Maximum Light Output Research Articles

Inorganic, Organic, and Perovskite Halides with Nanotechnology for High–Light Yield X- and γ-ray Scintillators

Trends in scintillators that are used in many applications, such as medical imaging, security, oil-logging, high energy physics and non-destructive inspections are reviewed. First, we address traditional inorganic and organic scintillators with respect of limitation in the scintillation light yields and lifetimes. The combination of high–light yield and fast response can be found in Ce 3 + , Pr 3 + and Nd 3 + lanthanide-doped scintillators while the maximum light yield conversion of 100,000 photons/MeV can be found in Eu 3 + doped SrI 2 . However, the fabrication of those lanthanide-doped scintillators is inefficient and expensive as it requires high-temperature furnaces. A self-grown single crystal using solution processes is already introduced in perovskite photovoltaic technology and it can be the key for low-cost scintillators. A novel class of materials in scintillation includes lead halide perovskites. These materials were explored decades ago due to the large X-ray absorption cross section. However, lately lead halide perovskites have become a focus of interest due to recently reported very high photoluminescence quantum yield and light yield conversion at low temperatures. In principle, 150,000–300,000 photons/MeV light yields can be proportional to the small energy bandgap of these materials, which is below 2 eV. Finally, we discuss the extraction efficiency improvements through the fabrication of the nanostructure in scintillators, which can be implemented in perovskite materials. The recent technology involving quantum dots and nanocrystals may also improve light conversion in perovskite scintillators.

Influence of light absorption on the performance characteristics of UV LEDs with emission between 239 and 217 nm

The development of ultraviolet AlGaN multiple quantum well (MQW) light emitting diodes (LEDs) in the wavelength range between 239 and 217 nm is presented. The effects of aluminum composition in the MQW active region and of the underlying AlxGa1−xN:Si current spreading layer on the emission characteristics and operating voltages are investigated. A strong reduction in output power is observed with decreasing emission wavelength which is partly attributed to light absorption within the underlying AlxGa1−xN:Si. Additionally, a reduced carrier injection efficiency is identified as the root cause for the reduced emission power with decreasing emission wavelength. Emission powers at a dc current of 20 mA between 310 and 0.15 μW have been achieved for LEDs emitting between 239 and 217 nm. The maximum light output in pulsed mode operation of these LEDs ranged between 4.6 mW and 3.6 μW, respectively.

Design and simulation of solutions to reduce the thermal resistance of lighting systems

The paper focuses on some essential steps in the development of high power line light. The research within the solved project deals with the development of a special extruded profile with a cooling channel and with heat removal by the circulation of the coolant and on the design of a cooling system with all the safety elements. The partial aim of the work is to determine the thermal resistance of individual parts of the device and to design modifications so that the total thermal resistance and maximum temperature are within the limits set by the effort to achieve maximum light output, but also the longevity, economy, safety, and reliability of operation. The analytical calculations, CFD simulations, and real testing were used to determine the required parameters. The main result presented in this paper is a novel developed PCB with a copper core and thermal vias.

High-precision characterization of the neutron light output of stilbene along the directions of maximum and minimum response

The scintillation light output response of stilbene crystals has been measured for protons recoiling along the a, b, and c’ crystalline axes with energies between 1.3 and 10 MeV using neutrons produced with the tandem Van de Graaff accelerator at Triangle Universities Nuclear Laboratory. The proton recoil energy and direction were measured using the coincident detection of neutrons between a stilbene scintillator and an array of EJ-309 liquid scintillators spanning arranged neutron recoil angles. The maximum light output was found to coincide with proton recoils along the a-axis, in disagreement with other published measurements, which reported the b-axis as the direction of the maximum light output. Additional measurements were conducted using two different stilbene crystals to confirm these results: a second measurement using the coincident detection of neutrons; measurements of neutron full energy deposition events along the a and b axes; and measurements of the count rate for 252Cf neutrons traveling along the a and b axes directions. All measurements found that recoils along the a-axis produce the maximum light output.

Unidirectionally Emitting Quantum Cascade Microcavity Lasers with Patterned Contacts

We investigate unidirectionally emitting quantum cascade microcavity lasers with semicircle-patterned top contacts. We employ novel patterned top contacts while fabricating notched ellipse-shaped cavity lasers. We study experimentally the microcavity-structure-based quantum cascade (QC) laser material with a long infrared wavelength of ~10 μm. Then we characterize microcavity lasers with patterned contacts and compare them with nonpatterned ones and observe a lower operating injection threshold current as a consequence of this relatively straightforward technique, with the unidirectional emission feature being kept. We obtain a maximum light output peak power of 16 mW with unidirectional emission at a far-field divergence angle of ~7° at a full width of half maximum, while the patterned device shows low threshold even in the microcavity laser with a size of 150 μm. Furthermore, we also carry out a reliability test of the QC microcavity lasers with semicircle-patterned top contacts, and the testing results show no sudden failure or severe light-output-power drop during an operating time of thousand hours.

A GaN‐Based VCSEL with a Convex Structure for Optical Guiding

In this paper, room‐temperature continuous‐wave operation of a GaN‐based vertical‐cavity surface‐emitting laser (VCSEL) with a convex structure for optical guiding is demonstrated. The GaN‐based VCSEL is designed and fabricated with the convex structure containing a 15‐nm step between the center area and the peripheral area of the VCSEL with a Nb2O5 spacer layer. In theory the convex structure with a 1.5% relative refractive index difference between the areas could provide more than 10 high‐order modes. Experimental emission spectra and near field pattern images clearly suggested a multi‐mode operation of the VCSEL with the convex structure. In addition, the VCSEL shows a lower threshold current, 2 mA, and a higher maximum light output power, 0.88 mW, which are superior to those of our standard VCSEL fabricated simultaneously without the convex structure. These results indicate that the optical guiding with the convex structure plays an important role in developments of GaN‐based VCSELs.

Comparative Study of Highly Reflective ITO/DBR and Ni/Ag ohmic Contacts for GaN-Based Flip-Chip Light-Emitting Diodes

Heat removal and current spreading uniformity are important for the realization of highly efficient light-emitting diodes (LEDs). Here we have compared two types of GaN-based flip-chip LEDs (FCLEDs) with highly reflective indium-tin oxide (ITO)/distributed Bragg reflector (DBR) and Ni/Ag p-type ohmic contacts. The via-based n-type contact structure was implemented to increase the utilization ratio of active region area and improve current spreading over the entire LEDs. Compared with FCLED with Ni/Ag, the FCLED with ITO/DBR shows a 6.3% higher light output power (LOP) at low current density of 31 A/cm2 (90 mA) due to higher reflectance of ITO/DBR ohmic contact. However, at high injection currents, the Ni/Ag ohmic contact in FCLED alleviates self-heating issue by means of its superior heat dissipation performance. We also found that the less area coverage between ITO and metallization layer have a negative influence on heat dissipation performance of FCLED with ITO/DBR. As a result, the maximum light output power (LOP) of the FCLED with Ni/Ag obtained at 292.8 A/cm2 is 1.3 times larger than that of FCLED with ITO/DBR obtained at 196.3 A/cm2.

GaN-based flip-chip LEDs with highly reflective ITO/DBR p-type and via hole-based n-type contacts for enhanced current spreading and light extraction

We demonstrate GaN-based double-layer electrode flip-chip light-emitting diodes (DLE-FCLED) with highly reflective indium-tin oxide (ITO)/distributed bragg reflector (DBR) p-type contact and via hole-based n-type contacts. Transparent thin ITO in combination with TiO2/SiO2 DBR is used for reflective p-type ohmic contact, resulting in a significant reduction in absorption of light by opaque metal electrodes. The finely distributed via hole-based n-type contacts are formed on the n-GaN layer by etching via holes through p-GaN and multiple quantum well (MQW) active layer, leading to reduced lateral current spreading length, and hence alleviated current crowding effect. The forward voltage of the DLE-FCLED is 0.31V lower than that of the top-emitting LED at 90mA. The light output power of DLE-FCLED is 15.7% and 80.8% higher than that of top-emitting LED at 90mA and 300mA, respectively. Compared to top- emitting LED, the external quantum efficiency (EQE) of DLE-FCLED is enhanced by 15.4% and 132% at 90mA and 300mA, respectively. The maximum light output power of the DLE-FCLED obtained at 195.6A/cm2 is 1.33 times larger than that of the top-emitting LED obtained at 93A/cm2.

Crystal growth and scintillation characterizations of Tl2LiYCl6: Ce3+

Single crystals of Tl2LiYCl6 with different mole% of Ce-concentration (0.5 and 5mol%) were presented. This material belongs to Chloro-elpasolite crystal family and was grown by two zone vertical Bridgman technique. X-ray diffraction pattern of the Tl2LiYCl6 single crystal was measured. X-ray diffraction analysis confirmed tetragonal crystal structure. The melting point of the sample was measured by DSC. Typical Ce3+ ion emission spectra were observed when samples were excited by an X-ray source at room temperature. The observed emission spectra were located between 350nm and 530nm wavelength range and peaking around 430nm. Pulse height spectra of the samples under 662keV γ-rays excitation shows improvement in energy resolution with the increase of Ce-concentration in the host lattice. The energy resolutions were obtained to be 10.6% and 7.2% (FWHM) for 0.5% and 5% Ce3+ concentration, respectively. Under γ-ray excitation, a maximum light yield of 23,800±2400ph/MeV was observed for 5% Ce-concentration. Studied samples of Tl2LiYCl6: Ce3+ showed three exponential decay time components under γ-ray excitation. From the measured scintillation properties, we consider this material is a promising scintillator for radiation detection.

: New Tl-based Elpasolite Scintillation Material

New single crystals of Ce-doped (1% and 10%) Tl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> LiYCl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> (TLYC) were investigated under X-ray and -y-ray excitation. This material belongs to Chloro-elpasolite crystal family and was grown by the two-zone vertical Bridgman technique. X-ray-induced luminescence shows typical Ce <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> - ion emission between 350 and 530 nm peaking at 430 nm. Under 662 keV -y-rays excitation, best energy resolution of 4.8% (FWHM) was found for 1% Ce-concentration. For the same Ce-concentration, a maximum light yield of ~30 500 ± 3500 ph/MeV was observed at room temperature. Under -y-ray excitation, three decay time components were observed for all Ce-doped samples. Effective Z-number and density of Tl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> LiYCl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> were found to be 69 and 4.58 g/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> , respectively. Due to highly hygroscopic nature of this compound, extra attention was devoted during handling and data taking processes. Overall, the scintillation properties confirm that this material is a promising candidate for medical imaging and radiation detection.

Scintillation Characterizations of Tin Doped Lithium Iodide Crystals at Room and Low Temperature

The crystals of 0.02%, 0.05%, 0.1%, and 0.5% Sn doped LiI are grown by the vertical Bridgman technique. The luminescence and scintillation properties of the grown crystals are investigated. X-ray excited emission spectrum of LiI (Sn) showed a broad emission band between 400–650 nm wavelength range at room temperature. Such emission is attributed to $\hbox {Sn}^{2 +}$ ion. Scintillation properties such as energy resolution, decay time profiles, and light yield are measured under 662 keV ( $^{137}\hbox {Cs}$ ) $\gamma $ -ray excitation at room temperature. A maximum light yield of $6000 \pm 600 \hbox{ph/MeV}$ is measured at room temperature. LiI (Sn) single crystals showed two exponential decay components under $\gamma $ -ray excitation. The dependence of scintillation properties on the temperature is also presented. Changes in the decay time and light yield are measured from 295 K down to 10 K. The light yield of LiI (Sn) at 50 K is found to be nine times higher than that at room temperature.

Enhanced light extraction and tunable chromaticity in white organic light-emitting devices with ZnO nanopillars.

Robust ZnO nanopillars (NPs) with well-controlled morphology were directly prepared on the glass surface of a white organic light-emitting diode (WOLED) substrate using a hydrothermal process. The NPs can effectively recover light loss in the glass substrate and the current efficiency of the device increased by 20%. In addition, the chromaticity coordinates of the WOLED can be tuned by selecting the dimensions of the ZnO NPs. As a consequence, it is possible to produce a solar light spectrum using this technique. Furthermore, the nanotextured glass surface was superhydrophobic, which caused self-cleaning properties to the surface and help maintain maximum light output.

Improved output power and low forward voltage for GaN-based light emitting diodes with an annealed ITO transparent conducting layer

This study performs various thermal treatments on Indium Tin Oxide (In2O3 + SnO2) films, which are the transparent conducting layers for GaN light emitting diodes (LEDs). Structural characterization by high-resolution transmission electron microscopy (HRTEM) indicates that the ITO films have the least crystalline grain boundary and greatest grain size after thermal treatment at 580 °C for 10 min, which gives a minimum forward voltage of LEDs of 2.95 V and the maximum light output power of LEDs is 177.51 mW at 125 mA. The high output intensity in the near horizontal direction is also greater than that for treatment at other temperatures.

Improved performance of AlGaN-based deep ultraviolet light-emitting diodes with n-AlGaN underlayers

We demonstrate notable performance improvement of 270 nm deep ultraviolet light-emitting diodes (DUV-LEDs) by inserting an n-AlGaN underlayer (UL), which was directly beneath an AlGaN multiple quantum well (MQW) active region. This DUV-LED exhibited significantly improved forward and reverse bias current-voltage characteristics, and spectral properties. As a result, the turn-on voltage was decreased from 7 to 4.6 V by introducing the n-AlGaN UL, and the maximum light output power and external quantum efficiency of DUV-LEDs with the n-AlGaN UL, obtained via on-wafer measurement, were increased by factors of 3.6 and 2.2, respectively. Further enhancements can be realized by using high-quality AlN/sapphire templates together with the n-AlGaN UL. These improvements were considered to be attributed to the introduction of the n-AlGaN UL which improved electron injection and reduced the quantum confined stark effect in the AlGaN MQWs.

Scintillation characterization of thallium-doped lithium iodide crystals

The paper discusses scintillation and luminescence properties of thallium-doped LiI crystals, grown by the Bridgman technique. X-ray induced emission spectrum is obtained between 380nm and 600nm, and is attributed to the Tl+ ion. The photoluminescence measurement with the excitation wavelength of 305nm revealed a similar emission spectrum. Light yield, energy resolution and scintillation decay time profiles were studied under 662keV (137Cs) γ-ray excitation. A maximum light yield of 14,000±1400ph/MeV and two exponential decay time components were obtained.

A novel theoretical model for broadband blue InGaN/GaN superluminescent light emitting diodes

A broadband superluminescent light emitting diode with In0.2Ga0.8N/GaN multiple quantum wells (MQWs) active region is investigated. The investigation is based on a theoretical model which includes the calculation of electronic states of the structure, rate equations, and the spectral radiation power. Two rate equations corresponding to MQW active region and separate confinement heterostructures layer are solved self-consistently with no-k selection wavelength dependent gain and quasi-Fermi level functions. Our results show that the superluminescence started in a current of ∼120 mA (∼7.5 kA/Cm2) at 300 K. The range of peak emission wavelengths for different currents is 423–426 nm and the emission bandwidth is ∼5 nm in the superluminescence regime. A maximum light output power of 7.59 mW is obtained at 600 mA and the peak modal gain as a function of current indicates logarithmic behavior. Also, the comparison of our calculated results with published experimental data is shown to be in good agreement.

Luminescence properties of Pr3+ doped Li2O–MO–B2O3 glasses

Lithium borate glasses of composition 30Li2O–10MO–59B2O3:1Pr2O3 (MO=ZnO, CaO and CdO) were prepared by a conventional melt quenching method. The amorphous nature of these glasses was confirmed by X-ray diffraction. Characterization of these glasses was carried out by differential scanning calorimetry and infrared spectra. Optical absorption and photoluminescence spectra of these glasses were investigated. Judd–Ofelt intensity parameters Ωλ (λ=2, 4, and 6) are evaluated from the intensities of various absorption bands of optical absorption spectra. From this theory various radiative properties like transition probability A, branching ratio βr, the radiative lifetime τR, and the stimulated emission cross-section σP for various emission levels of these glasses have been determined and reported. The relationship between the structural modifications and luminescence efficiencies of all the three glasses is explained. After X-ray irradiation on these glasses, the thermoluminescence emission properties were also studied. The thermoluminescence studies indicate that CdO modified glass gives out the maximum thermoluminescence light output among the three glasses studied.

Scintillator-Oriented Investigation of Y3Al5O12:Ce,Tb Powders

The spectral and kinetic characteristics of YAG:Ce,Tb powders (with Tb3+ concentration varying up to 100%, relative to Y) using cathode-ray and UV excitations were determined. The energy diagram of YAG:Ce,Tb was constructed and energy transfer processes in the compound were studied. The most effective resonance energy transfer takes place at Tb3+ concentrations of 20%. There is no evidence of Ce3+→Tb3+ energy back transfer for (Y,Tb)AG:Ce even in high resolution spectra. The optimal Tb3+ content for maximum light output of YAG:Ce,Tb was determined to be 20% and for lowest afterglow 100%.

Daylight enhancement using laser cut panels integrated with a profiled Fresnel collector

Performance of conventional vertical light pipes with acrylic domes is highly dependent on the solar altitude since for lower solar elevation the acceptance angle is limited due to the small inlet aperture. A profiled Fresnel collector with a multi-lens system is useful in enhancing light in a light pipe at low solar altitudes but suffers losses due to multiple reflections during transmission. In this paper we show that further enhancement of light during the morning and evening hours is possible by using a profiled Fresnel collector integrated with graded laser cut panels, i.e. laser cut panels with bands of varying depth to width ratio that can redirect low altitude sun rays axially down the light pipe. The effect of various depths to width ratios of the laser cut panel with respect to variation in incident light on the performance of a light pipe is also presented in this paper. The graded laser cut panel performed better than other laser cut panels. Optimum tilts of the laser cut panel at which maximum light is deflected for various incident angles of the source were identified. Maximum light output for the graded laser cut panel was for lower incidence of solar altitude angle. The graded/profiled laser cut panel was integrated with Fresnel north and south facing collectors and the performance was assessed in daylight. Good redirection of light was observed for the Fresnel collector in the morning and evening hours when compared to a light pipe with an acrylic dome or a Fresnel collector without a laser cut panel. The light is redirected more parallel to the axis of the pipe thus minimizing the reflections on the internal lining and thereby increasing the performance of the light pipe.

Effects of Asymmetric Quantum Wells on the Structural and Optical Properties of InGaN-Based Light-Emitting Diodes.

A metalorganic vapor phase epitaxy-grown InGaN/GaN multiple-quantum-well (MQW) with three graded-thickness wells (the first-grown well had the greatest width) near the n-GaN was used as the active layer of an LED. For LEDs with an asymmetric quantum well (AQW), high-resolution X-ray diffraction and transmission electron microscopic reveal that the modified MQWs with a reasonable crystalline quality were coherently strained on the underlying GaN epilayers without any relaxation. In addition, the slight increase of indium segregation in the LED with an AQW may be attributed to variations in indium contents experienced during epitaxial growth of the wide well-containing MQWs. By preventing the energetic electrons from accumulating at the topmost quantum well nearest the p-GaN, the presence of light intensity roll-off in the LED with an AQW is shifted to higher currents and the corresponding maximum light output power is increased with a ratio 7.9% higher than that of normal LEDs. Finally, similar emission wavelengths were observed in the electroluminescence spectra of both LEDs, suggesting that light emitted mostly from the top quantum wells (near the p-GaN) while the emissions from the AQW region were insignificant.