Abstract:
This invention describes new LEDs having light extraction structures on or within the LED to increase its efficiency. The new light extraction structures provide surfaces for reflecting, refracting or scattering light into directions that are more favorable for the light to escape into the package. The structures can be arrays of light extraction elements or disperser layers. The light extraction elements can have many different shapes and are placed in many locations to increase the efficiency of the LED over conventional LEDs. The disperser layers provide scattering centers for light and can be placed in many locations as well. The new LEDs with arrays of light extraction elements are fabricated with standard processing techniques making them highly manufacturable at costs similar to standard LEDs. The new LEDs with disperser layers are manufactured using new methods and are also highly manufacturable.

Description:
This application is a divisional of patent application Ser. No. 09/727,803 filed on Nov. 28, 2000, now U.S. Pat. No. 6,657,236, which claims the benefit of provisional application No. 60/168,817 to Thibeault et al., which was filed on Dec. 3, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to light emitting diodes and more particularly to new structures for enhancing their light extraction. 
     2. Description of the Related Art 
     Light emitting diodes (LEDs) are an important class of solid state devices that convert electric energy to light and commonly comprise an active layer of semiconductor material sandwiched between two oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. The light generated by the active region emits in all directions and light escapes the semiconductor chip through all exposed surfaces. Packaging of the LED is commonly used to direct the escaping light into a desired output emission profile. 
     As semiconductor materials have improved, the efficiency of semiconductor devices has also improved. New LEDs are being made from materials such as InAlGaN, which allows for efficient illumination in the ultraviolet to amber spectrum. Many of the new LEDs are more efficient at converting electrical energy to light compared to conventional lights and they can be more reliable. As LEDs improve, they are expected to replace conventional lights in many applications such as traffic signals, outdoor and indoor displays, automobile headlights and taillights, conventional indoor lighting, etc. 
     However, the efficiency of conventional LEDs is limited by their inability to emit all of the light that is generated by their active layer. When an LED is energized, light emitting from its active layer (in all directions) reaches the emitting surfaces at many different angles. Typical semiconductor materials have a high index of refraction (n≈2.2-3.8) compared to ambient air (n=1.0) or encapsulating epoxy (n≈1.5). According to Snell&#39;s law, light traveling from a region having a high index of refraction to a region with a low index of refraction that is within a certain critical angle (relative to the surface normal direction) will cross to the lower index region. Light that reaches the surface beyond the critical angle will not cross but will experience total internal reflection (TIR). In the case of an LED, the TIR light can continue to be reflected within the LED until it is absorbed. Because of this phenomenon, much of the light generated by conventional LEDs does not emit, degrading its efficiency. 
     One method of reducing the percentage of TIR light is to create light scattering centers in the form of random texturing on the LED&#39;s surface. [Shnitzer, et al., “30%  External Quantum Efficiency From Surface Textured, Thin Film Light Emitting Diodes” , Applied Physics Letters 63, Pgs. 2174-2176 (1993)]. The random texturing is patterned into the surface by using sub micron diameter polystyrene spheres on the LED surface as a mask during reactive ion etching. The textured surface has features on the order of the wavelength of light that refract and reflect light in a manner not predicted by Snell&#39;s law due to random interference effects. This approach has been shown to improve emission efficiency by 9 to 30%. 
     One disadvantage of surface texturing is that it can prevent effective current spreading in LEDs which have a poor electrical conductivity for the textured electrode layer, such as for p-type GaN. In smaller devices or devices with good electrical conductivity, current from the p and n-type layer contacts will spread throughout the respective layers. With larger devices or devices made from materials having poor electrical conductivity, the current cannot spread from the contacts throughout the layer. As a result, part of the active layer will not experience the current and will not emit light. To create uniform current injection across the diode area, a spreading layer of conductive material can be deposited on the surface. However, this spreading layer often needs to be optically transparent so that light can transmit through the layer. When a random surface structure is introduced on the LED surface, an effectively thin and optically transparent current spreader cannot easily be deposited. 
     Another method of increasing light extraction from an LED is to include a periodic patterning of the emitting surface or internal interfaces which redirects the light from its internally trapped angle to defined modes determined by the shape and period of the surface. See U.S. Pat. No. 5,779,924 to Krames et at. This technique is a special case of a randomly textured surface in which the interference effect is no longer random and the surface couples light into particular modes or directions. One disadvantage of this approach is that the structure can be difficult to manufacture because the surface shape and pattern must be uniform and very small, on the order of a single wavelength of the LED&#39;s light. This pattern can also present difficulties in depositing an optically transparent current spreading layer as described above. 
     An increase in light extraction has also been realized by shaping the LED&#39;s emitting surface into a hemisphere with an emitting layer at the center. While this structure increases the amount of emitted light, its fabrication is difficult. U.S. Pat. No. 3,954,534 to Scifres and Burnham discloses a method of forming an array of LEDs with a respective hemisphere above each of the LEDs. The hemispheres are formed in a substrate and a diode array is grown over them. The diode and lens structure is then etched away from the substrate. One disadvantage of this method is that it is limited to formation of the structures at the substrate interface, and the lift off of the structure from the substrate results in increased manufacturing costs. Also, each hemisphere has an emitting layer directly above it, which requires precise manufacturing. 
     U.S. Pat. No. 5,793,062 discloses a structure for enhancing light extraction from an LED by including optically non-absorbing layers to redirect light away from absorbing regions such as contacts, and also to redirect light toward the LED&#39;s surface. One disadvantage of this structure is that the non-absorbing layers require the formation of undercut strait angle layers, which can be difficult to manufacture in many material systems. 
     Another way to enhance light extraction is to couple photons into surface plasmon modes within a thin film metallic layer on the LED&#39;s emitting surface, which are emitted back into radiated modes. [Knock et al.,  Strongly Directional Emission From AlGaAs/GaAs Light Emitting Diodes , Applied Physics Letter 57, Pgs. 2327-2329 (1990)]. These structures rely on the coupling of photons emitted from the semiconductor into surface plasmons in the metallic layer, which are further coupled into photons that are finally extracted. One disadvantage of this device is that it is difficult to manufacture because the periodic structure is a one-dimensional ruled grating with shallow groove depths (&lt;0.1 μm). Also, the overall external quantum efficiencies are low (1.4-1.5%), likely due to inefficiencies of photon to surface plasmon and surface plasmon-to-ambient photon conversion mechanisms. This structure also presents the same difficulties with a current spreading layer, as described above. 
     Light extraction can also be improved by angling the LED chip&#39;s side surfaces to create an inverted truncated pyramid. The angled surfaces provide the TIR light trapped in the substrate material with an emitting surface [Krames, et. al.,  High Power Truncated Inverted Pyramid  ( Al   x   Ga   1-x ) 0.5   In   0.5   P/GaP Light Emitting Diodes Exhibiting &gt; 50%  External Qauntum Efficiency , Applied Physics Letters 75 (1999)]. Using this approach external quantum efficiency has been shown to increase by 35% to 50% for the InGaAlP material system. This approach works for devices in which a significant amount of light is trapped in the substrate. For GaN devices grown on sapphire substrates, much of the light is trapped in the GaN film so that angling the LED chip&#39;s side surfaces will not provide the desired enhancement. 
     Still another approach for enhancing light extraction is photon recycling [Shnitzer, et al., “ Ultrahigh Spontaneous Emission Quantum Efficiency,  99.7%  Internally and  72%  Externally, From AlGaAs/GaAs/AlGaAs Double Heterostructures” , Applied Physics Letters 62, Pgs. 131-133 (1993)]. This method relies on LEDs having a high efficiency active layer that readily converts electrons and holes to light and vice versa. TIR light reflects off the LED&#39;s surface and strikes the active layer, where it is converted back to an electron-hole pair. Because of the high efficiency of the active layer, the electron-hole pair will almost immediately be reconverted to light that is again emitted in random directions. A percentage of the recycled light will strike one of the LEDs emitting surfaces within the critical angle and escape. Light that is reflected back to the active layer goes through the same process again. 
     One disadvantage of this approach is that it can only be used in LEDs made from materials that have extremely low optical loss and cannot be used in LEDs having an absorbing current spreading layer on the surface. 
     SUMMARY OF THE INVENTION 
     The present invention provides new LEDs having light extraction structures that are disposed on an exposed surface or within the LED to increase the probability of light escaping from the LED; thereby increasing the LED&#39;s light extraction and overall efficiency. The new LED is easy to manufacture and provides numerous new options and combinations for extracting light. 
     The new LED generally comprises an LED structure having a p-type layer, an n-type layer, and an active layer between the p-type and n-type layers. The LED structure is sandwiched between a first spreader layer and a second spreader layer. The spreader layers are semiconducting or conducting layers that distribute current across the plane of the device so that current is efficiently injected into the active layer. Light extraction structures are included that are on or within the new LED (or substrate). The structures provide a spatially varying index of refraction and provides surfaces to allow light trapped within the LED to refract or reflect and escape. In most embodiments the LED structure and current spreading layers are grown on a substrate that is adjacent to the first spreader layer, opposite the LED structure. Respective contacts are included on the first and second spreader layers and a bias applied across the contacts causes the LED structure&#39;s active layer to emit light. The light extraction structures are preferably disposed in a plane parallel to the LED&#39;s layers and substantially cover the area of the LED 
     The light extraction structures are preferably either arrays of light extraction elements (LEEs) or disperser layers. In those embodiments having an LEE array on an exposed surface, the array is formed from a material that has a higher index of refraction than the LED&#39;s encapsulating material. The LEEs can be shaped using many different methods and provide many different surfaces for otherwise trapped light to escape. 
     Alternatively, the new LED can have the LEE arrays placed within the LED itself. The internal LEE arrays are also formed to provide a spatially varying index of refraction. The LEE array is formed during the LED growth process and once the array is formed the remaining layers of the LED structure are grown over the array by an epitaxial deposition technique to embed the LEE array within the LED. Light rays that would otherwise be trapped in the epitaxial layers or substrate can interact with the LEE array to refract and/or reflect into rays that can escape the LED. 
     Another embodiment of the new LED includes a disperser layer on one of the LED&#39;s exposed surfaces, with the layer formed of a material having a higher index of refraction than the LED encapsulating material. Light that hits the disperser layer on the LED has an increased chance of being scattered into an escaping direction. By using a surface material to form the light disperser layer the problems of patterning roughness into the semiconductor surface are eliminated, providing an advantage over the work of Schnitzer. 
     Alternatively, the new LED can have disperser layers disposed within the LED itself. The disperser layer can be formed in or on the substrate prior to epitaxial growth of the LED, or within the LED epitaxial structure itself. The disperser layer is made from a material with an index of refraction that is different from the substrate and/or epitaxial material so that light scattering can occur. 
     Most of the above embodiments can also be mounted using flip-chip mounting techniques, with the substrate becoming the LEDs primary emitting surface. 
     These and other further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings, in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of a new LED with an LEE array on the second spreader layer; 
     FIG. 2 is a sectional view of a new LED with an LEE array on the substrate&#39;s surface; 
     FIG. 3 shows sectional views of the basic shapes of LEEs that can be formed into arrays for integration with the new LEDs; 
     FIG. 4 is a sectional view of a new LED with an internal LEE array formed at the interface between the substrate and the first spreader layer; 
     FIG. 5 is a sectional view of a new LED with an LEE array formed within the first spreader layer; 
     FIG. 6 is a sectional view of a new LED with an LEE array formed with voids; 
     FIG. 7 is a sectional view of a new LED with an LEE array formed within the substrate at the interface with the first spreader layer; 
     FIG. 8 is a sectional view of a new LED with a surface disperser layer formed on the second spreader layer; 
     FIG. 9 is a sectional view of a new LED with a surface disperser layer formed on the substrate; 
     FIG. 10 is a sectional view of a new LED with an internal disperser layer formed within the first spreader at the interface with the substrate; 
     FIG. 11 is a sectional view of a new LED with an internal disperser layer formed within the first spreader layer; 
     FIG. 12 shows a sectional view of a new LED where the internal disperser layer is formed in-situ during the epitaxial growth; 
     FIG. 13 also shows a sectional view of a new LED where the internal disperser layer is formed in-situ during the epitaxial growth; 
     FIG. 14 is a sectional view of a new flip-chip mounted LED with an LEE array on the substrate&#39;s surface; and 
     FIG. 15 is a sectional view of a new flip-chip mounted LED with a surface disperser layer formed within one of the LED&#39;s layers. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     First Embodiment 
     FIG. 1 shows one embodiment of the new LED  10  constructed in accordance with the present invention. The new LED has a LED structure  12  which consists of a active layer  13  sandwiched between two oppositely doped layers  14 , 15 . In the preferred LED structure  12 , the top layer  14  is p-type and the bottom layer  15  is n-type, although opposite doping in the layers  14 , 15  will also work. The new LED has a first spreading layer  16  that is made of a conductive material which spreads current from a first contact pad  18  to the LED structure&#39;s bottom layer  15 . The first contact pad  18  is also referred to as the n-contact pad because in the preferred embodiment the bottom layer  15  is n-type. A second spreading layer  20  of conducting material is also included on the LED structure&#39;s top layer  14  to spread current from a second contact  22  to the top layer  14 . The second contact  22  is also referred to as the p-contact because in the preferred LED structure  12  the top layer  14  is p-type. The LED structure, spreading layers and contacts are formed on a substrate  24  with the first spreading layer adjacent to the substrate  24 . 
     The substrate  24  can be made of many materials and can be electrically conductive. When conductive, the substrate  24  can serve as the first spreader and an n-contact  28  can be deposited directly on the substrate. Current will spread through the substrate  24  to the bottom layer of the LED structure  12 . 
     An array of surface LEEs  26  are formed by standard semiconductor processing techniques on the second spreading layer  20 . The LEEs  26  provide surfaces for normally trapped TIR light to pass through or refract and escape, thereby increasing the efficiency of the LED  10 . To increase their effectiveness, the LEEs  26  should have a higher index of refraction (n2) than the LED encapsulating material (n1). The higher n2 allows more light to enter the LEEs the would normally enter the encapsulating material. The shaped surfaces of the LEEs then allow more light to escape into the encapsulating material. One advantage of the new LED  10  is easy to manufacture because the LEEs can be formed by standard process techniques on a wafer of LEDs before they are separated. 
     The new LED  10  is preferably made from AlInGaN materials. The second spreader  20  is preferably a thin semi-transparent metal such as Pd, Pt, Pd/Au, Pt/Au, Ni/Au, NiO/Au or any alloy thereof deposited on the LED structure&#39;s top layer  14 , which is preferably p-type AlInGaN. The second spreader  20  can be deposited on the new LED  10  by many conventional methods with the preferred methods being evaporation or sputtering. The first spreader  16  is preferably made of n-type AlInGaN and can be exposed for contact by reactive ion etching. Ni, Al/Ni/Au, Al/Ti/Au, or Al/Pt/Au is used as the n-contact  18  or  28  to the substrate  24  or first spreader  16 . 
     Sapphire, AlN, SiC, or GaN can be used as the substrate  24 , with SiC and GaN being conductive and AlN and sapphire being insulating. SiC has a much closer crystal lattice match to Group III nitrides such as GaN and results in Group III nitride films of high 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 is 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 a U.S. Pat. Nos. Re. 34,861; 4,946,547; and 5,200,022. 
     The LEEs  26  are preferably formed on the device using the following method. The LEE material is deposited on the surface by evaporation, chemical vapor deposition (CVD), or sputtering. The preferred LEE materials are SiC, SiN x , AlN, SiO x N y , Si 3 N 4 , ZnSe, TiO 2 , Ta 2 O 5 , GaN, or SiO, with ZnSe, TiO 2 , SiN x , AlN, and GaN being most preferable. The preferred LEE thickness is in the range of 100 nm to 10 μm. After the LEE material is deposited, a photosensitive polymer, such as photoresist, is first exposed and developed as a mask. 
     The LEEs  26  can then be formed in the LEE material in two ways. First, the LEE material can be etched away through the mask with a wet chemical etch. This etch will undercut the mask layer to form the LEE structures. Second, the mask can be reflowed in an oven to form curved or linear grade in the mask. Reactive ion etching is then used to transfer the pattern from the mask into the LEE material, forming the final LEE structures. The array patterns can be regular or irregular in nature, with preferred distances between individual LEEs in the range of 1 μm to 50 μm. 
     Other methods can also be used to form the LEE structures and this technique is applicable to all LED material systems. Also, the LEE formation described can be used in any of the following embodiments where LEE arrays are formed. 
     Second Embodiment 
     FIG. 2 shows a second embodiment of a new LED  30  constructed in accordance with the present invention. It is similar to the LED  10  in FIG. 1, having the same LED structure  12 , first spreader  16 , substrate  24 , second spreader  20  and n- and p-contact pads  18 ,  22 . It can also have the n-contact  28  on the substrate  24  when the substrate is conductive. 
     However, in this embodiment the LEEs  32  are formed on the surface of the substrate  24  opposite the first spreader  16 . Like the LED  10 , the LEEs  32  are formed during or after the fabrication of the devices, but before the die separation. To enhance the light extraction, the index of refraction (n2) of the LEEs should be larger than the index (n1) of the encapsulating material for the LEEs. The preferred materials and manufacturing processes used for the new LED  10  in FIG. 1, can also be used in this embodiment. 
     Alternatively, the LEEs  32  can be formed in the substrate  24 . This is particularly applicable to SiC substrates with AlInGaN-based LED structure. The LEEs are formed directly in the substrate by reactive ion etching through an etch mask, or by cutting the substrate with a laser or saw. The depth of the LEEs in this case is preferably in the range of 1 μm to 200 μm and the distance between elements is preferably in the range of 1 μm to 200 μm. 
     This new LED  30  is particularly applicable to LEDs having a majority of the trapped light within the substrate region, such as the case for a GaN-based LED on a SiC substrate. By forming the LEEs  32  in an array, the new LEDs  10  and  30  have the advantage of being scalable to larger LED chip sizes as compared to the inverted truncated pyramid process disclosed by Krames, et al (see above). 
     Different shapes can be used for all embodiments of the new LED to provide the best light extraction. FIG. 3 shows cross-sectional views of different examples of the shapes that can be used for the LEEs in the arrays. LEEs  42 , 44 , 46  have curved surfaces while LEEs  48 , 50 , 52 , 54  have piecewise linear surfaces. The shape can be chosen and adjusted to give the best light extraction for a given embodiment. The different shapes are formed by using different combinations of LEE materials and/or mask layers with standard wet chemical, dry etching, laser or wafer sawing techniques. The shapes shown in the figure represent only a small number of the possible shapes and the scope of this invention should not be limited to the shapes shown. 
     Third Embodiment 
     FIG. 4 shows a third embodiment of the new LED  60  constructed in accordance with the present invention. It also has a LED structure  62 , first spreader layer  64 , substrate  66 , n-contact  68 , second spreader layer  71 , and p-contact  72 , all disposed similarly to those in LEDs  10  and  20 . However, in this embodiment, the LEEs  74  are formed in an array that is internal to the LED device, preferably at the interface between the substrate  66  and first spreader  64 . The LEE material must be of a different index of refraction, n2, than the second spreader material, n1, to provide reflections and refractions that can redirect normally trapped light into a direction that allows the light to escape from the LED  60 . 
     The LEEs  74  are preferably formed using a photoresist mask with wet chemical etching of the LEE material. To form the internal LEE arrays, the epitaxial material must then be regrown over the LEEs. This is preferably done by metalorganic chemical vapor deposition (MOCVD), vapor phase epitaxy (VPE), or molecular beam epitaxy (MBE), with MOCVD being most preferable. The preferred mask materials are SiO 2 , SiN 2 , Si 3 N 4 , TiO 2 , AlN, and SiO. The preferred thickness of LEE mask material is 0.1 μm to 10 μm and the preferred distance between LEEs is 1 μm to 50 μm. In addition, the internal LEEs may be placed at different locations within the LED structure. 
     Fourth and Fifth Embodiment 
     FIG. 5 shows a fourth embodiment of the new LED  70  constructed in accordance with the present invention. It has the same LED structure  72 , spreader layers  75 , 76 , substrate  78  and contacts  80 , 82  as the above embodiments. However, in this embodiment a layer of epitaxial material  84  is grown on the substrate before formation of the LEEs  86 . The epitaxial layer  84  is grown by MOCVD, VPE, or MBE. The LEEs  86  are then formed in an array on the epitaxial layer&#39;s surface and the remainder of the second spreader  75  is formed over the LEEs  86 . This embodiment can be used to facilitate the regrowth of the LED structure  76  over the LEE array, but requires one extra epitaxial growth step. 
     In GaN-based LEDs having LEEs within one of their layers, the regrowth over the LEE material can be accomplished by lateral epitaxial overgrowth (LEO) in an MOCVD growth system. This offers superior material quality over the standard planar growth, leading to a further increased LED emission as a side benefit to the light extraction. 
     In addition, the LEO process provides for another embodiment of the LED  90  constructed in accordance with this invention and shown in FIG.  6 . In this embodiment, the LEO growth conditions are adjusted to create LEE voids  92  over the mask material  94 . The voids  92  serve as linear (or curved) LEEs internal to the first spreader layer  96 . The voids and the LEEs redirect the internally trapped light to enhance light extraction. The formation of voids in semiconductor material has been demonstrated by Fini. [See Fini et al.,  High Quality Coalescence of Laterally Overgrown GaN Stripes on GaN/Sapphire Seed Layers , Applied Physics Letters 75, 12, Pgs. 1706-1708 (1999)]. 
     Sixth Embodiment 
     FIG. 7 shows a sixth embodiment of the new LED  100  having the same layers as the above described LEDs. In this embodiment, the LEEs  102  are placed in an array at the interface between the substrate  104  and the first spreader  106 , but within the substrate  104 . The LEEs  102  are formed directly into the substrate  104  by etching the substrate through a mask with wet chemical or dry etching techniques. The LEEs are then grown in the etched areas and the remaining layers of the LED are grown over the LEEs by MOCVD, VPE, or MBE. The LEEs may be voids left in the substrate after regrowth of the epitaxial material or epitaxial material filled into the etched regions. 
     Seventh Embodiment 
     FIG. 8 shows the seventh embodiment of the new LED  110 , using a disperser layer  112  on top of the epitaxial side of the LED structure  114 , and on top of the second current spreading layer  116 . Again, the substrate, LED layers, and LED contacts are the same type as described in earlier embodiments. To be most effective, the disperser layer should have an index of refraction, n2, larger than the LED encapsulation material, n1. In general, the higher the index of refraction, n2, the better the light extraction. The disperser layer particles should have distances of 20 nm to 1 μm between adjacent particles. The particle size should be 20 nm to 1 μm. Alternatively, the disperser layer can be a series of holes in layer of material having a different index of refraction. 
     The disperser  112  can be formed by several different methods. The first method is to directly coat the surface of the LED structure with microspheres with the desired index of refraction. The preferred microspheres are ZnSe or TiO 2  or any high index, low optical absorption material. The LED can be coated by spraying or spin-coating spheres that are immersed in a solvent or water. 
     The second method of formation is to first deposit a disperser material uniformly or nearly uniformly over the LED&#39;s surface by evaporation, CVD, or sputtering. Preferred materials are SiN, AlN, ZnSe, TiO 2 , and SiO. Next, a mask material is coated over the surface, with the preferred mask materials being silica or polystyrene microspheres, or a thin polymer layer such as a spin coated photoresist. The mask material is used as a mask for wet chemical etching of the disperser material or as an ablative mask for dry etching, such as RIE. After the transfer of the pattern to the disperser material, the remaining mask material is removed, leaving a disperser on the LED surface. 
     The embodiments presented here are improvements to the LED described and demonstrated by Schnitzer, et al. They offer the advantage of not having to etch the disperser layer into the semiconductor material. This enables disperser technology to be easily used with the GaN-based material system, where the first spreader material is typically a very thin metallic layer that cannot be easily interrupted. 
     Eight Embodiment 
     FIG. 9 shows a new LED  120  that is a variation of the LED  110  in FIG.  8 . LED  120  has the same LED layers, but in this embodiment, the disperser layer  122  is applied to the bottom surface of the substrate  124 . This approach is particularly applicable to LED where the index of refraction of the substrate is similar to the LED epitaxial layers, such as AlInGaN epitaxial layers on SiC. 
     Ninth and Tenth Embodiments 
     FIGS. 10 and 11 show new LEDs  130  and  140 , where their respective disperser layers  134 ,  144  are placed within their first spreader layer  132 ,  142 . For these embodiments, the disperser layer index, n2, must differ from the first spreader layer&#39;s refractive index, n1, so that scattering of light can occur. The preferred material for this disperser layer is silica or TiO 2  microspheres. 
     For LED  130  in FIG. 10, the disperser layer  134  is disposed at the interface between the substrate  136  and the first spreader  132 . The LED layers are then grown over the disperser layer by MOCVD, VPE, or MBE. For LED  140  in FIG. 11, the disperser layer  144  is within the first spreader layer  142 . A layer of the first spreader is first grown and disperser layer  144  is then formed. The remainder of the first spreader and the LED layers are then grown over the disperser layer. 
     The disperser layer can also be formed within the other layers of the LEDs  130 ,  140 , including the layers of the LED structures and the substrates. The disperser can also be formed by other methods and with other materials. Accordingly, this invention should not be limited to the placement of the disperser layers as shown. 
     Eleventh Embodiments 
     Disperser layers can also be formed in LEDs using in-situ techniques when MOCVD is used as the epitaxial growth tool. This technique is particularly applicable to GaN based LEDs. FIGS. 12 and 13 show two LEDs  150  and  160  having disperser layers  152 ,  162  formed in-situ in the first spreader layers  154 ,  164 . In LED  150 , its substrate  155  is made of SiC or sapphire, and the first spreader  154  is formed of uncoalesced islands of material made of Al x In y Ga 1-x-y N, 0≦x≦1, 0≦y≦1. During the initial stages of the first spreader&#39;s growth, islands  156  are formed. Prior to coalescence of the islands  156 , growth is stopped and a layer  152  of lower index of refraction material, such as AlGaN, SiO 2 , or SiN, is deposited over and/or in between the islands, creating the required internal index discontinuities. Growth then proceeds as normal to finish the first spreader layer and the LED structure. 
     For the LED  160 , instead of using islands to form the discontinuities, growth conditions can be changed during the initial stages of the first spreader layer&#39;s growth, to introduce a roughness on its surface. For AlInGaN based LEDs the epitaxial layers can be grown rough by increasing the flow of disiline, changing the flow of ammonia, or increasing the rate that said first layer is grown. Once roughness is introduced, lower index of refraction AlGaN or other dielectric layer  162  is deposited. Growth then proceeds as normal to finish the first spreader and the LED structure. 
     Just as above, the disperser layers described can be placed in other layers including the LED structure&#39;s layers and the substrate, and the invention should not be limited to the placements shown. 
     Flip-chip Embodiments 
     Finally, in all embodiments listed above, the devices can be mounted using flip-chip bonding techniques. FIG. 14 shows a new LED  170  bonded in such a configuration. The LED structure  172  is coated with a conductive reflective layer  175  and a second spreader layer  189  is affixed to the reflective layer  175  by a conductive bonding media. A submount  176  is then mounted on the second spreader layer  189 . A p-contact  188  is included on a submount  176  and is bonded to the second spreader layer  189 . Current applied to the p-contact  188  spreads into the second spreader layer and into the LED structure&#39;s top layer. 
     An n-contact layer  178  is also included on the submount  176  and is coupled to the first spreader layer  180  through a second conductive bonding layer  182 . Current from the n-contact  178  passes through the layer  182  to the first spreader  180  and into the LED structure&#39;s bottom layer. LEEs  186  are formed on the bottom surface of substrate  184 . 
     Light emits from LED  170  primarily through its substrate  184  and light extraction from this structure can be improved over the conventional bonding structures, depending on the type of LEE array or disperser used. Here, the redirected light can escape the chip on the first pass through the LEEs  186 , reducing any chance of optical loss of light having to come back through the substrate after being redirected. 
     FIG. 15 shows a new LED  190 , using flip-chip bonding similar to LED  170 . However, instead of utilizing LEEs it has a disperser layer  192  at the interface of the second spreader  194  and the reflective layer  196 . 
     Although the present invention has been described in considerable detail with reference to certain preferred embodiments thereof, other versions are possible. Other LED configurations utilizing the LEE arrays can also be envisioned by one skilled in the art. The new LED can have different combinations of LEE arrays and disperser layer. LEEs can have different shapes, sizes, spaces between adjacent LEE, and can be placed in different locations. Similarly, the disperser layers can be made of different material and placed in different location. Therefore, the spirit and scope of the appended claims should not be limited to the preferred embodiments described above.