US20140203320A1 - Composite high reflectivity layer - Google Patents
Composite high reflectivity layer Download PDFInfo
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- US20140203320A1 US20140203320A1 US14/219,916 US201414219916A US2014203320A1 US 20140203320 A1 US20140203320 A1 US 20140203320A1 US 201414219916 A US201414219916 A US 201414219916A US 2014203320 A1 US2014203320 A1 US 2014203320A1
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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
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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
- H01L33/44—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 coatings, e.g. passivation layer or anti-reflective coating
- H01L33/46—Reflective coating, e.g. dielectric Bragg reflector
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B5/00—Optical elements other than lenses
- G02B5/08—Mirrors
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L2224/00—Indexing scheme for arrangements for connecting or disconnecting semiconductor or solid-state bodies and methods related thereto as covered by H01L24/00
- H01L2224/01—Means for bonding being attached to, or being formed on, the surface to be connected, e.g. chip-to-package, die-attach, "first-level" interconnects; Manufacturing methods related thereto
- H01L2224/42—Wire connectors; Manufacturing methods related thereto
- H01L2224/47—Structure, shape, material or disposition of the wire connectors after the connecting process
- H01L2224/48—Structure, shape, material or disposition of the wire connectors after the connecting process of an individual wire connector
- H01L2224/4805—Shape
- H01L2224/4809—Loop shape
- H01L2224/48091—Arched
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L2224/00—Indexing scheme for arrangements for connecting or disconnecting semiconductor or solid-state bodies and methods related thereto as covered by H01L24/00
- H01L2224/01—Means for bonding being attached to, or being formed on, the surface to be connected, e.g. chip-to-package, die-attach, "first-level" interconnects; Manufacturing methods related thereto
- H01L2224/42—Wire connectors; Manufacturing methods related thereto
- H01L2224/47—Structure, shape, material or disposition of the wire connectors after the connecting process
- H01L2224/48—Structure, shape, material or disposition of the wire connectors after the connecting process of an individual wire connector
- H01L2224/481—Disposition
- H01L2224/48151—Connecting between a semiconductor or solid-state body and an item not being a semiconductor or solid-state body, e.g. chip-to-substrate, chip-to-passive
- H01L2224/48221—Connecting between a semiconductor or solid-state body and an item not being a semiconductor or solid-state body, e.g. chip-to-substrate, chip-to-passive the body and the item being stacked
- H01L2224/48245—Connecting between a semiconductor or solid-state body and an item not being a semiconductor or solid-state body, e.g. chip-to-substrate, chip-to-passive the body and the item being stacked the item being metallic
- H01L2224/48247—Connecting between a semiconductor or solid-state body and an item not being a semiconductor or solid-state body, e.g. chip-to-substrate, chip-to-passive the body and the item being stacked the item being metallic connecting the wire to a bond pad of the item
Abstract
A high efficiency light emitting diode with a composite high reflectivity layer integral to said LED to improve emission efficiency. One embodiment of a light emitting diode (LED) chip comprises an LED and a composite high reflectivity layer integral to the LED to reflect light emitted from the active region. The composite layer comprises a first layer, and alternating plurality of second and third layers on the first layer, and a reflective layer on the topmost of said plurality of second and third layers. The second and third layers have a different index of refraction, and the first layer is at least three times thicker than the thickest of the second and third layers. For composite layers internal to the LED chip, conductive vias can be included through the composite layer to allow an electrical signal to pass through the composite layer to the LED.
Description
- This application is a continuation of U.S. patent application Ser. No. 14/047,566, to Ibbetson et al., filed on 7 Oct. 2013, which is a divisional of U.S. patent application Ser. No. 13/071,349, filed on 24 Mar. 2011, now U.S. Pat. No. 8,598,609, which is a continuation of U.S. patent application Ser. No. 12/316,097 to Ibbetson et al., filed on 8 Dec. 2008, now U.S. Pat. No. 7,915,629.
- 1. Field of the Invention
- This invention relates to light emitting diodes, and to light emitting diodes with a high reflectivity contact and method for forming the contact.
- 2. Description of the Related Art
- Light emitting diodes (LED or LEDs) are solid state devices that convert electric energy to light, and generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped n-type and p-type layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light is emitted from the active layer and from all surfaces of the LED.
- For typical LEDs it is desirable to operate at the highest light emission efficiency, and one way that emission efficiency can be measured is by the emission intensity in relation to the input power, or lumens per watt. One way to maximize emission efficiency is by maximizing extraction of light emitted by the active region of LEDs. For conventional LEDs with a single out-coupling surface, the external quantum efficiency can be limited by total internal reflection (TIR) of light from the LED's emission region. TIR can be caused by the large difference in the refractive index between the LED's semiconductor and surrounding ambient. Some LEDs have relatively low light extraction efficiencies because the high index of refraction of the substrate compared to the index of refraction for the surrounding material, such as epoxy. This difference results in a small escape cone from which light rays from the active area can transmit from the substrate into the epoxy and ultimately escape from the LED package. Light that does not escape can be absorbed in the semiconductor material or at surfaces that reflect the light.
- Different approaches have been developed to reduce TIR and improve overall light extraction, with one of the more popular being surface texturing. Surface texturing increases the light escape probability by providing a varying surface that allows photons multiple opportunities to find an escape cone. Light that does not find an escape cone continues to experience TIR, and reflects off the textured surface at different angles until it finds an escape cone. The benefits of surface texturing have been discussed in several articles. [See Windisch et al., Impact of Texture-Enhanced Transmission on High-Efficiency Surface Textured Light Emitting Diodes, Appl. Phys. Lett., Vol. 79, No. 15, October 2001, Pgs. 2316-2317; Schnitzer et al. 30% External Quantum Efficiency From Surface Textured, Thin Film Light Emitting Diodes, Appl. Phys. Lett.,
Vol 64, No. 16, October 1993, Pgs. 2174-2176; Windisch et al. Light Extraction Mechanisms in High-Efficiency Surface Textured Light Emitting Diodes, IEEE Journal on Selected Topics in Quantum Electronics, Vol. 8, No. 2, March/April 2002, Pgs. 248-255; Streubel et al. High Brightness AlGaNInP Light Emitting Diodes, IEEE Journal on Selected Topics in Quantum Electronics, Vol. 8, No. March/April 2002]. - U.S. Pat. No. 6,657,236, also assigned to Cree Inc., discloses structures formed on the semiconductor layers for enhancing light extraction in LEDs.
- Another way to increase light extraction efficiency is to provide reflective surfaces that reflect light so that it contributes to useful emission from the LED chip or LED package. In a
typical LED package 10 illustrated inFIG. 1 , asingle LED chip 12 is mounted on areflective cup 13 by means of a solder bond or conductive epoxy. One ormore wire bonds 11 connect the ohmic contacts of theLED chip 12 to leads 15A and/or 15B, which may be attached to or integral with thereflective cup 13. The reflective cup may be filled with anencapsulant material 16 which may contain a wavelength conversion material such as a phosphor. Light emitted by the LED at a first wavelength may be absorbed by the phosphor, which may responsively emit light at a second wavelength. The entire assembly is then encapsulated in a clearprotective resin 14, which may be molded in the shape of a lens to collimate the light emitted from theLED chip 12. While thereflective cup 13 may direct light in an upward direction, optical losses may occur when the light is reflected. Some light may be absorbed by the reflector cup due to the less than 100% reflectivity of practical reflector surfaces. Some metals can have less than 95% reflectivity in the wavelength range of interest. -
FIG. 2 shows another LED package in which one ormore LED chips 22 can be mounted onto a carrier such as a printed circuit board (PCB) carrier, substrate orsubmount 23. Ametal reflector 24 mounted on the submount surrounds the LED chip(s) 22 and reflects light emitted by theLED chips 22 away from thepackage 20. Thereflector 24 also provides mechanical protection to theLED chips 22. One or morewirebond connections 11 are made between ohmic contacts on theLED chips 22 andelectrical traces submount 23. The mountedLED chips 22 are then covered with anencapsulant 26, which may provide environmental and mechanical protection to the chips while also acting as a lens. Themetal reflector 24 is typically attached to the carrier by means of a solder or epoxy bond. Themetal reflector 24 may also experience optical losses when the light is reflected because it also has less than 100% reflectivity. - The reflectors shown in
FIGS. 1 and 2 are arranged to reflect light that escapes from the LED. LEDs have also been developed having internal reflective surfaces to reflect light internal to the LEDs.FIG. 3 shows a schematic of anLED chip 30 with anLED 32 mounted on asubmount 34 by ametal bond layer 36. The LED further comprises a p-contact/reflector 38 between theLED 32 and themetal bond 36, with thereflector 38 typically comprising a metal such as silver (Ag). This arrangement is utilized in commercially available LEDs such as those from Cree® Inc., available under the EZBright™ family of LEDs. Thereflector 38 can reflect light emitted from the LED chip toward the submount back toward the LED's primary emitting surface. The reflector also reflects TIR light back toward the LED's primary emitting surface. Like the metal reflectors above,reflector 38 reflects less than 100% of light and in some cases less than 95%. The reflectivity of a metal film on a semiconductor layer may be calculated from the materials' optical constants using thin film design software such as TFCalc™ from Software Spectra, Inc. (www.sspectra.com). -
FIG. 4 shows agraph 40 showing the reflectivity of Ag on gallium nitride (GaN) at different viewing angles for light with a wavelength of 460 nm. The refractive index of GaN is 2.47, while the complex refractive index for silver is taken from the technical literature. [See Handbook of Optical Constants of Solids, edited by E. Palik.] The graph shows the p-polarization reflectivity 42, s-polarization reflectivity 44, andaverage reflectivity 46, with theaverage reflectivity 46 generally illustrating the overall reflectivity of the metal for the purpose of LEDs where light is generated with random polarization. The reflectivity at 0 degrees is lower than the reflectivity at 90 degrees, and this difference can result in up to 5% or more of the light being lost on each reflection. In a LED chip, in some instances TIR light can reflect off the mirror several times before it escapes and, as a result, small changes in the mirror absorption can lead to significant changes in the brightness of the LED. The cumulative effect of the mirror absorption on each reflection can reduce the light intensity such that less than 75% of light from the LED's active region actually escapes as LED light. - The present invention discloses a higher reflectivity layer for use in LEDs and LED chips to increase emission efficiency. One embodiment of a light emitting diode (LED) chip according to the present invention comprises an active region between two oppositely doped layers, an n-type layer and a p-type layer arranged in a lateral geometry. A composite high reflectivity layer is on both the n-type layer and the p-type layer.
- These and other aspects and advantages of the invention will become apparent from the following detailed description and the accompanying drawings, which illustrate by way of example the features of the invention.
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FIG. 1 is a sectional view of one embodiment of a prior art LED lamp; -
FIG. 2 is a sectional view of another embodiment of a prior art LED lamp; -
FIG. 3 is a sectional view of another embodiment of a prior art LED chip; -
FIG. 4 is a graph showing the reflectivity of a metal reflector at different viewing angles; -
FIG. 5 a is a sectional view of one embodiment of an LED chip at a fabrication step in one method according to the present invention; -
FIG. 5 b is a sectional view of the LED chip inFIG. 5 a at a subsequent fabrication step; -
FIG. 6 is a sectional view of one embodiment of composite layer according to the present invention; -
FIG. 7 is a graph showing the reflectivity of a composite layer according to the present invention; -
FIG. 8 is a graph showing the reflectivity of a composite layer according to the present invention; -
FIG. 9 is a sectional view of another embodiment of a composite layer according to the present invention; -
FIG. 10 a is a sectional view of another embodiment of an LED according to the present invention; -
FIG. 10 b is a sectional view of the LED inFIG. 10 a at a subsequent fabrication step; -
FIG. 10 c is a sectional view of the LED inFIG. 10 b at a subsequent fabrication step; -
FIG. 10 d is a sectional view of the LED inFIG. 10 c at a subsequent fabrication step; -
FIG. 11 is a plan view of one embodiment of a composite layer according to the present invention; -
FIG. 12 is a plan view of another embodiment of a composite layer according to the present invention; -
FIG. 13 a is a sectional view of another embodiment of an LED chip according to the present invention; -
FIG. 13 b is a sectional view of the LED chip shown inFIG. 13 a at a subsequent fabrication step; -
FIG. 14 is a sectional view of another embodiment of an LED chip according to the present invention; and -
FIG. 15 is a sectional view of still another embodiment of an LED chip according to the present invention. - The present invention is directed to solid-state emitters and methods for fabricating solid-state emitters having one or more composite high reflectivity contacts or layers arranged to increase emission efficiency of the emitters. The present invention is described herein with reference to light emitting diodes (LED or LEDs) but it is understood that it is equally applicable to other solid-state emitters. The present invention can be used as a reflector in conjunction with one or more contacts, or can be used as a reflector separate from the contacts.
- The improved reflectivity of the composite contact/layer (“composite layer”) reduces optical losses that can occur in reflecting light that is emitted from the active region in a direction away from useful light emission, such as toward the substrate or submount, and also to reduce losses that can occur when TIR light is reflecting within the LED. Embodiments of the present invention provide various unique combinations of layers that can comprise a composite layer. In one embodiment according to the present invention, the composite layer can comprise a first relatively thick layer, with second and third layers having different indices of refraction and different thickness, and a reflective layer. The composite layer can be in many different locations such as on an outer surface of the LED or internal to the LED.
- Different embodiments of the invention also provide composite layers having conductive via or path arrangements that provide conductive paths through the composite layer. This allows an electric signal to pass through the composite layer along the vias so that the composite layer can be used as an internal layer, where an electrical signal passes through the composite layer during operation. This via arrangement can take many different shapes and sizes as described in detail below.
- The present invention is described herein with reference to certain embodiments but it is understood that the invention can be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. In particular, the composite layer can comprise many different layers of different material with many different thicknesses beyond those described herein. The composite layer can be in many different locations on different solid-state emitters beyond those described herein. Further, the composite layer can be provided with or without conductive structures to allow electrical signals to pass through.
- It is also understood that when an element such as a layer, region or substrate is referred to as being “on” another element, it can be directly on the other element or intervening elements may also be present. Furthermore, relative terms such as “inner”, “outer”, “upper”, “above”, “lower”, “beneath”, and “below”, and similar terms, may be used herein to describe a relationship of one layer or another region. It is understood that these terms are intended to encompass different orientations of the device in addition to the orientation depicted in the figures.
- Although the terms first, second, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the present invention.
- Embodiments of the invention are described herein with reference to cross-sectional view illustrations that are schematic illustrations of embodiments of the invention. As such, the actual thickness of the layers can be different, and variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances are expected. Embodiments of the invention should not be construed as limited to the particular shapes of the regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. A region illustrated or described as square or rectangular will typically have rounded or curved features due to normal manufacturing tolerances. Thus, the regions illustrated in the figures are schematic in nature and their shapes are not intended to illustrate the precise shape of a region of a device and are not intended to limit the scope of the invention.
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FIGS. 5 a and 5 b show one embodiment of anLED chip 50 according to the present invention, and although the present invention is described with reference to fabrication of a single LED chip it is understood that the present invention can also be applied to wafer level LED fabrication, fabrication of groups of LEDs, or fabrication of packaged LED chips. The wafer or groups of LEDs can then be separated into individual LED chips using known singulation or dicing methods. This embodiment is also described with reference to an LED chip having vertical geometry arrangement and that is flip chip mounted. As further described below the present invention can be used with other LED arrangements, such as lateral geometry LEDs and non flip-chip orientations. - The
LED chip 50 comprises anLED 52 that can have many different semiconductor layers arranged in different ways. The fabrication and operation of LEDs is generally known in the art and only briefly discussed herein. The layers of theLED 52 can be fabricated using known processes with a suitable process being fabrication using MOCVD. The layers of theLED 52 generally comprise an active layer/region 54 sandwiched between n-type and p-type oppositely dopedepitaxial layers growth substrate 60. It is understood that additional layers and elements can also be included in theLED 52, including but not limited to buffer, nucleation, contact and current spreading layers as well as light extraction layers and elements. Theactive region 54 can comprise single quantum well (SQW), multiple quantum well (MQW), double heterostructure or super lattice structures. - The
active region 54 and layers 56, 58 can be fabricated from different material systems, with preferred material systems being Group-III nitride based material systems. Group-III nitrides refer to those semiconductor compounds formed between nitrogen and the elements in the Group III of the periodic table, usually aluminum (Al), gallium (Ga), and indium (In). The term also refers to ternary and quaternary compounds such as aluminum gallium nitride (AlGaN) and aluminum indium gallium nitride (AlInGaN). In one embodiment, the n- and p-type layers active region 54 comprises InGaN. In alternative embodiments the n- and p-type layers - The
growth substrate 60 can be made of many materials such as sapphire, silicon carbide, aluminum nitride (AlN), GaN, with a suitable substrate being a 4H polytype of silicon carbide, although other silicon carbide polytypes can also be used including 3C, 6H and 15R polytypes. Silicon carbide has certain advantages, such as a closer crystal lattice match to Group III-nitrides than sapphire and results in Group III-nitride films of higher quality. Silicon carbide also has a very high thermal conductivity so that the total output power of Group-III nitride devices on silicon carbide is not limited by the thermal dissipation of the substrate (as may be the case with some devices formed on sapphire). SiC substrates are available from Cree Research, Inc., of Durham, N.C. and methods for producing them are set forth in the scientific literature as well as in U.S. Pat. Nos. Re. 34,861; 4,946,547; and 5,200,022. - Different embodiments of the
LED 52 can emit different wavelengths of light depending on the composition of theactive region 54 and n- and p-type layer LED 50 emits a blue light in the wavelength range of approximately 450 to 460 nm. TheLED chip 50 can also be covered with one or more conversion materials, such as phosphors, such that at least some of the light from the LED passes through the one or more phosphors and is converted to one or more different wavelengths of light. In one embodiment, the LED chip emits a white light combination of light from the LED's active region and light from the one or more phosphors. - In the case of Group-III nitride devices, current typically does not spread effectively through the p-
type layer 58 and it is known that a thin current spreadinglayer 64 can cover some or the entire p-type layer 58. The current spreading layer helps spread current from the p-type contact across the surface of the p-type layer 58 to provide improved current spreading across the p-type layer with a corresponding improvement in current injection from the p-type layer into the active region. The current spreadinglayer 64 is typically a metal such as platinum (Pt) or a transparent conductive oxide such as indium tin oxide (ITO), although other materials can also be used. The current spreading layer can have many different thicknesses, with one embodiment of an ITO spreading layer a thickness of approximately 115 nm. The current spreadinglayer 64 as well as the layers that comprise the composite layer described below can be deposited using known methods. It is understood that in embodiments where current spreading is not a concern, the composite layer can be provided without a current spreading layer. - Referring now to
FIG. 5 b, a compositehigh reflectivity layer 62 can be deposited on the p-type layer 58, and in the embodiment shown the current spreading layer is between thereflectivity layer 62 and the p-type layer. Thecomposite layer 62 according to the present invention has higher reflectivity to the wavelength of light generated by theLED 52 compared to standard metal contacts or distributed Bragg reflectors (DBRs). The composite layer generally comprises a thick layer of material followed by a plurality of thinner layers that combine to provide improved reflectivity. The present invention provides a composite layer with the desired reflectivity that also minimizes the number of layers to minimize the manufacturing complexities and cost. - Referring now to
FIG. 6 , the different layers that can comprise one embodiment of a composite layer according to the present invention are shown, but it is understood that many different materials, thicknesses and number of layers can also be used. Afirst layer 66 of the composite layer is provided on the current spreadinglayer 64, and the first layer can comprise many different materials, with the preferred material comprising a dielectric. Different dielectric materials can be used such as a SiN, SiO2, Si, Ge, MgOx, MgNx, ZnO, SiNx, SiOx, alloys or combinations thereof, with the material infirst layer 66 in the embodiment shown comprising SiO2. Thisfirst layer 66 should be relatively thick to provide a reliable viewing angle cut-off point after which the reflectivity of the composite layer is approximately 100%, and in one embodiment used with a blue emitting LED thefirst layer 66 can have a thickness in the range of 500 to 650 nm, with one embodiment having a thickness of approximately 591 nm. - Referring now to the
graph 72 inFIG. 7 , which shows the p-polarization reflectivity 74, s-polarization reflectivity 76, andaverage reflectivity 78 at different viewing angles for afirst layer 66 having a thickness in the range of 500 to 650 nm for blue wavelengths of light. The viewing angle cut-off is at a viewing angle of approximately 36 degrees. That is, the reflectivity of thecomposite layer 62 is approximately 100% at viewing angles greater than approximately 36 degrees, and below this viewing angle the reflectivity can be as low as 94% at certain viewing angles. - Referring again to
FIG. 6 , to improve reflectivity at lower viewing angles and to improve the angle averaged reflectivity (AAR), thecomposite layer 62 can also comprisesecond layers third layers third layers 70 a-b comprising TiO2. SiO2 has an index of refraction of 1.46, while TiO2 has an index of refraction of 2.34. The two SiO2 layers can have different thicknesses and the two TiO2 layers can have different thicknesses, which provide a composite layer that is different from standard DBRs where the layers of different materials have the same thickness. One such example of this type of DBR is a ¼ wavelength DBR where each of the second and third SiO2 and TiO2 layers can have essentially the same optical thickness approximately equal to a ¼ wavelength of the light. In other embodiments of the composite layer, Ta2O5 can be used in place of TiO2. - For the composite layer embodiment shown that is used in conjunction with a blue emitting LED, the second layers 68 a-b can have thicknesses in the range of 100 to 120 nm, and approximately 40 to 60 nm respectively, with one embodiment of the second layers being approximately 108 nm and 53 nm thick. The third TiO2 layers 70 a-b can have thicknesses in the range of 55 to 75 nm and 35 to 55 nm, respectively, with one embodiment having thicknesses of approximately 65 nm and 46 nm respectively.
- The
composite layer 62 can also comprise areflective layer 71 on thesecond layer 68 b, deposited using known methods such as sputtering. Thereflective layer 71 can have many different thicknesses and can comprise many different reflective materials, with suitable materials being Ag, Al and Au. The choice of material can depend on many factors with one being the wavelength of light being reflected. In the embodiment shown reflecting blue wavelengths of light, the reflective layer can comprise Ag having a thickness of approximately 200 nm. In other embodiments thereflective layer 71 can comprise composite metal layers such as TiAg, NiAg, CuAg or PtAg, and in some embodiments these composite layers can provide improved adhesion to the layer it is formed on, such as thesecond layer 68 b. Alternatively, a thin layer of material such as indium tin oxide (ITO), Ni, Ti or Pt can be included between thesecond layer 68 b and the reflective layer to also improve adhesion. - The structure of the
composite layer 62 provides improved AAR compared to standard ¼ wavelength DBRs. Although there may be a number of reasons why this arrangement provides this improvement, it is believed that one reason is that the different thicknesses of thesecond layers third layers composite layer 62 at many different angles, and at these different angles thesecond layers third layers -
FIG. 8 is agraph 80 showing the reflectivity of a composite layer similar to that shown inFIG. 6 , and shows the p-polarization reflectivity 82, s-polarization reflectivity 84, andaverage reflectivity 86 at different viewing angles. In this case, the reflectivity includes the effect of an ITO current spreadinglayer 64 having a thickness of 115 nm and finite absorption coefficient of 500/cm, which results in the reflectivity approaching but being slightly below 100% at viewing angles greater than approximately 36 degrees. The AAR across viewing angles of 0-90 degrees for the composite layer shown is approximately 98.79%, which provides an improvement over a standard DBR with a similar number of layers made of the same materials. At certain wavelengths of light the AAR of a standard ¼ wavelength DBR can be approximately 98.73% or less. This difference can have a significant impact on overall LED brightness because light can reflect off the composite layer multiple times before escaping from the LED. This compounding effect of multiple reflections amplifies even small differences in reflectivity. -
FIG. 9 shows another embodiment of thecomposite layer 100 that is similar tocomposite layer 62 described above and can be used with a blue emitting LED. Thecomposite layer 100 can have four layers instead of five. In this embodiment thefirst layer 102 is on a current spreadinglayer 64, although it is understood that thecomposite layer 100 can be used without a current spreading layer. Thefirst layer 102 is similar to thefirst layer 66 described above and can be made of many materials and many different thicknesses. In the embodiment shown thefirst layer 102 can comprise SiO2 with thickness in the range of 500 to 650 nm, with one embodiment having a thickness of approximately 591 nm. - In this embodiment, the
composite layer 100 comprises only onesecond layer 106 sandwiched between two third layers 108 a-b. Like the embodiment above. That is, there are not an equal number of alternating second layers and third layers as incomposite layer 62 described above, and as in conventional DBRs. This results in second and third layers combinations that comprise incomplete pairs or that are asymmetric. In embodiments with incomplete second and third layer pairs can comprise different numbers of each layer such as two second layers and three third layers, three second layers and four third layers, etc. - The second and
third layers 106, 108 a-b can comprise many different materials and can have many different thicknesses. In the embodiment shown, thesecond layer 106 can comprise SiO2 and can have a thickness in the range of approximately 100 to 120 nm, with one embodiment having a thickness of 107 nm. The third layers 108 a-b can comprise TiO2 and can have thicknesses of in the range of 45 to 65 nm and 65 to 85 nm respectively, with one embodiment having third layer thicknesses of approximately 56 and 75 nm, respectively. Thecomposite layer 100 can also comprise areflective layer 110 on thethird layer 108 b that can be deposited using known methods and can comprise the same materials asreflective layer 71 described above. - By having an asymmetric arrangement, the composite layer can have fewer layers with the corresponding reduction in manufacturing steps and costs. This can also provide the additional advantage of better adhesion to subsequent layers, such as a
reflective layer 110. In this embodiment the top layer comprisesthird layer 108 b, which is TiO2. This material can provide improved adhesion to reflective metals compared to thesecond layer 106 comprising SiO2. Thecomposite layer 100, however, can have a reduced AAR compared to a six-layer arrangement shown inFIG. 6 , with the AAR of one embodiment of a five-layer arrangement as shown inFIG. 9 being approximately 98.61%. This, however, represents an improvement over a standard five layer DBR where the AAR can be approximately 96.61%. Similar to the six-layer embodiment above, this difference can have a significant impact on overall LED brightness because of the compounding effect of multiple reflections. - It is understood that composite layers according to the present invention can have many different layers of different materials and thicknesses. In some embodiments the composite layer can comprise layers made of conductive materials such as conductive oxides. The conductive oxide layers can have different indices of refraction and the differing thicknesses to provide the improved reflectivity. The different embodiments can have different arrangements of complete and incomplete pairs of second and third layers. It is also understood that the composite layer can be arranged in different locations on an LED and can comprise different features to provide thermal or electrical conduction through the composite layer.
- Referring now the
FIGS. 10 a through 10 d, another embodiment of anLED 120 having many of the same features asLED 50 shown inFIGS. 5 a and 5 b and for those same features the same reference numbers will be used. TheLED 50 is fabricated such that is can be arranged in a flip-chip orientation, so for this embodiment the end LED chip will have the composite layer 62 (or composite layer 100) arranged as an internal layer as further described below. Accordingly, an electric signal should pass through thecomposite layer 62. Referring now toFIG. 10 a andFIG. 11 ,holes 122 can be formed through thecomposite layer 62 at random or regular intervals, with the holes sized and positioned so that a conductive material can be deposited in the holes to form conductive vias. In the embodiment shown theholes 122 are at regular intervals. - In different embodiments having a current spreading
layer 64, theholes 122 may or may not pass through the current spreadinglayer 64. Theholes 122 can be formed using many known processes such as conventional etching processes or mechanical processes such as microdrilling. Theholes 122 can have many different shapes and sizes, with theholes 122 in the embodiment shown having a circular cross-section with a diameter of approximately 20 microns.Adjacent holes 122 can be approximately 100 microns apart. It is understood that the holes 122 (and resulting vias) can have cross-section with different shapes such as square, rectangular, oval, hexagon, pentagon, etc. In other embodiments the holes are not uniform size and shapes and there can be different spaces between adjacent holes. - Referring now to
FIG. 12 , instead of holes aninterconnected grid 124 can be formed through thecomposite layer 62, with a conductive material then being deposited in thegrid 124 to form the conductive path through the composite layer. Thegrid 124 can take many different forms beyond that shown inFIG. 11 , with portions of the grid interconnecting at different angles in different embodiment. An electrical signal applied to thegrid 124 can spread throughout along the interconnected portions. It is further understood that in different embodiments a grid can be used in combination with holes. - Referring now to
FIG. 10 b, aconductive layer 126 can be deposited on thecomposite layer 62 covering its reflective layer and filling theholes 122 to form vias 128 through thecomposite layer 62. In other embodiments, the conductive layer can cover less than all of thecomposite layer 62. Theconductive layer 126 can comprise many different materials such as metals or conductive oxides, both of which can be deposited using known techniques. - Referring now to
FIG. 10 c, theLED 120 can be flip-chip mounted to asubmount 130 using known mounting techniques. In the embodiment shown, theLED 50 is flip-chip mounted to the submount by aconductive bond material 132. It is understood that in embodiments where the LEDs chips 120 are formed at the wafer level and then singulated, the LEDs chips 120 can be wafer bonded to thesubmount 130 using known wafer bonding techniques. Thesubmount 130 can be made of many different materials and can have many different thicknesses, with thepreferred submount 130 being electrically conductive so that an electrical signal can be applied to the active region of the LED through thesubmount 130. The signal also passes through the composite layer alongconductive vias 128. - Referring now to
FIG. 10 d, the growth substrate 60 (as shows inFIG. 10 c) can be removed using known grinding and/or etching processes. Afirst contact 134 can be deposited on the n-type layer 56 and asecond contact 136 can be deposited on thesubmount 130. The first andsecond contacts type layer 56 can be textured or shaped such as by laser texturing, mechanical shaping, etching (chemical or plasma), scratching or other processes, to enhance light extraction. - During operation, an electrical signal is applied to the
LED 50 across first andsecond contacts first contact 134 spreads into the n-type layer 56 and to theactive region 54. The signal on thesecond contact 136 spreads into thesubmount 130, throughcomposite layer 62 along thevias 128, through the current spreadinglayer 64, into the p-type layer 58 and to theactive region 54. This causes theactive region 54 to emit light and thecomposite layer 62 is arranged to reflect light emitted from the active region toward thesubmount 128, or reflected by TIR toward thesubmount 130, back toward the top of theLED chip 50. Thecomposite layer 62 encourages emission toward the top of theLED chip 50 and because of its improved reflectivity, reduces losses that occur during reflection. - It is understood that the composite layers can be used in many different ways and in many different locations on LEDs, LED chips, and other solid-state emitters. As shown in
FIGS. 13 a and 13 b, the composite layers can be used in conjunction with a lateralgeometry LED chip 150 where both contacts are on one side of the LEDs. The layers of theLED 150 are generally the same as those forLED chip 50 and can comprise an active layer/region 154 sandwiched between n-type and p-type oppositely dopedepitaxial layers growth substrate 160. For lateral geometry LEDs, a portion of the p-type layer 158 andactive region 154 is removed, such as by etching, to expose acontact mesa 161 on the n-type layer 156. In this embodiment, acomposite layer 162 similar to thecomposite layer 62 described above can be included on both the surface of the n-type layer 156 and the surface of the p-type layer 158, with the composite layer having ametal layer 164 andconductive vias 166 similar to themetal layer 126 and vias 128 described above. - Referring now to
FIG. 13 b, theLED chip 150 can be flip-chip mounted to asubmount 168 using known mounting processes preferably byconductive bonds 170 to the metal layers 164 on the composite layers 162. An electrical signal from thesubmount 168 is applied to the LED through theconductive bonds 170, andcomposite layers 162, causing the LED chip to emit light. Thecomposite layers 162 reflect light that is directed toward thesubmount 168 to reflect back toward the emission surface of theLED chip 150. The improved reflectivity of thecomposite layers 162 reduces reflectivity losses and improves overall emission efficiency of theLED chip 150. -
FIG. 14 shows still another embodiment ofLED 180 according to the present invention also having an active layer/region 184 sandwiched between n-type and p-type oppositely dopedepitaxial layers growth substrate 190. A portion of the p-type layer 188 andactive region 184 is removed, such as by etching, to expose a contact mesa on the n-type layer 186. In this embodiment, p- and n-type contacts type layer 188 and the contact mesa of the n-type 186, and acomposite layer 196 can be included on the bottom surface of thesubstrate 190. - In this embodiment, an electrical signal is not applied to the LED through the
composite layer 196. Instead, the electrical signal is applied through the p- and n-type contacts active region 184. As a result, an electrical signal does not need to pass through thecomposite layer 196 and thecomposite layer 196 does not need electrically conductive vias. Instead, an uninterrupted composite layer can be included across the substrate bottom surface to reflect light emitted from the active region toward the substrate and TIR light that reflects toward the substrate. It is understood that in different embodiments the composite layer can also cover all or part of the side surfaces of theLED 180, and a composite layer can be used in with the n- and p-type contacts - It is also understood that a composite layer can also be used on the bottom surface of submounts in flip-chip embodiments where the submounts are transparent. In these embodiments the desired reflectivity can be achieved without having internal
composite layers 162 as shown inFIG. 13 b. -
FIG. 15 shows still another embodiment of anLED chip 210 having an active layer/region 214; n-type and p-type oppositely dopedepitaxial layers growth substrate 220.LED 210 has vertical geometry with a p-type contact 224 on the p-type layer 218. A thin semitransparent current spreading layer (not shown) can cover some or the entire p-type layer 218, which is typically a metal such as Pt or a transparent conductive oxide such as ITO, although other materials can also be used. Acomposite layer 226 is included on thesubstrate 220 and, because theLED 210 has vertical geometry, an electrical signal can be applied to the LED through thecomposite layer 226. The composite layer comprisesconductive vias 228 similar to those described above that allow an electrical signal to pass through thecomposite layer 226. Thecomposite layer 226 can also comprise ametal layer 230 with an n-type contact 232 on the metal layer. This embodiment is particularly applicable to LEDs having electrically conductive substrates and an electrical signal applied to the p-type 224 and n-type contact 232 spreads to the LEDactive region 214 causing it to emit light. It is also understood that a composite layer can be included with the p-type contact to improve its reflectivity. - In different embodiments of the present invention the vias can serve additional purposes beyond conducting electrical signals. In some embodiments the vias can be thermally conductive to assist in thermal dissipation of heat generated by the LED. Heat can pass away from the LED through the vias where it can dissipate.
- Although the present invention has been described in detail with reference to certain preferred configurations thereof, other versions are possible. Therefore, the spirit and scope of the invention should not be limited to the versions described above.
Claims (12)
1. A light emitting diode (LED) chip, comprising:
an active region between an n-type layer and a p-type layer;
a composite high reflectivity layer on said n-type layer and on said p-type layer.
2. The LED chip of claim 1 , further comprising:
a first contact in electrical contact with said n-type layer on a first side of said LED chip;
a second contact in electrical contact with said p-type layer also on said first side of said LED chip.
3. The LED chip of claim 1 , wherein said LED chip is a lateral geometry LED chip.
4. The LED chip of claim 1 , said composite high reflectivity layer comprising a first portion on said n-type layer and a second portion on said p-type layer, said first and second portions electrically isolated from one another.
5. The LED chip of claim 1 , further comprising conductive vias through said composite high reflectivity layer.
6. The LED chip of claim 1 , further comprising a first metal layer on said composite high reflectivity layer over said n-type layer and a second metal layer on said composite high reflectivity layer over said p-type layer.
7. The LED chip of claim 1 , said composite high reflectivity layer comprising:
a first layer;
one or more second layers and a plurality of third layers on said first layer, said second layers having an index of refraction different from said third layers, said second and third layers alternating, and each of said third layers having different thicknesses compared to the other of said third layers, said third layers comprising at least a pair of layers comprising a same material and different thicknesses; and
a reflective layer on the topmost of said second and third layers.
8. The LED chip of claim 1 , further comprising a substrate on said n-type layer opposite said composite high reflectivity layer.
9. The LED chip of claim 1 , said composite high reflectivity layer comprising a plurality of layers and a conductive path through said composite layer through which an electrical signal can pass to said LED, said plurality of layers comprising at least a pair of layers comprising a same material and different thicknesses.
10. The LED chip of claim 1 , wherein said composite high reflectivity layer provides higher reflectivity to the wavelength of light emitted from said LED compared to a distributed Bragg reflector.
11. The LED chip of claim 1 , wherein said composite high reflectivity layer provides an angle averaged reflectivity greater than a quarter wavelength distributed Bragg reflector having the same number of layers.
12. The LED chip of claim 1 , further comprising a conductive grid electrically contacting said p-type layer or said n-type layer through said composite high reflectivity layer.
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Also Published As
Publication number | Publication date |
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CN102742037A (en) | 2012-10-17 |
JP5421386B2 (en) | 2014-02-19 |
EP2374163B1 (en) | 2022-02-02 |
WO2010077287A1 (en) | 2010-07-08 |
JP2012511252A (en) | 2012-05-17 |
US8710536B2 (en) | 2014-04-29 |
US7915629B2 (en) | 2011-03-29 |
EP2374163A1 (en) | 2011-10-12 |
US8598609B2 (en) | 2013-12-03 |
US20110169036A1 (en) | 2011-07-14 |
CN102742037B (en) | 2016-03-09 |
KR20110106347A (en) | 2011-09-28 |
US20100140635A1 (en) | 2010-06-10 |
KR101677520B1 (en) | 2016-11-18 |
US20140034987A1 (en) | 2014-02-06 |
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