Application of patterned sapphire substrate for III-nitride light-emitting diodes

Shadow-Assisted Sidewall Emission for Achieving Submicron Linewidth Light Source by Using Normal UV Photolithography

Micro light sources are crucial tools for studying the interactions between light and matter at the micro/nanoscale, encompassing diverse applications across multiple disciplines. Despite numerous studies on reducing the size of micro light sources and enhancing optical resolution, the efficient and simple fabrication of ultra-high-resolution micro light sources remains challenging due to its reliance on precise micro-nano processing technology and advanced processing equipment. In this study, a simple approach for the efficient fabrication of submicron light sources is proposed, namely shadow-assisted sidewall emission (SASE) technology. The SASE utilizes the widely adopted UV photolithography process, employing metal shadow modulation to precisely control the emission of light from polymer sidewalls, thereby obtaining photoluminescent light sources with submicron line widths. The SASE eliminates the need for complex and cumbersome manufacturing procedures. The effects of process parameters, including exposure dose, development time, and metal film thickness, on the linewidth of sources are investigated on detail. It is successfully demonstrated red, green, and blue submicron light sources. Finally, their potential application in the field of optical anti-counterfeiting is also demonstrated. We believe that the SASE proposed in this work provides a novel approach for the preparation and application of micro light sources.

High-performance GaN-based green resonant cavity-light emitting diodes with an Ag bottom mirror

Green resonant cavity light-emitting diodes (RCLEDs) are important components in optical communication and display applications. However, the light output power (LOP) is usually limited. In this study, GaN-based green RCLEDs with a large chip size (900 × 900 μm2) have been fabricated using a silver (Ag) metal mirror as the bottom reflector and a dielectric distributed Bragg reflector (DBR) as the top reflector. The device is characterized by a small spectral linewidth (3.0 nm) and divergence angle (95°) when compared with standard LEDs, which is attributed to the cavity effect. The turn-on voltage is 2.3 V at an injection current of 20 mA, with a LOP of 180 mW at a current density of 197 A/cm2, which is the highest LOP reported to date for green RCLEDs. The Ag bottom mirror can not only enhance the cavity effect but also improve heat dissipation and electrical injection. The utilization of array configuration for the n-side electrode has not only enabled uniform current injection but has also maximized the light output area that contributes to the high LOP.

Design of high-voltage deep ultraviolet LED sub-mesas toward improved optoelectronic performance

Here, we systematically investigate the effect of mesa/sub-mesa sidewall engineering on single-junction (SJ) and high-voltage (HV) deep ultraviolet light-emitting diodes (DUV LEDs). The configuration of ∼46° inclined angle of the mesa/sub-mesa sidewall and Al reflector optimally promotes light extraction of SJ/HV DUV LEDs. We further observe substantial improvements in the self-heating and external quantum efficiency (EQE) droop effects of HV DUV LEDs with an increasing number of sub-mesas. Specifically, the EQE droops are 27.6% and 34.6% for HV DUV LEDs with two and four sub-mesas, respectively, at an input power of 6.4 W. These values are markedly lower than the 51.6% droop observed in the SJ DUV LEDs, which is partly attributed to the superior heat dissipation and light extraction facilitated by a high perimeter-to-area ratio of the sub-mesas. This investigation sheds light on mesa design-related efficiency droop behaviors and contributes to enhancing the optoelectronic performance of AlGaN-based DUV LEDs.

Nonhalogen Dry Etching of Metal Carbide TiAlC by Low-Pressure N2/H2 Plasma at Room Temperature

Ternary metal carbide TiAlC has been proposed as a metal gate material in logic semiconductor devices. It is a hard-to-etch material due to the low volatility of the etch byproducts. Here, a simple, highly controllable, and dry etching method for TiAlC has been first presented using nonhalogen N2/H2 plasmas at low pressure (several Pa) and 20 °C. A capacitively coupled plasma etcher was used to generate N2/H2 plasmas containing active species, such as N, NH, and H to modify the metal carbide surface. The etch rate of TiAlC was obtained at 3 nm/min by using the N2/H2 plasma, whereas no etching occurred with pure N2 plasma or pure H2 plasma under the same conditions. The surface roughness of the TiAlC film etched by N2/H2 plasma was controlled at the atomic level. A smooth etched surface was achieved with a root-mean-square roughness of 0.40 nm, comparable to the initial roughness of 0.44 nm. The plasma properties of the N2/H2 plasmas were diagnosed by using a high-resolution optical emission spectrometer, detecting the NH molecular line at 336 nm. The etching behavior and plasma-surface reaction between N2/H2 plasma and TiAlC were investigated by using in situ spectroscopic ellipsometry, in situ attenuated total reflectance-Fourier transform infrared spectrometry, and X-ray photoelectron spectroscopy. The findings indicate that the N-H, C-N, and Ti(Al)-N bonds form on the TiAlC surface etched by the N2/H2 plasmas. The mechanism for etching of TiAlC involving transformation reactions between inorganic materials (metal carbides) and inorganic etchants (N2/H2 plasma) to form volatile organic compounds such as methylated, methyl-aminated, and aminated metals is proposed. Nonhalogen or nonorganic compound etchants were used during the etching process. The study provides useful insights into microfabrication for large-scale integrated circuits.

A Comprehensive Review of Group-III Nitride Light-Emitting Diodes: From Millimeter to Micro-Nanometer Scales

This review describes the development history of group-III nitride light-emitting diodes (LEDs) for over 30 years, which has achieved brilliant achievements and changed people's lifestyles. The development process of group-III nitride LEDs is the sum of challenges and solutions constantly encountered with shrinking size. Therefore, this paper uses these challenges and solutions as clues for review. It begins with reviewing the development of group-III nitride materials and substrates. On this basis, some key technological breakthroughs in the development of group-III nitride LEDs are reviewed, mainly including substrate pretreatment and p-type doping in material growth, the proposal of new device structures such as nano-LED and quantum dot (QD) LED, and the improvement in luminous efficiency, from the initial challenge of high-efficiency blue luminescence to current challenge of high-efficiency ultraviolet (UV) and red luminescence. Then, the development of micro-LEDs based on group-III nitride LEDs is reviewed in detail. As a new type of display device, micro-LED has drawn a great deal of attention and has become a research hotspot in the current international display area. Finally, based on micro-LEDs, the development trend of nano-LEDs is proposed, which is greener and energy-saving and is expected to become a new star in the future display field.

Limited impact of the sidewall effect in dependence of temperature for InGaN-based blue micro-LEDs grown on a silicon substrate

The electroluminescence (EL) properties of InGaN-based micro-LEDs grown on a silicon substrate are investigated in this Letter to reveal the dominant mechanism in dependence on different temperatures and dimensions. The invalidation of sidewall nonradiative recombination and the impact of localization-induced carrier tunneling on the external quantum efficiency (EQE) are analyzed systematically to realize high performance silicon-based micro-LEDs. Microscopic EL mapping exhibits that the localized carriers in the silicon-grown micro-LED mainly recombine in the central region of mesa. The defects in the multiple quantum wells (MQWs) grown on the silicon substrate can lead to carrier tunneling and EQE reduction at cryogenic temperatures below 200 K, which is more conspicuous for the 30 μm device with a larger inner area ratio. The low-temperature EQE evolution can be attributed to the trade-off between localization-induced tunneling and Shockley-Read-Hall (SRH) recombination.

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.

Enhanced light extraction efficiency for the inclined-sidewall-shaped AlGaN-based DUV LED by using an omni-directional reflector with a thin hybrid dielectric layer

In this Letter, an omni-directional reflector (ODR) with a thin hybrid dielectric layer (hybrid-ODR) is proposed to enhance the light extraction efficiency (LEE) for inclined-sidewall-shaped AlGaN-based deep ultraviolet light-emitting diode (DUV LED) by inserting a thin diamond with high refraction index into a conventional Al/Al2O3-based ODR. The three-dimensional finite-difference time-domain (3D FDTD) simulation results show that the LEE of TM-polarized light for the DUV LED with hybrid-ODR is enhanced by 18.5% compared with Al/Al2O3-based ODR. It is because the diamond can transform the evanescent wave in Al2O3 into the propagating light wave in diamond, thereby preventing effective excitation of the surface plasmon polariton (SPP) on the surface of the metal Al. Moreover, the Brewster's angle effect causes the TM-polarized light in diamond to propagate effectively into AlGaN. Furthermore, decreasing the total thickness of the dielectric layer also improves the scattering effect of the inclined sidewall. However, the utilization of hybrid-ODR results in a slight reduction in the LEE for transverse electric (TE) polarized light because the light is confined to the diamond layer and eventually absorbed by the metal Al.

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.

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.

Schottky-contact intrinsic current blocking layer for high efficiency AlGaInP-based red mini-LEDs

AlGaInP-based red light emitting diodes (LEDs) are considered as promising light sources in future full-color displays. At present, vertical chip configuration is still the mainstream device structure of AlGaInP-based red LEDs. However, current crowding around p-electrode severely hinders an efficient improvement. Here, we propose a Schottky-contact current blocking layer (SCBL) to enhance current spreading and to improve light extraction efficiency of AlGaInP-based red vertical miniaturized LEDs (mini-LEDs). By utilizing the Schottky contact between ITO and p-GaP, the SCBL can hinder current crowding around the p-electrode. The current is forced to inject into an active region through a p-GaP+ ohmic contact layer, avoiding light absorption by p-electrode. Through the transfer length method, the Schottky contact characteristics between the ITO and p-GaP as well as the ohmic contact characteristics between ITO and p-GaP+ are demonstrated. Benefiting from superior current spreading and improved light extraction, a mini-LED with SCBL realizes an enhancement of 31.8% in external quantum efficiency (EQE) at 20 mA in comparison with a mini-LED without SCBL.

High Power Pulsed LED Driver for Vibration Measurements

Vibration measurements pose specific experimental challenges to be faced. In particular, optical methods can be used to obtain full-field vibration information. In this scenario, stereo-camera systems can be developed to obtain 3D displacement measurements. As vibration frequency increases, the common approach is to reduce camera exposure time to avoid blurred images, which can lead to under-exposed images and data loss, as well as issues with the synchronization of the stereo pair. Both of these problems can be solved by using high-intensity light pulses, which can produce high-quality images and guarantee camera synchronization since data is saved by both cameras only during the short-time light pulse. To this extent, high-power Light-Emitting Diodes (LEDs) can be used, but even if the LED itself can have a fast response time, specific electronic drivers are needed to ensure the desired timing of the light pulse. In this paper, a circuit is specifically designed to achieve high-intensity short-time light pulses in the range of 1 µs. A prototype of the designed board was assembled and tested to check its capability to respect the specification. Three different measurement methods are proposed and validated to achieve short-time light pulse measurements: shunt voltage measurement, direct photodiode measurement with a low-cost sensor, and indirect pulse measurement through a low-frame-rate digital camera.

Effectiveness of light-emitting diodes for arsenic and mercury accumulation by Ceratophyllum demersum L.: An innovative advancement in phytoremediation technology

Light Emitting Diodes (LEDs) have emerged as a tool with great potential in the field of phytoremediation, offering a novel approach to enhance the efficiency of plant-based remediation techniques. In this work investigated the influence of LEDs on the phytoremediation of arsenic (As) and mercury (Hg) by Ceratophyllum demersum L., propagated using tissue culture methods. In addition, the biochemical properties of the plants exposed to metal toxicity were examined. Phytoremediation experiments employed concentrations of As (0.01-1.0 mg/L) and Hg (0.002-0.2 mg/L), with application periods set at 1, 7, 14, and 21 days. In addition to white, red and blue LEDs, white fluorescent light was used for control purposes in the investigations. A positive correlation was observed between higher metal concentrations, extended exposure times, and increased metal accumulation in the plants. Red LED light yielded the highest level of heavy metal accumulation, while white fluorescent light resulted in the lowest accumulation level. Examination of the biochemical parameters of the plants, including photosynthetic pigment levels, protein quantities, and lipid peroxidation, revealed a pronouncedly enhanced performance in specimens subjected to red and blue LED illumination, surpassing outcomes observed in other light treatments. The findings of this study introduce innovative avenues for the effective utilization of red and blue LED lights in the realm of phytoremediation research. Thus, the interaction between LEDs, tissue culture, and the phytoremediation process could lead to synergistic effects that contribute to more effective and sustainable remediation strategies.

Enhancing light extraction efficiency of the inclined-sidewall-shaped DUV micro-LED array by hybridizing a nanopatterned sapphire substrate and an air-cavity reflector

In this work, we hybridize an air cavity reflector and a nanopatterned sapphire substrate (NPSS) for making an inclined-sidewall-shaped deep ultraviolet micro light-emitting diode (DUV micro-LED) array to enhance the light extraction efficiency (LEE). A cost-effective hybrid photolithography process involving positive and negative photoresist (PR) is explored to fabricate air-cavity reflectors. The experimental results demonstrate a 9.88% increase in the optical power for the DUV micro-LED array with a bottom air-cavity reflector when compared with the conventional DUV micro-LED array with only a sidewall metal reflector. The bottom air-cavity reflector significantly contributes to the reduction of the light absorption and provides more escape paths for light, which in turn increases the LEE. Our investigations also report that such a designed air-cavity reflector exhibits a more pronounced impact on small-size micro-LED arrays, because more photons can propagate into escape cones by experiencing fewer scattering events from the air-cavity structure. Furthermore, the NPSS can enlarge the escape cone and serve as scattering centers to eliminate the waveguiding effect, which further enables the improved LEE for the DUV micro-LED array with an air-cavity reflector.

Improvement of light extraction efficiency in AlGaInP-based vertical miniaturized-light-emitting diodes via surface texturing

AlGaInP-based light-emitting diodes (LEDs) suffer from a low external quantum efficiency (EQE), which is mainly restrained by the poor light extraction efficiency. Here, we demonstrate AlGaInP-based vertical miniaturized-LEDs (mini-LEDs) with a porous n-AlGaInP surface using a wet etching process to boost light extraction. We investigated the effects of etching time on the surface morphology of the porous n-AlGaInP surface. We found that as the etching time is prolonged, the density of pores increases initially and decreases subsequently. In comparison with the vertical mini-LED with a smooth n-AlGaInP surface, the vertical mini-LEDs with the porous n-AlGaInP surface reveal improvement in light output power and EQE, meanwhile, without the deterioration of electrical performance. The highest improvement of 38.9% in EQE measured at 20 mA is observed from the vertical mini-LED with the maximum density of the pores. Utilizing a three-dimensional finite-difference time-domain method, we reveal the underlying mechanisms of improved performance, which are associated with suppressed total internal reflection and efficient light scattering effect of the pores.

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.

Investigation of highly reflective p-electrodes for AlGaN-based deep-ultraviolet light-emitting diodes

We proposed a "Ni sacrifice" method to fabricate Al-based highly reflective p-electrode in the ultraviolet spectral region for AlGaN-based deep-ultraviolet light-emitting diodes (DUV-LEDs). The "Ni sacrifice" p-electrode could have a high optical reflectivity of around 90% at the DUV spectral region below 300 nm. Compared to Ni/Au, indium tin oxide (ITO), and Pd p-contacts, the "Ni sacrifice" led to a higher resistivity of p-contacts and a slightly higher operated voltage of the DUV-LEDs (within 0.6 V at 20 mA). Although the electrical performance was degraded slightly, the light output power and external quantum efficiency of the DUV-LEDs could be improved by utilizing the "Ni sacrifice" p-electrode. Besides, we introduced a grid of vias in the device mesa and reduced the diameter of the vias to achieve an enhanced peak external quantum efficiency (EQE) up to 1.73%. And the wall-plug efficiency (WPE) of DUV-LEDs with a "Ni sacrifice" p-electrode was higher than that of Ni/Au p-electrode DUV-LEDs at low currents. These results highlight the great potential of the proposed "Ni sacrifice" reflective p-electrode for use in DUV-LEDs.

Uniformity improvement of a mini-LED backlight by a quantum-dot color conversion film with nonuniform thickness

Mini-LED backlights energized by quantum-dot color conversion (QDCC) hold great potential for technological breakthroughs of liquid crystal displays. However, luminance uniformity issues should still be urgently solved owing to the large interval of direct-lit mini-LEDs, especially when covering with a QDCC film (QDCCF) with uniform thickness. Herein, we propose a uniformity improvement approach of mini-LED backlights by employing a QDCCF with nonuniform thickness based on the Lambertian distribution of mini-LEDs, which is demonstrated by screen-printing preparation and ray-tracing simulation. Experimental results show that the luminance uniformity of the nonuniform QDCCF can reach 89.91%, which is 24.92% higher than the uniform one. Ray-tracing simulation further elaborates the mechanism of this significant improvement. Finally, by employing this nonuniform QDCCF, a mini-LED backlight prototype is assembled and achieves high uniformity of 92.15%, good white balance with color coordinates of (0.3482, 0.3137), and high color gamut of 109% NTSC. This work should shed some new light on mini-LED-based display technology.

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.

Nanoimprinted patterned sapphire with silica array for efficient InGaN-based green mini-LEDs

Here, we propose nanoimprinted patterned sapphire with a silica array (PSSA) with the aim to promote the efficiency of InGaN-based green (∼520 nm) mini-LEDs. According to x-ray diffraction measurements, the threading dislocation density of GaN epitaxial layers grown on nanoimprinted PSSA demonstrates a pronounced reduction compared with the epilayers on the conventional patterned sapphire substrate (PSS). Consequently, a mini-LED on PSSA exhibits a significantly boosted light output power (LOP) in comparison to a mini-LED on PSS. At 10 mA, the LOP of the mini-LED on PSS is 6.0 mW, and this is further improved to 6.8 mW for the mini-LED on PSSA. Moreover, the peak external quantum efficiencies of the mini-LEDs on PSS and PSSA are 41% and 47%, respectively. A three-dimensional (3D) finite-difference time-domain simulation demonstrates that the PSSA contributes enhanced light extraction for photons emitted from the active region. It is also highly feasible to use this nanoimprinted PSSA technology in red and blue mini-LEDs for the realization of full-color displays.

Comparative Analysis of Optoelectrical Performance in Laser Lift-Off Process for GaN-Based Green Micro-LED Arrays

This work explores the pivotal role of laser lift-off (LLO) as a vital production process in facilitating the integration of Micro-LEDs into display modules. We specifically investigate the LLO process applied to high-performance gallium nitride (GaN)-based green Micro-LED arrays, featuring a pixel size of 20 × 38 μm on a patterned sapphire substrate (PSS). Scanning electron microscopy (SEM) observations demonstrate the preservation of the GaN film and sapphire substrate, with no discernible damage. We conduct a comprehensive analysis of the optoelectrical properties of the Micro-LEDs both before and after the LLO process, revealing significant enhancements in light output power (LOP) and external quantum efficiency (EQE). These improvements are attributed to more effective light extraction from the remaining patterns on the GaN backside surface. Furthermore, we examine the electroluminescence spectra of the Micro-LEDs under varying current conditions, revealing a slight change in peak wavelength and an approximate 10% decrease in the full width at half maximum (FWHM), indicating improved color purity. The current-voltage (I-V) curves obtained demonstrate the unchanged forward voltage at 2.17 V after the LLO process. Our findings emphasize the efficacy of LLO in optimizing the performance and color quality of Micro-LEDs, showcasing their potential for seamless integration into advanced display technologies.

"Blue-free" orange ZnO-related light-emitting diode based on a natural interface layer of Ga2O3 and ZnGa2O4

To fabricate a ZnO-related light-emitting diode (LED) with zero emission at blue wavelengths ("blue-free"), an ingenious strategy is employed. Specifically, for the first time to the best of our knowledge, a natural oxide interface layer, possessing remarkable visible emission potential, is introduced into the Au/i-ZnO/n-GaN metal-insulator-semiconductor (MIS) structure. The unique Au/i-ZnO/interface layer/n-GaN structure successfully eliminated the harmful blue emissions (400-500 nm) from the ZnO film, and the remarkable orange electroluminescence is mainly attributed to the impact ionization process of the natural interface layer at high electric field. It is worth mentioning that the device achieved ultra-low color temperature (2101 K) and excellent color rendering index (92.8) under electrical injection, indicating that the device could fulfill the requirements of electronic display systems and general illumination, and might even play unexpected roles in special lighting domains. The results obtained provide a novel and effective strategy for the design and preparation of ZnO-related LEDs.

Improved crystal quality and enhanced optical performance of GaN enabled by ion implantation induced high-quality nucleation

Hetero-epitaxial growth of GaN often leads to high density of threading dislocations, which poses a significant challenge to the promotion of the performance of GaN-based devices. In this study, we address this issue by utilizing an Al-ion implantation pretreatment on sapphire substrates, which induces high-quality regularly arranged nucleation and promotes the crystal quality of GaN. Specifically, we demonstrate that an Al-ion dose of 10¹³cm^-2 leads to a reduction of full width at half maximum values of (002)/(102) plane X-ray rocking curves from 204.7/340.9 arcsec to 187.0/259.5 arcsec. Furthermore, a systematic investigation of GaN film grown on the sapphire substrate with various Al-ion doses is also performed, and the nucleation layer growth evolution on different sapphire substrates is analyzed. As confirmed by the atomic force microscope results of the nucleation layer, the ion implantation induced high-quality nucleation is demonstrated, which results in the improved crystal quality of the as-grown GaN films. Transmission electron microscope measurement also proves the dislocation suppression through this method. In addition, the GaN-based light-emitting diodes (LEDs) were also fabricated based on the as-grown GaN template and the electrical properties are analyzed. The wall-plug efficiency at 20 mA has risen from 30.7% to 37.4% of LEDs with Al-ion implantation sapphire substrate at a dose of 10¹³cm^-2. This innovative technique is effective in the promotion of GaN quality, which can be a promising high-quality template for LEDs and electronic devices.

Fabrication of Silver Nanobowl Arrays on Patterned Sapphire Substrate for Surface-Enhanced Raman Scattering

The current article discusses surface-enhanced Raman spectroscopy (SERS) as a powerful technique for detecting molecules or ions by analyzing their molecular vibration signals for fingerprint peak recognition. We utilized a patterned sapphire substrate (PSS) featuring periodic micron cone arrays. Subsequently, we prepared a three-dimensional (3D) PSS-loaded regular Ag nanobowls (AgNBs) array using self-assembly and surface galvanic displacement reactions based on polystyrene (PS) nanospheres. The SERS performance and structure of the nanobowl arrays were optimized by manipulating the reaction time. We discovered that the PSS substrates featuring periodic patterns exhibited superior light-trapping effects compared to the planar substrates. The SERS performance of the prepared AgNBs-PSS substrates was tested under the optimized experimental parameters with 4-mercaptobenzoic acid (4-MBA) as the probe molecule, and the enhancement factor (EF) was calculated to be 8.96 × 10⁴. Finite-difference time-domain (FDTD) simulations were conducted to explain that the AgNBs arrays' hot spots were distributed at the bowl wall locations. Overall, the current research offers a potential route for developing high-performance, low-cost 3D SERS substrates.

MicroLED/LED electro-optical integration techniques for non-display applications

MicroLEDs offer an extraordinary combination of high luminance, high energy efficiency, low cost, and long lifetime. These characteristics are highly desirable in various applications, but their usage has, to date, been primarily focused toward next-generation display technologies. Applications of microLEDs in other technologies, such as projector systems, computational imaging, communication systems, or neural stimulation, have been limited. In non-display applications which use microLEDs as light sources, modifications in key electrical and optical characteristics such as external efficiency, output beam shape, modulation bandwidth, light output power, and emission wavelengths are often needed for optimum performance. A number of advanced fabrication and processing techniques have been used to achieve these electro-optical characteristics in microLEDs. In this article, we review the non-display application areas of the microLEDs, the distinct opto-electrical characteristics required for these applications, and techniques that integrate the optical and electrical components on the microLEDs to improve system-level efficacy and performance.

Improved Crystallinity of Annealed 0002 AlN Films on Sapphire Substrate

AlN is a piezoelectric material used in telecommunication applications due to its high surface acoustic wave (SAW) velocity, stability, and mechanical strength. Its performance is linked to film quality, and one method to achieve high-quality films goes through the process of annealing. Consequently, c-orientated AlN film with a thickness of 1.1 μm deposited on sapphire was annealed at temperatures of 1100 °C and 1150 °C in a N₂ controlled atmosphere. This was compared to annealing at 1100 °C, 1450 °C, and 1700 °C with N₂ flow in an open atmosphere environment. Sample rotation studies revealed a significant impact on the ⍵-2θ X-ray rocking curve. A slight variation in the film crystallinity across the wafer was observed. After the annealing, it was found that the lattice parameter c was increased by up to 2%, whereas the screw dislocation density dropped from 3.31 × 10¹⁰ to 0.478 × 10¹⁰ cm^-2, and the full width at half maximum (FWHM) of reflection (0002) was reduced from 1.16° to 0.41° at 1450 °C. It was shown that annealing in a N₂-controlled atmosphere plays a major role in reducing the oxidation of the AlN film, which is important for acoustic wave devices where the electrodes are placed directly on the piezoelectric substrate. The face-to-face arrangement of the samples could further reduce this oxidation effect.

Deciphering the photophysical properties of near-infrared quantum emitters in AlGaN films by transition dynamics

Point defects in wide bandgap III-nitride semiconductors have been recently reported to be one kind of the most promising near-infrared (NIR) quantum emitters operating at room temperature (RT). But the identification of the point defect species and the energy level structures as well as the transition dynamics remain unclear. Here, the photophysical properties of single-photon emission from point defects in AlGaN films are investigated in detail. According to the first-principles calculations, a three-level model was established to explain the transition dynamics of the quantum emitters. An anti-site nitrogen vacancy complex (V_NN_Ga) was demonstrated to be the most likely origin of the measured emitter since the calculated zero-phonon line (ZPL) and the lifetime of V_NN_Ga in the AlGaN film coincide well with the experimental results. Our results provide new insights into the optical properties and energy level structures of quantum emission from point defects in AlGaN films at RT and establish the foundation for future AlGaN-based on-chip quantum technologies.

Unexpectedly Simultaneous Increase in Wavelength and Output Power of Yellow LEDs Based on Staggered Quantum Wells by TMIn Flux Modulation

Pursuing efficient long-wavelength InGaN LED has been a troublesome issue to be solved, which forms interesting subjects for fundamental research, but finds also motivation in extensive applications. Here, we investigate the effect of TMIn (trimethylindium) flux variation for growing bandgap-engineered staggered quantum wells (QWs) on corresponding LED properties and demonstrate the unexpectedly simultaneous increase in light output power (LOP) and emission wavelength. At 20 mA, LEDs based on staggered QWs grown under low flux show an increase of 28% in LOP and longer wavelength compared to that under high flux. The experimental results reveal that TMIn flux affects crystalline quality and indium composition of epilayers. Under high TMIn flux, high in-plane strain exists between adjacent layers, accompanied by the composition pulling effect, which reduces indium incorporation for the following staggered QW growth and hinders realization of yellow light emission. According to simulation results, low-flux-grown staggered QWs contribute to increased carrier wavefunction overlap as well as enhanced electric field. Notably, the former enables high LOP, while the latter results in emissions towards long wavelength, promising to solve an ever-present concern that LED performance deteriorates with increasing emission wavelength. Therefore, this work shows great significance in thoroughly understanding growth conditions for bandgap-engineered staggered QW structures, which offers a facile solution to achieve efficient long-wavelength optoelectronics devices.

Resonant Lasing Emission in Undoped and Mg-Doped Gallium Nitride Thin Films on Interfacial Periodic Patterned Sapphire Substrates

In this work, low-threshold resonant lasing emission was investigated in undoped and Mg-doped GaN thin films on interfacial designed sapphire substrates. The scattering cross-section of the periodic resonant structure was evaluated by using the finite difference time domain (FDTD) method and was found to be beneficial for reducing the threshold and enhancing the resonant lasing emission within the periodic structures. Compared with undoped and Si-doped GaN thin films, p-type Mg-doped GaN thin films demonstrated a better lasing emission performance. The lasing energy level system and defect densities played vital roles in the lasing emission. This work is beneficial to the realization of multifunctional applications in optoelectronic devices.

Improving the performance for flip-chip AlGaN-based deep ultraviolet light-emitting diodes using surface textured Ga-face n-AlGaN

Low light extraction efficiency (LEE), high forward voltage and severe self-heating effect greatly affect the performance for AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs). In this work, surface-textured Ga-face n-AlGaN is fabricated low-costly using self-assembled SiO₂ nanosphere as hard mask. The experimental results manifest that when compared with conventional DUV LEDs, the optical power, the forward voltage and the thermal characteristics for the DUV LEDs with surface-textured Ga-face n-AlGaN are improved obviously. It is because the surface-textured Ga-face n-AlGaN between mesa and the n-electrode can be used as the scattering center for trapped light, and this leads to the enhanced LEE. Furthermore, thanks to the surface-textured n-AlGaN under the n-electrode, the n-type ohmic contact area can be increased effectively. Therefore, the n-type ohmic contact resistance can be reduced and the better heat dissipation can be attained for the proposed flip-chip DUV LED.