This invention relates to light emitting diodes and more particularly to new LED structures for enhancing their light extraction.
Light emitting diodes (LEDs) are an important class of solid state devices that convert electric energy to light. LEDs typically provide an active layer of semiconductor material sandwiched between two oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. The light generated by the active region emits in all directions and light escapes the semiconductor chip through all exposed surfaces.
As semiconductor materials have improved, the efficiency of semiconductor devices has also improved. New LEDs are being made from materials such as InAlGaN, which allows for efficient illumination in the ultraviolet to amber spectrum. Many of the new LEDs are more efficient at converting electrical energy to light compared to conventional lights and they can be more reliable. As LEDs improve, they are expected to replace conventional lights in many applications such as traffic signals, outdoor and indoor displays, automobile headlights and taillights, conventional indoor lighting, etc.
The efficiency of conventional LEDs is limited by their inability to emit all of the light that is generated by their active layer. When an LED is energized, light emitting from its active layer (in all directions) reaches the emitting surfaces at many different angles. Typical semiconductor materials have a high index of refraction (n≈2.2-3.8) compared to ambient air (n≈1.0) or encapsulating epoxy (n≈1.5). According to Snell's law, light traveling from a region having a high index of refraction to a region with a low index of refraction that is within a certain critical angle (relative to the surface normal direction) will cross to the lower index region. Light that reaches the surface beyond the critical angle will not cross but will experience total internal reflection (TIR). In the case of an LED, the TIR light can continue to be reflected within the LED until it is absorbed. Because of this phenomenon, much of the light generated by conventional LEDs does not emit, degrading its efficiency.
One method of reducing the percentage of TIR light is to create light scattering centers in the form of random texturing on the LED's surface. The random texturing is patterned into the surface by using sub micron diameter polystyrene spheres on the LED surface as a mask during reactive ion etching. The textured surface has features on the order of the wavelength of light that refract and reflect light in a manner not predicted by Snell's law due to random interference effects. This approach has been shown to improve emission efficiency by 9 to 30%.
As discussed in U.S. Pat. No. 6,821,804, one disadvantage of surface texturing is that it can prevent effective current spreading in LEDs which have a poor electrical conductivity for the textured electrode layer, such as for p-type GaN. In smaller devices or devices with good electrical conductivity, current from the p and n-type layer contacts will spread throughout the respective layers. With larger devices or devices made from materials having poor electrical conductivity, the current cannot spread from the contacts throughout the layer. As a result, part of the active layer will not experience the current and will not emit light. To create uniform current injection across the diode area, a spreading layer of conductive material can be deposited on the surface. However, this spreading layer often needs to be optically transparent so that light can transmit through the layer. When a random surface structure is introduced on the LED surface, an effectively thin and optically transparent current spreader cannot easily be deposited.
Another method of increasing light extraction from an LED is to include a periodic patterning of the emitting surface or internal interfaces which redirects the light from its internally trapped angle to defined modes determined by the shape and period of the surface. See U.S. Pat. No. 5,779,924 to Krames et al. This technique is a special case of a randomly textured surface in which the interference effect is no longer random and the surface couples light into particular modes or directions. One disadvantage of this approach is that the structure can be difficult to manufacture because the surface shape and pattern must be uniform and very small, on the order of a single wavelength of the LED's light. This pattern can also present difficulties in depositing an optically transparent current spreading layer as described above.
An increase in light extraction has also been realized by shaping the LED's emitting surface into a hemisphere with an emitting layer at the center. While this structure increases the amount of emitted light, its fabrication is difficult. U.S. Pat. No. 3,954,534 to Scifres and Burnham discloses a method of forming an array of LEDs with a respective hemisphere above each of the LEDs. The hemispheres are formed in a substrate and a diode array is grown over them. The diode and lens structure is then etched away from the substrate. One disadvantage of this method is that it is limited to formation of the structures at the substrate interface, and the lift off of the structure from the substrate results in increased manufacturing costs. Also, each hemisphere has an emitting layer directly above it, which requires precise manufacturing.
U.S. Pat. No. 5,793,062 discloses a structure for enhancing light extraction from an LED by including optically non-absorbing layers to redirect light away from absorbing regions such as contacts, and also to redirect light toward the LED's surface. One disadvantage of this structure is that the non-absorbing layers require the formation of undercut strait angle layers, which can be difficult to manufacture in many material systems.
As further discussed in U.S. Pat. No. 6,821,804, another way to enhance light extraction is to couple photons into surface plasmon modes within a thin film metallic layer on the LED's emitting surface, which are emitted back into radiated modes. These structures rely on the coupling of photons emitted from the semiconductor into surface plasmons in the metallic layer, which are further coupled into photons that are finally extracted. One disadvantage of this device is that it is difficult to manufacture because the periodic structure is a one-dimensional ruled grating with shallow groove depths (<0.1 .mu.m). Also, the overall external quantum efficiencies are low (1.4-1.5%), likely due to inefficiencies of photon to surface plasmon and surface plasmon-to-ambient photon conversion mechanisms. This structure also presents the same difficulties with a current spreading layer, as described above.
As further discussed in U.S. Pat. No. 6,821,804, light extraction can also be improved by angling the LED chip's side surfaces to create an inverted truncated pyramid. The angled surfaces provide the TIR light trapped in the substrate material with an emitting surface. Using this approach external quantum efficiency has been shown to increase by 35% to 50% for the InGaAlP material system. This approach works for devices in which a significant amount of light is trapped in the substrate. For GaN devices grown on sapphire substrates, much of the light is trapped in the GaN film so that angling the LED chip's side surfaces will not provide the desired enhancement. Still another approach for enhancing light extraction is photon recycling. This method relies on LEDs having a high efficiency active layer that readily converts electrons and holes to light and vice versa. TIR light reflects off the LED's surface and strikes the active layer, where it is converted back to an electron-hole pair. Because of the high efficiency of the active layer, the electron-hole pair will almost immediately be reconverted to light that is again emitted in random directions. A percentage of the recycled light will strike one of the LEDs emitting surfaces within the critical angle and escape. Light that is reflected back to the active layer goes through the same process again.
U.S. Pat. No. 6,821,804 discloses LEDs having light extraction structures on or within the LED to increase its efficiency. The new light extraction structures provide surfaces for reflecting and refracting light into directions that are more favorable for the light to escape into the package. The structures can be arrays of light extraction elements or disperser layers that have quite higher refractive index than the LED encapsulating material. Both the light extraction elements and the disperser layer can have many different shapes and are placed on the spreader, within the epitaxy layers or under the conductive substrate to increase the efficiency of the LED over conventional LEDs.