Abstract:
An LED die ( 40 ) includes an N-type layer ( 18 ), a P-type layer ( 22 ), and an active layer ( 20 ) epitaxially grown over a first surface of a transparent growth substrate ( 46 ). Light is emitted through a second surface of the substrate opposite the first surface and is wavelength converted by a phosphor layer ( 30 ). Openings ( 42, 44 ) are etched in the central areas ( 42 ) and along the edge ( 44 ) of the die to expose the first surface of the substrate ( 46 ). A highly reflective metal ( 50 ), such as silver, is deposited in the openings and insulated from the metal P-contact. The reflective metal may conduct current for the N-type layer by being electrically connected to an exposed side of the N-type layer along the inside edge of each opening. The reflective metal reflects downward light emitted by the phosphor layer to improve efficiency. The reflective areas provided by the reflective metal may form  10 %- 50 % of the die area.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to the metallization of light emitting diodes (LEDs) having a wavelength conversion layer, such as phosphor layer, and, in particular, to a technique for metalizing surfaces of such an LED die to improve the upward reflection of light. 
       BACKGROUND 
       [0002]    One type of conventional LED is a blue-light-emitting LED with a phosphor layer deposited over its top light-emitting surface. The LED is usually GaN based. The blue light energizes the phosphor, and the wavelength-converted light emitted by the phosphor is combined with the blue light that leaks through the phosphor. Virtually any color may thus be created, such a white light. 
         [0003]    One issue regarding such phosphor-converted LEDs (PCLEDs), discussed in more detail below, is that the light emitted by the phosphor layer is isotropic, where some light is emitted upwards and exits the LED die and some light is emitted in a direction back into the semiconductor portion of the die. Most of this light is then reflected back upwards by the metal contacts on the bottom surface of the LED die. In order to minimize reflectivity losses, the metal contacts should feature very high reflectivity characteristics in the entire visible spectrum, which is often times difficult to achieve. 
         [0004]    LED package efficiency is the ability to extract light from the LED after it has been generated/converted. Improving such package efficiency is today considered one of the main obstacles in increasing the luminous efficacy of LEDs. 
         [0005]    Increased package efficiency in phosphor converted LEDs can be achieved by increasing the reflectivity of the LED die in architectures such as flip-chips. 
         [0006]    In flip-chip (FC) die architectures, light is extracted from the semiconductor N-type layer that typically is the “top” semiconductor surface. The P-type layer is the “bottom” semiconductor layer facing the mounting substrate (e.g., a printed circuit board). Metal contacts (electrodes) are formed on the bottom surface of the die. The N-contact is formed by etching away the P-type layer and active layers (i.e., quantum wells) to expose portions of the N-type layer. A dielectric layer is then patterned over the exposed P-type layer and active layer (in order to avoid short-circuits) within the openings, and a metal layer, such as aluminum, is deposited within the openings to contact the N-type layer. The N-contacts can be arranged in the form of vias across the die and/or grooves around the edge of the die, where current is then spread laterally across the N-type layer. No light is generated over the N-contacts since the active layer has been removed in those areas. 
         [0007]    The metal P-contact is usually the largest in surface area, and it is also functionally used as a mirror reflector. The P-contact usually consists of Ag (silver) material. Due to the ability of Ag to migrate, a metal guard sheet layer is generally used to prevent the Ag from migrating into any underlying dielectric layer. Aluminum, and not silver, is typically used for the N-contacts for improved electrical coupling to the N-type layer. 
         [0008]    In state-of-the-art technologies, thin-film-flip-chip (TFFC) architectures are achieved by further removing the growth substrate (e.g., the sapphire substrate) followed by a roughening process of the exposed N-layer surface, where the generated (blue) light is extracted from. The roughening improves light extraction by reducing internal reflections. 
         [0009]    In phosphor converted TFFC LEDs, a phosphor layer may further be deposited on, or attached to, the roughened N-layer surface, thus converting light from a narrow wavelength range into a well-defined wideband spectrum. 
         [0010]      FIGS. 1-3  illustrate one type of prior art phosphor converted TFFC LED. 
         [0011]      FIG. 1  is a bottom up view of an LED die  10  showing a large metal P-contact layer  12 , a narrow N-contact area  14  along the edge where electrical contact is made to the N-type layer, and distributed N-contact areas  16  where additional electrical contacts are made to the N-type layer for good current spreading. The metal layer contacting the N-type layer at areas  14  and  16  is typically Al, which has a reflectivity below 90% for the wavelengths of interest. 
         [0012]      FIG. 2  is a cross-sectional view of the edge portion along line  2 - 2  in  FIG. 1 . A semiconductor N-type layer  18  is epitaxially grown over a sapphire growth substrate, which has been removed. An active layer  20  and a P-type layer  22  are grown over N-type layer  18 . A highly reflectively metal layer (or a stack of metal layers), which may comprise Ag, is then deposited as a P-contact  12  to electrically contact the P-type layer  22 . The layers  22  and  20  are etched along the edge to expose the N-type layer  18 . A metal guard sheet layer  24  may be deposited on the metal P-contact layer  12  to block the migration of Ag atoms. A dielectric layer  26  is then deposited and etched to expose the N-type layer  18  at area  14 . A metal N-contact layer  13  (e.g., Al) is then deposited to electrically contact the N-type layer  18  at area  14  and form a metal ring along the edge of the die  10 . In the central area of the LED die  10 , the P-contact layer  12  is exposed (see  FIG. 1 ), by etching away the layers  13  and  26 , and further metalized to planarize the bottom surface of the LED die  10 . If the metal guard sheet layer  24  is used, the electrical contact to the P-contact layer  12  may be made through the metal guard sheet layer  24 . The P and N-metal contact layers  12  and  13  are ultimately bonded to corresponding metal anode and cathode pads on a mounting substrate. 
         [0013]      FIG. 3  is a cross-sectional view along line  3 - 3  in  FIG. 1  showing a portion of a distributed N-contact area  16 , where the N-type layer  18  is contacted by the metal N-contact layer  13 . The metallization and etching to create the N-type layer  18  contact at area  16  are performed at the same time the contact is made to the edge area  14 . 
         [0014]    The sapphire growth substrate may be removed by laser lift-off or other process. The exposed top N-type layer surface  28  is then roughened (e.g., by etching or grinding) to improve light extraction. A phosphor layer  30  is then deposited or otherwise affixed (as a tile) to the top surface. 
         [0015]    It will be assumed the phosphor layer  30  is a YAG phosphor that generates a yellow-green light, which, when combined with blue light, results in white light. When a photon generated by the active layer  20  energizes a phosphor particle  32  ( FIG. 2 ), the resulting wavelength-converted light is usually scattered isotropically, so a significant portion of the energy is direct back into the LED die.  FIG. 2  illustrates some energized phosphor particles  32  emitting light rays  34  upward and downward. The downward light is ideally reflected upward by the metal N-contact layer  13  at areas  14  and  16  and the P contact layer  12 . However, the N-contact layer  13  is typically aluminum, which is not a good reflector. Accordingly, light that impinges on the N-contact layer  13  at areas  14  and  16  is significantly attenuated. Good package efficiency relies upon a high metal contact reflectivity to avoid light absorption in the die. 
         [0016]    Besides the limited reflectivity of the N-contact layer  13  at areas  14  and  16 , the package efficiency of LED dies like shown above is also limited by the capability of the textured N-type layer surface  28  to extract light from the GaN semiconductor material (a high index material, e.g., n=2.5) to the lower index phosphor layer (e.g., n=1.6). 
         [0017]    Thus, what is needed is an LED die structure that mitigates such limitations, resulting in superior package efficiency. 
       SUMMARY 
       [0018]    One purpose of the proposed invention is to increase the effective reflectivity of the die area exposed to the phosphor layer light, where the wavelength-converted light is emitted isotropically. To achieve this, the following techniques are used in one embodiment of the present invention: 
         [0019]    Highly reflective regions are added to an otherwise conventional LED die that contribute to an overall higher die reflectivity. These highly reflective regions should be located at areas on the die for efficiently reflecting light generated by the phosphor layer. In one embodiment, the highly reflective regions are in areas that do not generate light. The percentage of the highly reflective area relative to the total die area should be significant (e.g., up to 50%). In order to keep the same area of quantum wells (where electrons are converted into photons) as in standard LED die sizes, the active layer area (and consequently the phosphor area) is increased generally in proportion to the added highly reflective area. 
         [0020]    The highly reflective regions can be formed around the edge of the die as well as distributed around the central portion of the die. The reflective regions may be used as electrical contact regions to the N-type layer, or even the P-type layer. 
         [0021]    In one embodiment, the transparent growth substrate (e.g., sapphire) is not removed, and the phosphor layer is ultimately provided over the top surface of the substrate. The highly reflective regions are within trenches etched through the semiconductor layers that expose the substrate. The exposed surfaces are then coated with a highly reflective material, such as Ag. If the reflective material is a metal, proper electrical isolation may be needed. The reflective metal in the trenches may or may not carry current to the N-type layer. In one embodiment, the electrical contacting of the P-type layer is not affected by the invention, since the P-contacts are already highly reflective. 
         [0022]    In another embodiment, a dielectric layer, having a relatively low index of refraction, is formed between the substrate and the highly reflective metal layer, or between the GaN and the metal layer, to create an index of refraction mismatch at the dielectric layer surface. Therefore, light incident on the interface at greater than the critical angle will reflect by total internal reflection without losses, and light that enters the dielectric layer will be reflected by the metal layer. 
         [0023]    Instead of, or in addition to, a reflective metal creating the highly reflective regions, the reflective layer may be a distributed Bragg reflector using stacked dielectric layers having thicknesses and indices of refraction selected so as to reflect 100% of the wavelengths of interest. 
         [0024]    By not removing the growth substrate (e.g., sapphire), the substrate helps to scatter the downward light from the phosphor layer to reduce internal reflections, the substrate provides good mechanical support, and the substrate (having an index of about n=1.8) reduces internal reflections by providing an index between that of the GaN (n=2.5) and the phosphor (n=1.6). 
         [0025]    The substrate may undergo texture patterning on its growth side prior to growing the epitaxial layers to improve light extraction at the epitaxial layer-substrate interface. 
         [0026]    Other embodiments are described. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]      FIG. 1  is a bottom up view of a prior art flip-chip LED die, showing the metal contact areas. 
           [0028]      FIG. 2  is a cross-sectional view of the LED die edge along line  2 - 2  in  FIG. 1 . 
           [0029]      FIG. 3  is a cross-sectional view of a portion of a distributed contact area along line  3 - 3  in  FIG. 1 . 
           [0030]      FIG. 4  is a bottom up view of an LED die in accordance with one embodiment of the invention. 
           [0031]      FIG. 5  is a cross-sectional view of a portion of the highly reflective region along line  5 - 5  in  FIG. 4 . 
           [0032]      FIG. 6  is a cross-sectional view of a portion of the highly reflective region along line  5 - 5  in  FIG. 4  in accordance with another embodiment of the invention where the highly reflective metal electrically contacts the N-type layer. 
           [0033]      FIG. 7  is a cross-sectional view of an edge portion of the highly reflective region along line  7 - 7  in  FIG. 4 . 
           [0034]      FIG. 8  is a cross-sectional view of an edge portion of the highly reflective region along line  7 - 7  in  FIG. 4 , where the edge of the substrate is coated with a reflector rather than phosphor. 
           [0035]      FIG. 9  is a cross-sectional view of a portion of a distributed N-contact along line  9 - 9  in  FIG. 4  showing how the highly reflective metal electrically contacts the N-type layer. 
           [0036]      FIG. 10  is an alternative cross-sectional view along line  5 - 5  of  FIG. 4 , illustrating how the dielectric layer may be between the substrate and the highly reflective metal for enhancing reflectivity. 
           [0037]      FIG. 11  is an alternative cross-sectional view along line  5 - 5  of  FIG. 4 , illustrating how the dielectric layer of  FIG. 10  may be opened so the reflective metal may contact the N-type layer. 
           [0038]      FIG. 12  is an alternative cross-sectional view along line  7 - 7  of  FIG. 4 , illustrating how a first metal layer may contact the N-type layer, and a higher reflectivity metal layer may be formed over a dielectric layer. The phosphor layer extends over the sides of the substrate. 
           [0039]      FIG. 13  is an alternative cross-sectional view along line  7 - 7  of  FIG. 4 , illustrating how a first metal layer may contact the N-type layer, and a higher reflectivity metal layer may be formed over a dielectric layer. A reflector is formed on the sidewalls of the substrate. 
           [0040]      FIG. 14  is an alternative cross-sectional view along line  7 - 7  of  FIG. 4 , illustrating how the metal reflective layer, formed over a dielectric layer, may contact the N-type layer, similar to  FIG. 7 . 
           [0041]      FIG. 15  is an alternative cross-sectional view along line  9 - 9  of  FIG. 4 , illustrating how the metal reflective layer, formed over a dielectric layer, may contact the N-type layer, similar to  FIG. 9 . 
           [0042]      FIG. 16  is a magnified view of a highly reflective area illustrating how dielectric layers may be stacked to form a distributed Bragg reflector (DBR) instead of, or in addition to, using a highly reflective metal layer. 
       
    
    
       [0043]    Elements that are the same or similar are labeled with the same numeral. 
       DETAILED DESCRIPTION 
       [0044]      FIG. 4  is a bottom up view of an LED die  40  in accordance with one embodiment of the invention. The LED die  40  includes an added highly reflective region  42  that may or may not serve as an electrical contact. Also, the perimeter of the LED die  40  includes a relatively wide highly reflective edge region  44 , in comparison to  FIG. 1 . In one embodiment, the area of the active layer  20  is the same as the prior art so that similar electrical specifications apply to both. However, the LED die  40  is made larger due to the added area for the regions  42  and  44 , and the light output is increased due to the increased package efficiency. 
         [0045]    In the embodiments shown, the prior art P-contact layer  12  is not significantly changed since the P-contact layer  12  (comprising of Ag) is already a good reflector. 
         [0046]    In one embodiment, the LED die  40  has sides on the order of 1 mm×1 mm. 
         [0047]    In the example of  FIG. 4 , the region  42  is formed as a cross; however, it may be any shape and preferably designed to provide a fairly uniform light output across the top surface of the LED die  40 . Region  42  may take up from 10%-50% of the die surface area. Since the region  42  removes a portion of the active layer  20 , the die may be made larger to compensate for the loss of light generation area. 
         [0048]      FIG. 5  is a cross-sectional view of a portion of the highly reflective region  42  along line  5 - 5  in  FIG. 4 . 
         [0049]    The transparent sapphire growth substrate  46  is not removed. The substrate  46  is optionally thinned prior to depositing the phosphor layer  30 . The phosphor layer  30  may be coated on the substrate  46  surface using any number of well-known techniques or may be affixed as a pre-formed tile to the substrate  46  surface. 
         [0050]    A trench  48  (formed as a cross in  FIG. 4 ) is then etched through the various layers to expose the transparent substrate  46  surface. 
         [0051]    The P-contact layer  12  metal (e.g., Ag) is deposited over the P-type layer  22  (which may be done prior to or after forming the trench  48 . The guard sheet layer  24  and dielectric layer  26  are then deposited and patterned to expose the substrate  46  but cover the P-contact layer  12 . 
         [0052]    On the exposed surface of the substrate  46  and over any portion of the dielectric layer  26 , a highly reflective layer  50 , such as Ag or an alloy, is then deposited and patterned. The reflectivity of Ag is about 95% for the wavelengths of interest, while the reflectivity of Al is less than 90% at the wavelengths of interest. 
         [0053]    A guard sheet layer  52  may then be deposited over the reflective layer  50  if Ag migration is a concern. 
         [0054]    The reflective layer  50 , guard sheet layer  24 , and dielectric layer  26  are patterned to expose the P-contact layer  12  at areas out of the view of  FIG. 5  so the exposed P-contact layer  12  can be used as an anode electrode when mounting to a submount or printed circuit board. Any reflective layer  50  under the P-contact layer  12  is not exposed to light and would only be used for electrically contacting the N-type layer  18 . 
         [0055]      FIG. 5  illustrates various phosphor particles  32  emitting light rays  34  in different directions. Light is shown being reflected off the Ag P-contact layer  12  as well as the Ag layer forming the reflective layer  50 . Elsewhere, light may also be reflected off the reflective layer  50  located in the distributed contact areas  54  ( FIG. 4 ) and along the edges of the die. 
         [0056]    In the example of  FIG. 5 , there is no electrical contact made between the reflective layer  50  and the N-type layer  18 . 
         [0057]      FIG. 6  is an alternative embodiment along line  5 - 5  in  FIG. 4  but where electrical contact is made to the N-type layer  18  by the reflective layer  50  at area  56 , where the dielectric layer  26  has been etched away. The narrow contact area  56  extends all the way around the edge of the cross-shaped pattern in  FIG. 4  for good current spreading. Accordingly, the guard sheet layer  52  and the reflective layer  50  may form part of the bottom cathode electrode that is bonded to a submount or printed circuit board. 
         [0058]      FIG. 7  is a cross-sectional view of an edge portion of the die  40  showing the highly reflective region  44  along line  7 - 7  in  FIG. 4 , with the addition of the phosphor layer  30  extending around the side walls of the substrate  46 . The manufacture of the various layers may be the same as described above. An edge of the die  40  is etched to expose the substrate  46 , and the various layers, including the reflective layer  50  (e.g., Ag), are deposited as shown.  FIG. 7  also shows the reflective layer  50  making electrical contact to the N-type layer  18  using a metal ring  58  that circumscribes the central portion of the die  40 . The metal used to form the ring  58  may comprise aluminum and may be a conventional metal stack conventionally used to make ohmic contact with N-type GaN. The ring  58  is deposited and patterned simultaneously with the metal used to make contact with the N-type layer  18  in the distributed contact areas  54  shown in  FIG. 4 , discussed later with respect to  FIG. 9 . Although electrical contact to the N-type layer by the reflective layer  50  along the edge may be made simply by opening up the dielectric layer  26 , as shown in  FIG. 6 , the interface metals forming the ring  58  and guard sheet layer portion  60  provide an interface for a better electrical connection. Such an interface may also be used in  FIG. 6 . 
         [0059]    To block migration of the Ag atoms from the reflective layer  50 , a guard sheet layer portion  60  is formed as a barrier between the metal ring  58  and the reflective layer  50 . The guard sheet layer portion  60  may be formed simultaneously with the guard sheet layer  24 . 
         [0060]    The dielectric layer  26  isolates the reflective layer  50  and metal ring  58  from the metal P-contact layer  12  (which may also comprise Ag for high reflectivity). 
         [0061]      FIG. 7  illustrates phosphor particles  32  emitting light rays  34  in various directions. Note how one particle  32  emits a light ray  62  that is reflected off the reflective layer  50  along the edge. If the reflective layer  50  is used for an N-contact, the reflective layer will typically extend to a bottom surface of the die to serve as a cathode electrode. Alternatively, the reflective layer  50  may be electrically connected to another type of less-reflective metal that extends along the bottom surface of the die  40 , since any metal that is below the metal P-contact layer  12  does not receive any light. For a cathode electrode, other well-known metals may be deposited over the reflective layer  50 , such as Ni and Au, to facilitate bonding to metal pads of a submount or printed circuit board. 
         [0062]    As seen by a comparison of  FIGS. 1 and 4 , the edge that is etched is much wider, and no light is generated along the edge. However, the die  40  may be made larger to compensate for the loss of the light generating area. The package efficiency will, however, be greater than that of the die  10  in  FIG. 1  since there is increased reflectance of the light generated by not only the phosphor layer  30  but by the active layer  20 . Therefore, the LED die  40  will have the same electrical specifications as the prior art LED die  10  of  FIG. 1  but will be brighter. 
         [0063]    In one embodiment, the area of the trench  48  around the edge of the die  40  that is covered by the reflective layer  50  is 10%-50% of the die  40  surface area. 
         [0064]      FIG. 8  is similar to  FIG. 7  but the edge of the substrate  46  is coated with a reflector  66  rather than phosphor. The substrate  46  may be many times thicker than the LED semiconductor layers and thus the light emitted from the sides is significant. If such side light is not desired, then forming the reflector  66  is recommended. The reflector  66  may be Ag or other suitable material.  FIG. 8  shows a light ray  68  from a phosphor particle  32  being reflected off both the reflective layer  50  and the reflector  66 . 
         [0065]      FIG. 9  is a cross-sectional view of a portion of a distributed N-contact along line  9 - 9  in  FIG. 4  showing how the reflective layer  50  electrically contacts the N-type layer  18  via a metal contact  70  forming a narrow ring within the circular etched opening in the LED layers. The metal contact  70  is the same metal forming the metal ring  58  in  FIG. 7  and formed at the same time. Although  FIG. 4  shows four identical distributed contact areas  54 , there may be many more for improved current uniformity. The distributed contact areas  54  may be circular or generally frustum-shaped, as shown, or may be rectangular or other shapes. The metal contact  70  would therefore take the shape of the edge of the contact area  54 . A guard sheet layer portion  72  is also shown, which is formed simultaneously with the guard sheet layer portion  60  in  FIG. 7 . Electrical contact to the N-type layer  12  is made by the various electrical contacts shown in  FIGS. 6, 7, and 9  to evenly spread current. 
         [0066]    Therefore, since the distributed contact areas  54  and the reflective edge region  44  will reflect about 95% of the impinging light from the phosphor layer  30 , and the P-contact layer  12  is also highly reflective, very little phosphor light is absorbed by the die  40 , in contrast to the die  10  of  FIG. 1  where there is significant absorption by the metal N-contact layer  13  at the areas  14  and  16 . Accordingly, the overall efficiency of the LED is improved. 
         [0067]    In another embodiment, instead of adding the trench  48  to form the cross-shaped reflective layer  50 , the distributed contact areas  54  are made larger than the distributed areas  16  in  FIG. 1 , where the electrical contact to the N-type layer  18  is made along the edges of the contact areas  54  (shown in  FIG. 9 ) and the central areas of the contact areas  54  are solely for adding the highly reflective areas. Note that, in the prior art  FIG. 3  and in contrast to  FIG. 9 , the distributed areas  16 , for contacting the N-type layer  18 , are solely for making electrical contact with the N-type layer  18 , and the contact metal used significantly absorbs the phosphor light. 
         [0068]    The areas of the highly reflective regions, using Ag, are preferably much larger than the areas where the N-contact metal, typically Al, contacts the N-type layer  18 , and the Al should only be used for the electrical interface between the reflective layer  50  and the N-type layer  18 . Preferably the Al should only occupy no more than the strictly necessary for good electrical contact to the N-type layer  18 , such as providing a contact width not larger than 2*Lt, where Lt is the transfer length of the metal-semiconductor contact, typically about 1 um. The remaining exposed regions are preferably covered by the highly reflective metal (e.g., Ag). The highly reflective layer  50  may or may not be used as a current carrier while still achieving the goals of the present invention. 
         [0069]      FIG. 10  illustrates how the dielectric layer  26  may be between the substrate  46  and the metal highly reflective layer  50  for enhancing reflectivity. The index of refraction of the dielectric layer  26  (e.g., 1-4-1.5) is selected to be lower than that of the substrate  46 .  FIG. 10  may illustrate any of the areas of high reflectivity, such as across lines  5 - 5 ,  7 - 7 , or  9 - 9  of  FIG. 4 . Therefore, light incident the interface at greater than the critical angle, such as light ray  74 , will reflect by total internal reflection without losses, and light that enters the dielectric layer  26 , such as light ray  76 , will be reflected by the reflective layer  50 . 
         [0070]    Further, in one example, a thinned N-type layer  18 , including the N-type layer surface  28 , may extend to the left edge of  FIG. 10 . If the dielectric layer  26  and reflective layer  50  are formed over the thinned N-type layer  18 , the relatively low index of the dielectric layer  26  will cause light incident at larger than the critical angle to reflect off the GaN/dielectric interface without losses. Light that enters the dielectric layer  26  will be reflected by the reflective layer  50 . The reflective layer  50  may or may not carry current for the N-type layer  18 . 
         [0071]    The lower the refractive index of the dielectric layer  26 , the lower the critical angle (in accordance with Snell&#39;s law) and hence the larger the range of the light rays that will be fully reflected at the interface by total internal reflection. 
         [0072]      FIG. 11  is an alternative cross-sectional view along line  5 - 5  of  FIG. 4  (or other edges of a reflective area), illustrating how the dielectric layer  26  of  FIG. 10  may be opened at area  80  so the metal reflective layer  50  may electrically contact the N-type layer  18  to carry N-type layer  18  current. 
         [0073]      FIG. 12  is an alternative cross-sectional view along line  7 - 7  of  FIG. 4 , illustrating how a first metal layer  84  (e.g., aluminum) may contact the N-type layer  18  at area  86  through an opening in the dielectric layer  26 . The reflective layer  50 , formed of a higher reflectivity metal such as Ag, may be formed the first metal layer  84  and over the dielectric layer  26 . As in  FIGS. 10 and 11 , the dielectric layer  26  contacting the substrate  46  reflects some light by total internal reflection. The phosphor layer  30  extends over the sides of the substrate. 
         [0074]      FIG. 13  is an alternative cross-sectional view along line  7 - 7  of  FIG. 4 , illustrating how the first metal layer  84  may contact the N-type layer  18  near the edges of the die.  FIG. 13  differs from  FIG. 12  in that a reflector  66  is formed over the sidewalls of the substrate  46 . 
         [0075]      FIG. 14  is an alternative cross-sectional view along line  7 - 7  of  FIG. 4 , and similar to  FIG. 7 , illustrating how the metal reflective layer  50 , formed over the dielectric layer  26 , may contact the N-type layer  18  via a metal ring  58  and a guard sheet layer portion  60 . 
         [0076]      FIG. 15  is an alternative cross-sectional view along line  9 - 9  of  FIG. 4 , illustrating how the metal reflective layer  50 , formed over the dielectric layer  26 , may contact the N-type layer  18  using a metal contact  70  and guard sheet layer portion  72 , similar to  FIG. 9 . 
         [0077]    Instead of, or in addition to, a reflective metal creating the highly reflective regions, the reflective layer may be a distributed Bragg reflector (DBR), as shown in  FIG. 16 , using stacked dielectric layers  90 A,  90 B, and  90 C, having thicknesses and indices of refraction selected so as to reflect 100% of the wavelengths of interest. In an actual embodiment, there may be many more stacked layers. Forming DBRs is well known for other applications. Light (e.g., light ray  94 ) that fully penetrates the DBR will be reflected by the metal layer forming the reflective layer  50 . The metal layer may be optional. The DBR may be formed below the P-type layer  22  for use as a dielectric layer and may be an extension of the dielectric layer  26 . 
         [0078]    Note that the DBR could also be extended over the mesa sidewalls to obtain mesa sidewall reflectance. 
         [0079]    By not removing the growth substrate  46 , the substrate helps to scatter the downward light from the phosphor layer to reduce internal reflections, the substrate  46  provides good mechanical support, and the substrate  46  (having an index of about n=1.8) reduces internal reflections by providing an index between that of the GaN (n=2.5) and the phosphor layer  30  (n=1.6). The growth surface of the substrate  46  may be roughed to further improve light extraction by reducing internal reflections. 
         [0080]    Additionally, since the phosphor layer  30  is separated from the semiconductor layers, there is less heat transferred to the phosphor layer  30 , allowing the use of phosphors that have lower temperature requirements. 
         [0081]    Instead of a phosphor layer, any other wavelength conversion layer may be located over the substrate  46 , such as a quantum dot layer. The wavelength conversion layer does not have to be in direct contact with the substrate  46 . 
         [0082]    While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from this invention in its broader aspects and, therefore, the appended claims are to encompass within their scope all such changes and modifications as fall within the true spirit and scope of this invention.