Abstract:
The current distribution across the p-layer ( 130 ) of a semiconductor device is modified by purposely inhibiting current flow through the p-layer ( 130 ) in regions ( 310 ) adjacent to the guardsheet ( 150 ), without reducing the optical reflectivity of any part of the device. This current flow may be inhibited by increasing the resistance of the p-layer that is coupled to the p-contact ( 140 ) along the edges and in the corners of contact area. In an example embodiment, the high-resistance region ( 130 ) is produced by a shallow dose of hydrogen-ion (H+) implant after the p-contact ( 140 ) is created. Similarly, a resistive coating may be applied in select regions between the p-contact and the p-layer.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to the field of semiconductor light emitting devices, and in particular to techniques for improving extraction efficiency and providing a more uniform current distribution across the light emitting region of the device. 
       BACKGROUND OF THE INVENTION 
       [0002]    The substantial increase in demand for semiconductor light emitting devices, and the corresponding increase in competition to satisfy demand has caused manufacturers to seek techniques that will reduce costs or improve performance. Of particular note, techniques that improve the efficiency or quality of the emitted light may serve to distinguish one competitor&#39;s product from the others. 
         [0003]      FIG. 1  illustrates an example prior art Thin Film Flip Chip (TFFC) InGaN Light Emitting Device (LED), such as disclosed in U.S. Pat. No. 6,828,596, “CONTACTING SCHEME FOR LARGE AND SMALL AREA SEMICONDUCTOR LIGHT EMITTING FLIp-CHIP DEVICES”, issued to Daniel A. Steigerwald, Jerome C. Bhat, and Michael J. Ludowise, and incorporated by reference herein. 
         [0004]    In this example device, a light emitting layer  120  is formed between an n-layer  110  and a p-layer  130 . An external power source (not illustrated) provides power to the device via connections to pads  160  and  170 . The p-pad  160  is coupled to the p-layer  130  via a p-contact  140 , through an optional guard layer  150  that inhibits migration of the p-contact material. The n-contact layer  170  is coupled directly to the n-layer  110  in this example. A boundary layer  180  isolates the n-contact layer  170  and n-layer  110  from the p-layer  130  and p-contact  140 . 
         [0005]    The p-contact  140  is provided over a large area to facilitate a uniform distribution of current through the p-layer  130 , which has a relatively higher resistance to current flow. The n-layer  110  does not exhibit a high resistance, and thus the n-contact covers a smaller area, which may be 10% or less of the device area. The p-contact  140  is preferably highly reflective to reflect the light toward the top, emitting surface of the light emitting device. Silver is commonly used as the p-contact  140 . The n-contact layer is also reflective and metals such as Aluminum are preferred. The guard layer  150  may be metallic, but is only partially reflective as a suitable highly reflective metal has not yet been found for this application. This partially reflective guard sheet fills the area adjacent to the p-contact, resulting in higher optical loss at the p-contact periphery. 
         [0006]    The inventors have recognized that the light generated within about 15 microns of the periphery of the p-contact may, with high probability, enter the guard layer area  150  and suffer optical absorption before having a chance to exit the device. Therefore, current injected at the edge of the p-contact will exhibit a lower external quantum efficiency than current injected at the center area of the p-contact. 
         [0007]    Despite the greater optical loss of the edges and corners of the device, the inventors have also noticed that more emitted light is produced at the periphery and in the corners than at the center of the device, because the voltage drop associated with the lateral flow of current through the n-contact layer, combined with the exponential dependence of vertical current flow upon junction voltage, provides a significantly higher current density at the edges and in the corners of the device. These relatively high injection currents create a slight halo-effect, with bright areas in the corners of the device. 
         [0008]    In addition to potentially introducing optical anomolies, such a non-uniform current injection pattern is inefficient, as the internal quantum efficiency is lower for higher current densities. The ‘over-emitting’ portions, particularly the corners, of the light emitting device will also be ‘hot-spots’ that draw more current in the device, which have been observed to lead to premature failure of devices operated at high current. 
       SUMMARY OF THE INVENTION 
       [0009]    It would be advantageous to distance the light emission regions away from the partially reflective guard layer and to further improve the uniformity of the injected current density light emissions across the surface of the active layer. 
         [0010]    To better address these concerns and others, in an embodiment of this invention, the current distribution across the p-layer of a semiconductor device is modified by purposely inhibiting current flow through the p-layer in regions adjacent to the guardsheet, without reducing the optical reflectivity of any part of the device. This current flow may be inhibited by increasing the resistance of the p-layer that is coupled to the p-contact along the edges and in the corners of contact area. In an example embodiment, the high-resistance region is produced by a shallow dose of hydrogen-ion (H+) implant after the p-contact is created. Similarly, a resistive coating may be applied in select regions between the p-contact and the p-layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The invention is explained in further detail, and by way of example, with reference to the accompanying drawings wherein: 
           [0012]      FIG. 1  illustrates an example prior-art light emitting device. 
           [0013]      FIG. 2  illustrates current distribution in the example light emitting device. 
           [0014]      FIGS. 3A-3B  illustrate an example light emitting device with a p-contact that includes a high resistance region and a low resistance region to improve current distribution. 
           [0015]      FIG. 3C  illustrates an alternative to  FIG. 3A . 
       
    
    
       [0016]    Throughout the drawings, the same reference numerals indicate similar or corresponding features or functions. The drawings are included for illustrative purposes and are not intended to limit the scope of the invention. 
       DETAILED DESCRIPTION 
       [0017]    In the following description, for purposes of explanation rather than limitation, specific details are set forth such as the particular architecture, interfaces, techniques, etc., in order to provide a thorough understanding of the concepts of the invention. However, it will be apparent to those skilled in the art that the present invention may be practiced in other embodiments, which depart from these specific details. In like manner, the text of this description is directed to the example embodiments as illustrated in the Figures, and is not intended to limit the claimed invention beyond the limits expressly included in the claims. For purposes of simplicity and clarity, detailed descriptions of well-known devices, circuits, and methods are omitted so as not to obscure the description of the present invention with unnecessary detail. 
         [0018]    This invention is presented in the context of the example prior art device of  FIG. 1 , for ease of illustration and understanding. One of skill in the art will recognize, however, that some or all of the principles of this invention may be applicable to a variety of different LED structures, or any structures that would benefit from a reduction in optical loss created by an absorbing region adjacent to a low loss current injection region. 
         [0019]    As noted above, the light emitting device of  FIG. 1 , the structure of which is repeated in  FIGS. 2 and 3 , includes a highly reflective, large area p-contact  140  that provides for a more uniform distribution of current through the p-layer  130 . The contact between the n-layer  110  and the n-pad  170  is along the perimeter of the n-layer  110 . A boundary layer  180  separates the n-type elements  110 ,  180  from the p-type elements  130 ,  140 ,  150 . 
         [0020]    As illustrated in  FIG. 2 , when connected to an external source via the n-pad  170  and p-pad  160 , the electron current  200  from the n-pad  170  spreads laterally through the n-layer  110 , crossing the boundary layer  180  and continuing down toward the p-contact  140  and the p-pad  160 . Because the current distribution across the n-layer  110  is not perfectly uniform, and because distance from the perimeter of the p-contact  140  and the source of the current  200  is shorter than the distance from the center of the p-contact  140 , the current flow  200   a  to the perimeter of the p-contact  140  will be greater than the current flow  200   b  to the center of the p-contact  140 . Depending upon geometry (corner vs. edge), n-GaN sheet resistance (thickness and doping), and operating conditions (current, temperature), a substantial fraction  200   a  of the current injection  200  may be concentrated near the boundary of the p-contact  140 . Accordingly, the current injection through the p-n junction of active layer  120  will be larger around the periphery of the active layer  120 , creating a higher emission of light at the periphery. 
         [0021]    In addition to potentially objectionable optical effects caused by this non-uniform light emission, this non-uniformity potentially reduces the overall light extraction efficiency, because the higher light emission occurs in regions where the optical losses are greatest. At the center of the light emitting active layer  120 , most of the emitted light will eventually exit the top surface of the light emitting device, either directly, or via reflections from the p-contact layer  140 . Light that is emitted from the center of the active layer  120  at severe angles (side-light) relative to the top surface will have a greater likelihood of exiting the top surface of the device than such light from other regions, because, from the center, there is less likelihood of encountering a light absorbing feature, such as the boundary layer  180 , before exiting the top surface. Conversely, along the perimeter of the active layer  120 , the likelihood of encountering the boundary layer  180  is significantly higher, with a corresponding increase in optical loss. 
         [0022]    In addition to the optical problems associated with the non-uniform current flow, the larger current flow  200   a  creates a “hot spot” that lowers the bandgap and draws even more current, resulting in the creation of failure prone areas in the device. 
         [0023]    Additionally, the uneven current injection into the light emitting region also reduces the overall chip internal quantum efficiency (IQE; a ratio of the number of photons emitted per injected electron), because the IQE decreases as the current density increases (known in that art as “IQE droop”). 
         [0024]    In an embodiment of this invention, hole current injection is inhibited in the periphery region  310  of the p-contact  140 , as illustrated in  FIGS. 3A-3B ,  FIG. 3B  being a cross section A-A′ of the device of  FIG. 3A . This hole current injection inhibition region  310  may be formed by using, for example, a shallow low dose H+ implant, or other means of reducing, or blocking, current flow in this region. Such an implant may be performed after a silver deposition to form the p-contact  140 , using a photo-resist pattern to form the region  310  that is subsequently processed to create the current-inhibiting region  310 . Sufficient energy and dose for this purpose depends upon the Ag thickness but a 15 keV energy and a dose of 2e14 cm−2 are nominal values. High energy that implants deeper than 50 nm into the p-layer and high doses will create excessive damage in the p-layer and increase optical absorption. 
         [0025]    Other means of inhibiting current flow to the p-layer  130  at the periphery may also be used, such as coating the periphery of the p-contact  140  with a resistive material  310 ′, such as a dielectric or other poorly conductive transparent material, as illustrated in  FIG. 3C . The p-contact layer  140  may run up over the edge of the dielectric layer  310 ′ overlapping  310 ′ to an extent of at least 5 μm, creating in the overlapped areas a highly reflective Ag-dielectric mirror. 
         [0026]    By inhibiting the current flow in the region  310 , the source current  300  is forced to be laterally diverted further through the n-layer  110 , as illustrated by the current flows  300   a ,  300   b  in  FIG. 3A . Because of the lateral diversion from the periphery of the p-contact  140 , the current  300   a  flows farther through the n-layer  110  before reaching the p-contact  140  than the current  200   a  in  FIG. 2 , and will correspondingly be reduced in magnitude. This reduction in current magnitude at the periphery will reduce the ‘hot-spot’ associated with the high current  200   a,  and will reduce the likelihood of premature failure caused by the high current  200   a.    
         [0027]    The reduction in current at the periphery of the p-contact  140  will correspondingly provide an increase in the current  300   b  that flows to the center of the light emitting layer  120 , compared to the current  200   b  in  FIG. 2 . The overall effect, for the same amount of total current in  FIGS. 2 and 3 , is a more uniform excitation of the light emitting layer  120  of  FIG. 3 , which provides for a more uniform light output from the device of  FIG. 3 . 
         [0028]    Additionally, by laterally shifting the current away from the periphery of the p-contact  140 , the edge of the light emission region is relocated away from the absorbing guard region  150 , thereby reducing the amount of light that is lost to this region  150 . 
         [0029]    It is desirable to maintain as small a radius of curvature as possible at the outer corners  320  of the p-contact layer, so as to provide a maximal reflective area below the light emitting layer  120 , thereby minimizing losses for any backscattered light. However, in a conventional device, a small radius of curvature maximizes the current crowding in the corners  320  of the device, causing even greater local hotspots at the corners. A reduction in the likelihood of local hot-spots may also be achieved by rounding the inner corners  330  of the inhibition region  310 . By creating a current inhibiting region of larger radius of curvature at the corner  330  upon a p-contact layer with a small radius of curvature at the corners  320 , the optical efficiency is maintained, and hot spots are mitigated. 
         [0030]    While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. 
         [0031]    For example, it is possible to operate the invention by situating a contact enhancing layer, such as NiO, beneath the regions of the Ag contact where an enhanced contact is desired and eliminating this layer in the regions where the enhancement is not desired. This embodiment may be combined with a reduction in Mg doping or other impairment in the typical p-contact to reduce the effectiveness of the Ag—GaN contact. 
         [0032]    Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.