Abstract:
A light emitting device with a light extraction layer and the fabricating method thereof are provided. Embedding a light extraction layer between an emitting layer and a packaging layer can reduce the difference in the refractive index between the emitting layer and the packaging layer. In addition, the refractive index of the light extraction layer is adjustable by doping various concentrations of metal materials in the light extraction layer. The mechanism helps improving the light emitting efficiency of the light emitting device.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The invention relates to a light emitting device and the fabricating method thereof. In particular, the invention relates to a light emitting device with a light extraction layer, that can reduce the difference in the refractive index between the emitting layer and the packaging layer by embedding a light extraction layer that matches the refractive index. The invention also relates to the fabricating method thereof. 
     2. Related Art 
     With prosperous development and energy-saving concept in semiconductor technology in recent years, using light emitting diodes (LED&#39;s) to replace traditional illumination and display devices has been widely accepted. It even has become one of the most noticeable industries. 
     Generally speaking, traditional LED does not have a matching refractive index between the light emitting material (i.e., emitting layer) and air or packaging glue (i.e., packaging layer). Therefore, most photons are totally reflected. Photons can penetrate through the packaging layer only when the incident angle is less than the critical angle, thus visible to eyes. The photons that cannot go through are absorbed by the LED, resulting in bad light emitting efficiency. Take the most popular blue LED material, GaN, as an example. Its refractive index n GaN  is roughly between 2.5 and 2.8. Another blue LED material InGaN has a refractive index n InGaN  around 3.3. The red LED material GaAs has a refractive index n GaAs  around 4. The refractive indices of the above-mentioned materials have a large difference from that of the air or packaging layer. It is helpful to extract photons inside the LED if we could reduce the difference between the refractive indices of the emitting layer and the external medium (e.g., air or packaging layer). 
     In view of this, some vendors propose to use packaging glue with a high refractive index as the packaging layer of the LED. The packaging glue may even be doped with materials with a scattering mechanism, such as fluorescent powders, TiO2 powders, etc. Besides, some vendors also change the packaging structure of the LED to reduce total reflection, thereby increasing light output efficiency. However, using packaging glue with a high refractive index as the packaging layer costs a lot and is not easy to obtain. Changing the packaging structure of the LED increases packaging difficulty and reduces the yield. Therefore, the above two methods cannot effectively solve the problem with bad light emitting efficiency. 
     In summary, the prior art always has the problem of being unable to effectively increase light emitting efficiency of LED. It is imperative to provide a better solution. 
     SUMMARY 
     In view of the foregoing, this specification discloses a light emitting device with a light extraction layer and the fabricating method thereof. 
     The disclosed light emitting device with a light extraction layer includes: an emitting layer, a light extraction layer, and a packaging layer. The emitting layer combines electrons and holes to produce multiple photons. The light extraction layer is formed on the emitting layer and doped with a metal material for adjusting the refractive index of the light extraction layer, thereby increasing the penetrating rate of photons. The packaging layer is formed on the light extraction layer for protecting the light emitting device and for the photons to go through. 
     The disclosed fabricating method includes the steps of: providing an emission later for combining electrons and holes and generating a plurality of photons; forming a light extraction layer on the emission layer and doping the light extraction layer with a metal material for adjusting the refractive index thereof in order to increase the penetration rate of the photons; forming a packaging layer on the light extraction layer for protecting the light emitting device and for the photons to go through the packaging layer. 
     The invention differs from the prior art in that the invention embeds a light extraction layer between the emission layer and the packaging layer, thereby reducing the refractive index difference in between. The light extraction layer is further doped with a metal material in order to adjust the refractive thereof. 
     Through the above-mentioned techniques, the invention achieves the goal of enhancing light efficiency of the light emitting device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will become more fully understood from the detailed description given herein below illustration only, and thus is not limitative of the present invention, and wherein: 
         FIG. 1  is a cross-sectional view of the first embodiment of the disclosed light emitting device with a light extraction layer; 
         FIG. 2  is a flowchart of the fabricating method of the disclosed light emitting device with a light extraction layer; 
         FIG. 3A  is a cross-sectional view of the second embodiment of the disclosed light emitting device with a light extraction layer; 
         FIG. 3B  is a cross-sectional view of the third embodiment of the disclosed light emitting device with a light extraction layer; 
         FIG. 3C  is a cross-sectional view of the fourth embodiment of the disclosed light emitting device with a light extraction layer; and 
         FIG. 4  is a schematic view showing better light emitting efficiency of the light emitting device. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will be apparent from the following detailed description, which proceeds with reference to the accompanying drawings, wherein the same references relate to the same elements. 
     First, please refer to  FIG. 1  for a cross-sectional view of the disclosed light emitting device with a light extraction layer. The light emitting device  10   a  includes: an emission layer  11 , a light extraction layer  12 , and a packaging layer  13 . The emission layer  11  combines electrons and holes to produce multiple photons. The emission layer  11  can be material layer such as light emitting diode (LED), organic light emitting diode (OLED), and so on that can generate photons. For example, suppose the emission layer  11  is an LED. Then the emission layer  11  can be an N-type semiconductor layer doped with N-type impurities (also called donors), such as silicon (Si), phosphorus (P), arsenic (As), and antimony (Sb) that belong to group-V elements, or a P-type semiconductor layer doped with P-type impurities (also called acceptor), such as magnesium (Mg), boron (B), aluminum (Al), gallium (Ga), and indium (In) that belong to group-III elements. The emission layer  11  can thus provide electrons or holes and allow the two to convert into photons. In practice, the N-type semiconductor layer and the P-type semiconductor layer can have an active layer with the structure of multi quantum wells (MQW) or quantum dots in between, so that electrons and holes are more likely to be confined together to increase the light emitting strength, i.e., increasing the conversion rate from electricity to optical energy. Since the LED structure and the MQW structure are well known in the prior art, they are not further described herein. It should be noted that the light emitting device  10   a  does not include a substrate. This is because after the emission layer  11  is successfully formed on the substrate, it can be removed by using laser or polishing. This increases the light emission efficiency and heat dissipation effect of the light emitting device  10   a.    
     If the emission layer  11  is an OLED, then it is a stack of indium tin oxide (ITO), an OLED layer, and a cathode layer (not shown). ITO is a transparent anode material, with such features as high transparent rate, low electric resistance, high chemical stability, and easy to etch. After a voltage is imposed on the ITO and the cathode layer, they provide holes and electrons, respectively. The holes and electrons combine in the OLED between the ITO and the cathode layer, providing energy to generate photons. The OLED layer can use a fluorescent or phosphorous material. The energy levels of the chemicals are adjusted by modifying the molecular structure or functional group. Since the layer structure of the OLED is well known, it is not further described herein. 
     The light extraction layer  12  is formed on the emission layer  11 , and is doped with a metal material, such as Al, Ag, etc, for adjusting the refractive index of the light extraction layer  12 , thereby increasing the penetration rate of photons produced by the emission layer  11 . The metal material can be doped into the light extraction layer  12  by co-evaporation, rendering a material with a refractive index between 1.5 and 2.5. The thickness of the light extraction layer  12  can be adjusted according to the harmonic oscillations of photons. The light extraction layer  12  may even cover the entire or sides of the emission layer  11 . In practice, the light extraction layer  12  can be 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (NPB), tris(8-hydroxyquinolinato)aluminum (Alq3), or consisting of NPB and Alq3, etc., whose refractive index is between those of the emission layer  11  and the packaging layer  13 . It should be noted that the invention is not restricted to use the above-mentioned material for the light extraction layer  12 . Any material whose refractive index is between those of the emission layer  11  and the packaging layer  13  belongs to the invention. In the case without the packaging layer  13 , the refractive index of the light extraction layer  12  is between those of the emission layer  11  and the packaging layer  13 . The refractive index can be appropriately adjusted according to the concentration of the doped metal material. 
     The packaging layer  13  is formed on the light extraction layer  12  to protect the light emitting device  10   a  and for the photons produced by the emission layer  11  to pass through. In practice, the material of the packaging layer  13  can be silicone or epoxy resin whose refractive index is around 1.5. Since the packaging layer  13  also belongs to the prior art, it is not further described here. 
       FIG. 2  is a flowchart of the disclosed fabricating method for the light emitting device with a light extraction layer. The method includes the following steps. Step  210  provides an emission layer  11  for combining electrons and holes to produce multiple photons. Step  220  forms a light extraction layer  12  on the emission layer  11 , and dopes a metal material therein, thereby adjusting the refractive index of the light extraction layer  12  to increase the penetration rate of photons. Step  230  forms the packaging layer  13  on the light extraction layer  12  for protecting the light emitting device  10   a  and letting the photons pass through the packaging layer  13 . Through the above-mentioned steps, the light extraction layer  12  is embedded between the emission layer  11  and the packaging layer  13  to reduce the difference between the refractive indices of the emission layer  11  and the packaging layer  13 . 
     Please refer to  FIG. 3A  for the cross-sectional view of a second embodiment of the disclosed light emitting device with a light extraction layer. As mentioned before, the substrate of the light emitting device  10   a  is removed by laser in order to increase the light emitting efficiency and heat dissipation effect. In practice, however, the substrate  14  under the emission layer  11  of the light emitting device  10   b  can be kept, as shown in  FIG. 3A . The substrate  14  can be sapphire, silicon, germanium (Ge), SiC, GaAs or glass for evaporation, sputtering, or printing a thin film. The evaporation method can be the metal-organic vapor deposition (MOCVD), thermal evaporator, etc. But the invention is not limited to this particular example. It should be noted that the substrate  14  is not limited to the above-mentioned optoelectric substrates. Any substrate that can be used as an optoelectric element should be covered by the invention. 
     Please refer to  FIG. 3B  for the cross-sectional view of a third embodiment of the disclosed light emitting device with a light extraction layer. As mentioned before, in addition to being formed on the emission layer  11 , the light extraction layer  12  further covers the entire or side of the emission layer  11 . Take the example of covering the emission layer  11 . The covering method is as shown in the light emitting device  10   c  in  FIG. 3B . The light extraction layer  12  is formed on the surface of the emission layer  11  to cover the entire emission layer  11 . Afterwards, the packaging layer  13  is formed on the light extraction layer  12  to protect the light emitting device  10   c . The difference between using the light extraction layer  12  to cover the side of the emission layer  11  and to cover the entire emission layer  11  is in that the bottom of the emission layer  11  is not covered by the light extraction layer  12 . It is not further described here. 
     Please refer to  FIG. 3C  for the cross-sectional view of a fourth embodiment of the disclosed light emitting device with a light extraction layer. Suppose the substrate  14  of the light emitting device  10   d  is not removed by laser, the light extraction layer  12  can cover the side of the light emitting layer  11 , as shown in  FIG. 3C . In this case, some part of the light extraction layer  12  is in touch with the substrate  14 . Afterwards, the packaging layer  13  is formed on the light extraction layer  12  to protect the light emitting device  10   d.    
     Finally,  FIG. 4  is a schematic view showing better light emitting efficiency of the disclosed light emitting device. When there is no light extraction layer  12 , the photons can penetrate through the boundary between the emission layer  11  and the packaging layer  13  when the incident angle is smaller than a first critical angle  41 . The propagation of photons is indicated by the solid arrow. Photons whose incident angle is greater than the first critical angle are reflected and restricted within the emission layer  11  until they are totally absorbed. However, when the light extraction layer  12  exists between the light emitting layer  11  and the packaging layer  13 , the photons generated from the emission layer  11  penetrate the boundary between the emission layer  11  and the packaging layer  13  when the incident angle is smaller than a second critical angle  42 . The propagation of such photons is indicated by the dashed arrow. Photons whose incident angle is greater than the second critical angle  42  are still reflected and restricted within the emission layer  11  until they are totally absorbed. However, since the second critical angle  42  is greater than the first critical angle  41 , there are more penetrating photons than the case without the light extraction layer  12 . The light emitting efficiency of the light emitting device is thus better. 
     In summary, the invention differs from the prior art in that the light extraction layer  12  is embedded between the emission layer  11  and the packaging layer  13  to reduce the difference in the refractive indices of the emission layer  11  and the packaging layer  13 . The light extraction layer  12  is doped with a metal material to adjust the refractive index thereof. The disclosed technique can solve problems in the prior art, thereby increasing the light emitting efficiency of the light emitting devices  10   a  to  10   d.    
     Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternative embodiments, will be apparent to persons skilled in the art. It is, therefore, contemplated that the appended claims will cover all modifications that fall within the true scope of the invention.