Abstract:
The present disclosure provides a III-nitride semiconductor light emitting device, including: a plurality of III-nitride semiconductor layers including an active layer for generating light by recombination of electrons and holes; and a substrate used to grow the plurality of III-nitride semiconductor layers, and including a protrusion with two opposite sides rounded.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of Korean Patent Application No. 10-2007-0077218 filed Jul. 31, 2007, and Korean Patent Application No. 10-2007-0106275 filed Oct. 22, 2007. The entire disclosures of these applications are hereby incorporated by reference. 
       FIELD 
       [0002]    The present disclosure generally relates to a III-nitride semiconductor light emitting device, and more particularly, to a III-nitride semiconductor light emitting device which can not only solve a lateral constraint of a protrusion but also improve crystallinity. 
         [0003]    The III-nitride semiconductor light emitting device means a light emitting device such as a light emitting diode including a compound semiconductor layer composed of Al (x) Ga (y) In (1−x−y) N (0≦x≦1, 0≦y≦1, 0≦x+y≦1), and may further include a material composed of other group elements, such as SiC, SiN, SiCN and CN, and a semiconductor layer made of such materials 
       BACKGROUND 
       [0004]    The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
         [0005]      FIG. 1  is a view illustrating an example of a conventional III-nitride semiconductor light emitting device. The III-nitride semiconductor light emitting device includes a substrate  100 , a buffer layer  200  epitaxially grown on the substrate  100 , an n-type nitride semiconductor layer  300  epitaxially grown on the buffer layer  200 , an active layer  400  epitaxially grown on the n-type nitride semiconductor layer  300 , a p-type nitride semiconductor layer  500  epitaxially grown on the active layer  400 , a p-side electrode  600  formed on the p-type nitride semiconductor layer  500 , a p-side bonding pad  700  formed on the p-side electrode  600 , an n-side electrode  800  formed on the n-type nitride semiconductor layer exposed by mesa-etching the p-type nitride semiconductor layer  500  and the active layer  400 , and a protective film  900 . 
         [0006]      FIG. 2  is a view illustrating a light emitting device disclosed in International Publication WO02/75821 and WO03/10831, particularly, showing a process of growing a nitride semiconductor layer  410  on a patterned substrate  400 . The nitride semiconductor layers  410  start to grow on lower and upper surfaces of the patterned substrate  400 , respectively, and are brought into contact with each other. The growth of the nitride semiconductor layer  410  is facilitated in the contact portions to thereby form a flat surface. Since the patterned substrate  400  is used, light is scattered to improve external quantum efficiency, and crystal defects are reduced to improve quality of the nitride semiconductor layer  410 . 
         [0007]      FIG. 3  is a view illustrating an example of a protrusion disclosed in WO03/10831, particularly, a protrusion with a hexagonal cross-section on a substrate. Meanwhile, according to WO03/10831, in a case where the side of the protrusion is positioned in parallel with a stably growing face of a nitride semiconductor (e.g., a flat zone  401  of a substrate; refer to  FIG. 4 ), the nitride semiconductor grows slowly, which results in increase of crystal defects. So as to decrease the crystal defects, the entire sides of the protrusion are formed, crossing the stably growing face. Such a lateral constraint of the protrusion makes it difficult to form protrusions and/or depressions on the substrate to improve crystallinity. 
         [0008]      FIG. 4  is an explanatory view illustrating stably growing faces of a nitride semiconductor, particularly, three stably growing faces  402 ,  403  and  404  with an angular difference of 60° on a C-surface sapphire substrate  400 . 
       SUMMARY 
       [0009]    This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features. 
         [0010]    Accordingly, the present disclosure has been made to solve the above-described shortcomings occurring in the prior art, and an object of the present disclosure is to provide a III-nitride semiconductor light emitting device which can solve the foregoing problems. 
         [0011]    Another object of the present disclosure is to provide a III-nitride semiconductor light emitting device which can not only solve a lateral constraint of a protrusion but also improve crystallinity. 
         [0012]    A further object of the present disclosure is to provide a III-nitride semiconductor light emitting device which can improve external quantum efficiency by adjusting an angle of a scribing line to a substrate having a protrusion. 
         [0013]    A still further object of the present disclosure is to provide a III-nitride semiconductor light emitting device which can secure a sufficient space for growing a nitride semiconductor on a substrate. 
         [0014]    To this end, according to an aspect of the present disclosure, there is provided a III-nitride semiconductor light emitting device including: a plurality of III-nitride semiconductor layers including an active layer for generating light by recombination of electrons and holes; and a substrate used to grow the plurality of III-nitride semiconductor layers, and including a protrusion with two opposite sides rounded. 
         [0015]    According to another aspect of the present disclosure, the protrusion includes two connection sides for connecting the two opposite rounded sides. Preferably, the two connection sides are composed of straight lines, but may be slightly outwardly or inwardly curved in accordance with its mask pattern. 
         [0016]    According to a further aspect of the present disclosure, the substrate is divided by at least one cutting surface, and extension lines of the two connection sides cross the at least one cutting surface. Here, the at least one cutting surface defines the contour of the light emitting device, and may be formed by means of a diamond cutting or scribing/breaking process. 
         [0017]    According to a still further aspect of the present disclosure, the substrate includes a first array with a plurality of protrusions formed thereon, and a second array with a plurality of protrusions formed thereon, and the plurality of protrusions of the first array and the plurality of protrusions of the second array are arranged in alternate positions. 
         [0018]    According to the present disclosure, the III-nitride semiconductor light emitting device can not only solve a lateral constraint of a protrusion but also improve crystallinity. 
         [0019]    According to the present disclosure, the III-nitride semiconductor light emitting device can improve external quantum efficiency by adjusting an angle of a scribing line to a substrate having a protrusion. 
         [0020]    According to the present disclosure, the III-nitride semiconductor light emitting device can improve crystallinity of a nitride semiconductor layer by securing a sufficient space for growing a nitride semiconductor on a substrate. 
         [0021]    Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0022]    The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure. 
           [0023]      FIG. 1  is a view illustrating an example of a conventional III-nitride semiconductor light emitting device. 
           [0024]      FIG. 2  is a view illustrating a light emitting device disclosed in International Publication WO02/75821 and WO03/10831. 
           [0025]      FIG. 3  is a view illustrating an example of a protrusion disclosed in WO03/10831. 
           [0026]      FIG. 4  is an explanatory view illustrating stably growing faces of a nitride semiconductor. 
           [0027]      FIG. 5  is a view illustrating an example of a III-nitride semiconductor light emitting device according to the present disclosure. 
           [0028]      FIG. 6  is an explanatory view illustrating shape and arrangement of protrusions according to the present disclosure. 
           [0029]      FIGS. 7 and 8  are explanatory views illustrating an example of a process of forming protrusions according to the present disclosure. 
           [0030]      FIG. 9  is a view illustrating an example of protrusions according to the present disclosure. 
           [0031]      FIG. 10  is an optical microscope photograph showing a GaN epitaxial layer grown on a prepared substrate according to the present disclosure. 
           [0032]      FIG. 11  is an explanatory view illustrating the relationship between a light emitting device and a scribing line. 
           [0033]      FIG. 12  is a graph showing a simulation result of analyzing light extraction efficiency variations by an angle y, using a quadrangular pyramidal protrusion with a base of 4.5 μm×3 μm. 
           [0034]      FIG. 13  is a view illustrating another example of arrangement of protrusions according to the present disclosure. 
       
    
    
       [0035]    Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings. 
       DETAILED DESCRIPTION  
       [0036]    Example embodiments will now be described more fully with reference to the accompanying drawings. 
         [0037]      FIG. 5  is a view illustrating an example of a III-nitride semiconductor light emitting device according to the present invention. The III-nitride semiconductor light emitting device includes a sapphire substrate  10  with protrusions  90  formed thereon, a buffer layer  20  grown on the sapphire substrate  10 , an n-type nitride semiconductor layer  30  grown on the buffer layer  20 , an active layer  40  grown on the n-type nitride semiconductor layer  30 , a p-type nitride semiconductor layer  50  grown on the active layer  40 , a light transmitting electrode  60  formed on the p-type nitride semiconductor layer  50 , a p-side electrode  70  formed on the light transmitting electrode  60 , and an n-side electrode  80  formed on an n-type nitride semiconductor layer  31  exposed by mesa-etching the p-type nitride semiconductor layer  50  and the active layer  40 . 
         [0038]      FIG. 6  is an explanatory view illustrating shape and arrangement of protrusions according to the present invention. The protrusions  90  are formed on the sapphire substrate  10 . Each protrusion  90  has two rounded sides  91 , and two connection sides  92  connecting the two rounded sides  91 . The rounded sides  91  face a flat zone  401  of the sapphire substrate  10 , i.e., a stably growing face of a nitride semiconductor. However, the sides  91  are rounded not to be parallel with the stably growing face, thereby preventing growth delay of a nitride semiconductor layer and subsequently reducing crystal defects. In the meantime, it is possible to align the rounded sides of the protrusions of array A and the rounded sides of the protrusions of array B. Here, it is necessary to guarantee a sufficient interval between arrays A and B for the growth of the nitride semiconductor. Accordingly, when the rounded sides of the protrusions of array A and the rounded sides of the protrusions of array B are arranged in alternate positions, the interval between arrays A and B can be reduced. Moreover, as compared with rectangular protrusions with straight line sides, the protrusions with the rounded sides  91  according to the present invention secure regions  93  between them. Therefore, although the interval between arrays A and B decreases, it is possible to secure a space for growing the nitride semiconductor layer. 
         [0039]      FIGS. 7 and 8  are explanatory views illustrating an example of a process of forming protrusions according to the present invention. First of all, a photoresist  11  is coated on a substrate  10 , and a patterning process is carried out thereon. The patterning process is performed by means of a photo-etching process. Here, a thickness of the photoresist  11  to be coated is changed according to a height of protrusions to be formed on the substrate  10 . That is, the thickness of the photoresist is adjusted according to the target height of the protrusions.  FIG. 8  shows a shape of the pattern. For example, the pattern can be 4.2 μm in height H, 2.4 μm in width W, 1.6 μm in protrusion interval D 1 , and 0.8 μm in array interval D 2 , and cut in edge portions.  FIG. 9  shows one example of protrusions formed by the above process. If necessary, the top surfaces of the protrusions can be flat. In this case, a nitride semiconductor is also grown on the top surfaces of the protrusions. 
         [0040]    Thereafter, the substrate  10  is etched normally by means of a reactive ion etching (RIE). Meanwhile, edges of the mask pattern are actively etched. Even if the mask pattern is not rounded, it is possible to form the protrusion  90  according to the present invention. A pattern with rounded edges or a baking process for rounding may be used. Here, connection sides are formed in a straight line shape by the etching. In a case where the baking process is performed, the connection sides can be slightly curved. 
         [0041]      FIG. 10  is an optical microscope photograph showing a GaN epitaxial layer grown on a prepared substrate according to the present invention. On the left side, a focus was fixed on a boundary surface between the sapphire substrate and the epitaxial layer, and processed protrusions were projected on the sapphire substrate. On the right side, a focus was fixed on a surface of the epitaxial layer, and a good epitaxial crystalline layer with a smooth surface was obtained without any pit or pinhole. A GaN layer with a thickness of about 4 μm was grown by means of MOCVD as follows. A prepared substrate was positioned on a susceptor made of graphite in an MOCVD reactor. A temperature was raised to 1100° C. in a hydrogen atmosphere to remove impurities from the surface of the substrate. SiC was grown by using 4.5 μmol of DTBSi per minute and 17 μmol of CBr 4  per minute as source material. The growth temperature was 950° C. and the growth time was 60 seconds, so that the layer was formed to an estimated thickness of 10 A. Thereafter, for a surface wetting layer, InGaN was grown by using 10 μmol of TMIn per minute, 400 μmol of TMGa per minute, and 12 I of NH 3  per minute as material source. The growth temperature was 500° C. and the growth time was 35 seconds, so that the layer was grown to a thickness of 500 A. Then, an undoped GaN was formed by using 870 μmol of TMGa per minute and 18 I of NH 3  per minute as source material. The growth temperature was 1050° C. and the growth time was 7200 seconds, so that the layer was grown to a thickness of 4 μm. Crystallinity of the grown epitaxial layer could be confirmed by an XRD rocking curve measurement result. A half width of XRD (002) was ˜250 arc-second, and a half width of XRD (102) was ˜300 arc-second. That is, the epitaxial layer showed very high crystallinity. 
         [0042]      FIG. 11  is an explanatory view illustrating the relationship between a light emitting device and a scribing line. Quadrangular protrusions  90  are formed parallel to flat zone  401  of a substrate  10 . Cutting surfaces  10   a  and  10   b  of each light emitting device, i.e., scribing lines are indicated by dotted lines. An amount of light to be extracted can be controlled by adjusting an angle y of the scribing line to the side of the quadrangular protrusion  90 . 
         [0043]      FIG. 12  is a graph showing a simulation result of analyzing light extraction efficiency variations by an angle y, using a quadrangular pyramidal protrusion with a base of 4.5 μm×3 μm. When the angle y is about 45°, light extraction efficiency increases. This result can be identically applied to a protrusion with rounded sides. Light Tools 5.1 program of Optical research associates was used in the simulation. 
         [0044]      FIG. 13  is a view illustrating another example of arrangement of protrusions according to the present invention. Protrusions  90  are formed on a sapphire substrate  10  in a position rotated against a flat zone  401 . In this configuration, when the result of  FIG. 12  is applied to a light emitting device, a scribing line is not adjusted according to a direction of the protrusions  90 , but the direction of the protrusions  90  is adjusted according to the scribing line. 
         [0045]    When introducing elements or features and the exemplary embodiments, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of such elements or features. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements or features other than those specifically noted. It is further to be understood that the method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed. 
         [0046]    The description of the disclosure is merely exemplary in nature and, thus, variations that do not depart from the gist of the disclosure are intended to be within the scope of the disclosure. Such variations are not to be regarded as a departure from the spirit and scope of the disclosure.