Abstract:
Aspects include electrodes that provide specified reflectivity attributes for light generated from an active region of a Light Emitting Diode (LED). LEDs that incorporate such electrode aspects. Other aspects include methods for forming such electrodes, LEDs including such electrodes, and structures including such LEDs.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent application is a continuation of U.S. patent application Ser. No. 12/888,379, filed on Sep. 22, 2010, now U.S. Pat. No. ______, entitled METHODS OF LOW LOSS ELECTRODE STRUCTURES FOR LEDs, which is a continuation of U.S. patent application Ser. No. 12/493,499, filed on Jun. 29, 2009, now U.S. Pat. No. 7,897,992, which is a divisional of U.S. patent application Ser. No. 11/437,570, filed on May 19, 2006, both of which are entitled LOW OPTICAL LOSS ELECTRODE STRUCTURES FOR LEDS; now U.S. Pat. No. 7,573,074, all of which are hereby expressly incorporated by reference in their entireties. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates generally to light emitting diodes (LEDs). The present invention relates more particularly to electrode structures that mitigate optical losses and thus tend to enhance the brightness and/or the efficiency of LEDs. 
       BACKGROUND 
       [0003]    Light emitting diodes (LEDs) for use as indicators are well known. LEDs have been used extensively for this purpose in consumer electronics. For example, red LEDs are commonly used to indicate that power has been applied to such devices as radios, televisions, video recorders (VCRs), and the like. 
         [0004]    Although such contemporary LEDs have proven generally suitable for their intended purposes, they possess inherent deficiencies that detract from their overall effectiveness and desirability. For example, the light output of such contemporary LEDs is not as great as is sometimes desired. This limits the ability of contemporary LEDs to function in some applications, such as providing general illumination, e.g., ambient lighting. Even high power contemporary LEDs do not provide sufficient illumination for such purposes. 
         [0005]    At least a part of this problem of insufficient brightness is due to inefficiency of contemporary LEDs. Efficiency of LEDs is a measure of the amount of light provided as compared to the electrical power consumed. Contemporary LEDs are not as efficient as they can be because some of the light generated thereby is lost due to internal absorption. Such internal absorption limits the amount of light that can be extracted from an LED and thus undesirably reduces the efficiency thereof. 
         [0006]    Thus, although contemporary LEDs have proven generally suitable for their intended purposes, they possess inherent deficiencies which detract from their overall effectiveness and desirability. As such, it is desirable to provide LEDs that have enhanced brightness and/or efficiency. 
       BRIEF SUMMARY 
       [0007]    Systems and methods are disclosed herein to provide brighter and/or more efficient LEDs. For example, in accordance with an embodiment of the present invention, an LED can comprise a reflective electrode structure comprising a metal electrode. 
         [0008]    More particularly, the electrode can be formed upon a semiconductor material that emits light having a central wavelength λ. This light is emitted in all directions. A comparatively thick, optically transmissive dielectric material can be formed upon the semiconductor material. A portion of the electrode can be formed over the comparatively thick dielectric material. Another portion of the same electrode can be in electric contact with the semiconductor material. The electrode cooperates with the thick dielectric to enhance reflection such that light emitted in the direction of the electrode is reflected back into the semiconductor material and thus has another opportunity to be extracted from the LED. 
         [0009]    The term wavelength (λ), as used herein, refers to the wavelength of light inside of the material that the light is traveling within. Thus, if light within a semiconductor material is being referred to, for example, then the wavelength of this light is its wavelength within the semiconductor material. 
         [0010]    The thick dielectric thickness can be greater than ½λ, where λ is the Wavelength of light inside of the thick dielectric material. The thick dielectric material can have an index of refraction that is lower than that of the semiconductor material and that is greater than or equal 1.0 The light emitting semiconductor material can comprise AlGaAs, AlInGaP, AlInGaN, and/or GaAsP, for example. Other materials can similarly be suitable. 
         [0011]    The optically transmissive thick dielectric layer can be a comparatively thick layer of material such as silicon dioxide, silicon monoxide, MgF2 and siloxane polymers, and/or air, for example. Other materials can similarly be suitable. 
         [0012]    There can be an ohmic contact layer between the metal electrode and the semiconductor. The ohmic contact layer can comprise indium tin oxide (ITO), nickel oxide, and/or RuO2, for example. Other materials can similarly be suitable. The ohmic contact layer can be part of the semiconductor device comprising of a heavily doped layer. 
         [0013]    There can be a current spreading layer between the metal electrode and the semiconductor. The current spreading layer is composed of indium tin oxide, nickel oxide, RuO2, for example. Other materials can similarly be suitable. 
         [0014]    A series of one or more pairs of DBR dielectric layers can be formed between the thick dielectric layer and the metal electrode such that each DBR dielectric layer of this pair can be optically transmissive, of different indices of refraction from each other, and/or odd multiples of about ¼λ thick. 
         [0015]    Each layer of the pairs of DBR dielectric material can comprise titanium dioxide TiO2, Ti3O5, Ti2O3, TiO, ZrO2, TiO2ZrO2Nb2O5, CeO2, ZnS, Al2O3, SiN niobium pentoxide (Nb2O5), tantalum pentoxide (Ta2O5), siloxane polymers SiO, SiO2, and/or MgF2, for example. Other materials can similarly be suitable. 
         [0016]    The metal electrode can be comprise one or more metal layers, wherein each metal layer can be selected from a group consisting of Al, Ag, Rh, Pd, Cu, Au, Cr, Ti, Pt nickel/gold alloys, chrome/gold alloys, silver/aluminum mixtures and combinations thereof. Other materials can similarly be suitable. 
         [0017]    The LED can have either a vertical or lateral structure. A portion of the metal electrode can form an area for wire bonding. A portion of the metal electrode can make an electrical contact to the semiconductor material at the edges of the thick dielectric material. A portion of the metal electrode makes an electrical contact to the semiconductor material through openings in the thick dielectric material. 
         [0018]    According to one embodiment of the present invention, a reflective electrode structure for an LED comprises a metal electrode. A GaN material emits light about some central wavelength λ. A comparatively thick silicon dioxide material can be formed upon the GaN material. A portion of the electrode can be formed over the thick dielectric material. Another portion of the same electrode can be in ohmic contact with a semiconductor material. The thick dielectric can have a thickness greater than ½λ. Both the dielectric material and the metal electrode can make physical contact to the semiconductor via an ITO layer or other materials than can be similarly suitable. 
         [0019]    According to one embodiment of the present invention, a reflective electrode structure comprises a metal electrode and a GaN material emits light about some central wavelength λ. A thick silicon dioxide material can be formed upon the GaN material. A series of at least one DBR pair can be formed upon the thick silicon dioxide material. 
         [0020]    A portion of the electrode can be formed over both the thick dielectric material and the DBR pairs. Another portion of the same electrode can be in ohmic contact with the semiconductor material. The thick dielectric thickness can be greater than ½λ. 
         [0021]    Each layer of the DBR pairs can be optically transmissive, of different indices of refraction with respect to one another, and can be odd multiples of about ¼λ in thickness. Both the thick dielectric and the metal electrode can make physical contact to the semiconductor via an ITO layer. 
         [0022]    Thus, according to one or more embodiments of the present invention a brighter and/or more efficient LED can be provided. Increasing the brightness and/or efficiency of LED enhances their utility by making them more suitable for a wider range of uses, including general illumination. 
         [0023]    This invention will be more fully understood in conjunction with the following detailed description taken together with the following drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  is a schematic diagram showing the concept of critical angle; 
           [0025]      FIG. 2  is a semi-schematic perspective view of a contemporary lateral LED structure; 
           [0026]      FIG. 3  is a semi-schematic perspective view of a contemporary vertical LED structure; 
           [0027]      FIG. 4A  is a semi-schematic diagram showing light reflection at a contemporary GaN/Cr/Au interface; 
           [0028]      FIG. 4B  is a chart showing reflectivity at the GaN/Cr/Au interface of  FIG. 4A  for different angles of incidence; 
           [0029]      FIG. 5A  is a semi-schematic diagram showing a contemporary electrode structure having an ohmic contact layer; 
           [0030]      FIG. 5B  is a semi-schematic diagram showing a contemporary electrode structure having an ohmic contact/current spreading layer; 
           [0031]      FIG. 6A  is a semi-schematic top view of a portion of a contemporary LED die showing a circular contact that can also function as a bond pad; 
           [0032]      FIG. 6B  is a semi-schematic top view of a portion of a contemporary LED dice showing a cross shaped contact with a circular bond pad; 
           [0033]      FIG. 6C  is a semi-schematic top view of a portion of a contemporary LED dice showing exemplary contact geometry that is suitable for use with larger LEDs and having a circular contact that can also function as a bonding pad; 
           [0034]      FIG. 7A  is a semi-schematic side view of a contemporary low aspect ratio electrode structure; 
           [0035]      FIG. 7B  is a semi-schematic side view of a high aspect ratio electrode structure; 
           [0036]      FIG. 8A  is a semi-schematic diagram showing light reflection at a contemporary Ag interface; 
           [0037]      FIG. 8B  is a chart showing reflectivity at the Ag interface of  FIG. 8A  for different angles of incidence; 
           [0038]      FIG. 9A  is a semi-schematic diagram showing light reflection at a contemporary GaN/SiO2/Ag interface of a vertical structure LED; 
           [0039]      FIG. 9B  is a chart showing reflectivity at the GaN/SiO2/Ag interface of  FIG. 9A  for different angles of incidence; 
           [0040]      FIG. 10A  is a semi-schematic diagram showing light reflection at a contemporary GaN/air interface; 
           [0041]      FIG. 10B  is a chart showing reflectivity at the GaN/air interface of  FIG. 10A  for different angles of incidence; 
           [0042]      FIG. 11A  is a semi-schematic diagram showing light reflection at a GaN/SiO2 interface, wherein the thick dielectric is thick according to an embodiment of the present invention; 
           [0043]      FIG. 11B  is a chart showing reflectivity at the GaN/SiO2 interface of  FIG. 11A  for different angles of incidence; 
           [0044]      FIG. 12A  is a semi-schematic diagram showing light reflection at a GaN/SiO2/Al interface according to an embodiment of the present invention; 
           [0045]      FIG. 12B  is a chart showing reflectivity at the GaN/SiO2/Al interface of  FIG. 12A  for different angles of incidence wherein thicknesses of the SiO2 layer are less than or equal to 1¾ the wavelength of incident light according to an embodiment of the present invention; 
           [0046]      FIG. 12C  is a chart showing reflectivity at the GaN/SiO2/Al interface of  FIG. 12A  for different angles of incidence wherein thicknesses of the SiO2 layer are greater than 1¾ the wavelength of incident light according to an embodiment of the present invention; 
           [0047]      FIG. 13A  is a semi-schematic diagram showing light reflection at a distributed Bragg reflector (DBR) comprised of alternating layers of SiO2 and TiO2 according to an embodiment of the present invention; 
           [0048]      FIG. 13B  is a chart showing reflectivity at the DBR layers of  FIG. 13A  for different angles of incidence according to an embodiment of the present invention; 
           [0049]      FIG. 14  is a chart showing reflectivity of several materials for different angles of incidence according to an embodiment of the present invention; 
           [0050]      FIG. 15A  is a semi-schematic diagram showing a first exemplary embodiment of a suspended electrode according to the present invention; 
           [0051]      FIG. 15B  is a semi-schematic diagram showing a second exemplary embodiment of a suspended electrode according to the present invention; 
           [0052]      FIG. 15C  is a semi-schematic diagram showing a third exemplary embodiment of a suspended electrode according to the present invention; 
           [0053]      FIG. 15D  is a semi-schematic diagram showing a fourth exemplary embodiment of a suspended electrode according to the present invention; 
           [0054]      FIG. 15E  is a semi-schematic diagram showing a fifth exemplary embodiment of a suspended electrode according to the present invention; 
           [0055]      FIG. 15F  is a semi-schematic diagram showing a sixth exemplary embodiment of a suspended electrode according to the present invention; 
           [0056]      FIG. 16A  is a semi-schematic diagram showing a first exemplary embodiment of a suspended electrode with an ohmic contact layer according to the present invention; 
           [0057]      FIG. 16B  is a semi-schematic diagram showing a second exemplary embodiment of a suspended electrode with an ohmic contact layer according to the present invention; 
           [0058]      FIG. 16C  is a semi-schematic diagram showing a third exemplary embodiment of a suspended electrode with an ohmic contact layer according to the present invention; 
           [0059]      FIG. 16D  is a semi-schematic diagram showing a fourth exemplary embodiment of a suspended electrode with an ohmic contact layer according to the present invention; 
           [0060]      FIG. 16E  is a semi-schematic diagram showing a fifth exemplary embodiment of a suspended electrode with an ohmic contact layer according to the present invention; 
           [0061]      FIG. 16F  is a semi-schematic diagram showing a sixth exemplary embodiment of a suspended electrode with an ohmic contact layer according to the present invention; 
           [0062]      FIG. 17A  is cross-section view of a contemporary lateral structure LED; 
           [0063]      FIGS. 17B-17D  are semi-schematic perspective views showing some steps in the process for fabricating the LED of  FIG. 17A ; 
           [0064]      FIG. 18A  is cross-section view of a lateral structure LED according to an embodiment of the present invention; 
           [0065]      FIGS. 18B-18E  are semi-schematic perspective views showing some steps in the process for fabricating the LED of  FIG. 18A ; 
           [0066]      FIG. 19A  is cross-section view of a lateral structure LED according to an embodiment of the present invention; 
           [0067]      FIGS. 19B-17E  are semi-schematic perspective views showing some steps in the process for fabricating the LED of  FIG. 19A ; 
           [0068]      FIG. 20A  is a semi-schematic perspective view showing another embodiment of suspended structure according to an embodiment of the present invention; 
           [0069]      FIG. 20B  is a semi-schematic perspective view showing another embodiment of suspended structure according to an embodiment of the present invention; 
           [0070]      FIG. 21A  is a semi-schematic diagram showing an exemplary embodiment of the present invention in an elongated LED; 
           [0071]      FIG. 21B  is a semi-schematic diagram showing an exemplary embodiment of the present invention in an elongated LED; 
           [0072]      FIG. 22A  is a semi-schematic diagram showing an exemplary embodiment of the present invention in an elongated LED; 
           [0073]      FIG. 22B  is a semi-schematic diagram showing an exemplary embodiment of the present invention in an elongated LED; 
           [0074]      FIG. 22C  is a semi-schematic diagram showing an exemplary embodiment of the present invention in an elongated LED; 
           [0075]      FIG. 23A  is a semi-schematic diagram showing an exemplary embodiment of the present invention in an elongated LED; 
           [0076]      FIG. 23B  is a semi-schematic diagram showing an exemplary embodiment of the present invention in an elongated LED; 
           [0077]      FIG. 23C  is a semi-schematic diagram showing an exemplary embodiment of the present invention in an elongated LED; and 
           [0078]      FIG. 24  is a semi-schematic diagram showing an exemplary embodiment of the present invention in an elongated LED. 
       
    
    
       [0079]    Embodiments of the present invention and their advantages are best understood by referring to the detailed description that follows. It should be appreciated that like reference numerals are used to identify like elements illustrated in one or more of the figures. 
       DETAILED DESCRIPTION 
       [0080]    Light emitting devices (LEDs) emit light in response to excitation by an electrical current. One typical LED has a heterostructure grown on a substrate by metal-organic vapor phase epitaxy or a similar technique. An LED heterostructure includes n-type and p-type semiconductor layers that sandwich a light producing layer, i.e., an active region. Exemplary active areas may be quantum wells surrounded by barrier layers. Typically, electrical contacts are attached to the n-type and p-type semiconductor layers. When a forward bias is applied across the electrical contacts electrons and holes flow from n-type and p-type layers to produce light in the active region. Light is produced according to well known principles when these electrons and holes recombine with each other in the active region. 
         [0081]    The efficiency with which a LED converts electricity to light is determined by the product of the internal quantum efficiency, the light-extraction efficiency, and losses due to electrical resistance. The internal quantum efficiency is determined by the quality of the semiconductor layers and the energy band structure of the device. Both of these are determined during deposition of the semiconductor layers. 
         [0082]    The light extraction efficiency is the ratio of the light that leaves the LED chip to the light that is generated within the active layers. The light extraction efficiency is determined by the geometry of the LED, self-absorption of light in semiconductor layers, light absorption by electrical contacts, and light absorption by materials in contact with the LED that are used to mount a device in a package. 
         [0083]    Semiconductor layers tend to have relatively high indices of refraction. Consequently, most of the light that is generated in the active region of an LED is internally-reflected by surfaces of a chip many times before it escapes. To achieve high light-extraction efficiency it is important to minimize absorption of light by the semiconductor layers and by electrical connections to the chip. When these layers are made to have very low optical absorption, by being transparent or highly reflective, the overall light extraction in an LED is improved substantially. 
         [0084]    Referring now to  FIG. 1 , light inside of a high index of refraction medium  11  is incident at interface to a lower index of refraction medium  12 . The light can be incident at different angles. When light from a high index of refraction medium  11  encounters the interface to a lower index of refraction medium  12  the light can either be transmitted into the lower index of refraction medium  12  or be reflected back into the higher index of refraction medium  11 . 
         [0085]    According to Snell&#39;s law, a portion of the light traveling from a material having an index of refraction n 1  into a material having a lower index of refraction n 2  at an angle less than the critical angle θc will pass into the lower index of refraction material. This is indicated by the arrow on the left that continues from the material having the lower index of refraction n 1  into the material having the higher index of refraction n 2 . 
         [0086]    Conversely, according to Snell&#39;s law, all of the light traveling from a material having a higher index of refraction n 1  toward a material having a lower index of refraction n 2  at an angle greater than the critical angle θc will be reflected back into the higher index of refraction material. This mechanism is know is total internal reflection (TIR) and is indicated by the arrow on the right that does not continue from the material having the higher index of refraction n 1  into the material having the lower index of refraction but the arrow rather extends back through the material having the higher index of refraction. 
         [0087]    Light within a material having a higher index of refraction than exists outside of the material (such as light within a semiconductor material where air or an encapsulating epoxy is the outside material) which is incident upon the interface surface at angles greater than θc will experience total internal reflection. Typical semiconductor materials have a high index of refraction compared to ambient air (which has an index of refraction of 1.0), or encapsulating epoxy (which can have an index of refraction of approximately 1.5). 
         [0088]    In an LED, this light is reflected back into the LED chip where further absorption can undesirably occur from other materials. This undesirable absorption reduces the efficiency of the LED by reducing the amount of light that the LED provides. 
         [0089]    For conventional LEDs, the vast majority of light generated within the structure suffers total internal reflection before escaping from a semiconductor chip. In the case of conventional Gallium Nitride (GaN) based LEDs on sapphire substrates, about 70% of emitted light can be trapped between the sapphire substrate and the outer surface of the GaN. This light is repeatedly reflected due to total internal reflection, thus suffering multiple absorptions by the metal electrodes and the other materials. It is thus desirable to create structures that tend to minimize this absorption. 
         [0090]    As used herein, the term electrode can refer to a conductor (such as a metal conductor) that supplies current to a semiconductor material of an LED. Thus, an electrode can be in electrical contact with the semiconductor material. However, not all portions of an electrode are necessarily in contact with the semiconductor material. Indeed, according to one or more embodiments of the present invention, a portion of an electrode is in electrical contact with the semiconductor material and another portion of an electrode is not in electrical contact with the semiconductor. 
         [0091]    Referring now to  FIG. 2 , a contemporary lateral structure LED is shown. Regions on the surface of a p-layer  21  and an n-layer  22  of an LED  20  can be metallized so as to form electrodes  23  and  24 . p-n junction or active region  26  is between p-layer  21  and an n-layer  22 . Electrodes  23  and  24  provide a means to provide electrical power to LED  20 . For device structures where the semiconductor is supported by an optically transparent, electrically non-conductive substrate  23 , comprised of a material such as sapphire, the electrical contact to p-layer  21  and n-layer  22  must be made from the top surface. 
         [0092]    In the configuration shown in  FIG. 2 , p-layer  21  is already exposed at top surface and electrical contact can be readily made therewith. However n-layer  22  is buried beneath both p-layer  21  and active region  26 . To make electrical contact to n-layer  22 , a cutout area  28  is formed by removing a portion of p-layer  21  and active layer  26  (the removed portion is indicated by the dashed lines) so as to expose n-layer  24  therebeneath. After the creation of cutout area  28 , the n-layer electrical contact or electrode  24  can be formed. 
         [0093]    Such device structures as that shown in  FIG. 2  result in the current flowing generally in the lateral direction. This is why they are referred to as lateral structures. One disadvantage of such lateral structures is that a portion of the active light producing region must be removed to produce the cutout structure  28  so the n-electrode  24  can be formed. Of course, this reduces the active region area and consequently reduces the ability of LED  20  to produce light. 
         [0094]    Referring now to  FIG. 3 , an LED  30  can alternatively comprise structures where the semiconductor (comprised of a p-layer  31  and an n-layer  32  that cooperate to define an active region  36 ) is supported by an electrically conductive substrate  37 . Substrate  37  can be formed of an optically transparent conductive material such as silicon carbide or can be formed of an optically non-transparent, electrically conductive substrate such as copper or molybdenum. Such LEDs can be configured to have either the n-layer, or p-layer in contact with the substrate. 
         [0095]    In such LEDs, electrically conductive substrate  37  serves as one electrode while the other electrode  33  can be readily formed on the top surface, e.g. p-layer  31 . Since the contacts or electrodes are on opposing surfaces of LED  30 , current flow is in a generally vertical direction. Such devices are thus referred to as vertical structures. 
         [0096]    Regardless of whether the metal electrodes are for vertical or lateral LED structures, they must satisfy similar requirements. These requirements include good adhesion, the ability to make ohmic contact to the semiconductor, good electrical conductivity, and good reliability. Often, these requirements are satisfied by using two or more layers. For example a first layer of metal such as chromium or titanium can provide good adhesion and ohmic contact. A second layer of metal such as silver or gold can provide good electrical conductivity. 
         [0097]    Although chromium has good adhesion and gold is a good electrical conductor. Neither material has good optical reflectivity in the visible region. The optical reflectivity and the corresponding optical absorption can be calculated from the refractive indices of these structures and their corresponding thicknesses. 
         [0098]    Where a material thickness has not been given herein, the thickness can be assumed to be great enough such that optical interference effects are not an issue. For example, such reflectivity calculations typically assume the incident and exit medium to be semi-infinite. In cases of metal reflector layers where their thickness have not been specified, they are assumed to be thick enough, typically a few thousand nanometers, so that an insignificant amount of light reaches the other surface of the metal. The refractive index values of Table 1 are used to calculate all reflectivity curves in this disclosure. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                 Refractive 
                 Refractive 
               
               
                 Dielectric 
                   
                 Wavelength 
                 Index 
                 Index 
               
               
                 Material 
                 Abbreviation 
                 (nm) 
                 (Real) 
                 (Imaginary) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Aluminum 
                 Al 
                 450 
                 0.49 
                 −4.7 
               
               
                 Titanium Dioxide 
                 TiO2 
                 450 
                 2.57 
                 −0.0011 
               
               
                 Silicon Dioxide 
                 SiO2 
                 450 
                 1.465 
                 0 
               
               
                 Air 
                 Air 
                 450 
                 1 
                 0 
               
               
                 Gold 
                 Au 
                 450 
                 1.4 
                 −1.88 
               
               
                 Chromium 
                 Cr 
                 450 
                 2.32 
                 −3.14 
               
               
                 Indium Tin Oxide 
                 ITO 
                 450 
                 2.116 
                 −0.0047 
               
               
                 Titanium 
                 Ti 
                 450 
                 2.27 
                 −3.04 
               
               
                 Silver 
                 Ag 
                 450 
                 0.132 
                 −2.72 
               
               
                 Gallium Nitride 
                 GaN 
                 450 
                 2.45 
               
               
                 Nano Porous 
                 SiO2_Nano 
                 633 
                 1.1 
                 0 
               
               
                 Silicon Dioxide 
               
               
                 Titanium Dioxide 
                 TiO2 
                 633 
                 2.67 
                 0 
               
               
                 Gallium 
                 GaP 
                 633 
                 3.31 
                 0 
               
               
                 Phosphide 
               
               
                 Silicon Dioxide 
                 SiO2 
                 633 
                 1.456 
                 0 
               
               
                   
               
             
          
         
       
     
         [0099]    The thickness of materials as referenced in this disclosure can be in absolute units, TABS, such as microns (□m) or nanometers (nm). Alternatively, the thickness of material can be given relative to the number of wavelengths in the medium, TIReI. When given as the number of wavelengths (λ), the parameter specifically refers to the wavelength of light within the material itself. This can be converted to the absolute thickness by multiplying by the index of refraction of the material (N) as indicated by Equation 1 below. For example a ¼λ of SiO2 at 450 nm would be 76.8 nm (0.25 450/1.465). 
         [0000]      TABS=( T |Rel/ N )·λ  (Equation 1)
 
         [0100]    The optically reflectivity curve as a function of incident angle has two components, i.e., P-polarized light and S-polarized light. P-polarized light experiences Brewster&#39;s angles and has a lower overall reflectivity than S-polarized light. 
         [0101]    Referring now to  FIG. 4A , a diagram of a contemporary semiconductor and electrode structure showing the reflectivity of an electrode  44  for light originating within the semiconductor  41  is provided. The electrode utilizes a typical chromium  42  and gold  43  electrode configuration and is formed upon a GaN semiconductor  41 . For a reflection at an incident angle of 45 degrees, an average of only 25% of the P-polarized and S-polarized light is reflected while, 75% of the light is absorbed. Thus, this contemporary configuration is undesirably highly absorbing. 
         [0102]    Although  FIG. 4A  shows a gold/chromium metal electrode structure formed upon GaN, other metals and semiconductor materials can alternatively be utilized. 
         [0103]    Referring now to  FIG. 4B , a chart shows reflectivity at the GaN/Cr/Au interface of the device of  FIG. 4A  for different angles of incidence. 
         [0104]    Referring now to  FIG. 5A , a more generic contemporary contact structure is shown. According to this more generic contact structure, there may be an ohmic contact and/or current spreading layer  52  between a metal contact  53  and a semiconductor material  51 . The metal contact  53  may have multiple layers for purposes for adhesion, diffusion barrier, solder, electrical conductivity, and ohmic contact. The layers can be fabricated from various metals and combinations of metals, including nickel, platinum, titanium, silver, aluminum, gold, tin, lead, and chromium. The semiconductor material  51  can be from the material systems such as AlGaAs, AlInGaP, AlInGaN, and GaAsP. The ohmic contact layer can be part of the metal electrode layers such as nickel oxide. 
         [0105]    Referring now to  FIG. 5B , an electrically conductive metal oxide such as indium tin oxide or nickel oxide can be deposited on entire surface of semiconductor  55  to define an ohmic contact/current spreading layer  56  upon which metal electrode  57  can be formed. In such a case, layer  56  serves both as an ohmic contact and current spreading layer. There can be a layer that allows for ohmic contact on the very top of the LED semiconductor material, such as a heavily doped region. 
         [0106]    Regardless of the exact metal electrode configuration, semiconductor material or LED structure, contemporary metal electrodes undesirably absorb some light. In additional, metal contacts are not transparent, they block the available surface area where light can escape. Thus, such contemporary electrodes have a double effect. They not only directly absorb a portion of the incident light, but the remaining reflected light is directed back into the device where it suffers further absorption by other materials. The total amount of absorption is highly dependent on the exact configuration of the electrode and tends to scale proportionally to the size of the electrode contact area. 
         [0107]    Referring now to  FIGS. 6A-6C , the principle of current spreading so as to mitigate the problem of current crowding is discussed. The p-layer and n-layer of contemporary LEDs are thin and have relatively low electrical conductivity. By themselves, these layers do not evenly distribute current to all regions of the p-n junction, i.e., the active region. For larger areas where portions of the active region are far away from the electrode, there will be less current flow in these distant areas than in areas close to the metal contact. This results in uneven current distribution and consequent uneven light emission. To reduce current crowding, the geometry of the metal electrodes is extended over the semiconductor surface. These extensions however lead to additional undesirable light absorption. 
         [0108]    With particular reference to  FIG. 6A , a circular contact or electrode  62  can be formed upon a semiconductor  61  and can serve as a wire bond pad. With particular reference to  FIG. 6B , a cross shaped contact  63  can be combined with electrode  62  to enhance current spreading. With particular reference to  FIG. 6C , various other geometrical structures  63  can similarly be combined with electrode  62  to facilitate current spreading, especially on larger LED dies. 
         [0109]    Typically, wire bonds are used as a means to provide electric power the LED. However the wire bond pad areas must be some minimum size of about 100□m by 100□m. Since the size of each wire bond pad is fixed regardless of device size, the absorbing and opaque wire bond areas can be a significant portion of the overall surface area and for same LED devices. 
         [0110]    One method for reducing the undesirable absorption of light by an electrode is to minimize the contact area or the width of the electrode. If electrical connection to the LED semiconductor material is the only consideration, then the contact width can be quite narrow, such as on the order of a few microns. However, an important consideration is the undesirable increase of electrical resistivity caused by decreasing the cross sectional area. In high power applications, the electrode may carry an amp or more of current. This requires the cross sectional area, width (W)×thickness (T) to be of some minimum value to minimize electrical resistance. Thus, the contact area or width of the electrode cannot merely be reduced without otherwise compensating for the increase in resistivity of the electrode. 
         [0111]    Referring now to  FIG. 7A , a typical dimension for a gold electrode is W=20□m and T=20□m for a total cross sectional area of 40□m2. Theoretically, one could keep a constant cross sectional area and therefore a constant electrical resistance by proportionally increasing thickness while decreasing the width as discussed with reference to  FIG. 7B  below. 
         [0112]    Referring now to  FIG. 7B , according to one embodiment of the present invention the aspect ratio of electrode  77  can be increased. That is, the height of electrode  77  can be increase as compared to the width thereof. For example, the height can be increase so as to provide a thickness greater than 2.5□m. In this manner, the area of electrode  74  that is in contact with semiconductor  75  (and is thus available for light absorption) is reduced and light absorption is consequently similarly reduced. Increasing the height of electrode  77  desirably maintains its conductivity. The contact area has been decreased and the thickness of the electrode has been increased so as to maintain desired conductivity. However manufacturing cost and practical process considerations typically limit electrode thickness to 2.5□m or below. Thus the electrode contact area and its associated absorption become much greater than would be necessary if the electrode was used for only electrical contact to the semiconductor material. 
         [0113]    Another method for reducing electrode absorption is to increase the reflectivity of the electrode. Several prior art approaches have been used to create reflective electrodes for LEDs. The simplest is to use a metal that has a high reflectivity. These include Al, Ag, Re and others known to one familiar with the art. 
         [0114]    The chosen metal needs to not only have a high reflectance, but must also make an acceptably low resistance ohmic contact to the semiconductor material. In the case of p-type AlInGaN, only Ag combines low electrical resistance with high reflectivity. 
         [0115]    Referring now to  FIG. 8A , an electrode structure comprised of Ag is shown. That is, an Ag electrode  82  is formed upon a semiconductor substrate  81 . Unfortunately, Ag presents a reliability concern because it is subject to tarnish and it is subject to electromigration during device operation. Also, the contact resistance of Ag-based contacts sometimes increases with time during device operation. 
         [0116]    Referring now to  FIG. 8B , the reflectance of the Ag electrode of  FIG. 8A  for different angles of incidence is shown. Even with a highly reflective metal electrode, silver, the absorption per reflection near normal incidence is about 10%. It would be desirable to further decrease absorption to well below 10%. 
         [0117]    Referring now to  FIG. 9A , it is known to use a ¼λlayer of dielectric  103 , i.e., SiO 2 , to enhance reflectivity in a vertical structure LED. The dielectric  103  is formed between a GaN semiconductor  104  and an Ag metal layer  102 , both of which are formed upon a conductive holder  101 . However, as discussed below, the use of a ¼λ of dielectric does not substantially enhance reflectivity. 
         [0118]    Referring now to  FIG. 9B , it can be seen that the use of the ¼λ layer of dielectric does provide enhanced reflectance for the S polarized light incident thereon, as indicated by curve  153 . However, the P polarized light incident upon this dielectric layer has a deep dip in the reflectance curve around 47°, as indicated by curve  152 . This dip substantially reduces the overall reflectivity, as indicated by the curve  151  for the average of the S polarized and the P polarized light. Therefore, the use of a ¼λ layer of dielectric is not a suitable solution to the problem of light absorption by an LED electrode. 
         [0119]    According to one embodiment of the present invention, a reflective electrode structure minimizes contact area between the electrode and the LED semiconductor material. A comparatively thick dielectric material is disposed between a conductive electrode and the semiconductor material so as to electrically isolate portions of the electrode while allowing for other portions to make electrical contact. The dielectric material can be of a lower index of refraction than the semiconductor and can be thick enough such that total internal reflection occurs for incident angles greater than the critical angle θc, as discussed below. 
         [0120]    Total internal reflection for dielectric materials provides the desirable capability for approximately 100% reflectivity. Total internal reflection occurs beyond the critical angle, θc. In the case of a GaN to air interface, the critical angle is approximately 24°. In the case of a GaN to SiO2 interface, the critical angle is approximately 37°. 
         [0121]    Referring now to  FIG. 10A , a semi-schematic diagram shows light reflection at a GaN/air. A ray of light is shown being reflected from the interface back into the GaN semiconductor material  121  because the angle of incidence is greater than the critical angle θc. 
         [0122]    Referring now to  FIG. 10B , a chart shows reflectivity at the GaN/air interface of  FIG. 10A  for different angles of incidence. 
         [0123]    Referring now to  FIG. 11A , a semi-schematic diagram shows light reflection at a GaN/SiO2 interface according to an embodiment of the present invention. A ray of light is shown being reflected from the interface of the GaN semiconductor material  131  and the SiO2 layer  132  back into the GaN semiconductor material  131  because the angle of incidence is greater than the critical angle θc. 
         [0124]    Referring now to  FIG. 11B , a chart shows reflectivity at the GaN/SiO 2  interface of  FIG. 11A  for different angles of incidence according to an embodiment of the present invention. 
         [0125]    Referring now to  FIG. 12A , is a semi-schematic diagram show light reflection at a GaN/SiO2/A 1  interface according to an embodiment of the present invention. A portion of electrode  173  is suspended over GaN substrate  171  and has a thick dielectric SiO2 layer  172  formed therebetween. Another portion of electrode  173  is formed directly upon GaN substrate  171 . 
         [0126]    Referring now to  FIG. 12B , is a chart showing the P-polarization reflectivity at the GaN/SiO2/A 1  interface of  FIG. 12A  for different angles of incidence wherein thicknesses of the SiO2 layer are less than or equal to 1¾λ according to an embodiment of the present invention. At a 1/16λ of SiO2 there is no total internal reflection effect and the reflectivity is marginally worse than without the SiO2 layer. At a ¼λ of SiO2 there is still no TIR effect and the reflectivity is dramatically worse. At ½λ of SiO2 total internal reflection does occur for large incident angles but a tremendous dip in reflectivity occurs at approximately 38°. At 1¾λ, total internal reflection occurs for the high angles of incidence and no noticeable dip in reflectivity. Since TIR begins at ½λ of SiO2, the term “thick” dielectric will refer to all dielectrics thicker or equal to ½λ. 
         [0127]    Referring now to  FIG. 12C , is a chart showing reflectivity at the GaN/SiO2/Al interface of  FIG. 12A  for different angles of incidence wherein thicknesses of the SiO2 layer are greater than 1¾ the wavelength of incident light according to an embodiment of the present invention. 
         [0128]    Once the dielectric layer is greater than this minimum thickness for total internal reflection, its exact thickness is not as critical as in conventional optical coatings based on interference. This allows for greater latitude in the manufacturing process. This is illustrated in  FIG. 12C , which shows the reflectivity curves of for a thick dielectric at two different thicknesses, one at 1.75λ, and the other at 1.85λ. The total internal reflection angle does not change. 
         [0129]    Referring now to  FIG. 13A , a semi-schematic diagram shows light reflection at a distributed Bragg reflector (DBR) comprised of alternating layers of SiO2  182  and TiO2  183  on top of the thick dielectric SiO2 base layer  185  according to an embodiment of the present invention. An electrode  184  makes electrical contact to semiconductor material  181  and is the final layer onto top of the DBR stack. Thick dielectric layer  185  is formed between the DBR stack and semiconductor material  181 . 
         [0130]    The thick dielectric creates an effective reflector at high angles. However, it does not substantially enhance the reflectivity below the critical angle. It is possible to add a distributed Bragg reflector (DBR) to reflect the light at these lower angles. DBRs are typically fabricated using a series of alternating high index/low index dielectric materials. As shown in  FIG. 13A , a series of 2 pairs of ¼λ SiO2 and ¼λ TiO2 over a thick layer of 1¾λ SiO2 enhances the reflectivity at lower angles. DBRs use optical interference to affect reflectivity, as result their thickness is more critical than the thickness of the underlying thick SiO2 layer. 
         [0131]    Table 2 below provides further information regarding the electrode materials utilized according to one or more embodiments of the present invention. The reference wavelength for the coating thickness is 0.4500 microns. The phase and retardance values are in degrees. The coating has six layers. The incident media is GaN. The wavelength of the light used is 0.4500 microns. 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Material 
                 Thickness 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Al1 
                 1.000000 
               
               
                   
                 SiO2 
                 0.250000 
               
               
                   
                 TiO2 
                 0.250000 
               
               
                   
                 SiO2 
                 0.250000 
               
               
                   
                 TiO2 
                 0.250000 
               
               
                   
                 SiO2 
                 0.750000 
               
               
                   
                   
               
             
          
         
       
     
         [0132]    Referring now to  FIG. 13B , is a chart showing reflectivity at the DBR layers of  FIG. 13A  for different angles of incidence according to an embodiment of the present invention compared to a design with only thick dielectric compared to a design with no thick dielectric and no DBR. 
         [0133]    Referring now to  FIG. 14 , is a chart showing reflectivity of several materials for different angles of incidence according to an embodiment of the present invention as compared to prior art. A Au metal layer with a Cr under layer has the worst reflectance as indicated by the lowest curve  1951 . Al is substantially better as indicated by curve  1952 . Ag is even better as indicated by curve  1953 . An Ag metal layer with a thick SiO2 dielectric under layer has generally better reflectance than Ag, although curve  1954  dips below curve  1953  in some places. An Ag metal layer with 2 pairs of DBR followed by with a thick SiO2 has the best reflectance, as indicated by curve  1955 . 
         [0134]    Referring now to  FIG. 15A , a semi-schematic diagram shows a first exemplary embodiment of a suspended electrode according to the present invention. Electrode  142   a  is suspended above a GaN substrate  141  such that a thick air gap  143   a  is formed therebetween. Electrode  142   a  is supported on both sides thereof. 
         [0135]    Referring now to  FIG. 15B , a semi-schematic diagram shows a second exemplary embodiment of a suspended electrode according to the present invention. Electrode  142   b  is suspended above the GaN substrate  141  such that a plurality of air gaps  143   b  are formed therebetween. Electrode  142   a  is supported on both sides and in the middle thereof. 
         [0136]    Referring now to  FIG. 15C , a semi-schematic diagram shows a third exemplary embodiment of a suspended electrode according to the present invention. Electrode  142   c  is suspended above the GaN substrate  141  such that a thick air gap  143   c  is formed therebetween. Electrode  142   c  is supported only on one side thereof. 
         [0137]    Referring now to  FIG. 15D , a semi-schematic diagram shows a fourth exemplary embodiment of a suspended electrode according to the present invention. Electrode  142   d  is suspended above the GaN substrate  141  and a thick SiO2 layer  143   d  is formed therebetween. Electrode  142   d  is supported on both sides thereof. 
         [0138]    Referring now to  FIG. 15E , a semi-schematic diagram shows a fifth exemplary embodiment of a suspended electrode according to the present invention. Electrode  142   e  is suspended above the GaN substrate  141  and a plurality of sections of a thick SiO2 layer  143   e  are formed therebetween. Electrode  142   e  is supported on both sides and in the middle thereof. 
         [0139]    Referring now to  FIG. 15F , a semi-schematic diagram shows a sixth exemplary embodiment of a suspended electrode according to the present invention. Electrode  142   f  is suspended above the GaN substrate  141  such that a thick SiO2 layer  143   f  is formed therebetween. Electrode  142   f  is supported only on one side thereof. 
         [0140]    Referring now to  FIG. 16A , a semi-schematic diagram shows a first exemplary embodiment of a suspended electrode with an ohmic contact layer according to the present invention. The structure of the electrode of  FIG. 16A  is similar to that of  FIG. 15A , except for the addition of indium tin oxide (ITO) layer  144 . 
         [0141]    Referring now to  FIG. 16B , a semi-schematic diagram shows a second exemplary embodiment of a suspended electrode with an ohmic contact layer according to the present invention. The structure of the electrode of  FIG. 16B  is similar to that of  FIG. 16B , except for the addition of indium tin oxide (ITO) layer  144 . 
         [0142]    Referring now to  FIG. 16C , a semi-schematic diagram shows a third exemplary embodiment of a suspended electrode with an ohmic contact layer according to the present invention. The structure of the electrode of  FIG. 16C  is similar to that of  FIG. 15C , except for the addition of indium tin oxide (ITO) layer  144 . 
         [0143]    Referring now to  FIG. 16D , a semi-schematic diagram shows a fourth exemplary embodiment of a suspended electrode with an ohmic contact layer according to the present invention. The structure of the electrode of  FIG. 16D  is similar to that of  FIG. 15D , except for the addition of indium tin oxide (ITO) layer  144 . 
         [0144]    Referring now to  FIG. 16E , a semi-schematic diagram shows a fifth exemplary embodiment of a suspended electrode with an ohmic contact layer according to the present invention. The structure of the electrode of  FIG. 16E  is similar to that of  FIG. 15E , except for the addition of indium tin oxide (ITO) layer  144 . 
         [0145]    Referring now to  FIG. 16F , a semi-schematic diagram shows a sixth exemplary embodiment of a suspended electrode with an ohmic contact layer according to the present invention. The structure of the electrode of  FIG. 15F  is similar to that of  FIG. 14F , except for the addition of indium tin oxide (ITO) layer  144 . 
         [0146]    Referring now to  FIGS. 17A-17D , an exemplary, contemporary, lateral LED structure and the process for forming it are shown. 
         [0147]    [With particular reference to  FIG. 17A , a pair of wire bond pads  1091  and  1092  facilitate the application of current to a semiconductor  1093 . Semiconductor  1093  is formed upon a substrate  1096 . Semiconductor  1093  comprises an p-layer  1097  and a n-layer  1098  (n-layer  1098  and p-layer  1097  are generally interchangeable for the purposes of this discussion) The current causes active region  1094  to produce light according to well known principles. 
         [0148]    With particular reference to  FIG. 17B , the fabrication of the LED of  FIG. 9A  comprises forming a semiconductor layer  1093  upon a substrate  1096 . Semiconductor layer  1093  comprises an n-layer  1098  and a p-layer  1097  (as shown in  FIG. 17A ). 
         [0149]    With particular reference to  FIG. 17C , a portion of p-layer  1097  is removed, such as by etching. A sufficient amount of p-layer  1097  is removed so as to expose a portion of n-layer  1098  therebeneath. Removal of the portion of p-layer  1097  defines a cutout portion  1099 . The formation of cut out  1099  leaves n-layer  1098  exposed. 
         [0150]    With particular reference to  FIG. 17D , wire bond pad  1091  is formed upon p-layer  1097  and wire bond pad  1092  is formed upon n-layer  1098 . wire bond pads  1091  and  1092  cover a comparatively large portion of the surface area of semiconductor  1093 . For example, the electrode wire bond pads of a contemporary LED can be 100□m×100□m. They thus absorb an undesirably large amount of the light produced by active region  1094 . Further, the comparatively large cut out area  1099  that is required for wire bond pads  1092  undesirably reduces the size of active area  1094  and thus further reduces the amount of light produced by such contemporary LEDs. Since the size of each electrode is fixed regardless of device size, the undesirable light absorption can be a significant portion of the overall surface area, particularly for smaller LEDs. 
         [0151]    It is worthwhile to appreciate that that the formation of such an electrode structure that is partially within and partially outside of a cutout offers substantial advantage, even if the electrode is not reflective. For example, the electrode structure described in connection with  FIGS. 18A-18B  below provides adequate bonding area while minimizing the size of the cutout such that less active area is removed and more light can be produced. 
         [0152]    Referring now to  FIGS. 18A-18E , an exemplary lateral LED structure and the process for forming it according to an embodiment of the present invention are shown. A thick dielectric layer  1101  and  1102  is formed beneath wire bond pads  1091   a  and  1092   a , respectively. Thick dielectric layers  1101  and  1102  enhance the reflectivity of wire bond pads  1091   a  and  1092   a  such that undesirable light absorption thereby is substantially decreased. A portion of each wire bond pad  1091   a  and  1092   a  remains in contact with semiconductor  1093  so as to facilitate current flow therethrough. 
         [0153]    As used herein, a thick dielectric layer is a layer having sufficient thickness such that effects of interference are not substantial. Moreover, as used herein a thick dielectric layer can have a thickness of greater than ¼λ. For example, a thick dielectric layer can have a thickness equal or great then ½λ, approximately 1.5λ, approximately 1.75λ, or greater than 1.75λ. 
         [0154]    With particular reference to  FIGS. 18B and 18C , semiconductor  1093  is formed upon substrate  1096  and cutout  1099  is formed in semiconductor  1093  as in  FIGS. 17B and 17C . 
         [0155]    With particular reference to  FIG. 18D , thick dielectric layers  1101  and  1102  are formed upon p-layer  1097  and n-layer  1098 , respectively. Thick dielectric layers  1101  and  1102  can be formed according to well known principles. 
         [0156]    With particular reference to  FIG. 18E , wire bond pad  1091   a  is formed so as to at least partially cover thick dielectric layer  1101  and wire bond pad  1092   a  is formed so as to at least partially cover thick dielectric layer  1102 . As mentioned above, a portion of wire bond pads  1091   a  and  1092   a  contacts semiconductor  1093  therebeneath. 
         [0157]    Referring now to  FIG. 19A-19E  an exemplary lateral LED structure and the process for forming it according to an embodiment of the present invention are shown. 
         [0158]    With particular reference to  FIGS. 19A  A thick dielectric layer  1101  and  1102   a  is formed beneath wire bond pads  1091   a  and  1092   b , respectively. Thick dielectric layers  1101  and  1102   a  enhance the reflectivity wire bond pads  1091   a  and  1092   b  such that undesirable light absorption thereby is substantially decreased. A portion of each wire bond pad  1091   a  and  1092   b  remains in contact with semiconductor  1093  so as to facilitate current flow. 
         [0159]    With particular reference to  FIGS. 19B and 19C , semiconductor  1093  is formed upon substrate  1096  and cutout  1099   a  is formed in semiconductor  1093  as in  FIGS. 17B and 17C . However, in this embodiment cutout  1099   a  is formed in an L-shaped configuration so as to mitigate the amount of surface area thereof. In this manner, less of the active area is sacrificed in the formation of cutout  1099   a  and the brightness of the LED is consequently enhanced. 
         [0160]    With particular reference to  FIG. 19D , a thick dielectric layer  1101  is formed upon the p-layer  1097 . Another thick dielectric layer  1102   a  is formed partially on the p-layer  1097  and partially on the n-layer  1098 . Thick dielectric layers  1101  and  1102   a  can again be formed according to well known principles. In this instance thick dielectric layer  1102   a  is formed downwardly, along the side of p-layer  1097  and active layer  1094  so as to electrically insulate wire bond pad  1092   b  therefrom. That is, thick dielectric layer  1102   a  is formed upon both p-layer  1097  and n-layer  1098 , as well as the interface therebetween, i.e., active layer  1094 . Thick dielectric layer  1102   a  stair steps downwardly from n-layer  1097  to n-layer  1098 . This configuration of thick dielectric layer  1102   a  is best seen in the cross section of  FIG. 19A . 
         [0161]    With particular reference to  FIG. 19E , wire bond pad  1091   a  is formed so as to at least partially cover thick dielectric layer  1101  and wire bond pad  1092   b  is formed so as to at least partially cover thick dielectric layer  1102   a . As mentioned above, a portion of wire bond pad  1091   a  contacts p-layer  1097  and a portion of wire bond pad  1092   b  contacts n-layer  1098 . In this instance, wire bond pad  1092   b  is formed downwardly, insulated by and covering thick dielectric layer  1102   a  and electrically contacting n-layer  1098 . The configuration of wire bond pad  1092   b  is best seen in  FIG. 19A . 
         [0162]    In this embodiment, thick dielectric layers  1101  and  1102   a  substantially mitigate light absorption by wire bond pads  1091   a  and  1092   b  so as to enhance the brightness of the LED. The reduced size of cutout  1099   a  provides a larger active area  1094 , so as to further enhance the brightness of the LED. 
         [0163]    According to the present invention, a thick dielectric can be formed between at least a portion of each bond pad and/or electrode and the semiconductor material. The thick dielectric material enhances reflectivity such that undesirable light absorption by the bond pad and/or electrode is substantially mitigated. 
         [0164]    Referring now to  FIG. 20A , a semi-schematic perspective view shows one embodiment of a suspended electrode structure according to an embodiment of the present invention. A metal electrode  162  is formed upon a semiconductor  161 . A thick dielectric  163  is formed between metal electrode  162  and semiconductor  161 . A portion of electrode  162  is formed over thick dielectric  163  and a portion of electrode  162  contacts semiconductor  161  such that electrode  162  is in electrical contact with semiconductor  161 . 
         [0165]    Referring now to  FIG. 20B , a semi-schematic perspective view shows another configuration of a suspended electrode structure according to an embodiment of the present invention. This structure is generally similar to that of  FIG. 20A  except that thick dielectric  163  is broken up such that portions of electrode  162  contact semiconductor is different places than in  FIG. 20A . As shown in  FIG. 20B , multiple contacts of electrode  162  to semiconductor  161  are provided. As those skilled in the art will appreciate, various configurations of electrode  162  and thick dielectric  163 , with electrode  162  contacting semiconductor  161  in various different places, are possible. 
         [0166]      FIGS. 21A-24  show exemplary electrode structures that utilize thick dielectrics according to one or more embodiments of the present invention. For example, one or more layers of insulating dielectric can be formed under the bonds pads. Some advantages of such construction include: the mitigation of current crowding, thus facilitating a simplified design; the minimization of light absorption because the dielectric layer(s) under the electrode can form a mirror; more efficient use of the emission area that is achieved by reducing the cutout area; a more easily scalable design for a large range of die sizes; comparatively low forward voltage; and more even current spreading. 
         [0167]    The exemplary embodiments of  FIGS. 21A-24  are implementations of an elongated chip. Such elongated chips can provide enhanced brightness with better efficiency. 
         [0168]    Referring now to  FIG. 21A , an electrode design for an elongated chip is shown. Thick dielectric layers  1002  and  1003  can be formed under each of the bond pads  1006  (the p-bond pad, for example) and  1007  (the n-bond pad, for example). N-bond pad  1007  and n-electrode extension  1001  are formed upon an etched away portion of semiconductor material  1008  or cutout  1004   
         [0169]    The thick dielectric layers  1002  and  1003  insulate the bond pads  1006  and  1007  from semiconductor material  1008  so as to mitigate current crowding. This results in an improved geometry for more even current flow. Hot spots that cause uneven brightness and can result in damage to the LED are substantially mitigated. 
         [0170]    Such thick dielectric layers are not formed under conductive extensions  1001  and  1005  that define n-wiring and p-wiring respectively. Extensions  1001  and  1005  thus more evenly distribute current throughout semiconductor  1008 . That is, the distance between the electrodes that provide current to the LED tends to be more equal according to one aspect of the present invention. 
         [0171]    It is worthwhile to appreciate that total internal reflection (TIR) provides a substantial advantage in enhancing light extraction for one or more embodiments of the present invention. The use of a DBR structure is optional and can be used, according to at least one embodiment of the present invention, to further enhance light extraction. 
         [0172]    The use of TIR and/or DBR structures as described above can substantially mitigate undesirable absorption of light under bond pads  1006  and  1007 . Such insulators (as well as insulating layers  1002  ands  1003 ) can be formed beneath bond pads  1006  and  1007  and not beneath extensions  1001  and  1005 , so that current flow through semiconductor (and consequently the active region thereof) is more evenly distributed. 
         [0173]    Bond pads  1006  and  1007 , as shown in  FIGS. 21A and 21B , are not located exactly at the end of the wire traces or extensions  1001  and  1005 . This is to show that bond pads  1006  and  1007  can be placed at any arbitrary location along the trace. Thus, bond pads  1006  and  1007  can be placed at the end, near the end, and/or in the middle of extensions  1001  and  1005 . Any desired location of bond pads  1006  and  1007  can be used. 
         [0174]    Referring now to  FIG. 21B , a potential improvement with respect to the configuration of  FIG. 21A  is shown. The area of cutout  1104  is reduced by putting the n-bond pad above the p-surface and separated form the p-surface by the thick dielectric. That is, at least a portion of the n-bond pad is not in cutout  1104  and cutout  1104  can thus be much smaller than in  FIG. 21A . This thick dielectric must also cover the edges of the cutout to ensure isolation of the n-bond pad from the p-Layer. That is, the area of the cutout is reduced such that the size of the active area is increased. The larger emission area facilitated by using a smaller cutout  1004  can enable a greater power output. 
         [0175]    In some applications, the distance between the p and n electrodes may be too great, thus resulting in an undesirably high forward voltage. In such cases, the use of multiple electrodes may be beneficial.  FIGS. 22A-23C  show various exemplary implementations of three electrode designs that can mitigate such undesirability high forward voltages. 
         [0176]    Referring now to  FIGS. 22A-22C , the n-bond pad is shown split into two electrically isolated pads  1217  and  1218 . In principle, they can be touching (and thus in electrical contact with one another) and thus effectively form a single pad. There can be two separate wire bonds, one to each of pads  1217  and  1218 . However if a gap  1220  between pad  1217  and  1218  is small enough, then a single bond pad can be used to electrically connect bond pads  1217  and  1218  together. In this manner, any desired number of such electrodes can be used. 
         [0177]    With particular reference to  FIG. 22A , two n-bond pads  1217  and  1218  and a single p-bond pad  1219  can be used. Two thick dielectric layers  1204  and  1283  can be formed between each bond pad  1219  and the semiconductor material  1280  disposed therebeneath. Similarly, a thick dielectric layer  1202  can be formed between bond pads  1217  and  1218  and the semiconductor material  1201  of cutout  1281 . As mentioned above, such construction results in more even current distribution. This is particularly true for larger and/or higher current LEDs. 
         [0178]    With particular reference to  FIG. 22B , the area of cutout  1201  is reduced with respect to that shown in  FIG. 22A  in a manner analogous to that of  FIG. 21B . Again, two thick dielectric layers  1204  and  1283  can be formed between each bond pad  1219  and the semiconductor material  1280  disposed therebeneath. Similarly, a thick dielectric layer  1202  can be formed between bond pads  1217  and  1218  and the semiconductor material  1201  of cutout  1281 . 
         [0179]    With particular reference to  FIG. 22C , p-wiring extension  1203  extends beneath n-bond pad thick dielectric  1202  such that a distal end  1230  of p-wiring extension extends to the right of thick dielectric  1202 . Again, two thick dielectric layers  1204  and  1283  can be formed between each bond pad  1219  and the semiconductor material  1280  disposed therebeneath. Similarly, a thick dielectric layer  1202  can be formed between bond pads  1217  and  1218  and the semiconductor material  1201  of cutout  1281 . 
         [0180]    With particular reference to  FIG. 23A-23C , the p-layer and the n-layer are reversed in position (with a consequent reversal in the respective bond pads, insulators, etc) to show that the construction of  FIGS. 22A-22C  is suitable with either type of device. Thus, n-bond  1507  and thick n-bond pad dielectric  1503  are formed on cutout  1504  and p-bond pads  1511  and  1512  and thick p-bond pad dielectric  1501  are not formed on cutout  1504  (which is the opposite of the construction shown in  FIGS. 22A-22C ). Thus, the electrodes are reversed with respect to those shown in  FIGS. 22A-22C . 
         [0181]    With particular reference to  FIG. 24 , a two electrode LED that facilitates more uniform current distribution is shown. An n-bond pad  2403  and a p-bond pad  2404  are formed upon a semiconductor material  2401 . n-bond pad  2403  has a thick dielectric layer  2406  form between itself and semiconductor material  2401 . Similarly, p-bond pad  2404  has a thick dielectric layer  2407  formed between itself and semiconductor material  2401 . 
         [0182]    A cutout  2402  facilitates contact of n-bond pad  2403  to the n-layer of semiconductor  2401 . A portion of n-bond pad  2403  can be formed outside of cutout  2402  (and thus upon the p-layer of semiconductor material  2401 ) and a portion of n-bond pad  2403  can be formed within cutout  2402  (to provide electrical contact with the n-layer). Similarly, a portion of thick dielectric layer  2406  can be formed outside of cutout  2402  (and thus upon the p-layer of semiconductor material  2401 ) and a portion of thick dielectric layer  2406  can be formed within cutout  2402 . 
         [0183]    n-bond pad  2403  and thick dielectric layer  2406  thus extend down the side of cutout  2402  from the n-layer to the p-layer of semiconductor material  2401 , in a fashion similar to that of  FIG. 21B . Such construction tends to minimize the size of cutout  2402  and thus tends to enhance the brightness and efficiency of the LED, as discussed above. 
         [0184]    p-wiring or extension  2407  extends from p-pad  2404  so as to more uniformly distribute current through the active region of semiconductor  2401 . A portion of p-pad  2404  and all of extension  2407  can be formed directly upon semiconductor material  2401  (without a thick dielectric layer therebetween). 
         [0185]    Although in  FIGS. 15-24  only a single thick dielectric layer is shown, a series of one or more DBR pairs can be deposed between the thick dielectric and the electrode. Similarly, although  FIGS. 15-24  show the electrode in direct contact with the semiconductor material, the contact can be via an ohmic contact layer or current spreading layer. 
         [0186]    According to one or more embodiments of the present invention, the thick dielectric can be non-perforated. That is, the dielectric can be continuous in cross-section. It can be formed such that it does not have any holes or perforations that would cause the thick dielectric to appear to be discontinuous in cross-section. 
         [0187]    The dielectric material can be porous. Thus, thick dielectric materials which may otherwise be too dense (and thus have to high of an index of refraction) can be used by effectively reducing the density (and the effective index of refraction, as well) by making the dielectric material porous or non continuous. 
         [0188]    In view of the foregoing, one or more embodiments of the present invention provide a brighter and/or more efficient LED. Increasing the brightness of an LED enhances its utility by making it better suited for use in a wide of applications. For example, brighter LEDs can be suitable for general illumination applications. Further, more efficient LEDs are desirable because they tend to reduce the cost of use (such as by reducing the amount of electricity required in order to provide a desire amount of light. 
         [0189]    Embodiments described above illustrate, but do not limit, the invention. It should also be understood that numerous modifications and variations are possible in accordance with the principles of the present invention. Accordingly, the scope of the invention is defined only by the following claims.