Investigation of Light Extraction Efficiency in AlGaN Deep-Ultraviolet Light-Emitting Diodes

Ryu, Han-Youl; Choi, Il-Gyun; Choi, Hyun-Woong; Shim, Jong‐In

doi:10.7567/apex.6.062101

Cited by 178 publications

(138 citation statements)

References 20 publications

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“…9 Finite-difference time-domain (FDTD) calculations suggest an increase in the light extraction efficiency from ~ 14% to 60% (transverseelectric polarized light) and from ~ 8% to 40% (transverse-magnetic polarized light) when the p-GaN thickness is reduced from 20 nm to 4 nm in a conventional UV LED device with surface roughening. 29 Therefore, similar enhancement in light extraction efficiency is expected by replacing p-GaN with a tunnel junction given that the absorption coefficients of In y Ga 1-y N (0 ≤ y ≤ 0.3) and GaN are similar for the emitted UV light. 30 The tunnel junction designs were tested experimentally by integrating a graded tunnel junction on a UV LED structure as shown in Since the compositional grading was achieved by varying Al cell temperature, we estimate a grading thickness of approximately 10 nm.…”

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confidence: 86%

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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confidence: 86%

Design and demonstration of ultra-wide bandgap AlGaN tunnel junctions

Zhang

Krishnamoorthy

Akyol

et al. 2016

Applied Physics Letters

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“…Although much research has been performed to improve the IQE of NUV-LEDs by improving crystal quality [13][14][15][16], it is still necessary to investigate means to enhance the IQE for high efficiency NUV-LEDs. In additional, most of the light will be trapped inside the NUV-LEDs, resulting in the low light extraction efficiency [17,18]. Since the light trapped inside, it will be reabsorbed eventually.…”

Section: Introductionmentioning

confidence: 98%

Improved UV LED performance using transparent conductive films embedded with plasmonic structures

Chuang

Lin

et al. 2015

SPIE Proceedings

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Recently, near-ultraviolet light-emitting diodes (NUV-LEDs) have been used in many applications such as light sources for ultraviolet curing, environmental cleaning, biomedical instrumentation, counterfeit bill detection and phosphor-based white LEDs. However, it is difficult to fabricate NUV-LEDs with high emission efficiency. As the wavelength of NUVLEDs decreases, the most dominant emission will be photons with transverse-magnetic (TM) polarization. For LED structures grown on a c-plane substrate, TM-light propagates mainly in the lateral direction, and it suffers strong effects of total internal reflection (TIR) due to the large incident angle on the interface. Therefore, light extraction efficiency (LEE) of NUV-LEDs is still lower than that of visible LEDs. In this study, a spin coating process in which the grating structure comprises the metallic nanoparticle layer coated on a p-GaN top layer was developed. Various sizes of metallic nanoparticles forming a suspended nanoparticle layer (SNL) embedded in a transparent conductive layer were clearly observed after the deposition of indium tin oxide (ITO). The SNL enhanced the light extraction efficiency of NUVLEDs. Light output power was 1.4 times the magnitude of that of conventional NUV-LEDs operating at 350 mA, but retained nearly the same current-voltage characteristic. Unlike in previous research on surface-plasmon-enhanced LEDs, the metallic nanoparticles were consistently distributed over the surface area. Device performance can be improved substantially by using the three-dimensional distribution of metallic nanoparticles in the SNL, which scatters the propagating light randomly and is coupled between the localized surface plasmon and incident light internally trapped in the LED structure through TIR.Keywords: Surface plasmon, nanoparticle, light-emitting diode (LED), internal quantum efficiency (IQE), external quantum efficiency (EQE), light extraction efficiency (LEE), electroluminescence spectrum, metal-organic chemical vapor deposition (MOCVD), localized surface plasmons (LSPs), transparent conductive layer.

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“…14 According to recent finite difference time domain calculations, the extraction of TM-polarized DUV light is .10 times lower than that of transverse electric (TE)-polarized light. 22 Thus, a new strategy that is totally different from conventional strategies, utilizing the strong sidewall emission in AlGaN DUV LEDs, should be developed to demonstrate DUV LEDs with high LEEs, and thus, high EQEs.…”

Section: Introductionmentioning

confidence: 99%

Overcoming the fundamental light-extraction efficiency limitations of deep ultraviolet light-emitting diodes by utilizing transverse-magnetic-dominant emission

et al. 2015

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While the demand for deep ultraviolet (DUV) light sources is rapidly growing, the efficiency of current AlGaN-based DUV light-emitting diodes (LEDs) remains very low due to their fundamentally limited light-extraction efficiency (LEE), calling for a novel LEE-enhancing approach to deliver a real breakthrough. Here, we propose sidewall emission-enhanced (SEE) DUV LEDs having multiple light-emitting mesa stripes to utilize inherently strong transverse-magnetic polarized light from the AlGaN active region and three-dimensional reflectors between the stripes. The SEE DUV LEDs show much enhanced light output power with a strongly upward-directed emission due to the exposed sidewall of the active region and Al-coated selective-area-grown n-type GaN micro-reflectors. The devices also show reduced operating voltage due to better n-type ohmic contact formed on the regrown n-GaN stripes when compared with conventional LEDs. Accordingly, the proposed approach simultaneously improves optical and electrical properties. In addition, strategies to further enhance the LEE up to the theoretical optimum value and control emission directionality are discussed.

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Investigation of Light Extraction Efficiency in AlGaN Deep-Ultraviolet Light-Emitting Diodes

Cited by 178 publications

References 20 publications

Design and demonstration of ultra-wide bandgap AlGaN tunnel junctions

Design and demonstration of ultra-wide bandgap AlGaN tunnel junctions

Improved UV LED performance using transparent conductive films embedded with plasmonic structures

Overcoming the fundamental light-extraction efficiency limitations of deep ultraviolet light-emitting diodes by utilizing transverse-magnetic-dominant emission

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