Improve characteristics of GaN-based green mini-LEDs with double dielectric sidewall passivation

Lee T., Huang Y., Chiang H., Chao C., Hung C., Kuo W., Fang Y., Chu M., Wu C., Lin C., Kuo H.

The effect of atomic-layer deposition (ALD) sidewall passivation on the enhancement of the electrical and optical efficiency of micro-light-emitting diode (µ-LED) is investigated. Various blue light µ-LED devices (from 5 × 5 µm2 to 100 × 100 µm2) with ALD-Al2O3 sidewall passivation were fabricated and exhibited lower leakage and better external quantum efficiency (EQE) comparing to samples without ALD-Al2O3 sidewall treatment. Furthermore, the EQE values of 5 × 5 and 10 × 10 µm2 devices yielded an enhancement of 73.47% and 66.72% after ALD-Al2O3 sidewall treatments process, and the output power also boosted up 69.3% and 69.9%. The Shockley-Read-Hall recombination coefficient can be extracted by EQE data fitting, and the recombination reduction in the ALD samples can be observed. The extracted surface recombination velocities are 551.3 and 1026 cm/s for ALD and no-ALD samples, respectively.

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.

Zhang P., Wang L., Zhu K., Yang Y., Fan R., Pan M., Xu S., Xu M., Wang C., Wu C., Zhang D.

A systematic study of the selective etching of p-GaN over AlGaN was carried out using a BCl3/SF6 inductively coupled plasma (ICP) process. Compared to similar chemistry, a record high etch selectivity of 41:1 with a p-GaN etch rate of 3.4 nm/min was realized by optimizing the SF6 concentration, chamber pressure, ICP and bias power. The surface morphology after p-GaN etching was characterized by AFM for both selective and nonselective processes, showing the exposed AlGaN surface RMS values of 0.43 nm and 0.99 nm, respectively. MIS-capacitor devices fabricated on the AlGaN surface with ALD-Al2O3 as the gate dielectric after p-GaN etch showed the significant benefit of BCl3/SF6 selective etch process.

Yeh Y., Lin S., Hsu T., Lai S., Lee P., Lien S., Wuu D., Li G., Chen Z., Wu T., Kuo H.

In recent years, the process requirements of nano-devices have led to the gradual reduction in the scale of semiconductor devices, and the consequent non-negligible sidewall defects caused by etching. Since plasma-enhanced chemical vapor deposition can no longer provide sufficient step coverage, the characteristics of atomic layer deposition ALD technology are used to solve this problem. ALD utilizes self-limiting interactions between the precursor gas and the substrate surface. When the reactive gas forms a single layer of chemical adsorbed on the substrate surface, no reaction occurs between them and the growth thickness can be controlled. At the Å level, it can provide good step coverage. In this study, recent research on the ALD passivation on micro-light-emitting diodes and vertical cavity surface emitting lasers was reviewed and compared. Several passivation methods were demonstrated to lead to enhanced light efficiency, reduced leakage, and improved reliability.

Yang F., Li L., Cai X., Li J., Tao J., Xu Y., Cao B., Xu K.

Microdisplays based on an array of micro-sized GaN-based light emitting diodes (μLEDs) are very promising for high brightness applications. As the size of Micro-LED decreases, the sidewall damage caused by plasma etching becomes an important factor in reducing the luminescence efficiency. Here, the photoluminescence, scanning electron microscope (SEM) and high‑resolution transmission electron microscopy (HR‑TEM) were combined to reveal physical defects on the sidewall surface, such as plasma-induced lattice disorder, the enrichment of impurity atoms such as oxygen, and the destruction of the exposed part of the quantum well during etching. The structure of the 20 um mesa after inductively coupled plasma (ICP) dry etching was characterized optically, and the luminescence intensity begins to decrease gradually at 5 um from the sidewall, which was caused by the surface non-radiative recombination. Finally, through the combination of tetramethylammonium hydroxide (TMAH) treatment and SiO2 passivation, the sidewall passivation process is optimized, and the luminous efficiency of Micro-LED edge is effectively improved 4.5 times. These results have reference significance for reducing sidewall defects to improve Micro-LEDs luminescence efficiency in the future.

Melanson B., Hartensveld M., Liu C., Zhang J.

We report on the realization of top-down fabricated, electrically driven, deep-ultraviolet (DUV) AlGaN micropillar array light emitting diodes (LEDs) with high output power density. Ordered arrays of micropillars with the inverse-taper profile were formed from an AlGaN epitaxial stack (epistack) using a Ni-masked Cl2 plasma dry etch and KOH-based wet etching. Following deposition of the n-contact, polydimethylsiloxane was spin-coated and etched-back to reveal the tips of the pillars to allow for formation of the p-contact. The DUV LEDs were tested at the wafer-level using a manual probe station to characterize their electrical and optical properties, revealing stable electroluminescence at 286 nm with a narrow 9-nm linewidth. Optical output power was found to be linearly related to current density, with output power densities up to 35 mW/cm2, comparable to the results reported for epitaxially grown DUV nanowire LEDs. Simulations revealed that the inverse-taper profile of the micropillars could lead to large enhancements in light extraction efficiency (ηEXT) of up to 250% when compared to micropillars with vertical sidewalls. The realization of ordered, electrically driven, top-down fabricated micropillar DUV LEDs with competitive output power represents an important step forward in the development of high-efficiency, scalable DUV emitters for a wide range of applications.

Yulianto N., Refino A.D., Syring A., Majid N., Mariana S., Schnell P., Wahyuono R.A., Triyana K., Meierhofer F., Daum W., Abdi F.F., Voss T., Wasisto H.S., Waag A.

The integration of gallium nitride (GaN) nanowire light-emitting diodes (nanoLEDs) on flexible substrates offers opportunities for applications beyond rigid solid-state lighting (e.g., for wearable optoelectronics and bendable inorganic displays). Here, we report on a fast physical transfer route based on femtosecond laser lift-off (fs-LLO) to realize wafer-scale top–down GaN nanoLED arrays on unconventional platforms. Combined with photolithography and hybrid etching processes, we successfully transferred GaN blue nanoLEDs from a full two-inch sapphire substrate onto a flexible copper (Cu) foil with a high nanowire density (~107 wires/cm2), transfer yield (~99.5%), and reproducibility. Various nanoanalytical measurements were conducted to evaluate the performance and limitations of the fs-LLO technique as well as to gain insights into physical material properties such as strain relaxation and assess the maturity of the transfer process. This work could enable the easy recycling of native growth substrates and inspire the development of large-scale hybrid GaN nanowire optoelectronic devices by solely employing standard epitaxial LED wafers (i.e., customized LED wafers with additional embedded sacrificial materials and a complicated growth process are not required).

Fan X., Wu T., Liu B., Zhang R., Kuo H., Chen Z.

Chen S.H., Huang Y., Chang Y., Lin Y., Liou F., Hsu Y., Song J., Choi J., Chow C., Lin C., Horng R., Chen Z., Han J., Wu T., Kuo H.

Light-emitting diodes (LEDs) have been regarded as promising candidates for visible light communication (VLC); however, strong internal polarization fields in common c-plane LEDs, especially green ...

Chen S.H., Huang Y., Singh K.J., Hsu Y., Liou F., Song J., Choi J., Lee P., Lin C., Chen Z., Han J., Wu T., Kuo H.

Red-green-blue (RGB) full-color micro light-emitting diodes (μ-LEDs) fabricated from semipolar (20-21) wafers, with a quantum-dot photoresist color-conversion layer, were demonstrated. The semipolar (20-21) InGaN/GaN μ-LEDs were fabricated on large (4 in.) patterned sapphire substrates by orientation-controlled epitaxy. The semipolar μ-LEDs showed a 3.2 nm peak wavelength shift and a 14.7% efficiency droop under 200  A/cm2 injected current density, indicating significant amelioration of the quantum-confined Stark effect. Because of the semipolar μ-LEDs’ emission-wavelength stability, the RGB pixel showed little color shift with current density and achieved a wide color gamut (114.4% NTSC space and 85.4% Rec. 2020).

Wong M.S., Lee C., Myers D.J., Hwang D., Kearns J.A., Li T., Speck J.S., Nakamura S., DenBaars S.P.

Micro-light-emitting-diodes (?LEDs) with size-independent peak external quantum efficiency behavior was demonstrated from 10×10 ?malt;supagt;2alt;/supagt; to 100×100 ?malt;supagt;2alt;/supagt; by employing a combination of chemical treatment and atomic-layer deposition (ALD) sidewall passivation. The chemical treatment and sidewall passivation improved the ideality factors of ?LEDs from 3.4 to 2.5. The results from the combination of chemical treatment and ALD sidewall passivation suggest the issue of size dependent efficiency can be resolved with proper sidewall treatments after dry etching.

Huang Y., Tan G., Gou F., Li M., Lee S., Wu S.

Wong M.S., Hwang D., Alhassan A.I., Lee C., Ley R., Nakamura S., DenBaars S.P.

Optoelectronic effects of sidewall passivation on micro-sized light-emitting diodes (µLEDs) using atomic-layer deposition (ALD) were investigated. Moreover, significant enhancements of the optical and electrical effects by using ALD were compared with conventional sidewall passivation method, namely plasma-enhanced chemical vapor deposition (PECVD). ALD yielded uniform light emission and the lowest amount of leakage current for all µLED sizes. The importance of sidewall passivation was also demonstrated by comparing leakage current and external quantum efficiency (EQE). The peak EQEs of 20 × 20 µm2 µLEDs with ALD sidewall passivation and without sidewall passivation were 33% and 24%, respectively. The results from ALD sidewall passivation revealed that the size-dependent influences on peak EQE can be minimized by proper sidewall treatment.

Hwang D., Mughal A., Pynn C.D., Nakamura S., DenBaars S.P.

Ultrasmall blue InGaN micro-light-emitting diodes (µLEDs) with areas from 10−4 to 0.01 mm2 were fabricated to study their optical and electrical properties. The peak external quantum efficiencies (EQEs) of the smallest and largest µLEDs were 40.2 and 48.6%, respectively. The difference in EQE was from nonradiative recombination originating from etching damage. This decrease is less severe than that in red AlInGaP LEDs. The efficiency droop at 900 A/cm2 of the smallest µLED was 45.7%, compared with 56.0% for the largest, and was lower because of improved current spreading. These results show that ultrasmall µLEDs may be fabricated without a significant loss in optical or electrical performance.

Park J., Shin D.S., Kim D.

We report on the improvement of light extraction efficiency in blue light-emitting diodes (LEDs) by the use of Al 2 O 3 /ZnO core/shell nanorods (NRs) grown on the surface of the LED epi-structure. The electroluminescence intensity at a current injection of 20 mA of LEDs with 40 nm-thick Al 2 O 3 -coated ZnO NR arrays is 2.1 and 4.0 times than those of LEDs with bare ZnO NRs and no NRs, respectively. The enhanced light extraction of LEDs with Al 2 O 3 -coated ZnO NRs can be attributed to an increase in transmission and improved light extraction probability for photons generated in blue LEDs due to a reduction of Fresnel reflection loss.

Liu Z., Cao H., Tang X., Liu T., Lu Y., Jiang Z., Xiao N., Li X.

Abstract The size of InGaN micro-LEDs is continuously decreasing to meet the demands of various emerging applications, especially in tiny micro-displays such as AR/VR. However, the conventional pixel definition based on plasma etching significantly damages the mesa sidewalls, leading to a severe reduction in efficiency as the micro-LED size decreases. This seriously impedes the development and application of micro-LEDs. In this work, we comprehensively explain the origin of micro-LED sidewall effects and corresponding physical models. Subsequently, we systematically review recent progress in micro-LED fabrication aiming at suppressing sidewall effects. Furthermore, we discuss advancements in micro-LED fabrication with “damage-free” techniques, which hold the potential to fundamentally address the issue of plasma damage in the micro-LED process. We believe this review will deepen the understanding of micro-LED sidewall effects and provide a better insight into the latest associated fabrication technologies for high-efficient InGaN micro-LEDs.

Lee I., Cho Y., Alexanyan L.A., Skorikov M.L., Vasilev A.A., Romanov A.A., Matros N.R., Kochkova A.I., Polyakov A.Y., Pearton S.J.

Arrays of nanorod multi-quantum-well (MQW) blue light emitting diodes (nLEDs) with diameter 800 nm were prepared by reactive ion etching (RIE) with subsequent surface treatment by KOH etching alone, and KOH etching followed by SiO2 passivation with the SiO2 prepared by the sol-gel technique. In addition, the effects of Ag/SiO2 core/shell nanoparticles prepared by sol-gel were studied for all surface treatments. For as etched nLEDs, the Internal Quantum Efficiency (IQE) of photoluminescence was low, 5.5%, because of the impact of surface damage introduced by RIE. KOH etching and KOH plus SiO2 passivation produced an increase of IQE to respectively 5.7 and 6.8%. When the Ag/SiO2 core/shell nanoparticles known to produce localized surface plasmon resonance at the wavelength well-matched to the light emission from the MQWs were added to the polymer filling the gaps between the MQW nanorods, the result was found to depend on finding a proper balance between the enhancement due to the interaction Ag/SiO2 nanoparticles adjacent to the MQW nanorods and the absorption by "idle" Ag/SiO2 particles in the bulk of the polymer. When optimal concentrations of LSP NPs giving rise to maximal enhancement of the MQW PL intensity were used, a considerable improvement of performance was obtained, with IQE of 10.6%.