Patent Application: US-94086607-A

Abstract:
light emitting diodes where the emission region , usually a n layer , is structured for efficient light extraction , are disclosed . the structuring is designed for light extraction from thin films , such as a photonic crystal acting as a diffraction grating . in addition , the structuring controls the in - plane emission and allows new modes into which light will be emitted . various electrode designs are proposed , including zno structures which are known to lead to both excellent electrical properties , such as good carrier injection , and high transparency . alternatively , the n layer can be replaced by structures with other materials compositions , in order to achieve efficient light extraction .

Description:
in the following description of the preferred embodiment , reference is made to the accompanying drawings that form a part hereof , and in which is shown by way of illustration a specific embodiment in which the invention may be practiced . it is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention . for conciseness throughout this disclosure , “( al , ga , in ) n layer ” will refer to an ensemble of layers deposited or grown by any technique , for example , by mbe ( molecular beam epitaxy ), mocvd ( metalorganic chemical vapour deposition ) or vpe ( vapour phase epitaxy ), and usually comprising : a buffer layer grown on a substrate , one or more active layers such as quantum wells , quantum dots , barriers , or any other light emitting semiconductor layer , current blocking layers , contact layers , and other layers typically grown for a led ( light emitting diode ) and well known in the state of the art . it is also well known in the art that these layers may be adapted for various specific implementations , in particular they may be adapted for each desired wavelength range being emitted by the led . the present invention discloses designs for structures which retain a high emission rate as well as high internal efficiency and extraction . typically , the emission rate can be 60 - 80 % of the value for a non - structured high index material . fig1 a is a graph 100 of relative radiative rate as a function of relative active ( light emitting ) layer position within a low index layer , wherein the relative radiative rate is the radiative rate divided by the radiative rate for a reference high index sample . fig1 b is a schematic of an ( al , in , ga ) n led 102 having a ( al , in , ga ) n layer 104 containing a high index layer 106 , a low index layer 108 , and an active layer 110 , wherein the a high index layer 106 and low index layer 108 are referred to as index modulation layers . led 102 further contains growth buffer layer 112 ( on top of a substrate , which is not shown ). the relative position of the active layer 110 within the ( al , in , ga ) n layer 104 is shown in fig1 a and 1b as z s . accordingly , fig1 a and 1b shows the dipole emission 100 , representing the light emitted by a quantum well placed at various positions in a structure 102 comprising two layers — a high refractive index layer 106 with n = 2 . 5 and a low index layer 108 having a refractive index of n = 1 . 9 . as can be seen , for given positions of the emitting species 110 , the emission rate is about 80 % of the rate for dipoles placed in a high index medium 106 . fig2 is a schematic cross - section of an ( al , ga , in ) n led structure 200 comprising a top transparent electrode 202 , ( al , ga , in ) n layers 204 , which includes a structured emitting region / active layer 206 and a buffer layer 208 , an n contact 210 , a substrate 212 , and an optional backside mirror 214 . thus , fig2 is a schematic of a typical structure 200 for the present invention , comprising a gan based led that has been grown with the ( al , ga , in ) n layers 204 comprising an ensemble of layers 206 , 208 grown or deposited by any technique , for example , by mbe , mocvd or vpe . preferably , the structured emitting region / active layer 206 includes one or more active layers embedded within a plurality of index modulation layers , wherein the index modulation layers are structured as a photonic crystal with an embedded active layer . specifically , the index modulation layers comprise a lower index layer and a higher index layer , and the active layer is positioned inside the lower index layer . moreover , the structured emitting region / active layer 206 may be structured by etching , according to the description found in [ 24 ]. alternatively , the structured emitting region / active layer 206 may be structured by direct organized growth . a transparent p contact layer 202 is placed on the structured emitting region / active layer 206 , either by direct deposition , for example , pendeo or canteliver epitaxy , or by attachment of a thin material layer such as zno . any type of bonding can be used as long as it leads to good electrical contact while preserving good optical properties . since light is also emitted towards the substrate 212 , it can prove advantageous to use a mirror 214 on the substrate 212 to reflect any light upwards that has been emitted downwards . accordingly , light emission may be up or up and down in accordance with the invention . a variant of the fig2 implementation is shown in fig3 . fig3 is a schematic cross - section of an ( al , ga , in ) n led 300 comprising a top reflective p contact electrode 302 , an ( al , ga , in ) n layer 304 containing a structured emitting region / active layer 306 and a buffer layer 308 , an n contact 310 , and a substrate 312 . the ( al , ga , in ) n led 300 uses a top reflecting contact 302 for emission through the substrate 312 . both the substrate 312 , and first buffer layer 308 grown on the substrate 312 , induce some loss . it can be useful to detach the ( al , in , ga ) n layer 304 from the substrate 312 using any substrate removal technique , for example , using laser lift off ( llo ), dry etching or chemical etching , and then to thin down the buffer layer 308 . thus , in fig3 , light emission is directed downward ( and the led is typically mounted as a flip - chip led ). fig4 is a schematic cross - section of an ( al , ga , in ) n led 400 comprising a top reflective p electrode 402 , an ( al , ga , in ) n layer 404 containing a structured emitting region / active layer 406 , a buffer layer 408 , and a bottom n contact electrode 410 which is a side electrode , where the substrate has been removed and the ( al , ga , in ) n buffer region 408 has been ( optionally ) thinned down . fig5 is a schematic cross - section of an ( al , ga , in ) n led 500 comprising a top reflective p electrode 502 , an ( al , ga , in ) n layer 504 containing a structured emitting region / active layer 506 , a buffer layer 508 , and a bottom n contact electrode 510 placed at the bottom of the structure , where the substrate has been removed and the ( al , ga , in ) n buffer region 508 may optionally be thinned down . in view of the above , fig4 and 5 show finished structures 400 and 500 comprising the structured emitting region / active layer 406 and 506 , a top reflective contact 402 and 502 , and a side or bottom second contact 410 and 510 , where the substrate has been removed . thus , light emission is directed downward ( and the led is typically mounted as a flip - chip led ). in order to improve optical and electrical performance further , it can be useful to grow a gan layer on top of the structured emitting region / active layer , as illustrated in fig6 . in this regard , fig6 is a schematic cross - section of an led structure 600 comprising a transparent top p contact 602 , an ( al , ga , in ) n layer 604 containing structured emitting region / active layer 606 and a buffer layer 608 , an n contact 610 , a substrate 612 , an optional backside mirror 614 , and an intermediate gan layer 616 that has been regrown over the structured region 606 . layer 616 can be grown during the same growth sequence as the structured emitting region / active layer ( s ) 606 of the device , by changing growth conditions from columnar growth to coalesced growth , once the structured emitting region / active layer 606 has been grown to a desired thickness . if the structured emitting region / active layer 606 is obtained by etching , then the contact gan layer 616 is re - grown under conditions for coalescence in order to obturate the holes ( e . g ., from less than 100 nm to several microns ). in other variants , light can be emitted upwards using a transparent top electrode 602 or up and down . similar to fig6 , fig7 is a schematic cross - section of an ( al , ga , in ) n led 700 comprising a top reflective p contact electrode 702 , an ( al , ga , in ) n layer 704 containing a structured emitting region / active layer 706 and a buffer layer 708 , a bottom n contact electrode 710 , a substrate 712 , and an intermediate gan layer 716 that has been regrown over the structured emitting region / active layer 706 ( e . g ., from less than 100 nm to several microns ). in the embodiment of fig7 , light can be emitted downwards using the reflective top electrode 702 ( and the led is typically mounted as a flip - chip led ). fig8 is a schematic cross - section of an ( al , ga , in ) n led 800 comprising a top reflective or transparent p contact electrode 802 , an ( al , ga , in ) n layer 804 containing a structured emitting region / active layer 806 and buffer layer 808 , and a bottom electrode 810 positioned on the side of the structure 804 , where an intermediate gan layer 816 has been regrown ( e . g ., from less than 100 nm to several microns ) over the structured emitting region / active layer 806 , where the substrate has been removed and the ( al , ga , in ) n buffer region 808 may be optionally thinned down . similarly , fig9 is a schematic cross - section of an ( al , ga , in ) n led 900 comprising a top reflective or transparent p contact electrode 902 , an ( al , ga , in ) n layer 904 containing a structured emitting region / active layer 906 and buffer layer 908 , and a bottom n contact electrode 910 placed at the bottom of the structure 904 , where an intermediate gan layer 916 has been regrown ( e . g ., from less than 100 nm to several microns ) over the structured emitting region / active layer 906 , the substrate has been removed , and the ( al , ga , in ) n buffer region 908 may optionally be thinned down . the purpose of the structured sio 2 in the gan layer 916 is to make a current aperture by having an insulating region under the p - type electrode 902 facing the n - type electrode 910 , so that the hole current current from the p - side is injected sideways from the sio 2 , and the light emission from the structure will not be blocked by the n - type electrode 910 , as it will occur sideways from it . consequently , device performance can be improved by removing the substrate and thinning the buffer layer 808 and 908 , again with the two possibilities of top or bottom emission through the choice of top contact , for example , a transparent contact 802 , as shown in fig8 , or reflecting contact 902 , as shown in fig9 . fig1 is a schematic cross - section of an ( al , ga , in ) n led 1000 comprising a top reflective or transparent electrode p contact 1002 , an ( al , ga , in ) n layer 1004 containing a structured emitting region / active layer 1006 and buffer layer 1008 , where the substrate has been removed . the ( al , ga , in ) n buffer region 1008 may eventually be thinned down , and the bottom n contact electrode 1010 is placed at the side of the structure 1004 . in addition , the structured emitting region / active layer 1006 contains a light emitting species 1018 . further , the structuring of region 1006 is such that a strong localized optical mode occurs in some part of the region 1006 . fig1 is a schematic cross - section of an ( al , ga , in ) n led 1100 comprising a top reflective or transparent p contact electrode 1102 , an ( al , ga , in ) n layer 1104 containing a structured emitting region / active layer 1106 and buffer layer 1108 , a bottom n contact electrode 1110 placed at the side of the structure 1104 , where an intermediate gan layer 1116 has been regrown over the structured emitting region / active layer 1106 , and the substrate has been removed . the ( al , ga , in ) n buffer region 1108 may ( optionally ) be thinned down . in addition , the structured emitting region / active layer 1106 contains a light emitting species 1118 . finally , the structuring of region 1106 is such that a strong localized optical mode occurs in some part of the region 11106 . the implementations of fig1 and 11 can lead to special devices , for example , using non - regular patterns for the structured emitting region / active layer 1006 and 1106 . for example , a pillar with larger diameter ( e . g ., the middle pillar with the labeled light emitting species 1018 and 1118 ) could result in a strongly confined mode , thus leading to a large enhancement of the radiative recombination rate according to the purcell effect . this can be implemented in the various devices described above , for example , in the thinned structures with or without a gan contact layer 1116 , as illustrated in fig1 and 11 , respectively . the conductive transparent electrodes can be shaped to increase light extraction . alternatively , the whole structure may be placed in an environment such as epoxy , which also provides for increased light extraction . this environment may also be shaped for optimal light extraction . the environment can be doped with species absorbing led light and re - emitting at longer wavelengths , thus providing white light emission . a preferred transparent electrode comprises zno . however , the zno can be replaced by another material having similar characteristics , namely good transmission properties , high refractive index for efficient light extraction , and good electrical properties . examples of materials comprise , for instance , silicon carbide ( sic ) or indium tin oxide ( ito ). the contacts used here can be made by any technique , for example , epitaxy , bonding , or sputtering . eventually , in the case of bonding , it can be advantageous to use an ultrathin metal layer to improve the electrical characteristics . the transparent contact can be situated on either side of the device when there is no substrate . the active layers can be comprised of one or several quantum wells , or one or several layers of other emitting species such as quantum dots . the vertical position of the emitting species is optimized to obtain the maximum emission outside the structure . fig1 is a schematic cross - section of an ( al , ga , in ) n and zno direct wafer - bonded led structure 1200 comprising a reflective ( or transparent ) p contact electrode 1202 , an ( al , ga , in ) n layer 1204 containing a structured emitting region / active layer 1206 and buffer layer 1208 , and a bottom n contact electrode 1210 placed at the side of the structure 1204 . the upper part of the ( al , ga , in ) n layer 1204 has been grown above randomly intermediate patterned regions 1212 . the structuring can be associated with other types of structuring , leading to improved properties . thus , fig1 shows the association of a structure 1200 according to the present invention with a lateral epitaxial overgrowth ( leo ) grown structure , as described in u . s . provisional application ser . nos . 60 / 802 , 993 and 60 / 774 , 467 ; and u . s . utility application ser . nos . 11 / 067 , 957 and 11 / 067 , 910 ; which applications are listed in the cross - reference section above . the design of the pattern within patterned regions 1212 can be optimized in order to obtain directional emission , due to the peculiar emission properties of patterned structures . also , one can use the properties of patterned structures to obtain low threshold lasers and lasing emission in given directions . fig1 is a schematic cross - section of an ( al , ga , in ) n led structure 1300 comprising a reflective ( or transparent ) p contact electrode 1302 , an ( al , ga , in ) n layer 1304 containing a structured emitting region 1306 and buffer layer 1308 , and a bottom n contact electrode 1310 placed at the side of the structure 1304 . the holes 1312 in the structured region 1306 have been filled with some material such as dielectric , metal , semiconductor or any other optically active material such as light emitting polymers or dyes . thus , fig1 shows how the holes 1312 in the structured region 1306 can be filled with materials selected to obtain other desired properties . for example , dielectrics can reinforce the structures , metals can enhance emission through plasmon effects or improve carrier injection , and emitting species such as dye impregnated polymers , light emitting polymers , phosphors and other similar species can be used to obtain overall white light emission . the shape , size and other parameters can be varied . for example , the crystal parameters of a photonic crystal used in a second light extractor can be varied along the structure in order to provide position - dependent light extraction behaviour . the typical parameter for the structuring , i . e ., the photonic crystal lattice period if one uses periodic patterning , is chosen to satisfy the first order diffraction condition of about 100 nanometer ( nm ) period , or up to higher orders , such as a few hundred periods . less ordered structures can also be used , such as archimedean tilings or quasi - periodic structures , or even disordered structures . the concepts described herein for ( al , ga , in ) n based materials can be used for other materials , such as other inorganic materials such as semiconductors , or organic materials such as light - emitting small molecules or polymers . u . s . pat . no . 6 , 538 , 371 , issued mar . 25 , 2003 , to duggal et al ., and entitled “ white light illumination system .” [ 2 ] u . s . pat . no . 6 , 525 , 464 , issued feb . 25 , 2003 , to y - c . chin , entitled “ stacked light - mixing led .” [ 3 ] u . s . pat . no . 6 , 504 , 180 , issued jan . 7 , 2003 , heremans et al ., and entitled “ method of manufacturing surface textured high efficiency radiating devices and devices obtained therefrom .” [ 4 ] u . s . pat . no . 6 , 163 , 038 , issued dec . 19 , 2000 to chen et al . and entitled “ white light emitting diode and method of manufacturing the same .” [ 5 ] u . s . pat . no . 5 , 779 , 924 , issued jul . 14 , 1998 , to krames et al . and entitled “ ordered interface texturing for a light emitting device .” [ 6 ] u . s . pat . no . 5 , 362 , 977 , issued nov . 8 , 1994 , to hunt et al . and entitled “ single mirror light emitting diodes with enhanced intensity .” [ 7 ] u . s . pat . no . 5 , 226 , 053 , issued jul . 6 , 1993 , to cho et al ., and entitled “ light emitting diode .” [ 8 ] m . r . krames et al ., “ high - 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[ 24 ] s . keller , c . schaake , n . a . fichtenbaum , c . j . neufeld , y . wu , k . mcgroody , a . david , s . p . denbaars , c . weisbuch , j . s . speck , and u . k . mishra , “ optical and structural properties of gan nanopillar and nanostripe arrays with embedded ingan / gan multi quantum wells ,” journal of applied physics 100 , 054314 ( 2006 ). this concludes the description of the preferred embodiment of the present invention . the foregoing description of one or more embodiments of the invention has been presented for the purposes of illustration and description . it is not intended to be exhaustive or to limit the invention to the precise form disclosed . many modifications and variations are possible in light of the above teaching , without fundamentally deviating from the essence of the present invention . it is intended that the scope of the invention be limited not by this detailed description , but rather by the claims appended hereto .