AlGaN-based Deep-ultraviolet Research Articles

Enhancing hole injection efficiency in an AlGaN-based deep ultraviolet laser diode by three-terminal structure with the AlScN ferroelectric layer

Here, we have designed a novel, to the best of our knowledge, three-terminal AlGaN-based deep-ultraviolet (DUV) laser diode (LD), featuring an AlScN ferroelectric layer beneath the third terminal and a stepped ridge structure. Our findings indicate that when the AlScN layer is negatively polarized by an external bias by the third terminal, the fixed polarization positive charges at the AlScN/p-GaN interface can enhance hole injection efficiency by modulating the energy band in the p-type region near the AlScN layer. The modulation does not deteriorate the electron injection and the optical field confinement, thus effectively increasing the stimulated recombination rate in the active region. As a result, there is a marked reduction in threshold current density, accompanied by a significant rise in output light power. Furthermore, our investigation suggests that the stepped ridge structure can further mitigate the hole lateral diffusion during the hole injection process, ensuring sufficient hole current density. This study, theoretically, offers a novel way to realize high-performance AlGaN-based DUV LD.

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.

Study on the Degradation Performance of AlGaN-Based Deep Ultraviolet LEDs under Thermal and Electrical Stress

AlGaN-based deep-ultraviolet (DUV) LEDs could realize higher optical power output when adopting a p-AlGaN contact layer instead of a p-GaN contact layer. However, this new type DUV LEDs exhibit poor reliability. Thus, this study thoroughly investigates the degradation behaviors of AlGaN-based DUV LEDs with a p-AlGaN contact layer through different aging tests, including single thermal stress, single electrical stress with air-cooling, single electrical stress, and thermoelectric complex stress. It can be found that both high temperature and large working current play crucial roles in accelerating the degradation of optoelectronic properties of the DUV LEDs, and the single high thermal stress without electrical stress can also bring obvious performance degradation to the DUV LEDs, which is a significantly different finding from previous studies. This is because thermal stress on DUV LED could bring some metal electrode elements entering the p-AlGaN layer. Thus, the degradation of optical and electrical properties under the thermal and electrical stress could be not only attributed to the degradation of the device’s ohmic contacts, but also due to the metal electrode elements entering the p-AlGaN layer through thermal diffusion, leading to the generation of tunneling current and the generation of defects within or around the active region. Despite that the peak wavelengths of the DUV LEDs remained stable, the turn-on voltage and series resistance increased. Particularly worth mentioning is that the value of the optical power degradation under thermoelectric conditions is larger than the sum of the single thermal and single electrical optical power degradation, which is a result of the mutual reinforcement of thermal and electrical stresses to exacerbate the defect generation and ohmic contact degradation. Based on the study above, preparing p-AlGaN layers with hyperfine gradient aluminum fractions and reducing the junction temperature may help to improve the reliability of AlGaN-based DUV LEDs with the p-AlGaN contact layer.

Highly reflective Ni/Pt/Al p-electrode for improving the efficiency of an AlGaN-based deep ultraviolet light-emitting diode.

In this work, we propose a highly reflective Ni/Pt/Al p-electrode for AlGaN-based deep ultraviolet (DUV) light-emitting diodes (LEDs) with a wavelength of 276 nm. AlGaN-based DUV LEDs with traditional Al-based reflectivity electrodes suffer from device degradation and wall-plug efficiency (WPE) droop due to the Al diffusion during electrode annealing. By inserting a Pt layer between the Ni contact layer and the Al reflective layer, the contact characteristics of the p-electrode can be optimized by blocking the diffusion of the O and Al atoms, maintaining a high reflectivity of over 80% near 280 nm. Compared to the AlGaN-based DUV LEDs with Ni/Au traditional p-electrodes and Ni/Al traditional reflective p-electrodes, the WPE of the LED with a highly reflective Ni/Pt/Al p-electrode is improved by 10.3% and 30.5%, respectively. Besides, compared to the other novel reflective p-electrodes using multiple annealing or evaporation processes reported for the AlGaN-based DUV LEDs, we provide a new, to the best of our knowledge, optimization method for single evaporation and annealing p-type reflective electrodes, featured with a simpler and more convenient process flow.

Effects of Electron Blocking Layer Thickness on the Electrical and Optical Properties of AlGaN-Based Deep-Ultraviolet Light-Emitting Diode

Effects of Electron Blocking Layer Thickness on the Electrical and Optical Properties of AlGaN-Based Deep-Ultraviolet Light-Emitting Diode

On the origin of the enhanced light extraction efficiency of DUV LED by using inclined sidewalls.

It is known that light extraction efficiency (LEE) for AlGaN-based deep ultraviolet (DUV) light-emitting diodes (LEDs) can be enhanced by using an inclined sidewall of mesa. However, the reported optimal inclined angles are different. In this work, to explore the origin for enhancing the LEE of DUV LED by using inclined sidewalls, we investigate the effect of an inclined sidewall angle on the LEE for AlGaN-based DUV LEDs with different mesa diameters by using ray tracing. It is found that when compared to large-size DUV LEDs with inclined sidewall, the LEE of small-size DUV LEDs with inclined sidewall is enhanced from both the bottom and side surfaces due to the reduced scattering length and material absorption. Additionally, the optimal inclined sidewall angle is recommended within the range of 25°-65°, and the optimal angle for DUV LEDs decreases as the chip size increases. It can be attributed to the fact that there are two scattering mechanisms for the inclined sidewall. For smaller chip sizes, most of the light is directly scattered into escape cones by the inclined sidewall, resulting in a larger optimal angle. For larger chip sizes, the light firstly experiences total internal reflections by the out-light plane and then is scattered into escape cones by the inclined sidewalls, leading to a smaller optimal angle.

Enhancing external quantum efficiency of deep ultraviolet micro-leds through geometry design and multi-physics field coupling analysis

Abstract High-efficiency deep-ultraviolet (DUV) micro light-emitting diodes (LEDs) are explored for inspiring development in numerous fields, such as non-line-of-sight solar-blind communication, optical pumping, and maskless lithography. In this study, we performed FDTD and SimuLED calculations to investigate the optimized DUV micro-LED structure geometry for high light extraction efficiency (LEE) by designing different mesa structures, including square, hexagonal, and circular geometries of micro-LEDs emitted at a wavelength of 275 nm. The results showed that a circular mesa of 5 μm diameter achieved a LEE of 27% from the bottom and sidewall emissions of as-prepared DUV micro-LED. And both the near- and far-field transverse magnetic polarized light intensities were enhanced by a factor of 1.5 over the square and hexagonal mesas. Meanwhile, the transverse electric (TE) polarized light of the circular mesa structure was enhanced and concentrated along the normal direction. Moreover, the internal quantum efficiency (IQE) of circular mesas with varied sizes was comprehensively investigated in the interactions of the thermal and electric fields. An AlGaN-based DUV micro-LED with a diameter of 5 μm was found to obtain the highest IQE owing to a high current-density distribution and its self-heating properties, thereby achieving a sufficiently high external quantum efficiency of 26.75%. This study provides a comprehensive technical report, including electrical, thermal, and optical analyses, and a new perspective for developing high-efficiency, high-performance DUV micro-LEDs in practical applications.

Ring geometric effect on the performance of AlGaN-based deep-ultraviolet light-emitting diodes.

In this study, we fabricated and characterized various parallel flip-chip AlGaN-based deep-ultraviolet (DUV) micro-ring LEDs, including changes in ring dimensions as well as the p-GaN-removed region widths at the outer micro-ring, respectively (PRM LEDs). It is revealed that the LED chips with smaller mesa withstand higher current density and deliver considerably higher light output power density (LOPD), under the same proportion of the hole to the entire mesa column (before it is etched into ring) within the limits of dimensions. However, as the ring-shaped mesa decreases, the LOPD begins to decline because of etching damage. Subsequently, at the same external diameter, the optical performance of micro-ring LEDs with varied internal diameters is studied. Meanwhile, the influence of different structures on light extraction efficiency (LEE) is studied by employing a two-dimensional (2D)-finite-difference time-domain (FDTD) method. In addition, the expand of the p-GaN-removed region at the outer micro-ring as well as the corresponding effective light emission region have some influence to LOPD. The PRM-23 LED (with an external diameter of 90 µm, an internal diameter of 22 µm, and a p-GaN-removed region width of 8 µm) has an LOPD of 53.36 W/cm2 with a current density of 650 A/cm2, and an external quantum efficiency (EQE) of 6.17% at 5 A/cm2. These experimental observations provide a comprehensive understanding of the optical and electrical performance of DUV micro-LEDs for future applications.

Fabrication of a freestanding AlN substrate via HVPE homoepitaxy on a PVT-AlN substrate

Hydride vapor phase epitaxy (HVPE) is employed for the homoepitaxial development of AlN thick films on AlN substrates grown via physical vapor transport (PVT). A freestanding AlN substrate with a 200 μm thickness is then obtained by mechanically grinding away the PVT-AlN substrate. The as-grown HVPE AlN layer has a smooth surface with long parallel atomic steps. The freestanding HVPE-AlN substrate is crack-free and stress-free. In comparison to PVT-AlN substrate, HVPE-AlN substrate not only has better crystal quality but also substantially lower C, O, and Si impurity concentrations. The deep ultraviolet (DUV) transmittance of the 200 μm thick freestanding AlN substrate is as high as 66% at 265 nm. This performance aligns perfectly with the demands of AlGaN-based DUV optoelectronic devices.

Efficient AlGaN-Based Deep-Ultraviolet LED With N-Side Located Tunnel Junction

Efficient AlGaN-Based Deep-Ultraviolet LED With N-Side Located Tunnel Junction

AlGaN-Based Deep-Ultraviolet Light-Emitting Diodes With Varied Thickness of Sidewall Passivation via Atomic Layer Deposition

AlGaN-Based Deep-Ultraviolet Light-Emitting Diodes With Varied Thickness of Sidewall Passivation via Atomic Layer Deposition

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.

Reverse compositional strategy of multiple quantum wells for the AlGaN-based deep-ultraviolet laser diodes

Due to the high activation energy/ionization energy as Mg increases, Al-rich AlGaN-based deep-ultraviolet (DUV) laser diodes have a structure that is difficult to perform Mg p-doping. The performance of this type of laser diode is affected by the content of Al in the quantum barriers of multiple quantum wells, and reducing the Al content in the laser diodes may simplify its structure and make Mg p-doping less difficult. The proposed device can be improved by adopting the reverse compositional strategy of multiple quantum wells with the Al content of quantum barriers 10% lower than that of quantum wells as opposed to the reference device where the Al content of the quantum barriers is 10% higher than that of the quantum wells. The simulation results have been observed theoretically in terms of the performance parameters such as band diagrams, optical confinement factor, peak gain, emitted power, stimulated recombination rate, and carrier concentration. Two deep ultraviolet laser diode devices have been analyzed in this study within the wavelength range of 267 nm. The proposed device has superior performance with a larger optical confinement factor and increased output power with reduced injection current.

High Hole Injection for Nitrogen-Polarity AlGaN-Based Deep-Ultraviolet Light-Emitting Diodes

High hole injection is desired for improving the performance of AlGaN-based deep-ultraviolet (DUV) light-emitting diodes (LEDs). In this work, we adopted a compositionally graded p-Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y1</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-y1</sub> N/Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.85</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.15</sub> N superlattice (SL) structure in a nitrogen-polarity DUV LED. Our numerical simulation results show that this SL structure is very beneficial to the hole injection into the active region, and leads to a low turn-on voltage and device resistance. This is significant because a low device resistance means a low power consumption and a high wall-plug efficiency. Meanwhile, a DUV LED with compositionally graded SL has a peak internal quantum efficiency (72%) that is much higher than that of the reference DUV LED without an SL (56%). This work provides an attractive approach to effectively injecting holes into the active region of a nitrogen-polarity AlGaN-based DUV LED.

AlGaN-Based Deep-Ultraviolet Laser Diodes with Quaternary AlInGaN Last Quantum Barrier

AlGaN-Based Deep-Ultraviolet Laser Diodes with Quaternary AlInGaN Last Quantum Barrier

Improving Light Extraction Efficiency of AlGaN-Based Deep Ultraviolet Light-Emitting Diodes by Combining Thinning p-AlGaN/p-GaN Layer With Ni/Au/Al High-Reflectivity Electrodes

Improving light extraction efficiency (LEE) of AlGaN-based deep-ultraviolet (DUV) light emitting diodes (LEDs) has been attempted by thinning the p-AlGaN/p-GaN layer and adopting Ni/Au/Al composite electrodes. It is found that the thin p-AlGaN/p-GaN layer can reduce the light absorption and the Ni/Au/Al electrodes achieve high reflectivity and Ohmic contact to ensure the enhancement of the light extraction and maintain fine electrical properties. By this approach, the maximum external quantum efficiency of the DUV-LEDs with optimized Ni/Au/Al reflective electrodes is increased by 40%, compared to that with conventional Ni/Au electrodes over the whole current range.

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.

Introducing an n-type electron deceleration layer to enhance the luminous efficiency of AlGaN-based DUV-LEDs

The internal quantum efficiency (IQE) of conventional AlGaN-based deep-ultraviolet (DUV) light-emitting diodes (LEDs) is seriously limited by the poor and inhomogeneous carrier injection. The typical solution is to optimize the structure parameters of p-type region and active region. In this work, however, we try to address this issue by introducing an n-type electron deceleration layer (EDL) underneath multiple quantum wells (MQWs). On one hand, the electron deceleration layer helps to decrease the electron velocity and thus increase the electron capture rate. On the other hand, it can also reduce barrier heights in the band valence and thus enhance the hole transport in the multiple quantum wells. As a consequence, the concentrations of electrons and holes in the multiple quantum wells were significantly increased, resulting in the enhancement of radiative recombination. Compared to the conventional structure, the DUV-LED structure with an electron deceleration layer achieves a higher internal quantum efficiency, leading to a 39% improvement in the light output power. It is believed that performing energy-band engineering in n-type region has great application prospects for high-performance DUV-LEDs.

Performance Enhancement of Algan-Based Deep Ultraviolet Laser Diodes with Step Superlattice Electron Blocking Layer and Wedge-Shaped Hole Blocking Layer

Performance Enhancement of Algan-Based Deep Ultraviolet Laser Diodes with Step Superlattice Electron Blocking Layer and Wedge-Shaped Hole Blocking Layer

Ohmic Contact Characteristics of AlGaN-based Deep-ultraviolet Light-emitting-diodes with NiAu Transparent Electrode

在p⁃AlGaN表面沉积Ni/Au/Ni/Aué€æ˜Žç”µæžä½“ç³»ï¼Œé€šè¿‡ä¼ è¾“çº¿æ¨¡åž‹æµ‹è¯•ï¼Œç ”ç©¶äº†é€€ç«æ¸©åº¦å¯¹Ni/Au/Ni/Au与p⁃AlGaN材料接触特性的影响。结果表明，AlGaN基深紫外LED采用Ni/Au/Ni/Au金属体系，在600 ℃空气氛围下退火3 min形成p型半导体材料NiO。进一步优化Ni/Au/Ni/Au体系金属厚度，当Ni/Au/Ni/Au各层厚度由20/20/20/20 nm减薄至2/2/5/5 nm，并在600 ℃空气氛围退火3 minï¼Œå ¶ä¸Žp⁃AlGaN材料的接触电阻率从3.23×10-1 Ω·cm2降到2.58×10-4 Ω·cm2。采用上述优化的Ni/Au/Ni/Au体系制备的深紫外LEDå™¨ä»¶ï¼Œå™¨ä»¶å ‰ç”µç‰¹æ€§å¾—åˆ°äº†æ”¹å–„ã€‚åœ¨150 mA驱动下工作电压低至5.8 Vï¼›é€šè¿‡æå‡ç”µæžé€è¿‡çŽ‡ï¼Œå ‰è¾“å‡ºåŠŸçŽ‡æå‡18.9%。