High‐power InGaN single‐quantum‐well‐structure blue and violet light‐emitting diodes
`Shuji Nakamura, Masayuki Senoh, Naruhito Iwasa, and Shin‐ichi Nagahama
`
`Citation: Applied Physics Letters 67, 1868 (1995); doi: 10.1063/1.114359
`View online: http://dx.doi.org/10.1063/1.114359
`View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/67/13?ver=pdfcov
`Published by the AIP Publishing
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`High-power InGaN single-quantum-well-structure blue and violet
`light-emitting diodes
`Shuji Nakamura,a) Masayuki Senoh, Naruhito Iwasa, and Shin-ichi Nagahama
`Department of Research and Development, Nichia Chemical Industries, Ltd., 491 Oka, Kaminaka, Anan,
`Tokushima 774, Japan
`共Received 28 March 1995; accepted for publication 31 July 1995兲
`High-power blue and violet light-emitting diodes 共LEDs兲 based on III–V nitrides were grown by
`metalorganic chemical vapor deposition on sapphire substrates. As an active layer, the InGaN
`single-quantum-well-structure was used. The violet LEDs produced 5.6 mW at 20 mA, with a sharp
`peak of light output at 405 nm, and exhibited an external quantum efficiency of 9.2%. The blue
`LEDs produced 4.8 mW at 20 mA and sharply peaked at 450 nm, corresponding to an external
`quantum efficiency of 8.7%. These values of the output power and the quantum efficiencies are the
`highest ever reported for violet and blue LEDs. © 1995 American Institute of Physics.
`
`Much research has been conducted on high-brightness
`blue light-emitting diodes 共LEDs兲 and laser diodes 共LDs兲 for
`use in full-color displays, full-color indicators, and light
`sources for lamps with the characteristics of high efficiency,
`high reliability, and high speed. For these purposes, II–VI
`materials such as ZnSe,1 SiC,2 and III–V nitride semicon-
`ductors such as GaN3 have been investigated intensively for
`a long time. However, it has been impossible to obtain high-
`brightness blue LEDs with brightness over 1 cd. As II–VI
`based materials, ZnMgSSe-, ZnSSe-, and ZnCdSe-based ma-
`terials have been intensively studied for blue and green light-
`emitting devices, and much progress has been achieved re-
`cently on green LEDs and LDs. The recent situation
`regarding performance of II–VI green LEDs is that the out-
`put power is 1.3 mW at 10 mA and that the peak wavelength
`is 512 nm.4 When the peak wavelength shortens to the blue
`region, the output power decreases dramatically to about 0.3
`mW at 489 nm.4 The lifetime of II–VI-based light-emitting
`devices is still short, which prevents the commercialization
`of II–VI-based devices at present. SiC is another wise band-
`gap material for blue LEDs. Current output power of SiC
`blue LEDs is only between 10 and 20 ␮W because it is an
`indirect band-gap material.2
`On the other hand, there are no suitable substrates for
`III–V nitride growth without sapphire considering its high
`growth temperature and the cost of the substrate although the
`sapphire has a large lattice mismatch between GaN and sap-
`phire. Despite this large lattice mismatch, recent research on
`III–V nitrides has paved the way for the realization of high-
`quality crystals of AlGaN and InGaN, and p-type conduction
`in AlGaN.5–8 Moreover, the hole-compensation mechanism
`of p-type AlGaN has been elucidated.9 High-power blue and
`blue-green LEDs with an output power over 1 mW have
`been achieved by using these techniques and are now com-
`mercially available.10,11 Although these
`InGaN/AlGaN
`double-heterostructure 共DH兲 LEDs produce a high-power
`light output in the blue and blue-green regions, they have a
`broad emission spectrum 关full width at half-maximum
`共FWHM兲⫽70 nm兴 with the light output ranging from the
`
`a兲Electronic mail: shuji@nichia.co.jp
`
`violet to the yellow-orange spectral region. This broad spec-
`trum, which results from the intentional introduction of Zn
`into the InGaN active region of the device to produce a deep-
`level emission peaking at 450 nm, makes the output appear
`whitish-blue, when the LED is viewed with the human eye.
`Therefore, blue LEDs, which produce a sharp blue emission
`at 450 nm with a narrow FWHM, have been desired for
`application to full-color LED displays. For this purpose, vio-
`let LEDs with a narrow spectrum 共FWHM⫽10 nm兲 at a peak
`wavelength of 400 nm originating from the band-to-band
`emission of InGaN were reported.12 However, the output
`power of these violet LEDs was only about 1 mW, probably
`due to the formation of misfit dislocation in the thick InGaN
`active layer 共about 1000 Å兲 by the stress introduced into the
`InGaN active layer due to lattice mismatch, and the differ-
`ence in thermal expansion coefficients between the InGaN
`active layer and AlGaN cladding layers. When the thickness
`of the InGaN active layer becomes small, the elastic strain is
`not relieved by the formation of misfit dislocation and that
`the crystal quality of the InGaN active layer improves. We
`reported the high-quality InGaN multiquantum-well structure
`共MQW兲 with the 30 Å well and 30 Å barrier layers.13 Here,
`we describe the single quantum-well structure 共SQW兲 blue
`LEDs which have a thin InGaN active layer 共about 20 Å兲 in
`order to obtain high-power blue emission with a narrow
`emission spectrum.
`III–V nitride films were grown by the two-flow metalor-
`ganic chemical vapor deposition 共MOCVD兲 method. Details
`of the two-flow MOCVD are described in other papers.14
`The growth was conducted at atmospheric pressure. Sapphire
`with 共0001兲 orientation 共c face兲, which had a 2 in. diameter,
`was used as a substrate. The growth conditions of each layer
`are described in other papers.10,11 In comparison with previ-
`ous InGaN/AlGaN DH LEDs, the major difference is that the
`InGaN active layer becomes a thin undoped InGaN layer.
`The blue LED device structures 共Fig. 1兲 consists of a
`300 Å GaN buffer
`layer grown at a low temperature
`共550 °C兲, a 4 ␮m thick layer of n-type GaN:Si, a 1000 Å
`thicklayer of n-type Al0.3Ga0.7N:Si, a 500 Å thick layer
`of n-type In0.02Ga0.98N:Si, a 20 Å thick active layer of un-
`doped In0.2Ga0.8N, a 1000 Å thick layer of p-type
`Al0.3Ga0.7N:Mg, and a 0.5 ␮m thick layer of p-type GaN:Mg.
`
`1868
`
`Appl. Phys. Lett. 67 (13), 25 September 1995
`
`0003-6951/95/67(13)/1868/3/$6.00
`
`© 1995 American Institute of Physics
`
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`VIZIO 1014
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`FIG. 1. The structure of SQW blue LED.
`
`The active region forms a SQW structure consisting of a 20
`Å In0.2Ga0.8N well
`layer sandwiched by 500 Å n-type
`In0.02Ga0.98N and 1000 Å p-type Al0.3Ga0.7N barrier layers.
`In violet LEDs, the active layer is In0.09Ga0.9N.
`Fabrication of LED chips was accomplished as follows.
`The surface of the p-type GaN layer was partially etched
`until the n-type GaN layer was exposed. Next, a Ni/Au con-
`tact was evaporated onto the p-type GaN layer and a Ti/Al
`contact onto the n-type GaN layer. The wafer was cut into a
`rectangular shape 共350 ␮m⫻350 ␮m兲. These chips were set
`on the lead frame, and were then molded. The characteristics
`of LEDs were measured under direct current 共dc兲-biased con-
`ditions at room temperature.
`Figure 2 shows the electroluminescences 共EL兲 of the
`SQW blue LEDs in comparison with the previous Zn-doped
`InGaN/AlGaN DH blue LEDs at forward current of 20 mA.
`The peak wavelengths of both LEDs are 450 nm. The
`FWHM of the EL spectrum of the SQW blue LEDs is about
`25 nm, while that of DH LEDs is about 70 nm. The peak
`wavelength and the FWHM of SQW LEDs are almost con-
`stant when the forward current is increased to 100 mA. On
`the other hand, the peak wavelength of DH LEDs becomes
`shorter with increasing forward current and a band-to-band
`emission 共around 385 nm兲 appears under a high-current in-
`jection condition.10,11 In the SQW blue LEDs, the active
`layer is an In0.2Ga0.8N whose band-edge emission wave-
`length is 420 nm under the stress-free.12 On the other hand,
`the emission peak wavelength of SQW blue LEDs is 450 nm.
`The energy difference between the peak wavelength of the
`EL and the stress-free band-gap energy is approximately 190
`
`FIG. 3. The output power of 共a兲 SQW violet LED, 共b兲 SQW blue LED, and
`共c兲 DH blue LED as a function of the forward current.
`
`meV. In order to explain this band-gap narrowing of the
`In0.2Ga0.8N active layer, the quantum size effects, the exciton
`effects 共Coulomb effects correlated to the electron-hole pair兲
`of the active layer, and the strained effects by the mismatch
`of the lattice and the difference in thermal expansion coeffi-
`cients between well layer and barrier layers must be consid-
`ered. Among these effects, the tensile stress in the active
`layer caused by the thermal expansion coefficient difference
`between well layer and barrier layers is probably responsible
`for the band-gap narrowing of the InGaN SQW structure.
`The output power of the SQW LEDs and the DH blue
`LEDs is shown as a function of the forward current under dc
`in Fig. 3. The output power of the SQW LEDs and that of the
`DH LEDs slightly increases sublinearly up to 40 mA as a
`function of the forward current. Above 60 mA, the output
`power almost saturates, probably due to the generation of
`heat. The output power of the SQW violet LEDs is 2.8 mW
`at 10 mA, and 5.6 mW at 20 mA, which is about twice as
`high as that of the DH blue LEDs. The external quantum
`efficiency is 9.2% at 20 mA. The output power of SQW blue
`LEDs with a peak wavelength of 450 nm is 4.8 mW at 20
`mA and the external quantum efficiency is 8.7%.
`A typical example of the I – V characteristics of the SQW
`blue LEDs is shown in Fig. 4. The forward voltage is 3.1 V
`
`FIG. 2. Electroluminescence spectra of 共a兲 SQW blue LED and 共b兲 DH blue
`LED at a forward current of 20 mA.
`
`FIG. 4. Typical I – V characteristics of SQW blue LED.
`
`Appl. Phys. Lett., Vol. 67, No. 13, 25 September 1995
`
`Nakamura et al.
`
`1869
`
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`VIZIO 1014
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`at 20 mA. This forward voltage is the lowest value ever
`reported for III–V nitride LEDs.
`In summary, high-power InGaN SQW blue and violet
`LEDs were fabricated. The output power of the violet LEDs
`was 5.8 mW and the external quantum efficiency was as high
`as 9.2% at a forward current of 20 mA at room temperature.
`The peak wavelength and the FWHM were 405 and 20 nm,
`respectively, and those of blue LEDs were 450 and 25 nm,
`respectively. Such LED performances of quantum well struc-
`tures will pave the way for the realization of blue LDs based
`on III–V nitride materials in the near future.
`
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`Appl. Phys. Lett., Vol. 67, No. 13, 25 September 1995
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`Nakamura et al.
`
` Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 198.65.204.101 On: Tue, 01 Nov 2016
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`
`VIZIO 1014