United States Patent [i9]
`Edmond et al.
`
`US005523589A
`[11] Patent Number:
`[45] Date of Patent:
`
`5,523,589
`Jun. 4, 1996
`
`[54] VERTICAL GEOMETRY LIGHT EMITTING
`DIODE WITH GROUP III NITRIDE ACTIVE
`LAYER AND EXTENDED LIFETIME
`
`[75]
`
`Inventors: John A. Edmond; Gary E. Bulman,
`both of Gary; Hua-Sbuang Kong,
`Raleigh; Vladimir Dmitriev,
`Fuquay-Varina, all of N.C.
`
`[73] Assignee: Cree Research, Inc., Durham, N.C.
`
`[21] Appl. No.: 309,251
`
`Sep. 20, 1994
`
`[22] Filed:
`[51] Int. CI.6
`[52] U.S. CI.
`
`[58] Field of Search
`
`H01L 31/0312; H01L 33/00
`257/77; 257/96; 257/97;
`257/103
`257/77, 96, 97,
`257/103; 372/43, 44, 45, 46
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`5,243,204
`5,247,533
`5,273,933
`5,290,393
`5,306,662
`5,313,078
`5,338,944
`5,387,804
`5,393,993
`5,416,342
`5,432,808
`
`9/1993 Suzuki et al
`9/1993 Okazaki et al. .
`12/1993 Hatano et al. ...
`3/1994 Nakamura
`4/1994 Nakamura et al.
`5/1994 Fujii et al
`8/1994 Edmond et al. ..
`2/1995 Suzuki et al
`2/1995 Edmond et al. ..
`5/1995 Edmond et al. ..
`7/1995 Hatano et al. ....
`
`. 257/77
`. 372/45
`437/127
`156/613
`437/107
`257/77
`257/76
`257/77
`257/77
`257/76
`372/45
`
`FOREIGN PATENT DOCUMENTS
`
`0541373A2
`
`5/1993 European Pat. Off. .
`
`OTHER PUBLICATIONS
`
`V. A. Dmitriev, SiC-Based Solid Solutions: Technology and
`Properties, Springer Proceedings in Physics, vol. 56, 1992,
`pp. 3-14.
`
`J. N. Kuznia et al.. Influence of Buffer Layers on the
`Deposition of High Quality Single Crystal GaN Over Sap­
`phire Substrates, J. Appl. Phys., vol. 73, No. 9, May 1993,
`pp. 4700-4702.
`M. Asif Khan et al., The Nature of Donor Conduction in
`n-GaN, J. Appl. Phys., vol. 74, No. 9, Nov. 1993, pp.
`5901-5903.
`Isamu Akasaki et al., Effects of AIN Buffer Layer on Crys-
`tallographic Structure and on Electrical and Optical Prop­
`erties of GaN and Ga1_xA\xN (0<x §0.4) Films Grown on
`Sapphire Substrate by MOVPE, Journal of Crystal Growth,
`vol. 98, 1989, pp. 209-219.
`Shuji Nakamura, GaN Growth Using GaN Buffer Layer,
`Japanese Journal of Applied Physics, vol. 30, No. 10A, OCt.
`1991, pp. L1705-L1707.
`Shuji Nakamura et al., Candela-Class High-Brightness
`InGaN/AlGaN Double-Heterostructure Blue-Light-Emit­
`ting Diodes, Appl. Phys. Lett., vol. 64, No. 13, Mar. 1994,
`pp. 1687-1689.
`Shuji Nakamura, InGaN/AlGaN Double-Heterostructure
`Blue LEDs (undated).
`Primary Examiner—Ngan V. Ngo
`Attorney, Agent, or Firm—Bell, Seltzer, Park & Gibson
`[57]
`ABSTRACT
`
`A light emitting diode emits in the blue portion of the visible
`spectrum and is characterized by an extended lifetime. The
`light emitting diode comprises a conductive silicon carbide
`substrate; an ohmic contact to the silicon carbide substrate;
`a conductive buffer layer on the substrate and selected from
`the group consisting of gallium nitride, aluminum nitride,
`indium nitride, ternary Group III nitrides having the formula
`A^Bj^N, where A and B are Group III elements and where
`x is zero, one, or a fraction between zero and one, and alloys
`of silicon carbide with such ternary Group III nitrides; and
`a double heterostructure including a p-n junction on the
`buffer layer in which the active and heterostructure layers
`are selected from the group consisting of binary Group HI
`nitrides and ternary Group III nitrides.
`
`36 Claims, 4 Drawing Sheets
`
`30
`
`26
`25
`27
`
`23
`
`21
`
`20
`
`24
`
`2 2
`
`EVERLIGHT ELECTRONICS CO., LTD. ET AL.
`Exhibit 1011
`
`

`

`U.S. Patent
`
`Jun. 4, 1996
`
`Sheet 1 of 4
`
`5,523,589
`
`30
`
`40
`
`46
`
`55
`
`1
`
`26
`25
`27
`
`23
`
`21
`
`37
`36
`
`33
`
`48
`47
`
`43
`
`54
`
`53
`58
`57
`
`51
`
`20
`
`24
`
`V 2 2
`32
`
`35
`
`X
`
`42
`
`45
`
`V 4 4
`50
`
`•52
`
`\ 56
`
`FIG. 1.
`
`FIG. 2 .
`
`FIG. 3 .
`
`FIG. 4 .
`
`EVERLIGHT ELECTRONICS CO., LTD. ET AL.
`Exhibit 1011
`
`

`

`U.S. Patent
`
`Jun. 4, 1996
`
`Sheet 2 of 4
`
`5,523,589
`
`GaN LEDS ON SiC AND SAPPHIRE
`
`GaN ON SiC
`
`GaN ON SAPPHIRE
`
`1 . 2
`
`1 . 0
`
`>-
`oo
`5 0.8
`z
`Ld
`>
`P 0.6
`<
`UJ cr
`
`' / / / / / / / / / / , 7 7 7 / / / / / / / ' f y 7 7 7 7 7 7 7 r / / / / / / / / / / ,
`0.4
`
`0.2
`1 0
`FIG. 5 .
`
`3
`25x10
`
`GaN
`
`2 0 -
`o
`UJ
`^ 15-
`co
`z
`=> 1 0 -
`o o
`
`100
`
`1000
`TIME (HR)
`
`10000
`
`100000
`
`FWHM = 85"
`
`SiC
`
`FWHM = 97"
`
`0
`
`0
`
`FIG. 6 .
`
`i
`1000
`2000
`ANGLE (ARCSEC)
`
`3000
`
`EVERLIGHT ELECTRONICS CO., LTD. ET AL.
`Exhibit 1011
`
`

`

`Jun. 4, 1996
`
`Sheet 3 of 4
`
`5,523,589
`
`GaN/SiC
`
`T=295 K
`EXCITATION AT 325 nm
`UNIFORM EMISSION OBSERVED
`ACROSS SURFACE.
`
`3.41
`
`eV I
`
`U.S. Patent
`
`3
`500x10
`
`oo 400-
`z:
`3
`DO cc 300
`<
`Ld o
`^ 200-
`o
`oo
`UJ
`2
`
`Z)
`
`100-
`
`FWHM = 38.6 meV
`
`2.24 eV
`
`FWHM = 444meV
`
`2.0
`
`2.5
`ENERGY (eV)
`
`3.0
`
`3.0
`
`0^—T
`1.5
`FIG. 7 .
`
`1 0 -
`
`T
`
`T
`
`T
`
`Zl
`
`0 6 -
`>-
`on
`Z4-
`LJ
`
`Z
`
`c
`
`N
`
`Al
`
`Si
`
`I
`
`Ga
`
`0 30
`FIG. 8 .
`
`i
`530
`
`i
`i
`1030
`1530
`KINETIC ENERGY, eV
`
`2030
`
`EVERLIGHT ELECTRONICS CO., LTD. ET AL.
`Exhibit 1011
`
`

`

`U.S. Patent
`
`Jun. 4, 1996
`
`Sheet 4 of 4
`
`5,523,589
`
`4.5
`
`4.0-
`
`>
`CD
`
`Z
`0 3.5-
`o
`x
`Q_
`
`bJ 3.0-
`
`2.5
`0
`
`J L
`
`I,
`
`I
`
`20
`
`.1
`60
`40
`S i C m o l %
`
`80
`
`100
`
`G. 9 .
`
`EVERLIGHT ELECTRONICS CO., LTD. ET AL.
`Exhibit 1011
`
`

`

`FIELD OF THE INVENTION
`
`35
`
`40
`
`5,523,589
`
`1
`VERTICAL GEOMETRY LIGHT EMITTING
`DIODE WITH GROUP III NITRIDE ACTIVE
`LAYER AND EXTENDED LIFETIME
`
`This invention relates to optoelectronic devices and more
`particularly to light emitting diodes formed from Group III
`nitrides (i.e., Group III of the Periodic Table of the Ele­
`ments) that will produce output in the blue to ultraviolet 10
`portions of the electromagnetic spectrum.
`
`2
`Pal. Nos. 4,918,497 and 5,027,168 to Edmond each titled
`"Blue Light Emitting Diode Formed In Silicon Carbide. "
`Other examples of such a blue LED are described in U.S.
`Pat. No. 5,306,662 to Nakamura et al. titled "Method Of
`5 Manufacturing P-Type Compound Semiconductor" and U.S.
`Pat. No. 5,290,393 to Nakamura titled "Crystal Growth
`Method For Gallium Nitride-Based Compound Semiconduc­
`tor." U.S. Pat. No. 5,273,933 to Hatano et al. titled "Vapor
`Phase Growth Method Of Forming Film In Process Of
`Manufacturing Semiconductor Device" also describes LEDs
`formed of GalnAIN on SiC substrates and Zinc Selenide
`(ZnSe) on gallium arsenide (GaAs) substrates.
`As known to those familiar with photonic devices such as
`BACKGROUND OF THE INVENTION
`LEDs, the frequency of electromagnetic radiation (i.e., the
`Light emitting diodes ("LEDs") are p-n junction devices is photons) that can be produced by a given semiconductor
`material are a function of the material's bandgap. Smaller
`that have been found to be useful in various roles as the field
`bandgaps produce lower energy, longer wavelength photons,
`of optoelectronics has grown and expanded over the years.
`while wider bandgap materials are required to produce
`Devices that emit in the visible portion of the electromag­
`higher energy, shorter wavelength photons. For example.
`netic spectrum have been used as simple status indicators,
`dynamic power level bar graphs, and alphanumeric displays 20 one semiconductor commonly used for lasers is indium
`in many applications, such as audio systems, automobiles,
`gallium aluminum phosphide (InGaAlP). Because of this
`household electronics, and computer systems, among many
`material's bandgap (actually a range of bandgaps depending
`others. Infrared devices have been used in conjunction with
`upon the mole or atomic fraction of each element present),
`spectrally matched phototransistors in optoisolators, hand-
`the light that InGaAlP can produce is limited to the red
`held remote controllers, and interruptive, reflective, and 25 portion of the visible spectrum, i.e., about 600 to 700
`fiber-optic sensing applications.
`nanometers (nm).
`Working backwards, in order to produce photons that
`An LED operates based on the recombination of electrons
`have wavelengths in the blue or ultraviolet portions of the
`and holes in a semiconductor. When an electron carrier in the
`spectrum, semiconductor materials are required that have
`conduction band combines with a hole in the valence band,
`it loses energy equal to the bandgap in the form of an emitted 30 relatively large bandgaps. Typical candidate materials
`photon; i.e., light. The number of recombination events
`include silicon carbide (SiC) and gallium nitride (GaN).
`Shorter wavelength LEDs offer a number of advantages in
`under equilibrium conditions is insufficient for practical
`addition to color. In particular, when used in optical storage
`applications but can be enhanced by increasing the minority
`and memory devices (e.g., "CD-ROM" or "optical disks"),
`carrier density.
`their shorter wavelengths enable such storage devices to
`In an LED, the minority carrier density is conventionally
`hold proportionally more information. For example, an
`increased by forward biasing the diode. Tlie injected minor­
`optical device storing information using blue light can hold
`ity carriers radiatively recombine with the majority carriers
`approximately 32 times as much information as one using
`within a few diffusion lengths of the junction edge. Each
`red light, in the same space.
`recombination event produces electromagnetic radiation, i.e,
`Gallium nitride, however, is an attractive LED candidate
`a photon. Because the energy loss is related to the bandgap
`material for blue and UV frequencies because of its rela­
`of the semiconductor material, the bandgap characteristics
`of the LED material has been recognized as being important.
`tively high bandgap (3.36 eV at room temperature) and
`because it is a direct bandgap material rather than an indirect
`As with other electronic devices, however, there exists
`45 bandgap material. As known to those familiar with semi­
`both the desire and the need for more efficient LEDs, and in
`conductor characteristics, a direct bandgap material is one in
`particular, LEDs that will operate at higher intensity while
`which an electron's transition from the valence band to the
`using less power. Higher intensity LEDs, for example, are
`conduction band does not require a change in crystal
`particularly useful for displays or status indicators in various
`momentum for the electron. In indirect semiconductors, the
`high ambient environments. There also is a relation between
`intensity output of the LED and the power required to drive 50 alternative situation exists; i.e., a change of crystal momen­
`tum is required for an electron's transition between the
`the LED. Low power LEDs, for example, are particularly
`valence and conduction bands. Silicon and silicon carbide
`useful in various portable electronic equipment applications.
`are examples of such indirect semiconductors.
`An example of an attempt to meet this need for higher
`intensity, lower power, and more efficient LEDs may be seen
`Generally speaking, an LED formed in a direct bandgap
`with the development of the AlGaAs LED technology for 55 material will perform more efficiently than one formed in an
`LEDs in the red portions of the visible spectrum. A similar
`indirect bandgap material because the photon from the direct
`continual need has been felt for LEDs that will emit in the
`transition retains more energy than one from an indirect
`blue and ultraviolet regions of the visible spectrum. For
`transition.
`Gallium nitride suffers from a different disadvantage,
`example, because blue is a primary color, its presence is
`either desired or even necessary to produce full color go however: the failure to date of any workable technique for
`displays or pure white light.
`producing bulk single crystals of gallium nitride which
`The common assignee of the present patent application
`could form appropriate substrates for gallium nitride pho­
`was the first in this field to successfully develop commer­
`tonic devices. As is known to those familiar with semicon-
`cially viable LEDs available in large quantities and that
`ductor devices, they all require some sort of structural
`emitted light in the blue color spectrum. These LEDs were 65 substrate. Typically, a substrate formed of the same materials
`formed in silicon carbide, a wide bandgap semiconductor
`as the active region of a device offers significant advantages,
`material. Examples of such blue LEDs are described in U.S.
`particularly in crystal growth and matching. Because gal-
`
`EVERLIGHT ELECTRONICS CO., LTD. ET AL.
`Exhibit 1011
`
`

`

`4
`consisting of gallium nitride, aluminum nitride, indium
`nitride, ternary Group III nitrides having the formula A^B,.
`tN, where A and B are Group III elements and where x is
`zero, one, or a fraction between zero and one, quaternary
`where A,
`Group III nitrides having the formula
`B, and C are Group III elements; x and y, are zero, one, or
`a fraction between zero and one, and 1 is greater than (x+y),
`and alloys of silicon carbide with such ternary and quater­
`nary Group III nitrides; and a double heterostructure includ-
`ing a p-n junction on the buffer layer in which the active and
`heterostructure layers are selected from the group consisting
`of binary Group III nitrides, ternary Group III nitrides,
`quaternary Group III nitrides, and alloys of silicon carbide
`with such nitrides.
`The foregoing and other objects, advantages and features
`of the invention, and the manner in which the same are
`apparent upon
`accomplished, will become more readily
`consideration of the following detailed description of the
`invention taken in conjunction with the accompanying draw-
`ings> which illustrate preferred and exemplary embodi­
`ments, and wherein;
`
`3
`lium nitride has yet to be formed in such bulk crystals,
`however, gallium nitride photonic devices must be formed in
`epitaxial layers on different—i.e., other than GaN—sub­
`strates.
`Using different substrates, however, causes an additional 5
`set of problems, mostly in the area of crystal lattice match­
`ing. In almost all cases, different materials have different
`crystal lattice parameters. As a result, when a gallium nitride
`epitaxial layer is grown on a different substrate, some crystal
`mismatch will occur, and the resulting epitaxial layer is 10
`referred to as being "strained" by this mismatch. Such
`mismatches, and the strain they produce, carry with them the
`potential for crystal defects which in turn affect the elec­
`tronic characteristics of the crystals and the junctions, and
`thus correspondingly tend to degrade or even prevent the 15
`performance of the photonic device. Such defects are even
`more problematic in higher power structures.
`To date, the most common substrate for gallium nitride
`devices—and the only substrate utilized in GaN LED's—
`has been sapphire; i.e., aluminum oxide (Al203). Sapphire is 20
`optically transparent in the visible and UV ranges, but is
`unfortunately insulating rather than conductive, and carries
`BRIEF DESCRIPTION OF THE DRAWINGS
`a lattice mismatch with gallium nitride of about 16%. In the
`absence of a conductive substrate, "vertical" devices (those
`FIG. 1 schematically illustrates a vertical sectional view
`with contacts on opposite sides) cannot be formed, thus 25
`of a first embodiment of an extended lifetime light emitting
`complicating the manufacture and use of the devices.
`diode according to the present invention;
`As a particular disadvantage, horizontal structures (those
`FIG. 2 schematically illustrates a vertical sectional view
`with contacts on the same side of the device), such as those
`of a second embodiment of an extended lifetime light
`required when gallium nitride is formed on sapphire, also
`30 emitting diode according to the present invention;
`produce a horizontal flow of current and therefore the
`FIG. 3 schematically illustrates a vertical sectional view
`current density through the layer is substantially increased.
`of a third embodiment of an extended lifetime light emitting
`This horizontal current flow puts an additional strain on the
`diode according to the present invention;
`already-strained (i.e., the 16% lattice mismatch) GaN-sap-
`FIG. 4 schematically illustrates a vertical sectional view
`phire structure and accelerates the degradation of the junc- 35
`of a fourth embodiment of an extended lifetime light emit­
`don and the device as a whole.
`ting diode according to the present invention;
`Gallium nitride also carries a lattice mismatch of about
`FIG. 5 graphically illustrates relative intensity over time
`2.4% with aluminum nitride (A1N) and a 3.5% mismatch
`of a prior art light emitting diode in comparison to a light
`with silicon carbide. Silicon Carbide has a somewhat lesser
`emitting diode according to the present invention;
`mismatch (only about 1 %) with aluminum nitride.
`FIG. 6 graphically illustrates a double crystal X-ray
`Group III ternary and quaternary nitrides (e.g., InGaN,
`rocking curve for a GaN layer on a SiC substrate as used in
`InGaAIN, etc.) have also been shown to have relatively wide
`a light emitting diode according to the present invention;
`bandgaps and thus also offer the potential for blue and
`FIG. 7 graphically illustrates phololuminescence as com-
`ultraviolet semiconductor lasers. Most of these compounds,
`however, present the same difficulty as gallium nitride: the 45 pared to energy output of a GaN layer on a SiC substrate as
`utilized in an extended lifetime light emitting diode accord­
`lack of an identical single crystal substrate. Thus, each must
`ing to the present invention;
`be used in the form of epitaxial layers grown on different
`substrates. Thus, they present the same potential for crystal
`FIG. 8 graphically illustrates intensity as compared to
`defects and their associated electronic problems.
`kinetic energy of an alloy of SiC-AlN-GaN; and
`FIG. 9 graphically illustrates crystal lattice peak energy as
`a function of silicon carbide concentration in a SiC-AlN-
`GaN alloy according to the present invention.
`
`50
`
`5,523,589
`
`OBJECT AND SUMMARY OF THE INVENTION
`Therefore, it is an object of the present invention to
`provide a light emitting diode that can emit in the blue and
`DETAILED DESCRIPTION OF A PREFERRED
`ultraviolet portions of the electromagnetic spectrum, that 55
`EMBODIMENT
`can be built in the vertical geometry that is most advanta­
`The present invention is a light-emitting diode that pro­
`geous for such devices, that has excellent brightness and
`duces light in the blue portion of the visible spectrum and
`efficiency and that can exhibit better physical and electronic
`that is characterized by an extended lifetime. As known to
`longevity and performance than can previously available
`those familiar with the performance, characteristics and
`diodes.
`ratings of such light-emitting diodes, the lifetime is gener­
`The invention meets this object with a light emitting diode
`ally defined as the time over which the LED's output will
`that emits in the blue portion of the visible spectrum and that
`degrade to about 50% of its original output.
`is characterized by an extended lifetime because of its
`FIG. 1 is a cross-sectional schematic view of a light-
`advantageous materials and structure. The light emitting
`diode comprises a conductive silicon carbide substrate; an 65 emitting diode according to the present invention and gen-
`erally designated at 20. The diode comprises a conductive
`ohmic contact to the silicon carbide substrate; a conductive
`silicon carbide substrate 21 which, in preferred embodi-
`buffer layer on the substrate and selected from the group
`
`6o
`
`EVERLIGHT ELECTRONICS CO., LTD. ET AL.
`Exhibit 1011
`
`

`

`5,523,589
`
`6
`5
`ments, is a single crystal silicon carbide substrate. As is well
`x is zero, one or a fraction between zero and one, and alloys
`understood by those of ordinary skill in this art, a high
`of silicon carbide with such ternary Group III nitrides.
`quality single crystal substrate provides a number of struc­
`In the heterostructure 24 illustrated in FIG. 1, the active
`tural advantages that in turn provide significant performance
`layer 25 can preferably comprise indium gallium nitride and
`and lifetime advantages. In preferred embodiments, the SiC 5
`the upper and lower heterostructure layers 26 and 27 will
`substrates can be formed by the methods described in U.S.
`preferably comprise aluminum gallium nitride. More spe­
`Pat. No. 4,866,005 (now U.S. Pat. No. RE 34,861) which is
`cifically, the aluminum gallium nitride heterostructure layers
`commonly assigned with the pending application.
`26 and 27 preferably have the formula Al/ja^N where x is
`An ohmic contact 22 is made to the silicon carbide
`zero, one or a fraction between zero and one. When the
`substrate and is one of the characteristics of the present 10 active layer 25 comprises indium gallium nitride, the com-
`invention that immediately distinguishes it from prior diodes
`position will be understood to be InXSa^N, where z is a
`of the materials discussed herein. As noted earlier, the
`fraction between zero and one.
`typical substrate for gallium nitride is sapphire, which
`As known to those of ordinary skill in this art, the
`cannot be made conductive, and thus cannot be connected to
`composition of the ternary Group III nitrides can affect both
`an ohmic contact. This prevents a sapphire-based device 15
`their refractive index and their bandgap. Generally speaking,
`from being formed in the vertical structure that is most
`a larger proportion of aluminum increases the bandgap and
`preferred for LEDs and many other devices.
`decreases the refractive index. Thus, in preferred embodi­
`FIG. 1 further illustrates that the LED 20 comprises a
`ments, in order for the heterostructure layers 26 and 27 to
`have a bandgap larger than the active layer 25 and a
`buffer layer 23 on the substrate 21. The buffer layer 23 is
`selected from the group consisting of gallium nitride, alu- 20 refractive index smaller than the active layer 25, the layers
`minum nitride, indium nitride, ternary Group III nitrides
`26 and 27 have a higher atomic or mole percentage of
`having the formula A^B^N, where A and B are Group III
`aluminum than does the active layer 25. The larger bandgap
`elements and where x is zero, one or a fraction between zero
`of the heterostructure layers 26 and 27 encourages electrons
`and one, quaternary Group III nitrides having the formula
`to be injected through the active layer 25 thus increasing the
`N where A, B, and C are Group III elements, x 25 efficiency of the device. Similarly, the lower refractive index
`AjtByC] -x-y
`and y, are zero, one, or a fraction between zero and one, and
`of the heterostructure layers 26 and 27 encourage the light
`1 is greater than (x+y) and alloys of silicon carbide with such
`to be more preferably emitted on an optical basis from active
`ternary and quaternary Group III nitrides. The buffer layer
`layer 25.
`23 and the substrate 21 are both conductive.
`In order to form the p-n junction, the upper and lower
`The LED 20 further includes a double heterostructure
`heterostructure layers 26 and 27 have opposite conductivity
`designated by the brackets 24, and specifically including a
`types from one another, and the active layer 25 has the same
`p-n junction, on the buffer layer 23. The structural designa­
`conductivity type as one of the two heterostructure layers.
`tion "double heterostructure" is used in a manner common
`For example, in a preferred embodiment, the upper hetero-
`structure layer 26 is p-type, the active layer 25 is n-type, the
`to, and well understood in, this art. Aspects of these struc-
`tures are discussed, for example, in Sze, Physics of Semi- 35
`lower heterostructure layer 27 is n-type, and the buffer and
`conductor Devices, Second Edition (1981) at pages
`the silicon carbide substrate are both also n-type. The p-n
`junction is thus formed between the active layer 25 and the
`708-710. The Sze discussion on those pages refers to lasers,
`upper heterostructure layer 26.
`but illustrates the nature of, and the distinction between,
`homostructure, single heterostructure, and double hetero­
`FIG. 2 illustrates a slightly different embodiment of the
`structure junctions.
`present invention broadly designated at 32. As in the pre­
`In the embodiment illustrated in FIG. 1, the double
`vious embodiment, the LED comprises a silicon carbide
`heterostructure 24 further comprises an active layer 25 along
`substrate 33 and its ohmic contact 34. The double hetero­
`with upper 26 and lower 27 heterostructure layers adjacent
`structure is designated by the brackets at 35. In the embodi­
`the active layer 25. The heterostructure layers 26 and 27 are 45
`ment of FIG. 2, the buffer layer is shown at 36 and comprises
`preferably formed of a composition selected from the group
`gallium nitride, and the overall structure further comprises a
`consisting of gallium nitride, aluminum nitride, indium
`gallium nitride epitaxial layer 37 on the buffer layer between
`nitride, ternary Group III nitrides having the formula A^B
`the gallium nitride buffer layer 36 and the double hetero­
`iN, where A and B are Group III elements and where x is
`structure 35. An ohmic contact 40 to the double heterostruc­
`zero, one or a fraction between zero and one, and alloys of
`ture 35 completes the advantageous vertical structure of the
`silicon carbide with such ternary Group III nitrides, e.g.,
`device.
`(SiCj^AyB, ^.N. Stated differently, the lowest heterostructure
`Although specific performance parameters will be dis­
`layer will be on top of the buffer layer. In FIG. 1, this is
`cussed later herein, the diodes described herein and illus­
`illustrated as lower heterostructure 27 being on top of buffer
`trated in these and the remaining drawings are expected to
`layer
`55 have lifetimes of greater than 10,000 hours operating at a
`An ohmic contact 30 can be applied to the upper hetero­
`forward bias current of 50 milliamps at room temperature,
`structure layer 26 to complete the advantageous vertical
`and lifetimes of greater than 10,000 hours operating at a
`structure of the invention. The ohmic contacts preferably are
`forward bias current of 30 milliamps at room temperature. It
`each formed of a metal such as aluminum (Al), gold (Au),
`will be recognized by those familiar with such devices that
`these specifications greatly exceed those of presently avail­
`platinum (Pt), or nickel (Ni), but may be formed of other go
`material for forming ohmic contacts as understood by those
`able devices.
`skilled in the art.
`FIG. 3 illustrates a third embodiment of the present
`In each of the embodiments illustrated herein, the double
`invention broadly designated at 42. As in the previous
`heterostructure comprises an active layer selected from the
`embodiments, the diode 42 includes a silicon carbide sub­
`group consisting of gallium nitride, aluminum nitride, 65
`strate 43, and an ohmic contact 44 to the substrate 43. The
`indium nitride, ternary Group III nitrides having the formula
`double heterostructure is again designated by the brackets 45
`A^B, ^N, where A and B are Group III elements and where
`and an upper ohmic contact 46 is made to the double
`
`50
`
`30
`
`40
`
`EVERLIGHT ELECTRONICS CO., LTD. ET AL.
`Exhibit 1011
`
`

`

`5,523,589
`
`8
`7
`second Group III nitride layer 54, and an ohmic contact 56
`heterostructure 45. In this embodiment, however, the buffer
`is formed on the silicon carbide substrate so that a current
`layer comprises first and second layers 47 and 48 respec­
`supplied across the first and second ohmic contacts to the p-n
`tively. The first layer 47 is on the substrate 43 and is formed
`junction device produces a high light intensity output there­
`of a graded composition of silicon carbide aluminum gal­
`from.
`lium nitride (SiCj^A^.Ga, ^ in which the portion adjacent 5
`As indicated by the dotted line in FIG. 4, the buffer 52
`the substrate 43 is substantially entirely silicon carbide and
`preferably comprises a first layer 57 on the substrate 51 and
`the portion furthest from the substrate is substantially
`formed of a graded composition of silicon carbide aluminum
`entirely aluminum gallium nitride, with the portions ther­
`gallium nitride in which the portion adjacent the substrate is
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`dominantly silicon carbide to predominantly aluminum gal- 10 substantially entirely silicon carbide and the portion furthest
`from the substrate is substantially entirely aluminum gallium
`lium nitride
`nitride with the portions therebetween being progressively
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`ded in content froin predominantly siiic0n carbide to
`The second layer 48 is on the first layer 47 md is formed
`predominantly aluminum gallium nitride.
`of another graded composition of aluminum gallium mtnde.
`In preferred embodiments, the composition of the graded
`A second buffer layer 58 is upon the first layer 57 and is
`second layer 48 is graded from a composition matching the 15
`formed of a graded composition of aluminum gallium
`composition of the first buffer layer 47 at the point where the
`nitride. As described with respect to earlier embodiments,
`layers 47 and 48 meet, to a composition matching the
`the composition of the graded second layer 58 is progres­
`composition of the lowest layer of the double heterostructure
`sively graded from a composition matching the composition
`45.
`20 of the first buffer layer 57 at the point where layers 58 and
`With respect to FIG. 3, the buffer layer can also be
`57 join, to a composition matching the composition of the
`described as having at least one graded layer of silicon
`lower Group III nitride layer 53 of the diode.
`carbide and a Group III nitride in which the graded layer is
`In the diode 50 illustrated in FIG. 4, the nitride layers 53
`silicon carbide at the interface with the substrate and then
`and 54 are selected from the group consisting of gallium
`progressively graded to a composition matching the com­
`nitride, aluminum nitride, indium nitride, ternary Group III
`position of the lowest layer of the double heterostructure at 25
`nitrides having the formula A^B^JM, where A and B are
`the interface with the double heterostructure.
`Group III elements and where x is zero, one or a fraction
`The invention can further comprise a strain-minimizing
`between zero and one, and alloys of silicon carbide with
`contact layer (not shown) above the active layer in the
`such ternary Group III nitrides. It will thus be understood
`that in this and the previous embodiment, the junction can be
`double heterostructure and that would have a lattice constant 30
`substantially the same as the respective buffer layers. Such
`a homostructure, a single-heterostructure, or a double-het-
`a strain-minimizing contact layer is set forth in an applica­
`erostructure.
`tion filed concurrently herewith by Edmond and Bulman for
`The buffer 52 can alternatively comprise a lower inter­
`"Low Strain Laser Structures with Group III Nitride Active
`mediate layer 57 formed of silicon carbide positioned on the
`intermediate layer
`Layers," which is commonly assigned with this application 35 silicon carbide substrale 51 and ^
`and which is incorporated entirely herein by reference.
`58 formed of a nitride alloy positioned on the lower inter­
`Briefly summarized, the overall strain of such a multi-
`mediate layer 57.
`layered crystalline device is a function of the average of the
`The buffer can include at least one graded layer of silicon
`individual strains based on the differences between their
`carbide and a Group III nitride in which the graded layer is
`lattice constants. Thus, by adding a layer with a lattice 40
`silicon carbide at the interface with the substrate 51, and the
`constant substantially the same as the buffer, the weighted
`graded layer is a composition matching the composition of
`average of the strains becomes more consistent and thus
`the lowest layer of the active device at the interface with the
`reduces the overall strain.
`junction structure.
`As some additional details, the upper surface of the silicon
`As in earlier embodiments, the light-emitting diode can
`carbide substrate in any of the embodiments can be doped 45
`have the upper surface of the silicon carbide substrate doped
`with aluminum to enhance the crystal growth. As already
`with aluminum.
`stated, the substrate and the buffer layers in each embodi­
`As discussed with reference to some of the other figures
`ment are conductive, and this is usually accomplished by
`herein, the characteristics of the crystals according to the
`doping each of the layers with appropriate dopants. The
`50 present invention are generally superior to any exhibited by
`silicon carbide substrate can be selected from several of the
`any prior devices. Thus, a