US20090140279A1

US20090140279A1 - Substrate-free light emitting diode chip

Info

Publication number: US20090140279A1
Application number: US12/221,304
Authority: US
Inventors: Scott M. Zimmerman; Karl W. Beeson; William R. Livesay; Richard L. Ross
Original assignee: Goldeneye Inc
Current assignee: Goldeneye Inc
Priority date: 2007-12-03
Filing date: 2008-07-31
Publication date: 2009-06-04
Also published as: US20090140272A1; US20110018011A1; US20220077355A1; US8362502B2; US7804099B2

Abstract

A light emitting diode (LED) chip has a multilayer semiconductor structure that is at least 10 microns thick and does not require an attached growth substrate or transfer substrate for structural rigidity or support. The multilayer semiconductor structure includes a first doped layer, a second doped layer and an active region interposed between the first doped layer and the second doped layer. Optionally, the multilayer semiconductor structure includes an undoped layer. At least one of the layers of the multilayer semiconductor structure is at least 5 microns thick and is preferably deposited by hydride vapor phase epitaxy.

Description

REFERENCE TO PRIOR APPLICATION

This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/005,258, which was filed on Dec. 3, 2007, and which is herein incorporated by reference.

TECHNICAL FIELD

The present invention is a thick light emitting diode chip. The chip is substrate-free or free-standing and requires no growth substrate or transfer substrate for structural support.

BACKGROUND OF THE INVENTION

Conventional light emitting diodes (LEDs) are fabricated by epitaxially growing multiple layers of semiconductors on a growth substrate. Inorganic light-emitting diodes can be fabricated from GaN-based semiconductor materials containing, for example, gallium nitride (GaN), aluminum nitride (AlN), aluminum gallium nitride (AlGaN), indium nitride (InN), indium gallium nitride (InGaN) and aluminum indium gallium nitride (AlInGaN). Other appropriate materials for LEDs include, for example, aluminum gallium indium phosphide (AlGaInP), gallium arsenide (GaAs), indium gallium arsenide (InGaAs), indium gallium arsenide phosphide (InGaAsP), diamond, boron nitride and zinc oxide (ZnO).
The total thickness of the semiconductor layers for a conventional GaN-based LED excluding a substrate is less than about 5 microns and usually only about 3 to 4 microns. The layers are fabricated by epitaxially growing a multilayered semiconductor structure on a growth substrate using metal organic chemical vapor deposition (MOCVD), which has a very slow growth rate of approximately 0.1 micron per hour. This results in deposition times of tens of hours and makes the growth of thicker layers prohibitively expensive. The approximately 3 to 4 micron thick multilayer semiconductor structure is very fragile and will break easily if removed from the growth substrate to form a free-standing or substrate-free die. The semiconductor layers must therefore either remain attached to the growth substrate or, alternatively, be attached to a transfer substrate, using wafer bonding techniques before being removed from the growth substrate. The wafer bonding techniques are expensive and can be unreliable. The added steps increase the cost of manufacturing LEDs. Removal of the growth substrate can be done by a laser liftoff process, chemical processing or mechanical polishing.
The growth substrate for GaN-based LEDs is usually sapphire or silicon carbide and is chosen to closely match the crystallographic structure of the epitaxial layers. A transfer substrate, if utilized, can be a metal, another semiconductor material such as silicon, or a ceramic material such as aluminum nitride. Such growth or transfer substrates may not suitable for the final LED device. For example, sapphire is a poor thermal conductor and is therefore not the most effective thermal conductor to direct heat away from the semiconductor layers. Thermal considerations are very important for LEDs, which generate a significant amount of heat during operation. The heat lowers the light output and operating lifetime of the LED. As LED sizes become larger, such heating effects become more important and can seriously degrade the light-output performance and lifetime of the LEDs.
Removing the growth substrate and attaching a transfer substrate to the semiconductor layers also adds thermal resistance to the device. The increased thermal resistance can come from the transfer substrate itself and from the bonding layer used to attach the transfer substrate.
In addition, the growth or transfer substrate may absorb some of the light emitted by the LED, thereby lowering the optical output. The substrate may also trap or reflect some of the light generated by the LED within the LED, resulting in an additional loss in optical output. Light trapping is caused by the high refractive index of the substrate relative to air and results in total internal reflection of emitted light back through the substrate and back through the epitaxial layers.
It would be desirable to develop thick, substrate-free or free-standing LED chips that do not need the original growth substrate or an attached transfer substrate for structural support. Such thick, substrate-free or free-standing LED chips could be easily manipulated, lifted or handled without breaking and could be subsequently bonded to other surfaces or leadframes in any desired pattern to form light emitting devices. Different growth techniques will be required to make such a structure since MOCVD is too slow to fabricate thick multilayer semiconductor structures.
In conventional LED designs, the back side of the LED opposite the light emitting side is a reflective surface. It would also be desirable to develop LED chips that do not have a back reflecting surface and that can emit light from all sides. Eliminating the back reflecting surface can reduce the average optical pathlength of the emitted light within the LED structure, thereby reducing optical absorption within the LED, increasing the external quantum efficiency and increasing the light output of the LED.
Conventional LEDs are cooled by a submount or heat sink in thermal contact with the LED die. The conventional LED die includes either a growth substrate or a transfer substrate. Heat flows from the LED semiconductor layers, through the growth or transfer substrate and through the submount or heat sink to ambient. The heat sink may include fins or other types of structures to transfer heat to an ambient fluid, such as air or water. It would be desirable to develop LED light sources where the LED die does not include a growth substrate or a transfer substrate and where the LED die can be cooled directly without having the added thermal resistance of a growth substrate or a transfer substrate.
The deficiencies of conventional LEDs described above can be eliminated by the various embodiments of this invention that are described below in the summary, the figures and the detailed descriptions of the preferred embodiments.

SUMMARY OF THE INVENTION

A substrate-free or free-standing LED chip of this invention is an LED chip that does not include a growth substrate or transfer substrate as an element of the LED chip. The growth substrate is defined as the substrate onto which the multilayer semiconductor structure is epitaxially grown. For LED chips of this invention, the growth substrate is removed after the multilayer semiconductor structure is fabricated and no transfer substrate is permanently bonded to the multilayer semiconductor structure prior to the removal of the growth substrate. The LED chips of this invention have multilayer semiconductor structures that are at least 10 microns thick and do not require an attached growth substrate or transfer substrate for structural rigidity or support. The LED chips can be handled without damage and without breaking.
After the LED chips of this invention are fabricated, the substrate-free or free-standing LED chips may be packaged into more complex LED light sources. If desired, the LED chips may be attached to a surface, a submount, a heat sink, a leadframe or to any other structure. However, none of these additional surfaces, submounts, heat sinks, leadframes or other structures are elements of the LED chip.
One embodiment of this invention is a thick light emitting diode chip. The LED chip is substrate-free, which means that the chip requires no growth substrate or transfer substrate for structural support. The LED chip includes a multilayer semiconductor structure that has a first side and an opposing second side. The multilayer semiconductor structure includes a first doped layer proximal to the first side, an active region, and a second doped layer proximal to the second side with the active region interposed between the first doped layer and the second doped layer. The first doped layer can be an n-doped layer and the second doped layer can be a p-doped layer or the polarities of the two layers can be reversed. The total thickness of the multilayer semiconductor structure is at least 10 microns, preferably at least 20 microns and more preferably at least 30 microns. The LED chip also includes a first electrode in electrical contact with the first doped layer and a second electrode in electrical contact with the second doped layer. The active region emits internally generated light when a voltage is applied between the first electrode and the second electrode.
An LED chip that has both electrodes on the first side or the second side of the multilayer semiconductor structure may optionally include a substantially undoped layer on the side of the multilayer semiconductor structure opposite the electrodes. The undoped layer can add additional thickness to the multilayer semiconductor structure without affecting the electrical properties of the structure. The additional thickness adds to the structural strength of the multilayer semiconductor structure. In addition, the heat transfer characteristics of an additional undoped layer fabricated from the same semiconductor material (for example, gallium nitride) as the remainder of the multilayer semiconductor structure will be better than the heat transfer characteristics of a growth substrate or transfer substrate of the prior art made from a different material.
The multilayer semiconductor structure of the thick LED chip has at least one thick semiconductor layer to provide structural support to the chip. The thick semiconductor layer can be the first doped layer, the second doped layer, the optional undoped layer or a combination of two or more thick layers. The thick semiconductor layers preferably are fabricated by hydride vapor phase epitaxy (HVPE). Each thick semiconductor layer is at least 5 microns thick, preferably at least 10 microns thick, more preferably at least 15 microns thick and most preferably at least 25 microns thick. When the LED chip includes at least one thick semiconductor layer to provide structural support, the growth or transfer substrate is no longer needed and the LED chip can be handled as a substrate-free or free-standing device without damage.
Another embodiment of this invention is a substrate-free LED chip where at least a portion of the first side of the multilayer semiconductor structure and a portion of the second side of the multilayer semiconductor structure transmit externally incident light. Externally incident light is light that is directed to the LED chip from another light source or light that is emitted by the LED chip and that is recycled back to the chip. Preferably the LED chip transmits at least 60 percent of the externally incident light, more preferably the LED chip transmits at least 70 percent of the externally incident light and most preferably the LED chip transmits at least 80 percent of the externally incident light.
Another embodiment of this invention is a substrate-free LED chip where substantially all of the first side of the multilayer semiconductor structure or substantially all of the second side of the multilayer semiconductor structure reflects light and where, respectively, the second side or the first side is a light emitting side of the LED. Preferably the LED chip reflects at least 60 percent of the externally incident light directed to the light emitting side of the chip, more preferably the LED chip reflects at least 70 percent of the externally incident light and most preferably the LED chip reflects at least 80 percent of the externally incident light.
In order for the substrate-free LED chip to have high external quantum efficiency and to have either high transmission or high reflectivity to externally incident light, the multilayer semiconductor structure should have low optical absorption. The absorption coefficient of the multilayer semiconductor structure should be less than 20 per centimeter, preferably less than 10 per centimeter, more preferably less than 5 per centimeter and most preferably less than 2 per centimeter.

BRIEF DESCRIPTION OF THE DRAWINGS

A more detailed understanding of the present invention, as well as other objects and advantages thereof not enumerated herein, will become apparent upon consideration of the following detailed description and accompanying drawings, wherein:

FIGS. 1A and 1B are illustrations of conventional LED chip of the prior art that has two electrodes on the upper side and includes a growth substrate. FIG. 1A is a top plan view of the LED. FIG. 1B is a side cross-sectional view along the I-I plane illustrated in FIG. 1A.

FIG. 2 is a side cross-sectional view of conventional LED chip of the prior art that has two electrodes on the lower side and includes a growth substrate.

FIG. 3 is a side cross-sectional view of conventional LED chip of the prior art that has two electrodes on the lower side and includes a transfer substrate.

FIGS. 4A and 4B are illustrations of conventional LED chip of the prior art that has one electrode on the upper side, one electrode on the lower side and includes a transfer substrate. FIG. 4A is a top plan view of the LED. FIG. 4B is a side cross-sectional view along the I-I plane illustrated in FIG. 4A.

FIG. 4C is a heat flow diagram of the conventional LED chip illustrated in FIGS. 4A and 4B.

FIGS. 5A, 5B and 5C illustrate a substrate-free LED chip of the present invention that has two upper electrodes, a thick first doped layer and a lower reflecting layer. FIG. 5A is a top plan view of the chip. FIGS. 5B and 5C are side cross-sectional views along the I-I plane illustrated in FIG. 5A.

FIG. 5D is a diagram of the heat flow from a substrate-free LED chip of the present invention to ambient.

FIGS. 6A and 6B are side cross-sectional views of substrate-free LED chip of the present invention that has two upper electrodes and no lower reflecting layer.

FIG. 7 is a side cross-sectional view of substrate-free LED chip of the present invention that has two upper electrodes, a thick second doped layer and a lower reflecting layer.

FIG. 8 is a side cross-sectional view of substrate-free LED chip that has two upper electrodes, a thick first doped layer, a thick second doped layer and a lower reflecting layer.

FIG. 9 is a side cross-sectional view of a substrate-free LED chip of the present invention that has two upper electrodes, a thick undoped layer and no lower reflecting layer.

FIG. 10 is a side cross-sectional view of a substrate-free LED chip of the present invention that has two lower electrodes.

FIG. 11 is a side cross-sectional view of another substrate-free LED chip of the present invention that has two lower electrodes.

FIGS. 12A and 12B illustrate a substrate-free LED of the present invention that has one upper electrode and one lower electrode. FIG. 12A is a top plan view. FIG. 12B is a side cross-sectional view along the I-I plane illustrated in FIG. 12A.

FIG. 13 is a side cross-sectional view of another substrate-free LED of the present invention that has one upper electrode and one lower electrode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention will be better understood by those skilled in the art by reference to the above listed figures. The preferred embodiments of this invention illustrated in the figures are not intended to be exhaustive or to limit the invention to the precise form disclosed. The figures are chosen to describe or to best explain the principles of the invention and its applicable and practical use to thereby enable others skilled in the art to best utilize the invention. For ease of understanding, the thicknesses of the layers in the semiconductor structures in the figures are not drawn to scale.
Light emitting diodes can be fabricated by epitaxially growing multiple layers of semiconductors on a growth substrate. Inorganic light-emitting diodes can be fabricated from gallium nitride (GaN) based semiconductor materials containing, for example, gallium nitride (GaN), aluminum nitride (AlN), aluminum gallium nitride (AlGaN), indium nitride (InN), indium gallium nitride (InGaN) and/or aluminum indium gallium nitride (AlInGaN). Other appropriate materials for LEDs include, for example, aluminum gallium indium phosphide (AlGaInP), gallium arsenide (GaAs), indium gallium arsenide (InGaAs), indium gallium arsenide phosphide (InGaAsP), diamond, boron nitride (BN) and zinc oxide (ZnO).
Especially important LEDs for this invention are GaN-based LEDs that utilize epitaxially-grown layers that can include, for example, GaN, AlN, AlGaN, InN, InGaN or AlInGaN. Depending on the composition of the semiconductor layers, GaN-based LEDs emit light in the ultraviolet, blue, cyan or green regions of the optical spectrum. The growth substrate for GaN-based LEDs is typically sapphire (Al₂O₃), silicon carbide (SiC), bulk gallium nitride or bulk aluminum nitride. Although the embodiments of this invention will be described using GaN-based LEDs, other types of LEDs including, but not limited to, AlGaInP and ZnO LEDs may also be utilized in the embodiments.
Typical epitaxial growth methods for thin semiconductor layers of GaN-based materials include chemical vapor deposition (CVD), metal-organic chemical vapor deposition (MOCVD), vapor phase epitaxy (VPE), hydride vapor phase epitaxy (HVPE) and molecular beam epitaxy (MBE). MOCVD is the most common method for conventional GaN-based LEDs where the total thickness of the epitaxial layers is less than about 5 microns. MOCVD is a relatively slow deposition method with growth rates of approximately 0.1 micron per hour. HVPE has much higher growth rates (10 microns per hour is possible). HVPE can be used to fabricate substrate-free LEDs that are described in this invention and where the thickness of one of the layers is at least 5 microns and could be as much as 30 microns or more.
Conventional GaN-based LED chips of the prior art are fabricated so that the total thickness of the epitaxial semiconductor layers, which include a first doped layer, an active region, and a second doped layer, with the active region interposed between the first doped layer and the second doped layer, is less than about 5 microns thick. The complete set of epitaxial semiconductor layers is denoted in this application as the multilayer semiconductor structure. The first doped layer and the second doped layers of the multilayer semiconductor structure can be, respectively, an n-doped layer and a p-doped layer or the layers can be reversed so that the first doped layer is a p-doped layer and the second doped layer is an n-doped layer.
When the multilayer semiconductor structure is less that about 5 microns thick, it is too fragile to form a self-supporting device. To provide structural support, a conventional LED chip of the prior art retains the growth substrate upon which the multilayer semiconductor structure is fabricated or includes a transfer substrate that is bonded to the multilayer semiconductor structure opposite the growth substrate during the fabrication process. The transfer substrate, if present, provides structural support to the epitaxial layers once the growth substrate is removed.
A conventional LED chip of the prior art usually emits light predominately from one side of the chip. The opposing side is substantially covered by one or more reflecting layers and emits little, if any, light.
Examples of conventional GaN-based LED chips of the prior art are illustrated in FIGS. 1 to 4. FIGS. 1A and 1B are illustrations of conventional LED chip 100. FIG. 1A is a top plan view of the chip. FIG. 1B is a side cross-sectional view along the I-I plane illustrated in FIG. 1A. Conventional LED chip 100 in FIG. 1 has both the n-electrode and p-electrode on the “top” surface of the device and the chip includes a growth substrate. Conventional LED chip 100 includes a first electrode 102, a multilayer semiconductor structure 104, a second electrode 114, a growth substrate 106 and a back reflector 115. The multilayer semiconductor structure 104 includes a first doped layer 108, an active region 110 and a second doped layer 112, which is on the opposite side of the active region 110 from the first doped semiconductor layer 108. Consequently, the active region is interposed between the first doped layer and the second doped layer. The active region is in electrical contact with the first doped layer and the second doped layer and the active region emits light in a first wavelength range when a current is applied through the first and second electrodes.
The first electrode 102 is in electrical contact with the first doped layer 108 and the second electrode 114 is in electrical contact with the second doped layer 112. The first electrode and the second electrode may be fabricated from reflecting metals.
The multilayer semiconductor structure 104 of the LED chip 100 can be fabricated from GaN-based semiconductor materials containing GaN, AlN, AlGaN, InN, InGaN and/or AlInGaN.
The active region 110 of the multilayer semiconductor structure 104 is a p-n homojunction, a p-n heterojunction, a single quantum well or a multiple quantum well of the appropriate semiconductor material for the LED.
A multilayer semiconductor structure 104 is fabricated on a growth substrate 106 of sapphire. The multilayer semiconductor structure includes a first doped layer 108, an active region 110 and a second doped layer 112. The growth substrate 106 has a first surface 120 and a second surface 122 opposite the first surface.
The first doped layer 108 is an n-doped GaN layer, which is epitaxially deposited or otherwise conventionally fabricated on the second surface 122 of the growth substrate. The first doped layer 108 has a first surface 124 and a second surface 126 opposite the first surface. The first surface 124 of the first doped layer is in contact with surface 122 of the growth substrate.
The active region 110 is a GaN-based multiple quantum well structure, which is epitaxially deposited or otherwise conventionally fabricated on the second surface 126 of the first doped layer 108. The active region 110 has a first surface 128 and a second surface 130 opposite the first surface. The first surface 128 of the active region is in electrical contact with the second surface 126 of the first doped layer.
The second doped layer 112 is a p-doped GaN layer, which is epitaxially deposited or otherwise conventionally fabricated on the second surface 130 of the active region 110. The second doped layer has a first surface 132 and a second surface 134 opposite the first surface. The first surface 132 of the second doped layer is in electrical contact with the second surface 130 of the active region.
A portion 116 of the second doped layer 112 and the active region 110 is removed to expose a portion 116 of the second surface 126 of the first doped layer. The first electrode 102 and the second electrode 114 are fabricated from aluminum. An aluminum layer is deposited on the second surface 134 of the second doped layer and the exposed portion 116 of the second surface 126 of the first doped layer. The aluminum layer is patterned by standard photolithographic techniques to form the first electrode 102 and the second electrode 114. First electrode 102 has a first surface 136 and a second surface 138. The first surface 136 of the first electrode is in electrical contact with the second surface 126 of the first doped layer. Second electrode 114 has a first surface 140 and a second surface 142. First surface 140 of the second electrode is in electrical contact with the second surface 134 of the second doped layer.
The first electrode 102 only partially covers the exposed portion 116 of the second surface 126 of the first doped layer. The second electrode 114 only partially covers the second surface 134 of the second doped layer. The remaining portion of the exposed portion 116 of the second surface 126 of the first doped layer and the second surface 134 of the second doped layer are an output or exit surface for the light emitted by the LED 100. To form a back reflector 115, a layer of silver is deposited on the first surface 120 of the growth substrate.
In summary, LED chip 100 has a first electrode 102, a multilayer semiconductor structure 104 that includes first-doped, active and second-doped layers, a growth substrate 106 and a second electrode 114. LED chip 100 has a first side 152 and a second side 154. The first side 152 is substantially adjacent to the first doped layer 108. The second side 154 is substantially adjacent to the second doped layer 112. The active region 110 emits internally generated light in a first wavelength range when a current is applied through the first electrode 102 and the second electrode 114. The light is emitted from the second side 154 of the LED.
The total thickness 150 of the multilayer semiconductor structure 104 for conventional LED chip 100 of the prior art is less than 5 microns. For example, the thickness of the first doped layer (the n-doped layer) is typically 3 microns, the thickness of the active region (a multi-quantum well structure) is typically 0.5 microns and the thickness of the second doped layer (the p-doped layer) is typically 0.5 microns, resulting in a total thickness of 4 microns. In prior art LEDs, the semiconductor layers are usually grown by MOCVD.
Another conventional LED design of the prior art is illustrated in FIG. 2. Conventional LED chip 200 in FIG. 2 is similar to LED chip 100 in FIG. 1 except that the LED chip 200 structure is inverted relative to LED chip 100 and has both the first electrode and second electrode on the lower surface of the device. This configuration is sometimes called a flip-chip design. LED 200 also includes a growth substrate.
Except for the back reflecting surfaces, most of the elements of LED chip 200 of the prior art are the same as LED chip 100. Conventional LED chip 200 includes a first electrode 102, a multilayer semiconductor structure 104, a second electrode 114 and a growth substrate 106. The multilayer semiconductor structure 104 includes a first doped layer 108, an active region 110 and a second doped layer 112, which is on the opposite side of the active region 110 from the first doped semiconductor layer 108.
The first electrode 102 is in electrical contact with the first doped layer 108 and the second electrode 114 is in electrical contact with the second doped layer 112. For LED chip 200, the second electrode is a reflecting electrode and covers substantially all of surface 134 of the second doped layer 112. The first electrode and the second electrode may be fabricated from reflecting metals.
The multilayer semiconductor structure 104 of LED chip 200 can be fabricated from GaN-based semiconductor materials containing GaN, AlN, AlGaN, InN, InGaN and/or AlInGaN. Alternatively, the multilayer semiconductor structure can be fabricated from any appropriate light-emitting semiconductor material.
The active region 110 of the multilayer semiconductor structure 104 is a p-n homojunction, a p-n heterojunction, a single quantum well or a multiple quantum well of the appropriate semiconductor material for the LED chip 200.
For purposes of illustration, LED chip 200 is assumed to be a GaN-based LED. The important fabrication steps for this GaN-based, illustrative example will be briefly summarized. Many of the fabrication steps are identical to the steps for LED chip 100 and will not be repeated.
First a multilayer semiconductor structure 104 of LED chip 200 is fabricated on a sapphire growth substrate 106 using the same methods that are described above for LED 100. The growth substrate 106 has a first surface 120 and a second surface 122 opposite the first surface. The multilayer semiconductor structure includes a first doped layer 108 that is n-doped GaN, an active region 110 that is a GaN-based multiple quantum well structure and a second doped layer 112 that is p-doped GaN.
A portion 116 of the second doped layer 112 and the active region 110 is removed to expose a portion 116 of the second surface 126 of the first doped layer. The first electrode 102 and the second electrode 114 are fabricated from aluminum. An aluminum layer is deposited on the second surface 134 of the second doped layer and the exposed portion 116 of the second surface 126 of the first doped layer. The aluminum layer is patterned by standard photolithographic techniques to form the first electrode 102 and the second electrode 114. The first surface 136 of the first electrode is in electrical contact with the second surface 126 of the first doped layer. First surface 140 of the second electrode is in electrical contact with the second surface 134 of the second doped layer.
The first electrode 102 partially covers the exposed portion 116 of the second surface 126 of the first doped layer. The second electrode 114 substantially covers the second surface 134 of the second doped layer. Surface 136 of the first electrode and surface 140 of the second electrode form the back reflector for LED chip 200.
In summary, LED chip 200 has a first electrode 102, a multilayer semiconductor structure 104 that includes first-doped, active and second-doped layers, a growth substrate 106 and a second electrode 114. LED chip 200 has a first side 252 and a second side 254. The first side 252 is substantially adjacent to the first doped layer 108. The second side 254 is substantially adjacent to the second doped layer 112. The active region 110 emits internally generated light in a first wavelength range when a current is applied through the first electrode 102 and the second electrode 114. The light is emitted from the first side 252 of the LED.
The total thickness 150 of the multilayer semiconductor structure 104 for LED chip 200 of the prior art is less than 5 microns. For example, the thickness of the first doped layer (the n-doped layer) is typically 3 microns, the thickness of the active region (a multi-quantum well structure) is typically 0.5 microns and the thickness of the second doped layer (the p-doped layer) is typically 0.5 microns, resulting in a total thickness of 4 microns. The semiconductor layers are usually grown by MOCVD.
When utilized as in a light source, LED chip 200 is normally attached to a submount (not shown). The submount acts as a heat transfer element or heatsink to remove heat generated by the device during operation. The submount also includes electrical interconnections that attach to the first electrode and second electrode.
Another conventional LED design of the prior art is illustrated in FIG. 3. Conventional LED chip 300 in FIG. 3 is similar to LED chip 100 in FIG. 1 and LED chip 200 in FIG. 2. The LED chip 300 structure is inverted and has both the n-electrode and p-electrode on the lower side of the device. This configuration is another version of a flip-chip structure. However, for the LED chip 300 design, the LED structure is bonded to a transfer substrate 302 that includes electrical connections (not shown) to the n-electrode and the p-electrode. The original growth substrate has been removed.
Except for the removal of the growth substrate and the addition of a transfer substrate 302, most of the elements of LED chip 300 are the same as LED chip 200. Conventional LED chip 300 of the prior art includes a first electrode 102, a multilayer semiconductor structure 104, a second electrode 114 and a transfer substrate 302 on the second side 354 of the chip. The multilayer semiconductor structure 104 includes a first doped layer 108 on the first side 352 of the chip, an active region 110 and a second doped layer 112, which is on the opposite side of the active region 110 from the first doped semiconductor layer 108.
The first electrode 102 is in electrical contact with the first doped layer 108 and the second electrode 114 is in electrical contact with the second doped layer 112. For LED 300, the second electrode is a reflecting electrode and covers substantially all of surface 134 of the second doped layer 112. The first electrode and the second electrode may be fabricated from reflecting metals.
The active region 110 of the multilayer semiconductor structure 104 can be a p-n homojunction, a p-n heterojunction, a single quantum well or a multiple quantum well of the appropriate semiconductor material for the LED chip 300.
For purposes of illustration, LED chip 300 is assumed to be a GaN-based LED. The important fabrication steps for this GaN-based, illustrative example will be briefly summarized. Many of the fabrication steps are identical to the steps for LED chip 100 and will not be repeated.
First a multilayer semiconductor structure 104 of LED chip 300 is fabricated on a sapphire growth substrate (not shown) using the same methods that are described above for LED 100.
A portion 116 of the second doped layer 112 and the active region 110 is removed to expose a portion 116 of the second surface 126 of the first doped layer. To form the electrodes, a metal layer is deposited on the second surface 134 of the second doped layer and the exposed portion 116 of the second surface 126 of the first doped layer. The metal layer is patterned by standard photolithographic techniques to form the first electrode 102 and the second electrode 114. The first electrode 102 partially covers the exposed portion 116 of the second surface 126 of the first doped layer. The second electrode 114 substantially covers the second surface 134 of the second doped layer. Surface 136 of the first electrode and surface 140 of the second electrode form the back reflector for LED chip 300.
A transfer substrate 302 is bonded to surface 138 of the first electrode and surface 142 of the second electrode. The transfer substrate includes electrical vias (not shown) in order to make electrical connections to the electrodes. After the transfer substrate is attached, the growth substrate is removed by standard processing steps. For example, the growth substrate can be removed by a laser liftoff process, a chemical process or by mechanical polishing.
The total thickness 150 of the multilayer semiconductor structure 104 for LED chip 300 of the prior art is less than 5 microns. For example, the thickness of the first doped layer (the n-doped layer) is typically 3 microns, the thickness of the active region (a multi-quantum well structure) is typically 0.5 microns and the thickness of the second doped layer (the p-doped layer) is typically 0.5 microns, resulting in a total thickness of 4 microns. The semiconductor layers are usually grown by MOCVD.
When utilized as in a light source, LED chip 300 may also include a submount (not shown), which acts as a heat transfer element or heatsink to remove heat generated by the device during operation.
Another conventional LED of the prior art has one electrode on the upper side of the device and one electrode on the lower side. LED chip 400 illustrated in FIGS. 4A and 4B is one example of such a device of the prior art.
FIGS. 4A and 4B are illustrations of a conventional LED chip 400 of the prior art that has a upper electrode, a lower electrode and includes a transfer substrate. FIG. 4A is a top plan view of the chip. FIG. 4B is a side cross-sectional view along the I-I plane illustrated in FIG. 4A. The multilayer semiconductor structure of LED chip 400 is inverted relative to the LED chip 100 structure. However, LED chip 400 has one electrode, in this case the n-electrode or first electrode 102, on the upper side of the device and the other electrode, the p-electrode or the second electrode 114, on the lower side of the device. In a similar manner as the LED chip 300 design, the LED chip 400 structure is bonded to a transfer substrate. The transfer substrate 402 is either an electrical conductor or includes an electrical connection (not shown) to the second or p-electrode. The original growth substrate has been removed.
Except for the arrangement of the electrodes, most of the elements of LED chip 400 of the prior art are the same as for LED chip 300. Conventional LED chip 400 includes a first electrode 102 on the first side 452 of the device, a multilayer semiconductor structure 104, a second electrode 114 proximal to the second side 454 of the device and a transfer substrate 402. The multilayer semiconductor structure 104 includes a first doped layer 108, an active region 110 and a second doped layer 112, which is on the opposite side of the active region 110 from the first doped semiconductor layer 108.
The first electrode 102 is in electrical contact with the first doped layer 108 and the second electrode 114 is in electrical contact with the second doped layer 112. For LED chip 400, the second electrode is a reflecting electrode and covers substantially all of surface 134 of the second doped layer 112. The first electrode and the second electrode may be fabricated from reflecting metals.
The active region 110 of the multilayer semiconductor structure 104 can be a p-n homojunction, a p-n heterojunction, a single quantum well or a multiple quantum well of the appropriate semiconductor material for the LED chip 400.
For purposes of illustration, LED chip 400 is assumed to be a GaN-based LED. The important fabrication steps for this GaN-based, illustrative example will be briefly summarized. Many of the fabrication steps, including the fabrication of the multilayer semiconductor structure, are identical to the steps for LED chip 100, LED chip 200 and LED chip 300 and will not be repeated.
The second electrode 114 is fabricated from a metal. A metal layer is deposited on the second surface 134 of the second doped layer of the multilayer semiconductor structure. The first surface 140 of the second electrode is in electrical contact with the second surface 134 of the second doped layer. Surface 140 of the second electrode also forms the back reflector for LED chip 400.
A transfer substrate 402, and in particular surface 404 of the transfer substrate, is bonded to surface 142 of the second electrode. The transfer substrate 402 is either an electrical conductor or includes an electrical interconnect (not shown) to the second electrode.
After the transfer substrate is attached, the growth substrate is removed by standard processing steps, exposing the first surface 124 of the first doped layer. For example, the growth substrate can be removed by a laser liftoff process, a chemical process or by mechanical polishing.
The first electrode 102 is fabricated from a metal. A metal layer is deposited on the first surface 124 of the first doped layer that was exposed by removing the growth substrate. The metal layer is patterned by standard photolithographic techniques to form the first electrode 102. The first electrode 102 partially covers surface 124 of the first doped layer.
In summary, LED chip 400 has a first electrode 102, a multilayer semiconductor structure 104 that includes first-doped, active and second-doped layers, a transfer substrate 402 and a second electrode 114. LED chip 400 has a first side 452 and a second side 454. The first side 452 is substantially adjacent to the first doped layer 108. The second side 454 is substantially adjacent to the second doped layer 112. The active region 110 emits internally generated light in a first wavelength range when a current is applied through the first electrode 102 and the second electrode 114. The light is emitted from the first side 452 of the LED.
The total thickness 150 of the multilayer semiconductor structure 104 for LED chip 400 is typically less than 5 microns. For example, the thickness of the first doped layer (the n-doped layer) is typically 3 microns, the thickness of the active region (a multi-quantum well structure) is typically 0.5 microns and the thickness of the second doped layer (the p-doped layer) is typically 0.5 microns, resulting in a total thickness of 4 microns. The semiconductor layers are usually grown by MOCVD.
When utilized as in a light source, LED chip 400 may also include a submount or heatsink (neither are shown) that acts as a heat transfer element to remove heat generated by the device during operation.
The heat flow from the multilayer semiconductor structure of LED chip 400 of the prior art is illustrated schematically in FIG. 4C. Heat flows from the multilayer semiconductor structure to the transfer substrate with thermal resistance 482, then from the transfer substrate to the submount or heatsink with thermal resistance 484 and finally from the submount or heatsink with thermal resistance 486 to the ambient 488. The transfer substrate, the bonding material (not shown) used to bond the transfer substrate to the multilayer semiconductor structure and the submount/heatsink increase the thermal resistance of the device.
FIGS. 1 to 4 illustrate conventional LED chips of the prior art that include multilayer semiconductor structures that are less than 5 microns thick and that include either a growth substrate or a transfer substrate.
In contrast to conventional LED chips of the prior are, the LED chips of this present invention are substrate-free. Alternatively, the LED chips of this invention may also be described as free-standing. A substrate-free or free-standing LED chip of this invention is an LED chip that does not include a growth substrate or transfer substrate as an element of the LED chip. The growth substrate is defined as the substrate onto which the multilayer semiconductor structure is epitaxially grown. For LED chips of this present invention, the growth substrate is removed after the multilayer semiconductor structure is fabricated and no transfer substrate is permanently bonded to the multilayer semiconductor structure prior to the removal of the growth substrate. The LED chips of this present invention have multilayer semiconductor structures that are at least 10 microns thick and do not require an attached growth substrate or transfer substrate for structural rigidity or support. The LED chips can be handled without damage and without breaking.
The substrate-free or free-standing LED chips of this present invention may later be packaged into more complex LED light sources. If desired, the LED chips may be attached to a surface, a submount, a heat sink, a leadframe or to any other structure. However, none of these additional surfaces, submounts, heat sinks, leadframes or other structures are elements of the LED chip.
Embodiments of this present invention are substrate-free LED chips that do not include a growth substrate or a transfer substrate. Substrate-free LED chips of this invention have multilayer semiconductor structures that are at least 10 microns thick. It is also within the scope of this invention that the multilayer semiconductor structures can have a thickness of at least 20 microns or at least 30 microns. The multilayer semiconductor structures utilized for the substrate-free LED chips are thick enough so that the LED chips can be handled as free-standing structures without breaking.
For the substrate-free LED chips of this invention, at least one layer of the multilayer semiconductor structure is at least 5 microns thick. The at least one thick layer in the multilayer semiconductor structure can be the first doped layer, the second doped layer or an undoped layer. Preferably the first doped layer, the second doped layer or the updoped layer is at least 10 microns thick. More preferably, the first doped layer, the second doped layer or the undoped layer is at least 15 microns thick. Most preferably, the first doped layer, the second doped layer or the undoped layer is at least 25 microns thick. Alternatively, two or more layers of the multilayer semiconductor structure are each at least 5 microns thick.
The total thickness of the multilayer semiconductor structure for the substrate-free LED chips of this invention is at least 10 microns thick. More preferably the total thickness of the multilayer semiconductor structure is at least 20 microns thick. Most preferably the total thickness of the multilayer semiconductor structure is at least 30 microns thick.
Since thicker semiconductor layers are utilized for the substrate-free LED chip, the optical absorption coefficients for the various layers must be low in order to prevent the absorption of a significant fraction of the internally generated light that is emitted by the active region of the chip and transmitted through the thicker semiconductor layers before exiting the chip. Lower optical absorption within the LED chip will result in higher light extraction from the chip, higher external quantum efficiency and higher light output from the LED.
In some applications, it is also important that the LED chip be highly reflective or highly transmissive to any externally incident light that comes from other light sources or from recycled light that is directed or reflected back toward the chip. The optical absorption coefficients for the various semiconductor layers must also be low in these cases so that any externally incident light that enters the chip will not undergo significant absorption by the semiconductor layers before exiting the chip. Lowering the optical absorption coefficients of the semiconductor layers will increase the reflectivity or transmissivity of the LED chip to externally incident light.
The multilayer semiconductor structure of the LED chip can absorb light and has an absorption coefficient that depends on wavelength. In many cases, the absorption coefficient is not uniform across the different semiconductor layers of the multilayer semiconductor structure. If the different semiconductor layers that make up the multilayer semiconductor structure have different absorption coefficients, the absorption coefficient for the multilayer semiconductor structure is defined in this specification as the thickness-weighted average absorption coefficient. The weighting function is the fractional thickness of each semiconductor layer in the multilayer semiconductor structure. For example, if 100 percent of the thickness of the multilayer semiconductor structure has a uniform absorption coefficient of 40 per centimeter in the emitting wavelength range of the internally generated light, then the thickness-weighted average absorption coefficient is 40 per centimeter. If 50 percent of the thickness of the multilayer semiconductor structure has an absorption coefficient of 30 per centimeter and 50 percent of the thickness of the multilayer semiconductor structure has an absorption coefficient of 50 per centimenter, then the thickness-weighted average absorption coefficient is also 40 per centimeter.
In order to improve the light extraction efficiency and external quantum efficiency of an LED chip and to improve the reflectivity or transmissivity of the LED chip to externally incident light, the absorption coefficient (i.e. the thickness-weighted average absorption coefficient) of the multilayer semiconductor structure in the emitting wavelength range of the internally generated light is less than 20 per centimeter. Preferably the absorption coefficient of the multilayer semiconductor structure in the emitting wavelength range of the internally generated light is less than 10 per centimeter. More preferably, the absorption coefficient of the multilayer semiconductor structure in the emitting wavelength range is less than 5 per centimeter. Most preferably, the absorption coefficient of the multilayer semiconductor structure in the emitting wavelength range is less than 2 per centimeter.
Minimizing the absorption coefficient of the multilayer semiconductor structure in the emitting wavelength range of the internally generated light can be accomplished by improving the deposition processes for the different semiconductor layers in order to reduce impurities or defects and to improve the crystalline structure of the layers.
Thick semiconductor layers can be grown by methods including, but not limited to, CVD, MOCVD, VPE, HVPE and MBE. MOCVD is the most common method for conventional GaN-based LEDs but it has relatively slow deposition rates of approximately 0.1 micron per hour. MOCVD deposited layers also have relatively high optical absorption coefficients due to impurities and defects. HVPE has much faster growth rates and is the preferred method for this invention in order to grow GaN layers that are more than 5 microns thick. HVPE can have growth rates of up to 10 microns per hour or more and can produce GaN-based LED layers that have optical absorption coefficients significantly less than 25 per centimeter.
For example, HVPE can be used to epitaxially grow the first doped layer, the second doped layer, both the first and the second doped layers, any undoped layer or the entire multilayer semiconductor structure of the LED. HVPE does not have the carbon impurities that can be present in the MOCVD processes normally used in GaN LED fabrication. MOCVD may optionally be used to grow active regions that are single- or multiple quantum wells. If the active region of the LED chip is a p-n homojunction or a p-n heterojunction, preferably the entire multilayer semiconductor structure is fabricated by HVPE.
A substrate-free LED chip may have two electrodes on one side of the chip, either the upper side or the lower side. Alternatively, the substrate-free LED chip may have one electrode on the upper side of the chip and one electrode on the lower side of the chip.
Both electrodes should be highly reflective to internally generated light to prevent excessive light absorption inside the chip. In addition, making the external surfaces of the electrodes highly reflective will result in an LED chip that has higher reflectivity to externally incident light.
Gold is a common electrode material. Gold has very good electrical properties, but is a poor optical reflector for visible light in the range of 400 mn to 550 mn. For LEDs that emit light in the 400 to 550 nm range or thereabouts, it is advantageous to replace gold with a more reflective material. Suitable electrode materials include, but are not limited to, aluminum and silver. The electrodes may also be omni-directional reflectors that include a dielectric layer and a metal layer and have electrically conducting pathways through the dielectric layer. Preferably the reflectivity of the electrodes is greater than 90 percent in the emitting wavelength range. More preferably, the reflectivity of the electrodes is greater than 95 percent in the emitting wavelength range. Most preferably, the reflectivity of the electrodes is greater than 98 percent in the emitting wavelength range.
Examples of substrate-free LED chips for this invention that have at least one thick epitaxial layer and that do not have either a growth substrate or a transfer substrate are illustrated in FIGS. 5 to 13. In the first set of examples illustrated in FIGS. 5 to 9, the LED chips each have both electrodes on the upper side of the chip. In the second set of examples illustrated in FIGS. 10 and 11, the LED chips each have both electrodes on the lower side of the chip. In the third set of examples illustrated in FIGS. 12 and 13, the LED chips each have one electrode on the upper side of the chip and one electrode on the lower side of the chip.
FIGS. 5 to 9 illustrate substrate-free LED chips that have both electrodes on the upper side. FIGS. 5 and 6 illustrate LED chips having a thick first doped layer. FIG. 7 illustrates an LED chip with a thick second doped layer. FIG. 8 illustrates an LED chip with both a thick first doped layer and a thick second doped layer. FIG. 9 illustrated an LED chip that has a thick undoped layer.
FIGS. 10 to 11 illustrate substrate-free LED chips that have both electrodes on the lower side of the chip and that have a thick first doped layer. It is also within the scope of this invention that chips that have both electrodes on the lower side may alternatively have a thick second doped layer or have both a thick first doped layer and a thick second doped layer or have a thick undoped layer. Only examples with a thick first doped layer are illustrated in the figures.
FIGS. 12 to 13 illustrate substrate-free LED chips that have one electrode on the upper side of the chip, one electrode on the lower side of the chip and have a thick first doped layer. It is also within the scope of this invention that chips that have one electrode on the upper side and one electrode on the lower side may alternatively have a thick second doped layer or have both a thick first doped layer and a thick second doped layer. Only examples with a thick first doped layer are illustrated in the figures.
First, examples of substrate-free LED chips that have two electrodes on the upper side of the chip are now described. The chips are illustrated in FIGS. 5 to 9. If the lower electrode or a reflector substantially covers the lower surface of the chip, the substrate-free LED chip will emit light from the upper side and emit little or no light from the lower side. If the lower electrode or reflector covers only a portion of the lower side of the chip, the chip will emit light from both the upper and lower sides, thereby increasing the extraction efficiency, the external quantum efficiency and the light output of the chip.
Substrate-free LED chip 500 illustrated in FIGS. 5A, 5B and 5C has both the first electrode and the second electrode on the upper surface of the device and has neither a growth substrate nor a transfer substrate. FIG. 5A is a top plan view of the chip. FIGS. 5B and 5C are side cross-sectional views along the I-I plane illustrated in FIG. 5A.
Substrate-free LED chip 500 includes a first electrode 102, a multilayer semiconductor structure 504, a second electrode 114 and a back reflector 115. The multilayer semiconductor structure 504 includes a first doped layer 108, an active region 110 and a second doped layer 112, which is on the opposite side of the active region 110 from the first doped semiconductor layer 108. Consequently, the active region is interposed between the first doped layer and the second doped layer. The active region is in electrical contact with the first doped layer and the second doped layer and the active region emits internally generated light when a voltage is applied between the first and second electrodes.
The first electrode 102 is in electrical contact with the first doped layer 108 and the second electrode 114 is in electrical contact with the second doped layer 112. The first electrode and the second electrode may be fabricated from reflecting metals. For example, the first electrode and the second electrode may be formed from one or more metals or metal alloys containing, but not limited to, silver, aluminum, nickel, titanium, chromium, platinum, palladium, rhodium, rhenium, ruthenium and tungsten. Preferred metals are aluminum and silver.
The multilayer semiconductor structure 504 of the LED chip 500 can be fabricated from GaN-based semiconductor materials containing, for example, GaN, AlN, AlGaN, InN, InGaN and/or AlInGaN. Alternatively, the multilayer semiconductor structure can be fabricated from any appropriate light-emitting semiconductor material.
The active region 110 of the multilayer semiconductor structure 504 is a p-n homojunction, a p-n heterojunction, a single quantum well or a multiple quantum well of the appropriate semiconductor material for the LED.
For purposes of illustration, LED chip 500 is assumed to be a GaN-based LED chip. The important fabrication steps for a GaN-based, illustrative example will be briefly summarized.
First a multilayer semiconductor structure 504 is fabricated on a sapphire growth substrate (not shown). The multilayer semiconductor structure illustrated in FIGS. 5B and 5C includes a first doped layer 108, an active region 110 and a second doped layer 112.
The first doped layer 108 is an n-doped GaN-based layer, which is epitaxially deposited or otherwise conventionally fabricated on a growth substrate. The first doped layer 108 has a first surface 124 and a second surface 126 opposite the first surface. The first doped layer is at least 5 microns thick. Preferably the first doped layer is at least 10 microns thick. More preferably, the first doped layer is at least 15 microns thick. Most preferably, the first doped layer is at least 25 microns thick. The first doped layer may be deposited by any standard GaN growth technique. Preferably, the first doped layer is deposited by HVPE. In this illustrative example, the first doped layer is 20 microns thick and is deposited by HVPE.
The active region 110 is a GaN-based multiple quantum well structure, which is epitaxially deposited or otherwise conventionally fabricated on the second surface 126 of the first doped layer 108. The active region 110 has a first surface 128 and a second surface 130 opposite the first surface. The first surface 128 of the active region is in electrical contact with the second surface 126 of the first doped layer. The active region may be deposited by any standard GaN growth technique. Preferably, the multiple quantum well structure is deposited by MOCVD or HVPE. In this illustrative example, the multiple-quantum-well active region is approximately 0.5 micron thick and is deposited by MOCVD.
The second doped layer 112 is a p-doped GaN-based layer, which is epitaxially deposited or otherwise conventionally fabricated on the second surface 130 of the active region 110. The second doped layer has a first surface 132 and a second surface 134 opposite the first surface. The first surface 132 of the second doped layer is in electrical contact with the second surface 130 of the active region. The second doped layer may be deposited by any GaN growth technique. In this illustrative example, the second doped layer is approximately 0.5 micron thick and is deposited by MOCVD.
A portion 116 of the second doped layer 112 and the active region 110 are removed to expose a portion 116 of the second surface 126 of the first doped layer. The first electrode 102 and the second electrode 114 are fabricated from aluminum. An aluminum layer is deposited on the second surface 134 of the second doped layer and the exposed portion 116 of the second surface 126 of the first doped layer. The aluminum layer is patterned by standard photolithographic techniques to form the first electrode 102 and the second electrode 114. First electrode 102 has a first surface 136 and a second surface 138. The first surface 136 of the first electrode is in electrical contact with the second surface 126 of the first doped layer. Second electrode 114 has a first surface 140 and a second surface 142. First surface 140 of the second electrode is in electrical contact with the second surface 134 of the second doped layer.
The first electrode 102 only partially covers the exposed portion 116 of the second surface 126 of the first doped layer. The second electrode 114 only partially covers the second surface 134 of the second doped layer. The remaining portion of the exposed portion 116 of the second surface 126 of the first doped layer and the second surface 134 of the second doped layer are output or exit surfaces for the light emitted by the LED chip 500. The first electrode is not in physical or electrical contact with the second doped layer and the active region. The air gap between the first electrode and the second doped layer and the active region may be filled with a non-conducting material or an insulating material (not shown).
The growth substrate (not illustrated) is removed by any conventional process including laser liftoff, chemical processes and mechanical polishing. Removing the growth substrate exposes first surface 124 of the first doped layer. For this illustrative example, the growth substrate is removed using an frequency-quadrupled Nd-YAG laser operating at 266 nanometers.
To form a lower reflector 115, a layer of silver or other reflective metal is deposited on the exposed first surface 124 of the first doped layer following the removal of the growth substrate. Optionally, the lower reflector may also be an omni-directional reflector that includes a dielectric layer (not shown) and a metal layer such as silver or aluminum.
In summary, substrate-free LED chip 500 illustrated in FIGS. 5A, 5B and 5C has a first electrode 102, a multilayer semiconductor structure 504 that includes a first doped layer, an active region and a second doped layer and a second electrode 114. LED chip 500 has neither a growth substrate nor a transfer substrate. LED chip 500 has a first side 552 and a second side 554. The first side 552 is proximal to the first doped layer 108. The second side 554 is proximal to the second doped layer 112. The active region 110 emits internally generated light when a voltage is applied across the first electrode 102 and the second electrode 114. Second side 554 of LED chip 500 is the light emitting side for internally generated light.
The total thickness 550 of the multilayer semiconductor structure 504 for LED chip 500 is at least 10 microns. In this illustrative example, the thickness 556 of the first doped layer (the n-doped layer) is approximately 20 microns, the thickness of the active region (a multi-quantum well structure) is approximately 0.5 microns and the thickness 558 of the second doped layer (the p-doped layer) is approximately 0.5 microns, resulting in a total thickness of 21 microns. In this example, the first doped layer is grown by HVPE and the remainder of the semiconductor layers are grown by MOCVD.
Example light rays 560, 562 and 564 in FIG. 5B illustrate internally generated light that is emitted by the active region 110 of the LED. Internally generated light ray 560 is emitted by active region 110 toward output surface 134 of the LED chip. Internally generated light ray 560 is directed at an angle to surface 134 that is less than the critical angle, which allows light ray 560 to exit the LED chip through surface 134.
Internally generated light ray 562 is emitted by active region 110 toward the lower reflector 115 of the LED. Internally generated light ray 562 is reflected by reflector 115 and directed to the output surface 134 at an angle less than the critical angle. Internally generated light ray 562 exits the LED chip through surface 134.
Internally generated light ray 564 is directed to surface 134 at an angle that is greater than the critical angle. Internally generated light ray 564 is reflected by total internal reflection and is redirected toward the rear reflector 115 of the LED chip.
Substantially all of the first side 552 of LED 500 is covered by lower reflector 115. Due to the reflectivity of reflector 115, the reflectivity of surface 138 of first electrode 102 and the reflectivity of surface 142 of second electrode 114, substrate-free LED chip 500 can reflect externally incident light. Externally incident light is light that is directed to the light emitting side of the LED from another light source or light that is emitted by the LED and is reflected back to the light emitting side of the LED as recycled light. For some applications, for example applications utilizing light recycling to increase the effective brightness of the LED, it is important that the LED have high reflectivity to externally incident light. High reflectivity to externally incident light will exist if the reflecting layers of the LED have high reflectivity (e.g. greater than 70%) and if the absorption coefficient of the multilayer semiconductor structure is low (e.g. less than 20 per centimeter). Preferably LED 500 reflects at least 60 percent of externally incident light directed to the light emitting side (second side 554) of the LED. More preferably, LED 500 reflects at least 70 percent of externally incident light. Most preferably, LED 500 reflects at least 80 of externally incident light.
Example light rays 570, 572 and 574 in FIG. SC illustrate externally incident light that is incident on the light emitting side or second side 554 of LED chip 500 and is reflected by the chip. Externally incident light ray 570 is incident on surface 134 of the LED chip. Externally incident light ray 570 passes through surface 134, passes through the multilayer semiconductor structure 504 a first time, is reflected by reflector 115, passes through the multilayer semiconductor structure 504 a second time and exits LED chip 500 through surface 134. Externally incident light ray 572 is directed to LED chip 500 and is reflected by surface 142 of the second electrode 114. Externally incident light 574 is directed to LED chip 500 and is reflected by surface 138 of the first electrode 102.
In the illustrative example in FIG. 5, the first doped semiconductor layer 108 is an n-doped layer and the second doped semiconductor layer 112 is a p-doped layer. However, the two layers can in principle be reversed. If the first doped semiconductor layer 108 is a p-doped layer, then the second doped semiconductor layer 112 is an n-doped layer. The two doped semiconductor layers 108 and 112 will have opposite n and p conductivity types.
It is well known by those skilled in the art that the multilayer semiconductor structure 504 may include additional layers in order to adjust and improve the operation of the LED chip 500. For example, a current spreading layer (not shown) may be inserted between surface 136 of the first electrode 102 and surface 126 the first doped layer 108. Such a current spreading layer will have the same conductivity type as the first doped layer and will improve the uniformity of current injection across the entire active region. In addition, a current spreading layer (not shown) may be inserted between surface 134 of the second doped layer and surface 140 of the second electrode 114. The latter current spreading layer will have the same conductivity type as the second doped layer. As another example, an electron blocking layer or a hole blocking layer (neither are shown) may be inserted either between surface 126 of the first doped layer 108 and surface 128 of the active region 110 or between surface 130 of the active region 110 and surface 132 of the second doped layer 112. An electron blocking layer reduces the escape of electrons from the active region into either doped layer. A hole blocking layer reduces the transfer of holes through the doped layer into the active region.
The substrate-free LED chips of this invention, including LED chip 500, preferably include light extraction elements (not shown) to aid in extracting internally generated light from the chips. The light extraction elements may be fabricated by any means, including chemical means, mechanical means such as grinding, or optical means such as laser ablation.
Substrate-free LED chip 500 does not have a growth substrate or a transfer substrate that can retard heat flow from the chip. If LED chip 500 is bonded to a surface, a submount, a heat sink or a leadframe, the thermal resistance for heat transfer is illustrated in FIG. 5D. Heat will flow from the LED chip to the surface, submount, heat sink or leadframe with thermal resistance R₄or 582. Heat will flow from the surface, submount, heat sink or leadframe to ambient with thermal resistance R₅or 584. The total thermal resistance, R₄plus R₅or, equivalently, the sum of the thermal resistances 582 and 584 of the substrate-free LED chip 500 will be less than for an LED chip such as LED chip 400 that includes a transfer substrate. For LED chip 400, the total thermal resistance illustrated in FIG. 4C is R₁plus R₂plus R₃or, equivalently, the sum of the thermal resistances 482, 484 and 486.
FIGS. 6 to 9 illustrate substrate-free LED chips that are variations of substrate-free LED chip 500 shown in FIG. 5.
Substrate-free LED chip 600 in FIGS. 6A and 6B is nearly identical to LED chip 500 except that LED chip 600 does not have a reflector on the first side 652. Internally generated light emitted by the active region 110 can exit from both the upper or second side 654 and the lower or first side 652. For example, light ray 660 illustrated in FIG. 6A and emitted by the active region exits LED chip 600 through surface 134 on the second side. Light ray 662 exits LED chip 600 through surface 124 on the first side. Light ray 664 undergoes total internal reflection at surface 134.
No portion of the first side 652 of LED 600 is covered by a reflecting layer. Only a portion of the second side 654 of LED 600 is covered by the first electrode and the second electrode. Both sides of the multilayer semiconductor structure are light emitting sides and will emit internally generated light. At least a portion of the first side of the multilayer semiconductor structure and at least a portion of the second side of the multilayer semiconductor structure will also transmit externally incident light. Externally incident light is light that is directed to a light emitting side of the LED from another light source or light that is emitted by an LED and is reflected back to the light emitting side of the LED as recycled light. For some applications, where it is desirable for light from a phosphor or light from another LED to pass through the LED, the LED should transmit a large portion of externally incident light. High transmissivity will exist if the absorption coefficient of the multilayer semiconductor structure is low (e.g. less than 20 per centimeter). Preferably LED 600 transmits at least 60 percent of externally incident light directed to a light emitting side (either the first side 652 or the second side 654) of the LED. More preferably, LED 600 transmits at least 70 percent of externally incident light. Most preferably, LED 600 transmits at least 80 of externally incident light.
FIG. 6B illustrates externally incident light rays 670 and 672 that are transmitted by LED chip 600. Externally incident light ray 670 is incident on surface 124 of the first side 652 of LED 600. Externally incident light ray 670 passes through the multilayer semiconductor structure 504 and exits LED chip 600 through surface 134 on the second side 654. Externally incident light ray 672 is incident on surface 134 of the second side 654 of LED 600. Externally incident light ray 672 passes through the multilayer semiconductor structure 504 and exits LED chip 600 through surface 124 on the first side 652.
In LED chip 600, the thick epitaxial layer is the first doped layer 108. Alternatively, the second doped layer 112 may be a thick epitaxial layer as illustrated by LED 700 in FIG. 7.
LED chip 700 in FIG. 7 illustrates a substrate-free LED chip that has a thick second doped layer 112. The thick second doped layer 112 is a p-doped GaN layer, which is epitaxially deposited or otherwise conventionally fabricated on the active region. The thick second doped layer 112 has a first surface 132 and a second surface 134 opposite the first surface. The first surface 132 of the second doped layer is in electrical contact with the second surface 130 of the active region. The thickness 558 of the second doped layer 112 is at least 5 microns. Preferably the second doped layer is at least 10 microns thick. More preferably, the second doped layer is at least 15 microns thick. Most preferably, the second doped layer is at least 25 microns thick. The thick second doped layer may be deposited by any standard GaN growth technique. In this illustrative example, the second doped layer 20 microns thick and is deposited by HVPE. The first doped layer 108 and the active region 110 may be deposited by any standard GaN growth technique. In this illustrative example, the first doped layer and the active region are deposited by MOCVD.
Example light rays 760, 762 and 764 illustrate internally generated light that is emitted by the active region 110 of the LED chip 700. Internally generated light ray 760 is emitted by active region 110 toward output surface 134 of the LED chip. Internally generated light ray 760 is directed at an angle to surface 134 that is less than the critical angle, which allows light ray 760 to exit the LED chip through surface 134.
Internally generated light ray 762 is emitted by active region 110 toward the lower reflector 115 of the LED. Internally generated light ray 762 is reflected by reflector 115 and directed to the output surface 134 at an angle less than the critical angle. Internally generated light ray 762 exits the LED chip through surface 134.
Internally generated light ray 764 is directed to surface 134 at an angle that is greater than the critical angle. Internally generated light ray 764 is reflected by total internal reflection and is redirected toward the rear reflector 115 of the LED chip.
FIG. 8 illustrates a side cross-sectional view of a substrate-free LED chip 800 that has both a thick first doped layer and a thick second doped layer. In this illustrative example, the thick first doped layer 108 is an n-doped layer and the thick second doped layer 112 is a p-doped layer. The thickness 556 of the first doped layer and the thickness 558 of the second doped layer are each at least 5 microns. Preferably the first doped layer and the second doped layer are each at least 10 microns thick. More preferably, the first doped layer and the second doped layer are each at least 15 microns thick. Most preferably, the first doped layer and the second doped layer are each at least 25 microns thick. The first doped layer and the second doped layer may have the same thickness or have different thicknesses. In this illustrative example, the first doped layer is 20 microns thick and the second doped layer is 5 microns thick. The first doped layer and the second doped layer may be deposited by any standard GaN growth technique. In this illustrative example, the first doped layer and the second doped layer are deposited by HVPE. For LED chip 800, the active region 110 may be deposited by any standard GaN growth technique. In this illustrative example, the active region is deposited by MOCVD.
FIG. 9 illustrates a side cross-sectional view of a substrate-free LED chip 900 that has a thick, substantially-undoped layer 902 adjacent to the first side 952 of the multilayer semiconductor structure. As illustrated in FIG. 9, an LED chip that has both electrodes on the first side or the second side of the multilayer semiconductor structure may optionally include a substantially undoped layer on the side of the multilayer semiconductor structure opposite the electrodes. The undoped layer adds additional thickness to the multilayer semiconductor structure without affecting the electrical properties of the structure. The additional thickness adds to the structural strength of the multilayer semiconductor structure. In addition, because the thermal resistance to heat transfer tends to be high at interfaces of dissimilar materials, a GaN-based multilayer semiconductor structure that contains a thick, substantially-undoped GaN layer will usually have lower thermal resistance and higher heat transfer than a prior art LED structure that includes a growth substrate or a transfer substrate of a different material. Furthermore, a thick, substantially undoped layer of GaN will have a lower optical absorption coefficient than, for example, a silicon carbide growth substrate of the prior art.
The thick substantially undoped layer 902 has a first or lower surface 904 and a second or upper surface 906. The first or lower surface 904 of the undoped layer is also the first or lower surface of the multilayer semiconductor structure. The lower surface 124 of the first doped layer is in contact with the second surface 906 of the undoped layer. In this illustrative GaN-based example, the first doped layer 108 is an n-doped layer with a thickness 556 of approximately 3 microns, the active region is a multiple quantum well approximately 0.5 microns thick and the second doped layer 112 is a p-doped layer with a thickness 558 of approximately 0.5 micron. The first doped layer, the active region and the second doped layer may be deposited by any standard GaN growth technique. In this illustrative example, the three layers are deposited by MOCVD. The thickness 910 of the undoped layer is at least 5 microns. Preferably the undoped layer is at least 10 microns thick. More preferably, the undoped layer is at least 15 microns thick. Most preferably, the undoped layer is at least 25 microns thick. In this illustrative example, the undoped layer is 20 microns thick. The undoped layer may be deposited by any standard GaN growth technique. In this illustrative example, the undoped layer is deposited by HVPE.
Example light rays 960, 962 and 964 illustrate internally generated light that is emitted by the active region 110 of the LED chip 900. Internally generated light ray 960 is emitted by active region 110 toward output surface 134 of the LED chip. Internally generated light ray 960 is directed at an angle to surface 134 that is less than the critical angle, which allows light ray 960 to exit the LED chip through surface 134 on the second side 954.
Internally generated light ray 962 is emitted by active region 110 toward the lower surface 904 of the LED at an angle less than the critical angle. Internally generated light ray 962 exits the LED chip through surface 904 on the first side 952.
Internally generated light ray 964 is directed to surface 134 at an angle that is greater than the critical angle. Internally generated light ray 964 is reflected by total internal reflection and is redirected toward the lower surface 904 of the LED chip.
For the remainder of this specification, the substrate-free LED chips will be illustrated as having a thick first doped layer. However, it will be apparent from the above discussion that any of the substrate-free chips illustrated in the following diagrams may instead have a thick second doped layer or may have both a thick first doped layer and a thick second doped layer. In addition, LED chips with both electrodes on the lower side of the chip may also have a thick undoped layer on the upper side of the chip.
FIGS. 10 and 11 illustrate substrate-free LED chips that have both electrodes on the lower side of the chip. LED chip 1000 and LED chip 1100 are flip-chip designs, but neither design includes a growth substrate nor a transfer substrate. The thick layer in FIGS. 10 and 11 is illustrated to be the first doped layer. However, the thick layer could also be the second doped layer or both the first doped layer and the second doped layer or an undoped layer. These latter examples are not illustrated.
LED chip 1000 in FIG. 10 includes a thick first doped layer 108. In this example design, the first doped layer is on the upper or first side 1052 of the chip. The first doped layer 108 is an n-doped GaN layer, which is epitaxially deposited or otherwise conventionally fabricated on a growth substrate (not shown). The growth substrate is later removed by a standard technique such as laser liftoff, chemical processing or mechanical polishing, thereby exposing the first surface 124 of the first doped layer. It is preferably to utilize a substrate removal process that results in an exposed surface 124 that is rough. A rough surface improves light extraction form LED chip 1000. The first doped layer 108 has a first surface 124 and a second surface 126 opposite the first surface. The thickness 556 of the first doped layer is at least 5 microns. Preferably the first doped layer is at least 10 microns thick. More preferably, the first doped layer is at least 15 microns thick. Most preferably, the first doped layer is at least 25 microns thick. The first doped layer may be deposited by any standard GaN growth technique. Preferably, the first doped layer is deposited by HVPE.
The first electrode 102 of LED chip 1000 is fabricated on a portion 116 of the second surface 126 of the first doped layer that was previously exposed by an etching process. The second electrode 114 is fabricated on the second surface 134 of the second doped layer. The second electrode substantially covers the second surface 134. Substantially all of the light emitted by LED chip 1000 is emitted through the upper or first side 1052 of the chip. For example, light ray 1060 is emitted through surface 124. Light ray 1062 is initially directed to the second side 1054 but is reflected by surface 140 of the second electrode. Light ray 1062 exits LED 1000 through the upper or first side 1052.
FIG. 11 illustrates example substrate-free LED chip 1100. LED chip 1100 is nearly identical to LED chip 1000 except that the second electrode 114 of LED chip 1100 covers only a portion of surface 134 of the second doped layer 112. Second electrode 114 is fabricated by depositing a layer of metal on surface 134 of the second doped layer followed by patterning the metal layer by standard photolithographic techniques. Light can exit LED chip 1100 through both the upper or first side 1152 and the lower or second side 1154 of the chip, thereby increasing the extraction efficiency, the external quantum efficiency and the light emission of the chip. For example, internally generated light ray 1160 exits LED chip 1100 on the upper or first side 1152 of the chip. Light ray 1162 exits LED chip 1100 on the lower or second side 1154. Preferably, surface 136 of the first electrode 102 and surface 140 of the second electrode 114 are reflective to internally generated light and to light externally incident on the first side 1152 of LED chip 1100.
FIGS. 12 and 13 illustrate substrate-free LED chips that have one electrode on the upper side of the chip and one electrode on the lower side of the chip. The thick layer in FIGS. 12 and 13 is illustrated to be the first doped layer. However, the thick layer could also be the second doped layer or both the first doped layer and the second doped layer.
LED chip 1200 in FIG. 12 includes a thick first doped layer 108. In this example design, the first doped layer is on the upper or first side 1252 of the chip. The first doped layer 108 is an n-doped GaN layer, which is epitaxially deposited or otherwise conventionally fabricated on a growth substrate (not shown). The growth substrate is later removed by a standard technique such as laser liftoff, chemical processing or mechanical polishing, thereby exposing the first surface 124 of the first doped layer. The thickness 556 of the first doped layer is at least 5 microns. Preferably the first doped layer is at least 10 microns thick. More preferably, the first doped layer is at least 15 microns thick. Most preferably, the first doped layer is at least 25 microns thick. The first doped layer may be deposited by any standard GaN growth technique. Preferably, the first doped layer is deposited by HVPE.
The first electrode 102 of LED chip 1200 is fabricated on the first surface 124 of the first doped layer. The first surface 124 was previously exposed by removing the growth substrate. The first electrode is fabricated by depositing a metal layer and patterning the layer using standard photolithographic techniques. The second electrode 114 is fabricated on the second surface 134 of the second doped layer. In this example, the second electrode substantially covers the second surface 134. Substantially all of the light emitted by LED chip 1200 is emitted through the upper or first side 1252 of the chip. For example, light ray 1260 is emitted through surface 124. Light ray 1262 is initially directed to the second side 1254 but is reflected by surface 140 of the second electrode. Light ray 1262 exits LED 1200 through the upper or first side 1252.
Substrate-free LED chip 1300 illustrated in FIG. 13 is nearly identical to LED chip 1200 except that the second electrode 114 for LED chip 1300 covers only a portion of the second surface 134 of the second doped layer 112. Second electrode 114 is fabricated by depositing a layer of metal on surface 134 of the second doped layer followed by patterning the metal layer by standard photolithographic techniques. Light can exit LED chip 1300 through both the upper or first side 1352 and the lower or second side 1354 of the chip, thereby increasing the extraction efficiency, the external quantum efficiency and the light emission of the chip. For example, internally generated light ray 1360 exits LED chip 1300 on the upper or first side 1352 of the chip. Light ray 1362 exits LED chip 1300 on the lower or second side 1354.
While the invention has been described in conjunction with specific embodiments and examples, it is evident to those skilled in the art that many alternatives, modifications and variations will be apparent in light of the foregoing description. Accordingly, the invention is intended to embrace all such alternatives, modifications and variations as fall within the spirit and scope of the appended claims.

Claims

1. An substrate-free, gallium-nitride-based light emitting diode chip comprising:

a multilayer semiconductor structure that has a first side and an opposing second side and that includes a first doped layer proximal to said first side, an active region, a second doped layer proximal to said second side with said active region interposed between said first doped layer and said second doped layer;

a first electrode in electrical contact with said first doped layer; and

a second electrode in electrical contact with said second doped layer;

wherein said multilayer semiconductor has a total thickness of at least 10 microns and wherein said active region emits internally generated light when an electrical voltage is applied between said first electrode and said second electrode.

2. The light emitting diode chip as in claim 1, wherein said multilayer semiconductor structure is at least 20 microns thick.

3. The light emitting diode chip as in claim 2, wherein said multilayer semiconductor structure is at least 30 microns thick.

4. The light emitting diode chip as in claim 1, wherein at least a portion of said first side of said multilayer semiconductor structure and at least a portion of said second side of said multilayer semiconductor structure transmit externally incident light.

5. The light emitting diode chip as in claim 4, wherein said light emitting diode chip transmits at least 60 percent of said externally incident light that is directed to said first side or said second side of said multiplayer semiconductor structure.

6. The light emitting diode chip as in claim 5, wherein said light emitting diode chip transmits at least 70 percent of said externally incident light.

7. The light emitting diode chip as in claim 6, wherein said light emitting diode chip transmits at least 80 percent of said externally incident light.

8. The light emitting diode chip as in claim 1, further comprising a reflective surface covering substantially all of said first side or substantially all of said second side of said multilayer semiconductor structure, wherein said reflective surface reflects externally incident light.

9. The light emitting diode chip of claim 8, wherein said reflective surface is said first electrode or said second electrode.

10. The light emitting diode chip as in claim 8, wherein said light emitting diode chip reflects at least 60 percent of said externally incident light that is directed to said multiplayer semiconductor structure.

11. The light emitting diode chip as in claim 10, wherein said light emitting diode chip reflects at least 70 percent of said externally incident light.

12. The light emitting diode chip as in claim 11, wherein said light emitting diode chip reflects at least 80 percent of said externally incident light.

13. The light emitting diode chip as in claim 1, wherein said multiplayer semiconductor structure has an absorption coefficient less that 20 per centimeter.

14. The light emitting diode chip as in claim 13, wherein said multiplayer semiconductor structure has an absorption coefficient less that 10 per centimeter.

15. The light emitting diode chip as in claim 14, wherein said multiplayer semiconductor structure has an absorption coefficient less that 5 per centimeter.

16. The light emitting diode chip as in claim 15, wherein said multiplayer semiconductor structure has an absorption coefficient less that 2 per centimeter.

17. The light emitting diode chip as in claim 1, wherein said first doped layer or said second doped layer is at least 5 microns thick and is fabricated by hydride vapor phase epitaxy.

18. The light emitting diode chip as in claim 17, wherein said first doped layer or said second doped layer is at least 10 microns thick.

19. The light emitting diode chip as in claim 18, wherein said first doped layer or said second doped layer is at least 15 microns thick.

20. The light emitting diode chip as in claim 19, wherein said first doped layer or said second doped layer is at least 25 microns thick.

21. The light emitting diode chip as in claim 1, wherein said multiplayer semiconductor structure includes a substantially undoped layer adjacent to said first side or said second side, wherein said undoped layer is at least 5 microns thick and wherein said undoped layer is fabricated by hydride vapor phase epitaxy.

22. The light emitting diode chip as in claim 21, wherein said undoped layer is at least 10 microns thick.

23. The light emitting diode chip as in claim 22, wherein said undoped layer is at least 15 microns thick.

24. The light emitting diode chip as in claim 23, wherein said undoped layer is at least 25 microns thick.

25. The light emitting diode chip as in claim 1, wherein said first doped layer is an n-doped layer and said second doped layer is a p-doped layer.