Nichia's 1 cd Blue LED Paves
`Way for Full-Color Display
`
`acturing
`
`by Shuji Nakamura, Nichia Chemical Industries Ltd
`
`The 1 cd blue LED, when used together with already available red and green
`LEOs with a high luminous intensity, can produce full-color LED display pan(cid:173)
`els, traffic signals and other outdoor displays.
`
`Nichia Chemical Industries Ltd of
`Japan has developed a blue light
`emitting diode (LED) registering as
`high as led in luminous intensity by
`using GaN -based materials. The
`blue diode is 100 times brighter than
`currently available blue LEDs. With
`a few tens of thousands of hours in
`service life, the blue LED is consid(cid:173)
`ered fit for commercial applications.
`Nichia started mass production of
`
`the LED in January 1994.
`
`Prolonged LED Life
`The newly developed diode has a
`peak wavelength of 450nm and a full
`width at half maximum of 70nm (Fig
`2). An output power of nearly
`1.5m W is recorded at a forward cur(cid:173)
`rent of 20mA (Fig 3). External
`quantum efficiency
`is 2.7%.
`
`Fig 1 Nichia's 1cd Blue LED The diode registered a luminous intensity 100 times stronger
`than that of currently available blue LEOs.
`
`Brightness reaches 1.2cd when
`packaged in a lens shape with a 15-
`degree full width at spread angle of
`emitted be~. At a forward current
`o{20mA~ the forward voltage drop
`is 3.6V. Lifespan is at the practical
`level of a few tens of thousands of
`hours until the brightness halves.
`Compared with commercially
`available blue LEDs, brightness
`performance is 100 times better
`(Table 1).
`Conventional brightness levels
`(about lOmcd) are insufficient for
`outdoor displays. With Nichia
`Chemical's led blue LEDs, however,
`such applications are feasible. Also,
`since the peak wavelength of 450nm
`is 20nm shorter than the SiC-based
`type, a pure blue light untainted by
`green is produced.
`Forward voltage drop, which pre(cid:173)
`sented a problem for GaN-based
`diodes with a conventional MIS·
`structure, is kept to 4V or less. This
`performance is equivalent to SiC(cid:173)
`based diodes.
`
`Green Diode Becomes Bottleneck
`
`Even in comparison to the widely(cid:173)
`used red LEDs, the brightness of
`the Nichia Chemical blue LED is up
`to par (Table 2). Compared with
`red-light diodes at 2cd and the new
`blue-light diode at led, green LEDs
`with only about 60mcd are under
`pressure to catch up.
`In order to achieve white-light
`
`The author: Shuji Nakamura, chief researcher, Department of Research and Development,
`Nichia Chemica/Industries Ltd, Anan City, Tokushima, Japan.
`
`NIKKEl ELECTRONICS ASIA I June1994 65
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`NICHIA EX2018
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`

`

`emission with high brightness by
`mixing these three colors, the per(cid:173)
`formance of green LEDs must be
`improved. The main issue for blue
`LEDs may be a broad full width at
`half maximum of the emission spec(cid:173)
`trum (Fig 2). Red and green LEDs
`offer sharper emission spectra.
`The blue LED's broad emission
`spectrum can be attributed to the
`emission centers of the emission lay(cid:173)
`er at deep-energy levels. If the
`broad emission spectrum presents
`difficulties for practical applications,
`this will have to be revised.
`In Fig 4, white-light emission is
`shown using the above-mentioned
`red, green and blue LEDs. The cur(cid:173)
`rents for these diodes are 5mA,
`20mA and 4mA, respectively. The
`highest current is required for the
`green LED to compensate for its
`lower brightness perfo~ance. With
`a balance among these three colors,
`white light is produced and a full(cid:173)
`color display with high brightness is
`possible. Using LEDs could greatly
`simplify maintenance of traffic sig(cid:173)
`nals. Other applications include
`medical equipment and lighting.
`
`Direct-Transition GaN Picked
`
`SiC, ZnSe, GaN and other materi(cid:173)
`als have been used to fabricate blue
`LEDs. The first of these materials
`to be commercialized has been SiC.
`There have also been research-level
`
`Fig 2 Light Emission Spectra-Blue LED Becomes More Blue Commercially available
`LEDs-AlGaAs red LED, GaP green LED and SiC blue LED-and the newly developed
`GalnN/AIGaN blue LED are shown.
`
`announcements that high power
`blue/green LEDs and laser diodes
`can be formed with ZnSe-based
`materials.
`The primary- reason for Nichia
`Chemical's selection of GaN is its
`direct transition energy bandgap.
`Semiconductor crystals can be clas(cid:173)
`sified as direct transition and indi(cid:173)
`rect transition types depending on
`the energy band structure. The
`direct transition type includes
`GaAs, ZnSe, ZnS, GaN, InN and
`AlN, and the indirect transition
`type Si, Ge, GaP and SiC.
`UV-to-Green Lights Covered
`
`from energy to light drops consider(cid:173)
`ably.
`As members of the direct transi(cid:173)
`tion group, ZnSe and Ga:N·have the
`potential to improve brightness.
`However, -ZnSe faces reliability
`problems. Its lifespan as a laser
`diode is just a few seconds.
`On top of this, forward voltage is
`high. Crystals of ZnSe-based mate(cid:173)
`rials are grown at temperatures of
`+30o·c or lower. This is a source of
`concern that crystals will break
`down if exposed to temperatures
`which exceed +300'C in post-growth
`steps. As a result, annealing pro(cid:173)
`cesses which involve high tempera(cid:173)
`tures cannot be used, making an
`ohmic contact between the electrode
`In the direct transition type,
`and ZnSe-based materials difficult
`almost all energy
`and requiring a high forward volt-.
`of electrons is con-
`age.
`verted into light
`The GaN -based materials also
`when the e1ec- ·
`trans transit from . have the direct transition bandgap,
`the conduction prompting expectations of high
`band to the lower
`brightness. The band gap energy for
`levels. This ener- GaN is 3.4e V. This value can be
`gy band structure
`adjusted from 2.0e V to 6.3e V by
`is suitable for
`compounding the GaN crystal with
`light-
`emitting
`InN (band gap = 2.0e V) or AlN
`devices. On the
`(band gap= 6.3eV). In other words,
`other hand, in the
`a GaN -based diode can emit light at
`indirect transition
`colors ranging from ultraviolet to
`·-type,_ a part of .the···--. green.
`electron energy
`converts to heat Efficient pn Junction
`and
`electron
`momentum
`is
`changed in this
`process. As a
`result, the conver(cid:173)
`sion
`efficiency
`
`Fig 3 New Blue LED-Output Power, Forward Current
`
`The biggest stumbling block for
`GaN -based materials has been the
`inability to produce p-type layers.
`This has forced LEDs to assume a
`metal-insulator-semiconductor
`
`66 NIKKEl ELECTRONICS ASIA I June 1994
`
`······- ·- ·7·:- .•···-·~-,_,..,-~
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`NICHIA EX2018
`
`

`

`Maximum Minimum Typical Maximum Minimum Typical Maximum Minimum Typical Maximuni -
`-
`-
`-
`3.6
`-
`-
`-
`-
`-
`-
`-
`-
`1,000
`
`-
`
`4.0
`
`3.0·1
`
`4.0·1
`
`100
`
`8
`
`8
`
`15
`
`50 "3
`
`-
`
`500
`
`4.0
`
`50
`
`-
`-
`-
`-
`
`·
`
`Design & Manufacturing
`
`Table 1 GalnN/AIGaN Blue LED vs SiC Blue LEOs-Specifications
`
`SiC Product A
`
`SiC Product B
`
`SiC Product C
`
`GalnN/AIGaN
`
`~
`
`Eiectricat and . ~
`.optical characteristics
`·(ambient temperature-
`·=2s·q
`",····:. :.:
`
`·~· ·~~i!fi,:~:-
`
`Luminous intensity (mcd)
`
`Minimum Typical
`
`-
`-
`
`5
`
`3.0
`
`-
`
`10
`
`3.5
`
`1oo·2
`
`Forward current = 20mA
`Forward voltage (V)
`Reverse current (IJA) Reverse voltage = SV
`I
`I
`-
`Forward current = 20mA
`-
`-
`I -
`-
`-
`-
`-
`Emission power (!JW) Forward current = 20mA
`-
`1-
`I 470
`-
`-
`Peak wavelength (nm) Forward current = 20mA
`I -
`I -
`I 470 I -
`-
`-
`I 70
`I -
`I -
`-
`-
`-
`-
`Spectrum FWHM (nm) Forward current = 20mA
`I 70
`I 70
`70
`I
`Catalog values are used for commercial products. *1 Forward current = 1 OmA. ·2 Reverse voltage = 4.3V. *3 Reverse voltage = 3.0V.
`
`-
`
`470
`
`-
`
`-
`
`600
`
`1,200
`
`450
`
`Table 2 Red, Green, Blue LEDs-Performance Balance
`·,';.{'~_!:~· ;~;_Jiesin ·:~~ -~"! Peak wavelength (nm)j Luminous intensity (mcd) Emission power(IJW)j Forward voltage (V) I External quantum efficiency(%};
`-~' .. ~l~~j~ ··~·~:~· , .:Material
`I
`I
`I
`I
`AIGaAs Colorless, transparent
`Red LED
`I
`I
`I
`!
`I
`I
`I
`I
`I
`I
`Values are measured by Nich1a Chemcal. Note) Electrical and optical measurement data show values at a forward current of 20mA and an ambi(cid:173)
`ent temperature of +2s·c.
`
`Green LED GaP
`
`Colorless, transparent
`
`Blue LED
`
`SiC
`
`Colorless, transparent
`
`GalnN Colorless, transparent
`
`660
`
`555
`
`466
`
`450
`
`1,790
`
`63
`
`9
`
`1,000
`
`4,855
`
`30
`
`11
`
`1,200
`
`1.8
`
`2.1
`
`2.9
`
`3.6
`
`12.83
`
`0.067
`
`0.021
`
`2.160
`
`structure. Diodes with the MIS
`structure 1) sandwich a light-emit(cid:173)
`ting insulation layer between an
`electrode and a semiconductor layer.
`The forward voltage of a GaN(cid:173)
`based LED with an MIS structure
`is about lOV. Furthermore, the thin
`insulation layer tends to break down
`under the application of a strong
`electric field. Another problem is
`that the achievement of high bright(cid:173)
`ness is difficult.
`Red LEDs and other widely-used
`LEDs all have pn-junction struc(cid:173)
`tures. If both n-type and p-type
`GaN can be pro~uced, it is possible
`to increase emission efficiency and ·
`lower forward voltage to about 3V.
`Two-Flow Method Developed
`
`A pn-junction structure requires
`the formation of a p-type GaN film.
`This in turn requires technology for
`fabricating GaN monocrystalline
`films with few defects.
`·
`Since GaN films are grown at a
`temperature of +l,OOOOC, the GaN
`has a tendency to dissolve and form
`N vacancies easily. Also, the 15.4%
`lattice mismatch between GaN and
`the substrate prevents the forma(cid:173)
`tion of an acceptable monocrys(cid:173)
`talline film using conventional met(cid:173)
`alorganic vapor phase epitaxy
`(MOVPE) techniques.
`
`~-.·-····
`
`Full-color LED display is made of AlGaAs red LEOs, GaP
`Fig 4 Full-Color LEO Display
`green LEOs and the new GalnN/AlGaN double-heterojunction blue LEOs. The LEOs' current
`values are SmA for red, 4mA for blue and 20mA for green. The red and green LEOs are com(cid:173)
`mercially available. ·
`
`To solve these problems, Nichia
`Chemical developed a new reactor
`which uses the two-flow MOVPE
`method and applied this to GaN
`crystal growth (Fig 5). This reactor
`features the introduction of a sub(cid:173)
`flow gas which pushes the main-flow
`reaction gas towards the substrate
`
`from above to overcome heat con(cid:173)
`vection which occurs during GaN
`growth. It supports the formation of
`GaN films with high mobility.2),3)
`NH3 is used as the source materi-
`al gas for N and trimethyl galliuiil----(cid:173)
`for Ga. H2 is the carrier gas. All
`source materials are delivered to
`
`NIKKEl ELECTRONICS ASIA I June 1994 67
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`NICHIA EX2018
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`

`

`(a) Cross section of reactor
`
`(b) Gas flow
`
`Quartz glass tube
`
`---
`
`H2+NH3+
`Trimethyl Ga
`
`Exhaust
`
`Fig 5 Two-Flow MOVPE Method The sub-flow gas forces the main flow gas onto the sub(cid:173)
`strate and reduces the effect of heat convection.
`
`the substrate in a gaseous form and
`the GaN film is grown at a substrate
`temperature of about + 1,000°C.
`However, when GaN is grown
`directly on a sapphire substrate at a
`temperature of +l,OOOOC, hexagon(cid:173)
`shaped pyramids form on the film
`surface creating cracks throughout
`the crystal (Fig 6(a)). This is· avoid(cid:173)
`ed by first growing a GaN thin
`buffer layer at a lower temperature
`of +500°C to +600°C and then grow(cid:173)
`ing the GaN film at +l,OOOOC (two(cid:173)
`stage growth method). With this
`technique, a film with a planar sur(cid:173)
`face can be grown (Fig 6(b)), and the
`number of defects within the film is
`reduced.
`Once a GaN single crystal film can
`be formed,. the next issue is a p-type
`
`film. In 1989, Professor Isamu
`Akasaki at Nagoya University of
`Japan discovered
`that
`low(cid:173)
`resistance p-type GaN can be pro(cid:173)
`duced by irradiating lVIg-doped GaN
`with electron beams. 4>
`p-Type Made with Annealing
`
`Subsequently, Nichia Chemical
`discovered that low-resistance, p(cid:173)
`type GaN could be obtained by
`annealing in a nitrogen atmosphere
`without the application of electron
`beams (Fig 7). S),S) Compared to the
`electron-beam approach, this tech(cid:173)
`nology makes p-type film more uni(cid:173)
`form at any depth in the wafer.
`In 1991, however, 3M Co of the
`US announced success with a blue
`
`(a) Direct growth
`
`(b) With buffer layer
`
`laser diode using II-VI family semi(cid:173)
`conductors. 7) This and other
`announcements of high-power blue
`LEDs shifted attention to II-VI
`family blue LED and laser diode
`research.
`Amid these activities, GaN -based
`material research lost ground tem(cid:173)
`porarily. To recover that ground, it
`was necessary to move beyond the
`homojunction type to double-hetero(cid:173)
`junction LEDs and laser diodes.
`A double-heterojunction diode
`has a structure which sandwiches
`the emission layer between a p-type
`layer and ann-type layer (cladding
`layers) with a larger bandgap ener(cid:173)
`gy than the emission layer. In this
`structure, the carrier injected in the
`emission layer is confined by the
`energy barrier between the emis(cid:173)
`sion layer and the cladding layers.
`The result is a considerable
`improvement in emission recombi(cid:173)
`nation probability and better emis(cid:173)
`sion efficiency compared to a simple
`homojunction structure.
`1 cd with Double Heterojunction
`
`In 1993, Nichia Chemical complet(cid:173)
`ed a GainN crystal growth process
`by applying this GaN crystal
`growth technique and produced a
`double-heterojunction LED with a
`GainN /GaN structure. 9>
`After that, the GainN layer was
`doped with Zn to form the emission
`
`i-\il'1,';:~'-:f':~c"'.f~'.l~-
`.:1-:-'tc:~~ ~~f .. :~.f~~(::=_j·.··.:.._1·:: ~.:.~~~ -~
`~;;-~;.~~;o.;,~ •• >-:,/~-"'::~---=~~~-lf1(J3/Z~,~: · ~
`,/ ;·:·,-~_ ;,~~~L~;::·~~.--~~~::_.,:~
`.. -~~;:;~; --... _;·~}.~.~i~~- -:
`
`(a) When crystal of the GaN layer is grown directly on the substrate, the crystal surface becomes rough. (b)
`Fig 6 GaN Surface Morphology
`In contrast, the surface becomes smooth with the introduction of a buffer layer.
`
`68 NIKKEl ELECTRONICS ASIA I June 1994
`
`___ .............. , ......... ,._...,.-~~ .. =-~····"--'
`
`NICHIA EX2018
`
`

`

`.
`
`Design & Manufacturing
`
`10 7
`
`1.0 6
`
`10 5
`
`10 4
`
`'E
`c, c:
`i!' 10 3
`:~ u;
`a;
`Q)
`cr:
`
`10 2
`
`10 1
`
`10°
`
`10"1
`
`-.. ·.
`
`.. . ·-:· .. ~. :.:·~~
`
`..... ~ -.. .
`
`•• .. r .. .:.
`
`GaN buffer layer
`
`Annealing temperature ("C)
`
`Fig 7 Resistivity Depends on Annealing Temperature
`doped GaN film is annealed in a nitrogen atmosphere.
`
`An Mg-
`
`Fig 8 GalnN/AlGaN Double-Heterojunction Blue LED-Str~cture
`
`center in order to raise spectral
`luminous efficacy of the LED.
`Doping with Zn increased the peak
`wavelength from 440nm to 450nm.
`Also, the cladding layers were
`changed to AlGaN to increase the
`energy difference between the emis(cid:173)
`sion layer band gap and the cladding
`layer bandgap. With these revisions,
`brightness performance of the GaN
`blue LED improved to over led.
`The structure of the newly devel(cid:173)
`oped GainN/ AIGaN double-hetero(cid:173)
`junction, high-brightness, blue LED
`is shown in Fig 8. First, a GaN
`buffer layer is grown at a tempera(cid:173)
`ture of about +550°C on the sapphire
`substrate and n-type GaN is
`deposited at about 1,000°C. Then, n(cid:173)
`type AlGaN, Zn-doped GainN, p(cid:173)
`type AIGaN and p-type GaN films
`are formed in that order. After film
`growth, the wafer is annealed to
`lower the p-type layer resistances.
`Next, a portion of the p-type GaN
`film is etched to the point of expos(cid:173)
`ing the n-type GaN film and elec(cid:173)
`trodes are metallized on the p-type
`GaN and n-type GaN films. The
`diode chip is then placed in a lead
`frame and molded with epoxy to
`complete the product.
`Sights Set on Blue Laser
`
`The next target is a blue laser
`diode using GaN -based materials. In
`this area as well, the GaN type10>.n>
`lags behind the II-VI family and
`
`other materials.
`In order to achieve laser opera(cid:173)
`tion with the new GaN blue LED,
`band-to-band emission and a cavity
`for light amplification are required.
`This diode does not use band-to(cid:173)
`band emission because of the Zn
`dopant in the GainN layer to
`lengthen the wavelength. In other
`words, if Zn is not doped, band-to(cid:173)
`band emission can be easily
`achieved even though the emission
`wavelength will be a bit shorter.
`To make the resonance cavity, it is
`not possible to use the cleavage
`facet as is done in conventional laser
`diodes because GaN does not have
`such cleavage characteristics. It is .
`necessary to create a resonance cav(cid:173)
`ity by etching or other methods.
`These are rather minor problems
`compared to the more fundamental
`one of how to make a double hetero(cid:173)
`junction to achieve high-power blue(cid:173)
`light emission. The completion of a
`laser diode is just a matter of time.
`
`Buffer Layers," Japanese Journal of
`Applied Physics, vol 30, no lOA, pp Ll705-
`L1707, October 1991.
`4) Amano, H, Kito, M, Hiramatsu, K and
`.Akasaki, I, "P-Type Conduction in Mg-Doped
`GaN Treated with Low-Energy Electron
`Beam Irradiation (LEEBI)," ibid., vol 28, no
`12, pp L2112-L2114, December 1989.
`5) Nakamura, S, Mukai, T, Senoh, M and
`Iwasa, N, "Thermal Annealing Effects on P(cid:173)
`Type Mg-Doped GaN Films," ibid., vol 31, no
`2B, pp L139-L142, February 15, 1992.
`6) Nakamura, S, Iwasa, N, Senoh, M and
`Mukai, T, "Hole Compensation Mechanism of
`P-Type GaN Films," ibid., vol 31, no 5A, pp
`1258-1266, May, 1992.
`7) Haase, M A, Qiu, J, DePuydt, J M and
`Cheng, H, "Blue-Green Laser Diodes,"
`Applied Physics Letters, vol 59, no 11, pp
`1272.:.1274, September 9, 1991.
`8) Nakamura, S and Mukai, T, "High(cid:173)
`Quality InGaN Films Grown on Gru.'J' Films,"
`Japanese Journal of Applied Physics, vol3t, ·
`no lOB, pp L1457-L1459, October 15, 1992.
`9) Nakamura, S, Senoh, M and Mukai, T,
`"P-GaN/N-InGaN/N-GaN
`Double(cid:173)
`Heterostructure Blue-Light-Emitting
`Diodes," ibid., vol 32, no 1AJB, pp L8-Lll,
`January 15, 1993.
`10) Amano, H, Asahi, T and Akasaki, I,
`References:
`"Stimulated Emission Near Ultraviolet at
`1) Ohki, Y, Toyoda, Y, Kobayashi, Hand Room Temperature from a GaN Film Grown
`on Sapphire by MOVPE Using an AIN
`A.kasaki, I, "Fabrication and Properties of a
`Practical Blue-Emitting GaN.M-I-8 Diode,"
`--Buffer Layer," ibid., vol 29, no 2, pp· L205-
`Institute of Physics Conference, Series
`L206, February 1990.
`11) Asif Khan, M, Olson, D T, VanHove, J
`Number 63, Chapter 10, pp 479-484, 1981.
`2) Amano, H, Sawaki, N, Akasaki, I and M and Kuznia, J N, "Vertical-Cavity, Room-
`Toyoda, Y; "Metalorganic Vapor Phase
`Temperature Stimulated Emission from
`Epitaxial Growth of High Quality GaN Film
`Photopumped GaN Films Deposited over
`Using an AIN Buffer Layer," Applied
`Sapphire Substrates Using Low-Pressure
`Physics Letters, vol 48~ no s;· pp 35:l-355-;---Metaforgan.lcCliemfcafVapor Deposition,''
`Applied Physics Letters, vol 58, no 14, pp
`February 3, 1986.
`3) Nakamura, S, "Gru.'J' Growth Using Gru.'l
`1515-1517, AprilS, 1991.
`
`*
`
`~·:-:·-·-..
`
`NIKKEl ELECTRONICS ASIA I June 1994 69.
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`NICHIA EX2018
`
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