Patent Publication Number: US-2007096121-A1

Title: Light emitting diode and method for manufacturing the same

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates generally to a light emitting diode and a method for manufacturing such a light emitting diode, and in particular to a light emitting diode made of III-V compounds and a method for manufacturing such a light emitting diode.  
      2. The Prior Arts  
      Conventionally, the quaternary AlGaInP compound made of III-V elements is grown on the substrate made of GaAs to form the LED with high brightness. However, the bandgap for GaAs is small (about 1.424 ev) so that the light emitted by the LED containing quaternary AlGaInP compound is absorbed by the substrate made of GaAs, and thus the brightness of the LED is greatly reduced. U.S. Pat. No. 5,376,580 disclosed that after completion of epitaxial growth of the light emitting layer, the substrate which can absorb light was completely removed, and the the light emitting layer was joined to the transparent substrate made of GaP using wafer bonding techniques. Moreover, U.S. Pat. No. 5,008,718 disclosed that after completion of epitaxial growth of the light emitting layer, a thick transparent substrate made of GaP was epitaxially grown on the the light emitting layer, and then the substrate which can absorb light was completely removed, and subsequently the window layer was grown. However, the manufacturing process is not only complicated but also difficult, and thereby the manufacturing cost is increased.  
      In order to overcome such shortcomings, the present invention provides a light emitting diode made of III-V compounds and a method for manufacturing such a light emitting diode.  
     SUMMARY OF THE INVENTION  
      Accordingly, the present invention provides a light emitting diode and a method for manufacturing such a light emitting diode in order to substantially obviate one or more of the problems due to limitations, shortcomings, and disadvantages of the related art.  
      The primary objective of the present invention is to provide a substrate with high light transmission by adjusting the proportions of the elements in the III-V compound of the substrate so as to minimize the brightness loss of the light emitting layer.  
      Another objective of the present invention is to provide a highly transparent window layer made of III-V compound and overlying the light emitting layer to minimize the brightness loss of the light emitting layer.  
      A further objective of the present invention is to provide a thick window layer made of III-V compound and overlying the light emitting layer to enhance the current distribution.  
      In order to achieve the above-mentioned objectives, the present invention provides a light emitting diode and a method for manufacturing such a light emitting diode. The light emitting diode comprises: a transparent substrate made of Al x Ga 1-x As; a light emitting layer made of AlGaInP, stacked on the transparent substrate, and having a multiple layered epitaxially growing structure; a window layer made of GaP, stacked on the light emitting layer, and having a transparent structure with a great bandgap; an upper electrode layer for making electrical contact with the window layer; and a lower electrode layer for making electrical contact with the transparent substrate, wherein the x-value in Al x Ga 1-x As of the transparent substrate is set to corresponding to the emission wavelengths of the light emitting layer so that the transparent substrate can have a great bandgap which make it to be transparent to the light emitted by the light emitting layer; and the window layer is used to increase the current diffusion from the upper electrode layer to the light emitting layer, and thereby the emission efficiency of the light laterally emitted by the light emitting layer is increased. 
    
    
      These and other objectives and functions of the present invention will be apparent to those skilled in the art by reading the following detailed description of a preferred embodiment thereof, with reference to the attached drawings.  
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIGS. 1A  to  1 E illustrate schematically the steps performed during the manufacture of the light emitting diode of the present invention; and  
       FIG. 2  is a cross-sectional view of the light emitting diode according to one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      The following descriptions are exemplary embodiments only, and are not intended to limit the scope, applicability or configuration of the invention in any way.  
       FIGS. 1A  to  1 E illustrate schematically the steps performed during the manufacture of the light emitting diode of the present invention.  
      Referring to  FIGS. 1A  to  1 E, the present invention provides a highly transparent light emitting diode made of III-V compounds and a method for manufacturing such a light emitting diode. As shown in  FIG. 1A , a first epitaxial layer  20  is grown on the substrate  10  using liquid phase epitaxy (LPE). The first epitaxial layer has a thickness in the range of 50 μm to 100 μm. Both of the first epitaxial layer  20  and the substrate  10  are made of the III-V compounds, wherein the substrate  10  is made of GaAs, and the first epitaxial layer  20  is made of AlGaAs. As shown in  FIG. 1B , a second epitaxial layer  30  is grown on the first epitaxial layer  20  using metalorganic chemical vapor deposition (MOCVD). The second epitaxial layer  30  is made of AlGaInP, and has a multiple layered structure as a light emitting layer. The light emitting layer can be any conventional light emitting layer made of the quaternary AlGaInP compound. As shown in  FIG. 1C , a third epitaxial layer  40  is grown on the second epitaxial layer  30  using MOCVD or hydride vapour phase epitaxy (HVPE). The third epitaxial layer  40  is made of GaP, and has a thickness in the range of 2 μm to 150 μm. As shown in  FIG. 1D , the substrate  10  made of GaAs is then removed by etching. A metal layer is formed on the top surface of the third epitaxial layer  40 , and another metal layer is formed on the bottom surface of the first epitaxial layer  20 . Both of the metal layers are patterned to respectively form the first electrode layer  50  and the second electrode layer  60 , which act as the excitation electrodes of the light emitting diode of the present invention. Each of the two electrode layers makes electrical contact with its corresponding epitaxial layer.  
      In the above step, the substrate  10  is made of GaAs which has a small bandgap of 1.424 eV so that the light emitted by the second epitaxial layer  30  can be absorbed by the substrate  10 , and thus the light emitting efficiency is greatly reduced. Therefore, the the substrate  10  made of GaAs is removed in the present invention.  
      The first epitaxial layer  20  is made of AlGaAs which has the formula AlGa 1-x As, wherein the larger the x-value is, the greater the bandgap of AlGaAs is. Because the degree of transparency is varied with the emission wavelength, the the x-value in Al x Ga 1-x As of the first epitaxial layer  20  is determined by the emission wavelength of the second epitaxial layer  30  so that the first epitaxial layer  20  is highly transparent to the light emitted by the second epitaxial layer  30 . For example, for the second epitaxial layer  30  made of AlGaInP, x is in the range of from 0.45 to 0.9. The first epitaxial layer  20  is highly transparent to any light emitted by the second epitaxial layer  30  of AlGaInP if x is set to about 0.8.  
       FIG. 2  is a cross-sectional view of the light emitting diode according to the embodiment of the present invention.  
      Referring to  FIG. 2 , The light emitting diode of the present invention comprises: a transparent substrate  1 , a light emitting layer  2 , and a window layer  3  sequentially formed in this order from bottom to top; a metal upper electrode  4  for making electrical contact with the top surface of of the window layer  4 ; and a lower electrode layer  5  for making electrical contact with the bottom surface of the transparent substrate  1 .  
      In the light emitting diode of the present invention, the transparent substrate  1  is an epitaxial layer made of Al x Ga 1-x As, wherein the x-value is corresponding to the emission wavelength of the light emitting layer  2 . The suitable x-value is set so that the transparent substrate  1  can highly transmit light. The light emitting layer  2  has a light emitting structure of conventional quaternary AlGaInP compound made of III-V elements. The light emitting layer  2  comprises at least an active layer, an upper confining layer, and a lower confining layer. The window layer  3  is made of GaP, and has a transparent structure with a great bandgap. The window layer  3  is used to increase the current diffusion from the upper electrode layer  4  to the light emitting layer  2 , and thereby the emission efficiency of the light laterally emitted by the light emitting layer  2  is increased.  
      In addition, the upper and lower electrodes can be made of transparent and conductive oxides, which can enhance the brightness of the light emitting diode of the present invention.  
      It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the present invention. Thus, it is intended that the present invention cover the modifications and the variations of this invention provided they come within the scope of the appended claims and their equivalents.