Photonics Online News for photonics professionals 11/24/1999 HomePyramidal LED Enhances Extraction Efficiency to 55%
|Shaping the chip minimizes internal losses due
to free-carrier absorption and active layer reabsorption.
By: Kristin Lewotsky
Conventional light-emitting diode (LED) chips suffer from a internal loss mechanisms and a high index of refraction, both of which act to limit the extraction efficiency of the devices to approximately 30%. By cutting the chips into a truncated inverted pyramid (TIP), as opposed to the commonly used rectangular parallelpiped shape, researchers at Hewlett-Packard (San Jose, CA) have achieved 55% extraction efficiency with a process suitable for volume production (see Figure 1).1
Internal loss mechanisms for conventional LEDs include free-carrier absorption, reabsorption by the active layer, and reflectivity of the Ohmic contact metallization. Chip-shaping methods developed previously, including hemispherical and conical configurations, have not been practical for volume production. The TIP approach skirts the disadvantages of those techniques, while redirecting photons via total internal reflection.
The group grew the devices on a gallium arsenide (GaAs) substrate, removing it after processing and bonding the chips to a transparent GaP:S substrate; the chip was bonded to the substrate in a p-down orientation to assist heat removal during DC operation. An aluminum bond pad applied to the n side provided Ohmic contact. A beveled dicing blade cut the sidewall angles at 35° with respect to the vertical.
In tests at 100 mA DC, the TIP devices generated 1.4 times the gain of standard designs, operating at 55% external quantum efficiency. The efficiency dropped off at higher currents, a behavior attributed to increased junction temperature and the rising thermal resistance of the test fixture. Luminous efficiency at the same drive current reached 100 lm/W; the 350 mA DC, the devices hit peak luminous flux of 60 lm, which is eight times that of conventional devices.
The best device performance was realized by orange TIP LEDs operating around 610 nm, which logged peak luminous efficiencies of 102 lm/W. Amber devices operating around 598 nm achieved photometric efficiencies of 68 lm/W. Accelerated degradation and stress tests showed the devices to be as robust as conventional designs.