Thin Phosphor Research Articles

Analysis of absorption signal and noise in thin phosphor detectorsfor high-energy transmission radiography

We investigated the detective quantum efficiency (DQE) of thin gadoliniumoxysulfide phosphor-based flat-panel detectors (FPDs) using cascaded-systemsanalysis and Monte Carlo (MC) simulations for applications in megavoltage(MV) x-ray industrial imaging. We decomposed the DQE formula into(dose-independent) upper-limit DQE and (dose-dependent) DQE-reductionfactors. We obtained the absorbed energy distributions (AEDs) forvarious x-ray detector designs and photon energies using MC simulationsand applied the AED analysis to the DQE formula. The investigationsexamined include the x-ray-detector-only DQE and the effect of thecoupling efficiency between the x-ray detector and readout panel,including electronic noise, on the upper-limit DQE. This study confirmsthat the design of the metal build-up layer on the phosphor is effectivefor MV imaging and emphasizes the importance of designing the readoutpanel to maintain the upper-limit DQE. We expect the proposed DQEanalysis to be suitable for designing and evaluating FPDs for high-energynondestructive x-ray testing.

Upconversion phosphor thermometry for use in thermal barrier coatings

Measuring the temperature below the surface of a thermal barrier coating (TBC) using a thin phosphor layer is challenging primarily due to the absorption and scattering of laser excitation light and phosphor luminescence as they propagate through the coating. One way to increase phosphor luminescence could be to use upconversion phosphor thermometry, which is investigated in the current study. It is attractive because using longer excitation wavelengths reduces the absorption and scattering in TBCs as 8% wt. yttria-stabilize zirconia (8YSZ) generally has lower scattering and absorption coefficients around 1000 nm than at 532 and 355 nm. Therefore, the viability of upconversion to measure the temperature at the bottom of a TBC was evaluated for the first time and was compared with the more conventional downconversion phosphor thermometry. The current work involved an experimental study of several phosphors with lanthanides doped in the 8YSZ host, which were excited through downconversion by pulsed 355 nm and 532 nm laser light and through upconversion with 965 nm laser light. The YSZ:Er,Yb and YSZ:Ho,Yb phosphors show promise for upconversion phosphor thermometry. The experimentally acquired optical phosphor characteristics were used to simulate laser light and phosphor luminescence propagation in TBCs using Kubelka–Munk theory. This was to evaluate the signal strength with upconversion excitation compared to downconversion excitation. Upconversion excitation resulted greater signal strength from an embedded phosphor layer than 532 nm excitation and much higher than 355 nm excitation. Upconversion lifetime phosphor thermometry also resulted in improved phosphor lifetime temperature sensitivity. Coupled with reduced interference from background luminescence from impurities in TBCs with upconversion, it is a promising method for temperature measurements with the thermographic phosphor embedded in or underneath a TBC.

Ultra-high-resolution greyscale fluorescence images via UV-exposure of thin flexible phosphor films.

Thin films of BaFCl:Sm nanocrystals prepared using a polymer binder were used to create fluorescence images. The phosphor films were exposed to a UV-C mercury lamp light source via chromium-coated quartz greyscale masks to create 4 μm resolution greyscale fluorescence images. The mechanism relies on the highly efficient conversion of Sm3+ to Sm2+ ions upon exposure to UV-C light which displays a large linear dynamic range. The red fluorescence around 688 nm of the Sm2+ is then read-out using blue-violet illumination under a laser scanning confocal microscope. The greyscale images with 16 greyscale levels had a resolution equivalent to ∼125 line pairs per mm or ∼6400 dpi. Improvements in the resolution would be possible using collimated UV-C laser exposure of the film or the use of higher resolution photomasks. Ultra-high resolution binary fluorescence images were also created with resolutions down to 2 μm (∼250 line pairs per mm, ∼12 700 dpi). Downstream applications of the technology could include tailored covert or overt anti-counterfeiting labelling.

Development of Thin and High-brightness Phosphor Film for LCD Backlight using a Dichroic Filter

Development of Thin and High-brightness Phosphor Film for LCD Backlight using a Dichroic Filter

P‐83: Oxygen Ratio's Effect on the Photoluminescence Property of Zinc‐Oxide Thin‐Film Phosphor

ZnO thin films phosphor was fabricated using reducing annealing of deposited thin films. The effect of oxygen ratios on the structural and optical properties of ZnO films as well as nanostructures were investigated. The crystallinity of ZnO nanostructures was significantly improved depending on the oxygen ratios increasing during the radio frequency magnetron sputtering process. The photoluminescence property was enhanced with increasing the density of ZnO nanostructures which obtained from high oxygen ratio deposited thin film.

Performance of Luminescent Solar Concentrators Integrated with Negative Replica Layers of Leaf Surface Microstructures.

In this study, a negative replica layer of leaf surface microstructures was used to cover the top surfaces of semitransparent thin-film luminescent solar concentrators (LSCs) to enhance the concentrators’ performance. With low reflection on the air–glass interface of the glass plate in a thin-film LSC, a negative replica layer enables the scattering of incident sunlight and increases the path of light transmitted into the LSC and the thin phosphor layer at the bottom surface of the LSC. The incident sunlight is therefore more likely to interact with the phosphor particles in the thin-film phosphor layer, thereby enhancing the performance of the LSC. In this study, semitransparent thin-film LSCs with different inorganic phosphors were examined. The experimental results revealed that the optical collection efficiency of semitransparent thin-film LSCs covered with negative replica layers of leaf surface microstructures was higher than that of the semitransparent thin-film LSCs without negative replica layers. Furthermore, the LSCs with negative replica layers with high haze ratios exhibited high optical collection efficiency. Integrating negative replica layers of leaf surface microstructures as semitransparent layers in thin-film LSCs may optimize the application of LSCs in building-integrated photovoltaics (BIPVs).

Preparation of boron carbon oxynitride phosphor film via laser chemical vapor deposition and annealing

The present study demonstrates the synthesis of a boron carbon oxynitride (BCNO) phosphor film by laser annealing treatment of a boron carbonitride (BCN) film via laser chemical vapor deposition (CVD) using a single precursor source. A dense and uniform BCN film (B0.38C0.37N0.25) was prepared on a fused quartz substrate at a high deposition rate of 18.4 μm h−1 via laser CVD at a deposition temperature of 1373 K using only tetrakis(dimethylamino) diboron with argon (Ar) as a carrier gas. The surface of the BCN film was subsequently annealed in air at 1223 K by laser irradiation for 300 s to obtain a BCNO thin film. A disordered boron nitride (BN) layered structure, turbostratic BN, with a one-dimensional orientation was formed in the BCNO thin film. Upon excitation at 375 nm, the BCNO thin film exhibited photoluminescence with a broad emission band at 580 nm. The combination of CVD using a less-hazardous precursor and short-time annealing using a high-power laser enabled the preparation of a thin BCNO phosphor film.

Power-law analysis of nonlinear active-pixel detector responses as a function of mammographic energy

Abstract This study investigates the imaging performance of a complementary metal-oxide-semiconductor (CMOS) active pixel sensor (APS) array coupled with a gadolinium oxysulfide phosphor. A wide range of mammographic energy was considered, which corresponds to an aluminum-equivalent thickness in the range of 0.515 to 0.689 mm in terms of half-value layers. The performance metrics include the modulation-transfer function (MTF), Wiener noise-power spectrum (NPS), and detective quantum efficiency (DQE). The detector showed a nonlinear response at low exposure ranges, so it was described using a power-law model to linearize image data before evaluating the MTF, NPS, and DQE. We show that the NPS, which is measured from images without linearization but corrected by the exponent of the power function (viz., nonlinearity correction), can be consistent with that obtained using the linearization procedure. Negligible difference in MTFs measured with and without linearization showed good agreement between the nonlinearity-corrected and linearized DQEs. The DQE performance of the detector was comparable to published data for conventional digital mammography detectors. However, the detector design with a CMOS APS array in conjunction with a thin phosphor was energy-sensitive. The overall spatial-resolution performance (the size of effective aperture) improved by about 13%, the noise performance was degraded by about 22%, and the zero-frequency DQE was reduced by about 26% when measured with a Mo-target beam quality of 25 kV to 31 kV and a similar exposure of about 6 mR. The detector design may be appropriate for operation in a narrow range of energy. Further design considerations of the phosphor with output that tolerates variations in energy could lead to applications in convertible mammography and tomosynthesis.

Depth profiling and photometric characteristics of Pr3+ doped BaMoO4 thin phosphor films grown using (266 nm Nd-YAG laser) pulsed laser deposition

Abstract Pr3+ doped BaMoO4 nano thin films have been deposited by pulsed laser deposition on a glass substrate using a Nd-YAG laser (λ = 266 nm). The thin phosphor films were deposited in an ultra-high vacuum at a substrate temperature of 400 °C at different oxygen partial pressures. The crystal structure, surface morphology, film thickness and composition were examined by X-ray diffraction patterns, scanning electron microscope and energy-dispersive X-ray spectroscopy, respectively. The binding energy of the core levels, chemical state, atomic fractions, depth profiling of the thin film were investigated by X-ray photo-electron spectroscopy. The photoluminescence excitation, emission spectra and energy transfer process involved in Pr3+ doped BaMoO4 thin films were analyzed. The color chromaticity co-ordinates, fluorescence decay time and the photometric properties of the as-deposited thin phosphor films were examined.

Thin-Film Luminescent Solar Concentrators Using Inorganic Phosphors

Because of the predicted shortage of fossil fuels in the future, public interest in utilizing solar energy for photovoltaic (PV) electricity has exponentially increased in recent years. In particular, interest in the building integration of PVs is growing worldwide, and the use of building-integrated PVs (BIPVs) is one of the fastest growing segments of the PV industry. Luminescent solar concentrators (LSCs) are one of the possible approaches for BIPVs. However, the high cost and poor stability of the dyes or quantum dots used in conventional LSCs prevent the widespread adoption of electricity generation through the LSC-based solar energy. Therefore, inorganic phosphors with long lifetime and photostability under ultraviolet (UV) excitation and moisture are favorable luminescent materials for LSCs. In this paper, thin-film LSCs using commercial inorganic phosphors are proposed. The thin-film LSCs with different inorganic phosphors were examined. The phosphor concentration and relative position of the thin phosphor layer were investigated to determine the suitable structure of a thin-film LSC. This paper aimed to achieve an understanding of thin-film LSCs by using inorganic phosphors and to provide useful guidelines for future commercialization.

Effect of Phosphor Layer Size on the Optical and Thermal Properties of Chip Scale Packaged Light-Emitting Diodes

Light-emitting diodes (LEDs) have been widely applied for backlight unit for displays, general lighting, and automotive illumination because they have a high light efficiency and long life time, and are environmentally friendly. These diverse applications require smaller and slim LED chip scale packages (CSPs). CSP LEDs have been developed to produce white LED chips by forming a thin phosphor film on blue LED chips. In this study, we investigated the effect of phosphor layer thickness and area on the optical and thermal properties of the CSP LEDs. The phosphor-conversion of chips is maximized by varying the thickness and area of the phosphor layer. Eight different types of phosphor-converted white CSPs with normal performance (CRI > 70, CCT ~ 5700 K) were prepared by using mixed multicolor phosphor materials. In other words, for five samples whose CSP area was fixed at 1.8 x 1.8 mm2, flip-chip area was 1.4 x 1.4 mm2, and flip-chip thickness was 250 μm, the thickness of the phosphor layers was varied from 100 to 500 μm at intervals of 100 μm. For other three samples where the phosphor layer thickness was fixed at 300 μm, the flip-chip area was 1.4 x 1.4 mm2, and flip-chip thickness was 250 μm, the CSPs had three different areas of 1.6 x 1.6, 1.8 x 1.8, and 2.0 x 2.0 mm2. First, the relative luminous flux and LED beam distribution were characterized using integrating sphere and a goniometer. It was shown that as the thickness of the phosphor layer increased, the luminous flux and the directivity angle increased. On the other hand, as the CSP area increased, the luminous flux and the directivity angle decreased. This could be attributed to the fact that CSP does not have a reflector (namely, a housing material) and so the blue light of the chip does not excite the whole area of the phosphor. This may be confirmed by the images showing light distribution and luminescence characteristics, which is currently under investigation. Furthermore, a junction temperature measurement system was used to characterize the junction temperatures of the five different samples whose phosphor layer thicknesses were varied from 100 to 500 μm. It was found that when the thickness of the phosphor layer exceeded the thickness of the chips (250 μm), the junction temperature of the samples dramatically increased. Further details regarding the phosphor layer thickness of the junction temperature characteristics will be investigated and described.

A design and qualification of LED flip Chip-on-Board module with tunable color temperatures

with the increasing requirements on guaranteeing the color uniformity and improving the luminous efficacy and manufacturing efficiency, a wafer level chip scale packaging (WLCSP) technology has been developed by thermally impressing a thin multiple phosphor film on a LED wafer, then being segmented into individual LED chips. In this paper, a high power white LED Chip-on-Board (COB) module with high color rendering index (CRI, Ra > 93) and tunable correlated color temperatures (CCTs) is prepared by the flip chip technology. In this COB module, the tunable color temperatures are achieved by using two types of white LED CSPs with different target CCTs of 3000 K and 5000 K. The thermal and photochromatic properties and the photochromatic stability of the COB module are studied through both experiments and simulations. The results show that: 1) The measured spectral power distribution (SPD) intensity of the prepared COB module is smaller than the arithmetic sum of SPD intensities of its each series connected CSPs, which may be attribute to the light absorption happened among CSPs; 2) The junction temperature and driven current have the different effects on the photochromatic properties of COB module (i.e. luminous flux, CCT and CRI); 3) The nonlinearity of luminous flux as a function of driven current and junction temperature should be considered in its modeling.

Analytic solution of the radiative transfer equation for the analysis of the efficiency and spatial resolution of a granular X-ray scintillator

In this study, we discussed the efficiency and spatial resolution of a granular X-ray scintillator using an analytic solution of the radiative transfer equation (RTE). A conventional X-ray scintillator employs a large high-density charge-coupled device (CCD) in direct contact with thin phosphor layers to remove imaging optics. To analyze the performance of the scintillator using the CCD, we derive the analytic solution of the RTE in order to calculate light propagation in a double-layer structure without imaging optics. The analytic solution calculates multiple light sources and scatterings in the phosphor layer as well as refraction in the dielectric-layer interface between the phosphor layer and the CCD. For the calculations, we introduced the scattering phase function, including the backward scattering due to micron-sized phosphor particles, and included the geometrical relations of the refraction at the interface. We analyzed the performance of the scintillator with respect to various scintillator design parameters, such as the size of phosphor particles, the thickness of the phosphor layer, and the thickness and refractive index of the dielectric interlayer. We confirmed the analytic calculation by comparing the results with those obtained using a Monte Carlo (MC) simulation tool. The results are in good agreement, with small differences of 1% and 3% on average for the total efficiency and spatial resolution of the scintillator, respectively.

A strategy to increase phosphor brightness: Application with Ce3+-doped Gd3Sc2Al3O12

Y3Al5O12 (YAG) doped with Ce3+ ions is widely used as a phosphor for the generation of white light in LEDs. However, the material presents intrinsic drawbacks: (1) a yellow emission band lacking a red component, leading to a “cold” white light and (2) limited cerium incorporation (~ 3mol%), resulting in poor absorption and thus limiting the external quantum efficiency (EQE) of the LED device. In order to increase phosphor absorption and thus phosphor brightness, we propose here an original strategy based on the increase of Ce content in a phosphor compound, while preserving a high internal luminescence quantum yield. For this purpose, we introduce Ce3+-doped Gd3Sc2Al3O12 (GSAG: Ce). Gd3(1-x)Ce3xSc2Al3O12 polycrystalline samples were prepared through a solid-state microwave-assisted reaction, with x varying from 0 to 0.5; Ce3+ ions can be incorporated into the GSAG matrix with a concentration up to approximately x=0.18 (i.e. 18mol%) without the formation of a parasitic phase. The maximum emission wavelength is located at 573nm for a doping concentration of x=0.1, giving an orange feature to the emission. The internal luminescence quantum yield (Φ) is 52% for Ce concentration up to x=0.03. The EQE, which is proportional to the product of Φ by the Ce concentration when considering a thin transparent phosphor layer is predicted to be about 30% stronger when using GSAG doped with 10mol% Ce (x=0.1), with respect to YAG doped with 2mol% Ce (x=0.02).

Thermal/luminescence characterization and degradation mechanism analysis on phosphor-converted white LED chip scale packages

According to the requirements on minimizing the package size, guaranteeing the performance uniformity and improving the manufacturing efficiency in LEDs, a Chip Scale Packaging (CSP) technology has been developed to produce white LED chips by impressing a thin phosphor film on LED blue chips. In this paper, we prepared two types of phosphor-converted white LED CSPs with high color rendering index (CRI>80, CCT~3000K and 5000K) by using two mixed multicolor phosphor materials. Then, a series of testing and simulations were conducted to characterize both short- and long-term performance of prepared samples. A thermal analysis through both IR thermometry and electrical measurements and thermal simulation were conducted first to evaluate chip-on-board heat dissipation performance. Next, the luminescence mechanism of multicolor phosphor mixtures was studied with the spectral power distribution (SPD) simulation and near-field optical measurement. Finally, the extracted features of SPDs and electrical current-output power (I-P) curves measured before and after a long-term high temperature accelerated aging test were applied to analyze the degradation mechanisms. The results of this study show that: 1) The thermal management for prepared CSP samples provides a safe usage condition for packaging materials at ambient temperature; 2) The Mie theory with Monte-Carlo ray-tracing simulation can be used to simulate the SPD of Pc-white LEDs with mixed multicolor phosphors; 3) The degradation mechanisms of Pc-white LEDs can be determined by analyzing the extracted features of SPDs collected after aging.

Fabrication, characterization and electroluminescence studies of SrS: Ce3+ ACTFEL device

The SrS: Ce3+ thin film active phosphor layer of different thickness based alternating current thin-film electroluminescent (ACTFEL) devices are fabricated by electron beam evaporation deposition (EBED) method. The morphology and chemical composition of the deposited films are investigated by field emission scanning electron microscopy (FESEM). The optical transmittance in visible range for the optimum film and optical band gap of SrS: Ce3+ film is described on the basis of Ce3+ doping concentration.

A CMOS-based high resolution fluoroscope (HRF) detector prototype with 49.5 μm pixels for use in endovascular image guided interventions (EIGI).

X-ray detectors to meet the high-resolution requirements for endovascular image-guided interventions (EIGIs) are being developed and evaluated. A new 49.5-micron pixel prototype detector is being investigated and compared to the current suite of high-resolution fluoroscopic (HRF) detectors. This detector featuring a 300-micron thick CsI(Tl) scintillator, and low electronic noise CMOS readout is designated the HRF-CMOS50. To compare the abilities of this detector with other existing high resolution detectors, a standard performance metric analysis was applied, including the determination of the modulation transfer function (MTF), noise power spectra (NPS), noise equivalent quanta (NEQ), and detective quantum efficiency (DQE) for a range of energies and exposure levels. The advantage of the smaller pixel size and reduced blurring due to the thin phosphor was exemplified when the MTF of the HRF-CMOS50 was compared to the other high resolution detectors, which utilize larger pixels, other optical designs or thicker scintillators. However, the thinner scintillator has the disadvantage of a lower quantum detective efficiency (QDE) for higher diagnostic x-ray energies. The performance of the detector as part of an imaging chain was examined by employing the generalized metrics GMTF, GNEQ, and GDQE, taking standard focal spot size and clinical imaging parameters into consideration. As expected, the disparaging effects of focal spot unsharpness, exacerbated by increasing magnification, degraded the higher-frequency performance of the HRF-CMOS50, while increasing scatter fraction diminished low-frequency performance. Nevertheless, the HRF-CMOS50 brings improved resolution capabilities for EIGIs, but would require increased sensitivity and dynamic range for future clinical application.

Facile hydrothermal synthesis and pulsed laser deposition of Ca0.5Y1−x(WO4)2:xEu3+ phosphors: investigations on the luminescence, Judd–Ofelt analysis and charge compensation mechanism

PL emission spectra of Ca0.5Y1−x(WO4)2:xEu3+ for powder and thin film phosphors, and PL spectra of Ca0.5Y1−x(WO4)2:xEu3+ co-doped with alkali metal ions.

Linear analysis of signal and noise characteristics of a nonlinear CMOS active-pixel detector for mammography

The imaging properties of a complementary metal-oxide-semiconductor (CMOS) active-pixel photodiode array coupled to a thin gadolinium-based granular phosphor screen with a fiber-optic faceplate are investigated. It is shown that this system has a nonlinear response at low detector exposure levels (<10mR), resulting in an over-estimation of the detective quantum efficiency (DQE) by a factor of two in some cases. Errors in performance metrics on this scale make it difficult to compare new technologies with established systems and predict performance benchmarks that can be achieved in practice and help understand performance bottlenecks. It is shown the CMOS response is described by a power-law model that can be used to linearize image data. Linearization removed an unexpected dependence of the DQE on detector exposure level.

Preparation and characterization of YAG:Ce thin phosphor films by pulsed laser deposition

We report the use of YAG:Ce phosphor as the raw material to make thin and transparent phosphor films with pulsed laser deposition including the effects of heating temperature, target–substrate distances, annealing times, and annealing atmosphere on the YAG:Ce3+ phosphor film crystal types and spectral properties. The results indicated that at a coating temperature of 350°C, the YAG:Ce3+ phosphor film had the best crystallinity with an intact film and maximum fluorescence emission. The crystallinity and fluorescence emission intensity of the film gradually decreased as a function of increasing target–substrate distances. As the annealing time increased, the crystallinity and the fluorescence emission intensity of the film first increased and then decreased. The film made with 5 h of annealing had the best crystallinity and the highest fluorescence emission intensity. The crystallinity of the film annealed under air was higher than that made under nitrogen; the fluorescence intensity of the film under air was slightly lower than the film under nitrogen. The emission peak of the prepared film was at 523 nm when excited at 450 nm. This is slightly blue‐shifted versus the emission of commercial phosphor powders. This study offers a theoretical basis for the development of transparent phosphor films.