Role of surface treatments and localized surface plasmon nanoparticles on internal quantum efficiency of 800 nm diameter blue GaN/InGaN nano light emitting diodes

Kim T., Uthirakumar P., Cho Y., Lee I.

Shen M., Guo W., Chen J., Lin S., Li S., Lai S., Lu T., Huang M., Kuo H., Chen Z., Wu T.

Abstract The technique of double dielectric sidewall passivation could improve the electroluminescence characteristics of green mini-LED, including the leakage current, electroluminescence intensity and external quantum efficiency (EQE). According to the analysis of EQE with ABC + f(n) model, both the increment of size and the sidewall passivation could reduce the fraction of SRH recombination. Then, the results of time-resolved photoluminescence measurement indicated that the carrier lifetime of mini-LEDs with higher surface-volume ratio and sidewall passivation would be reduced. Moreover, the communication performances such as modulation bandwidth and frequency response of green mini-LEDs could also be improved by adopting double dielectric sidewall passivation, and the smaller sized green mini-LEDs could achieve higher modulation bandwidth and frequency response.

Hsu Y., Lin Y., Wu M., Kuo H.C., Horng R.

Zhu Z., Lei L., Lin T., Li L., Lin Z., Jiang H., Li G., Wang W.

GaN-based light-emitting diodes (LEDs) are the ideal light sources for visible light communication (VLC). However, both the low modulation bandwidth (MB) and unstable lighting output power (LOP) of LEDs at high current density restrict the further development of VLC. In this work, micro-LEDs ( $\mu $ LEDs) with embedded ${N}$ electrodes have been proposed, possessing high MB and remarkable stability. The as-prepared $\mu $ LEDs show a high MB of 240 MHz at 8.5 kA/cm2 and a small LOP aging rate of 6% under high temperature and humidity conditions due to the embedded electrode structure, which can effectively improve the uniformity of current spreading and increase the injection saturation current density, thereby effectively broadening the −3-dB MB and improving the LOP stability of $\mu $ LEDs in harsh environments with high temperature and high humidity. Such high MB and reliable $\mu $ LEDs shed light on a promising solution for industrial fabrication and chip arrays in underwater VLC systems.

Park J., Pristovsek M., Cai W., Cheong H., Tanaka A., Furusawa Y., Han D., Seong T., Amano H.

Abstract The sidewall condition is a key factor determining the performance of micro-light emitting diodes (μLEDs). In this study, we prepared equilateral triangular III-nitride blue μLEDs with exclusively m-plane sidewall surfaces to confirm the impact of sidewall conditions. It was found that inductively coupled plasma-reactive ion etching (ICP-RIE) caused surface damages to the sidewall and resulted in rough surface morphology. As confirmed by time-resolved photoluminescence (TRPL) and X-ray photoemission spectroscopy (XPS), tetramethylammonium hydroxide (TMAH) eliminated the etching damage and flattened the sidewall surface. After ICP-RIE, 100 µm2-micro-LEDs (µLEDs) yielded higher external quantum efficiency (EQE) than 400 µm2-µLEDs. However, after TMAH treatment, the peak EQE of 400 µm2-µLEDs increased by around 10% in the low current regime, whereas that of 100 µm2-µLEDs slightly decreased by around 3%. The EQE of the 100 µm2-µLED decreased after TMAH treatment although the internal quantum efficiency (IQE) increased. Further, the IQE of the 100 µm2-µLEDs before and after TMAH treatment was insignificant at temperatures below 150 K, above which it became considerable. Based on PL, XPS, scanning transmission electron microscopy, and scanning electron microscopy results, mechanisms for the size dependence of the EQE of µLEDs are explained in terms of non-radiative recombination rate and light extraction.

Lee I., Kim T., Polyakov A.Y., Chernykh A.V., Skorikov M.L., Yakimov E.B., Alexanyan L.A., Shchemerov I.V., Vasilev A.A., Pearton S.J.

A matrix of blue GaN/InGaN multi-quantum-well (MQW) micro-Light-Emitting Diodes (micro-LEDs) with diode dimensions ranging from 2 to 100 µm was prepared by masked dry etching and characterized by Photoluminescence (PL), Microcathodoluminescence (MCL), capacitance-voltage profiling in the dark and under monochromatic illumination, current-voltage measurements, admittance spectra, Deep Level Transient Spectroscopy with electrical (DLTS) and optical (ODLTS) injection. The changes observed in the PL, MCL spectra are due to the formation of deep hole traps at E v + 0.75 eV and electron traps at E c -1 eV in the sidewalls of the micro-LEDs. The former give rise to the red defect band peaked near 600 nm and contribute to the increase of tunneling and leakage current with decreasing the diode diameter. The latter are prominent centers of nonradiative recombination. • DLTS spectra demonstrate an increase of the density of deep acceptors at E v + 0.7 eV and E v + 0.75 eV as the diode diameter decreases from 100 to 30 µm. • These deep acceptors have attributed to either carbon on nitrogen site C N acceptors or to gallium vacancy complexes with shallow donors, V Ga -D. • Experiments with LED degradation point to these being responsible for enhanced tunneling and increased leakage current. Experiments favor the attribution of the deep acceptors to V Ga -D defects rather than to C N • This mechanism explains the strong decrease in external quantum efficiency of micro-LEDs for diameters below ~30 µm, as the effect of trap states in the sidewalls becomes dominant. • The reported observation could form a basis for improvement of EQE of GaN-based micro-LEDs.

Lai S., Lu T., Lin S., Lin Y., Lin G., Pan J., Zhuang Y., Lu Y., Lin Y., Kuo H., Chen Z., Wu T.

By adopting atomic layer deposition (ALD) sidewall passivation, the electroluminescence (EL) and communication performances of mini-light emitting diodes (mini-LEDs) of different sizes were improved. In particular, the most significant improvement in the electroluminescence properties of the external quantum efficiency (EQE) (a 7.2% increase), leakage current, and the communication properties of the modulation bandwidth (a 20% increase) transmission rate and bit error rate (BER) was found in the smallest mini-LEDs (80 $\mu \text{m}\,\,\times $ 120 $\mu \text{m}$ ). According to the results of time-resolved photoluminescence (TRPL) measurements, the carrier lifetime of the samples can be affected by both the size and ALD sidewall passivation. In addition, the effects of ALD sidewall passivation for visible-light communication (VLC) were demonstrated.

Sheen M., Ko Y., Kim D., Kim J., Byun J., Choi Y., Ha J., Yeon K.Y., Kim D., Jung J., Choi J., Kim R., Yoo J., Kim I., Joo C., et. al.

Indium gallium nitride (InGaN)-based micro-LEDs (μLEDs) are suitable for meeting ever-increasing demands for high-performance displays owing to their high efficiency, brightness and stability1–5. However, μLEDs have a large problem in that the external quantum efficiency (EQE) decreases with the size reduction6–9. Here we demonstrate a blue InGaN/GaN multiple quantum well (MQW) nanorod-LED (nLED) with high EQE. To overcome the size-dependent EQE reduction problem8,9, we studied the interaction between the GaN surface and the sidewall passivation layer through various analyses. Minimizing the point defects created during the passivation process is crucial to manufacturing high-performance nLEDs. Notably, the sol–gel method is advantageous for the passivation because SiO2 nanoparticles are adsorbed on the GaN surface, thereby minimizing its atomic interactions. The fabricated nLEDs showed an EQE of 20.2 ± 0.6%, the highest EQE value ever reported for the LED in the nanoscale. This work opens the way for manufacturing self-emissive nLED displays that can become an enabling technology for next-generation displays. Using a sol–gel passivation method, the fabrication of blue InGaN nanorod-LEDs with the highest external quantum efficiency value ever reported for LEDs in the nanoscale is demonstrated.

Yao S., Chai H., Lei L., Zhu Z., Li G., Wang W.

Over the past few decades, GaN-based LEDs have been widely used in solid-state lighting (SSL) due to their energy-saving and long-lifetime characteristics. However, the limited modulation bandwidth of conventional commercial LED chips limits their application in a high-speed visible light communication (VLC) system. Herein, this work designs vertical parallel micro-LED arrays with different array numbers to develop LED sources for communication and lighting. The as-prepared micro-LEDs in a 2 × 2 array reveal a high modulation bandwidth of ∼785 MHz at 6.67 kA/cm2. Meanwhile, the light output power is over 7 mW, which is more suitable for VLC in free space.

Son K.R., Murugadoss V., Kim K.H., Kim T.G.

• The mechanism of sidewall defect passivation in GaN μLEDs is investigated. • SiO 2 , Si 3 N 4 , and Al 2 O 3 with a different bond dissociation energy are used for comparison. • Ga O bonds formed on the GaN surface effectively reduces sidewall defects. • SiO 2 exhibits the best passivation effect, enhancing overall performances of GaN μLEDs. Microscale light-emitting diodes (µLEDs) have been extensively employed for solid-state lighting applications. However, the ratio of the sidewall area to the emitting area increases as the pixel size of µLEDs decreases, which increases the non-radiative recombination probability on the sidewall surface and eventually degrades the performance of µLEDs. In this study, we investigate the nature of chemical bonds at the sidewall/passivation layer interface using three passivation materials (SiO 2 , Al 2 O 3 , and Si 3 N 4 ), to identify the underlying mechanism of passivation and thereby achieve high-performance InGaN-based µLEDs. According to the X-ray photoelectron spectroscopy results, the ratio of Ga O bonds on the sidewall/passivation layer interface to Ga N bonds varies with the passivation layer (1.1, 1.06, and 0.33 for SiO 2 , Al 2 O 3 , and Si 3 N 4 , respectively). This amount is a key factor affecting the passivation and directly influences the µLED performance. The µLED with SiO 2 passivation exhibits a 39% higher light output power and 192% higher current density compared to those associated with the µLED with Si 3 N 4 passivation. These results indicate that the suppression of non-radiative defects depends on the chemical states at the sidewall/passivation layer interface. The findings can provide guidance for optimizing the device performance of µLEDs by selecting appropriate passivation layers.

Anwar A.R., Sajjad M.T., Johar M.A., Hernández‐Gutiérrez C.A., Usman M., Łepkowski S.P.

The demand for high-performance displays is continuously increasing because of their wide range of applications in smart devices (smartphones/watches), augmented reality, virtual reality, and naked eye 3D projection. High-resolution, transparent, and flexible displays are the main types of display to be used in future. In the above scenario, the micro-LEDs (light-emitting diodes) display which has outstanding features, such as low power consumption, wider color gamut, longer lifetime, and short response-time, can replace traditional liquid crystal displays and organic LEDs-based display technologies. However, to attain a remarkable position in future display technology, the micro-LEDs need to overcome problems associated with mass transfer and its high cost of manufacturing. Besides micro-LEDs, the other option for future displays includes the usage of color conversion medium (phosphor/ quantum dots) to convert some of the blue light into other colors. In this review, the various mass transfer display technologies and color conversion strategies which are being used for the realization of a full-color display are discussed.

Chen P., Hsiao P., Chen H., Lee B., Chang K., Yen C., Horng R., Wuu D.

The mechanism of carrier recombination in downsized μ-LED chips from 100 × 100 to 10 × 10 μm2 on emission performance was systemically investigated. All photolithography processes for defining the μ-LED pattern were achieved by using a laser direct writing technique. This maskless technology achieved the glass-mask-free process, which not only can improve the exposure accuracy but also save the development time. The multi-functional SiO2 film as a passivation layer successfully reduced the leakage current density of μ-LED chips compared with the μ-LED chips without passivation layer. As decreasing the chip size to 10 × 10 μm2, the smallest chip size exhibited the highest ideality factor, which indicated the main carrier recombination at the high-defect-density zone in μ-LED chip leading to the decreased emission performance. The blue-shift phenomenon in the electroluminescence spectrum with decreasing the μ-LED chip size was due to the carrier screening effect and the band filling effect. The 10 × 10 μm2 μ-LED chip exhibited high EQE values in the high current density region with a less efficiency droop, and the max-EQE value was 18.8%. The luminance of 96 × 48 μ-LED array with the chip size of 20 × 20 μm2 exhibited a high value of 516 nits at the voltage of 3 V.

Polyakov A.Y., Alexanyan L.A., Skorikov M.L., Chernykh A.V., Shchemerov I.V., Murashev V.N., Kim T., Lee I., Pearton S.J.

Time-resolved photoluminescence (PL), current-voltage characteristics and deep trap spectra of nanopillar GaN/InGaN multi-quantum-well (MQW) light emitting diodes (LEDs) prepared by reactive ion etching (RIE) from planar samples were studied as a function of post-RIE surface treatment. Immediately after RIE, we observe a sharp drop of intensity of the 460 nm MQW PL band and a strong increase of intensity of the defect-related 600 nm MQW band similar to the yellow band in n-GaN. KOH etching and (NH4)2S sulfur passivation treatments increased the MQW PL band intensity above the level of the planar structure and decreased the intensity of the defect 600 nm PL band, but was not effective in decreasing the RIE-induced excessive leakage current of the LEDs. Only annealing at ≥ 700 °C following RIE was able to strongly suppress the excessive leakage. Deep trap spectra suggest that the RIE-induced drop of the 460 nm PL intensity and increase of the 600 nm PL intensity are accompanied by the increase of the Ec-0.7 eV electron trap density and of Ev+0.8 eV hole trap density in the quantum wells. The first of these defects is attributed to nitrogen interstitial acceptors believed to be effective nonradiative recombination centers, while the hole traps are attributed to gallium vacancy acceptor complexes with shallow donors. Both types of defects are produced on the sidewalls of nanopillars by RIE and can be largely annealed at 700 °C, although higher annealing temperatures are needed to fully suppress their negative impact. These results are also relevant to curing the effects of dry-etch damage for micro-LEDs with small chip dimensions, a serious problem in the micro-LED field.

Lee D., Han J.

Light-emitting diodes (LEDs), which emit efficient and bright light ranging from infrared to ultraviolet, have been adopted in various fields to improve quality of life. In particular, micro-LEDs, which are III–V inorganic-based micro-miniaturized light sources, which can form clear and vivid displays for smart portable devices and can be used with artificial intelligence, are drawing great attention. The micro-LEDs can be used in applications where it is difficult to use existing inorganic LEDs, such as in medicine, clothing, and miniature electronics. Research on developing micro-LEDs is underway around the world as the demand for micro-miniaturized light sources increases, especially for display applications. However, problems are also emerging that hinder the mass production of micro-LED displays. Herein, we introduce the current research trends and issues involved in overcoming various problems associated with using micro-LEDs as full-color light sources in display applications. We also briefly review newly presented research and development strategies and technologies for micro-LEDs by research institutes and companies, including their patents and products.

Parbrook P.J., Corbett B., Han J., Seong T., Amano H.

Typical light-emitting diodes (LEDs) have a form factor >(300 × 300) µm2. Such LEDs are commercially mature in illumination and ultralarge displays. However, recent LED research includes shrinking individual LED sizes from side lengths >300 µm to values

Kim T., Kim S., Uthirakumar P., Cho Y., Jang P., Baek S., Park J., Lee I.

Cho Y., Baek S., Jang P., Park J., Kim S., Kim T., Uthirakumar P., Lee I.