Abstract:
A GaN based three layer buffer structure disposed on a substrate, and having a GaN layer disposed on the three layer buffer structure, the GaN layer serving as a platform for growth of a light emitting structure thereon.

Description:
This is a continuation of application Ser. No. 09/626,442 filed Jul. 26, 2000, now U.S. Pat. No. 6,495,867. 
    
    
     TECHNICAL FIELD 
     This invention relates to GaN compound Light Emitting Diodes. 
     BACKGROUND OF THE INVENTION 
     A semiconductor light-emitting diode (LED) comprises: a substrate; a light emitting structure; and a pair of electrodes for powering the diode. The substrate may be opaque or transparent. Light Emitting Diodes which are based on Gallium Nitride compounds generally comprise: a transparent, insulating substrate, e.g. a sapphire substrate. Because of the substantial lattice mismatch between an insulating substrate, e.g., a sapphire substrate, and GaN compounds, it is common practice to provide a thin buffer or nucleation layer on the sapphire which is followed by a layer on which an LED structure is grown. Growth of single crystals on insulating substrates has been studied for over 30 years. Early work included growth of both silicon and III-V compounds on a variety of insulating substrates including sapphire and spinel. In these studies it was determined that use of nucleation or buffer layers reduces the occurrences of imperfections and the tendency towards twinning in the thicker layers grown thereon. 
     DISCLOSURE OF THE INVENTION 
     In accordance with one aspect of our present invention, we provide a new and novel structure for overcoming the mismatch of the lattices of a sapphire substrate and the nitride layers that follow. We provide three buffer layers on which we grow a high quality I Gallium Nitride layer as a substrate for growth of the light structure. Our first buffer layer is formed of Indium Gallium Nitride. The addition of Indium to the GaN compound provides a soft material with a superior surface diffusion coefficient. These factors facilitate the formation of high quality materials at the beginning of crystal growth. Since InGaN has a larger lattice constant than the target GaN, our second layer is formed of AlGaN to migrate to the lattice constant of GaN. The final buffer layer is formed of GaN to provide a template for the growth of our high quality I GaN layer which serves as a platform for growth of our light emitting structure  12 . 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIGS. 1 a  and  1   b  are schematic showings of the top and side views of an illustrative embodiment of our improved LED. 
    
    
     DETAILED DESCRIPTION 
     FIGS. 1 a  and  1   b  are not drawn to scale. 
     The illustrative LED of FIGS. 1 a  and  1   b  is a GaN based device. The structure of FIGS. 1 a  and  1   b  comprises sapphire substrate  101 ; buffer structure  11 ; GaN substitute substrate layer  105 ; light emitting structure  12 ; window layers  13 ; semi transparent conductive layer  111 ; bond pad adhesion layer  112 ; P electrode bond pad  113 ; and N electrode bond pad  115  which is not shown in FIG. 1 b.    
     Layers  102  through  110  are grown in a Metal Organic Chemical Vapor Deposition MOCVD reactor. The details of MOCVD growth of the stated layers are well known in the industry and will not be discussed herein except details of the growth process which are particularly relevant to our success. 
     The remaining components of our improved LED, namely, semi transparent layer  111 , adhesion pad  112 , P bond pad  113 , and N bond pad  115  are formed by evaporation in apparatus other than a MOCVD reactor. 
     Buffer  11  Between Sapphire Substrate  101  and GaN Layer  105   
     In the illustrative embodiment of our improved GaN based LED, the  0001  face of sapphire substrate  101  is exposed for growth of our first buffer layer  102 . Layer  102  is formed of InGaN to a thickness of approximately 8 nm. The range of Indium in the layer is 1 to 10%. As explained earlier herein, the addition of Indium to the GaN compound provides a soft material with a superior surface diffusion coefficient. These factors facilitate the formation of high quality materials at the beginning of crystal growth. 
     Since InGaN has a larger lattice constant than that of the target GaN layer  105 , our second buffer layer  103  is formed of AlGaN. to migrate to the lattice constant of GaN. The range of Aluminum in the compound of layer  103  ranges from 10 to 100%. Layer  103  is formed to a thickness of approximately 8 nm. 
     The final buffer layer  104  which is formed of GaN provides a template for the growth of our high quality I GaN layer  105 . Layer  104  is formed to a thickness of approximately 8 nm. 
     GaN layer  105  serves as a platform for growth of our light emitting structure  12 . Layer  105  is grown to a nominal thickness of 1 um. 
     Light Emitting Structure 
     In the illustrative embodiment of FIG. 1 a , light emitting structure  12  comprises N cladding layer  106 , active region  107 , and P cladding layer  108 . Other forms of light emitting structures, e.g., single heterojunction, quantum well, etc. may be equally well used with our invention. 
     Window Structure 
     The first window layer  109  is formed of GaN doped with Mg, and has a nominal thickness of 300 nm. The second window layer  110  is similarly formed of Mg doped GaN. However, layer  110  is more highly doped Mg+ to provide an ohmic contact between the layer and the Ni/Au layer  111 . 
     The invention has been described with particular attention to its preferred embodiment; however, it should be understood that variations and modifications within the spirit and scope of the invention may occur to those skilled in the art to which the invention pertains.