United States Patent [19]
`May 16, 1995
`[45] Date of Patent:
`Edmond et a1.
`
`5,416,342
`
`US005416342A
`[11] Patent Number:
`
`[54] BLUE LIGHT-EMITTING DIODE WITH
`HIGH EXTERNAL QUANTUM EFFICIENCY
`
`[75]
`
`Inventors:
`
`John A. Edmond, Apex; Hua-Shuang
`Kong, Raleigh, both of NC.
`
`[73] Assignee: Cree Research, Inc., Durham, NC.
`
`in the 555—620 nm Spectral Region Using a Thick GaP
`Window Layer, Appl. Phys. Lett., vol. 61, No. 9, Aug.
`1992, pp. 1045—1047.
`
`Primary Examiner—William Mintel
`Attorney, Agent, or Finn—Bell, Seltzer, Park & Gibson
`
`[21] Appl. No.: 81,668
`
`[22] Filed:
`
`Jun. 23, 1993
`
`Int. 01.6 ............................................. H01L 33/00
`[51]
`[52] us. Cl. ........................................ 257/76; 257/77;
`257/98; 257/99; 257/101; 257/103
`[58] Field of Search ....................... 257/76, 77, 91, 95,
`,
`257/98, 99, 101, 103, 102, 623
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`4,913,497 4/1990' Edmond ................................ 357/17
`6/1991 Edmond ........... 357/17
`5,027,168
`
`5,063,421 11/1991 Suzuki etal. ......... 257/77
`
`5,137,547 2/1993 Niina et a1. ........... 257/77
`5,243,204 9/1993 Suzuki etal. ......................... 257/77
`
`FOREIGN PATENT DOCUMENTS
`
`
`, 58-64074 4/1983 Japan ..................................... 257/77
`
`1/1990 Japan ................ 257/77
`0221673
`
`3/1992 Japan ............ 257/77
`0471278
`4112583 4/1992 Japan ..................................... 257/77
`
`OTHER PUBLICATIONS
`
`[57]
`
`ABSTRACT
`
`A light emitting diode is disclosed that emits light in the
`blue portion of the visible spectrum with high external
`quantum efficiency. The diode comprises a single crys-
`tal silicon carbide substrate having a first conductivity
`type, a first epitaxial layer of silicon carbide on the
`substrate and having the same conductivity type as the
`substrate, and a second epitaxial layer of silicon carbide
`on the first epitaxial layer and having the opposite con-
`ductivity type from the first layer. The first and second
`epitaxial layers forming a p-n junction, and the diode
`includes ohmic contacts for applying a potential differ-
`ence across the p-n junction. The second epitaxial layer
`has side walls and a top surface that forms the top sur-
`face of the diode, and the second 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, but less than the thickness at which
`internal absorption in said second layer would substan-
`tially reduce the light emitted from said top surface of
`the diode.
`
`K. H. Huang, et 31., Twofold Efficiency Improvement
`in High Performance AlGalnP Light—Emitting Diodes
`
`53 Claims, 5 Drawing Sheets
`
`
`
`Cree Ex. 1013
`
`Page 1
`
`Cree Ex. 1013
`
`Page 1
`
`

`

`US. Patent
`
`May 16, 1995
`
`Sheet 1 of 5
`
`5,416,342
`
`1o
`
`20
`
`17
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`30
`
`FIG. 2.
`
`
`Cree Ex. 1013
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`Page 2
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`Cree Ex. 1013
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`Page 2
`
`

`

`US. Patent
`
`May 16, 1995
`
`Sheet 2 of 5
`
`5,416,342
`
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`
`Cree Ex. 1013
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`Page 3
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`Cree Ex. 1013
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`Page 3
`
`

`

`US. Patent
`
`May 16, 1995
`
`Sheet 3 of 5
`
`5,416,342
`
`
`
`OUTPUT(7;).A O O
`
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`
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`
`LIGHTOUTPUT(2,)
`
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`
`Cree Ex. 1013
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`Page 4
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`Cree Ex. 1013
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`Page 4
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`

`

`US. Patent
`
`May 16, 1995
`
`Sheet 4 of 5
`
`5,416,342
`
`FLUX(yw)
`RADIANT
`
`10
`
`20
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`SURFACE P LAYER THICKNESS (pm)
`
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`
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`
`Cree Ex. 1013
`
`_
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`Page 5
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`Cree Ex. 1013
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`Page 5
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`

`

`US. Patent
`
`May 16, 1995
`
`Sheet 5 of 5
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`5,416,342
`
`(cm—1)
`ABSORPTIONCOEFFICIENT
`
`
`P—TYPE DOPANT CONCENTRATION (cm‘3)
`
`FIG. 9.
`
`Cree Ex. 1013
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`Page 6
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`Cree Ex. 1013
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`Page 6
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`

`

`1
`
`5,416,342
`
`BLUE LIGHT-EMITTING DIODE WITH HIGH
`EXTERNAL QUANTUM EFFICIENCY
`
`FIELD OF THE INVENTION
`
`The present invention relates to light-emitting diodes,
`and in particular relates to blue light-emitting diodes
`formed in silicon carbide. This invention was made with
`the support of The Defense Advanced Research
`Projects Agency under Contract or Grant No. N00014»
`92-C-0100. The government may have certain rights in
`this invention.
`
`BACKGROUND OF THE INVENTION
`
`Light-emitting diodes, commonly referred to as
`“LED’s” are semiconductor devices which convert
`electrical energy into emitted light.
`As is known to those familiar with atomic and molec-
`ular structure of semiconductor materials and electronic
`devices, electromagnetic radiation,
`including visible
`light, is produced by electronic transitions that occur in
`atoms, molecules, and crystals. Furthermore, 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 semicon-
`ductor material may be doped. As is further known to
`such persons, blue light represents one of the higher
`energy phenomena within the spectrum visible to the
`human eye. By way of comparison, higher energy tran-
`sitions such as ultraviolet light are invisible to the
`human eye. Similarly, red light represents the lower
`energy end of the visible spectrum, and infrared, far
`infrared, and microwave radiation represent even lower
`energy transitions that are out of the range of the visible
`spectrum.
`Only certain semiconductor materials have the capa-
`bility to permit the type of electronic transitions that
`will produce blue light in the visible spectrum. One of
`these materials is silicon carbide (SIC) which can pro-
`duce several different wavelengths of blue light. The
`characteristics of silicon carbide and the manner in
`which blue light can be produced using silicon carbide
`are thoroughly discussed in US. Pat. Nos. 4,918,497
`and 5,027,168, both entitled “Blue Light-Emitting
`Diode Formed in Silicon Carbide.” Both of these pa-
`tents are assigned to the assignee of the present applica-
`tion. These patents are incorporated entirely herein by
`reference (“the ’497 and ’168 patents”).
`The increased availability of blue LEDs has, how-
`ever, increased both the demand for the devices and for
`particular performance specifications. In particular, 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 effi-
`ciency. As the use of blue LEDs has increased, the
`demand for LEDs with higher quantum efficiencies has
`likewise increased.
`There are, however, some particular aspects of sili-
`con carbide which must be addressed when attempting
`to increase the quantum efficiency.
`LEDs formed in more conventional materials such as
`gallium phosphide (GAP) provide a comparative exam-
`ple. Gallium phosphide’s conductivity is generally suffi-
`cient for the entire device to light up as current passes
`across the p-n junction. Stated differently, current
`Spreads relatively easily in gallium phosphide,
`thus
`spreading the generated light relatively easily as well.
`For lower conductivity materials such as p-type silicon
`
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`2
`carbide, however, the current does not spread as effi-
`ciently throughout the entire device, thus reducing the
`amount of emitted light that could otherwise be gener-
`ated.
`The conductivity of silicon carbide can be increased,
`of course, by increasing its dopant concentration. In-
`creasing 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 car-
`bide requires various dopant and current injection con-
`siderations in a manner described thoroughly in the ’497
`and ’168 patents.
`OBJECT AND SUMMARY OF THE INVENTION
`
`Accordingly; it is an object of the present invention
`to provide a light-emitting diode that emits light in the
`blue portion of the visible spectrum with a relatively
`high external quantum efficiency.
`In one embodiment, 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.
`In another embodiment, the invention comprises in-
`creasing the surface area of the top surface of the light-
`emitting diode.
`the invention comprises
`In another embodiment,
`using a metal ohmic contact that will form a reflective
`surface from which light generated by the diode will
`reflect rather than be absorbed.
`
`In yet another embodiment, the invention comprises
`a method of forming a reflective ohmic contact on sili-
`con carbide.
`_
`
`In yet another embodiment, the invention comprises
`the use of a transparent conductive contact on a blue
`light-emitting diode formed in silicon carbide.
`The foregoing and other objects, advantages and
`features of the invention, and the manner in which the
`same are accomplished, will become more readily ap-
`parent upon consideration of the following detailed
`description of the invention taken in conjunction with
`the accompanying drawings, which illustrate preferred
`and exemplary embodiments, and wherein:
`DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a cross-sectional schematic view of a first
`embodiment of a blue LED according to the present
`invention;
`FIG. 2 is another cross-sectional schematic illustra-
`tion of a second embodiment of a blue LED according
`to the present invention;
`FIG. 3 is a plot of relative intensity plotted against
`wavelength for a blue LED according to the present
`invention;
`FIG. 4 is a combination plot of radiant flux in micro-
`watts and external power efficiency in percentage plot-
`ted against forward current in milliamps across the
`diode;
`FIG. 5 is a plot of light output expressed as a percent-
`age taken against the area of the top surface of the diode
`measured in mils along each side of a square mesa;
`FIG. 6 is a plot of light output as a percentage taken
`against the thickness of the top epitaxial layer of blue
`
`Cree Ex. 1013
`
`Page 7
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`Cree Ex. 1013
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`Page 7
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`

`

`3
`LEDs according to the present invention and measured
`in microns;
`FIG. 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 mea-
`sured in microns;
`FIG. 8 is a plot of the absorption coefficient of silicon
`carbide as a function of n-type dopant concentration;
`and
`
`FIG. 9 is a plot of the absorption coefficient of silicon
`carbide as a function of p-type dopant concentration.
`DETAILED DESCRIPTION
`
`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. In a first embodiment
`illustrated in FIG. 1, the blue LED is broadly desig-
`nated at 110. 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. As a result, the first and second
`epitaxial layers 12 and 13 form a p—n junction therebe-
`tween. Ohmic contacts 14 and 15 respectively complete
`the structure for providing the current injected across
`the junction to produce light from the diode.
`FIG. 1 schematically shows that in the first embodi-
`ment 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 emit-
`ted 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.
`As is known to those familiar with the basic princi-
`ples of optics, 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 adja-
`cent material to light of that wavelength. These proper-
`ties define a critical angle using the well-known rela-
`tionship of Snell’s Law. Summarized briefly, Snell’s
`Law defines a critical angle for any two adjacent mate-
`rials, 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 bound-
`ary. When light strikes the boundary between the mate-
`rials at an angle less than the critical angle, it changes
`direction somewhat (i.e., it refracts), but is nevertheless
`emitted. If the light strikes the boundary at an angle
`greater than the critical angle, however, the light is
`totally internally reflected rather than emitted.
`Because light from an LED is generated at the junc-
`tion, 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 me-
`dium. As is known to those familiar with light-emitting
`diodes, 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
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`5,416,342
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`4
`remaining light rays are reflected back into the device
`where most are absorbed.
`Alternatively, the amount of light emitted from the
`sides of the diode is determined by both the total inter-
`nal 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., l 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. As an approximation, where a ray of
`light is considered to have begun at a point source and
`headed toward a given sidewall,
`the proportion of
`transmitted light can be roughly estimated by the solid
`angle of intersection between the sidewall 16 of the
`second layer 13 and the cone of radiation for sideways-
`directed light. The total light from the sidewalls of the
`diode would then be the sum of the solid angles for all
`of the points on the entire light-emitting area of the
`junction for each of the sidewalls 16. A thorough dis-
`cussion of these principles is set forth in Huang, et al.,
`“Two-Fold Efficiency Improvement in High Perfor-
`mance AlGaInP Light-Emitting Diodes in the 555—620
`nm Spectral Region Using a Thick GaP Window
`Layer,” Appl. Phys. Lett. 61(9), Aug. 31, 1992, pp.
`1045—1047.
`As further known to those familiar with the interac-
`tion between light and materials, 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—l), 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.
`FIGS. 7—9 illustrate this effect. FIG. 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 ef-
`fects of absorption begin to overtake the benefits of the
`thicker layer. As indicated in FIG. 7, 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. FIGS. 8 and 9 illustrate how the absorp-
`tion coefficients for n-type and p-type silicon carbide
`increase as the dopant concentration (in cm—3)
`in-
`creases.
`
`In preferred embodiments of the present invention,
`the thickness of the second layer 13 is on the order of
`about 25 microns, a large increase over prior commer-
`cial devices for which such thicknesses are typically on
`the order of 1—3 microns. FIG. 6 shows how the in-
`crease in the surface layer 13 thickness increases the
`light output of a device according to the invention. In
`FIG. 6, 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 player
`thickness is then plotted as a function of thickness. As
`demonstrated by FIG. 6, 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.
`FIG. 1 illustrates a further aspect of the present in-
`vention, reflective contacts. FIG. 1 illustrates the ohmic
`
`Cree Ex. 1013
`
`Page 8
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`Cree Ex. 1013
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`Page 8
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`

`

`5
`contact to the substrate as a deposited metal 14. As is
`known to those familiar with such materials, in order to
`get appropriate ohmic behavior, 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. For exam-
`ple, aluminum, when deposited on silicon carbide, but
`not annealed or alloyed, will exhibit a reflectivity of 10
`90% or more. As deposited, however, aluminum gener-
`ally 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 unre- 15
`flective contact will tend to be absorbed rather than
`reflected or emitted.
`The invention, however, further increases the exter-
`nal efficiency of the LED by incorporating a reflective
`metal deposit as an ohmic contact which remains unal- 20
`loyed and unannealed and therefore maintains its reflec-
`tivity.
`As illustrated in FIG. 1, the reflective metal, such as
`aluminum, is deposited upon a very thin layer 18 that
`forms a top portion of the LED 10. The layer 18 is 25
`highly doped and is between the reflective metal de-
`posit 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. 30
`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. As a result, at least one of the 35
`ohmic contacts, and potentially both of them, can com-
`prise 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 ab-
`sorbed. Although it will be recognized that the re- 40
`flected light generated at the junction will be reflected
`back into the epitaxial layers 12 and 13, the fact that
`they are reflected at all, rather than simply absorbed,
`increases the amount of such light that will eventually
`escape from either the top surface 17 or the sidewalls 16 45
`of the LED 10. To date, the use of reflective contacts in
`this manner has resulted in increases as great as 50% in
`the overall efficiency of the devices as compared to
`prior LEDs.
`A thorough discussion of a method of producing an 50
`ohmic contact in this manner is set forth in US. applica-
`tion Ser. No. 07/943,043; filed Sep. 10, 1992 by Glass et
`al for “Method of Forming Ohmic Contacts to p-Type
`Wide Bandgap Semiconductors and Resulting Ohmic
`Contact Structure,” the contents of which are incorpo- 55
`rated entirely herein by reference. In preferred embodi-
`ments of the invention the dopant concentration of the
`highly doped layer is greater than about 2E19 (2X1019
`cm—3), and most preferably is greater than about 5E19
`(5 X 1019 cm-3).
`In the present invention, preferred metals for the
`ohmic contacts, including the reflective contacts, are
`aluminum, gold, platinum, and silver. Similarly,
`the
`preferred polytypes for the silicon carbide are the 6H,
`4H, and 15R polytypes.
`Some further features of the invention are shown in
`the embodiment illustrated in FIG. 2 in which the LED
`is broadly designated at 20. FIG. 2 illustrates a silicon
`
`6
`carbide single crystal substrate 21 having a first conduc-
`tivity type. A first epitaxial layer is formed of a compen-
`sated layer and a predominantly uncompensated layer
`As used herein, the term “compensated” refers to a
`portion of semiconductor material doped with both
`donor and acceptor dopants. Thus, a compensated p-
`type layer would include both p~type and n-type dop-
`ants, but with a sufficient excess of the p-type dopants to
`give the layer p-type characteristics overall. The rea-
`sons 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 refer-
`ence.
`
`As illustrated in FIG. 2, the substantially uncompen-
`sated layer 23 is adjacent to substrate 21 while the com-
`pensated layer 22 is adjacent a second epitaxial layer
`which has a conductivity type opposite from the con-
`ductivity type of layers 22, 23, and 21. The epitaxial
`layers 24 and form a p-n junction from which light is
`emitted when a potential difference is applied across the
`diode 20. The third epitaxial layer 25 is on the second
`epitaxial layer 24 and has the same conductivity type as
`the second epitaxial layer 24. As in the first embodi-
`ment, the third layer has sidewalls 26 and a top surface
`27 that forms the top surface of the diode 20. The third
`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 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.
`As stated above, the first epitaxial layer is formed by
`respective compensated and uncompensated layers 22
`and with the predominantly uncompensated layer 23
`being adjacent to substrate 21. The second 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.
`In a preferred embodiment, and to obtain a peak
`wavelength in the 460—475 nm range (and preferably
`465—470 nm), the substrate 21 and layers 22 and 23 are of
`n-type conductivity, while the second epitaxial layer 24
`and the third epitaxial layer 25 are p-type conductivity.
`In a most preferred embodiment, the third 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. FIGS. 3 and 4 illustrate
`these characteristics, with FIG. 3 showing the peak
`wavelengths and FIG. 4 showing radiant flux and exter-
`nal power efficiency, both as a function of forward
`current in milliamps. The undotted lower curve in FIG.
`4 represents the performance of prior diodes not specifi-
`cally incorporating the features of the present inven-
`tion.
`In other embodiments, and for which a slightly differ-
`ent wavelength of blue light is desired, 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. Prefera-
`bly, the third layer 25 is predominantly uncompensated
`as is the substrate.
`FIG. 2 further illustrates that in preferred embodi-
`ments, the epitaxial layers 22 through 25 form a mesa
`structure upon the substrate 21.
`In the embodiment illustrated in FIG. 2, the reflective
`metal contact is designated at 30, and is made to the
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`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. In preferred embodiments, the
`method can further comprise the step of heating the
`applied metal sufficiently to improve the ohmic charac-
`teristics of the metal contact, but less than the amount of
`heating that would substantially reduce the reflectivity
`of the applied metal.
`In preferred embodiments, the step of applying the
`metal can comprise sputtering or evaporation tech-
`niques.
`In use, the various aspects of the invention have given
`significant increases. For example, increasing the stan-
`dard thickness of the top epitaxial layer from 3 microns
`to approximately 25 microns increases the external out-
`put 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 ap-
`proximately 7%. Adding a reflective contact rather
`than an annealed or opaque contact gives a 50% in-
`crease 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.
`In the drawings and specification, there have been
`disclosed typical preferred embodiments of the inven-
`tion and, although specific terms have been employed,
`they have been used in a generic and descriptive sense
`only and not for purposes of limitation, the scope of the
`invention being set forth in the following claims.
`That which is claimed is:
`1. A light emitting diode that emits light in the blue
`portion of the visible spectrum with high external quan-
`tum efficiency, said diode comprising:
`a single crystal silicon carbide substrate having a first
`conductivity type;
`a first epitaxial layer of silicon carbide on said sub-
`strate and having the same conductivity type as
`said substrate;
`a second epitaxial layer of silicon carbide on said first
`epitaxial layer and having the opposite conductiv-
`ity type from said first layer,
`said first and second epitaxial layers forming a p-n
`junction; and
`ohmic contacts for applying a potential difference
`across said p-n junction; and wherein
`said second epitaxial layer has side walls and a top
`surface that define the top surface and top side
`walls of said diode, and said second epitaxial layer
`is sufficiently thick to increase the solid angle at
`which light emitted by the junction will radiate
`externally from said side walls, but less than the
`thickness at which internal absorption in said sec-
`ond layer would substantially reduce the light
`emitted from said top surface of the diode.
`2. A light emitting diode according to claim 1
`wherein said second epitaxial layer has a thickness of
`about 25 microns.
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`7
`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.
`In another embodiment, where the LED is to be
`placed in a plastic carrier material, as is quite common
`for such devices, the thickness of the third layer 25is
`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 inven-
`tion in which one or more of the contacts are formed of
`a substantially transparent conductive material. In FIG.
`2, the transparent contact is illustrated at 32 and is pref-
`erably formed of an indium-tin—oxide (ITO) material
`which is useful for transparent contacts in electro-opti—
`cal applications in a manner well known to those famil-
`iar with this art. Any other compounds having similar
`transparent characteristics would likewise be accept-
`able, provided that they can carry enough current to
`drive the LED without being applied in such thick-
`nesses that their transparency would be reduced beyond
`the point at which the transparent contact would be
`advantageous. As in the case of the reflective contact of
`the present invention, it will be understood that 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. Typically, an ITO
`contact is used in conjunction with a contact pad (not
`shown) of a more conventional metal or other conduc-
`tor.
`
`In yet another embodiment, the invention comprises
`an LED in which the cross-sectional area of the top
`surface, as indicated at 17 in FIG. 1 and at 27 in FIG. 2,
`is similarly increased. In preferred embodiments, the
`cross-sectional area is increased from approximately 170
`by 170 square microns of a typical LED to approxi-
`mately 200 by 200 square microns. Increasing this pa-
`rameter has given increases of at least 7% in present
`devices. FIG. 5 illustrates the increase in light output of
`a blue LED formed in silicon carbide when the mesa
`area is increased.
`In this regard, the use of a wider mesa in conjunction
`with a thicker “window layer” has synergistic advan-
`tages. As stated above, silicon carbide, particularly
`p-type, tends to be somewhat resistive. Thus, 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 junc-
`tion to produce a greater amount of visible light. Thus,
`in addition to providing advantages on an individual
`basis, the wider mesa and deeper window layer provide
`a synergistic effect that increases the external quantum
`efficiency of the resulting diode even more.
`In yet another embodiment, the invention provides a
`method of forming a reflective ohmic contact on a sili-
`con 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
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`3. A light emitting diode according to claim 1
`wherein said top surface has an area of about 200 by 200
`square microns.
`4. A light emitting diode according to claim 1
`wherein at least one of said ohmic contacts comprises an
`unannealed metal contact, said metal contact forming a
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`reflective surface from which light generated by said
`diode will reflect rather than be absorbed.
`5. A light emitting diode according to claim 4 and
`further comprising a highly doped layer between said
`unannealed metal contact and said diode, said highly
`doped layer having the same conductivity type as the
`portion of the diode to which it is adjacent, and said
`highly doped layer having a dopant concentration suffi-
`cient to lower the barrier between said metal contact
`and said diode portion sufficiently to provide ohmic
`behavior between said unannealed metal and said diode
`portion, said highly doped layer being thin enough to
`prevent the dopant concentration from substantially
`reducing the light emitted from said top surface of said
`diode.
`6