Increased Light Extraction of Thin-Film Flip-Chip UVB LEDs by Surface Texturing

Bergmann, Michael Alexander; Enslin, Johannes; Guttmann, Martin; Sulmoni, Luca; Ploch, Neysha Lobo; Hjort, Filip; Kolbe, Tim; Wernicke, Tim; Kneissl, Michael; Haglund, Åsa

doi:10.1021/acsphotonics.2c01352

Cited by 14 publications

(4 citation statements)

References 26 publications

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“…The reported maximum light output power (LOP) of 20 × 20 mil 2 UV-B chips at 304 nm was 57.2 mW under an operating current of 800 mA, with 17% degradation after 1000 h of operation . The major bottlenecks lie in proper strain management and light extraction. − For AlGaN grown on AlN bulk/templates, the huge misfit strain will trigger the strain relaxation inducing: (1) the generation of misfit dislocations (MDs) which enhance nonradiative recombination; (2) severe phase separation and surface roughness causing local current leakage; − (3) increasing point defects contributing to the nonradiative recombination and current leakage. ,, Although the strain relaxation of AlGaN-based LEDs has been reported widely, − the relaxation mechanisms including the critical condition and triggering sequence for high Al-content AlGaN have been less than decisive. Clarifying the relaxation mechanisms and engineering the misfit strain during AlGaN growth is crucial to defining the final structural quality, which is a vital step toward performance improvement of AlGaN-based UV-B LEDs and potential for practical applications.…”

Section: Introductionmentioning

confidence: 99%

Toward High-Performance AlGaN-Based UV-B LEDs: Engineering of the Strain Relaxation Process

Liu,

Huang

et al. 2024

Crystal Growth & Design

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Strain management is the key to achieving highquality aluminum gallium nitride (AlGaN) materials for fabricating AlGaN-based ultraviolet-B (UV-B) light-emitting diodes (LEDs). In this work, a kinetic defect evolution model to capture the strain relaxation process for AlGaN growth of UV-B LEDs was demonstrated. The relaxation started from the inclination of threading dislocations (TDs) dominantly over the coexistence of TD inclination, surface roughening, misfit dislocation (MD) generation, enhanced phase separation, etc. as the thickness of n-AlGaN increased. The critical thickness for strain relaxation by TD inclination, MD generation, and the theoretical MD density were obtained. Finally, flip-chip UV-B LEDs with properly engineered buffer strain were fabricated, which showed a maximum light output power of 49.1 mW under 500 mA and a high external quantum efficiency of 3.00% at 310.2 nm, without a high-reflective Rh electrode or any complex packaging process. This work improves our understanding of the relaxation mechanism and prompts further consideration of the strain management in AlGaN-based UV-B LEDs.

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Section: Introductionmentioning

confidence: 99%

Toward High-Performance AlGaN-Based UV-B LEDs: Engineering of the Strain Relaxation Process

Liu,

Huang

et al. 2024

Crystal Growth & Design

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“…Flip-chip LEDs with the growth substrate removed, often referred to as thin-film flip-chip (TFFC), are in demand when creating UV LEDs with low-conductivity buffer layers as part of the heterostructure [26,[28][29][30][31].…”

Section: Introductionmentioning

confidence: 99%

SiC/Si Hybrid Substrate Synthesized by the Method of Coordinated Substitution of Atoms: A New Type of Substrate for LEDs

et al. 2023

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This paper proposes a new type of substrate for manufacturing LEDs based on AlInGaN heterostructures. Instead of depositing SiC layers on the surface of Si using the conventional method, a new method involving the coordinated substitution of atoms (MCSA) to form the SiC layer is proposed. This new approach enables the growth of epitaxial GaN layers with low defect content and facilitates transfer to any surface. The paper details the technology of manufacturing LEDs on SiC/Si substrates obtained by the MCSA and elaborates on the benefits of using these substrates in LED production. Additionally, the advantages of the growth interface between SiC and Si materials are discussed. Moreover, it is found that thinner SiC layers (<200 nm) contribute to the scattering of the LED’s own radiation in the heterostructure waveguide, which decreases its absorption by silicon. For flip-chip LEDs with the substrate removed, substrates with thicker SiC layers (~400 nm) and a growth porous layer of several microns at the SiC-Si interface is utilized to simplify Si substrate removal and enhance the manufacturing process’s cost-effectiveness.

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“…Electrochemical etching is a substrate removal based on a tunneling process carried out at the semiconductor/electrolyte junction, where a sacrificial layer is etched, releasing the device membrane from the substrate. , The etch selectivity in this process is determined by the semiconductor bandgaps, n-doping concentrations, and the applied voltage, favoring the etching of more heavily doped layers with a lower bandgap . This technology has been proven compatible with doped devices such as UVB-LEDs, providing smoothly etched N-face surfaces and good cavity length control, , which are important features for the fabrication of microcavities. To our knowledge, the compatibility of electrochemical etching with devices containing heavily doped n-AlGaN structures, such as tunnel junctions (TJs), has not been proven up until now.…”

mentioning

confidence: 99%

Ultraviolet-B Resonant-Cavity Light-Emitting Diodes with Tunnel Junctions and Dielectric Mirrors

Torres,

Ciers,

Bergmann

et al. 2024

ACS Photonics

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We demonstrate the first electrically injected AlGaN-based ultraviolet-B resonant-cavity light-emitting diode (RCLED). The devices feature dielectric SiO2/HfO2 distributed Bragg reflectors enabled by tunnel junctions (TJs) for lateral current spreading. A highly doped n++-AlGaN/n++-GaN/p++-AlGaN TJ and a top n-AlGaN current spreading layer are used as transparent contacts, resulting in a good current spreading up to an active region mesa diameter of 120 μm. To access the N-face side of the device, the substrate is removed by electrochemically etching a sacrificial n-AlGaN layer, leading to a smooth underetched surface without evident parasitic etching in the n- and n++-doped layers of the device. The RCLEDs show a narrow emission spectrum with a full width at half-maximum (FWHM) of 4.3 nm compared to 9.4 nm for an ordinary LED and a more directional emission pattern with an angular FWHM of 52° for the resonance at 310 nm in comparison to ∼126° for an LED. Additionally, the RCLEDs show a much more stable emission spectrum with temperature with a red-shift of the electroluminescence peak of about ∼18 pm/K and a negligible change of the FWHM compared to LEDs, which shift ∼30 pm/K and show spectrum broadening with temperature. The demonstration of those devices, where a highly reflective mirror is spatially separated from an ohmic metal contact, opens up a new design space to potentially increase the poor light extraction efficiency in UV LEDs and is an important step toward electrically injected UV vertical-cavity surface-emitting lasers.

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Increased Light Extraction of Thin-Film Flip-Chip UVB LEDs by Surface Texturing

Cited by 14 publications

References 26 publications

Toward High-Performance AlGaN-Based UV-B LEDs: Engineering of the Strain Relaxation Process

Toward High-Performance AlGaN-Based UV-B LEDs: Engineering of the Strain Relaxation Process

SiC/Si Hybrid Substrate Synthesized by the Method of Coordinated Substitution of Atoms: A New Type of Substrate for LEDs

Ultraviolet-B Resonant-Cavity Light-Emitting Diodes with Tunnel Junctions and Dielectric Mirrors

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