Wafer-scale vertical injection III-nitride deep-ultraviolet light emitters

The study addresses long-standing barriers to high-performance, scalable AlGaN-based deep-UV LEDs. Despite AlGaN’s direct bandgap spanning the UV spectrum and recent efficiency records (WPE ~15.3% and EQE ~20.3% near 275 nm), conventional growth on AlN templates requires very high temperatures and specialized HT-MOCVD tools, raising costs and limiting accessibility. Additionally, deep-UV LEDs suffer from low light extraction efficiency (typically 6–8% without enhancements) due to internal absorption (e.g., p-GaN layers) and severe total internal reflection from substrate interfaces in flip-chip configurations. Thin-film approaches that remove the substrate (e.g., LLO) can alleviate TIR but are difficult for AlN/sapphire because short-wavelength, high-power lasers are needed and Al precipitation induces cracking. The research proposes using GaN/sapphire templates with a designed decoupling epitaxial scheme to enable wafer-scale vertical injection DUV-LEDs, combining improved light extraction, compatibility with standard MOCVD and lower-cost LLO, and crack-free large-area fabrication.

Prior work has pursued improving DUV LED efficiencies by optimizing p-layers, reflective electrodes, and backside treatments to mitigate absorption and TIR, achieving up to ~39% estimated LEE in best cases. Most devices are grown on AlN (on sapphire, SiC, or bulk AlN), but AlN epitaxy demands higher temperatures than GaN and often dedicated HT-MOCVD. Flip-chip devices retain sapphire, limiting escape cone and LEE despite encapsulation and backside roughening/patterning; internal AlN/sapphire TIR remains problematic. Thin-film devices via substrate removal methods (LLO, chemical lift-off, thinning) and emerging mechanical transfer with h-BN/graphene release layers have shown promise. However, applying LLO to AlN/sapphire is hampered by the need for 193 nm lasers for AlN decomposition and by rigid Al precipitation that causes cracking; AlGaN sacrificial layers or superlattices still face cracking due to Al precipitation. There is also work on AlGaN hetero-ELO on GaN to mitigate cracking up to ~30% Al, but it does not meet the requirements for higher Al compositions needed for ~280 nm emission. These gaps motivate a strategy that leverages GaN templates with an internal strain-decoupling scheme to enable scalable thin-film vertical injection DUV-LEDs.

Epitaxial growth: Samples were grown by close-coupled showerhead MOCVD (Aixtron 1×4 inch or 3×2 inch; repeated on AMEC Prismo HiT3). On sapphire, a ~4 µm GaN template was deposited (two-step method). A 120 nm Al0.8Ga0.2N pre-crack layer was then grown (1075 °C) to intentionally induce shallow, buried tensile-relief cracks (~100 nm depth); next, a 540 nm Al0.65Ga0.35N healing layer (1095 °C) was grown to fill cracks and restore a smooth surface while providing optical transparency. The LED structure comprised: 1.1 µm n-Al0.55Ga0.45N, 5× MQWs of Al0.5Ga0.5N barriers/Al0.37Ga0.63N wells (barrier ~8 nm, well ~1.8 nm), a 10 nm p-Al0.8Ga0.2N EBL, p-Al0.63Ga0.37N/Al0.46Ga0.54N superlattices, and a 6 nm p-GaN contact layer. A reference device with the same LED stack was grown on an AlN template for flip-chip comparison. Decoupling design and verification: AFM showed dense pre-cracks (spacing ~3 µm) along <11-20> after the pre-crack layer, and a crack-free, step-terrace surface after the healing layer (RMS roughness ~0.51 nm over 10×10 µm2). X-ray RSM ((10-15) reflection) indicated the healing layer was pseudomorphic to the pre-crack layer with approximately complete relaxation relative to GaN, implying the LED stack is effectively decoupled from the GaN template. Cross-sectional STEM identified V-shaped buried pre-cracks with critical thickness <80 nm for Al0.8Ga0.2N on GaN and confirmed progressive filling by the Al0.65Ga0.35N healing layer. In situ desorption-tailored indicator lines (growth pauses every 180 nm) tracked filling: bending at 180 nm indicated remaining crack sidewalls, while straightness at 360 nm indicated near-complete filling. EDS mapping/line scans showed Ga enrichment within filled cracks (peak Ga ~45%, Al ~55%), consistent with longer Ga diffusion lengths aiding sidewall migration; the composition remained transparent to 280 nm emission. Defect characterization and optical evaluation: Two-beam dark-field TEM (g=[0002], g=[11-20]) showed slightly increased TD density in the healing/LED layers compared to the GaN template, with some screw- and edge-type TDs originating at filled pre-cracks; overall impact was limited due to small crack coverage. XRC FWHM for Al0.65Ga0.35N was 294 arcsec (0002) and 387 arcsec (11-02), corresponding to a TD density ~1.35×10^9 cm^-2, comparable to AlGaN on AlN/sapphire. Plan-view STEM of the MQW region estimated TD density ~1.18×10^9 cm^-2. Temperature-dependent PL (213 nm excitation) from 10–300 K showed room-temperature emission at 280 nm and an RRE of 70.9%; resonant excitation at 266 nm yielded similar RRE. RRE increased with excitation power (1.6–11 mW) from 37.2% to 70.9%. Device fabrication (vertical injection, thin-film): After depositing the Ni/Au/Rh p-electrode, wafers were bonded to Si submounts and sapphire was removed via 355 nm frequency-tripled Nd:YAG LLO. The exposed N-face GaN was HCl-cleaned to remove Ga from GaN decomposition. The residual GaN was ICP-etched down to the Al0.8Ga0.2N pre-crack layer, then KOH roughening produced random hexagonal pyramid texturing to boost LEE. Etching windows exposed n-Al0.55Ga0.45N, and Ti/Al/Ni/Au n-electrodes were deposited. Surface and morphology after etch/roughening were verified by SEM. LOP and far-field patterns were measured with a UV integrating sphere; structural/defect analysis employed TEM/STEM/EDS (Themis Z), XRD, AFM, SEM, and crack mapping.

• Wafer-scale, crack-free 2- and 4-inch AlGaN DUV-LED wafers on GaN/sapphire were realized in a vertical injection, thin-film configuration using an internal pre-crack/healing decoupling structure.
• The decoupling enables sapphire removal by low-cost 355 nm LLO (Nd:YAG) without Al precipitation-induced fracturing; the structure provides a cushion against thermal shock during LLO.
• Surface morphology: Pre-crack layer showed dense cracks (spacing ~3 µm); the healing layer restored a crack-free step-terrace surface with RMS roughness ~0.51 nm (10×10 µm2).
• Strain and defects: Healing layer was pseudomorphic to the pre-crack layer and approximately fully relaxed relative to GaN (RSM). XRC FWHM (0002/11-02) for Al0.65Ga0.35N: 294/387 arcsec; inferred TD density ~1.35×10^9 cm^-2. Active region TD density ~1.18×10^9 cm^-2 (plan-view STEM).
• Crack filling mechanism: STEM/EDS showed Ga enrichment within filled cracks (peak Ga ~45%) and that greater Al content requires thicker healing layers for complete filling (e.g., near-complete at ~360 nm for Al0.65Ga0.35N).
• Optical efficiency: MQWs exhibited room-temperature emission at 280 nm with radiative recombination efficiency 70.9% (10 K as reference), comparable to LEDs on AlN templates; RRE increased with excitation power (37.2% to 70.9%).
• Device performance (0.5×0.5 mm2 dies): LOP 38.4 mW at 100 mA and 65.2 mW at 200 mA, significantly exceeding a flip-chip reference on AlN/sapphire (23.6 mW and 42.1 mW at 100 mA and 200 mA). Peak EQE reached 9.63% at 20 mA. Far-field pattern was Lambertian with enhanced on-axis intensity vs flip-chip, consistent with improved LEE from substrate removal and surface roughening.

The work directly addresses the dual challenge of tensile strain-induced cracking in high-Al-content AlGaN grown on GaN and the poor light extraction inherent to flip-chip DUV-LEDs on sapphire. By intentionally introducing and then healing buried pre-cracks, the epitaxial LED stack is effectively decoupled from the GaN template, enabling strain relaxation without compromising surface morphology or substantially degrading crystalline quality. This decoupling not only prevents macroscopic wafer cracking during growth but also provides resilience against thermal shock during laser lift-off, allowing use of a widely available 355 nm Nd:YAG source and avoiding brittle Al precipitation associated with AlN decomposition. Following sapphire removal and texturing, the thin-film vertical injection architecture eliminates internal TIR at the epi/substrate interface and enhances scattering, substantially improving LEE. Consequently, the devices demonstrate markedly higher LOP and a high peak EQE (9.63%), while PL-derived RRE (~70.9%) indicates that radiative efficiency remains competitive with AlN-template counterparts despite the modified growth strategy. Overall, the findings validate a scalable, cost-effective path to high-performance DUV-LEDs compatible with standard MOCVD infrastructure and mature LLO processes from the visible LED industry.

This study introduces a decoupling epitaxial strategy—pre-crack induction in Al0.8Ga0.2N followed by Al0.65Ga0.35N healing—that enables wafer-scale, crack-free, vertical injection AlGaN DUV-LEDs grown on GaN/sapphire. The approach allows efficient sapphire removal using 355 nm LLO, avoids fracture from Al precipitation, preserves good crystalline quality (TD ~10^9 cm^-2), and achieves robust radiative performance (RRE ~70.9%). Thin-film devices at 280 nm deliver 65.2 mW at 200 mA (38.4 mW at 100 mA) and peak EQE of 9.63%, outperforming flip-chip references primarily through improved light extraction. The method leverages standard MOCVD tools and existing LLO workflows, advancing manufacturability and scalability for DUV emitters. Future work should focus on reducing operating voltage via lower-resistance Ohmic contacts to etched n-AlGaN, further optimizing LEE and EQE, extending the approach across DUV wavelengths, and refining the decoupling layer design to minimize defect generation while maintaining complete crack filling.

• Electrical performance: The reported I–V characteristics indicate room for lowering operating voltage; forming low specific contact resistance Ohmic contacts on etched (0001) n-AlGaN remains challenging.
• Defect generation: Some screw- and edge-type threading dislocations originate at healed pre-cracks, though overall TD density remains ~10^9 cm^-2; further optimization may reduce these defects.
• Process sensitivity: Complete crack filling thickness depends on Al composition; tighter control over composition and growth conditions is required to ensure uniform filling across large wafers.
• Generalizability: While demonstrated at 280 nm, extending the approach to shorter wavelengths (higher Al content) may demand further adjustments to the pre-crack/healing design and could exacerbate strain/defect trade-offs.