Abstract:
An improved method of fabricating a semiconductor light emitting diode (LED) is disclosed. The current blocking layer and the contact area for the n-type layer are implanted at the same time. In some embodiments, a dopant, which may be an n-type dopant, is implanted into a portion of the p-type layer to cause that portion to become either u-type or n-type. Simultaneously, the same dopant is implanted into at least a portion of the exposed n-type layer to increase its conductivity. After this implant, the dopant in both portions of the LED may be activated through the use of a single anneal cycle.

Description:
FIELD 
     This invention relates to ion implantation of light emitting diodes (LEDs) and, more particularly, to ion implantation of LEDs to affect current spreading and contact doping. 
     BACKGROUND 
     LEDs are built on a substrate and are doped with impurities to create a p-n junction. A current flows from the p-side, or anode, to the n-side, or cathode, but not in the reverse direction. Electrons and holes flow into the p-n junction from electrodes with different voltages. If an electron meets a hole, it falls into a lower energy level and releases energy in the form of a photon. The wavelength of the light emitted by the LED and the color of the light may depend on the band gap energy of the materials forming the p-n junction. 
       FIG. 1  is a cross-sectional side view of an embodiment of a p-side up lateral LED. The LED  100  has an n-type layer  103  disposed on the substrate  104 . A multiple quantum well (MQW)  102  is disposed on the n-type layer  103  and the p-type layer  101  is disposed between the MQW  102  and a transparent-conductive layer (TCL)  113 . 
     One or more p electrodes  121  are disposed on the TCL  113 . Additionally, one or more n electrodes  106  are disposed on the n-type layer  103 . This LED  100  may be mounted on a metal alloy in one instance. The p-type layer  101  and n-type layer  103  may be, for example, GaN. The MQW  104  may be GaInN or AlGaInP. 
     One shortcoming of this configuration is that the current preferably flows directly toward the n electrodes  106 , as shown by arrows  109 . This means that light is generated in areas within the LED which are blocked by the p electrodes  121 . Electrodes, such as the p electrodes  121 , may be fabricated of metal and this metal may not be an efficient transparent material for light. Therefore, most light generated in the MQW  102  beneath the p electrode  121  will be reflected and eventually absorbed inside the LED  100 . This causes the input current versus light extraction ratio to be reduced, thereby reducing efficiency. 
     To overcome this shortcoming, a current blocking layer (CBL) may be used. The current blocking layer is disposed beneath the p electrode  121  to block current flow from the p electrode  121  to the p-type layer  101 .  FIG. 2A  shows a first embodiment of a CBL  114 . The CBL  114  is disposed above the p-type layer  101 , and forces the current to spread to either side of the CBL  114 , thereby reducing the amount of light that is generated directly beneath the p electrode. In some embodiments, the CBL  114  material is an insulating material such as SiO 2 , Si 3 N 4  or undoped GaN. This material may be deposited on the surface of the LED  100 , such as on the p-type layer  101 , using a mask to allow deposition only in the desired location. In another embodiment, the insulating material is deposited on the entire surface of the p-type layer  101 , and the unwanted material is then removed using a dry or wet etch process. 
     In another embodiment, shown in  FIG. 2B , the CBL  114  is created by implanting ions  119  into the p-type layer  101  to create CBL  114 . For example, argon or nitrogen may be implanted to create the non-conductive region represented by CBL  114 . Typically, a mask  116  is used to only allow the ions  119  to be implanted in the region where the CBL  114  is desired. 
     The use of a CBL  114  maximizes the amount of light that is produced which ultimately will be emitted outside of the LED  100 . 
     Unfortunately, the creation of the CBL  114  often requires additional process steps, which result in decreased throughout and increased cost. Therefore, it would be advantageous if the creation of the CBL  114  could be combined, or integrated with another process step to reduce manufacturing time and increase throughout. 
     SUMMARY 
     An improved method of fabricating a semiconductor LED is disclosed. The CBL and the contact area for the n-GaN region are implanted at the same time. In some embodiments, a dopant, which is an n-type dopant in GaN, is implanted into a portion of the p-type layer to cause that portion to become either u-type or n-type GaN. Simultaneously, the same dopant is implanted into at least a portion of the exposed n-type layer to increase its conductivity. After this implant, the dopant in both portions of the LED may be activated through the use of a single anneal cycle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present disclosure, reference is made to the accompanying drawings, which are incorporated herein by reference and in which: 
         FIG. 1  is a cross-sectional side view of the vertical LED structure of the prior art; 
         FIG. 2A  is a cross-sectional side view of a vertical LED structure having a CBL according to one embodiment in the prior art; 
         FIG. 2B  is a cross-sectional side view of the vertical LED structure having a CBL according to a second embodiment in the prior art; 
         FIG. 3  is a flowchart showing the processing steps used to manufacturing the LED according to one embodiment; and 
         FIGS. 4A-H  are cross-sectional side views of the LED during each of the processing steps according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The method is described herein in connection with ion implantation of LEDs. However, the method can be used with other semiconductor manufacturing processes. A beam-line ion implanter, plasma doping ion implanter, or other ion implantation system known to those skilled in the art may be used in the embodiments described herein. Thus, the invention is not limited to the specific embodiments described below. 
       FIG. 3  is a flowchart showing the sequence of processing steps that may be used to create the LED of the present disclosure. It should be realized that additional process steps may be added without departing from the spirit of the disclosure. 
       FIGS. 4A-H  show a sequence of cross-sectional views of a LED that correspond to the processing steps described in  FIG. 3 . As stated above, these process steps advantageous combine the creation of the CBL with the doping of the n-GaN contact region. 
     Referring to  FIGS. 3 and 4A , a LED is formed in step  300 . The LED has at least a p-type layer  430  disposed on a MQW  420 . The MQW  420  is disposed on an n-type layer  410 , which in turn is disposed on a substrate  400 . Various methods of creating such a LED are well known and will not be described herein. The p-type layer  430  and n-type layer  410  may be, for example, GaN, InGaN or AlGaInP. In some embodiments, a TCL is added on top of the p-type GaN layer  430 .  FIG. 4A  is a cross-sectional side view of such a LED structure. 
     A mesa etch mask  440  is disposed on the top surface of the LED structure, as shown in step  310 . In some embodiments, the etch mask  440  is disposed on a portion of the p-type GaN layer  430 , as shown in  FIG. 4B . In one embodiment, a photoresist or hard mask such as SiO 2  may be deposited on top of the LED surface as the etch mask  440 . A portion of this photoresist or hard mask is then removed to form the mesa window. In other embodiments, the etch mask  440  is disposed on the TCL, which is disposed on the p-type layer  430 . 
     An inductively coupled plasma (ICP) etch process is performed, as shown in step  320 . This etch may be, for example, a dry reactive ion etch (RIE). In other embodiments, a different etch process may be utilized, such as a wet etch using a KOH solution. The mesa etch mask  440  is used to protect a portion of the LED structure, while material from another portion of the LED structure is removed to expose the n-type GaN  410 . In other words, a portion of the p-type GaN layer  430  and the MQW  420  are removed, as shown in  FIG. 4C . Some of the n-type GaN  410  also may be removed during the etch. 
     The mesa etch mask  440  is then removed, as shown in step  330 . At this point, a portion of the n-type GaN layer  410  is exposed, as is the p-type GaN layer  430 , as shown in  FIG. 4D . 
     A CBL mask  450  is then deposited on the LED structure, as shown in step  340 . As best seen in  FIG. 4E , the CBL mask  450  has at least one opening disposed in the p-type GaN layer  430  and at least one opening disposed in the n-type GaN layer  410 . In some embodiments, the CBL mask  450  is deposited on the vertical sidewall  425  to prevent the implantation of ions in this area. In one embodiment, a photoresist or other mask material is deposited on the exposed top surface. An opening in the p-type GaN layer  430  and an opening in the n-type GaN layer  410  may be made using an etching process. 
     Ions  460  are then directed toward the LED structure, as shown in step  350 . The ions  460  only implant those portions of the LED structure without the CBL mask  450 , as shown in  FIG. 4F . The ions  460  used for this implant operate as an n-type dopant in the GaN layers. These ions thus serve to eliminate or reverse the conductivity of the implanted region in the p-type GaN layer  430 . In other words, the ions  460  change the implanted region into a u-type or n-type GaN region. Those ions  460  also implant the n-type GaN layer  410  and serve to increase the conductivity of the implanted region in that layer. 
     In other words, a single ion implantation serves to create two regions in the LED, a first region, which serves as a CBL  480 , in the p-type layer  430  and a second region, which is a highly conductive contact region  470 , in the n-type layer  410 , as shown in  FIG. 4G . Highly conductive contact region  470  serves to improve n-type GaN contact resistance, which reduces the forward voltage of the LED. In some embodiments, the ions  460  are silicon ions, although other species may also be used. 
     The use of silicon for the ion implant has several benefits. The use of silicon to form the u-type or n-type GaN CBL  480  in the p-type GaN  430  reduces the light absorption caused by material damage. Silicon atoms are larger than nitrogen atoms, but are smaller than argon atoms. Therefore, it may create more damage than nitrogen implantation and less damage than argon implantation. However, silicon induced damage can be much easier recovered by annealing, as compared with argon and nitrogen, in general. Thus, silicon can achieve the benefit from light absorption by reducing material damage via an anneal process while nitrogen and argon cannot achieve this result, even with added annealing. In addition, the creation of a region having u-type or n-type conductivity creates a p-n junction under the electrode. This makes use of the depletion layer in the p-n junction to block lateral current spreading under the CBL  480 . 
     As shown in step  360 , the CBL mask  450  is then removed. The completed LED substrate is shown in  FIG. 4H . After this processing step, other steps may be performed. In some embodiments, a TCL  487  is deposited on the exposed surfaces at this time. 
     After the implanted regions  470 , 480  are created, the p electrode  490  is added to the LED. In some embodiments, silver is applied to the p-type GaN layer  430 . Subsequent layers of metal, such as silver, can be applied using evaporation or electroplating. In operation, current travels through the p electrode  490  and then distributes laterally through the TCL  487  around the CBL  480 . The n electrode  491  can be added on contact region  470  using the same technique. 
     In addition, after the implanted regions  470 ,  480  are created, the regions may be activated through the use of a single anneal cycle. 
     While a CBL mask  450  is illustrated on the LED, other mechanisms for selectively implanting ions may also be used. For example, a stencil or shadow mask that is disposed a distance upstream of the LED may be utilized. In another embodiment, a selective implant system that does not use a mask (such as a focused ion beam) also may be used. 
     The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments of and modifications to the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Furthermore, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein.