US005523589A
`United States Patent 15
`(11] Patent Number:
`5,523,589
`Edmondetal.
`[45] Date of Patent:
`Jun. 4, 1996
`
`NIA AAEI
`
`[54] VERTICAL GEOMETRY LIGHT EMITTING
`DIODE WITH GROUPIII NITRIDE ACTIVE
`LAYER AND EXTENDED LIFETIME
`
`[75]
`
`Inventors: John A. Edmond; Gary E. Bulman,
`both of Cary; Hua-Shuang Kong,
`Raleigh; Vladimir Dmitriev,
`Fuquay-Varina, all of N.C.
`
`[73] Assignee: Cree Research, Inc., Durham, N.C.
`
`[21] Appl. No.: 309,251
`
`[22]
`
`Filed:
`
`Sep. 20, 1994
`
`.. HO1L 31/0312; HO1L 33/00
`Int. CLS ..
`[51]
`[52] U.S. Che oececcsecseesecensenns 257/77; 257/96; 257/97,
`257/103
`[58] Field of Search 0.0.0.0... 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...cesses 257/77
`9/1993 Okazaki et al.
`oo
`eeeeeeeee 372/45
`.
`12/1993 Hatanoet al.
`. 437/127
`3/1994 Nakamura......
`. 156/613
`4/1994 Nakamura etal.
`oo... 437/107
`
`5/1994 Fujii et ab. ee ceescseseenseeees 257/77
`
`eee 257/76
`8/1994 Edmondet al.
`........c
`
`sceesceeee 257/77
`2/1995 Suzuki et al. oc...
`2/1995 Edmondet al.
`ou...
`cessseeeee 257/77
`..........sssersesseseres 257/76
`5/1995 Edmondet al.
`
`soceeeenennee 372/45
`7/1995 Hatano etal. ..
`
`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.
`
`Influence of Buffer Layers on the
`J. N. Kuznia et al.,
`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 Ga,_,Al,N (Q<x 30.4) 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/AIGaN Double—Heterostructure Blue—Light-Emit-
`ting Diodes, Appl. Phys. Lett., vol. 64, No. 13, Mar. 1994,
`pp. 1687-1689.
`InGaN/AlGaN Double—Heterostructure
`Shuji Nakamura,
`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 portionof the visible
`spectrum and is characterized by an extendedlifetime. The
`light emitting diode comprises a conductive silicon carbide
`substrate; an ohmic contact to thesilicon carbide substrate;
`a conductive buffer layer on the substrate and selected from
`the group consisting of gallium nitride, aluminum nitride,
`indium nitride, temary GroupIII nitrides having the formula
`A,B,_,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 GroupIII 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 GroupIII nitrides.
`
`36 Claims, 4 Drawing Sheets
`
`30
`
`Yee 22
`
`Cree Ex. 1011
`
`Page 1
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`Cree Ex. 1011
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`Page 1
`
`

`

`5,523,589
`
`Jun. 4, 1996
`
`Sheet 1 of 4
`
`U.S. Patent
`
`
`
`Cree Ex. 1011
`
`Page 2
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`Cree Ex. 1011
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`

`

`U.S. Patent
`
`Jun. 4, 1996
`
`Sheet 2 of 4
`
`5,523,589
`
`GaN LEDS ON SiC AND SAPPHIRE
`
`
`© FWHM=97"
`
`
`
`RELATIVEINTENSITY
`
`FIG.
`
`“10
`5
`
`25x10°
`
`20
`
`15
`
`© A
`
`—n
`
`s2 10
`
`FIG. 6.
`
`100
`
`1000
`TIME (HR)
`
`10000
`
`100000
`
`FWHM=85”
`
`
`2000
`~~ 1000
`3000
`ANGLE (ARCSEC)
`
`Cree Ex. 1011
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`Page 3
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`Cree Ex. 1011
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`

`

`U.S. Patent
`
`Jun. 4, 1996
`
`Sheet 3 of 4
`
`5,523,589
`
`GaN /SiC
`
`~D 400
`KS
`
`300
`
`FWHM=38.6 meV
`
`“200
`WY
`Lu)
`Zz
`
`100
`
`0
`
`15
`
`2.24 OV wi 444meV
`
`20
`
`2'5
`
`3.0
`
`30
`
`= =
`
`“iC.
`
`7
`
`ENERGY (eV)
`
`C
`
`N
`
`Al
`
`Si
`
`Ga
`
`10
`
`8
`
`> O
`
`¢
`
`.F
`WY
`Z 4LJ
`_—
`=
`
`2
`
`O36
`rin 2
`
`530
`
`1030
`KINETIC ENERGY, eV
`
`7530
`
`7030
`
`Cree Ex. 1011
`
`Page 4
`
` 500x10°
`
`=> a&
`
`L L
`
`o© z
`
`Cree Ex. 1011
`
`Page 4
`
`

`

`U.S. Patent
`
`Jun. 4, 1996
`
`Sheet 4 of 4
`
`5,523,589
`
`4.5
`
`4.0
`
`3.5
`
`>O
`
`o +©
`
`©a
`
`n
`CL
`
`“3.0
`
`2.5
`0
`
`20
`
`80
`
`100
`
`40
`SiC mol%
`
`60
`
`FIG. 9.
`
`Cree Ex. 1011
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`Page 5
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`Cree Ex. 1011
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`Page 5
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`

`

`5,523,589
`
`1
`VERTICAL GEOMETRY LIGHT EMITTING
`DIODE WITH GROUP III NITRIDE ACTIVE
`LAYER AND EXTENDED LIFETIME
`
`FIELD OF THE INVENTION
`
`This invention relates to optoelectronic devices and more
`particularly to light emitting diodes formed from Group ITI
`nitrides (i.e., Group III of the Periodic Table of the Ele-
`ments) that will produce output in the blue to ultraviolet
`portions of the electromagnetic spectrum.
`
`BACKGROUNDOF THE INVENTION
`
`Light emitting diodes (“LEDs”) are p-n junction devices
`that have been foundto be useful in variousroles as the field
`of optoelectronics has grown and expanded overthe years.
`Devices that emit in the visible portion of the electromag-
`netic spectrum have been used as simple status indicators,
`dynamic powerlevel bar graphs, and alphanumeric displays
`in many applications, such as audio systems, automobiles,
`household electronics, and computer systems, among many
`others. Infrared devices have been used in conjunction with
`spectrally matched phototransistors in optoisolators, hand-
`held remote controllers, and interruptive, reflective, and
`fiber-optic sensing applications.
`An LEDoperates based on the recombinationof electrons
`and holes in a semiconductor. When an electron carrier in the
`conduction band combines with a hole in the valence band,
`it loses energy equal to the bandgapin the form of an emitted
`photon;
`i.e.,
`light. The number of recombination events
`under equilibrium conditions is insufficient for practical
`applications but can be enhanced by increasing the minority
`carrier density.
`In an LED,the minority carrier density is conventionally
`increased by forward biasing the diode. The injected minor-
`ity carriers radiatively recombine with the majority carriers
`within a few diffusion lengths of the junction edge. Each
`recombination event produces electromagnetic radiation,i.e,
`a photon. Because the energy loss is related to the bandgap
`of the semiconductor material, the bandgap characteristics
`of the LED material has been recognized as being important.
`As with other electronic devices, however, there exists
`both the desire and the need for more efficient LEDs, and in
`particular, LEDs that will operate at higher intensity while
`using less power. Higher intensity LEDs, for example, are
`particularly useful for displays or status indicators in various
`high ambient environments. Therealsois a relation between
`intensity output of the LED and the power required to drive
`the LED. Low power LEDs, for example, are particularly
`useful in various portable electronic equipmentapplications.
`An example of an attempt to meet this need for higher
`intensity, lower power, and moreefficient LEDs may be seen
`with the development of the AlGaAs LED technology for
`LEDsin the red portions of the visible spectrum. A similar
`continual need has been felt for LEDs that will emit in the
`blue and ultraviolet regions of the visible spectrum. For
`example, because blue is a primary color, its presence is
`either desired or even necessary to produce full color
`displays or pure white light.
`The common assignee of the present patent application
`was the first in this field to successfully develop commer-
`cially viable LEDs available in large quantities and that
`emitted light in the blue color spectrum. These LEDs were
`formed in silicon carbide, a wide bandgap semiconductor
`material. Examples of such blue LEDsare described in U.S.
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`65
`
`2
`Pat. Nos. 4,918,497 and 5,027,168 to Edmond eachtitled
`“Blue Light Emitting Diode Formed In Silicon Carbide.”
`Other examples of such a blue LEDare described in U.S.
`Pat. No. 5,306,662 to Nakamura etal. titled “Method Of
`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 Hatanoetal. titled “Vapor
`Phase Growth Method Of Forming Film In Process Of
`Manufacturing Semiconductor Device” also describes LEDs
`formed of GaInAIN on SiC substrates and Zinc Selenide
`(ZnSe) on gallium arsenide (GaAs) substrates.
`As knownto those familiar with photonic devices such as
`LEDs,the frequency of electromagnetic radiation (i.e., the
`photons) that can be produced by a given semiconductor
`material are a function of the material’s bandgap. Smaller
`bandgaps produce lowerenergy, longer wavelength photons,
`while wider bandgap materials are required to produce
`higher energy, shorter wavelength photons. For example,
`one semiconductor commonly used for lasers is indium
`gallium aluminum phosphide (InGaAIP). Because ofthis
`material’s bandgap (actually a range of bandgaps depending
`upon the mole or atomic fraction of each elementpresent),
`the light that InGaAIP can produce is limited to the red
`portion of the visible spectrum,
`i.e., about 600 to 700
`nanometers (nm).
`Working backwards, in order to produce photons that
`have wavelengths in the blue or ultraviolet portions of the
`spectrum, semiconductor materials are required that have
`relatively large bandgaps. Typical candidate materials
`include silicon carbide (SiC) and gallium nitride (GaN).
`Shorter wavelength LEDsoffer a numberof advantagesin
`addition to color. In particular, when used in optical storage
`and memory devices (e.g., “CD-ROM” or “optical disks’),
`their shorter wavelengths enable such storage devices to
`hold proportionally more information. For example, an
`optical device storing information using blue light can hold
`approximately 32 times as much information as one using
`red light, in the same space.
`Gallium nitride, however, is an attractive LED candidate
`material for blue and UV frequencies because ofits rela-
`tively high bandgap (3.36 eV at room temperature) and
`becauseit is a direct bandgap material rather than an indirect
`bandgap material. As known to those familiar with semi-
`conductor characteristics, a direct bandgap material is one in
`which an electron’s transition from the valence band to the
`conduction band does not require a change in crystal
`momentum for the electron. In indirect semiconductors, the
`alternative situation exists; i.e., a change of crystal momen-
`tum is required for an electron’s transition between the
`valence and conduction bands. Silicon and silicon carbide
`are examples of such indirect semiconductors.
`Generally speaking, an LED formed in a direct bandgap
`material will perform moreefficiently than one formed in an
`indirect bandgap material because the photon from the direct
`transition retains more energy than one from an indirect
`transition.
`
`Gallium nitride suffers from a different disadvantage,
`however: the failure to date of any workable technique for
`producing bulk single crystals of gallium nitride which
`could form appropriate substrates for gallium nitride pho-
`tonic devices. As is known to those familiar with semicon-
`ductor devices,
`they all require some sort of structural
`substrate. Typically, a substrate formed of the same materials
`as the active region of a deviceoffers significant advantages,
`particularly in crystal growth and matching. Because gal-
`
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`5,523,589
`
`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
`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
`referred to as being “strained” by this mismatch. Such
`mismatches, andthe 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
`performance of the photonic device. Such defects are even
`more problematic in higher power structures.
`To date, the most commonsubstrate for gallium nitride
`devices—and the only substrate utilized in GaN LED’s—
`has been sapphire; i.e., aluminum oxide (Al,O;). Sapphire is
`optically transparent in the visible and UV ranges, but is
`unfortunately insulating rather than conductive, and carries
`a lattice mismatch with gallium nitride of about 16%. In the
`absence of a conductive substrate, “vertical” devices (those
`with contacts on opposite sides) cannot be formed, thus
`complicating the manufacture and use of the devices.
`Asa particular disadvantage, horizontal structures (those
`with contacts on the same side of the device), such as those
`required when gallium nitride is formed on sapphire, also
`produce a horizontal
`flow of current and therefore the
`current density through the layer is substantially increased.
`This horizontal current flow puts an additional strain on the
`already-strained (i.e., the 16% lattice mismatch) GaN-sap-
`phire structure and accelerates the degradation of the junc-
`tion and the device as a whole.
`Gallium nitride also carries a lattice mismatch of about
`2.4% with aluminum nitride (AIN) and a 3.5% mismatch
`with silicon carbide. Silicon Carbide has a somewhat lesser
`mismatch (only about 1%) with aluminum nitride.
`Group Ill ternary and quaternary nitrides (e.g., InGaN,
`InGaAIN, etc.) have also been shownto have relatively wide
`bandgaps and thus also offer the potential for blue and
`ultraviolet semiconductor lasers. Most of these compounds,
`however, present the same difficulty as gallium nitride: the
`lack of an identical single crystal substrate. Thus, each must
`be used in the form of epitaxial layers grown on different
`substrates. Thus, they present the same potential for crystal
`defects and their associated electronic problems.
`
`OBJECT AND SUMMARYOF THE INVENTION
`
`invention to
`is an object of the present
`it
`Therefore,
`provide a light emitting diode that can emit in the blue and
`ultraviolet portions of the electromagnetic spectrum, that
`can be built in the vertical geometry that is most advanta-
`geous for such devices, that has excellent brightness and
`efficiency and that can exhibit better physical and electronic
`longevity and performance than can previously available
`diodes.
`
`The invention meetsthis object with a light emitting diode
`that emits in the blue portion of the visible spectrum and that
`is characterized by an extended lifetime because of its
`advantageous materials and structure. The light emitting
`diode comprises a conductive silicon carbide substrate; an
`ohmic contactto the silicon carbide substrate; a conductive
`buffer layer on the substrate and selected from the group
`
`10
`
`15
`
`20
`
`25
`
`35
`
`40
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`45
`
`50
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`55
`
`60
`
`65
`
`4
`indium
`consisting of gallium nitride, aluminum nitride,
`nitride, ternary GroupIII nitrides having the formula A,B,.
`xN, where A and B are Group III elements and where x is
`zero, one, or a fraction between zero and one, quaternary
`Group IH nitrides having the formula A,B,C, _,.,N where A,
`B, and C are Group HI elements; x and y, are zero, one, or
`a fraction between zero and one,and | 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 GroupIII 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
`accomplished, will become more readily apparent upon
`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:
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 schematically illustrates a vertical sectional view
`ofa first embodimentof an extendedlifetime light emitting
`diode according to the present invention,
`FIG. 2 schematically illustrates a vertical sectional view
`of a second embodiment of an extended lifetime light
`emitting diode according to the present invention;
`FIG. 3 schematically illustrates a vertical sectional view
`of a third embodimentof an extendedlifetime light emitting
`diode according to the present invention;
`FIG. 4 schematically illustrates a vertical sectional view
`of a fourth embodiment of an extended lifetime light emit-
`ting diode according to the present invention,
`FIG. 5 graphically illustrates relative intensity over time
`of a prior art light emitting diode in comparison to a light
`emitting diode according to the present invention;
`FIG. 6 graphically illustrates a double crystal X-ray
`rocking curve for a GaN layer on a SiC substrate as used in
`a light emitting diode according to the present invention,
`FIG. 7 graphically illustrates photoluminescence as com-
`pared to energy output of a GaN layer on a SiC substrate as
`utilized in an extended lifetime light emitting diode accord-
`ing to the present invention;
`FIG. 8 graphically illustrates intensity as compared to
`kinetic energy of an alloy of SiC-AIN-GaN; and
`FIG.9 graphically illustrates crystal lattice peak energy as
`a function of silicon carbide concentration in a SiC-AIN-
`GaN alloy according to the present invention.
`
`DETAILED DESCRIPTION OF A PREFERRED
`EMBODIMENT
`
`The present invention is a light-emitting diode that pro-
`duces light in the blue portion of the visible spectrum and
`that is characterized by an extended lifetime. As known to
`those familiar with the performance, characteristics and
`ratings of such light-emitting diodes, the lifetime is gener-
`ally defined as the time over which the LED’s output will
`degrade to about 50% ofits original output.
`FIG. 1 is a cross-sectional schematic view of a light-
`emitting diode according to the present invention and gen-
`erally designated at 20. The diode comprises a conductive
`silicon carbide substrate 21 which,
`in preferred embodi-
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`5,523,589
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`5
`ments,is a single crystal silicon carbide substrate. Asis well
`understood by those of ordinary skill
`in this art, a high
`quality single crystal substrate provides a numberofstruc-
`tural advantagesthat in turn provide significant performance
`and lifetime advantages. In preferred embodiments, the SiC
`substrates can be formed by the methods described in U.S.
`Pat. No. 4,866,005 (now U.S. Pat. No. RE 34,861) which is
`commonly assigned with the pending application.
`An ohmic contact 22 is made to the silicon carbide
`substrate and is one of the characteristics of the present
`invention that immediately distinguishes it from prior diodes
`of the materials discussed herein. As noted earlier,
`the
`typical substrate for gallium nitride is sapphire, which
`cannot be made conductive, and thus cannot be connected to
`an ohmic contact. This prevents a sapphire-based device
`from being formed in the vertical structure that is most
`preferred for LEDs and many other devices,
`FIG. 1 further illustrates that the LED 20 comprises a
`buffer layer 23 on the substrate 21. The buffer layer 23 is
`selected from the group consisting of gallium nitride, alu-
`minum nitride, indium nitride, ternary Group III nitrides
`having the formula A,B,_.N, where A and B are GroupIII
`elements and wherex is zero, one or a fraction between zero
`and one, quaternary Group III nitrides having the formula
`A,B,C,_,N where A, B, and C are Group III elements, x
`andy, are zero, one, or a fraction between zero and one, and
`1 is greater than (x+y) and alloys ofsilicon carbide with such
`ternary and quaternary Group III nitrides. The buffer layer
`23 and the substrate 21 are both conductive.
`The LED 20 further includes a double heterostructure
`designated by the brackets 24, and specifically including a
`p-n junction, on the buffer layer 23. The structural designa-
`tion “double heterostructure” is used in a manner common
`to, and well understood in, this art. Aspects of these struc-
`tures are discussed, for example, in Sze, Physics of Semi-
`conductor Devices, Second Edition (1981)
`at pages
`708-710. The Sze discussion on those pages refers to lasers,
`but illustrates the nature of, and the distinction between,
`homostructure, single heterostructure, and double hetero-
`structure junctions.
`the double
`illustrated in FIG. 1,
`In the embodiment
`heterostructure 24 further comprisesan active layer 25 along
`with upper 26 and lower 27 heterostructure layers adjacent
`the active layer 25. The heterostructure layers 26 and 27 are
`preferably formed of a composition selected from the group
`consisting of gallium nitride, aluminum nitride,
`indium
`nitride, ternary GroupIII nitrides having the formula A,B,_
`xN, 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, e.g.,
`(SiC),A,B _,N. Stated differently, the lowest heterostructure
`layer will be on top of the buffer layer. In FIG. 1, this is
`illustrated as lower heterostructure 27 being on top of buffer
`layer
`An ohmic contact 30 can be applied to the upper hetero-
`structure layer 26 to complete the advantageous vertical
`structure of the invention. The ohmic contacts preferably are
`each formed of a metal such as aluminum (Al), gold (Au),
`platinum (Pt), or nickel (Ni), but may be formed of other
`material for forming ohmic contacts as understood by those
`skilled in the art.
`
`In each of the embodimentsillustrated herein, the double
`heterostructure comprises an active layer selected from the
`group consisting of gallium nitride, aluminum nitride,
`indium nitride, ternary Group ITI nitrides having the formula
`A,B,_,N, where A and B are Group III elements and where
`
`10
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`6
`x is zero, one or a fraction between zero and one,and alloys
`of silicon carbide with such ternary GroupIII nitrides.
`In the heterostructure 24 illustrated in FIG. 1, the active
`layer 25 can preferably comprise indium gallium nitride and
`the upper and lowerheterostructure layers 26 and 27 will
`preferably comprise aluminum gallium nitride. More spe-
`cifically, the aluminum gallium nitride heterostructure layers
`26 and 27 preferably have the formula Al,Ga,_.N where x is
`zero, one or a fraction between zero and one. When the
`active layer 25 comprises indium gallium nitride, the com-
`position will be understood to be In,Ga,_,.N, where z is a
`fraction between zero and one.
`
`the
`in this art,
`As known to those of ordinary skill
`composition of the ternary GroupII] nitrides can affect both
`their refractive index and their bandgap. Generally speaking,
`a larger proportion of aluminum increases the bandgap and
`decreases the refractive index. Thus, in preferred embodi-
`ments, in order for the heterostructure layers 26 and 27 to
`have a bandgap larger than the active layer 25 and a
`refractive index smaller than the active layer 25, the layers
`26 and 27 have a higher atomic or mole percentage of
`aluminum than does the active layer 25. The larger bandgap
`of the heterostructure layers 26 and 27 encourageselectrons
`to be injected throughthe active layer 25 thus increasing the
`efficiency of the device. Similarly, the lowerrefractive index
`of the heterostructure layers 26 and 27 encourage the light
`to be more preferably emitted on an optical basis from active
`layer 25.
`In order to form the p-n junction, the upper and lower
`heterostructure layers 26 and 27 have opposite conductivity
`types from one another, and the active layer 25 has the same
`conductivity type as one of the two heterostructure layers.
`For example, in a preferred embodiment, the upper hetero-
`structure layer 26 is p-type, the active layer 25 is n-type, the
`lower heterostructure layer 27 is n-type, and the buffer and
`the silicon carbide substrate are both also n-type. The p-n
`junction is thus formed between the active layer 25 and the
`upper heterostructure layer 26.
`FIG,2 illustrates a slightly different embodiment of the
`present invention broadly designated at 32. As in the pre-
`vious embodiment, the LED comprises a silicon carbide
`substrate 33 and its ohmic contact 34. The double hetero-
`structure is designated by the brackets at 35. In the embodi-
`ment ofFIG.2,the buffer layer is shown at 36 and comprises
`gallium nitride, and the overall structure further comprises a
`gallium nitride epitaxial layer 37 on the buffer layer between
`the gallium nitride buffer layer 36 and the double hetero-
`structure 35. An ohmic contact 40 to the double heterostruc-
`ture 35 completes the advantageousvertical structure of the
`device.
`
`Although specific performance parameters will be dis-
`cussed later herein, the diodes described herein andillus-
`trated in these and the remaining drawings are expected to
`have lifetimes of greater than 10,000 hours operating at a
`forward bias current of 50 milliamps at room temperature,
`and lifetimes of greater than 10,000 hours operating at a
`forward bias current of 30 milliamps at room temperature.It
`will be recognized by those familiar with such devices that
`these specifications greatly exceed those of presently avail-
`able devices.
`
`FIG. 3 illustrates a third embodiment of the present
`invention broadly designated at 42. As in the previous
`embodiments, the diode 42 includes a silicon carbide sub-
`strate 43, and an ohmic contact 44 to the substrate 43. The
`double heterostructure is again designated by the brackets 45
`and an upper ohmic contact 46 is made to the double
`
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`5,523,589
`
`8
`second GroupIII nitride layer 54, and an ohmic contact 56
`is formed on the silicon carbide substrate so that a current
`supplied across the first and second ohmic contacts to the p-n
`junction device produces a highlight intensity output there-
`from.
`
`As indicated by the dotted line in FIG. 4, the buffer 52
`preferably comprises a first layer 57 on the substrate 51 and
`formedof a graded compositionofsilicon carbide aluminum
`gallium nitride in which the portion adjacent the substrate is
`substantially entirely silicon carbide and the portion furthest
`from the substrate is substantially entirely aluminum gallium
`nitride with the portions therebetween being progressively
`graded in content from predominantly silicon carbide to
`predominantly aluminum gallium nitride.
`A second buffer layer 58 is upon the first layer 57 and is
`formed of a graded composition of aluminum gallium
`nitride. As described with respect to earlier embodiments,
`the composition of the graded second layer 58 is progres-
`sively graded from a composition matching the composition
`of the first buffer layer 57 at the point where layers 58 and
`57 join, to a composition matching the composition of the
`lower Group III nitride layer 53 of the diode.
`In the diode 50 illustrated in FIG. 4, the nitride layers 53
`and 54 are selected from the group consisting of gallium
`nitride, aluminum nitride, indium nitride, ternary Group III
`nitrides having the formula A,B,..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 GroupIII nitrides. It will thus be understood
`that in this and the previous embodiment,the junction can be
`a homostructure, a single-heterostructure, or a double-het-
`erostructure.
`
`The buffer 52 can alternatively comprise a lower inter-
`mediate layer 57 formed ofsilicon carbide positioned on the
`silicon carbide substrate 51 and an upper intermediate layer
`58 formed ofa nitride alloy positioned on the lower inter-
`mediate layer 57.
`The buffer can include at least one graded layerof silicon
`carbide and a GroupIII nitride in which the graded layeris
`silicon carbide at the interface with the substrate 51, and the
`graded layer is a composition matching the composition of
`the lowestlayer of the active device at the interface with the
`junction structure.
`As in earlier embodiments, the light-emitting diode can
`have the upper surfaceof the silicon carbide substrate doped
`with aluminum.
`
`10
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`7
`heterostructure 45. In this embodiment, however, the buffer
`layer comprises first and second layers 47 and 48 respec-
`tively. The first layer 47 is on the substrate 43 and is formed
`of a graded composition of silicon carbide aluminum gal-
`lium nitride (SiC),Al,Ga,_,N in which the portion adjacent
`the substrate 43 is substantially entirely silicon carbide and
`the portion furthest
`from the substrate is substantially
`entirely aluminum gallium nitride, with the portions ther-
`ebetween being progressively graded in content from pre-
`dominantly silicon carbide to predominantly aluminum gal-
`lium nitride.
`
`The secondlayer 48 is on the first layer 47 and is formed
`of another graded composition of aluminum gallium nitride.
`In preferred embodiments, the composition of the graded
`second layer 48 is graded from a composition matching the
`composition ofthe first buffer layer 47 at the point where the
`layers 47 and 48 meet,
`to a composition matching the
`composition of the lowestlayer of the double heterostructure
`45.
`
`the buffer layer can also be
`to FIG. 3,
`With respect
`described as having at least one graded layer of silicon
`carbide and a Group IHnitride in which the graded layeris
`silicon carbide at the interface with the substrate and then
`progressively graded to a composition matching the com-
`position of the lowest layer of the double heterostructure at
`the interface with the double heterostructure.
`
`The invention can further comprise a strain-minimizing
`contact
`layer (not shown) above the active layer in the
`double heterostructure and that would havea lattice constant
`substantially the same as the respective buffer layers. Such
`a strain-minimizing contact layer is set forth in an applica-
`tion filed concurrently herewith by Edmond and Bulman for
`“Low Strain Laser Structures with GroupIII Nitride Active
`Layers,” which is commonly assigned with this application
`and which is incorporated entirely herein by reference.
`Briefly summarized,
`the overall strain of such a multi-
`layered crystalline device is a function of the average of the
`individual strains based on the differences between their
`lattice constants. Thus, by adding a layer with a lattice
`constant substantially the same as the buffer, the weighted
`average of the strains becomes more consistent and thus
`reduces the overall strain.
`
`Assomeadditional details, the upper surface of the silicon
`carbide substrate in any of the embodiments can be doped
`with aluminum to enhance the crystal growth. As already
`stated, the substrate and the buffer layers in each embodi-
`ment are conductive, and this is usually accomplished by
`doping each of the layers with appropriate dopants. The
`silicon carbide substrate can be selected from several of the
`silicon carbide polytypes specifically including 3C, 4H, 6H,
`and 15R.
`
`As discussed with reference to someof the other figures
`herein, the characteristics of the crystals according to the
`present invention are generally superior to any exhibited by
`any prior devices. Thus, a double crystal x-ray rocking curve
`for GaN grown on SiC substrates according to the present
`invention has a full width half maximum of about 85
`FIG. 4 illustrates another embodiment of the present
`arcseconds (FIG.6).
`invention broadly designated at 50. The LED 50 is formed
`As noted above, the lifetime of an LED is defined by the
`on a silicon carbide substrate 51 upon which a buffer layer
`LED degradation time to a light emitting output of only
`designated by the brackets 52 is formed. The buffer layer is
`about 50% of the initial ight emitting output of the LED. As
`selected from the group consisting of gallium nitride, alu-
`set forth above, FIG. 5 graphically illustrates relative inten-
`minum nitride, indium nitride, ternary Group III nitrides
`sity over timeof a prior art LED formed of GaN on sapphire
`having the formula A,B,_.N, where A and B are Group Il
`60
`as compared to an LED according to the present invention.
`elements and wherexis zero, one ora fraction between zero
`FIG. 5 best illustrates the significant improvement of the
`and one, and alloys of silicon carbide with such ternary
`lifetime of an LED according to the present invention. The
`GroupIII nitrides.