Abstract
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 stability1,2,3,4,5. However, μLEDs have a large problem in that the external quantum efficiency (EQE) decreases with the size reduction6,7,8,9. 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.
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Data availability
All data generated or analysed during this study are included in the paper.
Code availability
We used the commercially available software of Ansys Lumerical for the FDTD simulation and VASP for the DFT calculations. The simulation settings are presented in Methods. The atomic structure data for the defects are available at Zenodo, https://doi.org/10.5281/zenodo.6544988.
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Acknowledgements
We thank J. Kwag and D. Kim for encouragement and full support; Y. Choi and S. Yoon for support and technical advice on nLED fabrication; S.-C. Jo for his valuable advice; H. Cho for his help in nLED fabrication and characterization; H. Cha and S. Kim for their assistance with testing nLEDs; Y. Han and Y. Shim for the LEE simulation; M. Kim at Seoul National University for the deeply insightful discussion.
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Y.C. and J.H. prepared the epi-wafer and fabricated the nanorods. D.-u.K. and J.Y. fabricated the nLEDs and the pixels using the nanorods. Y.K., I.K., C.J. and N.H. synthesized the sol–gel SiO2 layer on the nanorods. J.K. and J.-h.B. analysed and interpreted the STEM-EELS data. K.Y.Y., D.K., J.J., J.C. and R.K. carried out the PL, DLTS, ESR and XPS analyses. Joohee Lee and S.H.J. calculated the defect levels of GaN. S.H.O. and Jaekwang Lee contributed to the interpretation of the experimental results and calculated the data. N.A. and C.L. supervised the research and coordinated the work. M.S. contributed to the experiments and analyses. M.S. and C.L. wrote the manuscript, with input from all other authors.
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Nature thanks Steven DenBaars, Zhaojun Liu and Shin-Tson Wu for their contribution to the peer review of this work.
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Extended data figures and tables
Extended Data Fig. 1 Size-dependent EQE of μLEDs.
The EQE values are presented according to recent results in the literature.
Extended Data Fig. 2 Comparison between thermal ALD SiO2 and plasma-enhanced ALD SiO2 passivation.
a, EQE curves for the nLEDs with the 60-nm-thick SiO2 passivation layer deposited with thermal ALD and plasma-enhanced ALD processes. b, Thermal desorption spectroscopy (TDS) for measuring H2 outgassing. c, X-ray reflectivity measurement for thin-film density. d, XPS core-level spectra for Si 2p and O 1s obtained from a 20-nm-thick SiO2 layer deposited on a Si substrate with thermal ALD and plasma-assisted ALD processes.
Extended Data Fig. 3 Influence of the GaN substrate on yellow luminescence in the PL spectra of nanorods.
a, b, PL images of the plasma-enhanced ALD SiO2 (60 nm thickness) coated nanorods dispersed on wafer (a) and glass (b). The PL spectra from the wafer and glass specimens are compared in c. The YL region of the PL spectra are enlarged in d. The sol–gel SiO2 (25 nm thickness) coated nanorods are presented in e–h in the same manner. The YL is compared in i for the wafer specimen and in j for the glass specimen.
Extended Data Fig. 4 LEE simulation and EQE measurement set-up for nLED devices.
a, Schematic diagram of the nLED pixel structure. b, Cross-sectional optical field distribution of the nLED device. c, Schematic diagram of the EQE measurement with an integrating sphere (ISP 40-101) and calibrated spectroradiometer system for the nLED TEG cell. Each TEG cell is composed of 60 pixels and each pixel is composed of 6–9 nanorods connected in parallel. d, Schematic diagram of the EQE measurement with a full integrating sphere (ISP 250-211) and calibrated spectroradiometer system for the nLED-array chip. e, EQE curves measured in the configurations of c and d are compared for the 60-nm-thick SiO2 passivation layer deposited with the plasma-enhanced ALD process.
Extended Data Fig. 5 Atomic structure between the GaN surface and SiO2 insulator.
a–e, STEM-HAADF images of the MQWs in the sidewall region according to the fabrication steps, that is, dry etch (a), wet etch (b), sol–gel SiO2 deposition (c) and plasma-enhanced ALD SiO2 deposition of thicknesses of 2 nm (d) and 60 nm (e). The increase in the plasma-induced amorphization at the quantum well sidewall with increasing thickness of the SiO2 layer from 2 nm (d) to 60 nm (e) is evident.
Extended Data Fig. 6 Surface defects of nLEDs according to their fabrication steps.
a, Optical micrographs of the nLEDs dispersed on Si substrates. b, Ga 3d state ratios estimated using XPS. Fitting of XPS core-level spectra of dry-etched nanorods for Ga 3d (c) and N 1s (d). XPS core-level spectra for Si 2p (e), O 1s (f) and C 1s (g).
Extended Data Fig. 7 Analysis of trap levels and their concentrations using DLTS.
a–f, Bulk LED chip. g–l, nLED-array chip. a,g, Schematic of the chip. DLTS spectra obtained at forward bias (b,h) and enlarged spectra (c,i) in the region around the peak. DLTS spectra obtained at reverse bias (e,k) and enlarged spectra (f,l) in the region around the peak. d,j, Trap levels with their concentrations. They clearly show that the hole traps are dominant, with high concentrations in the nLED-array chip.
Extended Data Fig. 8 Post-treatments of the sol–gel SiO2-coated nLEDs.
a, PL decay traces of the nLEDs averaged over the areas indicated in the images below. The carrier lifetime increased after baking the sample at 250 °C for 1 h. b, The EL efficiency versus current density of the nLEDs in a pixel structure. The EL performance is the same before and after the annealing. At a high current injection, the EL efficiency is slightly increased. c, Comparison of outgassing between the SiO2 layers using pyrolysis–gas chromatography–mass spectrometry with evolved gas analysis–mass spectrometry.
Extended Data Fig. 9 Synthesis of SiO2 passivation layer using the sol–gel method.
a–c, Transmission electron microscopy images of nLEDs passivated with sol–gel SiO2 at reaction times of 15 min (a), 30 min (b) and 60 min (c). d–f, In addition, the reaction was repeated twice under the same synthesizing conditions to increase their thicknesses. g, The thickness of the SiO2 layer was saturated at 23 nm after reaction for 60 min. h, PL intensities of the nanorods. We obtained the PL spectra from three different positions on a 4-inch wafer. The variation in the PL intensity with respect to the position is small. Furthermore, the PL intensity increased with an increase in the thickness of the sol–gel SiO2 layer.
Extended Data Fig. 10 Comparison between PL and EL spectra.
a, Plasma-enhanced ALD SiO2 passivation. b, Sol–gel SiO2 passivation. The EL spectra are obtained at a current density of 10 mA cm−2, which is near the peak EQE position.
Extended Data Fig. 11 Comparison between KOH wet etching and KOH wet etching plus plasma-enhanced ALD SiO2 passivation process.
a, b, PL spectra (a) and monochromatic CL images at 445 nm (b) of the nanorods for KOH wet etching and plasma ALD SiO2 deposited. c, HAADF-STEM images of the nanorods’ sidewalls. The yellow arrows indicate the native GaOx for the KOH case and the passivation-generated GaOx for the plasma SiO2 case. d, XPS core-level spectra of the nanorods: Ga 3d and N 1s.
Extended Data Fig. 12 Extraction of the ideality factor.
The dashed red lines represent the linear fittings for calculating the ideality factor in the J–V curves of devices with the plasma-enhanced ALD SiO2 passivation.
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Sheen, M., Ko, Y., Kim, Du. et al. Highly efficient blue InGaN nanoscale light-emitting diodes. Nature 608, 56–61 (2022). https://doi.org/10.1038/s41586-022-04933-5
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DOI: https://doi.org/10.1038/s41586-022-04933-5
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