiency organic solar cells (OSCs) based on low-temperature (LT) processed SnO2 electron transport layer (ETL) are promising for their commercial use. However, high density of traps and large contact barrier for carriers at the interface between LT SnO2 and the active layer has been reported. To solve the problem, various interface modifying layer materials, such as PFN, has been introduced. Currently, the fabrication process of such interface modifying layer materials is complex and expensive. Herein, a facile strategy involved a polar solvent ethanolamine (EA) is introduced to modify LT SnO2 surface. By soaking the SnO2 film into EA solution in 2-Methoxyethanol (2-ME), EA can easily anchor into SnO2 film surface and forms a continuous monomolecular layer via dehydration reaction. The whole process is green and highly compatible with a roll-to-roll process. Further study suggests that the deep trap centers on SnO2 surface are substantially reduced and the built-in potential in OSCs is reinforced. Finally, OSCs based on EA-modified SnO2 demonstrated an enhanced power conversion efficiency from 10.71% to 12.45% which was comparable to those based on ZnO (12.26%) under the same experiment parameters. Our work boosts the development of the inverted OSCs with easy fabrication and compatibility with roll-to-roll process.

Structure and photoluminescence properties of Dy3+ doping in Sr-Si-O-N frameworks for highly efficient white light and optical information storage applications

A series of Dy3+ doped Sr-Si-O-N frameworks with single phase were successfully developed by a two-step synthetic method. Under excitation of NUV light, Dy3+ singly-doped SrSi2O2N2 (a typical Sr-Si-O-N framework) phosphors exhibited two emission peaks at 478 nm and 571 nm, corresponding to blue (4F9-2→6H15-2) and yellow (4F9-2→6H13-2) emissions of Dy3+ ions. Compared with the traditional solid state reaction, the two-step method facilitated the formation of SrSi2O2N2: Dy3+ phosphor with pure phase and greatly enhanced the photoluminescence emission intensity (~4 times). The emission intensities of SrSi2O2N2: xDy3+ phosphors reached the maximum when x = 0.06 due to the concentration quenching, which was confirmed to be electric dipole-electric dipole interaction. The SrSi2O2N2: Dy3+ phosphors with an excellent thermal stability can be regarded as the potential single phase white-light phosphors when excited by NUV light. The optimal chromaticity coordinate of white light is (0.3347, 0.3966) for SrSi2O2N2: 0.06Dy3+ phosphor. Interestingly, the single green emission band at 525 nm was observed and a unique thermo-stimulated luminescence property was demonstrated for Dy3+/Eu2+ co-doped SrSi2O2N2 phosphors. A specific photographic pattern (for example pentagon pattern) was obtained by covered the mask on the surface of SrSi2O2N2: Eu2+, Dy3+ pellets under excitation of UV light. After UV lights turn off, the pentagon patterns were reproduced by heating the SrSi2O2N2: Eu2+, Dy3+ pellets. It was validated from the decrease of PL emission intensity and decay time with the increasing Dy3+ content that Dy3+ ions were acted as the deep trap centers. Similar thermal-stimulated luminescence was also found for Eu2+ singly-doped SrSi2O2N2 phosphor due to the oxygen vacancy in host lattice. Furthermore, the mechanism of thermal-stimulated luminescence has been proposed and Dy3+/Eu2+ co-doped SrSi2O2N2 phosphors with deep traps are suitable to be the good candidates for optical information storage applications.

High-temperature deep-level transient spectroscopy system for defect studies in wide-bandgap semiconductors.

Full investigation of deep defect states and impurities in wide-bandgap materials by employing commercial transient capacitance spectroscopy is a challenge, demanding very high temperatures. Therefore, a high-temperature deep-level transient spectroscopy (HT-DLTS) system was developed for measurements up to 1100 K. The upper limit of the temperature range allows for the study of deep defects and trap centers in the bandgap, deeper than previously reported by DLTS characterization in any material. Performance of the system was tested by carrying out measurements on the well-known intrinsic defects in n-type 4H-SiC in the temperature range 300-950 K. Experimental observations performed on 4H-SiC Schottky diodes were in good agreement with the literature. However, the DLTS measurements were restricted by the operation and quality of the electrodes.

Exfoliated graphitic carbon nitride self-recognizing CH3NH3PbI3 grain boundaries by hydrogen bonding interaction for improved perovskite solar cells

In CH3NH3PbI3 based perovskite solar cells (PSCs), the subtle pinholes and grain boundaries are hardly avoided in the formation of perovskite film. The defected film often leads to a reduced efficiency due to a larger amount of electron trap sites which induce carrier recombination leading to electron loss. In this work, ultrafine exfoliated graphitic carbon nitride (E-g-C3N4) nanoparticles are successfully synthesized by H2SO4 intercalation and NH3 stripping. The E-g-C3N4 has a lot of NH or OH groups, which could coil into nanoparticles spontaneous from nanosheets by hydrogen bonds. As an application in PSCs, the E-g-C3N4, as doping materials between MAPbI3 and the hole transport materials (HTMs), could magically self-recognize CH3NH3PbI3 grain boundaries and locate on them. Electron-hole recombination is greatly reduced even after incorporating trace of E-g-C3N4 by decreasing the deep electron trap centers at grain boundaries. As a result, the power conversion efficiency (PCE) of 15.8% is achieved for the MAPbI3 based PSCs, which is 35% higher than the reference cell. Thus, it is a universal and efficient method to weaken the influence of unavoidable defects existing in pinholes and grain boundaries, and develop the low-cost preparation for large-area PSCs in general conditions.

Thermoluminescent behavior of UV and γ rays irradiated Eu2+ and Er3+doped silicate phosphors

Abstract This paper reports the structural, morphological and thermoluminescent studies of Eu2+ and Er3+ ions activated ASiO3 (A = Ca, Ba, Sr) phosphors synthesized via conventional solid state reaction method. The impurity ions concentration of both Eu2+ and Er3+ ions varies from 0 mol% to 1 mol%. The study of crystallographic traits of the synthesized phosphors is done by Powder X-Ray Diffraction (PXRD) analysis. The crystalline nature, phase purity and appreciable homogeneity of the synthesized phosphors were confirmed by this study. The distinct morphological and topographical features were studied through Field Emission Scanning Electron Microscopy (FESEM) and Transmission Electron Microscopy (TEM). The thermoluminescent (TL) behavior of ultraviolet (UV) irradiated and gamma irradiated ASiO3:Eu2+, Er3+ (A = Ca, Ba, Sr) phosphors were studied here and the various kinetic parameters were estimated by Computerised glow curve deconvolution (CGCD) technique and peak shape method. The UV and gamma irradiated phosphors exhibit second order kinetics. The high values of activation energy (∼3 eV) confirm the trapping of electrons in deep trap centers. The incorporation of doping ions into the host lattice causes non-valence substitutions at the substitutional sites. This increases the number of defects and vacancies in the phosphors and more trap centers were generated. The increased TL intensity and high temperature glow peaks confirm the same. The synthesized phosphors show enhanced TL characteristics, efficient trapping mechanism and low fading effects. These characteristics confirm the suitability of the synthesized phosphors for TL mapping and sensing applications.

Development of efficient scintillator materials based on LiLaP4O12 doped with rare earth ions

Microstructural and luminescent properties of lithium lanthanum phosphate LiLaP4O12 (LiLaPO), undoped and doped with trivalent rare earth (RE3+, RE = Ce, Eu, and Tb) ions, were studied. LiLaPO powders were synthesized by sol-gel method assisted by polyvinyl alcohol (PVA). The structural phase analysis was confirmed by X-ray powder diffraction (XRD). Scintillator properties were investigated to evaluate the luminescent response of the material under excitation with X-rays. The scintillation efficiency of the doped samples were compared to the one measured for a known BGO scintillator and it was shown that Tb-doped LiLaPO showed efficiency 30% higher than the BGO one. The chromaticity diagram was obtained from radioluminescent (RL) measurements. The stability of the RL response was investigated during long term X-ray exposure. The results showed that the RL efficiency of the LiLaPO increased almost by 40%, for the Ce-doped sample, and almost 25%, for Eu and Tb-doped samples, as the exposure time increased. This behavior was attributed to the presence of the deep trapping centers that trap charge carriers during the first stage of the irradiation, with a subsequent release of the trapped charges increasing the probability of the electron-hole pair recombination as a function of the time. Time-resolved RL was carried out to investigate the luminescent decay time for doped samples and, for Ce-doped samples, a fast decay component with luminescent decay time of 17 ns dominates. Typical millisecond decay components with decay time constants of 3.12 ms and 5.17 ms were observed for Tb3+ and Eu3+-doped LiLaPO, respectively. Additionally, a slow component with decay time constant of 31.3 ms was also found for Eu3+-doped LiLaPO and this component was attributed to the existence very shallow traps in the material.

Identification of the spatial location of deep trap states in AlGaN/GaN heterostructures by surface photovoltage spectroscopy

Spatial and spectral origin of deep level defects in molecular beam epitaxy grown AlGaN/GaN heterostructures are investigated by using surface photovoltage spectroscopy (SPS) and pump-probe SPS techniques. A deep trap center ∼1 eV above the valence band is observed in SPS measurements which is correlated with the yellow luminescence feature in GaN. Capture of electrons and holes is resolved by performing temperature dependent SPS and pump-probe SPS measurements. It is found that the deep trap states are distributed throughout the sample while their dominance in SPS spectra depends on the density, occupation probability of deep trap states and the background electron density of GaN channel layer. Dynamics of deep trap states associated with GaN channel layer is investigated by performing frequency dependent photoluminescence (PL) and SPS measurements. A time constant of few millisecond is estimated for the deep defects which might limit the dynamic performance of AlGaN/GaN based devices.