Abstract
Despite its versatility in photocatalysis, the application of carbon nitride (CN) to optoelectronic devices, especially silicon (Si) optoelectronics, has been constrained by a lack of synthetic methods for producing large-scale, highly uniform and processable films. Here we report a large-scale synthesis of ultrathin amorphous carbon nitride (aCN) on Si, achieving high uniformity, ultralow surface roughness and a covalently bonded interface with Si. The ultrathin aCN on Si (aCN/Si) is synthesized via a two-step process involving a dual-heating-zone chemical vapour deposition and subsequent postannealing in a hydrogen atmosphere. During the postannealing process, the initially formed bilayer of polymeric CN and underlying aCN undergoes material transformations, including thinning of the CN film, increasing spatial uniformity and covalent bond formation between nitrogen and silicon atoms, producing a high-quality aCN/Si heterostructure. Based on this aCN/Si, we developed vertical photodiodes that function both as electrical diodes with high rectification ratio (3.8 × 108) and photodetectors with high specific detectivity (1.9 × 1012 Jones), fast photoresponse (6.7 µs) and a broad linear dynamic range (>130 dB). Integrating these aCN/Si vertical photodiodes with amorphous indium-gallium-zinc oxide switching thin-film transistors enables active-matrix-based multispectral imaging across the visible to near-infrared spectrum range.
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Acknowledgements
D.-H.K. and T.H. acknowledge support from the Institute for Basic Science (grant nos. IBS-R006-A1 and IBS-R006-D1, respectively). C.C. acknowledges support from the National Research Foundation of Korea grant funded by the Korean government (MSIT) (grant no. RS-2023-00209466) and the Future Resource Research Program of the Korea Institute of Science and Technology (KIST) (grant no. 2E33542).
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H.S. and J.B. conceived the idea. H.S. and J.B. designed the experiments, performed the material synthesis and analysed the data. C.C., D.-H.K. and T.H. supervised the project. H.S., J.B., J.S.K., C.C., T.H. and D.-H.K. wrote and revised the paper. H.S., J.B. and J.K. built the synthesis setup under the supervision of J.P. H.S. and H.C. fabricated the devices. H.S., C.C., J.A. and J.P.H. performed the device measurements under the supervision of D.K.H. L.C. performed the low-energy electron diffraction (LEED) analysis. E.Y., S.B. and S.P. analysed the band alignment using UPS and IPES. Y.Y.K. performed grazing incidence wide-angle X-ray scattering analysis. All authors discussed the results and commented on the manuscript.
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Nature Synthesis thanks Michael Bojdys and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editor: Alexandra Groves, in collaboration with the Nature Synthesis team.
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Extended data
Extended Data Fig. 1 Vertical structure of carbon nitride and silicon.
a, Cross-sectional TEM images of the edge parts of sample (1) (left), sample (4) (middle), and sample (7) (right). The sampling spots are marked with red stars in insets (sampling position 10 in Fig. 1e). b-e, Cross-sectional TEM images of the central parts of sample (1) (b), sample (6) (c), sample (7) (d), and sample (8) (e) grown on the p-Si wafer.
Extended Data Fig. 2 Surface morphologies of the samples (1) and (6)-(8).
a-d, AFM surface profile of sample (1) (a), sample (6) (b), sample (7) (c), and sample (8) (d). Three different points for each sample were measured.
Extended Data Fig. 3 Underlying aCN in sample (1).
a,b, Cross-sectional TEM images of the sample (1), after being washed with water to remove the top pCN layer. Three different spots were investigated, marked with red stars in insets; sampling position 8 (a, left), 10 (a, right), and 6 (b) as in Fig. 1e. The underlying aCN layer with the thickness of < 2 nm was visible in all three areas. However, in position 6 (b), the pCN residue was observed. Please see Supplementary Note 1 for details.
Extended Data Fig. 4 Crystallinity of sample (1) and sample (7) measured by grazing incidence wide-angle X-ray scattering (GIWAXS).
a,b, GIWAXS mapping diagrams (a) and horizontal cut graphs (b) of sample (1) (left) and sample (7) (right), measured at the center. c,d, GIWAXS mapping diagrams (c) and horizontal cut graphs (d) of sample (1) (left) and sample (7) (right), measured at the edge.
Extended Data Fig. 5 XPS N1s spectra of samples (1)-(5) and (7).
a,b, XPS N1s spectra of samples (1)-(5) and (7), measured at the center (a) and the edge (b) of each sample. Sample (7) (aCN (20 min)) represents the properties of ultrathin aCNs (samples (6)-(8)).
Extended Data Fig. 6 XPS C1s spectra of samples (1) and (7).
a,b, XPS C1s spectra of sample (1) (left) and sample (7) (right), measured at the center (a) and the edge (b) of each sample.
Extended Data Fig. 7 Patterning ultrathin aCN by conventional photolithography process.
a,b, Sample (7) was patterned into the shapes of the symbols of Seoul National University (a) and the circles with the diameters of 3 μm, 5 μm, 7 μm, and 10 μm (b).
Extended Data Fig. 8 Comparison of device performances of vertical photodiodes.
a, Histogram of rectification ratios measured from 120 random devices. b,c, I–V curves of vertical photodiodes with pCN/p-Si (b) and p-Si (c) sandwiched between the Ti/Au top electrode and the Au bottom electrode, under the irradiation of 638 nm light of different intensities. The lowest detectable intensity was 15.9 mW cm−2 and 0.2 mW cm−2 for each case. d, Iph of the aCN/p-Si device (red) compared to those of p-Si device (blue) and pCN/p-Si device (black) according to the 638 nm light intensities at Vd = 0 V. e, Specific detectivity (D*) of aCN/p-Si vertical photodiode (red) compared to those of p-Si device (blue) and pCN/p-Si device (black), according to the light intensities at Vd of 0 V. f,g, Spectral responsivity (f) and external quantum efficiency (EQE) (g) of aCN/p-Si device.
Extended Data Fig. 9 Fabrication and measurement set up of 1T1P active-matrix image sensor array.
a, Schematic illustration showing the fabrication procedure of 1T1P active-matrix image sensor array. Each pixel consists of an aCN/p-Si vertical photodiode and an a-IGZO TFT. b, Transfer curve of the a-IGZO TFT. c, Experimental setup for multispectral imaging demonstration performed by our active-matrix device. Photographs on the right show the shadow masks of “S” (top) and “N” (bottom) on top of the device. Each inset shows the 625 nm and 532 nm light shaped the alphabet “S” and “N”, respectively.
Supplementary information
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Seung, H., Bok, J., Kim, J.S. et al. Covalent heterostructures of ultrathin amorphous carbon nitride and Si for high-performance vertical photodiodes. Nat. Synth 4, 514–522 (2025). https://doi.org/10.1038/s44160-024-00730-2
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DOI: https://doi.org/10.1038/s44160-024-00730-2
