Abstract:
A light emitting diode comprising an epitaxial layer structure, a first electrode, and a second electrode. The first and second electrodes are separately disposed on the epitaxial layer structure, and the epitaxial layer structure has a root-means-square (RMS) roughness less than about 3 at a surface whereon the first electrode is formed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application for patent claims priority under 35 U.S.C. §119 to Provisional Application No. 61/041,172, filed Mar. 31, 2008. 
    
    
     BACKGROUND 
     1. Field 
     The present disclosure relates to a light emitting diode (LED) structure, and more particularly to a LED structure with a smooth surface for a reflective electrode. 
     2. Background 
     Light emitting diodes (LEDs) have been developed for many years and have been widely used in various light applications. As LEDs are light-weight, consume less energy, and have a good electrical power to light conversion efficiency, in some application areas, there have been intentions to replace conventional light sources, such as incandescent lamps and fluorescent light sources, with LEDs. Such LEDs produce light in a relatively narrow angular spread direction without side light so that the light cannot be easily collected by optical elements in a package. In other words, thin-film AlInGaN LEDs produce more light per steradian and photons generated therefrom can be efficiently utilized compared to the conventional lateral LEDs with sapphire substrate attached. However, the efficiency (Lumen/W) of the current LEDs is still not high enough to replace the conventional light source for general illumination or other light applications. 
     Therefore, there is a need in the art to improve the structure of the LEDs so that they emit light in more efficient ways than conventional LEDs. 
     SUMMARY 
     In an aspect of the disclosure, a light emitting diode includes an epitaxial layer structure, a first electrode, and a second electrode. The first and second electrodes are separately disposed on the epitaxial layer structure, and the epitaxial layer structure has a RMS (root-mean-square) roughness less than 3 nm on a surface wherein the second electrode is formed. 
     In another aspect of the disclosure, a method for manufacturing a light emitting diode includes forming an epitaxial layer structure, and separately depositing a first electrode and a second electrode on the epitaxial layer structure. The epitaxial layer structure has a RMS roughness less than about 3 nm on a surface whereon the second electrode is formed. 
     It is understood that other aspects of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein it is shown and described only exemplary aspects of the invention by way of illustration. As will be realized, the invention includes other and different aspects and its several details are capable of modification in various other respects, all without departing from the spirit and scope of the present invention. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not as restrictive. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       Various aspects of the present invention are illustrated by way of example, and not by way of limitation, in the accompanying drawings, wherein: 
         FIG. 1A  is a cross sectional view of a vertical LED structure. 
         FIG. 1B  is a top view of a vertical LED structure, in which a patterned n contact is shown. 
         FIG. 2A  is a cross sectional view of a flip-chipped lateral LED structure with metal joints and a sub-mount. 
         FIG. 2B  is a top view of a flip-chipped lateral LED structure with both p and n electrodes before flip-chipped to a sub-mount. 
         FIG. 3  illustrates an exemplary light extraction occurring in a vertical LED structure. 
         FIG. 4  is a schematic diagram of a rough interface between a p-GaN layer and a silver (Ag) layer, showing that the rough interface scatters an incident light as well as couples the light into the surface plasmon mode. 
         FIGS. 5A-5C  illustrate a process for manufacturing a vertical LED that has a smooth surface for forming reflective electrode. 
         FIGS. 6A-6C  illustrate a process for manufacturing a flip-chipped lateral LED structure that has a smooth surface for forming reflective electrode. 
     
    
    
     DETAILED DESCRIPTION 
     The detailed description set forth below in connection with the appended drawings is intended as a description of various aspects of the present invention and is not intended to represent all aspects in which the present invention may be practiced. The detailed description includes specific details for the purpose of providing a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced without these specific details. In some instances, well-known structures and components are shown in block diagram form in order to avoid obscuring the concepts of the present invention. 
       FIGS. 1A and 1B  illustrate a cross-sectional view and a top view of a vertical LED device, respectively. The vertical LED device  100 , as shown in  FIG. 1A , has a vertical current injection configuration, including a patterned n-type contact (or n-type electrode)  101 , an n-type GaN-based layer  102  with a roughened surface, an active region  103 , a p-type GaN-based layer  104 , a broad area reflective p-type contact (or p-type electrode or reflective p electrode)  105 , and a thermally and/or electrically conductive substrate  106  to support the device structure mechanically. 
     In the manufacturing process, the n-type GaN-based layer  102  is formed on a substrate (not shown), the active region  103  is formed on the n-type GaN-based layer  102 , and the p-type GaN-based layer  104  is formed on the active region  103 , however, other layers may be included. The p-type electrode  105  is directly or indirectly formed on p-type GaN-based layer  104 . The substrate on which the n-type GaN-based layer  102  is formed is removed so that the patterned n-type electrode  101  can be formed on the surface of the n-type GaN-based layer  102  that was attached to the removed substrate. The reflective p-type electrode  105  is mounted on the thermally conductive substrate  106  for mechanical support. 
     As the n-type GaN-based layer  102  and the p-type GaN-based layer  104  are opposite to each other, together they form a pair of carrier injectors relative to the active region  103 . Therefore, when a power supply is provided to the LED device  100 , electrons and holes will be combined in the active region  103 , thereby releasing energy in the form of light. In  FIG. 1A , arrows inside the LED device  100  show that an electrical path is generally vertically formed from the p-type electrode  105  to the patterned n-type electrode  101 .  FIG. 1B  shows a top view of the vertical LED of  FIG. 1A , in which an n-type contact with four fingers and a crossbar is shown. It will be recognized by those of ordinary skill in the art that the electrode pattern of the n-type contacts is not limited to the electrode pattern as illustrated. 
       FIG. 2A  illustrates a cross-sectional view of a flip-chipped lateral LED device  200 . As shown, the flip-chipped lateral LED device  200  is formed as a lateral LED device  200 ′ having a lateral current injection configuration that is flipped over and mounted on a sub-mount substrate  207  with matching metal contact pads  208 . The sub-mount substrate  207  may be electrically insulated or electrically conductive. The metal contact pads  208  are electrically isolated from each other either by forming an electrically insulating sub-mount  207  or an insulating dielectric coating formed over an electrically conducting sub-mount (not shown). The lateral LED device  200 ′ includes an n-type GaN-based layer  201  with a roughened surface, an active region  202  formed on the n-type GaN-based layer  201 , a p-type GaN-based layer  203 , a p-type electrode  204 , and an n-type electrode  205 . 
     In the manufacturing process, before forming the n-type electrode, parts of the p-type electrode  204 , the active region  202 , and the p-type GaN-based layer  203  are removed to allow the n-type electrode  205  to be formed on top of the n-type GaN-based layer  201 . In  FIG. 2A , the arrows inside the LED device  200 ′ show that an electrical path is formed from the p-type electrode  204  to the n-type electrode  205 . After the n-type electrode  205  is formed, the LED device  200 ′ is flipped over to mount on the sub-mount substrate  207  via solders or metal interconnects  206  to form the flip-chipped lateral LED device  200 . 
       FIG. 2B  shows a top view of the LED device  200 ′ of  FIG. 2A  before being flipped over to mount on the sub-mount substrate  207 .  FIG. 2B  shows that the p-type electrode  204  has an area larger than that of the n-type electrode  205 . 
     The n-type GaN-based layer, the p-type GaN-based layer, and the active layer in the LED devices of  FIGS. 1A-1B  and  2 A- 2 B are formed using a GaN-based material. When a voltage is applied to the LED devices, injected carriers (i.e., holes and electrons) recombine in the active layers, and generate light emission. The reflective index of the GaN-based material is around 2.4 at a wavelength of 460 nm. If an incident angle of light at the interface between the GaN-based layer and the ambient air (or other encapsulating material) is greater than a critical angle, a substantial portion of light generated inside the LED device is likely to get trapped inside the LED device due to total-internal-reflection (TIR). According to Snell&#39;s Law, the critical angle at the GaN/air interface is about 24.6 degrees. Conventionally, to increase the chance of light escaping from the LED device, the top surface of the LED device is randomly roughened to break up the limitation of the TIR. 
     Light extraction for an LED device will be described with reference to  FIG. 3 . The LED device  300  in  FIG. 3  is a vertical LED device that includes a GaN-based material structure  310 , including an n-type GaN-based layer  302 , an active region  303 , a p-type GaN-based layer  304 , an n-type electrode  301 , and a p-type electrode  305  mounted on a substrate  306 . Reference number  320  indicates the direction of travel of light generated inside the LED device  300 . Usually, the light emitting from the active region  303  has about a 50/50 chance of propagating toward the top surface or the bottom surface of the LED device  300 . The p-type electrode (also referred to interchangeably herein as the reflective electrode)  305  is used to re-direct the light propagating back to the top surface of the LED device  300 , as shown in  FIG. 3 . The reflective electrode  305  usually contains a metal, and the reflectivity of the reflective electrode  305  is made to be as high as possible to reduce reflection loss, since the light emission tends to be reflected multiple times before escaping the LED devices, as shown in  FIG. 3 . 
     Silver (Ag) is a suitable metal for forming a reflective electrode of an AlInGaN LED device because Ag has a high reflectivity in the wavelength range of interest (i.e., 400-700 nm), and this material can form ohmic contact with a p-type GaN-based layer. Due to an epitaxial growth process and growth conditions used in manufacturing the GaN-based material structure, there is generally some roughness on the p-type GaN-based layer. 
       FIG. 4  is an enlarged view of the p-type electrode  305  comprising Ag and the p-type GaN-based layer  304  of  FIG. 3 , showing light extraction inside the LED device  300 . Usually, the normal incidence of reflectance of Ag measured from the p-type GaN-based layer side is lower than an expected value from a simple optical model calculation because of scattering effects  403  and surface plasmon (SP) absorption  420  resulting from the rough GaN/Ag interface  410 , as shown in  FIG. 4 . The rough interface  410  scatters the normal incident light in random directions and reduces the specular reflection at all wavelengths. No photons are typically lost in the scattering process because the photons will continue to be reflected inside the LED device and will eventually escape the LED device  300  to, for example, ambient air. However, the photons coupled in the surface plasmon (SP) mode  420  typically will not be able to escape and eventually will be lost. As result, the SP absorption can have a significant effect on the reflectance measurement. Under experimental measurements, the strength of the SP absorption generally correlates to the degree of interface roughness. Even though the SP absorption peak may be below 400 nm, the width of the absorption may still be wide enough to significantly reduce Ag reflectance for wavelengths of around 460 nm. 
     The p-type electrode (e.g., Ag electrode) may be deposited on the p-type GaN-based layer by a physical (e.g., electron-beam or thermal) evaporation process, and Ag will conform to the p-type GaN-based layer surface without voids if the deposition is performed properly. The roughness of the p-type GaN-based layer/Ag interface is determined by the quality of the p-type GaN-based layer. The presence of voids created during the deposition process or contact annealing process, however, may increase the interface roughness and further enhance the SP absorption. 
     To achieve a high reflectance value at the p-type GaN layer/Ag electrode interface, an LED device is provided in which the p-type GaN-based layer has a root-mean-square (RMS) roughness less than about 3 nm to ensure appropriate smoothness of the p-type GaN-based layer/Ag electrode interface and to thereby minimize the SP absorption. 
     In a variation, a method for manufacturing the p-type GaN-based layer with a smooth surface is provided, such that the SP absorption can be reduced to a maximum extent. 
       FIGS. 5A-5C  and  6 A- 6 C illustrate manufacturing processes for a vertical LED device  500  and a flip-chipped lateral LED device  600 , respectively. The processes of  FIGS. 5A and 6A  basically follow similar manufacturing steps, except with regard to the formation of an n-type electrode and final mounting. 
     In  FIG. 5A , an n-type GaN-based layer  502  is formed on a substrate, such as a sapphire substrate  501 . Above the n-type GaN-based layer  502 , an active layer  503  and a p-type GaN-based layer  504  are formed. In an example, the RMS roughness of the surface of the p-type GaN-based layer  504  that will interface with a p-type electrode  505 , shown in  FIG. 5B , is preferably controlled to be less than about 3 nm. In an example, the p-type GaN-based layer  504  is formed by MOCVD (Metal-Organic Chemical Vapor Deposition) using metal-organic compound such as trimethyl gallium (TMGa), trimethyl indium (TMIn), trimethyl aluminum (TMAl), and ammonia, hydrogen, nitrogen as well as dopant precursors for silicon and magnesium in a reactor chamber with controlled pressure and temperature. Furthermore, the growth temperature in the deposition process is preferably greater than about 950° C. and the growth rate is preferably less than about 150 Å/min. 
     After depositing the p-type GaN-based layer with RMS roughness less than about 3 nm, the p-type electrode  505  is formed on the p-type GaN-based layer  504 , as shown in  FIG. 5B . Alternatively, prior to depositing the p-type electrode  505 , a transparent ohmic contact layer (not shown) may be formed on the p-type GaN-based layer  504 . The transparent ohmic contact layer may be formed by, for example, electron-beam evaporation, sputtering, MOCVD, etc., with doped metal oxides, such as indium tin oxide (ITO) or aluminum doped zinc oxide (AZO). Similarly, when the transparent ohmic contact layer is presented, the RMS roughness of the transparent ohmic contact layer may be controlled to be less than about 3 nm regardless of the RMS roughness of the p-GaN surface. 
     As described above, the p-type electrode  505  of  FIGS. 5B and 5C  is a reflective layer for reflecting light emitted downwardly back to the top surface of the LED device, as shown in  FIGS. 5B and 5C . Exemplary metals used in forming the p-type electrode  505  include Ag, Pt, Ni, Cr, Ti, Al, Pd, Ru, Rh, Mo, and their alloys. 
     In a variation, after the p-type electrode  505  is formed, the substrate  501  is removed from the n-type GaN-based layer  502  to allow an n-type electrode  506  to be formed on the surface of the n-type GaN-based layer  502  that was attached to the substrate  501 , as shown in  FIG. 5C . The surface of the n-type GaN-based layer  502  on which the n-type electrode  506  is formed is roughened according to a conventional roughening method to minimize the total internal reflection (TIR) effect and enhance light extraction efficiency. Also, as shown in  FIG. 5C , the p-type electrode  505  may be mounted on a sub-mount substrate  508  for a mechanical support. The sub-mount substrate  508  may include similar materials to those used in the substrate  501 . That is, the sub-mount substrate  508  may be selected from one or more of the followings: metals, such as Cu, Mo, W, and Al, or their alloys; semiconductor materials, such as Si, GaAs, GaP, InP, and Ge; and/or ceramics, such as Al 2 O 3  and AlN. 
       FIGS. 6A-6C  will now be described in further detail. Similar to  FIG. 5A , in  FIG. 6A , an n-type GaN-based layer  602 , an active layer  603 , and a p-type GaN-based layer  604  are formed on a substrate, such as a sapphire substrate  601 . As described above with reference to  FIG. 5A , the p-type GaN-based layer  503  may be formed by a MOCVD (Metal-Organic Chemical Vapor Deposition) using metal-organic compound such as trimethyl gallium (TMGa), trimethyl indium (TMIn), trimethyl aluminum (TMAl), and ammonia, hydrogen, nitrogen as well as dopant precursors for silicon and magnesium in a reactor chamber with controlled pressure and temperature. The growth temperature in the deposition process is preferably greater than about 950° C. and the growth rate is preferably less than about 150 Å/min. Furthermore, the RMS roughness of the p-type GaN-based layer may be, for example, less than about 3 nm. 
     Next, in  FIG. 6B , a transparent ohmic contact layer  609  and a reflective p-type electrode  605  are formed on the p-type GaN-based layer  604 . As described above, the transparent ohmic contact layer  609  is optional and does not limit the scope of the invention. The RMS roughness of the surface in direct contact with reflective electrode  605 , for example, the p-GaN surface or the transparent ohmic contact layer surface, may be less than 3 nm. 
     As a flip-chipped lateral LED device  600 ,  FIG. 6C  shows that parts of the reflective p-type electrode  605 , the transparent ohmic contact layer  609 , and the p-type GaN-based layer  604  are etched away to allow an n-type electrode  606  to be formed on top of the n-type GaN-based layer  602 , as shown in  FIG. 6C . The LED structure made by this process is then flipped over and mounted on a sub-mount substrate  608  with matching metal contact pads  611  via solder joints or metal interconnects  610 . The metal contact pads  611  are electrically isolated from each other either by an electrically insulating sub-mount  608  or an insulating dielectric coating formed over an electrically conducting sub-mount (not shown). 
     Example embodiments in accordance with aspects of the present invention have now been described in accordance with the above advantages. It will be appreciated that these examples are merely illustrative of aspects of the present invention. Many variations and modifications will be apparent to those skilled in the art. 
     The previous description is provided to enable any person skilled in the art to practice the various aspects described herein. Various modifications to these aspects will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other aspects. Thus, the claims are not intended to be limited to the aspects shown herein, but is to be accorded the full scope consistent with the language claims, wherein reference to an element in the singular is not intended to mean “one and only one” unless specifically so stated, but rather “one or more.” Unless specifically stated otherwise, the term “some” refers to one or more. All structural and functional equivalents to the elements of the various aspects described throughout this disclosure that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the claims. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the claims. No claim element is to be construed under the provisions of 35 U.S.C. §112, sixth paragraph, unless the element is expressly recited using the phrase “means for” or, in the case of a method claim, the element is recited using the phrase “step for.”