Photonics Online News for photonics professionals 11/24/1999 Home

Pyramidal 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 Figure 1: LED chip shaped into truncated inverted pyramid achieves improved extraction efficiency over conventional devices. (Courtesy of Hewlett-Packard) 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. Device fabrication The chips developed by Michael Krames and colleagues at HP are based on an epitaxially-grown aluminum gallium indium phosphide/gallium phosphide (AlGaInP/GaP) multiwell active region sandwiched between an n-type gallium phosphide (GaP) layer and a p-type GaP layer (see Figure 2). Figure 2. Total internal reflection in pyramidal LED chip redirects photons to minimize internal losses and increase extraction efficiency. 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. Performance The devices operate in the red spectral region, emitting about a center wavelength of 650 nm. The LEDs generated a maximum output power of 440 mW for a drive current of 600 mA DC, an order of magnitude greater than the output power of conventional devices (see Figure 3). Figure 3: Comparison of conventional large junction (LJ) device and TIP device efficiencies shows an increase of 1.4 times for the pyramidal structure. 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. References 1. M.R. Krames et. al., "High-Power truncated-inverted-pyramid (AlxGa1-x)0.5In0.5P/GaP light-emitting diodes exhibiting >50% external quantum efficiency," Applied Physics Letters, 75[16], pp. 2365, (1999).