Highly efficient AlGaN-based deep-ultraviolet light-emitting diodes: from bandgap engineering to device craft.

AlGaN-based light-emitting diodes (LEDs) operating in the deep-ultraviolet (DUV) spectral range (210-280 nm) have demonstrated potential applications in physical sterilization. However, the poor external quantum efficiency (EQE) hinders further advances in the emission performance of AlGaN-based DUV LEDs. Here, we demonstrate the performance of 270-nm AlGaN-based DUV LEDs beyond the state-of-the-art by exploiting the innovative combination of bandgap engineering and device craft. By adopting tailored multiple quantum wells (MQWs), a reflective Al reflector, a low-optical-loss tunneling junction (TJ) and a dielectric SiO2 insertion structure (IS-SiO2), outstanding light output powers (LOPs) of 140.1 mW are achieved in our DUV LEDs at 850 mA. The EQEs of our DUV LEDs are 4.5 times greater than those of their conventional counterparts. This comprehensive approach overcomes the major difficulties commonly faced in the pursuit of high-performance AlGaN-based DUV LEDs, such as strong quantum-confined Stark effect (QCSE), severe optical absorption in the p-electrode/ohmic contact layer and poor transverse magnetic (TM)-polarized light extraction. Furthermore, the on-wafer electroluminescence characterization validated the scalability of our DUV LEDs to larger production scales. Our work is promising for the development of highly efficient AlGaN-based DUV LEDs.

Flexible nanoimprint lithography enables high-throughput manufacturing of bioinspired microstructures on warped substrates for efficient III-nitride optoelectronic devices.

III-nitride materials are of great importance in the development of modern optoelectronics, but they have been limited over years by low light utilization rate and high dislocation densities in heteroepitaxial films grown on foreign substrate with limited refractive index contrast and large lattice mismatches. Here, we demonstrate a paradigm of high-throughput manufacturing bioinspired microstructures on warped substrates by flexible nanoimprint lithography for promoting the light extraction capability. We design a flexible nanoimprinting mold of copolymer and a two-step etching process that enable high-efficiency fabrication of nanoimprinted compound-eye-like Al2O3 microstructure (NCAM) and nanoimprinted compound-eye-like SiO2 microstructure (NCSM) template, achieving a 6.4-fold increase in throughput and 25% savings in economic costs over stepper projection lithography. Compared to NCAM template, we find that the NCSM template can not only improve the light extraction capability, but also modulate the morphology of AlN nucleation layer and reduce the formation of misoriented GaN grains on the inclined sidewall of microstructures, which suppresses the dislocations generated during coalescence, resulting in 40% reduction in dislocation density. This study provides a low-cost, high-quality, and high-throughput solution for manufacturing microstructures on warped surfaces of III-nitride optoelectronic devices.

Contact Engineering of III-Nitrides and Metal Schemes toward Efficient Deep-Ultraviolet Light-Emitting Diodes.

Throughout the development of III-nitride electronic and optoelectronic devices, electrically interfacing III-nitride semiconductors and metal schemes has been a long-standing issue that determines the contact resistance and operation voltage, which are tightly associated with the device performance and stability. Compared to the main research focus of the crystal quality of III-nitride semiconductors, the equally important contact interface between III-nitrides and metal schemes has received relatively less attention. Here, we demonstrate a comprehensive contact engineering strategy to realize low resistance to Al-rich n-AlGaN via pretreatment and metal scheme optimization. Prior to the metal deposition, the introduction of CHF3 treatment is conducive to the substantial resistance reduction, with the effect becoming more distinct by prolonging the treatment time. Furthermore, we compare different metal schemes, namely, Ti/Al/Ti/Au, Ti/Al/Ti/Pt/Au, and Cr/Ti/Al/Ti/Pt/Au, to form electrical contact on n-AlGaN. From microscale analysis based on multiple characterization methods, we reveal the correlation between electrical properties and the nature of the contact interface, attributing the contact improvement to the low-resistance Pt- and Cr-related alloy formation. Under the circumstance that no efforts have been devoted to optimizing the epitaxial growth, engineering the metal-semiconductor contact properties alone leads to a resistance value of 8.96 × 10-5 Ω·cm2. As a result, the fabricated deep-ultraviolet LEDs exhibit an ultralow forward voltage of 5.47 V at 30 A/cm2 and a 33% increase in the peak wall-plug efficiency.

Improved efficiency of AlGaN-based flip-chip deep-ultraviolet LEDs using a Ni/Rh/Ni/Au p-type electrode.

Here, we propose a thermally stable and high-reflectivity Ni/Rh/Ni/Au p-type electrode for AlGaN-based deep-ultraviolet (DUV) flip-chip light-emitting diodes (FCLEDs). We discover that the reflectance of Ni/Au electrode deteriorated significantly after rapid thermal annealing. Experiments show that Ni and Au agglomerate at high temperatures, and more incident photons traverse the gaps between the agglomerates, leading to a decrease in reflectance of Ni/Au after annealing. In contrast, the proposed Ni/Rh/Ni/Au p-type electrode shows remarkable thermal stability as a result of the suppression of Ni agglomeration by the Rh layer at high temperatures. Besides, due to the higher reflectivity of the Ni/Rh/Ni/Au electrode and its lower specific contact resistivity formed with p-GaN, the external quantum efficiency and wall-plug efficiency of a DUV FCLED with Ni/Rh/Ni/Au electrode are increased by 13.94% and 17.30% in comparison with the one with Ni/Au electrode at an injection current of 100 mA. The Ni/Rh/Ni/Au electrode effectively solves the long-standing dilemma of efficiency degradation of DUV FCLEDs with a Ni/Au electrode after high-temperature annealing.

Enhanced performance of 275-nm AlGaN-based deep-ultraviolet LEDs via internal-roughed sapphire and SiO2-antireflection film.

The internal-roughed sapphire in a 275-nm AlGaN-based deep-ultraviolet (DUV) LED is fabricated using a laser stealth dicing technique to improve the high-angle extraction. Furthermore, the low-angle extraction is enhanced by depositing a SiO2-antireflection film on the internal-roughed sapphire surface. Compared with conventional DUV LEDs with a light output power (LOP) of 33.05 mW at 350 mA, the LOP of DUV LEDs with internal-roughed sapphire and SiO2-antireflection film increases by 20.85% to 39.94 mW. In addition, combined with finite-difference time-domain simulations, the effect of internal-roughed sapphire on the transmission and light extraction efficiency (LEE) of the DUV LEDs is revealed. The combination of the internal-roughed sapphire substrate and SiO2-antireflection film improves the LEEs of transverse electric (TE) and transverse magnetic (TM) polarized light by 1.6% and 108%, respectively. These results offer the potential for large-scale, low-cost industrial production of high-efficiency DUV LEDs.

Ultra-narrowband resonant light absorber for high-performance thermal-optical modulators.

Herein, a tunable thermal-optical ultra-narrowband grating absorber is realized. Four ultra-sharp absorption peaks in the infrared region are achieved with the absorption efficiency of 19.89%, 98.41%, 99.14%, and 99.99% at 1144.34 nm, 1190.92 nm, 1268.58 nm, and 1358.70 nm, respectively. Benefiting from an extremely narrow bandwidth (0.27 nm), a maximum Q-factor over 4400 is obtained for the absorber. Moreover, the spectral response can be artificially tuned by controlling the temperature via the strong thermo-optic effect of silicon resonator. The high absorption contrast ratio of 23 dB is demonstrated by only increasing the temperature by 10 °C, showing an order of magnitude better than that of the previously demonstrated performance in the infrared image contrast manipulation. Also, the absorption intensity can be precisely regulated via tuning the polarization state of incident light. Strong tunability extending to temperature and polarization states makes this metasurface promising for applications in a high-performance switch, notch filter, modulator, etc.