Recent progress and future prospects of AlGaN-based high-efficiency deep-ultraviolet light-emitting diodes

Hasegawa, Hideki; Maeda, Noritoshi; Fujikawa, Sachie; Toyoda, Shiro; Kamata, Norihiko

doi:10.7567/jjap.53.100209

Cited by 505 publications

(451 citation statements)

References 43 publications

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“…However, the p-GaN contact layer causes the strong absorption of deep UV light. 6,131 Recently, transparent p-AlGaN contact layer was used to minimize the light absorption in p-GaN contact layer. 132,133 Consequently, the EQE of DUV LEDs (287 nm) increased from 2% to 5.5% by using a transparent p-AlGaN contact layer.…”

Section: 99mentioning

confidence: 99%

Review—Group III-Nitride-Based Ultraviolet Light-Emitting Diodes: Ways of Increasing External Quantum Efficiency

Park

Kim

Cho

et al. 2017

ECS J. Solid State Sci. Technol.

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There is a rapidly growing demand for highly efficient ultraviolet (UV) light sources for a wide variety of applications. In particular, state-of-the-art AlGaN deep UV light-emitting diodes (DUV LEDs) exhibit inadequately low external quantum efficiencies (EQEs). The low efficiencies are attributed to the inherent material properties of high-Al-content AlGaN including strained epitaxial layers, low carrier concentrations, and strong transverse magnetic (TM)-polarized light emission. Extensive efforts have been made to tackle these challenging issues and technological developments have been achieved and enabled the fabrication of reasonable EQE LEDs. In this review, recent advances in the growth of high-quality AlGaN epitaxial layers, transparent and reflective ohmic contacts, and light extraction for AlGaN-based UV LEDs are reviewed. Al x Ga 1-x N-based ultraviolet light-emitting diodes (UV LEDs) are of technological importance because of their applications in numerous areas such as water purification, biological analysis, sensing, epoxy curing, high-density optical storage, white light illumination, UV adhesives, 3-dimensional (3D) printer and UV-coating.1,2 These applications are made possible because of their wide bandgap energy range in the UV spectrum, which spans from 6.2 eV (AlN) to 3.4 eV (GaN), namely, from UV-A (320-400 nm), UV-B (290-320 nm) to UV-C (200-290 nm).3-7 The external quantum efficiency (EQE) of Al x Ga 1-x N-based UV LEDs decreases rapidly as the molar fraction (x) of Al increases, namely, as the wavelength decreases, as illustrated in Figure 1. 8 Thus, a great deal of effort has been made in order to improve the EQE of UV LEDs. The effort notwithstanding, however, the EQE of AlGaN-based UV LEDs is still insufficiently low. The typical EQE of UV LEDs, specifically in the UV-C spectral range (100-280 nm), is less than 5% and decreases down to 10 -6 % for 210-nm AlN-based UV LEDs. 4 These low EQEs are associated with the intrinsic material properties of Al x Ga 1-x N.1,2 Figure 2 exhibits a schematic diagram of a typical AlGaN UV LED structure grown on a sapphire substrate. The structure consists of a Si-doped n-type AlGaN, an Al x Ga 1-x N/Al y Ga 1-y N multiple-quantum well (MQW) active region, an AlGaN-based electron-blocking layer (EBL), a Mg-doped p-type AlGaN, and a Mg-doped p-type GaN. Almost every layer in the structure poses different technical issues, which limit the efficiency of UV LEDs, as shown in Figure 2. First, increasing the Al content generally results in a poor crystallinity of AlGaN epilayers, causing a poor internal quantum efficiency (IQE). Second, both n-type and ptype doping efficiencies of AlGaN are difficult to increase because the ionization energies of acceptor (Mg) and donor (Si) dopants largely increase with increasing Al content. The low doping efficiency causes several technological problems: (i) a poor electrical efficiency (EE) attributable to high-resistance contacts on resistive n-type and p-type AlGaN; (ii) current crowding causing non-uniform current in...

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Section: 99mentioning

confidence: 99%

Review—Group III-Nitride-Based Ultraviolet Light-Emitting Diodes: Ways of Increasing External Quantum Efficiency

Park

Kim

Cho

et al. 2017

ECS J. Solid State Sci. Technol.

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“…The large operation voltage of AlGaN quantum well deep UV LEDs [5,7,10], and moreover, the difficulty to obtain lasing shorter than 336 nm through direct electrical injection with AlGaN quantum wells [6], are partially attributed to poor p-type doping in Al-rich AlGaN alloys. As shown above, with the MBE-grown nanowire structures, Mg dopants can be efficiently incorporated; and free hole concentrations up to 6 × 10 17 cm −3 have been derived in AlN nanowires, which are orders of magnitude higher compared to the previously reported p-type AlN epilayers grown by MOCVD [39].…”

Section: Aln Nanowire-based Deep Uv Optoelectronic Devicesmentioning

confidence: 99%

Recent Advances on p-Type III-Nitride Nanowires by Molecular Beam Epitaxy

Zhao

2017

Crystals

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p-Type doping represents a key step towards III-nitride (InN, GaN, AlN) optoelectronic devices. In the past, tremendous efforts have been devoted to obtaining high quality p-type III-nitrides, and extraordinary progress has been made in both materials and device aspects. In this article, we intend to discuss a small portion of these processes, focusing on the molecular beam epitaxy (MBE)-grown p-type InN and AlN-two bottleneck material systems that limit the development of III-nitride near-infrared and deep ultraviolet (UV) optoelectronic devices. We will show that by using MBE-grown nanowire structures, the long-lasting p-type doping challenges of InN and AlN can be largely addressed. New aspects of MBE growth of III-nitride nanostructures are also discussed.

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“…Reported external quantum efficiencies (EQE) for group III-nitrides-based near and deep UV LEDs. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] In part courtesy of Prof. M. Kneissl of Technische Universität Berlin.…”

Section: Current Status and Challenges Of Group Iii-nitrde Duv Ledsmentioning

confidence: 99%

“…[56] In order to obtain AlN and high Al-molar fraction AlGaN with high crystalline quality and flat surface morphology by using moderate growth temperatures, pulsed-flow growth methods, which can effectively promote the surface migration of Al (and Ga and In) adatoms, have been developed instead of the conventional continuous growth. There are at least three types of pulsed-flow growth methods, namely (a) NH3 pulse-flow growth, [17,60] (b) pulsed atomic layer epitaxy (PALE), [61,62] and (c) migration-enhanced metalorganic chemical vapor deposition (MEMOCVD), [63,64] as shown schematically in Figure 5. For the NH3 pulse-flow growth of AlN, pulsed NH3 flow was employed to enhance the lateral migration of Al adatoms, while TMAl flow was kept constant during the NH3 pulsed-flow sequence to ensure Al-rich growth conditions.…”

Section: Pulsed-flow Growth Of Aln and High Al-molar Fraction Alganmentioning

confidence: 99%

Status of Growth of Group III-Nitride Heterostructures for Deep Ultraviolet Light-Emitting Diodes

Ding¹,

Avrutin²,

Özgür³

et al. 2017

Preprint

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Abstract:We overview recent progress in growth aspects of group III-nitride heterostructures for deep ultraviolet (DUV) light-emitting diodes (LEDs), with particular emphasis on the growth approaches for attaining high-quality AlN and high Al-molar fraction AlGaN. The discussion commences with the introduction of the current status of group III-nitride DUV LEDs and the remaining challenges. This segues into discussion of LED designs enabling high device performance followed by the review of advances in the methods for the growth of bulk single crystal AlN intended as a native substrate together with a discussion of its UV transparency. It should be stated, however, that due to the high-cost of bulk AlN substrates at the time of this writing, the growth of DUV LEDs on foreign substrates such as sapphire still dominates the field. On the deposition front, the heteroepitaxial growth approaches incorporate high-temperature metal organic chemical vapor deposition (MOCVD) and pulsed-flow growth, a variant of MOCVD, with the overarching goal of enhancing adatom surface mobility, and thus epitaxial lateral overgrowth which culminates in minimization the effect of lattice-and thermal-mismatches. This is followed by addressing the benefits of pseudomorphic growth of strained high Al-molar fraction AlGaN on AlN. Finally, methods utilized to enhance both p-and n-type conductivity of high Al-molar fraction AlGaN are reviewed.

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Recent progress and future prospects of AlGaN-based high-efficiency deep-ultraviolet light-emitting diodes

Cited by 505 publications

References 43 publications

Review—Group III-Nitride-Based Ultraviolet Light-Emitting Diodes: Ways of Increasing External Quantum Efficiency

Review—Group III-Nitride-Based Ultraviolet Light-Emitting Diodes: Ways of Increasing External Quantum Efficiency

Recent Advances on p-Type III-Nitride Nanowires by Molecular Beam Epitaxy

Status of Growth of Group III-Nitride Heterostructures for Deep Ultraviolet Light-Emitting Diodes

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