Abstract:
A method for enhancing light extraction efficiency of a light emitting diode is disclosed. The method includes the steps of providing a light emitting diode including in sequence a substrate, a first layer of a first conduction type, an active layer, and a second layer of a second conduction type opposite to the first conduction type; growing a number of protrusions on at least one layer selected from the first layer, the active layer, and the second layer of the light emitting diode to form a patterned oxide layer for protecting the light emitting diode from etch; controlling height of the protrusions to achieve a predetermined etching depth of the light emitting diode; dry etching through a portion of the light emitting diode which is not protected by the patterned oxide layer to form a plurality of depressions on the light emitting diode; and removing the oxide layer from the selected layer. The light emitting diode is patterned so that more light beams can be emitted. Therefore, light extraction efficiency is enhanced.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method for enhancing light extraction efficiency of a light emitting diode. More particularly, the present invention relates to a method for enhancing light extraction efficiency of a light emitting diode by roughening the surfaces of the light emitting diode. 
       BACKGROUND OF THE INVENTION 
       [0002]    Light emitting efficiency of a light emitting diode is dominated by its internal quantum efficiency and light extraction efficiency. Internal quantum efficiency relates to the light generated from an active layer. Light extraction efficiency is the ability that the light from the active layer emits to medium surrounded. With development of epitaxy technology, internal quantum efficiency can be up to 80%. However, light extraction efficiency is still low. For example, refraction index of GaN series materials is about 2.5. The air around them has refraction index of 1. Due to total reflection, the light extraction efficiency in the interface is only 10˜12%. 
         [0003]    In order to enhance light extraction efficiency, randomly etched cavities are formed on the surface of a transparent conductive layer. Therefore, most light beams from the active layer can emit out of the light emitting diode without being reflected. Alternatively, roughening a p-type layer can also achieve the same effect. 
         [0004]    Generally, thickness of the top epitaxial structure of a GaN or AlGaInP series light emitting diode for generating red or yellow beams is larger than 5 μm. Hence, plasma or chemical etching can be applied to make the cavities or a two dimensional pattern. However, for other light emitting diodes generating blue, green or UV light beams, the top epitaxial structure is very thin (about 0.2 μm). If the external quantum efficiency needs to be improved to enhance light extraction efficiency, depth of the cavities should be at least 0.2 μm. Therefore, traditional surface roughening methods are not proper for this purpose. 
         [0005]    Additionally, traditional etching roughening methods usually use photoresist as a mask. Due to low etching selectivity, these methods can not be used to make a desired etching depth. Not to mention a deeper depth. Therefore, it is not easy to pattern or roughen the light emitting diode. Moreover, while metallic materials, such as nickel, are used as a hot mask, a photoresist needs to be in advance spread on the light emitting diode before the hot mask is placed, such that manufacturing complexity and cost are increased. 
         [0006]    Traditionally, surface roughening methods may create patterns having protrusions with distance therebetween over 2˜3 um. This is disadvantageous for forming delicate patterns to improve light extraction efficiency. Moreover, traditional etching methods can only be used to roughen top surface of the light emitting diode and are not able to roughen the edges thereof. 
         [0007]    In order to overcome the problems mentioned above, an improved method is disclosed in U.S. Pat. No. 6,551,936. Please see  FIG. 1 . It is for etching a pattern in a semiconductor material based on the formation of an InP grating mask on the semiconductor material. The formation of the InP grating mask involves the formation of a multi-layered structure on the semiconductor material with an etch-stop layer between two InP layers. A photoresist grating mask corresponding to the pattern to be etched in the semiconductor material is then formed on the top InP layer. Subsequently, a non-selective etch is used to penetrate the top InP layer, the etch-stop layer, and the lower InP layer. A suitable stripping solvent is then used to remove the photoresist followed by a selective etch to clear the remaining exposed InP material, remove contaminated material and to expose the underlying semiconductor material in accordance with the pattern to be etched. Additional masking beyond the InP mask is, therefore, not required. The exposed semiconductor material is then etched such that the pattern is transferred to the semiconductor material. 
         [0008]    Although the above invention solves most of the problems, the pattern formed is still restricted. Not any desirable pattern for enhancing light extraction efficiency can be controlled and achieved. 
       SUMMARY OF THE INVENTION 
       [0009]    In order to overcome the problems mentioned above, the present invention provides a method for enhancing light extraction efficiency by patterning a light emitting diode. An oxide layer is used instead of photoresist for etching process. Thickness of the oxide layer can be easily controlled, so that a desired etching depth of the light emitting diode can be achieved readily, and therefore, the light emitting diode can be patterned into any shape. In this way, the present invention has a simpler manufacturing process compared to the traditional manufacturing process, thereby saving time and cost. 
         [0010]    In accordance with an aspect of the present invention, a method for enhancing light extraction efficiency includes the steps of: a) providing a light emitting diode including in sequence a substrate, a first layer of a first conduction type, an active layer, and a second layer of a second conduction type opposite to the first conduction type; b) growing a plurality of protrusions on at least one layer selected from the first layer, the active layer, and the second layer of the light emitting diode to form a patterned oxide layer for protecting the light emitting diode from etch; c) controlling height of the protrusions to achieve a predetermined etching depth of the light emitting diode; d) dry etching through a portion of the light emitting diode which is not protected by the patterned oxide layer to form a plurality of depressions on the light emitting diode; and e) removing the oxide layer from the selected layer. 
         [0011]    Preferably, the first conduction type is n-type and the second conduction type is p-type. 
         [0012]    Preferably, the active layer has a quantum well structure, a homojunction structure, or a heterojunction structure. 
         [0013]    Preferably, the patterned oxide layer is formed by hydrothermal treatment, sol-gel method, electro-plating, thermal evaporation, chemical vapor deposition (CVD), or molecular beam epitaxy (MBE). 
         [0014]    Preferably, the patterned oxide layer is made of indium tin oxide (ITO), al-doped zinc oxide (AZO), silicon dioxide (SiO 2 ), zinc oxide (ZnO), magnesium oxide (MgO), molybdenum oxide (MoO), aluminum oxide (Al 2 O 3 ), titanium dioxide (TiO 2 ), nickel oxide (NiO), calcium oxide (CaO), barium oxide (BaO), manganese oxide (MnO), copper oxide (CuO), tin dioxide (SnO 2 ), or a mixture thereof 
         [0015]    Preferably, the protrusion has a shape of hexagonal pyramid, truncated hexagonal pyramid, or hexagonal cylinder. 
         [0016]    Preferably, the patterned oxide layer is at least partially formed on a top surface or a side surface of the light emitting diode. 
         [0017]    Preferably, the dry etching step is performed by plasma etching, inductively coupled plasma (ICP) etching, ion beam etching or reactive ion etching. 
         [0018]    Preferably, the protrusions have a diameter ranging from 1 nm to 10 μm. 
         [0019]    Preferably, the predetermined etching depth is achieved by controlling reaction time of the dry etching step. 
         [0020]    Preferably, the present invention further comprises a step of d1) dry etching a portion of the oxide layer. 
         [0021]    Preferably, the light emitting diode has a cross-sectional shape of wedge, rectangle or step. 
         [0022]    Preferably, a distance between two adjacent protrusions is less than 1 micrometer. 
         [0023]    Preferably, the removing step is performed by hydrochloric acid, nitric acid, or hydrogen peroxide. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  shows a prior art light emitting diode. 
           [0025]      FIG. 2  is a flow chart of patterning a light emitting diode according to a first embodiment of the present invention. 
           [0026]      FIGS. 3A-3D  illustrate a manufacturing process for patterning a light emitting diode according to the first embodiment of the present invention. 
           [0027]      FIGS. 4A-4D  illustrate scanning electron microscope (SEM) images showing different shapes of protrusions on the light emitting diode. 
           [0028]      FIG. 5  shows a top view of the etched light emitting diode according to the first embodiment of the present invention. 
           [0029]      FIGS. 6A-6D  illustrate a manufacturing process for patterning a light emitting diode according to a second embodiment of the present invention. 
           [0030]      FIGS. 7A-7D  illustrate a manufacturing process for patterning a light emitting diode according to a third embodiment of the present invention. 
           [0031]      FIG. 8  illustrates a patterned light emitting diode according to a fourth embodiment of the present invention. 
           [0032]      FIGS. 9A-9D  illustrate a manufacturing process for patterning a light emitting diode according to a fifth embodiment of the present invention. 
           [0033]      FIG. 10  illustrates a patterned light emitting diode according to a sixth embodiment of the present invention. 
           [0034]      FIG. 11  illustrates a patterned light emitting diode according to a seventh embodiment of the present invention. 
           [0035]      FIGS. 12A-12C  illustrate a manufacturing process for patterning a light emitting diode according to an eighth embodiment of the present invention. 
           [0036]      FIGS. 13A-13B  illustrate scanning electron microscope (SEM) images showing a light emitting diode having protrusions formed thereon. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0037]    The present invention will now be described more specifically with reference to the following embodiments. It is to be noted that the following descriptions of preferred embodiments of this invention are presented herein for purpose of illustration and description only; it is not intended to be exhaustive or to be limited to the precise form disclosed. 
         [0038]    In order to have a full understanding of the present invention, eight embodiments are described below. 
       First Embodiment  
       [0039]    Please refer to  FIG. 2  and  FIGS. 3A-3D .  FIG. 2  is a flow chart of patterning a light emitting diode for enhancing light extraction efficiency. First, a light emitting diode  20  is provided (as shown in step S 101  in  FIG. 2 ). As shown in  FIG. 3A , the light emitting diode  20  includes from top to bottom a p-type layer  202 , an active layer  204 , a n-type layer  206  and a substrate  208 . Although in this embodiment, the p-type layer  202  is formed on the active layer  204  and the n-type layer  206  is formed below the active layer  204 , the p-type layer  202  and the n-type layer  206  can be exchanged. The active layer  204  has a quantum well structure. In practice, the active layer  204  can also have a homojunction structure or a heterojunction structure. 
         [0040]    In this embodiment, the p-type layer  202  is selected on which a number of protrusions  2102  are grown to form a patterned oxide layer  210  (S 102 ). The oxide layer  210  is formed by sol-gel method. The method is not limited to sol-gel method; it can be hydrothermal treatment, electro-plating, thermal evaporation, chemical vapor deposition (CVD), or molecular beam epitaxy (MBE). 
         [0041]    The material used for the oxide layer  210  is calcium oxide (CaO). Of course, in practice, it can be indium tin oxide (ITO), al-doped zinc oxide (AZO), silicon dioxide (SiO 2 ), zinc oxide (ZnO), magnesium oxide (MgO), molybdenum oxide (MoO), aluminum oxide (Al 2 O 3 ), titanium dioxide (TiO 2 ), nickel oxide (NiO), tin dioxide (SnO 2 ), barium oxide (BaO), manganese oxide (MnO), copper oxide (CuO) or a mixture of the materials mentioned above. 
         [0042]    As to the oxide layer  210 , the protrusions  2102  are micro-scaled to nano-scaled columns. Please see  FIGS. 4A-4D . With different methods used for growing the oxide layer  210 , the protrusions  2102  can be shaped as a hexagonal pyramid, a truncated hexagonal pyramid or a hexagonal cylinder. 
         [0043]    The height of the protrusions  2102  can be controlled to achieve a desired etching depth of the light emitting diode  20  (S 103 ). Next, a dry etching process is performed on the light emitting diode  20  through a portion of the light emitting diode  20  which is not protected by the patterned oxide layer  210  to form a lot of depressions on the light emitting diode  20  (S 104 ). When etching takes place, the portions which are not covered by the protrusions  2102  will be etched away. At the same time, the protrusions  2102  will be etched. When the protrusions  2102  are removed by etching process, the uncovered portions are etched to the desired depth. The higher the protrusions  2102  are, the deeper the etched depth will be. According to the present invention, distance between every two adjacent protrusions  2102  is less than 1 micrometer. 
         [0044]    In this embodiment, plasma etching is used. Of course, it can be replaced by inductively coupled plasma (ICP) etching, ion beam etching or reactive ion etching depending on what is suitable for etching the material used in the oxide layer  210 . When plasma keeps colliding with the protrusions  2102  and takes away the protrusions  2102  piece by piece, it also etches the light emitting diode  20 . Please refer to  FIG. 3C . After the dry etching process finishes, parts of the columns of the oxide layer  210  are gone by plasma colliding. The dry etching process makes depressions from the surface of the oxide layer  210 . 
         [0045]    Last, the oxide layer  210  is removed from the light emitting diode  20  (S 105 ). The agents used in removing the oxide layer  210  can be hydrochloric acid, nitric acid or hydrogen peroxide. In the present invention, nitric acid is used to wash away calcium oxide on the surface of the n-type layer  206 . A patterned surface  2022  is formed on the light emitting diode  20 . Since the protrusions  2102  have diameters ranging from 1 nm to 10 μm, the patterned light emitting diode  20  may correspondingly form a number of convexes which also have diameters ranging from 1 nm to 10 μm. The pattern surface  2022  allows light beams generated from the active layer  204  to be emitted out more easily via the depressions, thereby improving light extraction efficiency of the light emitting diode  20 . 
       Second Embodiment  
       [0046]    Please refer to  FIGS. 6A-6D . A light emitting diode  30  has a p-type layer  302 , an active layer  304 , a n-type layer  306  and a substrate  308 . In this embodiment, the active layer  304  has a quantum well structure. 
         [0047]    An oxide layer  310  made of calcium oxide protrusions  3102  are formed on the p-type layer  302 . In this embodiment, the thickness of the oxide layer  310  is thicker than that of the oxide layer  210  in the first embodiment. Therefore, when a dry etching process (such as inductively coupled plasma etching) is applied onto the oxide layer  310 , a depression caused by the dry etching process is formed. In comparison with the first embodiment, depth of the depression can extend to the n-type layer  306  via the p-type layer  302  and the active layer  304 . 
         [0048]    After removing process with nitric acid, a patterned surface  3022  is formed on the light emitting diode  30 . Since the light emitting diode  30  in this embodiment is etched to the n-type layer  306 , light extraction efficiency thereof is much better than that of the first embodiment. 
         [0049]    In this embodiment, time needed to make deeper depression is longer than that in the first embodiment. Because inductively coupled plasma etching has poor etching ability for the oxide layer  310 , before the oxide layer  310  is etched to a desired level, the deeper depression has already formed. In summary, depth of the depression can be controlled by thickness of the oxide layer  310  and reaction time for dry etching. Besides, when etching reaction passes by, distance between the protrusions  3102  is enlarged. It means that the pattern can be controlled by thickness of the oxide layer  310  or time for dry etching. 
       Third Embodiment  
       [0050]    For certain light emitting diodes, in order to easily form a pair of contacts thereon, a portion of the light emitting diode will be etched. Under this situation, the present invention is still applicable. 
         [0051]    Please see  FIG. 7A  to  FIG. 7D . A light emitting diode  40  has a p-type layer  402 , an active layer  404 , a n-type layer  406  and a substrate  408 . Since materials of respective components, method for etching and removing steps are fully disclosed in the previous two embodiments, no more details are illustrated hereafter. 
         [0052]    On the partially exposed n-type layer  406 , an oxide layer  410  having several oxide protrusions  4102  is formed. The oxide layer  410  is not provided on the top surface of the p-type layer  402  of the light emitting diode  40 . After etching and removing steps, the oxide layer  410  is removed. A n-type layer pattern  4062  is formed. The third embodiment shows that any specified region of the top surface of a light emitting diode can be patterned to enhance light extraction efficiency if etching process can apply to the region. 
       Fourth Embodiment  
       [0053]    Please see  FIG. 8 . A light emitting diode  50  has a p-type layer  502 , an active layer  504 , a n-type layer  506  and a substrate  508 . In contrast with the light emitting diode  40  in the third embodiment, the light emitting diode  50  has an exposed n-type layer  506 . After growing an oxide layer, dry etching the light emitting diode  50 , and removing the residual oxide layer, a n-type layer pattern  5062  and a p-type layer pattern  5022  are formed on the surfaces of the n-type layer  506  and p-type layer  502 , respectively. In the end, the top surface of the light emitting diode  50  is patterned regardless of its elevation. 
       Fifth Embodiment  
       [0054]    In some conditions, a second etching process can be used to deepen the depressions of the light emitting diode so that different light extraction efficiencies can be achieved. 
         [0055]    Please refer to  FIG. 9A  to  FIG. 9D . A light emitting diode  60  has a p-type layer  602 , an active layer  604 , a n-type layer  606  and a substrate  608 . In contrast with the light emitting diode  40  in the third embodiment, the light emitting diode  60  has an exposed n-type layer  606 . An oxide layer  610  with protrusions  6102  is formed over the top surface of the light emitting diode  60 . As shown in  FIG. 9B , after a first dry etching process is completed, thickness of the oxide layer  610  is decreased. The distance between each protrusion  6102  is increased. Depth of depression caused by the first dry etching is uniform. 
         [0056]    Now, the light emitting diode  60  is covered by a shelter (not shown) except the central portion. A second dry process is performed. As shown in  FIG. 9C , the central portion of the light emitting diode  60  is etched deeper. After removing process, a n-type layer pattern surface  6062 , a deeper p-type layer pattern surface  6022  and a shallow p-type layer pattern surface  6024  are formed as shown in  FIG. 9D . Obviously, the deeper p-type layer pattern surface  6022  has better light extraction efficiency than others. 
       Sixth Embodiment  
       [0057]    The oxide layer in the present invention is grown on side and top surfaces of the light emitting diode. Therefore, after etching and removing processes are finished, patterns can be obtained. If a light emitting diode has a sloped surface rather than a flat one, the present invention is still applicable. 
         [0058]    Please see  FIG. 10 . A light emitting diode  70  has a p-type layer  702 , an active layer  704 , a n-type layer  706  and a substrate  708 . Two sides of the light emitting diode  70  are sloped. The cross-sectional shape thereof is wedge. In practice, cross-sectional shape of the light emitting diode can be rectangular or stepped as mentioned above. 
         [0059]    After etching and removing processes, a p-type layer pattern surface  7022 , an active layer pattern surface  7042  and a n-type layer pattern surface  7062  are formed. The patterns can be formed on the sloped surface. Even though the patterns are formed on the sloped surface, depressions caused by dry etching are still formed downwardly. 
       Seventh Embodiment  
       [0060]    A combination of the fifth and sixth embodiments is disclosed. Please refer to  FIG. 11 . A light emitting diode  80  has a p-type layer  802 , an active layer  804 , a n-type layer  806  and a substrate  808 . Two sides of the light emitting diode  80  are sloped. 
         [0061]    By forming an oxide layer (not shown) over the light emitting diode  80 , etching the two sides of the light emitting diode  80 , etching the central portion of the light emitting diode  80  and removing the oxide layer, a p-type layer pattern surface  8022 , an active layer pattern surface  8042  and a n-type layer pattern surface  8062  are formed. Obviously, the depression of the p-type layer pattern surface  8022  is much deeper after the second etching process. 
       Eighth Embodiment  
       [0062]    Growing an oxide layer in different direction is disclosed in the last embodiment. Not only the top surface of a light emitting diode but also the side surfaces thereof can be formed with the oxide layer. 
         [0063]    Please refer to  FIG. 12A  to  FIG. 12C . A light emitting diode  90  has a p-type layer  902 , an active layer  904 , a n-type layer  906  and a substrate  908 . Different from the light emitting diodes mentioned in the other embodiments, the light emitting diode  90  is dry etched to remove its two sides. The shape of the p-type layer  902 , the active layer  904  and the n-type layer  906  is a reverse wedge. In order to enhance light extraction efficiency, top and side surfaces of the light emitting diode  90  need to be patterned. 
         [0064]    Please see  FIG. 12B . An oxide layer  910  covers the surfaces mentioned above. It should be noticed that oxide on the side surfaces, as shown in  FIG. 13A , can be formed along with that on the top surface, as shown in  FIG. 13B , at the same time. When dry etching is performed, etching particles should collide with the oxide layer  910  from the top and sides. After removing process, a pattern surface  9022  exits on the whole light emitting diode  90  except the substrate  908 . 
         [0065]    In the aforementioned embodiments, although the p-type layer is formed on the active layer and the n-type layer is formed below the active layer, the p-type layer and the n-type layer can be exchanged. 
         [0066]    While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not be limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims, which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.