Abstract:
A new transparent conducting oxide (TCO), which can be expressed as Al x Ga 3-x-y In 5+y Sn 2-z O 16-2z ; 0≦x&lt;1, 0&lt;y&lt;3, 0≦z&lt;2, has been used to improve the brightness and current spreading in GaN base LED process. The optical properties of this system are superior to regular Ni/Au transparent conducting layer in blue-green region, and the new Al 2 O 3 —Ga 2 O 3 —In 2 O 3 —SnO 2  system is able to increase the brightness at 1.5˜2.5 time to compare to regular process. Furthermore, the new transparent conducting oxide thin film has the highest conductivity, which is better than the Ni/Au transparent conducting thin film.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to the field of transparent conducting oxide (TCO) and solid-state Gallium Nitride-based light-emitting devices. 
   2. Description of the Related Art 
   In recent years, the p-type Gallium Nitride-based III-V group compound semiconductor layer usually has a carrier concentration of less than 1×10 18  cm −3 , and the lowest resistivity is no lower than 1 ohm-cm. Such poor conductivity cannot effectively distribute the current to the entire p-type compound semiconductor layer. Therefore, the p-electrode is usually formed to cover substantially entire surface of the p-type Gallium Nitride-based III-V group compound semiconductor layer in order to ensure the uniform spreading of current to the entire p-type compound semiconductor layer, thereby obtaining uniform light emission from the device. However, the p-electrode considered being a light transmitting and ohmic electrode. 
   Due to the end of 1993, the Nichia Chemical Industries Ltd. in Japan announced the successful fabrication of the solid-state Gallium Nitride-based light-emitting devices that employed the metallic thin film as the p-electrode. Also, Nichia Chemical Industries Ltd. in Japan announces a particularly preferable metallic thin film contains gold and nickel. Gold and nickel are preferable formed such that a layer of nickel is formed in direct contact with the p-semiconductor layer and a layer of gold is formed on the nickel layer. After an annealing treatment, such a multi-layered structure can form an alloy, which is light transmitting and ohmic to p-type Gallium Nitride-based III-V group compound semiconductor layer. 
   The prior art shown in  FIG. 1 , the Nichia Chemical Industries Ltd disclosed in U.S. Pat. No. 6,093,965 that the Gallium Nitride-based III-V group compound semiconductor uses a sapphire substrate  116 , an n-type Gallium Nitride-based cladding layer  15 , an n-type Titanium/Aluminum (Ti/Al) electrode bonding pad  14 , an Indium-Gallium Nitride system light emitting layer  13 , a p-type Gallium Nitride-based cladding layer  12 , a p-type metallic thin film contains Nickel-Gold (Ni/Au) light transmitting electrode  11 A and a p-type Nickel-Gold (Ni/Au) electrode bonding pad  10 . 
   A metallic thin film contains Nickel-Gold (Ni/Au) light transmitting electrode usually transmits 20 to 40% of the light emitted from device there through. Therefore, to improve the brightness and efficiency of the Gallium Nitride-based III-V group compound semiconductor light emitting device is to reduce the absorption from the light transmitting electrode. 
   Tin indium oxide has been used to be the light transmitting electrode to reduce the absorption from regular nickel-gold thin film light transmitting electrode as shown in  FIG. 2 , the Indium Gallium Nitride light emitting diode uses a sapphire substrate  116 , an n-type Gallium Nitride-based cladding layer  15 , an n-type Aluminum Gallium Nitride-based cladding layer  15 A, an n-type Titanium/Aluminum (Ti/Al) electrode bonding pad  14 , an Indium-Gallium Nitride system light emitting layer  13 , a p-type Gallium Nitride-based cladding layer  12 , a p-type high concentration contact layer  117 , an Indium-Tin Oxide (ITO) light transmitting electrode  11 C and a p-type Nickel-Gold (Ni/Au) electrode bonding pad  10 . 
   A Gallium Nitride-based contact layer with a p-type concentration of greater than 5×10 18  cm −3  and a thickness of less than 500 Angstroms. The contact layer  117  can be formed by Zinc (Zn) diffusion, Magnesium (Mg) diffusion, Zn or Mg ion implantation, etc, and the Epistar Co. uses Indium-Tin Oxide (ITO) to be the light transmitting electrode to improve the light efficiency. Usually, the light emitting diode of using this technique can only transmits 50 to 70% of the light emitted from light through the high concentration of p-type contact layer  117  by Zn, Mg diffusion or implantation process and ITO light transmitting electrode  11 C. Furthermore, In ITO film, charge carriers are from both Tin dopant and ionized oxygen vacancy donors. The humidity can easy diffuse into ITO film and destroy the interface between ITO film and Gallium Nitride-based contact layer; the contact resistivity of ohmic contact will increase a lot. So, it is unstable and unreliable in high humidity condition. 
   In the prior art, no light transmitting electrode has the light efficiency and good reliability at the same time for Gallium Nitride-based light emitting semiconductor device. 
   SUMMARY OF THE INVENTION 
   The present invention provides a method for producing high efficiency Gallium Nitride-based light emitting device comprising a new Al 2 O 3 —Ga 2 O 3 —In 2 O 3 —SnO 2  system transparent conducting oxide (TCO) such as an amorphous or nanocrystalline thin film. The new Al 2 O 3 —Ga 2 O 3 —In 2 O 3 —SnO 2  system thin film has the highest conductivity, which is approximately ten times higher then that observed in commercial Indium-Tin Oxide (ITO). 
   Another object of the present invention is to prove a Gallium Nitride-based contact layer with Gallium rich phase which can reduce the contact resistance between the Gallium Nitride-based III-V group compound semiconductor light emitting device and this new Al 2 O 3 —Ga 2 O 3 —In 2 O 3 —SnO 2  transparent conducting oxide system, also, this contact layer can be n-type, p-type or undoping. 
   Still another object of the present invention is to provide an intermediate layer which is between the p-type cladding layer and Gallium Nitride-based contact layer and the intermediate layer can be Indium Nitride-based materials and p-type, n-type or undoping. The material band-gap energy of this intermediate layer must be lower than the p-type Gallium Nitride-based cladding layer. The function of this layer is to reduce the electrical spiking effect between p-type Gallium Nitride-based cladding layer and Gallium Nitride-based contact layer with Gallium rich phase. 
   According to the first present invention, a high brightness Gallium Nitride-based light emitting semiconductor device can be produced by forming an Al 2 O 3 —Ga 2 O 3 —In 2 O 3 —SnO 2  system light transmitting layer in contact with the p-type Gallium Nitride-based cladding layer. To anneal the Al 2 O 3 —Ga 2 O 3 —In 2 O 3 —SnO 2  system light transmitting layer which establishing an ohmic contact with the p-type Gallium Nitride-based cladding layer. Moreover, a transparent conducting oxide window layer can be shaped in contact with this light transmitting layer to improve the light efficiency and current spreading. 
   According to the second present invention, a high brightness Gallium Nitride-based light emitting semiconductor device can be produced by growing a layer of Gallium Nitride-based contact layer with Gallium rich phase in between the p-type Gallium Nitride-based cladding layer and the Al 2 O 3 —Ga 2 O 3 —In 2 O 3 —SnO 2  system light transmitting layer. To annealing this high brightness Gallium Nitride-based light emitting semiconductor device at high temperature, which forming a firm interface between the Al 2 O 3 —Ga 2 O 3 —In 2 O 3 —SnO 2  system light transmitting layer and Gallium Nitride-based contact layer. This interface can reduce the contact resistivity of ohmic contact and improving the light efficiency. Furthermore, this firm interface as well can improve the reliability of contact resistivity of ohmic contact. 
   According to the third present invention, an intermediate layer which is in between the p-type cladding layer and Gallium Nitride-based contact layer and the intermediate layer can be Indium Nitride-based materials and p-type, n-type or undoping, the material band-gap energy of this intermediate layer must be lower than the p-type Gallium Nitride-based cladding layer. To low down the band-gap energy in between a p-type Gallium Nitride-based cladding layer and a Gallium Nitride-based contact layer, which form a good function to reduce the electrical spiking effect while current cross a p-type Gallium Nitride-based cladding layer and a Gallium Nitride-based contact layer. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which: 
       FIG. 1  is a cross-sectional view, schematically illustrating a light-emitting device according to a prior art with a metallic light transmitting electrode in contact with the p-type Gallium Nitride-based cladding layer. 
       FIG. 2  is a cross-sectional view, schematically illustrating a light-emitting device according to a prior art with an Indium-Tin Oxide (ITO) light transmitting layer in contact with the p-type Gallium Nitride-based cladding layer. 
       FIG. 3  is a cross-sectional view, schematically illustrating a light-emitting device according to the invention of Al 2 O 3 —Ga 2 O 3 —In 2 O 3 —SnO 2  system light transmitting layer  11 B and Gallium Nitride-based contact layer with Gallium rich phase  17 A on sapphire substrate. 
       FIG. 4  is a cross-sectional view, schematically illustrating a light-emitting device according to the invention of Al 2 O 3 —Ga 2 O 3 —In 2 O 3 —SnO 2  system light transmitting layer  11 B and Gallium Nitride-based contact layer with Gallium rich phase  17 A on a silicon carbide (SiC)  216  substrate. 
       FIG. 5  is a cross-sectional view, schematically illustrating a light-emitting device according to the invention of Al 2 O 3 —Ga 2 O 3 —In 2 O 3 —SnO 2  system light transmitting layer  11 B and Gallium Nitride-based contact layer with Gallium rich phase  17 A on a silicon (Si)  316  substrate. 
       FIG. 6  is a cross-sectional view, schematically illustrating a light-emitting device according to the invention of Al 2 O 3 —Ga 2 O 3 —In 2 O 3 —SnO 2  system light transmitting layer  11 B and Gallium Nitride-based contact layer with Gallium rich phase  17 A on Gallium Arsenide-based (GaAs based)  416  substrate. 
       FIG. 7  is a cross-sectional view, schematically illustrating a light-emitting device according to the invention of Al 2 O 3 —Ga 2 O 3 —In 2 O 3 —SnO 2  system light transmitting layer  11 B, Gallium Nitride-based contact layer with Gallium rich phase  17 A and Indium Gallium Nitride-based intermediate layer. 
       FIG. 8  is a cross-sectional view, schematically illustrating a light-emitting device according to the invention of Al 2 O 3 —Ga 2 O 3 —In 2 O 3 —SnO 2  system light transmitting layer  11 B, Gallium Nitride-based contact layer with Gallium rich phase  17 A and transparent conducting oxide window layer. 
       FIG. 9  is a graph showing a transmittance spectra (UV-V is region) of ITO, Ga 1.6 In 6.4 Sn 2 O 16 , Ga 2.8 In 5.2 Sn 2 O 16  and Al 0.1 Ga 2.7 In 5.2 Sn 2 O 16 . 
       FIG. 10  is a graph showing a current-voltage characteristic of an Al 2 O 3 —Ga 2 O 3 —In 2 O 3 —SnO 2  system light transmitting electrode in contact with the p-type Gallium Nitride-based cladding layer of the invention, wherein one division of X axis is 0.5 V, and one division of Y axis is 0.2 mA. 
       FIG. 11  is a graph showing a current-voltage characteristic of a Gallium Nitride-based contact layer with Gallium rich phase in between the p-type Gallium Nitride-based cladding layer and the Al 2 O 3 —Ga 2 O 3 —In 2 O 3 —SnO 2  system light transmitting electrode of the invention, wherein one division of X axis is 0.5 V, and one division of Y axis is 0.2 mA. 
       FIG. 12  is a graph showing a current-voltage characteristic of an intermediate layer which is in between the p-type cladding layer and Gallium Nitride-based contact layer of the invention, wherein one division of X axis is 0.5 V, and one division of Y axis is 0.2 mA. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   To prepare the samples under the same thickness condition of an Indium-Tin Oxide (ITO), Ga 1.6 In 6.4 Sn 2 O 16 , Ga 2.8 In 5.2 Sn 2 O 16  and Al 0.1 Ga 2.7 In 5.2 Sn 2 O 16  transparent conducting oxides. The color of the Al 2 O 3 —Ga 2 O 3 —In 2 O 3 —SnO 2  system samples ranged from light blue-green (slight Aluminum content) to light green (high Gallium content) to green (low Gallium content). The color of a polycrystalline Indium-Tin Oxide (ITO) sample was also green, but dark than any of the Al 2 O 3 —Ga 2 O 3 —In 2 O 3 —SnO 2  system samples. 
     FIG. 9  compares the transmittance spectra (UV-V is region) of ITO, Ga 1.6 In 6.4 Sn 2 O 16 , Ga 2.8 In 5.2 Sn 2 O 16  and Al 0.1 Ga 2.7 In 5.2 Sn 2 O 16 . Transmittances of the Al x Ga 3-x-y In 5+y Sn 2-z O 16-2z  compositions are slightly superior to Indium-Tin Oxide (ITO) at wavelength (λ)&gt;400 nm, in addition, the Al x Ga 3-x-y In 5+y Sn 2-z O 16-2z  composition have a lower absorption at UV region. The present invention shows, increasing the Gallium concentration or slight Aluminum concentration to Al x Ga 3-x-y In 5+y Sn 2-z O 16-2z  compositions, which can have higher transmission in blue-green region. 
   The transmission property of transparent conducting oxide is determined by material band gap at lower wavelength limit:
 
λ bg   =hc/E   g ,
 
and upper wavelength limit is determined by charge carrier density:
 
λ p =2 π[mc   2 /4π( N/V ) e   2 ] 1/2 ,
 
Also, material defect density and phase relation determine the transmission property of transparent conducting oxide material. In the present invention of Al x Ga 3-x-y In 5+y Sn 2-z O 16-2z  composition, when the Aluminum concentration is increasing to x&gt;1, the transmittance of Al x Ga 3-x-y In 5+y Sn 2-z O 16-2z  composition is decay tremendously.
 
   The Al x Ga 3-x-y In 5+y Sn 2-z O 16-2z  composition, where x&gt;1, the sheet resistance is increasing and the carriers concentration is decreasing a lot, from this invention result, if the Aluminum concentration is too high in the Al x Ga 3-x-y In 5+y Sn 2-z O 16-2z  composition, the defect density will increase and the monoclinic β-Gallia phase structure will be destroyed by rising Aluminum concentration to over x&gt;1 in the Al x Ga 3-x-y In 5+y Sn 2-z O 16-2  composition. The preferred consist of Al 2 O 3 —Ga 2 O 3 —In 2 O 3 —SnO 2  system may be represented by the formula:
 
Al x Ga 3-x-y In 5+y Sn 2-z O 16-2z ,
 
where 0≦x&lt;2, 0&lt;y&lt;3, 0≦z&lt;2.
 
   Furthermore, in present invention, the Al x Ga 3-x-y In 5+y Sn 2-z O 16-2z  have a tetragonal structure phase in which Sn is incorporated as a structural element rather than as a substitution dopant as it is in Indium-Tin Oxide (ITO). For this reason, the new Al 2 O 3 —Ga 2 O 3 —In 2 O 3 —SnO 2  system has more stable and reliable in comparison to Indium-Tin Oxide (ITO). 
   In present invention, the light transmitting layer of Al 2 O 3 —Ga 2 O 3 —In 2 O 3 —SnO 2  system has to be deposited in contact with the p-type Gallium Nitride-based cladding layer and annealing at a temperature 200° C. or more. There still have a shocky barrier and formed a poor ohmic contact between the light transmitting electrodes of Al 2 O 3 —Ga 2 O 3 —In 2 O 3 —SnO 2  system and p-type Gallium Nitride-based cladding layer.  FIG. 10  shows a current-voltage characteristic of an Al 2 O 3 —Ga 2 O 3 —In 2 O 3 —SnO 2  system light transmitting layer in contact with the p-type Gallium Nitride-based cladding layer of the invention. 
   In present invention, to consider the conductivity of light transmitting layer of Al 2 O 3 —Ga 2 O 3 —In 2 O 3 —SnO 2  system, the thickness must over 5 Angstroms. 
   In present invention, to grow a Gallium Nitride-based contact layer with Gallium rich phase in between the p-type Gallium Nitride-based cladding layer and the Al 2 O 3 —Ga 2 O 3 —In 2 O 3 —SnO 2  system light transmitting layer, and annealed at a temperature 200° C. or more, the Gallium of a Gallium Nitride-based contact layer with Gallium rich phase is partially diffused into the Al x Ga 3-x-y In 5+y Sn 2-z O 16-2z  light transmitting layer, and formed a preferable high Gallium content of Al x Ga 3-x-y In 5+y Sn 2-z O 16-2z  interface, and establishes a good ohmic contact with a Gallium Nitride-based contact layer, and the thickness of this Gallium Nitride-based contact layer is between 5 Angstroms to 1000 Angstroms. A current-voltage characteristic of a Gallium Nitride-based contact layer with Gallium rich phase in between the p-type Gallium Nitride-based cladding layer and the Al 2 O 3 —Ga 2 O 3 —In 2 O 3 —SnO 2  system light transmitting layer is shown in  FIG. 11 . 
   The Gallium Nitride-based contact layer with Gallium rich phase has highly disorder structure in comparison to an p-type Gallium Nitride-based cladding layer, which a current cross a p-type Gallium Nitride-based cladding layer and a Gallium Nitride-based contact layer that the current spiking will appear in between two layer, and the thickness of this Indium Nitride-based intermediate layer is between 5 Angstroms to 500 Angstroms. 
     FIG. 7  shows to grow a Indium Nitride-based intermediate layer  17 B which is in between the p-type cladding layer  12  and Gallium Nitride-based contact layer  17 B and the Indium Nitride-based intermediate layer  17 B can be AlGaInN, AlInN, InGaN or InN materials and p-type, n-type or undoping, the material band-gap energy of this Indium Nitride-based intermediate layer must be lower than the p-type Gallium Nitride-based cladding layer  12 . 
     FIG. 12  shows a current-voltage characteristic of the lower material band-gap energy of a Indium Nitride-based intermediate layer between a p-type Gallium Nitride-based cladding layer and a Gallium Nitride-based contact layer, which come out a good function to reduce the electrical spiking effect in which current cross a p-type Gallium Nitride-based cladding layer and a Gallium Nitride-based contact layer. 
     FIG. 3  schematically shows an embodiment of the light-emitting device according to the invention of Al 2 O 3 —Ga 2 O 3 —In 2 O 3 —SnO 2  system light transmitting layer  11 B and Gallium Nitride-based contact layer with Gallium rich phase  17 A on a transparent insulating substrate  116 , such as Al 2 O 3 , LiGaO 2 , LiAlO 2  and MgAl 2 O 4 . And an n-type GaN as a first lower cladding layer  15  directly over said transparent insulating substrate  116 , an InGaN light-emitting layer  13  directly over said lower cladding layer  15 , a p-type GaN as second upper cladding layer  12  directly over said light-emitting layer  13 , an n-type electrode  14  formed on the partially exposed area of the n-type GaN  15 , a p-type electrode  10  formed on top of the light transmitting layer  11 B. 
     FIG. 4  schematically shows an embodiment of the light-emitting device according to the invention of Al 2 O 3 —Ga 2 O 3 —In 2 O 3 —SnO 2  system light transmitting layer  11 B and Gallium Nitride-based contact layer with Gallium rich phase  17 A on a transparent and electrically conducting substrate  216 , such as silicon carbide (SiC), Gallium Nitride-based (GaN) and Aluminum Nitride-based (AlN) substrate. The difference between this structure and that of  FIG. 3  is that an n-type electrode  14  formed underneath the conductivity type substrate  216 . 
     FIG. 5  schematically shows an embodiment of the light-emitting device according to the invention of Al 2 O 3 —Ga 2 O 3 —In 2 O 3 —SnO 2  system light transmitting layer  11 B and Gallium Nitride-based contact layer with Gallium rich phase  17 A on a silicon (Si)  316  substrate or ZnSe substrate. 
     FIG. 6  schematically shows an embodiment of the light-emitting device according to the invention of Al 2 O 3 —Ga 2 O 3 —In 2 O 3 —SnO 2  system light transmitting layer  11 B and Gallium Nitride-based contact layer with Gallium rich phase  17 A on a light absorption and electrically conducting substrate  416 , such as the Gallium Arsenide-based (GaAs based), Gallium phosphide-based (GaP) substrate. 
     FIG. 8  schematically shows an embodiment of the light-emitting device according to the invention of Al 2 O 3 —Ga 2 O 3 —In 2 O 3 —SnO 2  system light transmitting layer  11 B and Gallium Nitride-based contact layer with Gallium rich phase  17 A and a transparent conducting oxide window layer  11 D, such as SnO 2 , In 2 O 3 , ITO, Cd 2 SnO 4 , ZnO, CuAlO 2 , CuGaO 2 , SrCu 2 O 2 , NiO, AgCoO 2 , etc, can be formed in contact with this light transmitting electrode to improve the light efficiency and current spreading, and to consider the light efficiency and current spreading of transparent conducting oxide, the thickness must over 5 Angstroms. 
   Many changes and modifications in the above-described embodiments of the invention can, of course, be carried out without departing from the scope thereof. Accordingly, to promote the progress in science and the useful arts, the invention is disclosed and is intended to be limited only by the scope of the appended claims.