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Ce:Y3Al5O12 transparent ceramics (TCs) with appropriate emission light proportion and high thermal stability are significant to construct white light emitting diode devices with excellent chromaticity parameters. In this work, strategies of controlling crystal-field splitting around Ce3+ ion and doping orange-red emitting ion, were adopted to fabricate Ce:(Y,Tb)3(Al,Mn)5O12 TCs via vacuum sintering technique. Notably, 85.4 % of the room-temperature luminescence intensity of the TC was retained at 150 °C, and the color rendering index was as high as 79.8. Furthermore, a 12 nm red shift and a 16.2 % increase of full width at half maximum were achieved owing to the synergistic effects of Tb3+ and Mn2+ ions. By combining TCs with a 460 nm blue chip, a warm white light with a low correlated color temperature of 4155 K was acquired. Meanwhile, the action mechanism of Tb3+ ion and the energy transfer between Ce3+ and Mn2+ ions were verified in prepared TCs.

Elaborating a high performance and thermostable LED lighting source based on YAG:Ce ceramic photo-convertor is mainstream for outdoor lighting. However, it still suffers from narrow luminous angle using traditional molding method. In this paper, high quality YAG:Ce tubular ceramic was prepared using a low pressure injection molding (LPIM) and vacuum sintering. The results showed the viscosity of suspension with 75.0 wt.% solid content was as low as 190.34 Pa·s at the shear rate of 100 s−1, and the molding pressure and holding time were adopted to be 0.8 MPa and 8 s, respectively. Impressively, the final YAG:Ce ceramic (double polished to 1.0 mm) with relatively high in-line transmittance of 72.5 % was produced. Furthermore, constructing the YAG:Ce ceramic tube with the hexahedral “columnar blue chip arrays’’, a 4π white light emission with a luminous efficiency of 126.46 lm/W and low correlated-color temperature of 4333 K and was attained.

Advancement of optoelectronic and high-power devices is tied to the development of wide band gap materials with excellent transport properties. However, bipolar doping (n-type and p-type doping) and realizing high carrier density while maintaining good mobility have been big challenges in wide band gap materials. Here P-type and n-type conductivity was introduced in beta-Ga2O3, an ultra-wide band gap oxide, by controlling hydrogen incorporation in the lattice without further doping. Hydrogen induced a 9-order of magnitude increase of n-type conductivity with donor ionization energy of 20 meV and resistivity of 10-4 Ohm.cm. The conductivity was switched to p-type with acceptor ionization energy of 42 meV by altering hydrogen incorporation in the lattice. Density functional theory calculations were used to examine hydrogen location in the Ga2O3 lattice and identified a new donor type as the source of this remarkable n-type conductivity. Positron annihilation spectroscopy confirmed this finding and the interpretation of the results. This work illustrates a new approach that allows a tunable and reversible way of modifying the conductivity of semiconductors and it is expected to have profound implications on semiconductor field. At the same time it demonstrates for the first time p-type and remarkable n-type conductivity in Ga2O3 which should usher in the development of Ga2O3 devices and advance optoelectronics and high-power devices, Comment: 15 pages, 7 figures

Positron annihilation spectroscopy is a powerful tool to quantify the amount of vacancies and vacancy clusters in materials. The technique has been utilized to study the induced defects in materials after ion beam and neutron irradiations. This paper makes the case for how the technique can and should be utilized to quantify the defects created by irradiation in situ during irradiation to foster a more thorough understanding of the surviving defects after initial collision cascades. This paper outlines a future experimental approach and its meaning for the nuclear materials community, being able to benchmark commonly used rate theory models of damage evolution.

The interaction of positronium (Ps) with molecular oxygen dissolved in liquids is experimentally investigated. Computer software has been developed for fitting the positron annihilation lifetime spectra in liquids using parameters with clear physical meaning: rate constants of the Ps chemical reactions, annihilation rate constants of the different positron states, probability of Ps formation in a quasi-free state, typical formation time of a Ps nanobubble. Such processing of the spectra allowed identification of the dominant interaction of the Ps atom with dissolved oxygen. It turns out to be mainly ortho–para-conversion (Ps → 1/4 p-Ps + 3/4 o-Ps), but not oxidation (Ps + O2 → e+ + O2−). Values of the reaction rate constants are obtained.

Transparent ceramics (TCs) are incredibly promising color converters for high-power white LEDs/LDs. However, the preparation process of multiple structured TCs with a high color rendering index (CRI) is a complicated technical challenge, and the inability of single-structured TCs to achieve a high CRI significantly limits their real applications. In this study, high quality single-structured Ce,(Pr,Mn):YAG TCs with “wide peak” and “narrow peak” red light emissions were designed and fabricated via a solid-state reaction and a vacuum sintering method. Compared with the emission spectra of Ce:YAG TC, the synchronous doping of Pr3+ and Mn2+ ions into Ce:YAG TC resulted in inhomogeneous broadening of the full width at half maximum (FWHM) from 91.7 nm to 102.2 nm. Impressively, the CRI of a single Ce,(Pr,Mn):YAG TC-based high-power white LED was as high as 84.8, and the correlated-color temperatures (CCTs) of the white LEDs/LDs were 5450 K and 3550 K, respectively. Furthermore, when the addition amounts of Pr3+ and Mn2+ were 0.2 at% and 0.8 at%, respectively, the as-prepared Ce,(Pr,Mn):YAG TC displayed a high quantum efficiency (IQE = 48.14%) and excellent color stability (only 5% fluctuation). Therefore, this study not only demonstrates how to overcome the spectrum deficiencies of single-structured TCs that restrain intrinsic CRI improvement, but also provides a reference for the pursuit of high luminescence properties. This work significantly reinforces the understanding of the CRI problems of TC-based high-power lighting, which is crucial for the real application of white LEDs/LDs.

Transparent ceramics (TCs) with high quantum efficiency and excellent thermal stability are essential for the applications in high-power remote excitation white light-emitting diode or laser diode (LED/LD) lighting devices as color convertors. In this study, based on the “local complexity of Ce3+ ion coordination environment effect” and “effective ionic radius match effect”, an effective regulation of the phase composition and spectral properties was realized in LuAG TCs by incorporating Y3+ and Sc3+ ions. A series of Ce:(Lu,Y)3(Al,Sc)2Al3O12 (Ce:LuYSAG) TCs were fabricated with a high internal quantum efficiency (IQE) of 86.7%. The prepared TCs exhibited a distinct red-shift (520 → 541 nm) and a 10.2% increase of full width at half maximum (FWHM) in the emission spectra. Surprisingly, 89.9% of the room temperature emission intensity of the TC was retained at 200 °C. By combining the Ce:LuYSAG TCs with a 460 nm blue chip or a 455 nm laser source, white LED/LD devices in a remote excitation mode were constructed. A tunable hue from cyan-green to pale-green was obtained with increasing Y3+ concentration of TCs. The surface temperature of TCs varied from 112.6 °C to 62 °C under 1.0 W LD irradiation. Upon increasing the power of the blue laser from 1.0 W to 3.5 W, the luminous flux (LF) of the TC with 30 at% Sc3+ and 50 at% Y3+ ions substitution tremendously increased from 250.9 lm to 877.4 lm, and the optimized luminous efficiency of radiation (LER) was as high as 251 lm W−1. Therefore, this work opens up new opportunities for the development of TCs with excellent luminescence performance through manipulation of their local coordination structure for high-power white LEDs/LDs.

Ce:YAG transparent ceramics have great potential in white LEDs, especially under high power packaging technology, due to their superior thermal performance. However, their interactions and the synergistic effects among multiple factors limit the in-depth understanding and higher efficiency conversion of these ceramics for popular applications, and the single variable research approach is incapable of addressing the complex and diverse requirements for luminescence performance in as-constructed LED devices. In this work, for the first time, orthogonal experiments were designed to comprehensively evaluate the composite effects of thickness, doping concentration and surface roughness on the luminous efficiency and ideal white light of a constructed device with the assistance of the Taguchi method and one-way variance analysis. High throughput fabrication of ceramics was achieved using the novel Isobam gel casting method plus vacuum sintering. The Ce3+ doping concentration was proved to be the most dominant of these factors for excellent luminescence performance. The optimized factors of ceramics for luminous efficiency were 1.5 mm thickness, 0.2 at% Ce3+ concentration and double-polishing of surface roughness, while the optimized factors for good CIE were 0.4 mm thickness, 0.05 at% Ce3+ concentration and double-polishing. Therefore, this work not only shows how the Taguchi method can achieve a better group of multiple factors, but also how the synergistic effects of these factors work together. They significantly affect the optical properties of Ce:YAG ceramics and their performance in high power LED applications. These results are crucial for advancing transparent ceramics applications.

Ce:YAG transparent ceramic, a promising photo-converter for white LED lighting, faces a significant obstacle of post-annealing treatments, which allow the simultaneous elimination of defections and the regulation of the redox state of Ce. In this paper, vacuum as-sintered high quality Ce:YAG ceramics were subjected to various annealing conditions, and their microstructural evolution, and optical and luminescence properties were monitored and studied systematically. The results showed that air annealing efficiently benefited the microstructure and optical transmittance. In addition, the absorption band below 300 nm was strongly dependent on the annealing strategies due to the elimination of defects (e.g. F/F+-type color centers) and charge transfer (CT) between Ce(III) and Ce(IV). By constructing with 1350 °C air annealed Ce:YAG ceramics, a high-performance white LED with a luminous efficiency radiation (LER) exceeding 220 lm W−1 was achieved, thanks to the “protected”-annealing condition, which guaranteed moderate CT from Ce(IV) to Ce(III) due to weakened pump light absorption by defects. Differing from traditional Ce:YAG phosphors, excrescent oxygen vacancies were introduced by annealing Ce:YAG ceramics in hydrogen, leading to deteriorated luminescence properties. Therefore, it is expected that this refined preparation of Ce:YAG photo-convertors could drive the development of white LED lighting.

High lumen-density laser lighting has become an advanced lighting technology. However, as an important component, transparent light-conversion materials exhibit low efficiency and luminescence declining behaviors, especially under high power density lasers, seriously restricting their potential applications. Herein, high quality YAG:Ce transparent ceramics (TCs) with different Ce concentrations and sizes were designed and fabricated for comparison with YAG:Ce single crystals to reveal the nature behind the above phenomena under the assistance of different encapsulated models of LED/LD devices. The highest luminous efficiency for the TCs of over 248 lm W−1 was achieved in the LED device, while only 170 lm W−1 could be reached for the LD devices. The strong thermal quenching and concentration quenching worked synergistically to result in the luminescence declining behavior in the 0.5 at% YAG:Ce ceramics, which have 80% and 20% responsibility after smart experimental comparison, respectively. Besides, the potential factors for the low efficiency in TC-based LD lighting were also revealed. Therefore, this work not only shows the optical performance of TCs in LED and LD applications, but also presents how the deep influence factors and excitation models lead to differences in their performance. It significantly reinforces the understanding of fundamental problems for TC-based high power laser lighting. These results are crucial for advancing TC-supported technologies in illumination, projection and scintillation.

Realizing a high color rendering index (CRI) in Ce:LuAG transparent ceramics (TCs) with desired thermal stability is essential to their applications in white LEDs/LDs as color converters. In this study, based on the scheme of configuring the red component by Cr3+ doping, an efficient spectral regulation was realized in Ce,Cr:LuAG TCs. A unilateral shift phenomenon could be observed in both photoluminescence (PL) and photoluminescence excitation (PLE) spectra of TCs. By constructing TC-based white LED/LD devices in a remote excitation mode, luminescence properties of Ce,Cr:LuAG TCs were systematically investigated. The CRI values of Ce:LuAG TC based white LEDs could be increased by a magnitude of 46.2%. Particularly, by combining the as fabricated Ce,Cr:LuAG TCs with a 0.5 at% Ce:YAG TC, surprising CRI values of 88 and 85.5 were obtained in TC based white LEDs and LDs, respectively. Therefore, Ce,Cr:LuAG TC is a highly promising color convertor for high-power white LEDs/LDs applied in general lighting and displaying.

Monoclinic gallium oxide (β-Ga2O3) is attracting intense focus as a material for power electronics, thanks to its ultra-wide bandgap (4.5–4.8 eV) and ability to be easily doped n-type. Because the holes self-trap, the band-edge luminescence is weak; hence, β-Ga2O3 has not been regarded as a promising material for light emission. In this work, optical and structural imaging methods revealed the presence of localized surface defects that emit in the near-UV (3.27 eV, 380 nm) when excited by sub-bandgap light. The PL emission of these centers is extremely bright—50 times brighter than that of single-crystal ZnO, a direct-gap semiconductor that has been touted as an active material for UV devices.

$\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}$ is emerging as an excellent potential semiconductor for high power and optoelectronic devices. However, the successful development of $\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}$ in a wide range of applications requires a full understanding of the role and nature of its point and extended defects. In this work, high quality epitaxial $\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}$ films were grown on sapphire substrates by metal-organic chemical vapor deposition and fully characterized in terms of structural, optical, and electrical properties. Then defects in the films were investigated by a combination of depth-resolved Doppler broadening and lifetime of positron annihilation spectroscopies and thermally stimulated emission (TSE). Positron annihilation techniques can provide information about the nature and concentration of defects in the films, while TSE reveals the energy level of defects in the bandgap. Despite very good structural properties, the films exhibit short positron diffusion length, which is an indication of high defect density and long positron lifetime, a sign for the formation of Ga vacancy related defects and large vacancy clusters. These defects act as deep and shallow traps for charge carriers as revealed from TSE, which explains the reason behind the difficulty of developing conductive $\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}$ films on non-native substrates. Positron lifetime measurements also show nonuniform distribution of vacancy clusters throughout the film depth. Further, the work investigates the modification of defect nature and properties through thermal treatment in various environments. It demonstrates the sensitivity of $\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}$ microstructures to the growth and thermal treatment environments and the significant effect of modifying defect structure on the bandgap and optical and electrical properties of $\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}$.

Positron annihilation spectroscopy and transmission electron microscopy yield previously unknown insights on radiation damage., The nondestructive investigation of single vacancies and vacancy clusters in ion-irradiated samples requires a depth-resolved probe with atomic sensitivity to defects. The recent development of short-pulsed positron beams provides such a probe. Here, we combine depth-resolved Doppler broadening and positron annihilation lifetime spectroscopies to identify vacancy clusters in ion-irradiated Fe and measure their density as a function of depth. Despite large concentrations of dislocations and voids in the pristine samples, positron annihilation measurements uncovered the structure of vacancy clusters and the change in their size and density with irradiation dose. When combined with transmission electron microscopy measurements, the study demonstrates an association between the increase in the density of small vacancy clusters with irradiation and a remarkable reduction in the size of large voids. This, previously unknown, mechanism for the interaction of cascade damage with voids in ion-irradiated materials is a consequence of the high porosity of the initial microstructure.

Monoclinic gallium oxide (β-Ga2O3) is attracting intense focus as a material for power electronics, thanks to its ultra-wide bandgap (4.5–4.8 eV) and ability to be easily doped n-type. Because the holes self-trap, the band-edge luminescence is weak; hence, β-Ga2O3 has not been regarded as a promising material for light emission. In this work, optical and structural imaging methods revealed the presence of localized surface defects that emit in the near-UV (3.27 eV, 380 nm) when excited by sub-bandgap light. The PL emission of these centers is extremely bright—50 times brighter than that of single-crystal ZnO, a direct-gap semiconductor that has been touted as an active material for UV devices.

Planar YAG:Ce transparent ceramics have great potential in the high power white LEDs due to the superior optical property and thermal conductivity. However, it is still a challenging issue to explore a simple method to enhance the forward light extraction of device. In this paper, the high quality YAG:Ce ceramics were fabricated by tape casting plus vacuum sintering, and the different surface textures, thicknesses of YAG:Ce ceramics and their assembly modes with high power LED chips were designed to enhance the light extraction. A fully dense and pore free microstructure with uniform grain size of 5.40 μm was obtained, and the convex and concave impressions formed the texture and resulted in the photoluminescent enhancement of near six times in the single polished sample. The enhanced “light trapping effect” and reduced “TIR effect” worked together to enhance the forward light extraction of 22.5% in the SP-Up mode assembled device. The color coordinates of (0.3393, 0.3512) that close to white light were observed at the thickness of 0.6 mm. The high efficacy of ∼93.0 lm/W and low CCT of 4144 K could be obtained at the thickness of 0.8 mm in the simple remote mode. YAG:Ce planar transparent ceramics with special surface texture and assembly mode provided in this study greatly enhanced the extractions efficacy and improved the emission color, and then promoted their industrial application in high power white LEDs.

Cr: YAG and Cr, Nd: YAG transparent ceramics have significant application prospects in solid state lasers, therefore a controllable charge state of Cr ion in Cr doped YAG transparent ceramics is necessary. In this study, a successful regulation of Cr charge state in both Cr, Nd: YAG and Cr: YAG transparent ceramics was achieved, by a simple optimizing the sintering additives. Both ceramics with the Cr doping concentration of 0.3 at% reached to the theoretical transmittance, after the vacuum sintering and the subsequent annealing process. It was found that by adopting silica additive, divalent charged Cr 2+ ions could be detected from the vacuum sintered samples, and they were transferred into trivalent state after further annealing in air. Meanwhile, by vacuum sintering ceramics with divalent additives (CaO and MgO), a stable trivalent charged Cr ion could be obtained, and the subsequent air annealing process indicated a significant conversion from Cr 3+ to Cr 4+ . Further increasing the Cr concentration was not benefit to the optical quality as well as the conversion of Cr 3+ ion in Cr, Nd: YAG transparent ceramics.

We report on the observation of an unusual negative magnetic hysteresis loop in ZnO thin film codoped with cobalt and aluminum (Co-Al:ZnO), while other transition-metal-doped ZnO films, such as Cu-doped ZnO and Mn-doped ZnO, exhibit normal hysteresis loops. The unusual magnetic behavior is ascribed to the presence of double magnetic layers with different magnetic moments due to the change of structural defects across the film layers. Positron annihilation measurements confirmed the presence of unique microstructural changes in the Co-Al:ZnO film. This study shows that defects in diluted magnetic semiconductors may induce not only ferromagnetism but also novel magnetic behaviors.

In current study, various amounts of MgO single dopant was adopted to fabricated high quality transparent YAG ceramics, by utilizing a simple one-step solid state reaction sintering method in vacuum. At a MgO doping amount of only 0.03 wt.%, YAG transparent ceramics with a transmittance of 84.5% at 1064 nm could be obtained, after sintering at 1820 °C for 8 h. The microstructure evolution and optical property of as-fabricated YAG ceramics as a function of MgO doping concentration were systematically investigated. MgO dopant could effectively promote densification of YAG ceramics when the sintering temperature was lower than 1660 °C, and dramatically accelerate its grain growth between 1540 °C and 1660 °C. Further increase the doping amount of MgO would not benefit to the optical quality of YAG ceramics, and the intragranular pores as well as the Mg-riched secondary phase were observed from the MgO heavily doped ceramics.

Titanium oxide (TiO2) is a semiconducting oxide of increasing interest due to its chemical and thermal stability and broad applicability. In this study, thin films of TiO2 were deposited by pulsed laser deposition on sapphire and silicon substrates under various growth conditions, and characterized by x-ray diffraction (XRD), atomic force microscopy (AFM), optical absorption spectroscopy and Hall-effect measurements. XRD patterns revealed that a sapphire substrate is more suitable for the formation of the rutile phase in TiO2, while a silicon substrate yields a pure anatase phase, even at high-temperature growth. AFM images showed that the rutile TiO2 films grown at 805°C on a sapphire substrate have a smoother surface than anatase films grown at 620°C. Optical absorption spectra confirmed the band gap energy of 3.08 eV for the rutile phase and 3.29 eV for the anatase phase. All the deposited films exhibited the usual high resistivity of TiO2; however, when employed as a buffer layer, anatase TiO2 deposited on sapphire significantly improves the conductivity of indium gallium zinc oxide thin films. The study illustrates how to control the formation of TiO2 phases and reveals another interesting application for TiO2 as a buffer layer for transparent conducting oxides.

Sn dopant in ZnO may significantly improve the n-type conductivity of ZnO through a characteristic double effect. However, studies on bulk Sn-doped ZnO are rare, and the effect of Sn doping on the optoelectronic properties of bulk ZnO is not well understood. In this work, the effect of Sn doping on the optical and electrical properties of ZnO bulk single crystals was investigated through optical absorption spectroscopy, Hall-effect measurements, and thermoluminescence (TL) spectroscopy. Undoped and Sn-doped ZnO single crystals were grown by chemical vapor transport method and characterized by x-ray diffraction analysis. The Sn doping level in the crystals was evaluated by inductively coupled plasma mass spectroscopy measurements. Hall-effect measurements revealed an increase in conductivity and carrier concentration with increasing Sn doping, while TL measurements identified a few donor species in the crystals with donor ionization energy ranging from 35 meV to 118 meV. Increasing Sn doping was also associated with a color change of single crystals from colorless to dark blue.

Electron/hole traps alter the optical and electrical properties of materials by creating additional recombination pathways, trapping or providing charge carriers and modifying exciton dynamics. Understanding the defect/trap dynamics is crucial to control the optoelectronic properties of materials, and measuring donor/acceptor ionization energy is critical in semiconductor research. Here, we developed a highly sensitive thermally stimulated emission (TSE) spectrometer for the low temperature regime of 9–325 K to detect and characterize shallow traps in bandgap materials with enhanced sensitivity. It provides a powerful characterization tool for a wide range of semiconductors and electronic and photonic materials. This technique is ideal where electrical methods cannot be used for donor/acceptor characterization as in powder, irregular shape and thickness, and high resistive samples. The performance of the spectrometer was tested on Ce doped Y3Al5O12 single crystals, and the measurements identified several shallow levels that cannot be detected with conventional methods. Then, its capabilities were further demonstrated by detecting a shallow level in Ga2O3, which is emerging as an exceptional semiconductor for high-power devices and optoelectronics. A sophisticated data analysis technique based on the three-point analysis (TPA) approach was applied to deconvolute the highly overlapped TSE signals. The developed ultra-low temperature spectrometer together with the TPA deconvolution method provides a unique tool for studying exciton dynamics in photonic materials and measuring donor/acceptor ionization energies and densities in luminescent semiconductors. It will advance material characterization and development for a wide range of applications including lasers, electronic and illumination devices, and detectors for medical diagnostic and nuclear applications.

Epitaxial Ga2O3 films were grown by Metal Organic Chemical Vapor Deposition (MOCVD) on native substrates at growth rate of 1 µm/hour. The electron conductivity was introduced in the films through Si doping during deposition and the growth and doping parameters were optimized to control the electrical transport properties. The films were characterized in terms of structure, surface morphology, electrical transport properties, dopant concentrations and trapping defects. It was found that electron densities are not solely dependent on dopant concentrations and the use of metal organic precursor seems to induce additional donors of carbon. The work shows that the electron density and conductivity of MOCVD Ga2O3 films are mainly governed by the interplay between dopant concentration, C concentration and the presence of trapping defects in the films, which is most likely applicable for other oxide films grown by MOCVD. Conductive films of Ga2O3 with resistivity in the order of 0.07 Ω.cm were successfully grown. The electron density in most of these films was in the range of 1019 cm−3 but the mobility was limited to 1.5 cm2/V⋅s. Higher mobility of 30 cm2/V⋅s was obtained in some films at the expense of carrier concentration by reducing Si doping level resulting in resistivity in the order of 0.3 Ω.cm. This range of conductivity and mobility is relevant for field-effect transistors (FET) and the applications of Ga2O3 as transparent FET in Deep Ultra-Violet (DUV) technology.

Understanding the irradiation-induced defects in oxides is of interest for a wide range of applications. ZnO is an interesting oxide with mixed ionic and covalent bonding that contains a variety of point defect structures—making it an excellent model for studying irradiation-induced defects and their impact on properties. Here, we investigate the effects of neutron irradiation on the formation of defects and on the structural, optical, and electrical properties of ZnO single crystals. We observe the formation of vacancies and voids via positron annihilation spectroscopy. Neutron irradiation led to a significant deterioration of the ZnO structure and formed a high concentration of point defects, vacancy clusters, and voids with large disparities in their structure across variable irradiation times. It also led to significant changes in the optical properties and sample color. Irradiation for 444 h induced a high concentration of Cu acceptors as well as a high concentration of Ga donors. Temperature-dependent Hall effect measurements revealed the competing production of donors and acceptors and showed an increase in the slope of the carrier freeze-out curve with increasing irradiation dose. This work demonstrates the combined effects of neutron irradiation in producing a wide range of structural defects, impurities, and dopants in oxides and their enormous impact on modifying the oxide structure and both the optical and electronic properties. It particularly emphasizes the importance of considering the production of new impurities and dopants during the neutron irradiation of oxides.

Thermally stimulated current (TSC), thermally stimulated depolarization current (TSDC), and thermally stimulated luminescence (TSL) spectroscopies were combined to study trapping phenomena in undoped bulk SrTiO3 crystals. Electrical measurements were also performed and showed that the crystals are highly resistive in the dark but exhibited an unusually high photocurrent upon 400-nm illumination. Several traps were revealed in both TSC and TSDC spectra between 83 K and 450 K in such a broad temperature range and their activation energies were extrapolated from the trap positions (peaks). TSL spectra demonstrate similar characteristics comparable to TSC and TSDC spectra, though there are some differences because of different excitation and recombination mechanisms. This work reveals the presence of large number of traps in SrTiO3 single crystals, which are most likely the source of many of the interesting phenomena in SrTiO3 such as transient and persistent photoconductivity.

Tetraethyl orthosilicate (TEOS) was commonly served as a sintering additive to promote the densification of transparent Y 3 Al 5 O 12 (YAG) ceramics. However, Si 4+ that decomposed from TEOS would restrain the conversion of dopants into a higher valence state (e.g., Cr 3+ → Cr 4+ ). In this study, by using divalent sintering additives (CaO and MgO), the colorless and highly transparent YAG ceramics (T = 84.6%, at 1064 nm) were obtained after vacuum sintering at 1840 °C for 8 h and without subsequent annealing in air. An absorption peak centered at ∼320 nm was observed before annealing, and it extended to ∼550 nm after annealing at 1450 °C for 10 h in air. A discoloration phenomenon occurred and more scattering centers were observed with the formation of new [Mg/Ca 2+ F + ] color centers. Air annealing did not improve the optical quality of the as-fabricated YAG ceramics with divalent dopants as sintering additives, owing to the formation of scattering centers.

Monodispersed and spherical β-TCP granules for 3D printing were prepared by spray drying with the surface modification of PVB. Modifier PVB attached to the particle surface to form the crosslinked network structure and keep them combine with each other into aggregates. The morphology, size, forming performance and crushing behavior of granules as well as the microstructure and density of as-fabricated β-TCP ceramics were used to optimize the PVB contents. Promising results could be expected that PVB contents were 1.0 wt.% and the average particle size of granules was 37.36 μm. The modified granules exhibited excellent forming performance with monodispersed and spherical aggregates for 3D printing. They could be completely crushed under 80 MPa without obvious inter-granules void. These granules with 1.0 wt.% PVB had defined strengths for ceramic forming. The good dispersity and flowability assured the high density and uniform porous structure of β-TCP ceramics, which were used as biological ceramic materials and completely met the standard for bone tissue engineering.

In this work, a small amount of CaO single dopant was adopted to realize the densification and microstructure control of fine grained YAG ceramic with excellent optical quality, by a simple solid-state reaction and one-step vacuum sintering method. Then, highly transparent YAG ceramics (T = 84.4% at 1064 nm) were obtained just after vacuum sintering at 1820°C for 8 hours. The average grain size was only 2.7 μm, when the total amount of CaO was as low as 0.045 wt%. The effect of CaO on the microstructural evolution and optical property of the as-fabricated YAG ceramics was systematically investigated in detail. It was found that CaO dopant promoted both densification and grain growth of YAG ceramics when the sintering temperature was lower than 1660°C, however, it dramatically inhibited grain growth when the sintering temperature was further increased.

α-Al2O3 powder is widely used in numerous applications, especially in optical ceramics and crystals growth. However, achieving perfect dispersibility is essential in these applications and requires a complete understanding of the dispersion mechanism as well as high technology processing. In this paper, a step-by-step horizontal ball-milling method was adopted to improve the dispersibility of α-Al2O3 powders through coprecipitation. Effects of ball milling processes and Al3+ concentration on the particle size and the dispersity of alumina precursor were systematically investigated. The presence of absorbed sulfate ion on the surface of the particles could inhibit aggregation processes. Precursors were evolved into α-Al2O3 phase after subsequent calcination of 1180 °C in air. The work shows that step-by-step horizontal ball-milling achieves a well-dispersible powder with quasi-spherical morphology and narrow size distribution in the range of 150 ~ 200 nm.

High sinterability nano-Y2O3 powders for transparent ceramics were successfully synthesized via the decomposition of hydroxyl-carbonate precursors from spray coprecipitation. The chemical composition of the precursor was determined as Y(CO3)(OH)·nH2O (n = 1–1.5), and it was evolved into Y2O3 particles with clear facets after calcination with the assistance of sulfate. Two dispersion mechanisms, “absorption” and “intercalation,” were proposed to work together to provide the dispersion effect. Microstructural and optical characterization of powders and as-fabricated transparent ceramics was employed to evaluate the sintering behavior of powders. The nanopowders calcined at 1250 °C had weakly agglomerated morphology with the mean particle size of ~140 nm and exhibited excellent sinterability. The in-line transmittance of Y2O3 ceramic of 1 mm thickness that was vacuum sintered at 1800 °C for 8 h without any sintering additives reached 78.7% at 1064 nm.

Hydrogen does not exist as a neutral impurity in semiconductors; it always gives up an electron becoming positively charged impurity or acquires an electron becoming negatively charged impurity. This has profound effects on the electronic properties of materials.

A model that allows interpretation of positron annihilation spectra (LT and CDB) in nanodispersed dielectric powders is developed. Model takes into account e+ capture by vacancy-type defects, formation of e+ surface-bound states, as well as the formation of quasi-free positronium (inside crystallites) which may escape into intergranular space where it becomes thermalized during elastic collisions. The process of annihilation of ortho-Ps is narrowed down to its conversion into the para-state as a result of collisions with oxygen molecules (O2) adsorbed on the surface of crystallites. A computer program for lifetime spectra fitting that accounts on the parameters of the above model is developed (with ROOT framework). Combination of the program with our CDB software allows joint processing of the LT and CDB annihilation spectra of nanopowders.

Software that allows fitting positron lifetime (LT) and coincidence Doppler broadened (CDB) positron annihilation spectra of nanodispersed dielectric powders is developed. Computer program takes into account following effects: possibility of e+ trapping by vacancy-type defects, formation of the surface-bound e+ states, formation and thermalization of Ps atoms in inter- granular space. In order to test the developed software, lifetime and coincident Doppler broadened spectra of Al2O3 nanopowders with different grain sizes are acquired. Suggested physical model of the processes involved gives self-consistent interpretation of the spectra. Parameters of the thermalization kinetics of Ps and its ortho-para conversion due to presence of O2 molecules adsorbed on the crystallite surface are obtained.Software that allows fitting positron lifetime (LT) and coincidence Doppler broadened (CDB) positron annihilation spectra of nanodispersed dielectric powders is developed. Computer program takes into account following effects: possibility of e+ trapping by vacancy-type defects, formation of the surface-bound e+ states, formation and thermalization of Ps atoms in inter- granular space. In order to test the developed software, lifetime and coincident Doppler broadened spectra of Al2O3 nanopowders with different grain sizes are acquired. Suggested physical model of the processes involved gives self-consistent interpretation of the spectra. Parameters of the thermalization kinetics of Ps and its ortho-para conversion due to presence of O2 molecules adsorbed on the crystallite surface are obtained.

Positron is the only probe that can detect individual atomic vacancies and small and large vacancy clusters induced by irradiation with remarkable sensitivity, providing information about their size, concentration, and chemical environment. The focus of this review article is to provide guidance to facilitate applications of positron annihilation spectroscopy (PAS) in irradiation-induced defect studies to advance the development of new radiation-tolerant materials. The principle of PAS, its techniques, and data analysis methods are described. PAS studies of defects in nuclear and irradiated materials are reviewed and discussed in depth. Future developments to advance PAS applications in nuclear materials research and studies of materials under extreme environments are presented.

The discovery of single structure Ce3+ doped garnet transparent ceramics (TCs) with a broad full width at half maximum (FWHM) is essential to realize a high CRI for high-power white light emitting diodes (LEDs) and laser diodes (LDs). In this work, by utilizing the ion substitution engineering strategy, pure phase Gd3Sc2Al3O12:Ce3+ (GSAG:Ce) TC with a broad FWHM of 132.4 nm and a high CRI value of 80.7 was fabricated through the vacuum sintering technique for the first time. The optimized in-line transmittance of TCs was 58.4% @ 800 nm. Notably, the GSAG:Ce TCs exhibited a remarkable red shift from 546 nm to 582 nm, with a high internal quantum efficiency (IQE) of 46.91%. The degraded thermal stability in Ce:GSAG TCs was observed compared with that of Ce:YAG TC, owing to the narrowed band gap of GSAG. Additionally, remote excitation white LEDs/LDs were constructed by combining GSAG:Ce TCs with blue LED chips or laser sources. A tunable color hue from yellow to shinning white was achieved in white LEDs, whereas the acquired CRI and CCT of the white LDs were 69.5 and 7766 K, respectively. This work provides a new perspective to develop TCs with high CRI for their real applications in high-power white LEDs/LDs.

β-phase titanium pentoxide (β-Ti3O5) has great potential applications in sensors, electrodes and laser devices, due to its unique phase-transforming behaviors. However, just this reversible phase transition behaviors between λ and β phases made its high purity preparation be challenging, and the high energy consumption of present industrialized method also limited the high-level applications of β-Ti3O5. In this work, a novel carbothermal reduction method has been acquired to produce highly pure β-Ti3O5 compacts from titanium dioxide “wrapped” by phenolic resin served as carbon source. Their phase and microstructure evolutions during the whole reaction process were systematically characterized under the assistance of Rietveld refinement and SEM images. The highly active pyrolytic carbon of phenolic resin could deposit on the surface of TiO2 and this compacted state could promote the generated CO to accelerate the reaction process and decrease the addition of reductant, simultaneously. The reduction sequences were TiO2→ Ti4O7→ Ti3O5→ Ti2O3. The weight ratio of phenolic to precursor of 9.0% could achieve the pure β-Ti3O5 as high as 98.06% under the calcination temperature of 1250 °C for 4 h. This work provided a novel and industrialized route to prepare β-Ti3O5 for applications in sensor and electrode in electrochemical reactions.

Fourier transform infrared (FTIR) measurements were carried out on thin films and bulk single crystals of ZnO over a wide temperature range to study the free-carrier and multi-phonon infrared absorptions and the effects of hydrogen incorporation on these properties. Aluminum-doped ZnO thin films were deposited on quartz substrates using atomic-layer deposition (ALD) and sol–gel methods. Hall-effect measurements showed that the ALD films have a resistivity of ρ = 1.11 × 10−3 Ω cm, three orders of magnitude lower than sol–gel films (ρ = 1.25 Ω cm). This result is consistent with the significant difference in their free-carrier absorption as revealed by FTIR spectra obtained at room temperature. By reducing the temperature to 80 K, the free carriers were frozen out, and their absorption spectrum was suppressed. From the FTIR measurements on ZnO single crystals that were grown by the chemical vapor transport method, we identified a shoulder around 3350 cm−1 and associated it with the presence of two or more hydrogen ions in a Zn vacancy. After reducing the hydrogen level in the crystal, the measurements revealed the multi-phonon absorption of ZnO in the range of 700–1200 cm−1. This study shows that the multi-phonon absorption bands can be completely masked by the presence of a large concentration of hydrogen in the crystals.

Aluminum-doped zinc oxide (ZnO:Al) thin films were synthesized by atomic layer deposition on silicon, quartz and sapphire substrates and characterized by x-ray diffraction (XRD), high-resolution scanning electron microscopy, optical spectroscopy, conductivity mapping, Hall effect measurements and positron annihilation spectroscopy. XRD showed that the as-grown films are of single-phase ZnO wurtzite structure and do not contain any secondary or impurity phases. The type of substrate was found to affect the orientation and degree of crystallinity of the films but had no effect on the defect structure or the transport properties of the films. High conductivity of 10−3 Ω cm, electron mobility of 20 cm2/Vs and carrier density of 1020 cm−3 were measured in most films. Thermal treatments in various atmospheres induced a large effect on the thickness, structure and electrical properties of the films. Annealing in a Zn and nitrogen environment at 400°C for 1 h led to a 16% increase in the thickness of the film; this indicates that Zn extracts oxygen atoms from the matrix and forms new layers of ZnO. On the other hand, annealing in a hydrogen atmosphere led to the emergence of an Al2O3 peak in the XRD pattern, which implies that hydrogen and Al atoms compete to occupy Zn sites in the ZnO lattice. Only ambient air annealing had an effect on film defect density and electrical properties, generating reductions in conductivity and electron mobility. Depth-resolved measurements of positron annihilation spectroscopy revealed short positron diffusion lengths and high concentrations of defects in all as-grown films. However, these defects did not diminish the electrical conductivity in the films.

Cerium doped Barium Fluoride (BaF 2 Ce) transparent ceramic was fabricated and its luminescence and scintillation properties were studied. The photoluminescence shows the emission peaks at 310 nm and 323 nm and is related to the 5d-4f transitions in Ce 3+ ion. Photo peak at 511 keV and 1274 keV were obtained with BaF 2 Ce transparent ceramic for Na-22 radioisotopes. Energy resolution of 13.5% at 662 keV is calculated for the BaF 2 Ce transparent ceramic. Light yield of 5100 photons/MeV was recorded for BaF 2 Ce(0.2%) ceramic and is comparable to its single crystal counterpart. Scintillation decay time measurements shows fast component of 58 ns and a relatively slow component of 434 ns under 662 keV gamma excitation. The slower component in BaF 2 Ce(0.2%) ceramic is about 200 ns faster than the STE emission in BaF 2 host and is associated with the dipole-dipole energy transfer from the host matrix to Ce 3+ luminescence center.

An intense green luminescence is observed from single crystals of LiGaO2 doped with copper. Czochralski-grown undoped crystals are wrapped in thin copper foil and then held at 900 °C for 1 h in a flowing nitrogen atmosphere. Large concentrations of Cu+ ions enter the crystals during this process and occupy Li+ sites. These copper-diffused crystals are characterized with optical absorption, photoluminescence (PL), photoluminescence excitation (PLE), thermoluminescence (TL), and electron paramagnetic resonance (EPR). An optical absorption band peaking near 350 nm is assigned to the Cu+ ions at Li+ sites and represents an excitation from a 3d10 ground state to a 3d94s1 excited state. A broad PL emission from these excited Cu+ ions has a peak near 523 nm and the related PLE band has a peak near 356 nm (this PLE band links the emission to the optical absorption band). Illuminating a Cu-diffused crystal at room temperature with 325 nm laser light converts a portion of the Cu+ ions to Cu2+ ions. EPR spectra from these 3d9 ions are easily seen at low temperatures and their angular dependence is used to determine the g matrix and the 63Cu hyperfine matrix. Subsequent heating produces a TL peak near 122 °C with a maximum in its spectral dependence near 535 nm. Correlated EPR measurements show that this TL peak occurs when trapped electrons are thermally released from unintentionally present transition-metal ions (most likely Fe) and recombine with holes at the Cu2+ ions.

Scintillation properties are often studied by photo-luminescence (PL) and scintillation measurements. In this work, we combine X-ray-induced luminescence (XRIL) spectroscopy [Review of Scientific Instruments 83, 103112 (2012)] with PL and standard scintillation measurements to give insight into the scintillation properties of un-doped ZnO single crystals. XRIL revealed that ZnO luminescence proportionally increases with X-ray power and exhibits excellent linearity - indicating the possibility of developing radiation detectors with good energy resolution. By coupling ZnO crystals to fast photomultiplier tubes and monitoring the anode signal, rise times as fast as 0.9 ns were measured.

Realizing the high brightness solid-state lighting with the emission spectra similar to that of sun light is significantly challenged by the inherent deficiency of the luminescence spectrum and thermal robustness in the single sturctured Ce3+:Y3Al5O12 (Ce:YAG) transparent ceramic (TC). In this study, based on the idea of energy band engineering, the energy splitting intensity of 5d level and the energy gap between the ground band and the 5d1 energy level of Ce3+ ion in Ce:YAG TC were effectively modified via Sc doping. A series of Ce:Y3Al5−xScxO12 (Ce:YASG) TCs were fabricated with impressive properties, showing a decreased stokes shift and a distinct blue-shift (541 → 520 nm) with broadened full width in emission spectra. Particularly, the prepared TCs exhibited a desired thermal stability (only ~6% luminescent intensity loss at 150 °C) and high internal quantum efficiency (IQE) of 84.2%. The optimal composition between Ce3+ and Sc3+ ions was investigated by constructing the Ce:YASG and (Y1−yCey)3Al4ScO12 (YCASG) TC-based white LEDs/LDs in a remote excitation mode. Both the luminous efficiency of radiation (LER) as high as 218 lm/W and tunable color from pale-green (pale-white) to yellowish white (pale-green) region were obtained. Therefore, the Ce:YASG TC is a highly promising color conversion material for high power lighting and displaying applications in the future.

Intrinsic and extrinsic point defects often act as electron traps in oxide-based semiconductors and significantly impact their electrical and optical properties. Here, we show how to measure the density, energy level, and trapping cross section of the compensating acceptors that act as electron traps in Ga2O3 films, and we introduce the sheet trap number or the sheet compensating acceptor number as an essential parameter to fully describe the electrical transport properties of semiconductors. Si-doped β-Ga2O3 thin films were fabricated homoepitaxially by metalorganic chemical vapor deposition and studied by thermally stimulated luminescence spectroscopy, temperature dependent Hall-effect measurements, and secondary ion mass spectroscopy to investigate the compensating acceptor defects responsible for suppressing conductivity in the films. A deep level defect of energy in the range of 0.50–0.65 eV was identified as a compensating acceptor. The correlation between the electrical properties and its concentration and characteristics was established. This work shows how to quantify the density of compensating acceptors in semiconductors and directly relate it to the electrical transport properties, which should significantly advance the development of semiconductors and devices.