Classifications

• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10H—INORGANIC LIGHT-EMITTING SEMICONDUCTOR DEVICES HAVING POTENTIAL BARRIERS
  • H10H20/00—Individual inorganic light-emitting semiconductor devices having potential barriers, e.g. light-emitting diodes [LED]
  • H10H20/80—Constructional details
  • H10H20/83—Electrodes
  • H10H20/832—Electrodes characterised by their material
  • H10H20/833—Transparent materials
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10H—INORGANIC LIGHT-EMITTING SEMICONDUCTOR DEVICES HAVING POTENTIAL BARRIERS
  • H10H20/00—Individual inorganic light-emitting semiconductor devices having potential barriers, e.g. light-emitting diodes [LED]
  • H10H20/80—Constructional details
  • H10H20/81—Bodies
  • H10H20/816—Bodies having carrier transport control structures, e.g. highly-doped semiconductor layers or current-blocking structures
  • H10H20/8162—Current-blocking structures
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10H—INORGANIC LIGHT-EMITTING SEMICONDUCTOR DEVICES HAVING POTENTIAL BARRIERS
  • H10H20/00—Individual inorganic light-emitting semiconductor devices having potential barriers, e.g. light-emitting diodes [LED]
  • H10H20/80—Constructional details
  • H10H20/81—Bodies
  • H10H20/819—Bodies characterised by their shape, e.g. curved or truncated substrates
  • H10H20/82—Roughened surfaces, e.g. at the interface between epitaxial layers
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10H—INORGANIC LIGHT-EMITTING SEMICONDUCTOR DEVICES HAVING POTENTIAL BARRIERS
  • H10H20/00—Individual inorganic light-emitting semiconductor devices having potential barriers, e.g. light-emitting diodes [LED]
  • H10H20/80—Constructional details
  • H10H20/83—Electrodes
  • H10H20/832—Electrodes characterised by their material
  • H10H20/835—Reflective materials

Definitions

• This invention relates to light emitting diodes (LEDs) and more particularly to new structures for enhancing the extraction of light from LEDs .
• LEDs are an important class of solid state devices that convert electric energy to light and generally comprise an active layer of semiconductor material sandwiched between two oppositely doped layers . When a bias is applied across the doped layers , holes and electrons are injected into the active layer where they recombine to generate light . Light is emitted omnidirectionally from the active layer and from all surfaces of the LED .
• LEDs formed of Group-Ill nitride based material systems because of their unique combination of material characteristics including high breakdown fields , wide bandgaps (3.36 eV for GaN at room temperature) , large conduction band offset, and high saturated electron drift velocity.
• the doped and active layers are typically formed on a substrate that can be made of different materials such as silicon (Si) , silicon carbide (SiC) , and sapphire (Al 2 O 3 ) .
• SiC wafers are often preferred because they ttave" a" TmTch closer crystal lattice match to Group-III nitrides , which results in Group III nitride films of higher quality.
• SiC also has a very high thermal conductivity so that the total output power of Group III nitride devices on SiC is not limited by the thermal resistance of the wafer (as is the case with some devices formed on sapphire or Si) . Also, the availability of semi insulating SiC wafers provides the capacity for device isolation and reduced parasitic capacitance that make commercial devices possible .
• SiC substrates are available from Cree Inc . , of Durham, North Carolina and methods for producing them are set forth in the scientific literature as well as in U . S . Patents , Nos . Re . 34 , 861 ; 4 , 946 , 547 ; and 5 , 200 , 022.
• TIR total internal reflection
• LEDs with SiC substrates have relatively low light extraction efficiencies because the high index of refraction of SiC (approximately 2.7 ) compared to the index of refraction for the surrounding material , such as epoxy (approximately 1.5) . This difference results in a small escape cone from which light rays from the active area can transmit from the SiC substrate into the epoxy and ultimately escape from the LED package .
• U. S . Patent No . 6 , 410 , 942 assigned to Cree Inc . , discloses an LED structure that includes an array of electrically interconnected micro LEDs formed between first and second spreading layers . When a bias is applied across the spreaders , the micro LEDs emit light . Light from each of the micro LEDs reaches a surface after traveling only a short distance, thereby reducing TIR .
• U. S . Patent No . 6 , 657 , 236 also assigned to Cree Inc . , discloses structures for enhancing light extraction in LEDs through the use of internal and external optical elements formed in an array.
• the optical elements have many different shapes , such as hemispheres and pyramids , and may be located on the surface of, or within, various layers of the LED .
• the elements provide surfaces from which light refracts or scatters .
• the invention is directed to LEDs having enhanced light extraction features .
• the LED includes a p-type layer of material with an associated p-contact , an n-type layer of material with an associated n-contact and an active region between the p-type layer and the n-type layer .
• the LED further includes a confinement structure that is formed within at least one of the p-type layer of material and the n-type layer of material .
• the confinement structure substantially prevents the emission of light from the area of the active region that is coincident with the area of the confinement structure .
• the LED also includes a roughened surface that is associated with one of the p-type and n-type layers of material .
• the LED includes a first layer of material with an associated first contact and first surface through which light is emitted, a second layer of material with an associated second contact and an active region between the first layer and the second layer .
• the LED further includes a confinement structure that is integral with one of the first layer and the second layer .
• the confinement structure is generally axially aligned with the first contact and substantially prevents the emission of light in the area of the active region that is coincident with the area of the confinement structure .
• the LED includes a first layer of material with an associated first contact and first surface through which light is emitted, a second layer of material , an active region between the first layer and the second layer, and a conducting substrate adj acent the second layer of material that has an associated substrate contact .
• the LED further includes at least one confinement structure tenatr is"' ⁇ ' ⁇ 'within one of the first layer, the second layer and the substrate .
• the confinement structure is generally axially aligned with the first contact and directs current flowing toward the active region away from the area of the active region that is coincident with the area of the confinement structure .
• the LED includes a first layer of material with an associated first contact and first surface through which light is emitted, a second layer of material with an associated second contact and an active region between the first layer and the second layer .
• the LED further includes a confinement structure that is associated with the second contact .
• the confinement structure directs current flowing toward the active region away from the area of the active region that is coincident with the area of the confinement structure .
• FIG . 1 is a cross section of one general embodiment of an LED including an active region between two layers of conducting material and a current confinement structure that may be located in either layer of the conducting material ;
• FIG . 2 is a cross section of another general embodiment of an LED including an active region between two layers of conducting material , a substrate , a roughened top surface and a current confinement structure that may be located in either layer of the conducting material or a substrate; ⁇ O 0 ⁇ 5J"FTCJ . "3' Is a cross section of a configuration of the LED of FIG. 1 , including a current confinement structure in a bottom layer of n-type material ;
• FIG . 4 is a cross section of a configuration of the LED of FIG. 2 , including a current confinement structure in a top layer of p-type material and a layer of transparent conducting material having a roughened top surface;
• FIG . 5 is a cross section of a configuration of the LED of FIG. 1 , including a current confinement structure in a bottom layer of p-type material and a layer of n-type material having a roughened top surface;
• FIG . 6 is a cross section of a configuration of the LED of FIG. 1 , including a current confinement structure in a top layer of n-type material and a layer of n-type material having a roughened top surface; and
• FIG . 7 is a cross section of another general embodiment of an LED including an active region between two layers of conducting material , a top side contact , a bottom side contact and a current confinement structure located in the layer of the bottom side contact .
• the present invention provides improved light extraction for light emitting diodes (LEDs) through a confinement structure that is formed within at least one of the p-type layer of material and the n-type layer of material of a base LED structure .
• the confinement structure is generally aligned with the contact on the top and primary emission surface of the LED and substantially prevents the emission of light from the area of the active region that is coincident with the area of the confinement structure and the top-surface contact .
• light that would otherwise emit under and be absorbed by fehS r 'tcp- ⁇ surface" contact is redirected to other regions of the active layer and the emitting side where the absorbing affect of the contact is substantially reduced.
• the current confinement structure is formed within the based LED structure using ion implantation.
• the current confinement structure may also be formed in the LED base structure using selective oxidation .
• the current confinement structure may also be formed as part of the LED structure using epitaxial regrowth .
• the LED may further include a roughed surface around the absorbing contact .
• the roughened surface may be included in all or a portion of the surface area of a layer of the base LED structure or in all or a portion of the surface area of an additional layer of material applied to the base LED structure .
• a layer of transparent material may be added to the p-type layer and roughen that layer .
• a layer of transparent material may also be added to the n-type layer of an n-side up LED structure .
• the combination of a roughened surface and current confinement structure that directs current toward the roughened surface and away from the absorbing contact provides further enhanced light extraction .
• the roughened surface improves light extraction by providing a varying surface that allows light that would otherwise be trapped in the LED by total internal reflection to escape and contribute to light emission.
• an LED 10 including a first layer of material 12 having a first surface 14 through which light is emitted, a secdhd"" ⁇ ayeT 'of" ma'E ' er ⁇ al 16 , and a layer of active material 18 sandwiched between the first layer and the second layer .
• the first layer 12 , second layer 16 and active layer 18 form a base LED structure that is positioned on a support structure 36.
• the base LED structure may be fabricated from different semiconductor material systems such as the Group III nitride based material systems .
• Group III nitrides refer to those semiconductor compounds formed between nitrogen and the elements in the Group III of the periodic table, usually aluminum (Al) , gallium (Ga) , and indium ( In) .
• the term also refers to ternary and tertiary compounds such as AlGaN and AlInGaN.
• the layer of active material 18 is in adjacent contact with the first layer 12 and the second layer 16 , and the material forming the first and second layers is GaN, with either of the first or second layers being p-type material and the other layer being n-type material .
• the material forming the active layer is InGaN.
• the first and second layer material may be AlGaN, AlGaAs or AlGaInP .
• the support structure 36 which may be a substrate or a submount .
• the support structure 36 would be a substrate, with a suitable substrate being a 4H polytype of silicon carbide, although other silicon carbide polytypes can also be used including 3C, 6H and 15R polytypes .
• Silicon carbide has a much closer crystal lattice match to Group III nitrides than sapphire and results in Group III nitride films of higher quality.
• Silicon carbide also has a very high thermal conductivity so that the total output power of Group III nitride devices on silicon carbide is not limited by the thermal dissipation of the substrate (as may be the case with some devices formed on sapphire) . Also, the availability of s ⁇ Picd ⁇ provides the capacity for device isolation and reduced parasitic capacitance that make commercial devices possible .
• SiC substrates are available from Cree, Inc . , of Durham, North Carolina and methods for producing them are set forth in the scientific literature as well as in a U . S . Patents , Nos . Re . 34 , 861 ; 4 , 946 , 547 ; and 5 , 200 , 022. For an n-side up LED configuration, the support structure 36 would be a submount .
• a first contact 22 is associated with the first layer 12 and a second contact 24 is associated with the second layer 16.
• the association of the contacts 22 , 24 with their respective layers 12 , 16 may be direct or indirect .
• a direct association wherein the first contact is in direct contact with the first layer 12 and the second contact 24 is in direct contact with the second layer 16 , is shown in FIG . 1.
• this association may be present when the substrate 36 is formed of a non-conducting material .
• Indirect associations are shown in FIG. 2 and may be present , with respect to the first contact 22 , if the LED includes a layer of transparent conducting material 25 , and for the second contact 24 , if the support structure 36 is a substrate formed of a conducting material .
• the second contact may be formed of a reflective material , such as silver (Ag) , aluminum (Al) or rhodium (Rh) .
• a current confinement structure 20 is integral with and can be formed in different location within the LED, such as within at least one of the first layer 12 , the second layer 16 or the substrate 36 (as shown in FIG. 2) .
• more than one current confinement structure may be used and in one embodiment the current confinement structure 20 may be formed in both the first layer 12 and the second layer 16 with a portion of the active material 18 between the confinement "structu ⁇ fesr" 411 S 1 OV Ih" ' : s" ⁇ m"e 'embodiments, the confinement structure
• the confinement structure 20 may be a region of the layer of material whose crystal structure or molecular properties have been altered through processes known in the art , such as ion implantation or oxidation .
• the confinement structure 20 may be a current blocking layer formed from a material that is oppositely doped relative to the material of the first or second layers 12 , 16. This current blocking layer of material may be incorporated into one or more of the first and second layers through the known process of epitaxial regrowth .
• the current confinement structure 20 is positioned relative to the first contact 22 such that the center 26 or axis of the first contact is generally aligned with the center or axis 28 of the confinement structure .
• the cross-sectional area size of the confinement structure 20 essentially mirrors that of the first contact 22.
• the thickness of the current confinement structure 20 may range anywhere from between 0.1% to 80% of the total thickness of the layer . For example, in an n-type layer of material 1 micron thick, the current confinement structure 20 may be between 0.001 and 0.8 microns thick .
• the current confinement structure 20 directs current 30 that is flowing toward the active region 18 away from the portion 32 of the active region that is substantially coincident with and aligned with the first contact 22. This redirection of current substantially prevents the recombination of current charges , i . e. , "holes” and “electrons , " in the portion 32 of the active region aligned with the first contact 22 , thus essentially rendering the region inactive .
• Light 34 is emitted from the active material 18 and propagates through the LED structure .
• light emits rrotn" CnS 'SCtlve material 18 in all directions for ease of illustration, light in the figures is shown only in the upward direction toward the top or primary emission surface of the LED .
• the top surface is the surface 14 of the top layer of material 12.
• the top surface is a layer of roughened material 25.
• one embodiment of the general LED of FIG . 1 is shown comprising a p-side up LED that includes a first layer 40 of p-type material and a second layer 42 of n-type material .
• the material is GaN.
• the current confinement structure 44 is incorporated into the n- type material layer 42.
• the structure 44 is formed by introducing impurities into the n-type material .
• the introduction of impurities may be done by ion implantation. For example, for an n-type GaN material , either Al or Ga ions may be implanted.
• the current confinement structure 44 may be positioned at any one of several locations along the depth of the n-type layer 42. This may be done by interrupting the growth process of the n-type layer 42 , implanting the impurities into the incomplete n-type layer and then resuming the growth process to complete the rest of the n-type layer .
• the growth process may be any one of various known processes including metal oxide chemical vapor deposition (MOCVD) , hybrid vapor phase epitaxy (HVPE) or molecular beam epitaxy (MBE) .
• An exemplary confinement structure formation process includes implanting the n-layer with 180 keV aluminum ions in doses of 10 13 , 10 14 and 10 15 cm "2 .
• the current confinement structure 44 is located close to active region 50 to effectively prevent n-type current from moving completely around the current confinement structure and back toward the inactive region 54 , and to increase the affect of the structure on the current in the p-type layer on the opposite side of the active region 50.
• FIG . 4 another embodiment of the general LED 10 of FIG . 2 according to the present invention is shown comprising a p-side up LED that is essentially the same as that described with reference to FIG . 3 , except the current confinement structure 44 is located in the p-type layer 40. Also, the substrate 58 is conducting thus allowing for an ifLcLxrecfi """association ' b ' etween the n-contact 48 and the n-type layer 42. A layer of transparent conducting material 56 is included on the p-type layer 40 with a portion of the conducting material layer sandwiched between the p-contact 46 and the p-type layer .
• This layer of material may be formed from ZnO, In 2 O 3 and indium tin oxide (ITO) . At least part of the conducting material layer 56 not covered by the p-contact 46 is roughened, with all of the top surface of the conducting material layer as shown in FIG . 4 being roughened. The combination of the layer of transparent conducting material 56 and localized light generation away from the absorbing contact 46 , provided by the current confinement structure 44 , increases the light extraction efficiency of the LED .
• ITO indium tin oxide
• FIG. 5 Another embodiment of the general LED 10 of FIG . 1 is shown, comprising an n-side up, flip-chip LED that includes a first layer 60 of n-type material and a second layer 62 of p-type material .
• the substrate which is typically adj acent to the first n-type layer 60 is removed to reveal the top primary emitting surface of the LED.
• the LED material is GaN.
• the current confinement structure 64 is incorporated into the p-type material layer 62 prior to flipping layers 60 , 62 , 64 and p-contact substructure onto a submount 78.
• the confinement structure 64 is formed by introducing impurities into the p-type material by ion implantation during growth. For example, for a p-type GaN material , either Al or Ga ions may be implanted.
• the top surface 66 of the n-type layer is roughened to form a roughened light extraction surface .
• the roughened surface may be provided by etching, using any one of several methods known in the art , such as photoelectrochemical (PEC) etching .
• PEC photoelectrochemical
• the roughened surface is added directly to ' the n-type layer instead of through a separately added layer of transparent conducting material , as is typically required for p-side up LEDs , due to the relative thinness of the p- layer .
• the current confinement structure 64 may be positioned at various locations along the depth of the p-type layer 62 by interrupting the growth process of the p-type layer, implanting the impurities into the incomplete p-type layer and then resuming the growth process to complete the layer .
• the current confinement structure 64 is located close to active region 72 to effectively prevent p- type current from moving completely around the current confinement structure and back toward the inactive region 76 , and to increase the affect of the structure on the current in the n-type layer on the opposite side of the active region 72.
• an LED 100 including a first layer of material 102 having a first surface 104 through which most of the light is emitted, a second layer of material 106 and a layer of active material 108 sandwiched between the first layer and the second layer .
• the layer of active material 108 is in adj acent contact with the first layer 102 and the second layer 106 , and the material forming the first and second layers is GaN and the material forming the active layer is InGaN.
• the first and second layer material may be AlGaN, AlGaAs or AlGaInP .
• a first contact 110 is associated with the first layer 102 and a second contact 112 is associated with the second layer 106.
• a current confinement structure 114 is included in the layer of the second contact 112 and is positioned relative to the first contact 110 such that the center 116 or axis of the first contact is generally aligned with the center or axis 118 of the confinement structure .
• the layer of the second contact 112 and the confinement structure 114 is formed by depositing a layer of contact material , etching away a portion of the layer of contact material and replacing it with the material of the confinement structure .
• the confinement structure 114 is formed of an insulating, non-conducting material , such as SiO 2 , AlN and SiN, and has a cross-sectional area size essentially the same as that of the first contact 110.
• the current confinement structure 114 directs current 120 that is flowing toward the active region 108 away from the portion 122 of the active region that is substantially coincident with and aligned with the first contact 110. This redirection of current substantially prevents the recombination of current charges , i . e. , "holes” and “electrons , " in the portion 122 of the active region aligned ' with ' t ' he " " "£irst contact 110 , thus essentially rendering the region inactive .
• the general LED of FIG. 7 may be formed such that the first layer 102 is either one of an n-type layer or a p-type layer and the second layer 106 is a type of layer opposite that of the first layer .
• the LED 100 may also include a surface roughening, either in the form of a roughened top surface of the first layer 102 or an additional layer of transparent conducting material having a roughened top surface .
• FIGs . 1-7 may be incorporated into other LED configurations .
• the LEDs of FIGs . 1-7 have contacts in a vertical arrangement , i . e. , on opposite sides of the LED
• the invention may be applied to LEDs having laterally arranged contacts, i . e. , on the same side of the LED, such as resonant cavity LEDs .

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• 2005
  • 2005-01-24 US US11/042,030 patent/US7335920B2/en not_active Expired - Lifetime
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