WO2000065667A1 - Led having embedded light reflectors to enhance led output efficiency - Google Patents

Led having embedded light reflectors to enhance led output efficiency Download PDF

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Publication number
WO2000065667A1
WO2000065667A1 PCT/US2000/010285 US0010285W WO0065667A1 WO 2000065667 A1 WO2000065667 A1 WO 2000065667A1 US 0010285 W US0010285 W US 0010285W WO 0065667 A1 WO0065667 A1 WO 0065667A1
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Prior art keywords
distance
reflector layer
light emitting
emitting diode
active region
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PCT/US2000/010285
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French (fr)
Inventor
Yu-Hwa Lo
Chen Ji
Chen Li
Zuhua Zhu
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Nova Crystals, Inc.
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Application filed by Nova Crystals, Inc. filed Critical Nova Crystals, Inc.
Priority to AU43552/00A priority Critical patent/AU4355200A/en
Publication of WO2000065667A1 publication Critical patent/WO2000065667A1/en

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L33/00—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L33/02—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies
    • H01L33/10—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies with a light reflecting structure, e.g. semiconductor Bragg reflector

Definitions

  • the invention pertains to the field of high-brightness LEDs. More particularly, the invention pertains to high-brightness LEDs that use light reflectors to reflect and guide side emissions outside the LED.
  • High-brightness visible LEDs have found wide-range applications in displays, traffic lights, signs, automobile rear lights, and many other illuminating devices.
  • InAlGaP quantum wells produce the highest brightness red, orange, and yellow LEDs while InGaN quantum wells produce the brightest green and blue LEDs.
  • the internal quantum efficiency is defined as the ratio of photon generation rate to the carrier injection rate. If no other nonradiative carrier recombination mechanisms can compete with the radiative spontaneous emission and if carrier leakage is negligible, the internal quantum efficiency approaches 100%, which is nearly what state-of-the-art AlInGaP quantum wells can achieve. In this case, it is particularly important to enhance the external light coupling efficiency of the LEDs to achieve high brightness.
  • AlInGaP light emitting diodes find wide applications in display, traffic light, signs, and other illuminating devices.
  • the key device issue is to increase the LED brightness through enhanced output efficiency of the side emission.
  • This invention discloses designs that use light reflectors embedded in the LED epitaxial layers to effectively reflect and guide the side emission to the outside world. These integrated light reflectors may not work for normal incident beams as standard multi-layer distributed Bragg reflectors (BRRs). Instead they are particularly designed to reflect side beams with a shallow incident angle. Placing these side-beam reflectors in proper positions in the LED epitaxial layers, the side-emitting beams can be isolated from the absorptive substrate and active layer in their paths to the outside world.
  • BRRs distributed Bragg reflectors
  • the LEDs in this invention use light reflectors embedded in the epitaxial layers of the LED to guide the side emitting beams and to prevent them from passing the absorptive materials, i.e., the absorptive substrate and the active region. As long as the light beam does not pass any absorptive region, it eventually emits from the side of the mesa without significant attenuation.
  • Fig. 1 shows an example of a conventional LED die according to the prior art.
  • Fig. 2 shows an example of a conventional LED die according to the prior art.
  • Fig. 3 shows an LED die according to an embodiment of the present invention.
  • Fig. 4 shows an LED die according to an embodiment of the present invention.
  • Fig. 5 shows an LED die according to an embodiment of the present invention.
  • Fig. 6 shows an LED die according to an embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENT
  • a conventional LED die 1 includes a plurality of layers on top of a substrate 100.
  • An active region 200 is within the plurality of layers, with a top interface 500 on top.
  • Light beams 600 and 601 are absorbed by active region 200 and substrate 100, yielding low output from the sides of the device.
  • a conventional LED die includes an absorptive substrate 100, an active region for light generation 200, a top interface 500, and a bottom reflector 700.
  • a light beam 602 takes a zig-zag path between top reflecting surface 500 and bottom reflector 700 towards a side of the device.
  • Each time the light passes active region 200 its intensity gets attenuated, resulting in a low output coupling efficiency from the sides of the device.
  • the current density in the active layer is much lower than the transparency current so the light can be reabsorbed by the active region every time it passes the region.
  • Light absorption also occurs to the top contact metal layer due to the fact that the metal (Zn, Ti, etc.) making immediate contact with the semiconductor has relatively low reflectivity.
  • the LEDs in this invention use light reflectors embedded in the epitaxial layers of the LED to guide the side emitting beams and to prevent them from passing the abso ⁇ tive materials, i.e., the abso ⁇ tive substrate and the active region. As long as the light beam does not pass any abso ⁇ tive region, it eventually emits from the side of the mesa without significant attenuation.
  • the LED is made of multiple mesas each of which is less than 100 ⁇ m, much smaller than the typical size of an LED (about 250 ⁇ m);
  • each mesa contains at least one side beam reflector embedded in the epitaxial layer; (4) the magnitude of the refractive index of the side beam reflector is at least 20% different from that of the active region to satisfy the following relation,
  • fi r and n a are the complex indices of the reflector and the active region, respectively, measured at the LED operation wavelength
  • At least one reflector has higher resistivity than its neighboring epitaxial layers.
  • Figs. 3-6 show some device structures for the LED design using embedded reflectors.
  • an LED mesa 3 is on an abso ⁇ tive substrate 100 as part of a light-emitting diode.
  • the LED mesa 3 includes an active region 200, a top interface 500, and light reflectors 301, 302 above active region 200 by a distance d ⁇ and below top interface 500 by a distance do-
  • beam 603 is first reflected at top interface 500 of LED mesa 3 and then reflected by embedded reflector 302.
  • Light beam 603 is thus guided towards the side of mesa 3 without passing active region 200.
  • the distance "d ⁇ " between reflectors 301, 302 and active layer 200 should be kept as small as practically possible.
  • the distance do between reflectors 301, 302 and top interface 500 of LED mesa 3 is preferably as large as possible to reduce the number of bounces before light beam 603 reaches the side of the mesa.
  • the ratio of distance do/d] is preferably greater than 2.
  • Reflector layers 301 , 302 preferably have a higher resistivity than the surrounding material so that they act as a current confinement layer. The effect of the current confinement layer is discussed in detail in U.S. Application Serial No. 09/240,801 filed 01/28/99 and entitled HIGH PERFORMANCE LIGHT EMITTING DIODES, inco ⁇ orated herein by reference.
  • LED mesa 4 on an abso ⁇ tive substrate 100 is part of a light emitting diode.
  • LED mesa 4 includes an active region 200, a top interface 500, and reflectors 301, 302, 321, and 322. This additional pair of reflectors 321, 322 underneath active layer 200 enhances coupling of the side beams in the lower half plane of LED mesa 4.
  • a light beam 604 shows the function of the bottom reflectors 321, 322.
  • a reflector 700 is added to the structure of Fig. 3, as are reflectors 311, 312.
  • An LED mesa 5 on an abso ⁇ tive substrate 100 is part of a light- emitting diode.
  • LED mesa 5 includes an active region 200, a top interface 500, reflectors
  • Reflectors 311 , 312 are separated from reflector 700 by a distance s where the ratio s/ti is greater than 2.
  • Three side beams 603, 604, and 605 do not cross either substrate 100 or active region 200 in their paths to the sides of mesa 5.
  • Reflector 700 may be made of a single layer or multiple layers that are current conductive. Whatever the entire layer structure is for reflector 700, the first layer of reflector 700 has to have a lower refractive index than the transparent region immediately above it so that reflector 700 can reflect the side beams 604, 605 effectively. Beam 604 is reflected by another embedded reflector 312 below the active layer so the beam takes a zig-zag pattern towards the side of mesa 5 without crossing active layer 200. On the other hand, a single bounce off reflector 700 allows beam 605 to leave LED mesa 5.
  • LED mesa 6 on an abso ⁇ tive substrate 100 is part of a light emitting diode.
  • LED mesa 6 includes an active region 200, a top interface 500, reflectors 301, 302 above active region 200, reflectors 311, 312, 321, 322 below active region 200, and a bottom reflector 700. All reflectors are designed for reflection of side beams except reflector 700, which in this embodiment is designed for reflection of a bottom beam 606.
  • the ratio of distance tn/t! is preferably greater than 2.
  • a bottom reflector that can reflect bottom emitting beam 606 so that it leaves the mesa from the top surface.
  • This function can be fulfilled by reflector 700 if it has a structure of a distributed Bragg reflector (DBR).
  • DBR distributed Bragg reflector
  • the design that maximizes the reflectivity of normal incident beam 606 is different from the design that optimizes the reflectivity of a side emitting beam 607. Therefore, it is best to use two pairs of the reflectors 311, 312 and 321, 322 to reflect the side emitting beams while using reflector 700 for reflection of the normal incident beams.
  • the spacing t_ between active region 200 and reflectors 311, 312 is preferably as small as possible. To minimize the number of reflections in the optical path, it is also preferred to have a large spacing, to, between reflectors 311, 312 and 321, 322. The ratio to / ti is preferably greater than 2. If reflector 700 is used to serve the function of reflectors 321, 322 as it is in Fig. 5, the spacing between reflector 700 and reflectors 311, 312 should preferably be at least twice of ti.
  • the design of the present invention works well only if at least one of the reflectors 301, 302, 311, 312, 321, 322 has higher resistivity than its neighboring epitaxial layers. Because of its higher resistivity, most current is funneled to the active region near the center of the mesa. The light generated near the center of the active region has a higher side coupling efficiency than that generated elsewhere. Furthermore, the high resistivity of reflectors extended all the way to the side of the mesa reduces the carrier concentration near the boundary of the mesa where the surface carrier recombination problem is most serious. Surface carrier recombination not only reduces the internal quantum efficiency but also adversely affect the reliability of the device particularly in high temperature and high humidity environment. Suppressing surface carrier recombination is a very significant side benefit of our design. In fact it is already well worth the effort of placing high resistivity side reflectors in the LED solely for the reason of suppressing surface recombination.
  • Al x Gai- x As layers with x > 90% can be oxidized rapidly at high temperatures (e.g., 400° C) in water vapor, and the oxidation rate increases rapidly as the Al concentration, x, increases from lower than 90% towards 100%.
  • the refractive index changes drastically from about 3.3 to 1.6, making the AlOx layer a very effective reflector for side beams.
  • a mesa structure is first etched to expose the AlGaAs layers. After leaving the sample at around 400° C in water vapor for several minutes, the AlGaAs layers are turned into AlOx layers from the sides of the mesa to form side beam reflectors. Those regions that are not oxidized remain as high quality AlGaAs layers that are transparent and current conductive.
  • the lateral oxide can enter the mesa by a distance as deep as 40 to 50 micrometers from each side before the oxidation process slows down significantly. This is a large enough distance to form the structures shown in Figs. 3-6 having a typical mesa size of about 40 ⁇ m.
  • InAlGaP can also be oxidized laterally in a similar way to AlGaAs although the process is slower and takes place at a higher temperature (e.g., 550° C).
  • a higher temperature e.g., 550° C
  • AlGaP with high Al concentration to increase the oxidation rate.
  • InAlP (50% Al) without affecting the active layer growth.
  • lateral oxidation of high Al concentration AlInGaP is feasible and may be a viable approach for yellow and yellow/green LEDs.

Abstract

The LEDs in this invention use light reflectors embedded in the epitaxial layers of the LED to guide the side emitting beams and to prevent them from passing the absorptive materials, i.e., the absorptive substrate and the active region. As long as the light beam does not pass any absorptive region, it eventually emits from the side of the mesa without significant attenuation.

Description

LED HAVING EMBEDDED LIGHT REFLECTORS TO ENHANCE LED
OUTPUT EFFICIENCY
FIELD OF THE INVENTION
The invention pertains to the field of high-brightness LEDs. More particularly, the invention pertains to high-brightness LEDs that use light reflectors to reflect and guide side emissions outside the LED.
BACKGROUND OF THE INVENTION
High-brightness visible LEDs have found wide-range applications in displays, traffic lights, signs, automobile rear lights, and many other illuminating devices. Today, InAlGaP quantum wells produce the highest brightness red, orange, and yellow LEDs while InGaN quantum wells produce the brightest green and blue LEDs. Besides selecting the right materials for specific colors, two major concerns for any LED design are enhancing the internal quantum efficiency and the external light coupling efficiency. The internal quantum efficiency is defined as the ratio of photon generation rate to the carrier injection rate. If no other nonradiative carrier recombination mechanisms can compete with the radiative spontaneous emission and if carrier leakage is negligible, the internal quantum efficiency approaches 100%, which is nearly what state-of-the-art AlInGaP quantum wells can achieve. In this case, it is particularly important to enhance the external light coupling efficiency of the LEDs to achieve high brightness.
The best opportunity for enhancing the external light coupling efficiency is through efficient side emissions. Common approaches to realize this objective include using transparent substrates, growing thick window layers, and using mesa structures. The fundamental idea behind all these approaches is that the side emission light beams within the light cones defined by Snell's law can have few or no intersections with any light absorptive material before leaving the LED die. The worst light absorptive material is an opaque substrate where 100% of the light that enters the substrate is absorbed. Besides substrate absorption, the active region and the top metal contact both absorb light as the beam passes them. SUMMARY OF THE INVENTION
AlInGaP light emitting diodes find wide applications in display, traffic light, signs, and other illuminating devices. The key device issue is to increase the LED brightness through enhanced output efficiency of the side emission. This invention discloses designs that use light reflectors embedded in the LED epitaxial layers to effectively reflect and guide the side emission to the outside world. These integrated light reflectors may not work for normal incident beams as standard multi-layer distributed Bragg reflectors (BRRs). Instead they are particularly designed to reflect side beams with a shallow incident angle. Placing these side-beam reflectors in proper positions in the LED epitaxial layers, the side-emitting beams can be isolated from the absorptive substrate and active layer in their paths to the outside world.
The LEDs in this invention use light reflectors embedded in the epitaxial layers of the LED to guide the side emitting beams and to prevent them from passing the absorptive materials, i.e., the absorptive substrate and the active region. As long as the light beam does not pass any absorptive region, it eventually emits from the side of the mesa without significant attenuation.
BRIEF DESCRIPTION OF THE DRAWING
Fig. 1 shows an example of a conventional LED die according to the prior art.
Fig. 2 shows an example of a conventional LED die according to the prior art.
Fig. 3 shows an LED die according to an embodiment of the present invention.
Fig. 4 shows an LED die according to an embodiment of the present invention.
Fig. 5 shows an LED die according to an embodiment of the present invention.
Fig. 6 shows an LED die according to an embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to Fig. 1, a conventional LED die 1 includes a plurality of layers on top of a substrate 100. An active region 200 is within the plurality of layers, with a top interface 500 on top. Light beams 600 and 601 are absorbed by active region 200 and substrate 100, yielding low output from the sides of the device.
Referring to Fig. 2, a conventional LED die includes an absorptive substrate 100, an active region for light generation 200, a top interface 500, and a bottom reflector 700. A light beam 602 takes a zig-zag path between top reflecting surface 500 and bottom reflector 700 towards a side of the device. Each time the light passes active region 200, its intensity gets attenuated, resulting in a low output coupling efficiency from the sides of the device.
In normal LED operation condition, the current density in the active layer is much lower than the transparency current so the light can be reabsorbed by the active region every time it passes the region. Light absorption also occurs to the top contact metal layer due to the fact that the metal (Zn, Ti, etc.) making immediate contact with the semiconductor has relatively low reflectivity.
The LEDs in this invention use light reflectors embedded in the epitaxial layers of the LED to guide the side emitting beams and to prevent them from passing the absoφtive materials, i.e., the absoφtive substrate and the active region. As long as the light beam does not pass any absoφtive region, it eventually emits from the side of the mesa without significant attenuation.
Our LED design contains the following key features;
(1) an LED is on an absoφtive substrate;
(2) the LED is made of multiple mesas each of which is less than 100 μm, much smaller than the typical size of an LED (about 250 μm);
(3) each mesa contains at least one side beam reflector embedded in the epitaxial layer; (4) the magnitude of the refractive index of the side beam reflector is at least 20% different from that of the active region to satisfy the following relation,
( 2 | nr - na |) / (| nr + na |) > 0.2
where fir and na are the complex indices of the reflector and the active region, respectively, measured at the LED operation wavelength; and
(5) at least one reflector has higher resistivity than its neighboring epitaxial layers.
Figs. 3-6 show some device structures for the LED design using embedded reflectors.
Referring to Fig. 3, an LED mesa 3 is on an absoφtive substrate 100 as part of a light-emitting diode. The LED mesa 3 includes an active region 200, a top interface 500, and light reflectors 301, 302 above active region 200 by a distance d} and below top interface 500 by a distance do- For a light beam 603 propagating in the upper half plane of active region 200, beam 603 is first reflected at top interface 500 of LED mesa 3 and then reflected by embedded reflector 302. Light beam 603 is thus guided towards the side of mesa 3 without passing active region 200. For the best performance, the distance "d\" between reflectors 301, 302 and active layer 200 should be kept as small as practically possible. At the same time, the distance do between reflectors 301, 302 and top interface 500 of LED mesa 3 is preferably as large as possible to reduce the number of bounces before light beam 603 reaches the side of the mesa. The ratio of distance do/d] is preferably greater than 2. Reflector layers 301 , 302 preferably have a higher resistivity than the surrounding material so that they act as a current confinement layer. The effect of the current confinement layer is discussed in detail in U.S. Application Serial No. 09/240,801 filed 01/28/99 and entitled HIGH PERFORMANCE LIGHT EMITTING DIODES, incoφorated herein by reference.
Referring to Fig. 4, two bottom reflectors 321, 322 are added to the structure of
Fig. 3. An LED mesa 4 on an absoφtive substrate 100 is part of a light emitting diode. LED mesa 4 includes an active region 200, a top interface 500, and reflectors 301, 302, 321, and 322. This additional pair of reflectors 321, 322 underneath active layer 200 enhances coupling of the side beams in the lower half plane of LED mesa 4. A light beam 604 shows the function of the bottom reflectors 321, 322.
Referring to Fig. 5, a reflector 700 is added to the structure of Fig. 3, as are reflectors 311, 312. An LED mesa 5 on an absoφtive substrate 100 is part of a light- emitting diode. LED mesa 5 includes an active region 200, a top interface 500, reflectors
301, 302 above active region 200 by a distance d\, reflectors 311, 312 below active region 200 by a distance t\, and bottom reflector 700. Reflectors 311 , 312 are separated from reflector 700 by a distance s where the ratio s/ti is greater than 2. Three side beams 603, 604, and 605 do not cross either substrate 100 or active region 200 in their paths to the sides of mesa 5.
Reflector 700 may be made of a single layer or multiple layers that are current conductive. Whatever the entire layer structure is for reflector 700, the first layer of reflector 700 has to have a lower refractive index than the transparent region immediately above it so that reflector 700 can reflect the side beams 604, 605 effectively. Beam 604 is reflected by another embedded reflector 312 below the active layer so the beam takes a zig-zag pattern towards the side of mesa 5 without crossing active layer 200. On the other hand, a single bounce off reflector 700 allows beam 605 to leave LED mesa 5.
Referring to Fig. 6, the features of Figs. 4 and 5 are combined. An LED mesa 6 on an absoφtive substrate 100 is part of a light emitting diode. LED mesa 6 includes an active region 200, a top interface 500, reflectors 301, 302 above active region 200, reflectors 311, 312, 321, 322 below active region 200, and a bottom reflector 700. All reflectors are designed for reflection of side beams except reflector 700, which in this embodiment is designed for reflection of a bottom beam 606. The ratio of distance tn/t! is preferably greater than 2.
In practical LED devices on absoφtive substrates, it is preferable to have a bottom reflector that can reflect bottom emitting beam 606 so that it leaves the mesa from the top surface. This function can be fulfilled by reflector 700 if it has a structure of a distributed Bragg reflector (DBR). However, the design that maximizes the reflectivity of normal incident beam 606 is different from the design that optimizes the reflectivity of a side emitting beam 607. Therefore, it is best to use two pairs of the reflectors 311, 312 and 321, 322 to reflect the side emitting beams while using reflector 700 for reflection of the normal incident beams.
To minimize the waveguide effects of the active layer that trap light in the active region, the spacing t_ between active region 200 and reflectors 311, 312 is preferably as small as possible. To minimize the number of reflections in the optical path, it is also preferred to have a large spacing, to, between reflectors 311, 312 and 321, 322. The ratio to / ti is preferably greater than 2. If reflector 700 is used to serve the function of reflectors 321, 322 as it is in Fig. 5, the spacing between reflector 700 and reflectors 311, 312 should preferably be at least twice of ti. It should be pointed out that the design of the present invention works well only if at least one of the reflectors 301, 302, 311, 312, 321, 322 has higher resistivity than its neighboring epitaxial layers. Because of its higher resistivity, most current is funneled to the active region near the center of the mesa. The light generated near the center of the active region has a higher side coupling efficiency than that generated elsewhere. Furthermore, the high resistivity of reflectors extended all the way to the side of the mesa reduces the carrier concentration near the boundary of the mesa where the surface carrier recombination problem is most serious. Surface carrier recombination not only reduces the internal quantum efficiency but also adversely affect the reliability of the device particularly in high temperature and high humidity environment. Suppressing surface carrier recombination is a very significant side benefit of our design. In fact it is already well worth the effort of placing high resistivity side reflectors in the LED solely for the reason of suppressing surface recombination.
Here we discuss a few methods to implement our design. One key concern is that the introduction of the side beam reflectors 301, 302, 311, 312, 321, 322 should not degrade the quality of other epitaxial layers in the LED, and particularly active layer 200. The best way to achieve this is by using selective lateral oxidation process. AlxGai-xAs layers with x > 90% (preferably x > 95%) can be oxidized rapidly at high temperatures (e.g., 400° C) in water vapor, and the oxidation rate increases rapidly as the Al concentration, x, increases from lower than 90% towards 100%. When the AlGaAs layer is converted into AlOx by the wet oxidation process, the refractive index changes drastically from about 3.3 to 1.6, making the AlOx layer a very effective reflector for side beams. In the fabrication process, a mesa structure is first etched to expose the AlGaAs layers. After leaving the sample at around 400° C in water vapor for several minutes, the AlGaAs layers are turned into AlOx layers from the sides of the mesa to form side beam reflectors. Those regions that are not oxidized remain as high quality AlGaAs layers that are transparent and current conductive. Experiment shows that the lateral oxide can enter the mesa by a distance as deep as 40 to 50 micrometers from each side before the oxidation process slows down significantly. This is a large enough distance to form the structures shown in Figs. 3-6 having a typical mesa size of about 40 μm. Experiment also shows that InAlGaP can also be oxidized laterally in a similar way to AlGaAs although the process is slower and takes place at a higher temperature (e.g., 550° C). For reflectors
301, 302 on top of the active region 200, one can use AlGaP with high Al concentration to increase the oxidation rate. However, for reflectors 311, 312, 321, 322, one can at most use InAlP (50% Al) without affecting the active layer growth. Nonetheless, lateral oxidation of high Al concentration AlInGaP is feasible and may be a viable approach for yellow and yellow/green LEDs.
Accordingly, it is to be understood that the embodiments of the invention herein described are merely illustrative of the application of the principles of the invention. Reference herein to details of the illustrated embodiments are not intended to limit the scope of the claims, which themselves recite those features regarded as essential to the invention.

Claims

What is claimed is:
1. A light emitting diode, comprising:
a) an absoφtive substrate;
b) an LED mesa structure on said absoφtive substrate;
c) an active region within said LED mesa structure;
d) a top interface layer on an upper surface of said LED mesa structure; and
e) a first reflector layer within said LED mesa structure, wherein said first reflector layer is between said active region and said top interface layer, and said first reflector layer does not interfere with light emitted from a central portion of said active region.
2. A light emitting diode according to claim 1, wherein:
a first distance is defined as being between said top interface layer and said first reflector layer;
a second distance is defined as being between said first reflector layer and said active region; and
a ratio between said first distance and said second distance is greater than or equal to two.
3. A light emitting diode according to claim 1, further comprising:
a second reflector layer, wherein said second reflector layer is within said LED mesa structure between said absoφtive substrate and said active region.
4. A light emitting diode according to claim 3, wherein:
a first distance is defined as being between said top interface layer and said first reflector layer; a second distance is defined as being between said first reflector layer and said active region; and
a ratio between said first distance and said second distance is greater than or equal to two.
5. A light emitting diode according to claim 3, wherein said second reflector layer is substantially adjacent an interface between said absoφtive substrate and said LED mesa structure.
6. A light emitting diode according to claim 3, further comprising:
at least one third reflector layer, wherein said at least one third reflector layer is at an interface between said absoφtive substrate and said LED mesa structure.
7. A light emitting diode according to claim 6, wherein:
a first distance is defined as being between said top interface layer and said first reflector layer;
a second distance is defined as being between said first reflector layer and said active region; and
a ratio between said first distance and said second distance is greater than or equal to two.
8. A light emitting diode according to claim 7, wherein:
a third distance is defined as being between said second reflector layer and said at least one third reflector layer;
a fourth distance is defined as being between said second reflector layer and said active region; and
a ratio between said third distance and said fourth distance is greater than or equal to two.
. A light emitting diode according to claim 6, wherein:
a first distance is defined as being between said second reflector layer and said at least one third reflector layer;
a second distance is defined as being between said second reflector layer and said active region; and
a ratio between said first distance and said second distance is greater than or equal to two.
10. A light emitting diode according to claim 6, wherein said at least one third reflector layer has a refractive index lower than a refractive index of a portion of said LED mesa structure immediately above said at least one third reflector layer.
11. A light emitting diode according to claim 6, further comprising:
a fourth reflector layer, wherein said fourth reflector layer is within said LED mesa structure and is between said third reflector layer and said active region.
12. A light emitting diode according to claim 11 , wherein:
a first distance is defined as being between said top interface layer and said first reflector layer;
a second distance is defined as being between said first reflector layer and said active region; and
a ratio between said first distance and said second distance is greater than or equal to two.
13. A light emitting diode according to claim 11, wherein:
a first distance is defined as being between said second reflector layer and said fourth reflector layer; a second distance is defined as being between said fourth reflector layer and said active region; and
a ratio between said first distance and said second distance is greater than or equal to two.
14. A light emitting diode according to claim 13, wherein:
a third distance is defined as being between said top interface layer and said first reflector layer;
a fourth distance is defined as being between said first reflector layer and said active region; and
a ratio between said first distance and said second distance is greater than or equal to two.
15. A light emitting diode according to claim 11, wherein at least one of said first, second, third, and fourth reflector layers has a higher resistivity than its neighboring epitaxial layers within said LED mesa structure.
16. A light emitting diode, comprising:
a) an absoφtive substrate;
b) an LED mesa structure on said absoφtive substrate;
c) an active region within said LED mesa structure;
d) a top interface layer on an upper surface of said LED mesa structure; and
e) a first reflector layer within said LED mesa structure, wherein said first reflector layer directs light emitted from a central portion of said active region towards a side of said LED mesa structure, wherein a magnitude of a refractive index of said first reflector layer differs from a magnitude of a refractive index of said central portion of said active region by more than 20 percent.
17. A light emitting diode according to claim 16, wherein:
a first distance is defined as being between said top interface layer and said first reflector layer;
a second distance is defined as being between said first reflector layer and said active region; and
a ratio between said first distance and said second distance is greater than or equal to two.
18. A light emitting diode according to claim 16, further comprising:
a second reflector layer, wherein said second reflector layer is within said LED mesa structure between said absoφtive substrate and said active region.
19. A light emitting diode according to claim 18, wherein:
a first distance is defined as being between said top interface layer and said first reflector layer;
a second distance is defined as being between said first reflector layer and said active region; and
a ratio between said first distance and said second distance is greater than or equal to two.
20. A light emitting diode according to claim 18, wherein said second reflector layer is substantially adjacent an interface between said absoφtive substrate and said LED mesa structure.
21. A light emitting diode according to claim 18, further comprising:
at least one third reflector layer, wherein said at least one third reflector layer is at an interface between said absoφtive substrate and said LED mesa structure.
22. A light emitting diode according to claim 21, wherein:
a first distance is defined as being between said top interface layer and said first reflector layer;
a second distance is defined as being between said first reflector layer and said active region; and
a ratio between said first distance and said second distance is greater than or equal to two.
23. A light emitting diode according to claim 22, wherein:
a third distance is defined as being between said second reflector layer and said at least one third reflector layer;
a fourth distance is defined as being between said second reflector layer and said active region; and
a ratio between said third distance and said fourth distance is greater than or equal to two.
24. A light emitting diode according to claim 21, wherein:
a first distance is defined as being between said second reflector layer and said at least one third reflector layer;
a second distance is defined as being between said second reflector layer and said active region; and
a ratio between said first distance and said second distance is greater than or equal to two.
25. A light emitting diode according to claim 21, wherein said at least one third reflector layer has a refractive index lower than a refractive index of a portion of said LED mesa structure immediately above said at least one third reflector layer.
26. A light emitting diode according to claim 21, further comprising: a fourth reflector layer, wherein said fourth reflector layer is within said LED mesa structure and is between said third reflector layer and said active region.
27. A light emitting diode according to claim 26, wherein:
a first distance is defined as being between said top interface layer and said first reflector layer;
a second distance is defined as being between said first reflector layer and said active region; and
a ratio between said first distance and said second distance is greater than or equal to two.
28. A light emitting diode according to claim 26, wherein:
a first distance is defined as being between said second reflector layer and said fourth reflector layer;
a second distance is defined as being between said fourth reflector layer and said active region; and
a ratio between said first distance and said second distance is greater than or equal to two.
29. A light emitting diode according to claim 28, wherein:
a third distance is defined as being between said top interface layer and said first reflector layer;
a fourth distance is defined as being between said first reflector layer and said active region; and
a ratio between said first distance and said second distance is greater than or equal to two.
30. A light emitting diode according to claim 26, wherein at least one of said first, second, third, and fourth reflector layers has a higher resistivity than its neighboring epitaxial layers within said LED mesa structure.
31. A light emitting diode, comprising:
a) an absoφtive substrate;
b) an LED mesa structure on said absoφtive substrate;
c) an active region within said LED mesa structure;
d) a top interface layer on an upper surface of said LED mesa structure; and
e) a first reflector layer within said LED mesa structure, wherein said first reflector layer is between said active region and said absoφtive substrate, and said first reflector layer does not interfere with light emitted from a central portion of said active region.
32. A light emitting diode according to claim 31, wherein:
a first distance is defined as being between said absoφtive substrate and said first reflector layer;
a second distance is defined as being between said first reflector layer and said active region; and
a ratio between said second distance and said first distance is greater than or equal to two.
33. A light emitting diode according to claim 31, further comprising:
a second reflector layer, wherein said second reflector layer is within said LED mesa structure between said top interface layer and said active region.
34. A light emitting diode according to claim 33, wherein: a first distance is defined as being between said top interface layer and said second reflector layer;
a second distance is defined as being between said second reflector layer and said active region; and
a ratio between said first distance and said second distance is greater than or equal to two.
35. A light emitting diode according to claim 33, further comprising:
at least one third reflector layer, wherein said at least one third reflector layer is at an interface between said absoφtive substrate and said LED mesa structure.
36. A light emitting diode according to claim 35, wherein:
a first distance is defined as being between said top interface layer and said second reflector layer;
a second distance is defined as being between said second reflector layer and said active region; and
a ratio between said first distance and said second distance is greater than or equal to two.
37. A light emitting diode according to claim 36, wherein:
a third distance is defined as being between said first reflector layer and said at least one third reflector layer;
a fourth distance is defined as being between said first reflector layer and said active region; and
a ratio between said third distance and said fourth distance is greater than or equal to two.
38. A light emitting diode according to claim 35, wherein: a first distance is defined as being between said first reflector layer and said at least one third reflector layer;
a second distance is defined as being between said first reflector layer and said active region; and
a ratio between said first distance and said second distance is greater than or equal to two.
39. A light emitting diode according to claim 35, wherein said at least one third reflector layer has a refractive index lower than a refractive index of a portion of said LED mesa structure immediately above said at least one third reflector layer.
40. A light emitting diode according to claim 35, further comprising:
a fourth reflector layer, wherein said fourth reflector layer is within said LED mesa structure and is between said third reflector layer and said active region.
41. A light emitting diode according to claim 40, wherein:
a first distance is defined as being between said top interface layer and said second reflector layer;
a second distance is defined as being between said second reflector layer and said active region; and
a ratio between said first distance and said second distance is greater than or equal to two.
42. A light emitting diode according to claim 40, wherein:
a first distance is defined as being between said first reflector layer and said fourth reflector layer;
a second distance is defined as being between said fourth reflector layer and said active region; and a ratio between said first distance and said second distance is greater than or equal to two.
43. A light emitting diode according to claim 42, wherein:
a third distance is defined as being between said top interface layer and said second reflector layer;
a fourth distance is defined as being between said second reflector layer and said active region; and
a ratio between said first distance and said second distance is greater than or equal to two.
44. A light emitting diode according to claim 40, wherein at least one of said first, second, third, and fourth reflector layers has a higher resistivity than its neighboring epitaxial layers within said LED mesa structure.
AMENDED CLAIMS
[received by the International Bureau on 25 September 2000 (25.09.00); original claims 1-44 replaced by amended claims 1-11 (3 pages)]
1. A light emitting diode, comprising:
a) an absorptive substrate (100); b) an LED mesa structure (3, 4, 5, 6) on the absorptive substrate (100); c) an active region (200) within the LED mesa structure (3, 4, 5, 6); d) a top interface layer (500) on an upper surface of the LED mesa structure (3, 4, 5, 6); and e) at least one reflector layer (301, 302; 311, 312; 321, 322) within the LED mesa structure (3, 4, 5, 6), wherein the reflector layer (301, 302; 311 , 312; 321 , 322) does not interfere with light emitted from a central portion of the active region (200).
2. The light emitting diode according to claim 1 , wherein the reflector layer (301, 302; 311 , 312; 321 , 322) directs light emitted from a central portion of the active region (200) towards a side of the LED mesa structure (3, 4, 5, 6), and wherein a magnitude of a refractive index of the reflector layer (301, 302; 311, 312; 321, 322) differs from a magnitude of a refractive index of the central portion of the active region (200) by more than 20 percent.
3. A light emitting diode according to claim 1, wherein the reflector layer (301, 302) is located between the active region (200) and the top interface layer (500).
4. A light emitting diode according to claim 1, wherein the reflector layer (311, 312; 321, 322) is located between the active region (200) and the absorptive substrate (100).
5. A light emitting diode according to one of claims 1 to 3, wherein:
a distance (do) is defined as being between the top interface layer (500) and the reflector layer (301, 302); a distance (di) is defined as being between the reflector layer (301 , 302) and the active region (200); and a ratio between the distance (do) and the distance (di) is greater than or equal to two.
6. A light emitting diode according to any of claims 1 to 5, further comprising:
at least one other reflector layer (700), wherein the at least one other reflector layer (700) is substantially adjacent to an interface between the absorptive substrate (100) and the LED mesa structure (5, 6).
7. A light emitting diode according to any of claims 6, wherein the additional reflector layers (311, 312; 321, 322) are located within the LED mesa structure (6) between the other reflector layer (700) and the active region (200).
S. A light emitting diode according to claim 6, wherein:
a distance (s) is defined as being between the reflector layer (311, 312) and the at least one other reflector layer (700);
a distance (ti) is defined as being between the reflector layer (311, 312) and the active region (200); and
a ratio between the distance (s) and the distance (ti) is greater than or equal to two.
9. A light emitting diode according to claim 7, wherein:
a distance (to) is defined as being between the reflector layer ( 11, 31 ) and the reflector layer (321 , 322);
a distance (ti) is defined as being between the reflector layer (311, 312) located closest to the active region (200) and the active region (200); and
a ratio between the distance (to) and the distance (ti) is greater than or equal to two.
10 A light emitting diode according to claim 6, wherein the at least one other reflector layer (700) has a refractive index lower than a refractive index of a portion of the LED mesa structure (5, 6) immediately above the at least one other reflector layer (700).
11. A light emitting diode according to at least one of claims 1 to 10, wherein at least one of the reflector layers (301, 302; 311 , 312; 321, 22; 700) has a higher resistivity than its neighboring epitaxial layers within the LED mesa structure (3, 4, 5, 6).
PCT/US2000/010285 1999-04-28 2000-04-14 Led having embedded light reflectors to enhance led output efficiency WO2000065667A1 (en)

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