Abstract:
A light-emitting diode is based on an undoped intrinsic SiC substrate on which are grown: an insulating buffer or nucleation structure; a light-emitting structure; window layers; a semi-transparent conductive layer; a bond pad adhesion layer; a p-type electrode bond pad; and an n-type electrode bond pad. In one embodiment, the light-emitting surface of the substrate is roughened to maximize light emission.

Description:
The present invention relates to a light-emitting diode (LED) device and a method for producing and operating the same. More particularly, the present invention relates to an LED having an improved design and output characteristics. Even more particularly, the present invention relates to an LED formed on a high resistivity silicon carbide substrate with a lateral device structure. 
     BACKGROUND OF THE INVENTION 
     The efficiency of a light-emitting diode (LED) is limited by a number of factors that constitute recurring challenges for LED device engineers. Among them, generated light can be absorbed by the layers of semiconductor material that constitute the LED, and it can be occluded by the electrodes that are required to bring activation current to the active region of the device. 
     Silicon carbide (SiC) with relatively low resistance (i.e., highly doped) has been commonly used as a conductive substrate material for high brightness LEDs in the blue, green, and near-ultraviolet spectral range. For LEDs in this spectral range, Gallium nitride (GaN) has been used as a basic light-emitting material. GaN-based LED structures are normally grown on the substrate, layer by layer, through vapor deposition processing, which is generally a metal-organic chemical vapor deposition process (MOCVD). 
       FIG. 1  depicts a schematic diagram of a conventional LED device  10 , or an LED chip, built on a substantially conductive SiC substrate  20 . Two electrodes  21 ,  22 , serving as ohmic contacts, are disposed at opposite sides of the substrate  20 . One of the electrodes  21 , which is referred to herein as the top electrode  21 , is positioned at the side of the substrate  20  upon which the LED is built (i.e., the MOCVD or epitaxial layer side). The other electrode  22  is referred to herein as the bottom electrode  22  and is positioned at the side of the substrate  20  opposite the epitaxial layer side. A buffer layer  23  is disposed on the SiC substrate  20 , and a light-emitting structure  24  is disposed on the buffer layer. The light-emitting structure  24  includes an active region  26  flanked by an n-type cladding layer  25  and a p-type cladding layer  27 . 
     There are performance issues associated with this device design. To grow high quality GaN material on SiC substrate a 3% lattice mismatch needs to be considered. Lattice mismatches induce strain in the crystal structure that leads to performance limiting crystal structure defects or degrades electronic device reliability. 
     Usually, an aluminum nitride (AlN) layer with only 1% lattice mismatch to the SiC is used as a transition layer between SiC and GaN. Since AlN is highly resistive, LEDs made with an AlN transition layer exhibit very high forward voltage that results in high power consumption and low efficiency. 
     In order to reduce the resistivity of the transition layer, an aluminum-gallium-nitride (AlGaN) layer can be employed. AlGaN can be doped n-type and create much higher conductivity than AlN. However, since the lattice mismatch issue must be addressed, the AlGaN compound used still requires a high aluminum (Al) composition. This results in a limited improvement of the forward voltage. 
     A second issue is that in order to form a low resistance current flow path from the top electrode to the bottom electrode during device operation, the SiC substrate is required to be highly doped. When the substrate is highly doped the SiC becomes more absorptive of light energy, especially in the blue-green and near-ultraviolet range light, with a wavelength of about 400-550 nm, which reduces the substrate&#39;s efficiency as a light transmitter. A compromise between light output and the forward voltage is therefore unavoidable. Therefore, what is needed is an LED architecture that provides for improved light output. 
     SUMMARY OF THE INVENTION 
     An LED consistent with the present invention emits light in about the 400-550 nm range of the light spectrum and is characterized by a high-energy conversion efficiency between the device driving current and output optical energy. The LED has a substrate side including a substantially non-conductive SiC substrate; a nucleating buffer structure disposed on the substrate; an epitaxial layer side comprising an n-type layer, an active region and a p-type layer. The active region can be a double heterostructure (DH), a single quantum well (SQW), or a multiple quantum well (MQW) structure. The n-type and p-type layers can be n-doped and p-doped Al x In y Ga 1-x-y N, 0≦x,y≦1. The epitaxial layer side abutting the buffer structure has at least one electrode electrically connected to each of a p-side and an n-side of the LED. 
     The epitaxial layer side includes a plurality of layers containing GaN and is disposed on an upper surface of the substrate, and the preponderance of light emitted from the LED emerges from through a lower, or light-emitting surface of the substrate. 
     Because the undoped SiC substrate is highly resistive, the LED consistent with the present invention includes the two necessary electrodes positioned at the same side (i.e., the epitaxial layer side of the LED), and spaced apart from the substrate. Because the current does not need to pass through either the buffer structure or the substrate, the forward voltage is not degraded by the high resistivity of the buffer material or the substrate. Also, since current is not passed through the substrate, it can remain substantially undoped, and the light emission from the substrate side is enhanced without degrading the low biasing voltage. Further, since light generated in the active region exits more readily from the substrate side than from the other faces of the LED chip, a reflector can be formed on the epitaxial layer side so that substantially all light originally propagating toward the epi/epoxy interface is reflected towards the substrate side, and therefore more light is emitted from the LED. The SiC substrate is to be substantially undoped, preferably having a resistivity of at least 0.09 ohm-cm and as such, the substrate is minimally absorptive of output light energy having wavelengths in the range greater than 400 nm. 
     Additionally, the light-emitting surface of the substrate can be roughened by mechanical processing, improving optical transmission from the substrate into and through the epoxy packaging material into open space. 
     Light emission is additionally maximized in an LED consistent with the current invention by the placement of the electrodes at the epitaxial layer side of the device. 
     Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of a conventional LED. 
         FIG. 2A  is a top plan view of an LED consistent with the present invention. 
         FIG. 2B  is a cross-sectional schematic view of the LED of FIG.  2 A. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 2  depicts a cross-sectional schematic view of one embodiment of an LED consistent with the present invention and generally designated at  100 . The LED is constructed on a substrate  101 . Preferably, the substrate  101  is undoped and is single crystal SiC having a resistivity of greater than 0.09 ohm-cm. SiC is selected because of its high index of refraction and its close lattice match to gallium nitride (3.5% mismatch) and related III-V nitride compounds. Other substrates known to those skilled in the art to substantially match the characteristics of SiC may be used as well. 
     The substrate  101  is commonly grown by vapor transfer, a technique well known to those skilled in the art and not discussed further herein. Such substrates may be purchased from Sterling Semiconductor, located at 22660 Executive Drive, Suite 101 Sterling, Va. 20166-9535, or II-VI Inc. located at 375 Saxonburg Blvd., Saxonburg, Pa. 16056. Additional semiconductor layers described in this section are grown using metalorganic chemical vapor deposition (MOCVD), a technique well known in the art and also not discussed further herein. Other well-known growth techniques and processes may be employed as well, to grow the epitaxial layers upon the substrate  101 . 
     The light-emitting diode  100  includes the substrate  101  having a lower or light-emitting surface  130  and an upper surface  132 . The LED  100  further includes a nucleating buffer structure  102  having which may abut the substrate  101  and is preferably formed from GaN, AlN, indium nitride (InN), ternary Group III nitrides having the formula A x B 1-x N, where A and B are Group III elements and where x is one of zero, one, and a fraction between zero and one, quaternary Group III nitrides having the formula A x B y C 1-x-y N where A, B, and C are Group III elements, the sum of x and y is one of zero, one, and a fraction between zero and one, and 1 is greater than the sum of x and y, and alloys of SiC with such ternary and quaternary Group III nitrides. 
     The buffer structure  102  is disposed between the substrate  101  and the light-emitting structure  112  to mitigate the physical stress induced by the crystal lattice mismatch between the two materials. The light emitting diode  100  has a horizontal architecture light-emitting structure  112 , and neither the buffer structure  102  nor the substrate  101  is situated between the electrodes  110 ,  115  disrupting the intended path of the activation current. 
     In one embodiment consistent with the present invention as depicted in  FIGS. 2A and 2B , the buffer structure  102  includes a single non-conducting nucleation layer, but may include other layers. The layer  102  is formed from AlN, however other materials may be used including AlGaN or other materials known to those skilled in the art. The buffer structure material may be non-conductive as vertical current conduction through the buffer structure  102  is not required. A single layer buffer design reduces manufacturing complexity and improves diode  100  performance by minimizing absorption and internal reflection. Other embodiments consistent with the present invention may employ different or layered nucleation material layers, or other layers, to emphasize different device performance characteristics. 
     Disposed on the buffer structure  102 , is a layer of undoped GaN which may be grown to serve as a GaN substrate  103  in the light-emitting structure  112 . The GaN substrate  103  serves to complete the lattice buffer function establishing the GaN crystal lattice and creating a high quality, low defect foundation for the formation of a cladding layer that is disposed on the substrate  103 . 
     A light-emitting structure  112  is formed on the GaN substrate  103 , the light-emitting structure  112  being a double heterostructure including a p-n junction in which the active and heterostructure layers are selected from the group of binary Group III nitrides, ternary Group III nitrides, quaternary Group III nitrides, and alloys of SiC with such nitrides. 
     The light-emitting structure  112  includes a first cladding layer  104 , an active region  105 , and a second cladding layer  106 . The first cladding layer  104  is disposed on the GaN substrate  103 . The cladding layers  104 ,  106  must each be doped to either a different one of a p-type or n-type. The active region  105  is disposed on the first cladding layer  104 . The active region  105  preferably has a bandgap smaller than the bandgap of either of the cladding layers  104 ,  106 . 
     The second cladding layer  106  is disposed on the active region  105 . In the illustrative example of  FIG. 3 , the first cladding layer  104  is preferably formed of silicon doped GaN, the active region  105  is preferably formed from a silicon doped n-type gallium-indium-nitride/gallium nitride (GaInN/GaN) multi quantum well (MQW) structure, and the second cladding layer is preferably formed of Mg doped aluminum gallium nitride (AlGaN). 
     In one embodiment consistent with the present invention as depicted in  FIG. 2B , a first window layer  107  is formed of Mg doped GaN, and a second window layer  108  is formed of another Mg doped GaN layer to permit an ohmic contact between the window layers  107 ,  108  and a first electrode  110 . The second window layer  108  is disposed on the first window layer  107 , the first window layer  107  being disposed on the light-emitting region  112 . 
     A semitransparent conductive layer  119  formed of nickel oxide/gold (NiO/Au) is disposed on the second window layer  108  to further spread current from the first electrode  110  over the surface of the window layers  107 ,  108  to maximize the reach of the drive current and make optimal use of the available active region  105 . The upper surface  132  of the semitransparent conductive layer  119  is also the upper surface of the LED  100 . 
     The first electrode  110  is seated upon a reflective bond pad adhesion layer  109  formed on the upper surface of the second window layer  108 . The first electrode  110  is deposited on an upper surface of the adhesion pad  109  to facilitate wire bonding in the packaging process. Each of the first cladding layer  106  and the second cladding layer  104  have a bandgap larger than the active region  105 . 
     To provide an ohmic contact to the first cladding layer  104 , the window layers  107 ,  108  and several layers of the light-emitting structure  112  are etched to form an opening  113  through the window layers  107 ,  108  and the several layers of the light-emitting structure  112  to expose the upper surface of the first cladding layer  104  as shown by the dotted line in  FIG. 2B. A  reflective bond pad  111  is deposited on the upper surface  150  of the first cladding layer  104 , and a conductive contact, such as gold, is deposited on the bond pad  111  to form a second electrode  115 . 
     The lower surface  130  of the substrate  101  can be roughed using a chemical or mechanical process to minimize reflection back into the substrate and LED structure. This promotes light transmission out of the device. Potential roughening techniques include sawing (mechanical), RIE (chemical) and LE4 (chemical). 
     Because of the very close match of the lattice constants of SiC and GaN related III-V semiconductor compounds, the substrate  101  is preferably formed from SiC. SiC is suited to the construction, high performance and durability requirements, and production efficiency of the GaN LED structure. Minimizing the lattice mismatch between layers in the LED device reduces crystal defects that limit the performance of the device. The use of lateral conduction LED device structure allows for a non-conductive buffer structure  102  and the process of producing it. An LED consistent with the present invention has a power output rating of about at least 1 mW when operating with a driving current of 20 mA. 
     Light absorption within the substrate  101  consistent with the current invention is minimized, as the SiC substrate is preferably undoped, and having a resistance of at least 0.09 Ohm-cm. The electrodes  110 ,  115  do not significantly obstruct light emitted from the semiconductor structure as they are on the epitaxial side of the device, opposite the lower surface of the substrate  130 , from which the preponderance of emitted light is to escape. A further refinement consistent with the current invention roughens the lower surface of the SiC substrate  130  by a technique known in the art to further improve emission efficiency. 
     In view of the cumulative effect of these device features and refinements, a light-emitting device of high-energy conversion efficiency is consistent with the present invention. 
     While the invention has been described in conjunction with several embodiments, it is evident to those skilled in the art that many alternatives, modifications, and variations will be apparent in light of the foregoing description. Accordingly, the invention is intended to embrace all such alternative, modifications, and variations that fall within the spirit and scope of the appended claims.