Abstract:
A light emitting epi-layer structure which contains a temporality light absorption substrate on one side, the other side thereof can be adhered to a light absorption free transparent substrate in terms of a transparent adhesive layer which is light absorption free too. After that, the light absorption substrate portion is removed by means of an etching process. The resulted light emitting diode has significant improvement in light emitting efficiency. Moreover, the transparent conductive layer is a low resistance and high transparency layer. The current flow can thus be distributed evenly than conventional one.

Description:
[0001]    This application incorporates by reference Taiwanese application Serial No 90132394, filed on Dec. 26, 2001  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a light emitting device, and more particularly to an AlGalnP light emitting diode structure.  
           [0004]    2. Description of the Prior Art  
           [0005]    The conventional AlGaInP LED has a double heterostructure (DH), as shown in FIG. 8. The LED stacked sequentially, from a bottom thereof, has an n-type ohmic contact electrode  2 , a GaAs substrate  3 , an n-type (Al x Ga 1−x ) 0.5 In 0.5 P lower cladding layer  4  with an Al composition between about 70%-100%, an (Al x Ga 1−x ) 0.5 In 0.5 P active layer  5  with an Al composition of 0%-45%, a p-type (Al x Ga 1−x ) 0.5 In 0.5 P upper cladding layer  6  with an Al composition 70%-100%, a p-type high energy band gap current spreading layer  7  such as layers of GaP, GaAsP, AlGaAs or GalnP, and a p-type ohmic contact layer  8  as well as a bonding pad  9 .  
           [0006]    With the composition alternation of the active layer  5 , the wavelengths of the light emitted are varied from 650 nm: red to 555 nm: green. A drawback is generally found in the conventional LED, that is: while the light emitted from the active layer  5  towards the substrate  3  will be totally absorbed by GaAs substrate  3 . It is because the GaAs substrate has an energy gap smaller than that of the active layer  5 . Therefore, the light generated is absorbed resulted in lower light generated efficiency for this kind of conventional AlGaInP LED.  
           [0007]    To overcome the substrate  3  light absorption problem, several conventional LED fabrication technologies have been disclosed. However, those conventional technologies still accompany with several disadvantages and limitations. For example, Sugawara et al. disclosed a method published in Appl. Phys. Lett. Vol. 61, 1775(1992), Sugawara et al. inserted a distributed Bragg reflector (DBR) layer in between GaAs substrate and lower cladding layer so as to reflect those light emitted toward the GaAs substrate. However, the reflectivity of DBR layer is usefully only for those light which almost vertically towards the GaAs substrate. With the decrease of injection angle, the reflectivity is drastically decreased. Consequently, the improvement of external quantum efficiency is limited.  
           [0008]    Kish et al. disclosed a wafer-bonded transparent-substrate (TS) (Al x Ga 1−x ) 0.5 In 0.5 P/GaP light emitting diode, entitled “Very high efficiency semiconductor wafer-bonded transparent-substrate (Al x Ga 1−x ) 0.5 In 0.5 P/GaP light emitting diodes” on Appl. Phys. Lett. Vol. 64, No. 21, 2839 (1994). The TS AlGaInP LED was fabricated by growing a very thick (about 50 μm) p-type GaP window layer by hydride vapor phase epitaxy (HVPE) formed on epi-layers light emitting structure. Subsequently, the temporary n-type GaAs substrate is selectively removed using conventional chemical etching techniques. After removing the GaAs substrate, the LED epilayer structure is then bonded to an 8-10 mil thick n-type GaP substrate.  
           [0009]    For the light illuminated concerned, the TS AlGaInP LED exhibits a two fold improvement in light output compared to absorbing substrate (AS) AlGaInP LEDs. However, the fabrication process of TS AlGaInP LED is very complicated. Since the bonding process is to make two III-V semiconductor wafers directed bond together by heating and pressing for a period of time. Even worse, a non-ohmic contact interface between them is generally found to have high resistance. To manufacture these TS AlGaInP LEDs in high yield and low cost is difficult as a result.  
           [0010]    Another conventional technique was proposed by Horng et al., on Appl. Phys. Lett. Vol. 75, No. 20, 3054 (1999) entitled “AlGaInP light-emitting diodes with mirror substrates fabricated by wafer bonding.” Horng et al., reported a mirror-substrate (MS) of AlGaInP/metal/SiO 2 /Si LED fabricated by wafer-fused technology. In LED, AuBe/Au stack layer function as a bonding layer for silicon substrate and epi-layer LED. However, the intensity of the AlGaInP LED is only about 90 mcd under 20 mA injecting current. The light intensity is at least lower than that of TS AlGaInP LED by 40%. It could not be satisfied.  
         SUMMARY OF THE INVENTION  
         [0011]    An object of the present invention is thus to provide a LED structure which is composed a newly bonding layer and a transparent substrate.  
           [0012]    Firstly, a temporary semiconductor substrate having epi-layers thereon sequentially formed, from a bottom thereof, with an n-type etch stop layer, an n-type cladding layer, an active layer epi-layers, a p-type cladding layer, and a p-type ohmic contact epi-layer is prepared. And then a first metal electrode is formed on the p-type ohmic contact epi-layer.  
           [0013]    Thereafter the temporary semiconductor substrate is bonded to a transparent substrate with the p-type ohmic contact epi-layer and the first metal electrode face to the transparent substrate by a BCB, a transparent resin or the like. Next the temporary semiconductor substrate is removed by etching and stopping at the etch stop layer.  
           [0014]    After that, two steps of lithographic and etching methods are carried out successively so as to form an opening that exposes the first metal electrode. In the first lithographic and etching step, a trench of about 3 to 6 mils in width is formed, which exposes a portion of the p-type ohmic contact epi-layer. In the second lithographic and etching step, a contact channel of about 0.5 to 3 mils in width is formed to contact the first metal electrode. Thereafter, the processes are performed to form a transparent conductive layer atop the etch stop layer, to form a first boding metal on the contact channel, and a second boding metal (or called second electrode) on the transparent conductive layer.  
           [0015]    The second preferred embodiment is modified from the first preferred embodiment. The approaching of forming a trench and a contact channel are as the first preferred embodiment The modified portion is the second boding metal, which is refilled in a preserved hole constructed by photoresist and transparent conductive layer, Thus after the photoresist removal, the second bonding layer is higher than a surface level of the transparent conductive layer.  
           [0016]    In the third preferred embodiment, the two step etchings to form a trench and a contact channel are the same as prior two preferred embodiments. Thereafter a contact hole or a recess region is formed in the etch stop layer first, and then a transparent conductive layer is formed on the etch stop layer including refilled the contact hole or the recess region.  
           [0017]    In the fourth preferred embodiment, the two step etchings to form a trench and a contact channel are the same as before, A dielectric region is then formed in the etch stop layer. Thereafter the processes of forming the transparent conductive layer and two bonding metals are as the first prior embodiment.  
           [0018]    In the fifth preferred embodiment, the processes are modified from the fourth preferred embodiment. Instead of forming a dielectric region, a high resistance region is formed in the etch stop layer by ion implant with nonconductive ions. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:  
         [0020]    [0020]FIG. 1 is a schematic cross-sectional view of the light emitting diode before bonding with a transparent substrate according to the present invention.  
         [0021]    [0021]FIG. 2 is a schematic cross-sectional view of the transparent substrate coated with transparent adhesive layer according to the present invention.  
         [0022]    [0022]FIG. 3A to FIG. 3E are schematic cross-sectional views of a series of fabricating process for a light emitting diode according to the first preferred embodiment of the present invention.  
         [0023]    [0023]FIG. 4A to FIG. 4C are schematic cross-sectional views of a series of fabricating process for a light emitting diode according to the second preferred embodiment of the present invention.  
         [0024]    [0024]FIG. 5 is a schematic cross-sectional view of fabricating a light emitting diode according to the third preferred embodiment of the present invention.  
         [0025]    [0025]FIG. 6 is a schematic cross-sectional view of fabricating a light emitting diode according to the fourth preferred embodiment of the present invention.  
         [0026]    [0026]FIG. 7 is a schematic cross-sectional view of fabricating a light emitting diode according to the fifth preferred embodiment of the present invention.  
         [0027]    [0027]FIG. 8 is a schematic cross-sectional view of fabricating a light emitting diode according to the prior art. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0028]    The present invention disclosed a new LED structure and the making method. The detailed descriptions accompany with the FIG. 1 to FIG. 7 are as follows.  
         [0029]    Referring to FIG. 1, the cross-sectional view shows an epi-LED stack structure comprises, from a bottom thereof, an n-type temporary GaAs substrate  26 , an etching stop layer  24 , a lower cladding layer  22 , an active layer  20  an upper cladding layer  18 , a p-type ohmic contact epi-layer  16  and a p-type metal electrode  28 . The shape of the metal electrode  28  is arbitrary, shown in the figure is a ring shape, so two electrode blocks  28  are observed in a cross-sectional view.  
         [0030]    The lower cladding layer  22  is an n-type (Al x Ga 1−x ) 0.5 In 0.5 P. The active layer  20  is an undoped (Al x Ga 1−x ) 0.5 In 0.5 P layer and the upper cladding layer  18  is a p-type (Al x Ga 1−x ) 0.5 In 0.5 P layer.  
         [0031]    The p-type ohmic contact epi-layer  16  can be a layer selected from GaP, GaAsP, AlGaAs or GaInP. All of the candidates for serving as the p-type ohmic contact epi-layer  16  require having an, energy band gap higher than those of the active layer thereby alleviating the light absorption. Moreover, the p-type ohmic contact epi-layer  16  usually has high carrier concentrations doped therein so as to form a good ohmic contact. The (Al x Ga 1−x ) 0.5 In 0.5 P active layer  20  is with Al composition of about x=0 to 0.45. The Al dosage in the upper cladding layer  18  and lower cladding layer  22  is of about x=0.5 to 1.0. For situation of without Al containing, the wavelength of the light emitted from Ga 0.5 In 0.5 P LED is about 635 nm, which is in range of red visible light.  
         [0032]    As is known by skilled in the art, the ratio of forgoing compound is, for example of the preferred embodiment only, not intended to limit the claim scope. The invention is also applied to any ratio of the composition. Furthermore, the structure of active layer  20  can be a single hetero-structure (SH), a double hetero-structure (DH), or multiple quantum wells (MQW). For DH, it comprised: the n-type (Al x Ga 1−x ) 0.5 In 0.5 P lower cladding layer  22 , the (Al x Ga 1−x ) 0.5 In 0.5 P active layer  20  and the p-type (Al x Ga 1−x ) 0.5 In 0.5 P upper cladding layer  18 . The typical thickness of the layer  22 ,  20 , and  18  are respectively, between about 0.5-10 μm, 0.5-2.0 μm and 0.5-3.0 μm in thickness, The thicker of lower cladding layer  22  is with, the more evenly spread the current will be. Since, the lower cladding layer  22  does not absorb the light emitting from the active layer. Thus it does not affect the light emitting efficiency.  
         [0033]    The preferred material of the etching stop layer  24  according to the present invention can be any III-V compound semiconductor material that has an etching selectively to that of the GaAs substrate  26 . As to the lattice matched with that of the GaAs substrate  26  is not crucial. It&#39;s for sure, if the lattice matched well is also preferred because it can reduce the dislocation density. The good candidates of those satisfied above conditions, for examples, InGaP or AlGaAs can be served as the etch stop layer  24 . The lower cladding layer  22  can also be served as the etching stop layer  24  since it has a high selectivity to GaAs substrate  26 , and thus if the thickness of the lower cladding layer  22  is thick enough, the etch stop layer  24  becomes optional.  
         [0034]    Subsequently, a structure as shown in FIG. 2 is prepared. The structure comprises a transparent adhesive layer  14 , for example, a BCB (B-staged bisbenzocyclobutene; BCB) layer and a transparent substrate (TS)  10 . The material of the adhesive layer  14  is not limited to BCB. Any adhesive material with similar property, such as epoxy is also applicable to the invention. The transparent substrate  10  can be a substrate selected from glass, sapphire, SiC, GaP, GaAsP, ZnSe, ZnS, or ZnSSe. Other materials can also be chosen as the transparent substrate  10  as long as the light absorbed by the material is minor. One advantage of the present invention is that the transparent substrate  10  is not limited to be a single crystal substrate. The transparent substrate herein is used for supporting the LED epitaxial layer and avoids the LED epi-layers from breaking. In addition, the injected current does not need to flow through the transparent substrate  10 . In other words, either poly-crystal or amorphous crystal can be used as the TS  10 . Accordingly, the manufacture can be cost down.  
         [0035]    Thereafter, the epi-layer structure as shown in FIG. 1 is bonded together with the TS  10  by BCB layer  14 . The adhesion process is carried out at a temperature of about 250° C. with pressure and heat for a while. To improve the adhesion well, prior to coat a BCB layer  14  on the surface of the TS  10 , a step of coating an adhesion prompter on the surface of TS  10  can be optionally done. Alternatively, after epi-layers bonded with transparent surface by BCB, the thermal process can be firstly performed at 60 to 100° C. for a while to evaporate organic solvent away, and then heated and pressed at a temperature of about 200-600° C.  
         [0036]    Thereafter, the opaque n-type GaAs substrate  26  is then removed and stopped at the etching stop layer  24  by an etchant mixture, for example, 5H 3 PO 4 :3H 2 O 2 :3H 2 O or 1NH 4 OH:35H 2 O 2 .  
         [0037]    Referring to FIG. 3A, a first photoresist pattern  29  is then coated on the etch stop layer  24  to define a first trench  30 . A dry etching, for example, RIE (reactive ion etching), is then applied to sequentially remove the exposed portion of the etch stop layer  24 , the lower cladding layer  22 , the active layer  20  and upper cladding layer  18  and slightly etch the p-type ohmic contact epitaxial layer  16  so as to further remove a portion thickness thereof, as is shown in FIG. 3B. Two step levels are formed.  
         [0038]    After stripping the first photoresist pattern  29 , as is shown in FIG. 3B, a second photoresist pattern  32  having a strip of opening of about 0.5 to 3 mil is formed on the p-type ohmic contact epitaxial layer  16  so as to define a contact channel  33  therein to connect the p-type ohmic contact metal electrode  28 . Thereafter, a second dry etch is performed using the second photoresist pattern  32  as a mask to form a contact channel  33 .  
         [0039]    After etching is implemented, the second photoresist pattern  32  is removed, as is shown in FIG. 3C, a third photoresist pattern  34  is coated on the exposed sidewall surface and bottom surface of first trench  30 . Next, a lithographic process is performed to expose the surface of the etch stop layer  24 . Then, an n-type ohmic contact transparent electrode  35  is deposited on the etch stop layer  24  and on the third photoresist resist pattern  34 . The n-type ohmic contact transparent conductive layer  35  is selected from materials with properties of low resistance, and high transparent oxide layer such as a layer of indium tin oxide (ITO), indium oxide, tin oxide, zinc oxide or magnesium oxide. The thickness of ohmic contact transparent electrode  35  of about 100 Å to 10000 Å is preferred. Alternatively, a thin metal layer with 30 Å to 300 Å in thickness can replace for the ohmic contact transparent conductive layer  35 . The thin metal layer  35  can be chosen from Au, GeAu, Al, Ti, Ni and the combination thereof. The metal layer  35  is transparent for a layer with such thickness. Since the adhesion of the transparent oxide layer or thin metal layer  35  on the photoresist layer  34  is much weaker than those on the etch stop layer  24 , a lift off technique which is tapping an adhesive tap and then pull it up is thus easier to strip the weaker adhesive portions layer away. Thereafter, referring to FIG. 3D, a lithographic is carried out to coat a fourth photoresist pattern  37  on the resulting surfaces. The fourth photoresist pattern  37  includes opening  39 A and  39 B. The opening  39 A is slightly larger than the contact channel  33  to expose the metal electrode  28  and the opening  39 B is to define the position of electrode on the transparent conductive layer  35 . Thereafter, a meal layer  40  is deposited on the resulting surface by sputtering or by E-beam process as is shown in FIG. 3D. A lift off technique is then done to strip the portion of metal layer  40  on the fourth photoresist pattern  37  and then removes the residue fourth photoresist  37 , as is shown in FIG. 3E. Thus, the remnant metal layer  40 A is only left on the contact channel  33  and the metal layer  40 B on transparent conductive layer  35 .  
         [0040]    The method of second preferred embodiment according to the present invention is shown in FIG. 4A. After two etching steps are sequentially implemented for forming the first trench  30  and the contact channel  33  as the first preferred embodiment, a fifth photoresist pattern  42  is coated on a side wall, on a bottom of the first trench  30  and contains also a photoresist block  42 A on an etch stop layer  24  by lithographic method. A transparent oxide layer or thin metal layer is then formed on the resulting surface, including the exposed portion of the etch stop layer  24  and on the fifth photoresist layer  42 , the photoresist block  42 A. Then a lift off process is performed so that a transparent conductive layer  44  contains an opening refilled is formed on the etch stop layer  24 . Finally, as is shown in FIG. 4B a sixth photoresist pattern  46 , including an opening  45 A exposed contact channel  33 , and an opening  45 B exposed etch stop layer  24  on the transparent conductive layer  44 , is formed by coating and lithographic method. Next a metal layer  48  formed on all surfaces, a lift off process to strip away those of weak bonding portions and remnant photoresist removal are successively carried out. The result is shown in FIG. 4C, where the metal layer  48 A and  48 B are not only filled in the contact channel  33  and opening,  45 B, respectively but also have an altitude higher than the surface level of the transparent conductive layer  44 . Note, the contact between the metal layer  48 B and the transparent electrode  44  is of Shockley contact. Consequently, the metal layer  48 B is served as a current block so as to distribute the current evenly.  
         [0041]    The third preferred embodiment according to the present invention is shown in FIG. 5. Slightly different from two prior preferred embodiments, before forming the transparent electrode  55 , a lithographic and an etching step are sequentially performed to form a recessive region 50  in the etch stop layer  24  and expose the lower cladding layer  22  thereto. Then transparent conductive layer  52  and metal layer  55 A and  55 B are formed as second preferred embodiment. The transparent electrode  52  filled in the etch stop layer is to distribute the injunction current uniformly.  
         [0042]    The fourth preferred embodiment according to the present invention is shown in FIG. 6, which is modified from the third preferred embodiment. After forming a recess region  50  in the etch stop layer  24 , a dielectric layer  51  is refilled in the recession region  50 . The dielectric layer  51  is chosen from silicon dioxide silicon nitride or aluminum oxide. The dielectric layer  51  is a current block which makes the current flow distributed out of the second electrode  57 B. The successively steps of forming the transparent electrode  56  and the metal bonding layer  57 A and  57 B are similar to those steps of forming the metal bonding layer  40 A and the transparent electrode  35 .  
         [0043]    The fifth preferred embodiment according to the present invention is shown in FIG. 7, which is modified from the fourth preferred embodiment. Instead of forming a recess region and then refilled in by a dielectric layer  51 , the current block of a high resistance region  61  is formed by performing a lithographic process to pattern a region and then performing ion implantation with oxygen ions or hydrogen ions into the etch stop layer  24 . The transparent conductive layer  63  and the metal bonding layer  65 A and  65 B are formed as the same steps as depicted in the first preferred embodiment. Another approaching of forming high resistance region  61  is through diffusion of oxygen and hydrogen instead of ion implant.  
         [0044]    The power output of the AlGaInP four components LED, operated at 20 mA in accordance with the present invention is of about 4 mW which is about two times as light intensity as conventional LED including absorption substrate.  
         [0045]    While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.