Classifications

• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10H—INORGANIC LIGHT-EMITTING SEMICONDUCTOR DEVICES HAVING POTENTIAL BARRIERS
  • H10H20/00—Individual inorganic light-emitting semiconductor devices having potential barriers, e.g. light-emitting diodes [LED]
  • H10H20/80—Constructional details
  • H10H20/81—Bodies
  • H10H20/822—Materials of the light-emitting regions
  • H10H20/826—Materials of the light-emitting regions comprising only Group IV materials
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10H—INORGANIC LIGHT-EMITTING SEMICONDUCTOR DEVICES HAVING POTENTIAL BARRIERS
  • H10H20/00—Individual inorganic light-emitting semiconductor devices having potential barriers, e.g. light-emitting diodes [LED]
  • H10H20/80—Constructional details
  • H10H20/81—Bodies
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10H—INORGANIC LIGHT-EMITTING SEMICONDUCTOR DEVICES HAVING POTENTIAL BARRIERS
  • H10H20/00—Individual inorganic light-emitting semiconductor devices having potential barriers, e.g. light-emitting diodes [LED]
  • H10H20/80—Constructional details
  • H10H20/83—Electrodes
  • H10H20/832—Electrodes characterised by their material
  • H10H20/835—Reflective materials

Definitions

• the present invention relates to light- emitting diodes, and in particular relates to blue light-emitting diodes formed in silicon carbide.
• LED's Light-emitting diodes, commonly referred to as “LED's” are semiconductor devices which convert electrical energy into emitted light.
• electromagnetic radiation including visible light
• visible light is produced by electronic transitions that occur in atoms, molecules, and crystals.
• the color of light that can be produced from an LED is a function of the basic semiconductor material from which the LED is formed, and the manner in which the semiconductor material may be doped.
• blue light represents one of the higher energy phenomena within the spectrum visible to the human eye.
• higher energy transitions such as ultraviolet light are invisible to the human eye.
• red light represents the lower energy end of the visible spectrum
• infrared, far infrared, and microwave radiation represent even lower energy transitions that are out of the range of the visible spectrum.
• blue LEDs have, however, increased both the demand for the devices and for particular performance specifications.
• one important performance characteristic of an LED is the amount of light it can produce from a given amount of electricity, a relationship referred to as quantum efficiency.
• quantum efficiency the amount of light it can produce from a given amount of electricity
• Gallium phosphide GaP
• GaP gallium phosphide
• the conductivity of silicon carbide can be increased, of course, by increasing its dopant concentration. Increasing the doping level is a less desirable solution, however, because the increased doping lowers the transparency of the device, thus detracting from its overall performance. Furthermore, producing a blue LED in silicon carbide requires various dopant and current injection considerations in a manner described thoroughly in the '497 and '168 patents.
• the invention comprises an LED in which the top epitaxial layer has a thickness sufficient to increase the solid angle at which light emitted by the junction will radiate externally from the side walls of the layer, but less than the thickness at which internal absorption in the layer would substantially reduce the light emitted from the top surface of the diode.
• the invention comprises increasing the surface area of the top surface of the light-emitting diode. In another embodiment, the invention comprises using a metal ohmic contact that will form a reflective surface from which light generated by the diode will reflect rather than be absorbed.
• the invention comprises a method of forming a reflective ohmic contact on silicon carbide.
• the invention comprises the use of a transparent conductive contact on a blue light-emitting diode formed in silicon carbide.
• Figure 1 is a cross-sectional schematic view of a first embodiment of a blue LED according to the present invention
• Figure 2 is another cross-sectional schematic illustration of a second embodiment of a blue LED according to the present invention
• Figure 3 is a plot of relative intensity plotted against wavelength for a blue LED according to the present invention.
• Figure 4 is a combination plot of radiant flux in micro-watts and external power efficiency in percentage plotted against forward current in milliamps across the diode
• Figure 5 is a plot of light output expressed as a percentage taken against the area of the top surface of the diode measured in mils along each side of a square mesa;
• Figure 6 is a plot of light output as a percentage taken against the thickness of the top epitaxial layer of blue LEDs according to the present invention and measured in microns;
• Figure 7 is a second plot of light output as a percentage taken against the thickness of the top epitaxial layer of blue LEDs according to the present invention and measured in microns;
• Figure 8 is a plot of the absorption coefficient of silicon carbide as a function of n-type dopant concentration
• Figure 9 is a plot of the absorption coefficient of silicon carbide as a function of p-type dopant concentration.
• the present invention is a light-emitting diode that emits light in the blue portion of the visible spectrum (i.e., approximately 400-480 nanometers (nm) ) with high external quantum efficiency.
• the blue LED is broadly designated at 10.
• the diode 10 includes a single crystal silicon carbide substrate 11 having a first conductivity type.
• a first epitaxial layer of silicon carbide 12 is on the substrate 11 and has the same conductivity type as the substrate.
• a second epitaxial layer 13 is on the first epitaxial layer and has the opposite conductivity type from the first layer.
• the first and second epitaxial layers 12 and 13 form a p-n junction therebetween.
• Ohmic contacts 14 and 15 respectively complete the structure for providing the current injected across the junction to produce light from the diode.
• Figure 1 schematically shows that in the first embodiment of the invention, the second epitaxial layer 13 has sidewalls designated at 16, and a top surface designated at 17.
• the second epitaxial layer 13 has a thickness sufficient to increase the solid angle at which light emitted by the junction will radiate externally from the sidewall 16, but less than the thickness at which internal absorption in the layer 13 would substantially reduce the light emitted from the top surface 17 of the diode.
• the extent to which light will be reflected or refracted at the surface of a material is a function of the refractive index of the material to the particular wavelength of light and the refractive index of the adjacent material to light of that wavelength.
• These properties define a critical angle using the well-known relationship of Snell's Law. Summarized briefly, Snell's Law defines a critical angle for any two adjacent materials, based upon their respective indexes of refraction. The critical angle is defined as the angle between the direction of propagation of the light, and a line normal to the boundary at the point the light strikes the boundary.
• the amount of light that will be emitted from the top surface 17 of the diode is limited by total internal reflection, which, as stated above, is determined by the critical angle defined by Snell's Law and the refractive indexes of the semiconductor and the surrounding medium.
• the surrounding medium will usually be either a plastic material, or the ambient surroundings; i.e., air.
• Light rays that are incident on the top surface 17 at an angle less than the critical angle are transmitted. The remaining light rays are reflected back into the device where most are absorbed.
• the amount of light emitted from the sides of the diode is determined by both the total internal reflection and by the thickness of the second layer 13. Light rays that strike the sidewalls 16 at angles less than the critical angle will be transmitted. Nevertheless, if the layer 13 is • relatively thin (e.g., 1 or 2 microns as set forth in the '497 and '168 patents), some light rays heading towards the sidewalls will instead be totally internally reflected off the top surface of the diode and will be absorbed in the substrate before they reach one of the sidewalls 16.
• any given material through which light passes will absorb some of the light.
• the amount absorbed is based on the wavelength of the light, the absorption coefficient of the material (usually expressed in units of reciprocal length; e.g. cm "1 ) , and the distance the light travels through the material . Accordingly, although a thicker top epitaxial layer offers more external emission as described herein, the optimum thickness is limited by the corresponding absorption of the layer.
• Figures 7-9 illustrate this effect.
• Figure 7 illustrates that the radiant flux of two diodes according to the present invention increases with an increase in the thickness of the top epitaxial layer up to a maximum, and then begins to decrease as the layer becomes too thick and the effects of absorption begin to overtake the benefits of the thicker layer.
• one diode had a 6.8 mil by 6.8 mil mesa, and the other had an 8.0 mil by 8.0 mil mesa.
• Figures 8 and 9 illustrate how the absorption coefficients for n-type and p-type silicon carbide increase as the dopant concentration (in cm "3 ) increases.
• the thickness of the second layer 13 is on the order of about 25 microns, a large increase over prior commercial devices for which such thicknesses are typically on the order of 1-3 microns.
• Figure 6 shows how the increase in the surface layer 13 thickness increases the light output of a device according to the invention.
• the output of a device with a 3 micron surface layer is taken as a baseline value of 100%.
• the increased light output resulting from increased surface p-layer thickness is then plotted as a function of thickness.
• the increase in the thickness of the second layer 13 has a very effective positive impact on the light output of the device. To date, a thickness of about 25 microns has been found to be appropriate.
• Figure 1 illustrates a further aspect of the present invention, reflective contacts.
• Figure 1 illustrates the ohmic contact to the substrate as a deposited metal 14.
• the metal of the ohmic contact 14 must normally be alloyed or annealed after being deposited on the appropriate substrate, epitaxial layer, or other portion of the device 10. Such alloying or annealing, however, tends to reduce the reflectivity that a deposited metal will normally exhibit.
• aluminum when deposited on silicon carbide, but not annealed or alloyed, will exhibit a reflectivity of 90% or more. As deposited, however, aluminum generally will not exhibit ohmic behavior with respect to silicon carbide. Thus, in devices to date, reflectivity must be sacrificed in exchange for ohmic behavior. In turn, light generated by the diode that strikes the unreflective contact will tend to be absorbed rather than reflected or emitted.
• the invention further increases the external efficiency of the LED by incorporating a reflective metal deposit as an ohmic contact which remains unalloyed and unannealed and therefore maintains its reflectivity.
• the reflective metal such as aluminum
• the layer 18 is highly doped and is between the reflective metal deposit 15 and the diode, in particular the second layer 13.
• the highly doped layer has the same conductivity type as the portion of the diode to which it is adjacent, in this case the same conductivity type as the second layer 13.
• the highly doped layer 18 has a dopant concentration sufficient to lower the barrier between the metal contact 15 and the second layer 13 enough to provide ohmic behavior between the unannealed reflective metal and the diode.
• At least one of the ohmic contacts can comprise an unannealed metal contact, such as the contact 15, which forms a reflective surface from which light generated by the diode will reflect rather than be absorbed.
• an unannealed metal contact such as the contact 15, which forms a reflective surface from which light generated by the diode will reflect rather than be absorbed.
• the dopant concentration of the highly doped layer is greater than about 2E19 (2 X 10 19 cm “3 ) , and most preferably is greater than about 5E19 (5 X 10 19 cm “3 ) .
• preferred metals for the ohmic contacts, including the reflective contacts are aluminum, gold, platinum, and silver.
• the preferred polytypes for the silicon carbide are the 6H, 4H, and 15R polytypes.
• a first epitaxial layer is formed of a compensated layer 22 and a predominantly uncompensated layer 23.
• the term "compensated" refers to a portion of semiconductor material doped with both donor and acceptor dopants.
• a compensated p-type layer would include both p-type and n-type dopants, but with a sufficient excess of the p-type dopants to give the layer p-type characteristics overall.
• the reasons for using compensated layers in silicon carbide LEDs are set forth in an appropriate manner in the '497 and '168 patents already incorporated herein by reference.
• the substantially uncompensated layer 23 is adjacent to substrate 21 while the compensated layer 22 is adjacent an intermediate epitaxial layer 24 which has a conductivity type opposite from the conductivity type of layers 22, 23, and 21.
• the second epitaxial layer 25 is on the intermediate epitaxial layer 24 and has the same conductivity type as the intermediate epitaxial layer 24. As in the first embodiment, the second layer 25 has sidewalls 26 and a top surface 27 that forms the top surface of the diode 20.
• the second epitaxial layer 25 has a thickness sufficient to increase the solid angle at which light emitted by the junction will radiate externally from the sidewalls 26, but less than the thickness at which internal absorption in the layer 25 would substantially reduce the light emitted from the top surface 27 of the diode.
• the first epitaxial layer is formed by respective compensated and uncompensated layers 22 and 23, with the predominantly uncompensated layer 23 being adjacent to substrate 21.
• the intermediate epitaxial layer 24 is likewise compensated so that injection of carriers across the junction between layers 24 and 22 takes place between compensated portions of silicon carbide to give the appropriate energy transitions and wavelengths as described in the '497 and '168 patents.
• the substrate 21 and layers 22 and 2 are of n-type conductivity, while the intermediate epitaxial layer 24 and the second epitaxial layer 25 are p-type conductivity.
• the second layer 25 and the uncompensated layer 23 are doped slightly more heavily than their respective adjacent layers 24 and 22 in a manner which encourages the proper transition of carriers across the junction.
• Figures 3 and 4 illustrate these characteristics, with Figure 3 showing the peak wavelengths and Figure 4 showing radiant flux and external power efficiency, both as a function of forward current in milliamps.
• the undotted lower curve in Figure 4 represents the performance of prior diodes not specifically incorporating the features of the present invention.
• layers 22 and 23, and the substrate 21 can be of p-type conductivity while layers 24 and 25 can be of n-type conductivity.
• the second layer 25 is predominantly uncompensated as is the substrate.
• Figure 2 further illustrates that in preferred embodiments, the epitaxial layers 22 through 25 form a mesa structure upon the substrate 21.
• the reflective metal contact is designated at 30, and is made to the semiconductor 21.
• the heavily doped layer that permits the ohmic contact is likewise illustrated as 31. It will be understood that the use of reflective contacts and the appropriate highly doped layer can be made to one or both of the ohmic contacts to the device 20.
• the thickness of the second layer 25 is less than the thickness at which internal absorption in the layer would substantially reduce the light emitted from the top surface of the diode.
• FIG. 2 illustrates another embodiment of the invention in which one or more of the contacts are formed of a substantially transparent conductive material.
• the transparent contact is illustrated at 32 and is preferably formed of an indium-tin-oxide (ITO) material which is useful for transparent contacts in electro-optical applications in a manner well known to those familiar with this art.
• ITO indium-tin-oxide
• Any other compounds having similar transparent characteristics would likewise be acceptable, provided that they can carry enough current to drive the LED without being applied in such thicknesses that their transparency would be reduced beyond the point at which the transparent contact would be advantageous .
• the transparent contact can be used for one or both of the contacts to an LED, and can be used in combination with either the reflective contact or a more typical annealed or alloyed ohmic contact.
• an ITO contact is used in conjunction with a contact pad (not shown) of a more conventional metal or other conductor.
• the invention comprises an LED in which the cross-sectional area of the top surface, as indicated at 17 in Figure 1 and at 27 in Figure 2, is similarly increased.
• the cross-sectional area is increased from approximately 170 by 170 square microns of a typical LED to approximately 200 by 200 square microns. Increasing this parameter has given increases of at least 7% in present devices.
• Figure 5 illustrates the increase in light output of a blue LED formed in silicon carbide when the mesa area is increased.
• the use of a wider mesa in conjunction with a thicker "window layer” has synergistic advantages.
• silicon carbide, particularly p-type tends to be somewhat resistive.
• an applied current spreads in a less than ideal manner throughout the epitaxial layers.
• Making the mesa wider as well as thicker increases the opportunity for current to spread when a potential difference is applied across the diode.
• the greater ⁇ area of the diode gives the current more initial room to spread, and the deeper window layer adds for even further lateral spreading as the current moves axially through the diode.
• the result is a greater flow of current through the diode and across the junction to produce a greater amount of visible light.
• the wider mesa and deeper window layer provide a synergistic effect that increases the external quantum efficiency of the resulting diode even more.
• the invention provides a method of forming a reflective ohmic contact on a silicon carbide light-emitting diode to thereby increase the light emitted externally from the diode.
• the method comprises applying a metal to a surface portion of the silicon carbide diode and in which the portion of the diode having the surface is a highly doped layer of silicon carbide, and wherein the highly doped layer has a dopant concentration sufficient to lower the barrier between the metal contact and the diode portion enough to provide ohmic behavior between the metal and the diode portion.
• the method can further comprise the step of heating the applied metal sufficiently to improve the ohmic characteristics of the metal contact, but less than the amount of heating that would substantially reduce the reflectivity of the applied metal.
• the step of applying the metal can comprise sputtering or evaporation techniques.
• the various aspects of the invention have given significant increases. For example, increasing the standard thickness of the top epitaxial layer from 3 microns to approximately 25 microns increases the external output of the LED by at least about 30% Increasing the surface area from 170 by 170 square microns to 200 by 200 square microns increases the external output approximately 7%. Adding a reflective contact rather than an annealed or opaque contact gives a 50% increase in external output. When combined, all of these improvements have increased the efficiency of blue LEDs, according to the present invention, nearly 100% over earlier devices such as those described in the '497 and '168 patents.

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• 1993
  • 1993-06-23 US US08/081,668 patent/US5416342A/en not_active Expired - Lifetime
• 1994
  • 1994-04-22 WO PCT/US1994/004497 patent/WO1995000974A1/en not_active Ceased
  • 1994-04-22 AU AU68184/94A patent/AU6818494A/en not_active Abandoned
  • 1994-05-30 TW TW083104919A patent/TW272320B/zh not_active IP Right Cessation

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