Progress of high-power deep-ultraviolet LEDs

Fujioka, Akira; Asada, Katsuhiko; Yamada, Hirohito; Ohtsuka, Takumi; Ogawa, Takuma; Kosugi, Takao; Kishikawa, D.; Mukai, Takashi

doi:10.1117/12.2078620

Cited by 9 publications

(13 citation statements)

References 14 publications

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“…1 However, while AlGaN-based UV LEDs with peak wavelength ranging between 210 nm and 365 nm have been demonstrated, the wall-plug efficiency (WPE) for UV LEDs below 365 nm is still significantly lower than that for visible GaN LEDs. [2][3][4][5][6][7] The underlying reason for this is the high acceptor ionization energy in AlGaN, which makes p-type contacts and layers highly resistive.…”

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confidence: 99%

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Design and demonstration of ultra-wide bandgap AlGaN tunnel junctions

Zhang

Krishnamoorthy

Akyol

et al. 2016

Applied Physics Letters

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Ultra violet light emitting diodes (UV LEDs) face critical limitations in both the injection efficiency and light extraction efficiency due to the resistive and absorbing p-type contact layers. In this work, we investigate the design and application of polarization engineered tunnel junctions for ultra-wide bandgap AlGaN (Al mole fraction > 50%) materials towards highly efficient UV LEDs. We demonstrate that polarization-induced 3D charge is beneficial in reducing tunneling barriers especially for high composition AlGaN tunnel junctions. The design of graded tunnel junction structures could lead to low tunneling resistance below 10 -3 Ω cm 2 and low voltage consumption below 1 V (at 1 kA/cm 2 ) for high composition AlGaN tunnel junctions. Experimental demonstration of 292 nm emission was achieved through non-equilibrium hole injection into wide bandgap materials with bandgap energy larger than 4.7 eV, and detailed modeling of tunnel junctions shows that they can be engineered to have low resistance, and can enable efficient emitters in the UV-C wavelength range.a) Authors to whom correspondence should be addressed. Electronic mail: zhang.3789@osu.edu, rajan@ece.osu.edu 2The large range of direct bandgaps in the III-Nitride system makes these semiconductors uniquely suitable for achieving ultra violet emission over a wide wavelength range down to 210 nm. III-Nitride ultra-violet light emitting diodes (UV LEDs) have a large variety of applications including water purification and air disinfection, and they provide distinct advantages over traditional gas-based lamps. We propose an approach to realize highly efficient hole injection based on interband tunneling. In this approach, a transparent n-type AlGaN layer is connected to the p-AlGaN layer using an interband tunnel junction ( Fig. 1(b)). Non-equilibrium tunneling injection of holes overcomes limits imposed by thermal ionization of deep acceptors. 9,10 At the same time, it leads to efficient light extraction since it eliminates absorbing p-type top contact layers. 11Making efficient interband p-n tunnel junctions is challenging for ultra-wide bandgap AlGaN materials due to the large band gap, wide depletion barrier, and doping limitations. In this work, we show that nanoscale heterostructure and polarization engineering of tunnel junctions can enable highly efficient tunneling even for ultra-wide bandgap materials where ordinary dopant-based homojunctions would have very low tunneling current density. In this letter, we discuss the design of AlGaN tunnel junctions over the entire Al composition range. We then experimentally demonstrate highly efficient tunnel junctions for AlGaN with Al composition greater than 50%. was used for tunneling probability calculation, and tunneling current was calculated by considering all possible contributions by carriers with different energies. Bandtail states and bandgap narrowing effect due to heavy impurity doping are not included in the simulations, though they could lead to increase in the tunneling probability. ...

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“…1(a)) as the hole injection layer. [2][3][4][5][6][7] However, the poor ptype conductivity of p-AlGaN leads to low injection efficiency and high operation voltage 8 , while the pGaN cap layer and metal contact absorb UV light, resulting in reduced light extraction 4,5 .…”

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Design and demonstration of ultra-wide bandgap AlGaN tunnel junctions

Zhang

Krishnamoorthy

Akyol

et al. 2016

Applied Physics Letters

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“…In addition, AlN has recently been considered as an optoelectronic material due to its direct wide bandgap of 6.0 eV at room temperature and piezoelectricity. In an effort to develop energy saving devices, AlN-based deep-ultraviolet light-emitting devices 1 2 3 4 5 6 7 8 9 10 11 12 13 , high electron mobility transistors 14 15 , sensors 16 , etc. have already been exploited.…”

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“…(It is noteworthy that EQE of InGaN blue LEDs has exceeded 80% 17 ). Because AlN substrates are immature and quite expensive, foreign substrates such as sapphire and silicon carbide (SiC) are widely used in devices 1 2 3 4 5 6 7 10 11 14 15 . Unfortunately, the lattice and crystallographic mismatches between AlN and a foreign substrate induce threading dislocations, which are on the order of 10 9 –10 11 /cm 2 in the epilayer.…”

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Environmentally friendly method to grow wide-bandgap semiconductor aluminum nitride crystals: Elementary source vapor phase epitaxy

Funato

Kawakami

2015

Sci Rep

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Aluminum nitride (AlN) has attracted increasing interest as an optoelectronic material in the deep ultraviolet spectral range due to its wide bandgap of 6.0 eV (207 nm wavelength) at room temperature. Because AlN bulk single crystals are ideal device substrates for such applications, the crystal growth of bulky AlN has been extensively studied. Two growth methods seem especially promising: hydride vapor phase epitaxy (HVPE) and sublimation. However, the former requires hazardous gases such as hydrochloric acid and ammonia, while the latter needs extremely high growth temperatures around 2000 °C. Herein we propose a novel vapor-phase-epitaxy-based growth method for AlN that does not use toxic materials; the source precursors are elementary aluminum and nitrogen gas. To prepare our AlN, we constructed a new growth apparatus, which realizes growth of AlN single crystals at a rate of ~18 μm/h at 1550 °C using argon as the source transfer via the simple reaction Al + 1/2N2 → AlN. This growth rate is comparable to that by HVPE, and the growth temperature is much lower than that in sublimation. Thus, this study opens up a novel route to achieve environmentally friendly growth of AlN.

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“…According to the research results by different groups, however, the external quantum efficiency (EQE) is still lower than 10% for DUV LEDs when the peak emission wavelength is shorter than 280 nm, and most of the reported efficiency droop is higher than 10% for the LEDs at deep ultraviolet range [see Figure (a,b)]. DUV LEDs with low EQE and significant efficiency droop effect cannot efficiently kill all the bacteria in the drinking water with high flow rate . As a result, before the massive penetration into the market to replace the conventional mercury based deep ultraviolet light source, it is essentially important to enhance the EQE for DUV LEDs.…”

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confidence: 99%

Progress in External Quantum Efficiency for III‐Nitride Based Deep Ultraviolet Light‐Emitting Diodes

Chu

Tian

Zhang

et al. 2019

Physica Status Solidi (a)

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AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs) are featured with small size, DC driving, no environmental contamination etc., and they are now emerging as the excellent solid-state light source to replace the conventional mercury based light tubes. Nevertheless, the DUV LEDs are currently affected by the poor external quantum efficiency (EQE), which is caused by the low internal quantum efficiency (IQE) and the very unsatisfying light extraction efficiency (LEE). In this work, the authors disclose the underlying mechanism for the low EQE and summarize the technologies that have been adopted so far for enhancing the EQE.

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Progress of high-power deep-ultraviolet LEDs

Cited by 9 publications

References 14 publications

Design and demonstration of ultra-wide bandgap AlGaN tunnel junctions

Design and demonstration of ultra-wide bandgap AlGaN tunnel junctions

Environmentally friendly method to grow wide-bandgap semiconductor aluminum nitride crystals: Elementary source vapor phase epitaxy

Progress in External Quantum Efficiency for III‐Nitride Based Deep Ultraviolet Light‐Emitting Diodes

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