Abstract:
A light-emitting device with improved optical efficiency is disclosed. A semiconductor substrate underlies active p-n junction layers, and has an internal scattering/reflecting surface near the bottom surface of the semiconductor substrate. Accordingly, the light originated at the active p-n junction layers is internally reflected from the internally curved reflecting surface, and substantially passes though the top surface of the semiconductor substrate.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention discloses a light-emitting device with improved optical efficiency, in particular to a light-emitting diode having a substrate with a light scattering/reflecting surface.  
         [0003]     2. Description of the Prior Art  
         [0004]      FIGS. 1A and 1B  are cross-sectional view and top view, respectively, of a conventional light-emitting diode (LED) respectively. As shown in  FIG. 1A , an n-type layer  120 , an undoped active layer  125 , and a p-type layer  130  are sequentially grown on a substrate  110  by epitaxial growth process. When the structure is under forward biased current, photons are emitted due to the recombination of the minority carriers in the active layer. As shown in  FIGS. 1A and 1B , a transparent electrode layer  140  is disposed on the p-type layer  130 . Layer  140  firstly acts as an ohmic contact layer between the p-type layer  130  and a p-electrode (anode)  1501 ; secondly, it enhances the current spreading through the p-type layer  130 . An n-electrode (cathode)  1502  is disposed on the exposed surface of the n-type layer  120 , as preferably shown in  FIG. 1B .  
         [0005]     Part of the light generated from the active layer  125  passes through the transparent electrode layer  140 , and is partly absorbed by layer  140 . Another part of the light generated from the active layer  125  propagates toward the substrate  110 . Some of the propagated light is emitted out of the LED from the bottom surface of the substrate  110  when the incident angle is less than the critical angle of total reflection, while light having incident angle greater than critical angle is repetitively reflected inside the substrate  110 , as indicated by arrow  160  in  FIG. 1A . The totally reflected light  160  is eventually absorbed inside the substrate  110 . To increase the optical efficiency, the above mentioned are the two major loss mechanisms that the current invention aims to overcome.  
       SUMMARY OF THE INVENTION  
       [0006]     It is an object of the present invention to provide a light-emitting device with improved optical efficiency.  
         [0007]     It is another object of the present invention to provide a light-emitting device having a substrate with an internal scattering/reflecting surface, such that the light originated at the active layer is substantially reflected or scattered from the substrate, and eventually emitted out of the light-emitting device, thereby increasing optical efficiency of the light-emitting device.  
         [0008]     It is a further object of the present invention to provide a light-emitting device having an electrode layer or transparent conducting layer with openings formed therein, such that the light is minimally blocked or absorbed, thereby increasing optical efficiency of the light-emitting device.  
         [0009]     In accordance with the present invention, a light-emitting device with improved optical efficiency is disclosed. A semiconductor substrate underlies active p-n junction layers, and has an internally scattering surface near the bottom surface of the semiconductor substrate. In one embodiment, the internal scattering/reflecting surface is formed, for example, by implanting process; in other embodiment, the bottom surface of the substrate is roughened or curved. Accordingly, the light originated at the active p-n junction layers is internally reflected from the internal scattering/reflecting surface, and substantially passes through the top surface of the semiconductor substrate, instead of internal total reflection as occurred in the conventional LEDs. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1A  is a cross-sectional view of a conventional light-emitting diode (LED);  
         [0011]      FIG. 1B  is a top view of the conventional LED of  FIG. 1A , showing the arrangement of the p- and n-electrode;  
         [0012]      FIG. 2  is a cross-sectional view illustrating the structure of an LED in accordance with one embodiment of the present invention;  
         [0013]      FIG. 3A  is a cross-sectional view illustrating the structure of an LED with a substrate having a rough bottom surface in accordance with the present invention;  
         [0014]      FIG. 3B  is a cross-sectional view illustrating the structure of an LED with a substrate having a semicircular geometric shape in accordance with the present invention;  
         [0015]      FIG. 3C  is a cross-sectional view illustrating the structure of an LED with a substrate having a triangular geometric shape in accordance with the present invention;  
         [0016]      FIG. 3D  is a cross-sectional view illustrating the structure of an LED with a substrate having a polyhedron geometric shape in accordance with the present invention;  
         [0017]      FIG. 3E  is a cross-sectional view illustrating the structure of an LED with a reflecting layer in accordance with the present invention;  
         [0018]      FIG. 4A  is a top view illustrating the structure of an LED in accordance with one embodiment of the present invention;  
         [0019]      FIG. 4B  is a top view illustrating the structure of an LED in accordance with another embodiment of the present invention;  
         [0020]      FIG. 4C  is a cross-sectional view of  FIG. 4A , showing the structure of the LED;  
         [0021]      FIG. 5  is a cross-sectional view illustrating the structure of an LED with a substrate having implanted regions in accordance with the present invention;  
         [0022]      FIG. 6A  is a cross-sectional view illustrating the structure of an LED with a substrate having a rough bottom surface in accordance with the present invention;  
         [0023]      FIG. 6B  is a cross-sectional view illustrating the structure of an LED with a substrate having a semicircular geometric shape in accordance with the present invention;  
         [0024]      FIG. 6C  is a cross-sectional view illustrating the structure of an LED with a substrate having a triangular geometric shape in accordance with the present invention;  
         [0025]      FIG. 6D  is a cross-sectional view illustrating the structure of an LED with a substrate having a polyhedron geometric shape in accordance with the present invention;  
         [0026]      FIG. 6E  is a cross-sectional view illustrating the structure of an LED with a reflecting layer in accordance with the present invention; and  
         [0027]      FIG. 7  is a cross-sectional view illustrating the structure of an LED in accordance with an additional embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0028]     It should be recognized that the present invention can be practiced in a wide range of other variations besides those explicitly described, and the scope of the present invention is expressly not limited except as specified in the accompanying claims.  
         [0029]      FIG. 2  is a cross-sectional view illustrating the structure of a light-emitting device, particularly a light-emitting diode (LED), in accordance with one embodiment of the present invention. This LED is structurally similar to that shown in  FIG. 1A , where an n-type layer  220 , an undoped active layer  225 , and a p-type layer  230  are sequentially formed on a semiconductor substrate  210 , for example, by an epitaxial growth process. The n-type layer  220 , the undoped active layer  225 , and the p-type layer  230  altogether also referred to as active p-n junction layers in this disclosure. A transparent electrode layer  240  is disposed on the p-type layer  230 , and a p-electrode (anode)  2501  and an n-electrode (cathode)  2502  are disposed respectively on the transparent electrode layer  240  and the exposed surface of the n-type layer  220 .  
         [0030]     A number of regions  270  are defined and formed near the bottom surface of a substrate  210 , such as sapphire. These defined regions  270  are formed, for example, by implanting ions different from the doped ions inside the substrate  210 , if the substrate  210  is doped. Accordingly, these regions  270  have a refractive index different from that of the substrate  210  for that the material characteristic, composition, or density is changed. In operation, while the light  260  generated from the active layer  225  reaches the defined regions, it is scattered or reflected at a different angle, as indicated by arrows  2601 , as compared to the conventional substrate  110  without the defined regions ( FIG. 1A ). The change of the path of the reflected light  2601  would increase the probability for the light to escape from the LED due to the change of incident angle.  
         [0031]      FIG. 3A  illustrates the cross section of a light-emitting diode (LED) in accordance with another embodiment of the present invention. In this embodiment, the bottom surface of the substrate  210  is roughened, for example, by polishing technique, resulting in a randomly distributed rough surface  270 - 1 . In operation, while the light  260  generated from the active layer  225  reaches the rough surface, it is scattered or reflected at a different angle of reflection, as indicated by arrows  2601 , than the conventional substrate  110  with the smooth surface ( FIG. 1A ), therefore increasing the probability that the reflected light further passes through the n-type layer  220 , the active layer  225 , the p-type layer  230 , the transparent electrode layer  240 , and eventually emits out of the LED. Accordingly, an LED with improved optical efficiency is also attained.  
         [0032]     Alternatively, the roughening processing of the bottom surface of the substrate  210  could be performed by other techniques, such as dry etching, wet etching, micromachining, micro replication, or laser techniques. Diverse geometric patterns or shapes in cross-sectional view, such as semicircular  270 - 2  ( FIG. 3B ), triangular  270 - 3  ( FIG. 3C ), or polyhedron  270 - 4  ( FIG. 3D ) could alternatively be used instead. As illustrated in  FIG. 3E , a reflecting layer  280  could be further formed on the rough surface  270 - 1 ,  270 - 2 ,  270 - 3 , or  270 - 4 , resulting in a mirror surface, and further enhancing the reflection or scattering. The reflecting layer  280  could be made of materials such as sliver (Ag), platinum (Pt), molybdenum (Mo), Aluminum (Al), palladium (Pd), or a distributed Bragg reflector consisting of multiple dielectric layers, such as TiO 2 /SiO 2 .  
         [0033]     As mentioned in the Background of the Invention of this disclosure, the light generated from the active layer  125 / 225  passes through the transparent electrode layer  140 / 240 , and is somewhat blocked or absorbed by the transparent electrode layer  140 / 240 . In order to overcome this drawback, the present invention discloses further embodiments, which are described as follows.  
         [0034]      FIG. 4A  is a top view illustrating the arrangement of the p-electrode (anode)  2501 , the n-electrode (cathode)  2502 , and the transparent electrode layer  240  in accordance with one embodiment of the present invention.  FIG. 4B  is a top view in accordance with another embodiment of the present invention.  FIG. 4C  is a cross-sectional view of  FIG. 4A , showing the structure of the LED. Specifically, a number of openings  2401  are defined and formed in the transparent electrode layer  240 , so that some of the light generated from the active layer  225  could be emitted out of the LED without being blocked, while the current through the LED could also be effectively spread by the transparent electrode layer  240 .  FIG. 4A  shows elongated openings  2401  for instance, while  FIG. 4B  demonstrates hexagonal openings  2401 . The implanted regions  270  as described in  FIG. 2  are brought together with the specific transparent electrode layer  240  as described in  FIGS. 4A-4C , resulting in a configuration of  FIG. 5 . The implanted regions  270  could be preferably arranged primarily under the openings  2401  to maximizing the optical efficiency. Although the bundle of the transparent electrode layer  240  and the p-electrode (anode)  2501  as shown in  FIG. 4C  and  FIG. 5  possesses a two-layer structure, other structure having more than two layers is also possible.  
         [0035]     Referring to the embodiments of  FIG. 6A -D, the bottom surface of the substrate  210  ( FIG. 6A ) is roughened as described accompanying  FIG. 3A , or the bottom surface of the substrate  210  is curved with geometric patterns or shapes such as semicircular  270 - 2  ( FIG. 6B ), triangular  270 - 3  ( FIG. 6C ), or polyhedron  270 - 4  ( FIG. 6D ). As mentioned above, these geometric shapes could be preferably arranged primarily under the openings  2401  to maximizing the optical efficiency. Furthermore, a reflecting layer  280  could be further formed on the rough surface  270 - 1 ,  270 - 2 ,  270 - 3 , or  270 - 4 , to enhance the reflection or scattering as illustrated in  FIG. 6E . The reflecting layer  280  is made of, for example, Ag, Pt, Mo, Al, Pd, or a distributed Bragg reflector consisting of multiple dielectric layers, such as TiO 2 /SiO 2 .  
         [0036]      FIG. 7  illustrates a further embodiment which is similar to that of  FIG. 6E , except that the top surface of the p-type layer  230  is roughened, which reduces the possibility that the light coming from the active layer  225  is reflected back. The rough surface of the p-type layer  230  could be made, for example, by changing the process parameters during the epitaxial process, or could be formed by an appropriate process after the epitaxial process. It is appreciated that rough surface of the p-type layer  230  shown in  FIG. 7  could be adapted into other embodiments as discussed above.  
         [0037]     The invention is not limited to the specific embodiments illustrated and described here, as it is obvious to those skilled in the art that various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.