Light Output Power Research Articles

Ion implantation induced nucleation and epitaxial growth of high-quality AlN

AlN materials have a wide range of applications in the fields of optoelectronic, power electronic, and radio frequency. However, the significant lattice mismatch and thermal mismatch between heteroepitaxial AlN and its substrate lead to a high threading dislocation (TD) density, thereby degrading the performance of device. In this work, we introduce a novel, cost-effective, and stable approach to epitaxially growing AlN. We inject different doses of nitrogen ions into nano patterned sapphire substrates, and then deposit the AlN layers by using metal-organic chemical vapor deposition. Ultraviolet light-emitting diode (UV-LED) with a luminescence wavelength of 395 nm is fabricated on it, and the optoelectronic properties are evaluated. Compared with the sample prepared by the traditional method, the sample injected with N ions at a dose of 1×10<sup>13</sup> cm<sup>–2</sup> exhibits an 82% reduction in screw TD density, the lowest surface roughness, and a 52% increase in photoluminescence intensity. It can be seen that appropriate dose of N ion implantation can promote the lateral growth and merging process in AlN heteroepitaxy. This is due to the fact that the process of implantation of N ions can suppress the tilt and twist of the nucleation islands, effectively reducing the density of TDs in AlN. Furthermore, in comparison with the controlled LED, the LED prepared on the high quality AlN template increases 63.8% and 61.7% in light output power and wall plug efficiency, respectively. The observed enhancement in device performance is attributed to the TD density of the epitaxial layer decreasing, which effectively reduces the nonradiative recombination centers. In summary, this study indicates that the ion implantation can significantly improve the quality of epitaxial AlN, thereby facilitating the development of high-performance AlN-based UV-LEDs.

Influence of period number of superlattice electron barrier layer on the performance of AlGaN-based deep ultraviolet LED

The development of AlGaN-based deep ultraviolet light emitting diodes (DUV-LEDs) is currently limited by poor external quantum efficiency (EQE) and wall-plug efficiency (WPE). Internal quantum efficiency (IQE), as an important component of EQE, plays a crucial role in improving the performance of DUV-LEDs. The IQE is related to the carrier injection efficiency and the radiation recombination rate in the active region. In order to improve the IQE of AlGaN-based DUV-LEDs, this work proposes a scheme to optimize the period number of superlattice electron barrier layer (SL-EBL) to achieve better carrier injection efficiency and confinement capability. The effect of the period number of SL-EBL on the luminous efficiency, reliability and carrier recombination mechanism of AlGaN-based DUV-LEDs with an emission wavelength of 273 nm are investigated. The experimental results show that the light output power (LOP), external quantum efficiency (EQE) and wall-plug efficiency (WPE) of the DUV-LEDs tend to first increase and then decrease with the period number of SL-EBL increasing, while the leakage current decreases and the reliability is enhanced. The maximum EQE and WPE of the DUV-LED are 3.5% and 3.2%, respectively, at an injection current of 7.5 mA when the period number of SL-EBL is fixed at 7 (the thickness is 28 nm). Meanwhile, the numerical simulation results show that the electron potential barrier height is enhanced with the period number of SL-EBL increasing, and the variation of the hole potential barrier height is negligible. Therefore, increasing the period number of SL-EBL is beneficial to shielding the dislocations and suppressing the leakage of electrons into the p-type layer, which improves the luminous efficiency and reliability of DUV-LEDs. However, when the period number of SL-EBL exceeds 7, the excessively thick hole potential barrier prevents the holes from entering into the activation region and reduces the radiative recombination efficiency. Therefore, EQE and WPE will show an inflection point with the variation of the period number of SL-EBL. In addition, to investigate the carrier recombination mechanism of the active region, the experimental EQE curves are fitted by the ABC model as well as the different slopes in logarithmic light output power-current (<i>L-I</i> ) curves are calculated after aging. It can be found that increasing the period number of SL-EBL can effectively suppress the non-radiative combination of carriers in the active region. This investigation can provide an alternative way to enhance the photoelectric performance of DUV-LEDs.

On the integrated p-type region free of electron blocking layer for AlGaN-based deep-ultraviolet light emitting diodes

AlGaN-based deep-ultraviolet light emitting diodes (DUV-LEDs) with thin p-GaN capping layer have been one of the most promising configurations, thanks to their excellent light extraction potential, which are, however, generally accompanied by insufficient hole supply. In this work, multi-graded p-AlGaN layers are adopted as an integrated p-type region, in which the electron blocking layer (EBL) is taken off to promote the carrier transport. The experimental results show that both the operation voltage and light output power of the DUV-LEDs are improved compared to the traditional ones, leading to a remarkable increase (by 114%) in their wall-plug efficiency. Further analysis confirms that the integrated p-type region serving as the hole supply layer helps holes to avoid the barrier between the p-GaN and integrated p-type region during their transport, and the absence of the EBL further eliminates part of the obstacles for hole drifting, giving rise to an enhanced hole concentration in quantum wells. Meanwhile, thanks to the modulation of the negative polarization induced bulk charges for conduction band, which is introduced within the integrated p-type region, the electron leakage is effectively suppressed even without the EBL, thus improving the device performance dramatically.

Determining the Turn-On Voltage of GaN-Based Light-Emitting Diodes: From Near-Ultraviolet to Green Spectra

This work investigates the turn-on voltage (V on) estimation methods from experimental data of InGaN-based multiple-quantum-well light-emitting diodes (LEDs) with emission spectra from near-ultraviolet to green wavelengths. Three different methods are examined: fixed current intercept, linear fitting of current-voltage curve, and the minimum ideality factor (n ideal). The voltage at minimum n ideal consistently represents the on-state of LEDs since it describes the carrier recombination processes reflected in light output power (LOP) and the external quantum efficiency (EQE). Beyond the voltage at minimum n ideal, samples’ LOPs start to increase abruptly, with the EQEs reaching ≥ 54% of the peak EQE values. For V on determined by other methods, samples’LOPs are already turned on and the EQEs exceed their peak value. Therefore, using the minimum n ideal is a suitable method for determining Von of an LED.

Study on the Influence of KOH Wet Treatment on Red μLEDs

InGaN-based red micro-light-emitting diodes (µLEDs) of different sizes were prepared in this work. The red GaN epilayers were grown on 4-inch sapphire substrates through metal-organic chemical vapor deposition (MOCVD). Etching, sidewall treatment, and p- and n-contact deposition were involved in the fabrication process. Initially, the etching process would cause undesirable damage to the GaN sidewalls, which leads to an increase in leakage current. Hence, we employed KOH wet treatment to rectify the defects on the sidewalls and conducted a comparative and systematic analysis of electrical as well as optical properties. We observed that the µLEDs with a size of 5 µm exhibited a substantial leakage current, which was effectively mitigated by the application of KOH wet treatment. In terms of optical performance, the arrays with KOH demonstrated improved light output power (LOP). Additionally, while photoelectric performance exhibited a decline with increased current density, the devices treated with KOH consistently outperformed their counterparts in terms of optoelectronic efficiency. It is noteworthy that the optimized devices displayed enhanced photoelectric characteristics without significantly altering their original peak wavelength and FWHM. Our findings point to the elimination of surface non-radiative recombination by KOH wet treatment, thereby enhancing the performance of small-sized red µLEDs, which has significant potential in realizing full-color micro-displays in near-eye projection applications.

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.

Improved external quantum efficiency of deep UV LEDs with an ultra-thin AlGaN last quantum barrier by controlling the desorption-kinetics process

An ultra-thin AlGaN last quantum barrier (LQB) with a high Al content is effective in enhancing the hole injection in AlGaN-based deep UV LEDs (DUV-LEDs). However, it is very challenging to realize the ultra-thin LQB by the metalorganic chemical vapor deposition technique. In this work, a well-defined 1.0 nm thick AlGaN LQB with unintentionally high Al content was achieved by controlling the surface desorption kinetics for the growth of multiple quantum wells. The light output power of the DUV-LEDs with 1 nm thick LQB was increased by 44% to 7.16 mW at 40 mA, and the peak external quantum efficiency reached 4.04%.

Enhancing current spreading performance of GaN-based ultraviolet light-emitting diode chips through periodic doping composite barrier structures

In response to the issue of current crowding in GaN-based ultraviolet (UV) light-emitting diode (LED) chips, a periodic doping composite barrier structure is proposed. The results of the optical intensity distribution demonstrate that the device with the periodic doping composite barrier structure exhibits a more uniform light distribution compared to the device with traditional structures. Moreover, when the injected current is 20 mA, the light output power (LOP) and wall-plug efficiency (WPE) are improved by 11.99% and 13.5%, respectively. Experimental and simulation results indicate that the periodic doping composite barrier structure enhances the current spreading performance through two mechanisms: (1) the introduction of an electron barrier layer enhances lateral current transport; (2) increase the ratio of longitudinal to transverse resistivity of the material to further enhances current spreading performance.

Advanced Design of a III‐Nitride Light‐Emitting Diode via Machine Learning

AbstractGallium nitride (GaN)‐based light‐emitting diodes (LEDs) have obtained great market success in the past 20 years. However, the traditional research paradigm, i.e., experimental trial‐and‐error method, no longer adapts to the industry development. In this work, an efficient approach is demonstrated to design and optimize GaN‐based LED structures via machine learning (ML). By using the dataset of GaN‐based LED structures over the past decade to train four typical ML models, it is found that the convolutional neural network (CNN) provides the most accurate prediction, with a root mean square error (RMSE) of 1.03% for internal quantum efficiency (IQE) and 11.98 W cm−2 for light output power density (LOPD). Based on the CNN model, 1) the feature importance analysis is adopted to reveal the critical features for LED performance; 2) the predicted trends of IQE and LOPD match well with the physical mechanism, being consistent with the experimental and simulation results; and 3) a high‐throughput screening is demonstrated to predict the properties of over 20 000 structures within seconds to obtain high efficiency LED structures. This ML‐based LED design method enables direct guiding of the LED structure optimization in terms of key parameter selection during manufacturing and greatly accelerates the development cycle of GaN‐based LEDs.

Bridging optical fiber communication and underwater wireless optical communication via third harmonic generation.

In this paper, we propose an optical module, consisting of an Erbium/Ytterbium co-doped fiber amplifier (EYDFA) and a cascaded periodically poled lithium niobate (cascaded-PPLN), to bridge the conventional telecommunication and the emerging underwater wireless optical communication (UWOC). Compared with using two discrete crystals to achieve the third harmonic generation (THG), using a cascaded crystal simplifies the optical system. Under a fundamental power of 5 W at 1550 nm, we have generated an optical power of 6.54 mW at 516 nm, corresponding to a conversion efficiency of 0.1308%. Furthermore, we added a 5-km single-mode fiber (SMF) before the EYDFA, and by adjusting the seed laser power, we successfully maintained the efficiency of the THG process and the output power of the green light. Afterwards, the nonlinearity of the THG process is analyzed, and a simplified nonlinear pre-compensation method has been proposed to tailor the 4-pulse amplitude modulation (PAM4) signals. In such case, the bit error rate (BER) of the modified PAM4 (m-PAM4) can reduce by 69.3% at a data rate of 12 Gbps. Finally, we demonstrate the practicality of our proposed system by achieving a 7-m UWOC transmission in a water tank at a data rate of 13.46 Gbps in an optical dark room. This result demonstrates the feasibility of the hybrid fiber/UWOC system, highlighting its potential for practical implementation.

Efficient employment of VCSEL light sources in high speed dispersion compensation system

Abstract This paper demonstrated the efficient employment of vertical cavity surface emitting laser (VCSEL) light sources in high speed dispersion compensation systems. VCSEL are compared with CW laser and distributed feedback laser in the modulated light output power, signal quality factor and minimum data error rates through the fiber system. Light signal base band modulated power is clarified with spectral frequencies after LiNb MZ modulators. Light signal base band modulated amplitude is simulated against base band time frequencies after LiNb MZ modulators. The total light signal index base band modulated power/amplitude value is estimated after LiNb MZ modulators for various light sources. The light signal base band modulated power is demonstrated with both spectral frequencies and time interval after fiber loop control length with compensation system. Total light signal index base band modulated power/amplitude value is estimated numerically after fiber loop control length with compensation system. The modulated electronic signal base band modulated power is indicated with spectral frequencies after PIN receiver. The light signal base band modulated amplitude is studied clearly against base band time frequencies after PIN receiver. The signal quality factor, BER, and modulated lighted output power are clarified and sketched against fiber system lengths for various light sources.

Monolithic integrated all-GaN-based µLED display by selective area regrowth.

This work demonstrates an all-GaN-based µLED display with monolithic integrated HEMT and µLED pixels using the selective area regrowth method. The monochrome µLED-HEMT display has a resolution of 20 × 20 and a pixel pitch of 80 µm. With the optimized regrowth pattern, the µLED-HEMT achieves a maximum light output power of 36.2 W/cm2 and a peak EQE of 3.36%, mainly due to the improved crystal quality of regrown µLED. TMAH treatment and Al2O3 surface passivation are also performed to minimize the impact of nonradiative recombination caused by the dry etching damage. With a custom-designed driving circuit board, images of "HKUST" are successfully shown on the µLED-HEMT display.

Increasing Eye Injuries Following Exposure to Laser Beams, the Silent Epidemic

Increasing Eye Injuries Following Exposure to Laser Beams, the Silent Epidemic

Improved Efficiency Droop of 370 nm UV LEDs with ITO/Au/ITO Structure

In this paper, a method of indium-tin oxide/Au/indium-tin oxide (IAI) structure as transparent conductive layers (TCLs) to improve the current spreading and suppress efficiency droop of ultraviolet light-emitting diodes (UV-LEDs) is investigated. The morphology and distribution of Au clusters formed by annealing are investigate by atomic force microscope and scanning electron microscope. Compared with the UV-LED without Au clusters, the forward voltage of UV-LED is reduced by 5.9% at 20 mA, light output power and wall plug efficiency increase by 64.2% and 84.2% at 300 mA for the UV-LED with IAI structure as the thickness of the Au interlayer is 1 nm. In addition, light emission distribution results show that by inserting Au interlayer, current distributes more uniform and the current spreading characteristics of UV-LED are improved. Ultimately, efficiency droop of the LED has been suppressed.

Effect of Template-Mediated Alumina Nanoparticle Morphology on Sapphire Wafer Production via Heat Exchange Method.

The sapphire crystal, the most commonly used LED substrate material, has excellent optical and chemical properties and has rapidly developed in recent years. However, the challenge of growing large-size sapphire crystals remains. This paper presents a novel approach using alumina nanoparticles synthesized with abietic acid as a template to enhance sapphire growth via the heat exchange method. This study explores the effects of temperature, time, and template amount on the structure and morphology of the synthesized alumina nanoparticles. The results show that the morphology of the raw material, particularly spherical alumina nanoparticles, positively affects the quality and yield stability of sapphire products. Furthermore, the light output power of GaN-based LED chips made with the experimentally fabricated sapphire substrate increased from 3.47 W/µm2 to 3.71 W/µm2, a 6.9% increase compared to commercially available sapphire substrates. This research highlights the potential of using abietic acid as a template for alumina nanoparticle synthesis and their application in sapphire growth for LED production.

Red InGaN Micro-LEDs on Silicon Substrates: Potential for Multicolor Display and Wavelength Division Multiplexing Visible Light Communication

Red micro light-emitting diodes (micro-LEDs) on silicon substrates are crucial for the realization of large-scale, high-quality, low-cost micro-LED displays, and are also beneficial for high-speed visible light communication (VLC). In this letter, micro-LEDs with different sizes were fabricated. The maximum light output power density of the 20 μm pixel can reach 3.36 W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , which is 14 times higher than that of the largest pixel (150 μm). By adjusting the pixel size and injection current, a direct color shift from red to green can be observed. Moreover, multicolor emission with uniform brightness can be realized by adjusting duty cycle, which has broad application prospect in monolithic, multicolor displays. Despite the significant blue shift phenomenon, the peak wavelength of all pixels is still greater than 630 nm at the current density of 100 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , which means that red light can be emitted in a large current density range to meet various application scenarios requiring different driving conditions. We have realized red, yellow and green micro-LED transmitters on Si substrate by changing the driving current density and modulation bandwidth up to 533.15 MHz is achieved when emitting green light. This is the report of red-emission micro-LEDs on Si substrates for visible light communication for the first time. And we proposed a proof-of-concept monolithic, multicolor wavelength division multiplexing scheme that achieved a total allowable transmission data rate of 2.35 Gbps. Such micro-LED is expected to be applied in multicolor displays, high-speed multi-channel VLC, color-tunable light sources, etc. In particular, due to its high integration and miniaturization, it is expected to be used in future practical application scenarios such as wearable communication devices and smartwatches.

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.

Enhanced light output power from AlGaN-based deep ultraviolet LEDs achieved by a four-in-one mesa structure

This study aimed to investigate the impact of mesa geometry on the light output characteristics of AlGaN-based 275 nm deep ultraviolet light-emitting diodes (DUV-LEDs). By dividing the original single-junction mesa into four parts and connecting them serially (four-in-one), high-voltage (HV) DUV-LEDs with rectangular, hexagonal, circular, triangular, and square submesas were realized, achieving significant enhancement of the light output power (LOP) and wall-plug efficiency (WPE). The LOP of HV DUV-LEDs with hexagonal submesas has been promoted substantially compared to that of the original DUV-LEDs. Among the investigated five different types of submesas, hexagonal-type HV DUV-LEDs can achieve the highest LOP and WPE due to the higher sidewall light extraction. Furthermore, it is also demonstrated that pulse current driving can reduce the self-heating effect of HV DUV-LEDs.

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.

A high SNR system for intensity demodulation of fiber Bragg grating based on phase-locked loop

The intensity demodulation technology of fiber Bragg gratings (FBG) is typically realized by detecting the output light power. Traditional methods, such as DC power detection, have a low signal-to-noise ratio and sensitivity. Thus, a novel intensity demodulation system based on the phase-locked loop is proposed in this study. Theoretical analysis and experiments show that the system has a high signal-to-noise ratio, linearity, and stability with a low-temperature sensitivity of the FBG sensor head. Compared to traditional light power detection, the proposed system increases the signal-to-noise ratio by more than 24 dB. According to the repeated temperature experimental results, the proposed sensor with an accuracy of ± 0.5 °C has a repeatability error of 1.8%, alinearity error of 3%, and ahysteresis error of 1.5% in the temperature range from 10 to 90 °C, which indicates that the system is of high-sensitivity and high-precision.