Abstract
Micro-LEDs (µLEDs) have been explored for augmented and virtual reality display applications that require extremely high pixels per inch and luminance1,2. However, conventional manufacturing processes based on the lateral assembly of red, green and blue (RGB) µLEDs have limitations in enhancing pixel density3,4,5,6. Recent demonstrations of vertical µLED displays have attempted to address this issue by stacking freestanding RGB LED membranes and fabricating top-down7,8,9,10,11,12,13,14, but minimization of the lateral dimensions of stacked µLEDs has been difficult. Here we report full-colour, vertically stacked µLEDs that achieve, to our knowledge, the highest array density (5,100 pixels per inch) and the smallest size (4 µm) reported to date. This is enabled by a two-dimensional materials-based layer transfer technique15,16,17,18 that allows the growth of RGB LEDs of near-submicron thickness on two-dimensional material-coated substrates via remote or van der Waals epitaxy, mechanical release and stacking of LEDs, followed by top-down fabrication. The smallest-ever stack height of around 9 µm is the key enabler for record high µLED array density. We also demonstrate vertical integration of blue µLEDs with silicon membrane transistors for active matrix operation. These results establish routes to creating full-colour µLED displays for augmented and virtual reality, while also offering a generalizable platform for broader classes of three-dimensional integrated devices.
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Acknowledgements
The team at Massachusetts Institute of Technology (MIT) acknowledges support from the National Science Foundation (award no. 2001231), the Defense Advanced Research Projects Agency Young Faculty Award (no. 029584-00001), the Air Force Research Laboratory (award no. FA9453-21-C-0717) and the US Department of Energy’s Office of Energy Efficiency and Renewable Energy under the Solar Energy Technologies Office (award no. DE-EE0008558). The team at MIT also acknowledges support, in part, by LG electronics and Rohm Semiconductor. The team at Georgia Tech-Lorraine acknowledges partial funding by the French National Research Agency under the GANEXT Laboratory of Excellence project. The work by Y.J.H., J.J. and J.C. was supported by a National Research Foundation of Korea grant funded by the Ministry of Science and ICT (nos. 2018K1A4A3A01064272, NRF-2021R1A5A1032996 and 2022M3D1A2050793) and by the Ministry of Education (no. 2022R1A6C101A774).
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Contributions
Jeehwan Kim, A.O., Y.J.H., K.C. and K. Lee conceived the idea and directed the team. J.S. coordinated and designed the experiments and characterization. H.K., S.S., J.J., B.-I.P., J.C. and K. Lu developed and performed epitaxial growth of red, blue and green LED films under the guidance of Jeehwan Kim, A.O. and Y.J.H. 2D material-coated substrates were prepared by H.K., S.S., J.J., B.-I.P., K. Lu, Y.L., K.Q. and Jekyung Kim. T.K. developed Si TFTs under the guidance of K.J.Y. M.S. developed the set-up and codes for measurement of luminance, EQE and radiation pattern under the guidance of V.B., and J.S., J.J., M.S. and J.H.K. collected the data. J.S., H.K., J.J., M.-K.S., K. Lu, S.K., J.L., J.M.S., J.-H.K., J.S.K. and D.L. carried out layer transfer and fabrication of LEDs. J.S. collected I–V curves, EL microscopy images and optical transmission spectra. H.K., S.S. and K. Lu performed EBSD and XRD analyses. XPS and AFM data were collected by K.S.K. EL spectra were obtained by J.S. and H.K. All SEM, EDX and STEM imaging and analyses were performed by C.S.C. J.M.S. designed all three-dimensional schematic illustrations. H.E.L., H.Y., Y.K., H.S.K., S.-H.B. and K. Lee provided feedback throughout experiments and data analysis. The manuscript was written by J.S., H.K., S.S., J.J., Y.J.H., A.O. and Jeehwan Kim. All authors contributed to the analysis and discussion of results leading to the manuscript.
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Extended data figures and tables
Extended Data Fig. 1 Schematic illustrations of the 2D material-based layer transfer and vertical heterointegration process for constructing full-color vertical µLEDs.
The layers are transferred and stacked in the order of red, green, and blue LEDs using polyimide as adhesion layer.
Extended Data Fig. 2 XRD, EBSD, and AFM measurements of ultrathin RGB LED films obtained via 2DLT.
a-c, XRD 2θ-ω scan results for red (a), green (b), and blue (c) LED films. d-f, XRD Φ scan results for red (d), green (e), and blue (f) LED films. g-i, EBSD analysis results for red (g), green (h), and blue (i) LED films. j-l, AFM morphology images for red (j), green (k), and blue (l) LED films.
Extended Data Fig. 3 Luminance and external quantum efficiencies (EQEs) of RGB µLEDs.
a-d, Measured luminance of red (a), green (b), blue (c), and reference (d) µLEDs of varying size. e-h, Measured EQEs of red (e), green (f), blue (g), and reference (h) µLEDs of varying size. Red, green, and blue µLEDs are obtained via 2DLT, while reference devices are blue µLEDs grown directly on sapphire substrate without 2D layer.
Extended Data Fig. 4 Verification of reusability of sapphire substrates after 2DLT.
a-b, XPS spectra of B 1s (a) and N 1s (b) regions for hBN-coated sapphire wafer (reference; black), used sapphire wafer following exfoliation of LED (LED exfol.; blue), and used sapphire wafer after removal of residual hBN layer (hBN removal; red). c-d, SEM images of blue LEDs on hBN grown on pristine (d) and reused (e) sapphire wafers.
Extended Data Fig. 5 Optimization of optical transmission characteristics of absorber interlayers.
a-e, Optical transmission spectra obtained from absorber layers with varying concentrations of dyes dissolved in the PI precursor (a), film curing temperatures (b), types of dye product (c), film thickness (represented as spin-coating rate) (d), and types of PI (colorless PI from Kolon, Inc. and PI-2545 from HD Microsystems, Inc.) (e). Parameters in parentheses represent conditions that apply for all data in each plot. (a, Insets) Photographs of representative absorber layers coated on glass slide.
Extended Data Fig. 6 Si TFTs on silicon-on-insulator wafer.
a, b, Optical microscopy images of a 30 × 30 array of silicon TFTs fabricated on silicon-on-insulator wafer. Each TFT has dimensions of 2.2 μm × 9.2 μm. c, Transfer characteristics of the silicon TFT with W/L of 1.5 μm/2.2 μm, driven at VDS of 2 V. d, Output characteristics of a Si TFT showing current saturation at VGS values ranging from 0.5 V to 2.5 V.
Extended Data Fig. 7 Si TFT-integrated blue InGaN μLEDs.
a, b, Optical microscopy and tilted SEM images of a 30 × 30 array of blue μLEDs vertically integrated with silicon TFTs. c, Cross-sectional SEM image of a blue μLED transferred on a 300 nm-thick silicon TFT on silicon-on-insulator wafer by PI adhesive layer and electrically interconnected by sputtered metal. d, Optical images of the active matrix μLED display displaying the ‘mit’ logo. Scale bar, 200 μm.
Extended Data Fig. 8 Schematic illustrations and optical microscope images of 2DLT-based, selective µLED mass transfer process for manufacturing large-scale displays.
The process involves the fabrication of blue LED chips (size ~10 µm) on hBN-coated sapphire substrate (step i), photolithography of a partially-developed photoresist (PR) pattern that exposes only the upper bodies of µLEDs to be transferred (step ii), deposition of Ni stressor layer and attachment of handling tapes (step iii), and mechanical lift-off of the exposed µLEDs via cleavage through the hBN layer (steps iv), which leaves behind the PR-coated chips on sapphire substrate that can undergo cleaning and additional lift-off (step ii). The slippery surface of 2D materials, combined with photolithography-based selection approach, enables facile yet highly resolved extraction of µLED chips among a densely-packed array (chip-to-chip separation ~10 µm). Cleaning the PR residue, transferring the µLEDs onto a secondary substrate, and removing Ni and underlying PI layers complete the process (steps v-vii).
Extended Data Fig. 9 Blue LED grown via remote epitaxy on GaN wafer.
a, Schematic illustrations of the epitaxial structures of InGaN-based blue LED grown on GaN wafer. b, AFM morphology image of a GaN film grown via remote epitaxy. c, SEM images of the surfaces of GaN films grown via remote (left) and van der Waals (right) epitaxy techniques. d-e, Measured EQEs and luminance of blue µLEDs of varying size grown on GaN substrate.
Supplementary information
Supplementary Information
Supplementary Notes 1–6, Figs. 1–11 and Table 1.
Supplementary Video 1 Exfoliation of blue LED film from hBN-sapphire wafer.
Video recordings of the exfoliation process for 2-inch-wafer-sized blue LED membrane, highlighting the high speed and yield.
Supplementary Video 2 Electroluminescence of mixed colours for vertical micro-LEDs.
Video recordings of the emission of various mixed colours by vertical µLEDs.
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Shin, J., Kim, H., Sundaram, S. et al. Vertical full-colour micro-LEDs via 2D materials-based layer transfer. Nature 614, 81–87 (2023). https://doi.org/10.1038/s41586-022-05612-1
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DOI: https://doi.org/10.1038/s41586-022-05612-1
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