Hybrid LEDs Offer High Brightness and Output Powers
Daniel S. Burgess
By combining thin-film and flip-chip approaches to LED design, scientists at Philips Lumileds Lighting Co. of San Jose, Calif., have demonstrated blue, green and white devices with higher surface brightness and output powers than are produced by devices that employ only one of the technologies. The researchers suggest that the hybrid approach may enable the development of improved LEDs for applications such as projection displays, automotive headlamps and general illumination.
To improve the brightness and output powers of LEDs, manufacturers have turned to thin-film and flip-chip methods, respectively.
In the thin-film technique, the substrate upon which the LED structure is grown is removed by laser-assisted liftoff, the N-type side of the resulting semiconductor film is patterned and wire-bonded to a cathode, and the P-type side is bonded to a semiconductor substrate that acts as the anode.
In the flip-chip technique, the LED die is inverted and bonded to a thermally conductive submount.
Oleg B. Shchekin, a member of the research team, explained that employing a flip-chip architecture enables the electrical connections to the LED to be moved out of the path of light, thereby increasing output. It also enhances the removal of heat — and, therefore, the maximum output — by locating the active area more closely to the heat sink of the LED package.
The typical vertical thin-film LED structure, he said, requires an intermediate substrate that is approximately 100 μm or more thick. This increases the thermal resistance and, thus, the temperature of the active region, with detrimental effects on output.
The thin-film approach, on the other hand, increases the extraction efficiency by reducing losses to total internal reflection within the device structure, so an ideal LED instrument might employ both methods.
In the work, the investigators fabricated 1 × 1-mm InGaN/GaN multiple-quantum-well LEDs using thin-film, flip-chip, or both thin-film and flip-chip technologies. The main hurdle that they had to overcome in realizing a thin-film flip-chip LED, Shchekin said, involved the mechanical stabilization of the microns-thick film layer into the flip-chip submount.
Using both thin-film and flip-chip approaches to the manufacture of LEDs yields devices that outperform those employing only one of the technologies.
“We have utilized proprietary technologies to achieve this,” he said.
At a drive current of 1 A, the 441-nm hybrid LEDs displayed a radiance of 168 to 191 mW/mm2 sr, which was 1.2 and 2.3 times that of the thin-film and flip-chip devices, respectively.
The optical power of the blue hybrid emitters at 1 A was 750 mW, which was 51 percent and 15 percent higher than that of the thin-film and flip-chip devices, respectively. The 517-nm hybrid LEDs demonstrated similar improvements in performance compared with green thin-film and flip-chip lamps, producing a maximum luminance of 37 million nits.
White LEDs that incorporated the blue hybrid lamps and that used YAG:Ce3+ as the down-conversion phosphor exhibited a peak efficiency of 96 lm/W at 20 mA and an efficiency of 60 lm/W at 350 mA. The total lumen output was 45 percent higher than that of devices that incorporated blue flip-chip LEDs, and the luminance at 1 A was 38 million units, which the scientists noted is approximately 50 percent higher than the halogen filaments used in standard automotive headlamps.
Shchekin said that the investigators are continuing to pursue means of further increasing the light-extraction efficiency and the surface brightness of the LEDs while reducing the forward voltage and thermal loading.
“While we cannot quantify the overall impact at this time, we can say that all improvements to the performance metrics of LEDs invariably lead to the opening of new market segments and to overall market growth for LEDs,” he said.
Applied Physics Letters, online Aug. 14, 2006, 071109.