Te-Yuan Chung

Solid-state laser spectral narrowing using a volumetric photothermal refractive Bragg grating cavity mirror

Dramatic spectral narrowing of two normally broadband lasers, Ti:sapphire and Cr:LiSAF, was achieved by simply replacing the output mirror with a reflective, volumetric Bragg grating recorded in photothermal refractive glass. The output power of each laser was unchanged from that obtained using dielectric coated output mirrors with the same output coupling as the Bragg grating while spectral brightness increased by 3 orders of magnitude.

High uniformity in angular correlated-color-temperature distribution of white LEDs from 2800K to 6500K.

In this paper, to our best knowledge, it is the first time to present a precise simulation and detailed design of angular correlated color temperature (CCT) distribution of white LEDs covering a range of CCT from 2800K to 6500K. An optimized design of packaging structure with a silicone lens covering a phosphor dome performed an extreme small angular CCT deviation of 105K in the simulation and 182K in a corresponding real sample for a white LED with the CCT near 6500K.

Electromigration-induced failures at Cu/Sn/Cu flip-chip joint interfaces

We observed various EM-induced failures in current-stressed Cu/Sn/Cu flip-chip solder joints. EM-induced Cu-pad consumption occurred at the current-entry point (maximum current-density) on the cathode interface, and voiding occurred at the other joint corner away from the current-entry point (minimum current-density). We believe that the above various EM-induced failure modes were the result of different current-stressing densities and joule heating at the cathode joint interfaces.

Diamond Heat Spreader Layer for High-Power Thin-GaN Light-Emitting Diodes

A diamond-coating layer is studied as a heat spreading layer for a thin-GaN light-emitting diode (LED) chip. Our results show that this diamond layer can effectively spread the heat and improve the temperature uniformity of a thin-GaN LED chip, which enhances the heat dissipation efficiency down to the heat sink. With the diamond heat spreading layer, the junction temperature of the thin-GaN LED was reduced by 20 C at 1-A current input and the uniformity of the temperature distribution is also greatly improved.

A study of large area die bonding materials and their corresponding mechanical and thermal properties

Abstract We tested the ability of Au/Sn eutectic, silver paste, and solder paste to bond to a large area as well as the bonding of a high power LED die to a highly conductive submount. The samples ran through several tests including ultrasound image, shear force, and thermal resistance measurement. Finite element analysis (FEA) models were built for comparison and analysis. Au/Sn bonding shows the best thermal and mechanical properties. Silver paste shows lower contact thermal resistance compared with solder paste. Although the thickness of the silver paste bonding layer is greater than the solder paste bonding layer, the average total thermal resistance is noticeably lower than the solder paste bonded samples.

Precise Simulation of Spectrum for Green Emitting Phosphors Pumped by a Blue LED Die

This paper proposes an effective approach for spectrum shaping in the optical modeling of phosphor-converted white light-emitting diodes (LEDs) in which overlapping of the absorption and emission spectra of the phosphor are considered. The spectrum shaping method was applied to explore the wavelength-dependent absorption effect and reabsorption by the green phosphor. The experimental results indicated that the adjustment factor for blue light can enable the blue light spectrum to fit well with the measured spectrum. The adjustment factor was linearly related to the transmission ratio of the blue light. The blue light emitted by the blue die and the green light emitted by the phosphor were simulated and predicted in an accurate way as compared with the experimental measurements. The enhanced accuracy in assessing the spectra resulted in enhanced precision of chromaticities. In the experiments, the color differences were one order smaller in CIE 1931 chromaticity (Δx, Δy) than in models without spectrum shaping and were almost imperceptible to the human eye. The novel optical modeling of green phosphor pumped by a blue die facilitates the application of these materials in high-color rendering in white LEDs and projection displays.

Study of Temperature Distribution Within pc-WLEDs Using the Remote-Dome Phosphor Package

Both theoretical and simulation approaches were used to obtain the thermal power and temperature distribution in phosphor-converted white LEDs (pc-WLEDs) using a remote-dome phosphor package (RDPP). Different material and geometric parameters were systematically and thoroughly studied. An experiment was performed to measure the temperature distribution in an RDPP pc-WLED, which confirmed the simulation results. Given practical limitations of the material parameters, the most feasible method to reduce the phosphor temperature is to extend the dome radius and the thickness of the phosphor region.

Influence of Minute Self-Absorption of a Volume Bragg Grating Used as a Laser Mirror

A laser system with low output power using a volume Bragg grating (VBG) of photothermal refractive glass and of high diffraction efficiency (0.994) as one of the end mirrors is demonstrated. The minute absorption of the VBG causes changes in various aspects of the laser performance including laser output power, shifting of laser wavelength, chirped grating period, and surface deformation of the VBG. Finite element analysis and T-matrix simulations indicate that the minute absorption of the VBG can induce changes in the thermal, mechanical, and optical properties, which are responsible for the phenomena observed in the experiments.

High-voltage thin-film GaN LEDs fabricated on ceramic substrates: the alleviated droop effect at 670 W/cm(2).

High-voltage thin-film GaN LEDs with the emission wavelength of 455 nm were fabricated on ceramic substrates (230 W/m · K). The high-voltage operation was achieved by three cascaded sub-LEDs with dielectric passivation and metal bridges conformally deposited on the side walls. Under the driving power of 670 W/cm(2), the high-voltage LEDs exhibit much alleviated efficiency droop and the operative temperature below 80 °C. The excellent performances were attributed to the improved current spreading within each sub-LED and the superior heat sinking of the ceramic substrate.

Study of temperature distributions in pc-WLEDs with different phosphor packages

Phosphor converted white LEDs (pc-WLED) with different geometries, namely remote-dome, remote-plate, half-dome, and conformal-coating packages, were studied using simulations to obtain the thermal power and the temperature distribution. The results of an experiment carried out to measure the temperature distribution in these pc-WLEDs validate the simulation results. A hotspot always occurs in the phosphor region. The conformal-coating package is the most feasible choice to reduce the hotspot temperature comparing with the other geometries.