Abstract:
A light-emitting diode (LED) for both AlGaInP- and GaN-based materials needs a good transparent current spreading layer to disseminate electrons or holes from the electrode to the active layer. The present invention utilizes a conductive and transparent ITO (Indium Tin Oxide) thin film with an ultra-thin (to minimize the absorption) composite metallic layer to serve as a good ohmic contact and current spreading layer. The present invention avoids the Schottky contact due to direct deposition of ITO on the semiconductor. For AlGaInP materials, a thick GaP current spreading layer is omitted by the present invention. For GaN-based LEDs with the present invention, semi-transparent Ni/Au contact layer is avoided. Therefore, the light extraction of LED can be dramatically improved by the present invention. Holes may be etched into the semiconductor cladding layer forming a Photonic Band Gap structure to improve LED light extraction.

Description:
FIELD OF THE INVENTION 
   The invention relates to the structure and fabrication of light emitting diodes. In particular, the invention can improve the light extraction efficiency and increase the light output. 
   BACKGROUND OF THE INVENTION 
   AlGaInP-based materials grown on lattice-matched GaAs substrates and GaN-based materials grown on sapphire or SiC substrates have led to major advances in high-brightness LEDs. That LED produces high brightness and posses complete visible spectrum to make solid-state lighting possible. The advancement of LED technology is attributed to the developments of advanced epitaxial growth technologies such as molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD). The conventional growth techniques such as liquid phase epitaxy (LPE) and hydride vapor phase epitaxy (HVPE) were not suitable for growing AlGaInP crystal layer. The advanced epitaxial growth technologies enable the formation of high-quality alloy of III-V materials. They facilitate band gap engineering such as heterostructure and multiple quantum wells (MQWs) structure, which in turn increase the internal quantum efficiency and produce more light output. However, some other technical issues such as current spreading, light extraction efficiency, and heat dissipation must be resolved in order to obtain high-brightness LEDs (high wall-plug efficiency and large power output). 
   The most popular technique to solve the current spreading problem is presented in the U.S. Pat. No. 5,008,718. The LED structure cited in this patent is illustrated in FIG.  1 A. Epitaxial layers are grown on the top of the n-GaAs substrate  10  in the following order: n-type AlGaInP cladding layer  13 , active layer  14  in double heterostructure with p-type AlGaInP cladding layer  15  over the active layer and then a thick window layer GaP  19  (15-60 μm). The electrodes  21  and  22  are deposited on both sides of the structure. The advantages of using the window layer GaP  19  are that it is transparent and highly electrically conductive. But, its drawback is the extremely high thickness, which results in increasing manufacturing cost. The thick window layer is also not suitable for some device configurations—such as resonant cavity LEDs (RCLEDs) and creating photonic bandgap (PBG) in LED devices. 
   Alternatively, a conductive transparent material—Indium Tin Oxide (ITO) is studied and applied as a current spreading layer. FIG.  1 B and  FIG. 1C  show the prior art of the LEDs with ITO current spreading layer. The ITO layer  20  in  FIGS. 1B and 1C  replaces the GaP layer  19  in  FIG. 1A  to serve as current spreading layer.  FIG. 1B  structure was disclosed in U.S. Pat. No. 5,481,122. The epitaxial structure of  FIG. 1B  is same as that in  FIG. 1A  except that a p-type contact layer  16  is inserted between the ITO layer  20  and cladding layer  15 . The transmission coefficient of ITO layer  20  is about 90% in the visible range. The electrical resistivity of n-type ITO (around 2˜5×10 −4  Ω-cm) is 100 times smaller than that of p-type GaP. However, a Schottky contact is formed between the ITO layer  20  and p-type contact layer  16 . It degrades performance of the LEDs. 
     FIG. 1C  shows the prior art disclosed in U.S. Pat. No. 6,580,096. Compared to the  FIG. 1A , a Distributed Bragg Reflector (DBR) layer  12  is added between the layer  13  and the substrate  10  to reduce the absorption of light in the absorbed substrate  13 . There are two lightly p-doped window layers  17  (GaP) and  18  (GaAs) to be added between the ITO layer  20  and the p-type cladding layer  15 . The layer  17  is used to form an ohmic contact and to facilitate current spreading. The ohmic contact issue is perhaps resolved by such structure. However, the process is much more complicated and current spreading is still an issue due to the lateral contact. 
   For GaN material, the semi-transparent p-type ohmic contact NiO/Au (transparency is about 60%) is used as current spreading. But it suffers from low transmission.  FIG. 2  shows the prior art LED presented in the paper  Semicond Sci. Technol . 18 (2003) L21-L23. An ITO layer  117  is deposited on the GaN-based LED structure, which contains in the following order: sapphire substrate  110 , a thin GaN nucleation layer  111 , n-type GaN cladding layer  112 , active layer  113 , p-type cladding layer AlGaN  114 , and p-type GaN contact layer  115 . The electrodes  121  and  122  are fabricated on the ITO layer  117  and n-GaN  112 , respectively. The major drawback is the Schottky contact formed between ITO  117  and the p-type GaN contact layer  115 ; such contact causes reliability problems. 
   SUMMARY OF THE INVENTION 
   According to one embodiment of the invention, a light emitting diode comprises an active light emission layer and a semiconductor layer over the active layer which pases light emitted by the active layer as light output from the diode. A current spreading composite layer is employed on the semiconductor layer. The composite layer includes a first metallic layer in contact with said semiconductor layer, and a second current spreading layer comprising indium tin oxide in contact with said first metallic layer. The two layers in the current spreading composite layer are substantially transparent to light emitted by the active layer. 
   According to an additional embodiment of the invention, a light emitting diode comprises an active light emission layer and a semiconductor layer over the active layer which pases light emitted by the active layer as light output from the diode. A current spreading composite layer is employed on the semiconductor layer. The composite layer includes a first metallic layer of not more than about 8 nm in total thickness in contact with said semiconductor layer, and a second current spreading layer in contact with said first metallic layer. The two layers in the current spreading composite layer are substantially transparent to light emitted by the active layer. 
   Another embodiment of the invention covers a method for making a light emitting diode. An active light emission layer is formed over a substrate and a semiconductor layer is formed over the active layer. A current spreading composite layer is provided on said semiconductor layer. The composite layer comprises a first metallic layer in contact with said semiconductor layer, and a second current spreading layer in contact with said first metallic layer. The two current spreading layers are substantially transparent to light emitted by the active layer. In one implementation, the second current spreading layer comprising indium tin oxide, and in another, the first metallic layer provided is not more than about 8 nm in total thickness 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  shows a cross-section view of a first prior art AlGaInP-based LED; 
       FIG. 1B  shows a cross-section view of a second prior art AlGaInP-based LED; 
       FIG. 1C  shows a cross-section view of a third prior art AlGaInP-based LED; 
       FIG. 2  shows a cross-section view of an addiitonal prior art GaN-based LED; 
       FIG. 3A  is schematic top view of AlGaInP-based light emitting diode to illustrate an embodiment of the invention; 
       FIG. 3B  shows the bonding pad in a mesh configuration to illustrate an embodiment of the invention; 
       FIG. 4  is a cross section view of AlGaInP-based light emitting diode to illustrate an embodiment of the invention; 
       FIG. 5  is a cross section view of AlGaInP-based light emitting diode with current blocking layer under bonding pad to illustrate an alternative embodiment of the invention; 
       FIG. 6  is a graphical illustration of I-V characteristics of the light emitting diode with different contact configurations to illustrate I-V characteristics of an embodiment of the invention. 
       FIG. 7  is schematic top view of GaN-based light emitting diode to illustrate an alternative embodiment of the invention; 
       FIG. 8  is a cross section view of GaN-based light emitting diode to illustrate yet another alternative embodiment of the invention; 
       FIG. 9  is a cross section view of GaN-based light emitting diode with current blocking layer under bonding pad to illustrate still yet another alternative embodiment of the invention; 
       FIG. 10  is a perspective view of a LED structure inscribed with a photonic band gap pattern and covered with a layer of ITO to illustrate another one of an alternative embodiment of the invention; 
       FIG. 11  is the cross-sectional view of the structure of  FIG. 10  along the line A—A in FIG.  10 . 
   

   For simplicity in description, identical components are labeled by the same numerals in this application. 
   DETAILED DESCRIPTION OF THE INVENTION 
   One embodiment of the present invention provides novel LED structures and a method for manufacturing such light emitting diode structures comprising a transparent conductive ITO current spreading layer with an ultra-thin composite metallic layer beneath it on p-type semiconductor to form a good ohmic contact. High transparency of ITO and low absorption of ultra-thin composite metallic layer does not degrade light extraction. On the contrary, the light extraction is significantly increased due to better current spreading and ohmic contact. 
   Light emitting diodes may be manufactured with a simple current spreading layer on wafer level using e-beam and sputtering techniques. The light emitting portions of the diode structures can be the same as those in any conventional LED structures. Also, the concept described herein can be applied to RCLEDs structures and PBG-inscribed LED structure. The process is simple, controllable, and reproducible. Therefore, it is suitable for low cost mass manufacturing. Also the current spreading layer with intermediate metallic layer can be applied to the texture surfaces or two-dimensional PBG slabs of certain LEDs. 
   In a first embodiment of the present invention, a light emitting diode comprises an ultra-thin composite metallic layer between the transparent current spreading layer and p-type semiconductor layer to form a good ohmic contact without sacrificing the light transparency. The ultra-thin metallic layer comprises at least one type of metal. Typically two metal layers such as a first layer of titanium and a second layer of gold may be deposited to form the ultra-thin composite layer. Here, the titanium layer serves as surface cleaning and adhesive agent when deposited over or on the p-type semiconductor layer. The second transparent current spreading layer is a thick ITO layer or its equivalents. The ultra-thin composite metallic layer will facilitate connection to the ITO layer to form an excellent ohmic contact after Rapid Thermal Annealing (RTA) treatment around 330˜440° C. for 0.5 to 2 minutes, which causes some of the metallic layer to diffuse into the ITO layer. The metal thickness of the ultra-thin composite metallic layer is optimized to form an excellent ohmic contact and minimize the light absorption from the active layer. 
   The p-type bonding pad can be formed on the top of the ITO layer by titanium and gold layers. 
   The present invention is equally applicable to n-type semiconductor LEDs with the ITO combined with ultra-thin composite metallic layer. 
   The first embodiment in the present invention is illustrated schematically in  FIG. 3-5  for the LED structure  200  and LED device of AlGaInP-based materials.  FIG. 3  shows the top metal (Ti/Au etc.) bonding pad  221  on ITO layer  218  of the LED  200 . In order to obtain better current spreading in any one of the embodiments described herein, the bonding pad  221  can be constructed in a mesh configuration  241  as indicated in FIG.  3 B. 
   As illustrated by the structures in FIG.  4  and  FIG. 5 , the LED structure is first grown on the lattice-matched n-GaAs substrate  210 . An n-GaAs buffer layer  211  is grown followed by a Distributed Bragg Reflector (DBR)  212  of AlGaAs-based or AlGaInP-based materials to reflect the light out of being absorbed by the GaAs substrate. The n-AlGaInP bottom cladding layer  213  is grown and followed by the active layer  214 , which can be double heterostructure or multi-quantum well (MQW) to optimize the internal quantum efficiency. Then the p-AlGaInP upper cladding layer  215  is formed with an appropriate optimized doping profile (to maximize current injection and avoid out-diffusion). For the conventional LED structure, the layer  216  will be a thick current spreading semiconductor layer such as GaP and AlGaAs. In the present invention, the layer  216  is replaced by a heavily p-type doped GaP, very thin InGaP, or GaAs contact layer. In addition, an ultra-thin composite metallic layer  217  having at least one metal such as titanium (Ti), gold (Au), zinc (Zn), indium (In) beryllium (Be) or nickel (Ni) within it. For example, one of these metals may be deposited as a first layer on the top of the layer  216 . When Ti is used, for example, as the metal in such layer, Ti has the functions of cleaning up and increasing the adhesion with the layer  216 . The composite metallic layer may also include metals such as Ti/Au, Ti/Au—Zn, Ti/Au—Be, Ni/Au, Au—Zn, or In—Be, such as in a layer different from the first layer. Such alloys are a few examples for the composition of another layer in the ultra-thin composite metallic layer  217 . The total thickness of the ultra-thin composite metallic layer is not more than about 8 nm, and preferably in a range of about 0.4 to 8 nm. An ITO layer  218  of thickness of about 40 to 1,000 nm is applied to the top of the composite metallic layer  217 . The ITO layer  218  can have transmission more than 90% and conductivity p about 2˜5×10 −4  Ω-cm. The composite metallic layer  217  and ITO layer  218  can be deposited using e-beam (electron beam directed to a metallic material), sputtering or other deposition techniques. Such techniques are known to those skilled in the art and will not be described here. Thereafter, the full substrate is subjected to a Rapid Thermal Annealing (RTA) treatment at 330˜440° C. to assist the composite metallic layer  217  to form a good ohmic contact with ITO and the layer  216 . To further enhance the current spreading to cover the area of the active layer  214 , a dielectric film such as SiO 2  or Si 3 N 4    220  can be deposited locally underneath the bonding pad  221  as shown in FIG.  5 . The bottom ohmic contact layer  222  is formed out of Ni/Au—Ge. 
     FIG. 6  shows the I-V characteristics for different device configurations for AlGaInP-based LEDs—(a) contact metal formed on the semiconductor directly, (b) ITO coated with an ultra-thin composite metallic layer of the present invention, and (c) ITO coated on the semiconductor directly. It can be seen that the LED forward voltage V f  measured with a 20 mA current injection is 2.0V, 2.2V, and 2.85V for cases (a), (b), and (c), respectively. It is clear that the forward voltage Vf of Case (b) is 0.65 V less than that of Case (c). 
   A second embodiment in the present invention is illustrated schematically in  FIG. 7-9  for the LED structure and LED device of GaN-based materials.  FIG. 7  shows the top metal (Ni/Au etc.) bonding pad  321  of the LED. 
   As illustrated by the structures shown in FIG.  8  and  FIG. 9 , the LED structure is first grown on the lattice-mismatched insulating sapphire substrate  310  using MOCVD. A very thin low temperature GaN nucleation layer  311  is grown, followed by a thick n-type GaN  312 . The active layer  313  comprises several InGaN/GaN strained MQWs. Finally, the p-AlGaN upper cladding layer  314  for electrical confinement and p-GaN contact layer  315  are deposited. 
   In the conventional LED structure, the current spreading layer is a thin semi-transparent metal layer such as NiO/Au (transmission is around 60% with thickness of 10/30 nm). In the present invention, an ultra-thin composite metallic layer such as Ti/Au, Ti/Au—Zn, Ti/Au—Be, Ti/In—Be or Ni/Au  316 , followed by a transparent and conductive ITO layer  317  can be coated using e-beam and sputtering techniques. Thereafter, the wafer is processed to define the contact areas for positive and negative electrodes utilizing typical semiconductor fabrication techniques. The device structure of the GaN-based LED is shown in FIG.  7  and FIG.  8 . The top bonding-pad  321  can be formed out of Ti/Au. The n-type ohmic contact  322  can be formed out of Ti/Al/Au. In order to increase the adhesion of the metal, metal Ti is applied with sputtering techniques to increase the bonding energy. 
   To further enhance the current spreading to cover the area of the active layer  214 , a dielectric film such as SiO 2  or Si 3 N 4    320  can be deposited locally underneath the bonding pad  321  as shown in FIG.  9 . 
   LED generally suffers from low extraction efficiency due to multiple total internal reflections at the walls of the high index semiconductor materials. Photonic band gap structure or so-called photonic crystal is one the methods proposed herein to increase the extraction efficiency of the LED. A lattice of holes is formed in the semiconductor layers of the LED as indicated in FIG.  10 . For LEDs emitting visible light, the hole diameter ranges from 80 to 300 nm and lattice constant from 100 to 400 nm. The lattice pattern can be triangular as indicated by  501  or other patterns such as rectangle and hexagon. The numerals  511 ,  512 ,  513 ,  514  (the shaded layer), and  516  refer to respectively the substrate, DBR, cladding layer, waveguide layer, and cladding layer, respectively. The active layer  515  comprised of multiple quantum well (MQW) or strain MQW is imbedded within the waveguide layer  514 . The ultra-thin composite metallic layer as described in  FIGS. 4 ,  5 ,  8  and  9  is deposited on the top of the semiconductor layer  516 . The semiconductor layer  516  in general may consist of multiple semiconductor layers to serve functions such as enhancing electric property in addition to confining the optical power in the waveguide layer  514 . ITO  518  is thereafter applied to the top of the ultra-thin composite metallic layer  517 . Transparency property and high conductivity of ITO  518  are nicely utilized to this photonic band gap LED. After layers  517 ,  518  and cladding layer  516  have been formed, holes  531  are etched into these three layers to form the Photonic Band Gap structure, as shown in  FIGS. 10 and 11 , such as by an electron beam lithography process. The holes etched may extend into and through the semiconductor layer  516 . The etching process does not have to etch holes to precise depths. Thus, the LED will still perform adequately even if the holes are etched into the waveguide and active layer  514 . Layer  520  is the metal pad for conducting the current to the external electrical terminal. The metal pad  520  is deposited on the top of ITO  518 . 
     FIG. 11  is a cross-section view A—A of the LED structure of  FIG. 10 , after the ITO layer  518  has been applied to the structure. The light merges from the top of the wafer as indicated by  533 . The holes  531  are drilled by etching through the ITO layer  518 , the ultra-thin metallic layer  517  and then cladding layer  516 . In some cases, the holes  531  may deepen into the waveguide layer  514 . 
   In the fabrication process, the holes of the photonic crystal can also be drilled by etching into the cladding layer  516  first, and then apply the ultra-thin composite metallic and ITO layer later. Therefore holes in the ITO layer are avoided. This can increase the current spreading area without affecting the extraction function of the photonic band gap structure. Also, avoiding etching through ITO makes the hole diameter, depth and shape in the cladding layer more controllable. 
   While the invention has been described by reference to various embodiments, it will be understood that modification changes may be made without departing from the scope of the invention which is to be defined only by the appended claims or their equivalents. For example, while the embodiments are illustrated with an n-type semiconductor material used as the substrate and the LED is terminated with a p-type semiconductor material from which light is emitted as the light output of the LED, it will be understood that the invention is equally applicable where a p-type semiconductor material is used as the substrate and the LED is terminated with an n-type semiconductor material from which light is emitted as the light output of the LED. Such and other variations are within the scope of the invention. The LED may be implemented in AlGaInP-. AlGaAs-, AlGaN-, InGaN-, or GaN-based or other suitable materials. All references referred to herein are incorporated by reference in their entireties.