Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – Plural light emitting devices
Reexamination Certificate
2000-11-14
2002-06-25
Crane, Sara (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
Plural light emitting devices
C257S091000, C257S093000, C257S098000
Reexamination Certificate
active
06410942
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to light emitting diodes and more particularly to new structures for enhancing their light extraction.
2. Description of the Related Art
Light emitting diodes (LEDs) are an important class of solid state devices that convert electric energy to light and commonly comprise 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 device through all exposed surfaces. Packaging of the LED is commonly used to direct the escaping light into a desired output emission profile.
As semiconductor materials have improved, the efficiency of semiconductor devices has also improved. New LEDs are being made from materials such as GaN, which provides efficient illumination in the ultra-violet 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.
However, 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, or it can escape out surfaces other than the emission surface. Because of this phenomenon, much of the light generated by conventional LEDs does not emit, degrading efficiency.
One method of reducing the percentage of TIR light is to create light scattering centers in the form of random texturing on the surface. [Shnitzer, et al., “30% External Quantum Efficiency From Surface Textured, Thin Film Light Emitting Diodes”, Applied Physics Letters 63, Page 2174-2176 (1993)]. 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 9-30%.
One disadvantage of surface texturing is that it can prevent effective current spreading in LEDs which have poor electrical conductivity for the textured electrode layer, such as the case of p-type GaN. In smaller devices or devices with good electrical conductivity, current from the p and n-type layer contacts spreads 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 does not experience the current and will not emit light. To create uniform current injection across the diode area, a spreading layer of conductive material is deposited on its 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 in 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 at. 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 shape and pattern of the surface must be uniform and very small, on the order of a single wavelength of the LED's light. The 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 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 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.
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. [Knock et al., ‘Strongly Directional Emission from AlGaAs/GaAs Light Emitting Diodes’
Applied Physics Letter
57, pgs. 2327-2329 (1990)]. 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 &mgr;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.
Light extraction can also be improved by angling the LED chip's side surfaces to create an inverted truncated pyramid. The angled surfaces provide TIR light trapped in the substrate material with an emitting surface within the critical angle [Krames et al., ‘High Power Truncated Inverted Pyramid (Al
x
Ga
1−x
)
0.5
In
0.5
P/GaP Light Emitting Diodes Exhibiting >50% External Quantum Efficiency’
Applied Physics Letters
75 pgs. 2365-2367 (1999)]. Using this approach external quantum efficiency has been shown to increase from 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 the case of GaN on sapphire, much of the light is trap
DenBaars Steven
Thibeault Brian
Crane Sara
Cree Lighting Company
Koppel, Jacobs Patrick & Heybl
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