Patent Publication Number: US-2011049468-A1

Title: Led and led display and illumination devices

Description:
FIELD OF INVENTION 
     The present invention relates to light emitting diodes (LEDs) and, more particularly, to light emitting unit cells and light emitting chips which recycle total internal reflection (TIR) light as a photocurrent source, and methods of forming the same. 
     BACKGROUND OF THE INVENTION 
     Light emitting diodes (LEDs) generally convert electrical energy to light, and are known to be used as light sources. For example, LEDs may be used in full-color displays, image scanners, optical communication systems and various signal systems. LEDs are generally formed from semiconductor materials and typically include an active layer of semiconductor material located 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. The light generated by the active region may be emitted in all directions and may escape from the LED through any exposed surfaces. The material of the active layer may be selected for emission of a particular wavelength of light. For example, gallium nitride (GaN) and zinc selenide (ZnSe) semiconductor materials may be used to emit green or blue light. Other examples of semiconductor materials include gallium phosphide (GaP) for green light, gallium arsenide phosphide (GaAsP) for yellow, orange and red light, and gallium aluminum arsenide (GaAlAs) for red light. 
     The efficiency of conventional LEDs may be limited by their inability to emit all of the light that is generated by the active layer. When an LED is energized, the light that is emitted from the active layer may reach the emitting surfaces/adjacent surfaces at many different angles. LEDs are typically formed from semiconductor materials having relatively high refractive indices (for example, a refractive index of about 2.2-3.8) compared to a refractive index of air (of about 1.0). According to Snell&#39;s law, light traveling from a region with a high index of refraction (the semiconductor material) to a region with a low index of refraction (air) that is less than a critical angle (relative to the surface normal direction) may propagate out of the LED. Light that reaches the surface at an angle greater than the critical angle does not pass, but instead experiences total internal reflection (TIR). Because of total internal reflection, much of the light generated by conventional LEDs is not emitted, thereby reducing the external quantum efficiency of the LED. 
     SUMMARY OF THE INVENTION 
     The present invention relates to light emitting chips and methods of forming light emitting chips. The light emitting chip includes a light emission structure comprising a p-type semiconductor layer, an n-type semiconductor layer and an active layer between the p-type semiconductor layer and the n-type semiconductor layer. The light emitting chip includes at least one light emitting unit comprising a light emitting diode (LED) portion formed from the light emission structure and a plurality of light receiving diode (LRD) portions formed from the light emission structure. The plurality of LRD portions are serially connected and configured to surround the LED portion. The plurality of LRD portions are optically coupled to the LED portion to receive total internal reflection (TIR) light from the LED portion and are configured to convert the TIR light to a photocurrent. 
     The present invention also relates to a light emitting unit cell comprising a first light emitting diode (LED) electrically connected to a power source, a plurality of light receiving diodes (LRDs) connected in series and a second LED. The plurality of LRDs are optically coupled to the first LED to receive total internal reflection (TIR) light from the first LED and are configured to convert the TIR light to a photocurrent. The second LED is electrically connected in parallel with the plurality of LRDs. 
     The present invention further relates to a light emitting unit cell comprising a light emitting diode (LED) electrically connected to a power source and a plurality of light receiving diodes (LRDs) connected in series. The plurality of LRDs are optically coupled to the LED to receive total internal reflection (TIR) light from the LED and are configured to convert the TIR light to a photocurrent. The plurality of LRDs feed back the photocurrent to the LED. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be understood from the following detailed description when read in connection with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawing are the following figures: 
         FIG. 1A  (Prior Art) is a top-plan view diagram of a conventional LED; 
         FIG. 1B  (Prior Art) is a cross-section diagram along lines  1 B- 1 B′ of the conventional LED shown in  FIG. 1A ; 
         FIG. 2  is a cross-section diagram of a portion of an LED illustrating reflection of light rays within the LED and propagation of light rays out of the LED; 
         FIG. 3  is a circuit diagram of an exemplary light emitting unit cell, according to an example embodiment of the present invention; 
         FIG. 4  is a top-plan view diagram of a structure of an exemplary light emitting unit cell shown in  FIG. 3 , according to an example embodiment of the present invention; 
         FIGS. 5A ,  5 B,  5 C,  5 D,  5 E,  5 F and  5 G are respective cross-section and top-plan views diagrams illustrating an exemplary method of forming a light emitting chip, according to an embodiment of the present invention; 
         FIG. 6  is a cross-section diagram along lines  6 - 6 ′ of the exemplary light emitting chip shown in  FIG. 5G , according to an embodiment of the present invention; 
         FIGS. 7A and 7B  are graphs of theoretical extraction efficiency for an exemplary light emitting chip for different conversion efficiencies, according to an embodiment of the present invention; 
         FIG. 8  is a circuit diagram of an exemplary light emitting unit cell, according to another embodiment of the present invention; 
         FIG. 9  is a top-plan view diagram of a structure of the light emitting unit cell shown in  FIG. 8 , according to another embodiment of the present invention; 
         FIGS. 10A ,  10 B,  10 C,  10 D,  10 E and  10 F are top-plan views diagrams illustrating an exemplary method of forming a light emitting chip, according to another embodiment of the present invention; 
         FIG. 11A  is a cross-section diagram along lines  11 A- 11 A′ of the exemplary light emitting chip shown in  FIG. 10F , according to another embodiment of the present invention; 
         FIG. 11B  is a cross-section diagram along lines  11 B- 11 B′ of the exemplary light emitting chip shown in  FIG. 10F , according to another embodiment of the present invention; 
         FIG. 12  is a graph of extraction efficiency for an exemplary light emitting chip for different conversion efficiencies, according to an exemplary embodiment of the present invention; and 
         FIG. 13  is a top-plan view diagram of an exemplary micro-pixelated LED, according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIGS. 1A and 1B , a conventional LED  100  is shown. In particular,  FIG. 1A  is a top-plan view diagram of conventional LED  100  and  FIG. 1B  is a cross-section diagram of LED along lines  1 B- 1 B′. 
     Conventional LED  100  includes substrate  110 , buffer layer  112 , n-type GaN layer  104 , active layer  114  containing a multi-quantum well (MQW) structure, p-type GaN layer  116  and transparent electrode  108 , which are sequentially laminated on substrate  110 . Transparent electrode  108  may be used, for example, to enhance a current spreading effect. 
     Portions of transparent electrode  108 , p-type layer  116  and active layer  114  may be removed by mesa-etching such that a portion of the upper surface of n-type layer  104  is exposed. A negative electrode (n-electrode)  102  is formed on the exposed upper surface of n-type layer  104 . A positive electrode (p-electrode)  106  is formed on an upper surface of transparent electrode  108 . 
     In active layer  114 , electrons and holes are recombined so as to generate and emit light. The MQW structure of active layer  114  is formed by alternately laminating well layers and barrier layers (not shown). The well layer includes a semiconductor layer with a smaller band gap than n-type layer  104 , p-type layer  116 , and the barrier layer, thereby providing quantum wells in which electrons and holes may be recombined. 
     Referring to  FIG. 2 , a portion of an LED  200  is shown. LED  200  includes n-type layer  208 , active layer  210 , p-type layer  212  and transparent electrode  214 .  FIG. 2  illustrate the propagation of light generated from active layer  210 . Light generated by active layer  210  may propagate out of LED  200  as light ray  202  or light ray  204 , for angles less than the critical angle (relative to the surface normal direction). The remainder of light rays  206  have an angle greater than the critical angle, and experience TIR. In general, most of the generated light inside of active layer  210  may be trapped inside LED  200  due to total internal reflection, without escaping outside to be extracted. Accordingly, the extraction efficiency of conventional LEDs tends to be poor (for example, about 40%). 
     One conventional technique to improve the extraction efficiency is related to ray redirection using, for example, surface roughening, gratings and volume holograms to circumvent TIR. However, these techniques tend to improve the extraction by no more than about 60% from 40% (which is only a 50% increase in efficiency). 
     In general, LED efficiency may include an internal quantum efficiency, an extraction efficiency and an external quantum efficiency (also referred to herein as power efficiency). The internal quantum efficiency, extraction efficiency and external quantum efficiency may be defined by respective equations (1)-(3), below as: 
     
       
         
           
             
               
                 
                   
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     Because the external quantum efficiency (power efficiency) is the product of internal quantum efficiency (eq. 1) and extraction efficiency (eq. 2), the external quantum efficiency may be improved by improving either the internal quantum efficiency or the extraction efficiency. 
     As shown in  FIG. 2 , light ray  202  is emitted from a top surface of transparent electrode  214  and propagates out of LED  200 . Light ray  204  also propagates from a lateral surface of LED  200 , such as through active layer  210  and/or p-type layer  212 . According to aspects of example embodiments of the present invention, exemplary light emitting unit cells includes a plurality of light receiving diodes (LRDs) optically coupled to the LED to absorb TIR light rays  206  from the LED. The LRDs may convert the absorbed light to a photocurrent. The light emitting unit cell may use the photocurrent to power a further LED or may feed the photocurrent back to the LED. Accordingly, by reusing the TIR light as an applied photocurrent, the current-voltage (I-V) characteristics of the exemplary light emitting unit cell may be improved, such that a lower current may be used to obtain a same light output. Exemplary light emitting unit cells of the present invention may theoretically achieve as much as 80% extraction efficiency, resulting in a 100% increase in extraction efficiency relative to a conventional LED. 
     Referring next to  FIG. 3 , a circuit diagram of an example light emitting unit cell  300  is shown. Light emitting unit cell  300  includes first LED  304  connected in parallel with power source  302 . Light emitting unit cell  300  also includes a plurality of light receiving diodes (LRDs)  306  which are optically coupled to first LED  304  and electrically connected to second LED  308 . 
     LRDs  306 - 1 ,  306 - 2 ,  306 - 3 ,  306 - 4  are connected to each other in series. Anode p 4  of LRD  306 - 4  is electrically connected to anode p 5  of second LED  308 . Cathode n 1  of LRD  306 - 1  is electrically connected to cathode n 5  of second LED  308  and cathode n 0  of first LED  304 . 
     In operation, first LED  304  is powered by power source  302  and second LED  308  is powered by LRDs  306  (i.e., LRDs  306  supply current and voltage to second LED  308  for light emission). LRDs  306  may absorb light trapped inside the layers of first LED  304  (i.e., due to TIR) and convert the absorbed light to a photocurrent. LRDs  306  may be configured to absorb the TIR light without emitting light. Thus, each of LRDs  306  may act as a photodiode. Accordingly, light emitting unit cell  300  may recycle photocurrent that would be lost due to TIR and apply the photocurrent to power second LED  308 . 
     Although four LRDs  306  are shown in  FIG. 3 , it is understood by the skilled person that light emitting unit cell  300  may include two or more LRDs  306  configured to absorb TIR light from first LED  304 , to provide a suitable photocurrent for powering second LED  308 . Because second LED  308  may also lose a portion of the respective light emission due to TIR, further LRDs  306 ′ and a third LED  308 ′ (shown in phantom) may also be included in light emitting unit cell  300 , coupled to second LED  308 . 
     Although one light emitting unit cell  300  is shown in  FIG. 3 , an LED chip (described below with respect to  FIGS. 5A-5G ) may include a plurality of unit cells (such as described below with respect to  FIG. 13 ), which is referred to herein as a micro-pixelated LED. 
     Referring to  FIG. 4 , a top-plan view diagram of a structure  400  of light emitting unit cell  300  is shown. Structure  400  illustrates the layout and electric connections of unit cell  300  of an example LED chip. First LED  304  is powered by a system power source through anode p 0  and cathode n 0 . Cathode n 0  is formed on a fabricated mesa-etched part of an n-type layer (for example of GaN) and is connected to the anode side of the power source. 
     A first LRD,  306 - 1 , is connected to first LED  304  at cathodes n 0 , n 1 . Cathode n 1  is the n-side of LRD  306 - 1  which is the same as cathode n 0 . A p-side of LRD  306 - 1 , at anode p 1 , is connected to the n-side of LRD  306 - 2 , at cathode n 2 , to form a series connection. LRDs  306 - 2 ,  306 - 3 ,  306 - 4  are similarly connected to each other. Because a p-type layer is formed as an upper layer and an n-type laser is formed as a lower layer, non-planar contacts  402  are provided for serial connection of LRDS  306 - 1 ,  306 - 2 ,  306 - 3 ,  306 - 4 . 
     The p-side of LRD  306 - 4 , at anode p 4 , is in contact with the p-side of second LED  308 , at anode p 5 . The n-side of LED 2 , at cathode n 5 , is connected to cathodes n 0 , n 1 . Because the photocurrent received from LRDs  306  may be a fraction of the power supplied to first LED  304 , it may be desirable for second LED  308  to be formed with an area that is smaller than first LED  304 . 
     As shown in  FIG. 4 , LRDs  306  are formed proximate to first LED  304  such that LRDs  306  surround first LED  304 , in order to receive TIR light from first LED  304 . In general, LRDs  306  are formed from a same light emission structure (described below with respect to  FIGS. 5A-5G ) used to form first and second LEDs  304 ,  308 . Because the LRD&#39;s  306  are connected in series and the series connected LRD&#39;s  306  are connected in parallel with second LED  308 , the potential across any of the LRD&#39;s  306  cannot be sufficient to cause the LRD to emit light. Thus, each of the LRDs acts as a photodiode, converting received light into an electrical current. 
     Referring to  FIGS. 5A-5G , an exemplary method of forming light emitting chip  500  is shown. As shown in  FIG. 5A , buffer layer  504 , n-type semiconductor layer  506 , active layer  508  and p-type semiconductor layer  510  are sequentially grown on substrate  502 , to form light emission structure  501 . 
     Substrate  502  may be formed of a transparent material such as sapphire (for example with a (0001) plane orientation). Substrate  502  may be formed from other materials including, but not limited to, silicon carbide (SiC), GaN or magnesium aluminum oxide (MgAlO 2 ). 
     Buffer layer  504  may be used to enhance a lattice matching between substrate  502  and n-type semiconductor layer  506 . Buffer layer  504  may be omitted depending on a process condition and diode characteristic. According to an exemplary embodiment, buffer layer  504  may be formed from about a 10 nm thickness undoped GaN semiconductor layer on a (0001) surface of sapphire substrate  502  for the lattice matching. In addition to GaN, buffer layer  504  may be formed from, but not limited to, (undoped) GaN, aluminum nitride (AlN), aluminum gallium nitride (AlGaN) or aluminum indium nitride (AlInN). 
     N-type semiconductor layer  506  and p-type semiconductor layer  510  may be formed of semiconductor materials including, but not limited to, Al m Ga 1-m N (for 0≦m≦1), to emit blue light. According to an example embodiment, n-type semiconductor layer  506  of about 2 μm thickness may be grown from GaN semiconductor material doped with n-type conductive impurities, such as silicon (Si) or germanium (Ge). According to an example embodiment, p-type semiconductor layer  510  of about 200 nm thickness may be grown from GaN semiconductor material doped with p-type conductive impurities, such as magnesium (Mg), zinc (Zn) or beryllium (Be). 
     To produce light emitting chips for emitting other colors, n-type and p-type semiconductor layers  506 ,  510  may be formed from different materials. For example, to emit blue light: ZnSe or indium gallium nitride (InGaN) may be used. To emit green light: InGaN, GaP, aluminum gallium indium phosphide (AlGaInP) or aluminum gallium phosphide (AlGaP) may be used. To emit yellow light: gallium arsenide phosphide (GaAsP), AlGaInP or GaP may be used. To emit orange light: GaAsP, AlGaInP or GaP may be used. To emit red light: aluminum gallium arsenide (AlGaAs), GaAsP, AlGaInP or GaP may be used. 
     Active layer  508  may include a single quantum well (SQW) structure or a MQW structure. According to an exemplary embodiment, active layer  508  of about 50 nm total thickness may be formed from alternating layers of InGaN/GaN. Active layer  508  may be formed of semiconductor material including, but not limited to, In m Al n Ga 1-m-n N (for 0&lt;m≦1, 0≦n≦1, 0&lt;m+n≦1) or In m Ga 1-m N (for 0&lt;m&lt;1). Active layer  508  may be omitted depending on a desired process condition and a desired diode characteristic. According to another embodiment, active layer  508  may be omitted for portions of the light emitting structure corresponding to the LEDs or to the LRDs. According to a further embodiment, light emitting structure  501  may be formed with different active layer materials for the portions corresponding to the respective LEDs and LRDs. 
     Buffer layer  504 , n-type semiconductor layer  506 , active layer  508  and p-type semiconductor layer  510  may be grown by using any suitable deposition process, including, but not limited to, metal organic chemical deposition (MOCVD) or molecular beam epitaxy (MBE). 
     As shown in  FIG. 5B , after forming light emitting structure  501 , insulating pattern  512  may be formed on p-type semiconductor layer  510 , for example, by any suitable photolithographic technique. 
     According to an exemplary embodiment, a silicon dioxide (SiO 2 ) film (not shown) may be deposited on the p-type semiconductor layer  510  layer, for example, by chemical vapor deposition (CVD). A photoresist (not shown) may be subsequently spun on the SiO 2  film. A binary chromium (Cr) mask with a desired insulating pattern may be applied for patterning the photoresist. The photoresist may be exposed to ultraviolet (UV) light by a mask aligner or stepper and may be subsequently developed by a developer. The SiO 2  film may be etched through the photoresist by, for example, reactive ion etching (RIE) with insulating pattern  512 . 
     The patterned SiO 2  film may be used as a mask for a GaN full etching process. All layers of light emitting structure  501  may be etched via the SiO 2  mask through to substrate  502 . The SiO 2  film may be subsequently removed after the etching process is completed. 
     As shown in  FIG. 5C , a GaN mesa-etching process is performed. According to an exemplary embodiment, a similar photolithographic method as described for the full etching process (shown in  FIG. 5B ) may be used for n-GaN mesa-etching. For example, a thickness of about 0.55 μm may be etched by RIE from a top of n-type semiconductor layer  506  on predetermined portions. The n-GaN mesa-etching may be performed to securely supply free electrons from n-type layer  506  to p-type layer  510  through the interface between the two layers. 
     The patterns for n-GaN mesa-etching may include: an entire n-electrode portion  514 , a portion  516  of a first LED portion  520 , and a portion  518  of each LRD portion  522 . On each portion ( 514 ,  516 ,  518 ), a top 0.55 μm thickness may be etched away such that the n-type layer  506  is partially exposed. 
     As shown in  FIG. 5D , a transparent electrode layer  528  may be deposited by any suitable process such as sputtering, CVD and evaporation. According to an exemplary embodiment, transparent electrode layer  528  is formed with a thickness of about 200 nm. Transparent electrode layer  528 , may be formed from, for example, indium tin oxide (ITO), titanium (Ti), gold (Au), a combination of Ti and Au, tin oxide (SnO) or zinc oxide (ZnO). Transparent electrode layer  528  may be deposited and patterned on predetermined portions by any suitable photolithography process. 
     Transparent electrode layer  528  may be deposited to cover p-electrode portion  526 , first LED portion  520  (except for the mesa-etched portion  516 ), LRD portion  522  (except for mesa-etched portion  518 ), and second LED portion  524 . Transparent electrode layer  528  is desirably formed to be substantially transparent to light having a predetermined wavelength. Transparent electrode layer  528  may be formed so that light escaping from the underlying layer (p-type layer  510 ) may be effectively extracted and so that electrons are spread over transparent electrode layer  528  from p-type layer  510  to p-electrode portion  526 . P-electrode portion  526  may also be formed from any suitable metallic materials, such as Au, copper (Cu), a combination of platinum (Pt) and Au, a combination of nickel (Ni) and Au or a combination of chromium (Cr) and Au, for example by sputtering, CVD and evaporation. 
     As shown in  FIG. 5E , n-electrode portion  514  may be formed, for example, by any suitable photolithography process on the mesa-etched part of n-type layer  506 , to provide an ohmic contact. According to an exemplary embodiment, n-electrode portion  514  is formed with a thickness of about 20 nm. The ohmic contact desirably includes a linear I-V curve and low resistance. N-electrode portion  514  may be formed from any suitable metallic materials, such as Au, Cu, a combination of Pt and Au or a combination of Ni and Au, for example by sputtering, CVD and evaporation. Alternatively, ITO may be used as n-electrode portion  514 , in which case the patterning of n-electrode portion  514  may be performed together with the transparent electrode patterning ( FIG. 5D ). 
     As shown in  FIG. 5F , passivation layer  530  may be formed to provide insulation between n-type layer  506  and p-type layer  510  at the series connection of LRD portions  522 . Passivation layer  530  may help to prevent an electrical short circuit when bridge contact layer  532  ( FIG. 5G ) is formed between p-type layer  510  and an adjacent mesa-etched part of n-type layer  506 . Passivation layer  530  may be formed, for example, from SiO 2  by a sputtering, CVD and/or an evaporation process. 
     As shown in  FIG. 5G , bridge contact layer  532  is then formed to provide an electrical connection between p-type layer  510  on one side of LRD portion  522  and the mesa-etched n-type layer  506  (portion  518 ) on a side of an adjacent LRD portion  522 , thus forming light emitting chip  500 . Any metallic material, such as Au, Cu or Ni, may be used for bridge contact layers  532 . Bridge contact layer  532  may be formed by any suitable process such as electroless/electro plating or sputtering and CVD. 
     Referring to  FIG. 6 , cross-section diagram along lines  6 - 6 ′ of light emitting chip  500  is shown. Bridge contact layer  532  connects the mesa-etched n-type layer  506  on one side of one LRD  522  to a p-type layer  510  of an adjacent LRD  522 . Bridge contact layer  532  is formed over passivation layer  530 . Accordingly, photo-electrons created in one LRD  522  travel from n-type layer  506  to a p-type layer  510  of an adjacent LRD  522 ′ through bridge contact layer  532 . 
     Next, a theoretical extraction efficiency (η extr ) for light emitting unit cell  300  ( FIG. 3 ) is compared with a conventional extraction efficiency (η o ) for number m of LEDs  304 ,  308  (where m is an integer). For m LEDs  304 ,  308 , extraction efficiency η extr  is shown in equation (4) as: 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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                               α 
                                
                               
                                 ( 
                                 
                                   1 
                                   - 
                                   
                                     η 
                                     o 
                                   
                                 
                                 ) 
                               
                             
                           
                           + 
                           
                             … 
                              
                             
                                 
                             
                              
                             
                               η 
                               o 
                             
                              
                             
                               
                                 
                                   α 
                                   
                                     m 
                                     - 
                                     1 
                                   
                                 
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                                   ( 
                                   
                                     1 
                                     - 
                                     
                                       η 
                                       o 
                                     
                                   
                                   ) 
                                 
                               
                               
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                                 - 
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                         = 
                         
                           
                             η 
                             o 
                           
                            
                           
                             
                               1 
                               - 
                               
                                 
                                   
                                     α 
                                     m 
                                   
                                    
                                   
                                     ( 
                                     
                                       1 
                                       - 
                                       
                                         η 
                                         o 
                                       
                                     
                                     ) 
                                   
                                 
                                 m 
                               
                             
                             
                               1 
                               - 
                               
                                 α 
                                  
                                 
                                   ( 
                                   
                                     1 
                                     - 
                                     
                                       η 
                                       o 
                                     
                                   
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                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     where the conversion efficiency (α) is the respective efficiencies for converting light to a photocurrent within LRDs  306 . 
     A summary of theoretical extraction efficiencies under different conversion efficiencies (α=0.8 and α=1.0) and a different number of LEDs  304 ,  308  (m=2, m=3) are shown in Table 1 below. Referring to  FIGS. 7A and 7B , graphs are shown which summarizing the theoretical extraction efficiencies of Table 1. In particular,  FIG. 7A  shows the extraction efficiencies for α=0.8 and  FIG. 7B  shows the extraction efficiencies for α=1.0. As shown in  FIGS. 7A and 7B  and Table 1, the extraction efficiencies are improved by a ratio between about 1.4-2.3 as compared with the conventional extraction efficiency. In general, the extraction efficiency increases with an increasing number of LEDs  304 ,  308 . 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Theoretical Extraction Efficiency for Light Emitting Unit Cell 300 
               
            
           
           
               
               
               
               
               
            
               
                   
                 m 
                 α 
                 η o   
                 η extr   
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 2 
                 0.8 
                 0.25 
                 0.4 
               
               
                   
                   
                   
                 0.5 
                 0.7 
               
               
                   
                   
                 1.0 
                 0.25 
                 0.4375 
               
               
                   
                   
                   
                 0.5 
                 0.75 
               
               
                   
                 3 
                 0.8 
                 0.25 
                 0.49 
               
               
                   
                   
                   
                 0.5 
                 0.78 
               
               
                   
                   
                 1.0 
                 0.25 
                 0.5781 
               
               
                   
                   
                   
                 0.5 
                 0.8750 
               
               
                   
                   
               
            
           
         
       
     
     Referring next to  FIG. 8 , a circuit diagram of an exemplary light emitting unit cell  800  is shown. Light emitting unit cell  800  includes LED  804  connected in parallel with power source  802 . Light emitting unit cell  800  also includes a plurality of light receiving diodes (LRDs)  806  which are optically coupled to LED  804  and are electrically connected in parallel with LED  804 . 
     LRDs  806 - 1 ,  806 - 2 ,  806 - 3 ,  806 - 4  are connected to each other in series. Anode p 4  of LRD  806 - 4  is electrically connected to anode p 0  of LED  804 . Cathode n 1  of LRD  806 - 1  is electrically connected to cathode n 0  of LED  804 . Because LRDs  806  are serially connected, each LRD  806  is powered by a fraction of the received voltage (based on the number of LRDs  806 ). Consequently, none of the LRD&#39;s can emit light. Instead, each of the LRD&#39;s  806  operates as a photodiode. 
     In operation, LED  804  is powered by power source  802 . LRDs  806  may absorb light trapped inside the layers of LED  804  (i.e., due to TIR) and convert the absorbed light to a photocurrent. The photocurrent generated in LRDs  806  is fed back to LED  804 , to supply a photocurrent to light emitting unit cell  800 . Because LRDs  806  are serially connected (and receive a fraction of the voltage), LRDs  806  may be configured to absorb the TIR light without emitting light. Accordingly, light emitting unit cell  800  may recycle photocurrent that would be lost due to TIR and apply the photocurrent to further power light emitting unit cell  800 . 
     Although four LRDs  806  are shown in  FIG. 8 , it is understood by the skilled person that light emitting unit cell  800  may include two or more LRDs  806  configured to absorb TIR light from LED  804 , to feed back a suitable photocurrent to LED  804 . 
     Although one light emitting unit cell  800  is shown in  FIG. 8 , an LED chip (described below with respect to  FIGS. 10A-10F ) may include a plurality of unit cells (such as described below with respect to  FIG. 13 ), which is referred to herein as a micro-pixelated LED. 
       FIG. 9 , shows a top-plan view diagram of a structure  900  of light emitting unit cell  800 . Structure  900  illustrates the layout and electric connections of unit cell  800  of an example LED chip. LED  804  is powered by a system power source through anode p 0  and cathode n 0 . Cathode n 0  is formed on a fabricated mesa-etched part of an n-type layer (for example of GaN) and is connected to the anode side of the power source. 
     A first LRD,  806 - 1 , is connected to LED  804  at respective cathodes n 0 , n 1 . A p-side of LRD  806 - 1 , at anode p 1 , is connected to the n-side of LRD  806 - 2 , at cathode n 2 , to form a series connection. LRDs  806 - 2 ,  806 - 3 ,  806 - 4  are similarly connected to each other. Because a p-type layer is formed as an upper layer and an n-type laser is formed as a lower layer, non-planar contact  902  is provided for serial connection of LRDs  806 - 1 ,  806 - 2 ,  806 - 3 ,  806 - 4 . 
     The p-side of LRD  806 - 4 , at anode p 4 , is in contact with the p-side of LED  804 , at anode p 0 , and a cathode of the system power source. The n-side of LRD  806 - 1 , at cathode n 1 , is also connected with an anode of the system power source. 
     As shown in  FIG. 9 , LRDs  806  are formed proximate to LED  804  such that LRDs  806  surround LED  804 , in order to receive TIR light from LED  804 . In general, LRDs  806  are formed from a same light emission structure (described below with respect to  FIGS. 10A-10F ) used to form LED  804 . 
     Referring to FIGS.  5 A and  10 A- 10 F, an exemplary method of forming light emitting chip  1000  ( FIG. 10F ) is shown. In general, light emitting chip  1000  may be formed by a process similar to light emitting chip  500 , the details of which are described above. 
     Light emitting chip  1000  may be formed from the same light emission structure  501  described above with light emitting chip  500 . As described with respect to  FIG. 5A , buffer layer  504 , n-type semiconductor layer  506 , active layer  508  and p-type semiconductor layer  510  are sequentially grown on substrate  502 , to form light emission structure  501 . 
     As shown in  FIG. 10A , after forming light emitting structure  501 , insulating pattern  1002  may be formed on p-type semiconductor layer  510 , for example, by any suitable photolithographic technique. For example, as described above, a patterned SiO 2  film may be used as a mask for a GaN full etching process. The SiO 2  film may be etched through the photoresist with insulating pattern  1002 . All layers of light emitting structure  501  may be etched via the SiO 2  mask through to substrate  502 . The SiO 2  film may be subsequently removed after the etching process is completed. 
     As shown in  FIG. 10B , a GaN mesa-etching process is performed. According to an exemplary embodiment, a similar photolithographic method as described for the full etching process (shown in  FIG. 10A ) may be used for n-GaN mesa-etching. The n-GaN mesa-etching may be performed to securely supply free electrons from n-type layer  506  to p-type layer  510  through the interface between the two layers. The patterns for n-GaN mesa-etching may include: an entire n-electrode portion  1004 , a portion  1006  of LED portion  1010 , and a portion  1008  of each LRD portion  1012 . On each portion ( 1004 ,  1006 ,  1008 ), a top 0.55 μm thickness, for example, may be etched away such that the n-type layer  506  is partially exposed. 
     As shown in  FIG. 10C , a transparent electrode layer  1018  may be deposited and patterned. Transparent electrode layer  1018  may be deposited to cover p-electrode portion  1016 , LED portion  1010  (except for the mesa-etched portion  1006 ) and LRD portion  1012  (except for mesa-etched portion  1008 ). Transparent electrode layer  1018  may be formed so that light escaping from the underlying layer (p-type layer  510 ) may be effectively extracted and so that electrons are spread over transparent electrode layer  1018  from p-type layer  510  to p-electrode portion  1016 . 
     As shown in  FIG. 10D , n-electrode portion  1004  may be formed, for example, by any suitable photolithography process on the mesa-etched part of n-type layer  506 , to provide an ohmic contact. The ohmic contact desirably includes a linear I-V curve and low resistance. 
     As shown in  FIG. 10E , passivation layer  1020  may be formed to provide insulation between n-type layer  506  and p-type layer  510  at the series connection of LRD portions  522 . Passivation layer  1020  may help to prevent an electrical short circuit when bridge contact layer  1022  ( FIG. 10F ) is formed between p-type layer  510  and an adjacent mesa-etched part of n-type layer  506 . 
     As shown in  FIG. 10F , bridge contact layer  1022  is formed to provide an electrical connection between p-type layer  510  on one side of LRD portion  1012  and the mesa-etched n-type layer  506  (portion  1008 ) on a side of an adjacent LRD portion  1012 , thus forming light emitting chip  1000 . 
     Referring to  FIGS. 11A and 11B , cross-section diagrams of light emitting chip  1000  are shown. In particular,  FIG. 11A  is a cross-section diagram along lines  11 A- 11 A′; and  FIG. 11B  is a cross-section diagram along lines  11 B- 11 B′. 
     Along line  11 A- 11 A′, bridge contact layer  1022  connects the mesa-etched n-type layer  506  on one side of one LRD  1012  to a p-type layer  510  of an adjacent LRD  1012 . Bridge contact layer  1022  is formed over passivation layer  1020 . Accordingly, photo-electrons created in one LRD  1012  travel from n-type layer  506  to a p-type layer  510  of an adjacent LRD  1012  through bridge contact  1022 . Along line  11 B- 11 B′, bridge contact layer  1022  bridges over passivation layer  1020  to connect a p-type layer  510  of LRD  1012  to a p-type layer  510  of LED portion  1010 . 
     Next, a theoretical extraction efficiency (η extr ) for light emitting unit cell  800  ( FIG. 8 ) is compared with a conventional extraction efficiency (η o ). For unit cell  800 , extraction efficiency η extr , based on Kirchhoff&#39;s law, is shown in equation (5) as: 
     
       
         
           
             
               
                 
                   
                     η 
                     extr 
                   
                   = 
                   
                     
                       η 
                       o 
                     
                      
                     
                       
                         η 
                         o 
                       
                       
                         1 
                         - 
                         
                           α 
                            
                           
                             ( 
                             
                               1 
                               - 
                               
                                 η 
                                 o 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     where the conventional extraction efficiency (η o ) and the conversion efficiency (α) are the respective efficiencies for converting light to a photocurrent within LRDs  806 . In equation (5), for the sake of simplicity, any series resistances of LED  804  and LRDs  806  have been neglected. 
     A summary of theoretical extraction efficiencies under different conversion efficiencies (α=0.6, α=0.8 and α=1.0) are shown in Table 2 below. Referring to  FIG. 12 , a graph is shown which summarizing the theoretical extraction efficiencies of Table 2. As shown in  FIG. 12  and Table 2, the extraction efficiencies are improved by a ratio between about 1.4-4.0 as compared with the conventional extraction efficiency. In general, the extraction efficiency increases with increasing conversion efficiency. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Theoretical Extraction Efficiency for Light Emitting Unit Cell 800 
               
            
           
           
               
               
               
            
               
                 α 
                 η o   
                 η extr   
               
               
                   
               
            
           
           
               
               
               
            
               
                 0.6 
                 0.25 
                 0.4545 
               
               
                   
                 0.5 
                 0.7143 
               
               
                 0.8 
                 0.25 
                 0.6250 
               
               
                   
                 0.5 
                 0.8333 
               
               
                 1.0 
                 0.25 
                 1.0 
               
               
                   
                 0.5 
                 1.0 
               
               
                   
               
            
           
         
       
     
     Referring to  FIG. 13 , a micro-pixelated LED  1300  is shown. Micro-pixelated LED  1300  is similar to light emitting chips  500 ,  1000  (respective  FIGS. 5G and 10F ), except that micro-pixelated LED  1300  includes LED arrays  1310 , having a plurality of light emitting unit cells  1312 . Each of the light emitting unit cells  1312  may be similar to light emitting unit cell  300  or  800  (respective  FIGS. 3 and 8 ) described above. 
     Micro-pixelated LED  1300  includes an n-electrode  1302  formed on an exposed upper surface of n-type layer  1304 . A p-electrode  1306  is formed on an upper surface of transparent electrode  1308 . 
     Although p-side up configurations (where light is emitted from the p-type layer) of light emitting unit cells  300 ,  800  are described above, n-side up configurations (where light is emitted from the n-type layer) of unit cells  300 ,  800  may also be used. Light emitting unit cells  300 ,  800  of the present invention may be applied to any configuration which provides photocurrent recycling. 
     An n-side up configuration may be formed, for example, by direct growth of a p-type layer and an n-type layer in reverse order. An n-side up configuration may also be formed by growing a p-side up configuration first and further depositing a suitable metal layer (such as Au and/or Sn)) and an appropriate submount (such as Au-coated Si) on top of the p-type layer. A laser lift-off (LLO) process may be used to detach the n-type layer from the substrate. For example, a laser beam (such as a krypton fluoride (KrF) laser beam) may be applied through the transparent substrate side to detach the n-type layer from the substrate. 
     An n-side up configuration may also be formed using a flip-chip technique. In this configuration, a p-side up configuration is initially formed, except that the transparent electrode may be replaced by a thick reflective electrode layer. The device may be flipped and bonded onto a submount, typically a Si wafer, by using a bonding material such as solder, a Au bump or a Au ball. In this device, light is emitted from the substrate bottom surface. 
     Several embodiments of the invention have been described herein. It is understood that the present invention is not limited to these embodiments and that different embodiments may be used together. In addition, it is understood that any conventional technique such as surface roughening, grating, volume hologram and photonic crystal may be incorporated with embodiments of the present invention, for example, for further enhancement of the extraction efficiency. 
     Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.