Abstract:
A substrate for semiconductor light emitting devices is provided. The substrate is characterized in that the substrate is a single crystal material and has a nanocrystal structure capable of diffracting an electromagnetic wave. The nanocrystal structure is disposed on a surface portion of the substrate and includes an etched region and an unetched region, wherein the etched region has a depth of 10-200 nm. Due to the periodicity of the nanocrystal structure, the semiconductor material grown on the substrate has fewer defects, and the material stress is reduced.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the priority benefit of Taiwan application serial no. 95121557, filed Jun. 16, 2006. All disclosure of the Taiwan application is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a semiconductor light emitting device structure and a method of fabricating the same. More particularly, the present invention relates to a semiconductor light emitting device structure having a nanocrystal structure and a method of fabricating the same. 
         [0004]    2. Description of Related Art 
         [0005]    Distinct from the light emitting theory of regular fluorescent lamps or incandescent lamps generating heat to emit light, semiconductor light emitting devices such as light emitting diodes takes advantage of the specific property of semiconductor to emit light, and thus the light emitted by light emitting diodes is referred to as cold luminescence. The light emitting diodes have advantages of long service life, light weight, and low power consumption, and being free of harmful substance such as mercury, so the light emitting diodes used instead to illuminate can save a large amount of energy. 
         [0006]    Currently, a nanocrystal light emitting diode is proposed to improve the light emitting diode. Researchers found that like the frequency band structure in the state of electromagnetic wave being transmitted in periodic dielectric, a photonic band substance can be achieved by the periodic changing of more than two kinds of materials with different refraction index (or dielectric constant), thereby paving a way for developing the above nanocrystal light emitting diode. 
         [0007]      FIG. 1  is a schematic sectional view of the conventional nanocrystal light emitting diode, in which a periodically arranged nanocrystal structure is fabricated on the semiconductor layer on the light emitting layer. 
         [0008]    Referring to  FIG. 1 , the conventional light emitting diode mainly includes a substrate  100 , an N-type GaN layer  102 , a P-type GaN layer  104 , a light emitting layer  106 , a transparent conductive layer  108 , electrodes  110 ,  112 , and an insulating layer  114 , wherein the surface of the P-type GaN layer  104  has a pattern  104   a . The N-type GaN layer  102  and the P-type GaN layer  104  are successively disposed on the substrate  100 , and the light emitting layer  106  is disposed between the N-type GaN layer  102  and the P-type GaN layer  104 . The transparent conductive layer  108  is disposed on the surface of the P-type GaN layer  104 , and the electrodes  110  and  112  are respectively disposed on the N-type GaN layer  102  and the transparent conductive layer  108 , wherein the insulating layer  114  is disposed beneath the transparent conductive layer  108  and separates the electrode  112  and the P-type GaN layer  104 . 
         [0009]    The nanocrystal mainly functions as changing the refraction of light, such that the light emitted from the active light emitting layer can be successfully sent out, and is not totally reflected inside the light emitting diode. Therefore, the nanocrystal light emitting diode has higher extraction efficiency than the conventional light emitting diode. 
         [0010]    However, the pattern  104   a  (i.e., the nanocrystal structure) of the P-type GaN layer  104  is usually fabricated in manner of etching, such that the defect density at the etched portions increases, leading to the increase of resistance, thus influencing the electrical property of the light emitting diode. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention is related to a semiconductor light emitting device substrate, so as to obtain higher light emitting efficiency by the use of the semiconductor light emitting device grown on the substrate. 
         [0012]    Moreover, the present invention is related to a semiconductor light emitting device substrate, so as to obtain higher output power by the use of the semiconductor light emitting device grown on the substrate. 
         [0013]    Furthermore, the present invention is related to a method of fabricating a semiconductor light emitting device substrate, which can decrease the defect and stress of the semiconductor light emitting device grown on the substrate. 
         [0014]    The present invention provides a semiconductor light emitting device substrate, which is characterized in that the substrate is a single crystal material and has a nanocrystal structure capable of diffracting an electromagnetic wave, the nanocrystal structure is disposed on a surface portion of the substrate and has an etched region and an unetched region, wherein the etched region has a depth of 10-200 nm. 
         [0015]    The present invention provides another semiconductor light emitting device substrate, which comprises a single crystal material and an undoped nitride semiconductor layer. The single crystal material has a nanocrystal structure suitable for diffracting an electromagnetic wave, and the nanocrystal structure is disposed on a surface portion of the single crystal material and has an etched region and an unetched region, wherein the etched region has a depth of 10-200 nm. The undoped nitride semiconductor layer is disposed on the surface of the single crystal material, wherein the single crystal material and the undoped nitride semiconductor layer form a substrate. 
         [0016]    The present invention further provides a method of fabricating a semiconductor light emitting device substrate, which comprises providing a single crystal material. Next, a nanocrystal structure is formed on a surface of the single crystal material, wherein the nanocrystal structure has an etched region and an unetched region. Next, a nitride semiconductor material is grown on the surface of the single crystal material with an epitaxial process, so as to form a substrate. 
         [0017]    Because to the periodic nanocrystal structure is disposed on the surface of the substrate, the present invention has the following advantages. (1) The semiconductor material grown on the substrate has fewer defects. (2) The semiconductor light emitting device grown on the substrate has higher light emitting efficiency. (3) The semiconductor light emitting device grown on the substrate has higher optical output power. (4) The material stress of the semiconductor light emitting device grown on the substrate is reduced. 
         [0018]    In order to make the aforementioned and other objects, features and advantages of the present invention comprehensible, preferred embodiments accompanied with figures are described in detail below. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is a schematic sectional view of the conventional nanocrystal light emitting diode. 
           [0020]      FIG. 2  is a schematic sectional view of the semiconductor light emitting device substrate according to a first embodiment of the present invention. 
           [0021]      FIG. 3  and  FIG. 4  are top views of two kinds of tetragonal lattice patterns on the surface of the substrate according to the first embodiment of the present invention. 
           [0022]      FIG. 5  and  FIG. 6  are top views of two kinds of hexagonal lattice patterns on the surface of the substrate according to the first embodiment of the present invention. 
           [0023]      FIG. 7  is a schematic sectional view of a light emitting diode fabricated on the semiconductor light emitting device substrate of  FIG. 2 . 
           [0024]      FIG. 8  is a schematic sectional view of the semiconductor light emitting device substrate according to a second embodiment of the present invention. 
           [0025]      FIG. 9  is a schematic sectional view of a light emitting diode fabricated on the semiconductor light emitting device substrate of  FIG. 8 . 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0026]      FIG. 2  is a schematic sectional view of the semiconductor light emitting device substrate according to the first embodiment of the present invention. 
         [0027]    Referring to  FIG. 2 , the semiconductor light emitting device substrate  200  of the first embodiment has a nanocrystal structure  210 , which is a periodic structure. The nanocrystal structure  210  is disposed on a surface  200   a  portion of the substrate  200 , and has an etched region  202  and an unetched region  204 . The material of the substrate  200  is a single crystal material, for example, transmissive and does not absorb in wavelength range of visible light and infrared light. The single crystal material is, for example, Al 2 O 3  (sapphire), LiAlO 2 , LiGaO 2 , SiC, GaN, AlN, AlGaN, or another suitable single crystal material. 
         [0028]    Referring to  FIG. 2  again, the step of forming the nanocrystal structure  210  includes defining a pattern on the surface  200   a  of the single crystal material with a lithographic process, which is a pattern such as a network, columnar, or another periodically arranged geometric pattern exhibited by the nanocrystal structure  210 , as shown in  FIG. 3  to  FIG. 6 . 
         [0029]    The geometric pattern as shown in  FIG. 3  is a tetragonal packed nanocrystal  300 , the geometric pattern as shown in  FIG. 4  is a tetragonal packed network nanocrystal  400 , the geometric pattern as shown in  FIG. 5  is a hexagonal close-packed columnar nanocrystal  500 , and the geometric pattern as shown in  FIG. 6  is a hexagonal close-packed network nanocrystal  600 . Moreover, in the periodic structure having a plurality of crystals, the size of each of the crystals is about 100-900 nm, and each crystal is quadrilateral-shaped, pentagon-shaped, hexagon-shaped, or polygon-shaped, as shown in the figure. 
         [0030]    Referring to  FIG. 2  again, the lithographic process is a process such as laser interference lithography, holography-lithography, E-beam lithography, X-ray lithography, nano lithography, and nano imprinting. Next, an etching process is performed on the surface  200   a  of the single crystal material to form the nanocrystal structure  210 . The etching process includes dry etching or wet etching. Moreover, the surface roughness of the etched region  202  is greater than that of the unetched region  204 . 
         [0031]    The substrate of the first embodiment can be directly applied in all commonly-used blue, green, and white light emitting diodes. A semiconductor light emitting device fabricated by the use of the substrate of the first embodiment of the present invention is illustrated with reference to an embodiment below. However, it is not intended to limit the application scope of the present invention. 
         [0032]    Referring to  FIG. 7 , a schematic sectional view of a light emitting diode fabricated on the semiconductor light emitting device substrate of  FIG. 2  is shown. The light emitting diode in the figure includes a substrate  200 , a first-type doped semiconductor layer  702 , a second-type doped semiconductor layer  704 , a light emitting layer  706 , a transparent conductive layer  708 , electrodes  710 ,  712 , and an insulating layer  714 , wherein a surface  200   a  of the substrate  200  has a nanocrystal structure  210 . 
         [0033]    Referring to  FIG. 7  again, the first-type doped semiconductor layer  702  and the second-type doped semiconductor layer  704  are successively disposed on the substrate  200 , the light emitting layer  706  is disposed between the first-type and second-type doped semiconductor layers  702  and  704 , wherein the first-type doped semiconductor layer  702  is, for example, N-type GaN layer, and the second-type doped semiconductor layer  704  is, for example, P-type GaN layer. The transparent conductive layer  708  is disposed on the surface of the second-type doped semiconductor layer  704 , the electrodes  710  and  712  are respectively disposed on the first-type doped semiconductor layer  702  and the transparent conductive layer  708 , wherein the insulating layer  714  is disposed below the transparent conductive layer  708  and separates the electrode  712  and the second-type doped semiconductor layer  704 . 
         [0034]    The substrate  200  has the nanocrystal structure  210 , so the semiconductor light emitting device fabricated by the use of the substrate  200  has higher optical output power and higher light emitting efficiency. Particularly, when the semiconductor light emitting device is applied in a flip-chip process, the extraction efficiency can be further improved. 
         [0035]      FIG. 8  is a schematic sectional view of a semiconductor light emitting device substrate according to the second embodiment of the present invention. 
         [0036]    Referring to  FIG. 8 , the second embodiment is similar to the first embodiment, and only the difference is described below. In addition to a single crystal material  801  having a nanocrystal structure  810 , the substrate  800  of the second embodiment further includes an undoped nitride semiconductor layer  806  on the surface of the single crystal material  801 . The single crystal material  801  and the undoped nitride semiconductor layer  806  form a substrate, and the nanocrystal structure  810  also has an etched region  802  and an unetched region  804 . The material of the substrate is, for example, transmissive and does not absorb in wavelength range of visible light and infrared light. The undoped nitride semiconductor layer  806  is, for example, a nitride semiconductor material containing at least one of In, Al, or Ga, such as GaN, AlN, InN, AlGaN, InGaN, AlInN, and InGaAlN. Furthermore, the nitride semiconductor material (i.e., the undoped nitride semiconductor layer  806 ) is formed with an epitaxial process, wherein the epitaxial process includes MBE, MOCVD, OMVPE, HVPE, PECVD, or sputter. 
         [0037]    A semiconductor light emitting device fabricated by using the substrate of the second embodiment of the present invention is illustrated with reference to the embodiment below. However, it is not intended to limit the application scope of the present invention. 
         [0038]    Referring to  FIG. 9 , a schematic sectional view of a light emitting diode fabricated on the semiconductor light emitting device substrate of  FIG. 8  is shown. The light emitting diode in the figure includes a substrate  800  and the first-type doped semiconductor layer  702 , the second-type doped semiconductor layer  704 , the light emitting layer  706 , the transparent conductive layer  708 , the electrodes  710 ,  712 , and the insulating layer  714  which are the same as those in  FIG. 7 , and the position of each of the above layer is the same as that in  FIG. 7 . The substrate  800  is the same as that in  FIG. 8 , wherein an undoped nitride semiconductor layer  806  is disposed on the surface of the single crystal material  801 . 
         [0039]    The substrate  800  has a periodic nanocrystal structure  810 , so with the lateral overgrowth property in the epitaxial growth, the grown undoped nitride semiconductor layer  806  has fewer defects. In addition, due to the nanocrystal structure  810 , the semiconductor light emitting device fabricated on the substrate  800  has higher optical output power and higher light emitting efficiency. Particularly, when the semiconductor light emitting device is applied in the flip-chip process, the extraction efficiency can be further improved. 
         [0040]    To sum up, the substrate of the present invention has a periodic nanocrystal structure on the surface, so the defect of the semiconductor material grown on the substrate may be reduced during the epitaxial process, and the material stress of the semiconductor light emitting device grown on the substrate can also be reduced. In addition, due to the inherent advantages of the nanocrystal, the semiconductor light emitting device fabricated on the substrate has higher optical output power and higher light emitting efficiency. Furthermore, the light emitting diode formed on the substrate of the present invention does not have the pattern  104   a  (i.e., the nanocrystal structure) as shown in  FIG. 1 , thus the problem of the increasing defect density caused by etching can be avoided, thereby preventing the increase of resistance. 
         [0041]    It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.