Chi-Shen Lee

A Novel Tunable Green- to Yellow-Emitting β-YFS:Ce3+ Phosphor for Solid-State Lighting

A Ce(3+)-activated fluorosulfide phosphor (β-YFS:Ce(3+)) was synthesized by solid-state reaction in a sealed tube. The crystal structure has been refined from the XRD profiles and there are two different crystallographic rare earth sites, namely, Y(1) and Y(2), where the Ce(3+) ions occupied. The emission band with a maximum at 495 nm of β-Y(0.99)Ce(0.01)FS phosphor was characterized by the 4f-5d transitions of Ce(3+) ion. With increasing Ce(3+) concentration, the emission variations were observed from 495 to 547 nm. When β-YFS:Ce(3+) phosphors were utilized to incorporate with n-UV/blue chip, greenish-white light with color rendering index of 65-77 were obtained. The results indicate that the tunable green- to yellow-emitting β-YFS:Ce(3+) can serve as a potential phosphor for incorporation in fabrication for solid-state lighting. The preparation, spectroscopic characterization, quantum efficiency, thermal-quenching behavior, and related LED device data are also presented.

High-Performance GaN-Based Vertical-Injection Light-Emitting Diodes With TiO

We have designed and fabricated a new type of GaN-based thin-film vertical-injection light-emitting diode (LED) with TiO2-SiO 2 omnidirectional reflector (ODR) and n-GaN roughness. The associated ODR designed for LED operation wavelength at 455 nm was integrated with patterned conducting channels for the purpose of vertical current spreading. With the help of laser lift-off and photo-electrochemical etching technologies, at a driving current of 350 mA and with chip size of 1 mm times 1 mm, the light-output power and the external quantum efficiency of our thin-film LED with TiO2-SiO2 ODR reached 330 mW and 26.7%. The result demonstrated 18% power enhancement when compared with the results from the thin-film LED with Al reflector replace

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Enhancement of light output intensity for GaN-based vertical light-emitting diodes (LEDs), combining wafer bonding and the laser lift-off (LLO) process, employing an omnidirectional extraction surface with synthesized single-crystal ZnO nanorod arrays in aqueous solution at room temperature is presented. The light output intensity and wall-plug efficiency of the GaN-based LLO vertical LED with the omnidirectional extraction surface by ZnO nanorod arrays shows 38.9 and 41.2% increases, respectively, at 200 mA current injections compared to that of a vertical LED without ZnO nanorod arrays. The ZnO nanorod arrays not only support a current spreading layer but enhance the probability of photon escape through the omnidirectional extraction surface.

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The flip-chip light-emitting diodes (FC-LEDs) with geometric sapphire shaping structure were investigated. The sapphire shaping structure was formed on the bottom side of the sapphire substrate by a chemical wet etching technique for light extraction purpose. The crystallography-etched facets were (1010) M-plane, (1102) R-plane, and (1120) A-plane against the (0001) c-axis with the angles range between 29deg~60deg. These large slope oblique sidewalls are useful for light extraction efficiency enhancement. The light-output power of sapphire shaping FC-LEDs was increased 55% (at 350-mA current injection) compared to that of conventional FC-LEDs.

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In this paper, a novel GaN-based thin-film vertical injection light-emitting diode (LED) structure with a TiO2 and SiO2 omnidirectional reflector (ODR) and an n-GaN rough surface is designed and fabricated. The designed ODR, consisting of alternating TiO2 and SiO2, layers possesses a complete photonic band gap within the blue region of interest. The arrays of the conducting channels are integrated into the TiO2/SiO2 ODR structure for vertically spreading the current. Assisted by the laser lift-off and photo-enhanced chemically etched surface roughening process, the light output power and the external quantum efficiency of our thin-film LED with a TiO2/SiO2 ODR (at a driving current of 350 mA and with chip size of 1 mm × 1 mm) reached 330 mW and 26.7%, increased by 18% and 16%, respectively, compared with the results from the thin-film LED with an Al mirror. By examining the radiation patterns of the LEDs, the optical output power mainly increased within the 120 deg cone due to the higher reflectance of the TiO2/SiO2 ODR within the blue regime.

Omnidirectional Reflector and n-GaN Roughness

Abstract Two novel materials, [M(C6H5O7)(C6H6O7)(C6H7O7)(H2O)] . H2O (M=La(1a), Ce(1b)) and [Ce2(C2O4)(C6H6O7)2] . 4H2O (2), with a metal-organic framework (MOF) were prepared with hydrothermal reactions and characterized with photoluminescence, magnetic susceptibility, thermogravimetric analysis and X-ray powder diffraction in situ. The crystal structures were determined by single-crystal X-ray diffraction. Compound 1 crystallized in triclinic space group P1 (No. 2); compound 2 crystallized in monoclinic space group P21/c (No. 14). The structure of 1 is built from a 1D MOF, composed of deprotonated citric ligands of three kinds. Compound 2 contains a 2D MOF structure consisting of citrate and oxalate ligands; the oxalate ligand arose from the decomposition in situ of citric acid in the presence of CuII ions. Photoluminescence spectra of compounds 1b and 2 revealed transitions between the 5d1 excited state and two levels of the 4f1 ground state (2F5/2 and 2F7/2). Compounds 1b and 2 containing CeIII ion exhibit a paramagnetic property with weak antiferromagnetic interactions between the two adjacent magnetic centers.

Enhancement of Light Output Intensity by Integrating ZnO Nanorod Arrays on GaN-Based LLO Vertical LEDs

GaN-based flip-chip LEDs (FC-LEDs) with geometric sapphire shaping structure were fabricated. The sapphire shaping structure was formed on the bottom side of the sapphire substrate by the chemical wet etching technique for the purpose of light extraction. The crystallography-etched facets were (1 0 1 0) M-plane, (1 1 0 2) R-plane and (1 1 2 0) A-plane against the (0 0 0 1) c-axis with the angles range between ∼29 ◦ and ∼60 ◦ . It is demonstrated that the geometrical shape of the sapphire windows layer improves the light extraction efficiency. Compared to the conventional FC-LED, the sapphire-shaped FC-LED significantly enhanced the output power. The light output power of sapphire-shaped FC-LEDs was increased by 55% (at 350 mA current injection) compared to that of conventional FC-LEDs. (Some figures in this article are in colour only in the electronic version)

Luminance Enhancement of Flip-Chip Light-Emitting Diodes by Geometric Sapphire Shaping Structure

New ternary and quaternary chalcogenides, Sn 3− δ Pb δ Bi 2 Se 6 ( δ =0.0–0.7), were synthesized from pure elements using the solid-state method. Their crystal structures, determined using single crystal X-ray diffraction, belong to the orthorhombic space group Pnma (No. 62). The structure is related to Pb 3 Bi 2 S 6 , which contains NaCl [311] layer units and zigzag arrays of metal atoms along the c -axis. A correlation between the Pb composition and the shifted position of a metal site was observed. Band structure calculations confirmed that the structure is stabilized when the position of the M 5 site is farther from the mirror plane. Thermopower and conductivity measurements indicated that all of the compounds are n-type semiconductors with small band gaps.

Fabrication and characteristics of thin-film InGaN–GaN light-emitting diodes with TiO2/SiO2 omnidirectional reflectors

AlGalnP-based visible 650-nm GalnP-AlGalnP resonant-cavity light-emitting diodes (RCLEDs) with high-temperature stability were fabricated by wafer-bonding techniques on Si substrates. In this study, the metal-bonding RCLEDs (MBRCLEDs) devices were designed with 84-mum apertures for light output. The MBRCLEDs with a maximum wall-plug efficiency of 13.7% were demonstrated at an injection current of 2.5 mA. In addition, the improved heat sinking of MBRCLEDs led to lower junction temperature, and resulted in a very low power decay of 0.31 dB from room temperature to 100degC at an injection current of 20 mA.

New metal-organic frameworks of [M(C6H5O7)(C6H6O7)(C6H7O7)(H2O)] . H2O (M=La, Ce) and [Ce2(C2O4)(C6H6O7)2] . 4H2O

Small-scale GaN-based LEDs with a single electrode pad enjoying such properties as low cost, low series resistance, high efficiency and high yield were fabricated on a sapphire substrate by a novel and simple method. The devices present not only lower series resistance but higher light output power due to a specific n-contact design and better current spreading properties. Furthermore, higher ESD resistance (> −800 V at machine-mode operation) was demonstrated. The single-pad electrode of small-scale GaN-based LEDs has a chip size of 180 × 180 µm2, and showed a lower forward voltage of 3.15 V and 53.4% output power enhancement at 20 mA.