Abstract:
The disclosure provides a light-emitting device. The light-emitting device comprises: a substrate having a first patterned unit; and a light-emitting stack on the substrate and having an active layer with a first surface; wherein the first patterned unit, protruding in a direction from the substrate to the light-emitting stack, has side surfaces abutting with each other and substantially non-parallel to the first surface in cross-sectional view, and has a non-polygon shape in top view.

Description:
RELATED APPLICATION 
       [0001]    This application is a continuation application of U.S. patent application Ser. No. 12/646,553, entitled “A LIGHT-EMITTING DEVICE HAVING A PATTERNED SURFACE”, filed on Dec. 23, 2009, which is a continuation-in-part of U.S. patent application Ser. No. 12/222,548, entitled “Stamp Having Nanoscale Structure And Applications Thereof In Light-Emitting Device”, filed on Aug. 12, 2008 claiming the right of priority based on TW application Ser. No. 097150633 filed on Dec. 24, 2008; the contents of which are incorporated herein by reference in their entirety. 
     
    
     BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present disclosure relates to a light-emitting device having a patterned surface. 
         [0004]    2. Description of the Related Art 
         [0005]    Recently, efforts have been devoted to promote the luminance of the light-emitting diode (LED) in order to implement the device in the lighting domain, and further procure the goal of energy conservation and carbon reduction. There are two major aspects to promote luminance. One is to increase the internal quantum efficiency (IQE) by improving the epitaxy quality to enhance the combination efficiency of electrons and holes. The other is to increase the light extraction efficiency (LEE) that emphasizes on the light which is emitted by the light-emitting layer capable of escaping outside the device, and therefore reducing the light absorbed by the LED structure. 
         [0006]    Surface roughening technology is one of the efficient methods to enhance luminance.  FIG. 7  shows a known LED  700  having a patterned substrate. LED  700  comprises a growth substrate  701 , an epitaxial stack, a first electrode  707 , and a second electrode  708 . The surface  701   a  of the growth substrate  701  has a plurality of trapezoid depression for improving the light-extraction efficiency. The epitaxial stack comprises a buffer layer  702  grown on the growth substrate, a non-doped semiconductor layer  703  grown on the buffer layer  702 , a first semiconductor layer  704  with first conductivity-type grown on the non-doped semiconductor layer  703 , an active layer  705  grown on the first semiconductor layer  704 , a second semiconductor layer  706  with second conductivity-type grown on the active layer  705 . The first electrode  707  is formed on the exposed first semiconductor layer  704 , and the second electrode  708  is formed on the second semiconductor layer  706 . 
         [0007]    The ratio of the pattern width to the width between patterns of the substrate surface  701   a  is generally designed to be around 1. Therefore, a considerable portion of the substrate surface  701   a  is still parallel to the surface of the active layer  705   a,  and the light emitted from the active layer  705  to the parallel substrate surface is easily reflected back to the epitaxial stack because of total internal reflection (TIR) effect and absorbed by the epitaxial stack to generate heat. It worsens both the light extraction efficiency and the heat dissipation problems. Nevertheless, the pattern is usually formed deeper in order to compensate the light loss due to the parallel (unpatterned) region, but the high aspect ratio of the deeper pattern causes difficulty for subsequently epitaxial growth and adversely affects the epitaxial quality. 
         [0008]    Another prior technique for roughen surface is to utilize mechanically polishing method to form a randomly distributed rough patterns on the substrate surface. By this method, it is hard to control the roughened dimension, such as the depth or the width. Moreover, the epitaxial quality is not good by growing an epitaxial layer on the randomly rough surface. 
       SUMMARY OF THE DISCLOSURE 
       [0009]    The disclosure provides a light-emitting device. The light-emitting device comprises: a substrate having a first patterned unit; and a light-emitting stack on the substrate and having an active layer with a first surface; wherein the first patterned unit, protruding in a direction from the substrate to the light-emitting stack, has side surfaces abutting with each other and substantially non-parallel to the first surface in cross-sectional view, and has a non-polygon shape in top view. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  shows a light-emitting device in accordance with the first embodiment of the present disclosure. 
           [0011]      FIG. 2  shows a light-emitting device in accordance with the second embodiment of the present disclosure. 
           [0012]      FIG. 3  shows a light-emitting device in accordance with the third embodiment of the present disclosure. 
           [0013]      FIG. 4  shows a light-emitting device in accordance with the fourth embodiment of the present disclosure. 
           [0014]      FIG. 5  shows a light-emitting device in accordance with the fifth embodiment of the present disclosure. 
           [0015]      FIG. 6A to 6E  show embodiments of the top views of the patterned surface in accordance with the present disclosure. 
           [0016]      FIG. 7  shows a known structure of a light-emitting diode. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0017]      FIG. 1  shows a light-emitting device  100  in accordance with a first embodiment of the present disclosure. The light-emitting device  100  comprises a growth substrate  101 , an intermediate layer comprising a buffer layer  102  and/or an undoped semiconductor layer  103  epitaxially grown on the growth substrate  101 , a first contact layer  104  with first conductivity-type epitaxially grown on the intermediate layer, a first cladding layer  105  with first conductivity-type epitaxially grown on the first contact layer  104 , an active layer  106  epitaxially grown on the first cladding layer  105 , a second cladding layer  107  with second conductivity-type epitaxially grown on the acitve layer  106 , a second contact layer  108  with second conductivity-type epitaxially grown on the second cladding layer  107 , a current spreading layer  109  formed on the second contact layer  108  and forming an ohmic contact with the second contact layer  108 , a first electrode  110  formed on the exposed first contact layer  104  by evaporation or sputtering method, and a second electrode  111  formed on the current spreading layer  109  by evaporation or sputtering method; wherein the growth substrate  101  has a patterned surface  101   a  comprising a plurality of ordered pattern units, and each of the plurality of ordered pattern units is compactly disposed, for example, at least one of the plurality of pattern units is substantially contacted with the neighboring units. According to the embodiment, any region of the patterned surface  101   a,  e.g. A1 region, is substantially not parallel to the corresponding region of the surface of the active layer, e.g. A2 region. The plurality of the ordered pattern units is disposed in a fixed period, variable period, or quasi-period. The top views of the plurality of pattern units comprise a polygon, or at least one pattern selected from the group consisting of triangle, rectangle, hexagon, and circle. The cross-sections of the plurality of pattern units comprise at least one pattern selected from the group consisting of V-shape, semicircle, arc, and polygon. Each of the plurality of pattern units has a width and a depth, and the depth is preferable less than the width for facilitating the subsequently grown buffer layer  102  and/or the undoped semiconductor layer  103  to fill into the depressed region of the patterned surface  101   a.    
         [0018]      FIG. 2  shows a light-emitting device  200  in accordance with a second embodiment of the present disclosure. In comparison with the light-emitting device  100  shown in  FIG. 1 , the cross-section of the patterned surface  101   b  comprises a plurality of ordered patterned units, and each of the patterned units comprises a smooth curve for facilitating the subsequently grown buffer layer  102  and/or the undoped semiconductor layer  103  to fill into the depressed area of the patterned surface  101   b.  The method for forming the cross-section with a smooth curve comprises firstly forming a mask layer of photoresist on a plane substrate, patterning the mask layer by lithographic process, then curing the patterned mask layer in a baking machine under an appropriate temperature to reflow the patterned mask layer of photoresist to form a profile with smooth curve, finally dry-etching or wet-etching the substrate with the patterned mask layer to transfer the smooth curve profile to the substrate to form a patterned surface  101   b  with a smooth curve as shown in  FIG. 2 . The top views of the plurality of pattern units comprise polygon, or at least one pattern selected from the group consisting of triangle, rectangle, hexagon, and circle. 
         [0019]      FIG. 3  shows a light-emitting device  300  in accordance with a third embodiment of the present disclosure. In comparison with the light-emitting device  200  shown in  FIG. 2 , the patterned surface  101   c  of the light-emitting device  300  comprises a plurality of patterned units with variable dimensions or variable patterns disposed in a fixed period, variable period, or quasi-period. The top views of the plurality of the pattern units comprise polygon, or at least one pattern selected from the group consisting of triangle, rectangle, hexagon, and circle. The cross-section of the plurality of pattern units comprises at least two curves with different curvatures. 
         [0020]      FIG. 4  shows a light-emitting device  400  in accordance with a fourth embodiment of the present disclosure. In comparison with the light-emitting device  200  shown in  FIG. 2 , the second contact layer  108  of the light-emitting device  400  further comprises an exterior surface  108   a  having the patterned surface as disclosed in the foregoing embodiments for further enhancing the light extraction efficiency, and any region of the patterned surface  108   a  is substantially not parallel to the corresponding region of the upper surface  106   a  of the active layer. The method for forming the exterior surface  108   a  of the second contact layer  108  comprises naturally growing the second contact layer  108  with hexagonal depressions by adjusting the epitaxial growth parameters, such as lowering the growth temperature, or changing the gas concentration ratio of Hydrogen to Nitrogen, or performing a traditional lithographic and etching process to form the patterned surface  108   a  with protrusions and/or depressions. The subsequently formed current spreading layer  109  is conformable with the patterned surface  108   a  and forms a good ohmic contact with the second contact layer  108 . 
         [0021]      FIG. 5  shows a light-emitting device  500  in accordance with a fifth embodiment of the present disclosure. In comparison with the light-emitting device  200  shown in  FIG. 2 , the intermediate layer  502  of the light-emitting device  500  comprises a bonding layer, e.g. a transparent adhesive layer or a transparent conductive layer. The first contact layer  104  is joined to the second substrate  501  by a bonding technique, e.g. a direct bonding method or a thermo-compression bonding method. According to the present disclosure, the second substrate  501  is not limited to a material for epitaxial growth, and is flexible as long as the material meets the purpose, e.g. a material with high conductivity, a material with high transparency, a conductive material, or a material with high reflectivity. 
         [0022]      FIG. 6A  to  FIG. 6D  shows the top views of the patterned surface in accordance with the present disclosure. As shown in  FIG. 6A , the patterned surface comprises a plurality of hexagonal pattern units. Each of the pattern units is composed of six inclined surfaces  601   a  depressed or protruded from the substrate. The six inclined surfaces  601   a  are commonly joined at a vertex  601   c,  and mutually joined at six connecting sides  601   b  such that the patterned surface of the substrate is substantially not parallel to the corresponding region of the upper surface  106   a  of the active layer. As shown in  FIG. 6B , the patterned surface comprises a plurality of triangular pattern units. Each of the pattern units is composed of three inclined surfaces  602   a  depressed or protruded from the substrate. The three inclined surfaces  602   a  are commonly joined at a vertex  602   c,  and mutually joined at three connecting sides  602   b  such that the patterned surface of the substrate is substantially not parallel to the corresponding region of the upper surface  106   a  of the active layer. As shown in  FIG. 6C , the patterned surface comprises a plurality of rhombus pattern units. Each of the pattern units is composed of four inclined surfaces  603   a  depressed or protruded from the substrate. The four inclined surfaces  603   a  are commonly joined at a vertex  603   c,  and mutually joined at four connecting sides  603   b  such that the patterned surface of the substrate is substantially not parallel to the corresponding region of the upper surface  106   a  of the active layer. As shown in  FIG. 6D , the patterned surface comprises a plurality of square pattern units defined by overlapped circles. Each of the pattern units is composed of four inclined surfaces  604   a  protruded from the substrate and a rounded top surface  604   c.  The plurality of pattern units are mutually joined at the connecting sides  604   b  such that the patterned surface of the substrate is substantially not parallel to the corresponding region of the upper surface  106   a  of the active layer. The statement of “the patterned surface of the substrate is substantially not parallel to the corresponding region of the upper surface of the active layer” as described in the foregoing embodiments does not exclude the circumstances caused by the various process deviations, such as the photoresist pattern distortion by lithographic deviation or pattern distortion by etching deviation such that portion of the to-be-patterned surface is not patterned or portion of the patterned region still comprises surface parallel to the active layer. For example, the vertices  601   c,    602   c,    603   c,  or rounded top surface  604   c  still possibly comprises a small mesa under the various process deviations, but the process deviations are preferred to be controlled to have the total surface area that is parallel to the active layer and the total surface area of the unpatterned surface do not exceed 3% of the total substrate area. As shown in  FIG. 6E , the patterned surface comprises a plurality of circular pattern units. Each of the pattern units is disposed side by side in a tightest disposition such that the patterned surface area of the substrate that is parallel to the corresponding region of the upper surface  106   a  of the active layer is about 9.3% or not over 10% of the total substrate area, i.e. the ratio of the area of the triangular area subtracting the area of the three sectors to the area of the triangular area is about 9.3% or not over 10%. 
         [0023]    The pattern units as disclosed in the foregoing embodiments have a relative higher patterned proportion, therefore increase the difficulty to epitaxially grow the subsequently buffer layer and the undoped semiconductor layer. In order to fulfill both light extraction efficiency and internal quantum efficiency, the cross-section of each of the pattern units has a width and a depth smaller than the width, i.e. the ratio of the depth to the width is lower than 1, therefore a pattern unit with a lower aspect ratio is achieved. The subsequently epitaxially grown buffer layer and/or the undoped semiconductor layer are easily filled into the depressed region of the patterned surface to enhance the epitaxial growth quality. 
         [0024]    The patterned surface described in the above-mentioned embodiments is not limited to be formed on any surface of any specific structure of the light-emitting device in accordance with the present disclosure. It is still under the scope of the disclosure to form the patterned surface on any structure of the light-emitting device in accordance with the present disclosure. For example, the patterned surface can be formed on the light output surface of the light-emitting device contacting with the surroundings. The neighboring materials neighbored to the patterned structure includes but not limited to any structure of the light-emitting device, the encapsulating material, or the environmental medium having a different refraction index from the patterned structure. The difference of the refraction indexes of the patterned structure and the neighboring material is at least 0.1. 
         [0025]    The materials of the buffer layer, the undoped semiconductor layer, the first contact layer, the first cladding layer, the second cladding layer, the second contact layer, and the active layer comprise III-V compound materials, e.g. Al p Ga q In (l-p-q) P or Al x In y Ga (l-x-y) N, wherein, 0≦p, q, x, y≦1; (p+q)≦1; (x+y)≦1. The first conductivity-type comprises n-type or p-type. The second conductivity-type comprises n-type or p-type and is different to the first conductivity-type. The current spreading layer comprises metal oxide, e.g. ITO, or well-conductive semiconductor layer of phosphide or nitride having high impurity concentration. The growth substrate comprises at least one material selected from the group consisting of GaP, sapphire, SiC, GaN, and AlN. The second substrate comprises a transparent material selected from the group consisting of GaP, sapphire, Sic, GaN, and AlN, or a heat dissipating material selected from the group consisting of diamond, diamond-like-carbon (DLC), ZnO, Au, Ag, Al, and other metals. 
         [0026]    It will be apparent to those having ordinary skill in the art that various modifications and variations can be made to the methods in accordance with the present disclosure without departing from the scope or spirit of the disclosure. In view of the foregoing, it is intended that the present disclosure cover modifications and variations of this disclosure provided they fall within the scope of the following claims and their equivalents.