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Photo-excited hot carrier dynamics in hydrogenated amorphous silicon imaged by 4D electron microscopy

Nature Nanotechnology volume 12, pages 871–876 (2017)Cite this article

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Abstract

Charge carrier dynamics in amorphous semiconductors has been a topic of intense research that has been propelled by modern applications in thin-film solar cells1, transistors and optical sensors2. Charge transport in these materials differs fundamentally from that in crystalline semiconductors3,4 owing to the lack of long-range order and high defect density. Despite the existence of well-established experimental techniques such as photoconductivity time-of-flight5,6,7,8 and ultrafast optical measurements9,10,11,12, many aspects of the dynamics of photo-excited charge carriers in amorphous semiconductors remain poorly understood. Here, we demonstrate direct imaging of carrier dynamics in space and time after photo-excitation in hydrogenated amorphous silicon (a-Si:H) by scanning ultrafast electron microscopy (SUEM)13,14. We observe an unexpected regime of fast diffusion immediately after photoexcitation, together with spontaneous electron–hole separation15 and charge trapping3 induced by the atomic disorder. Our findings demonstrate the rich dynamics of hot carrier transport in amorphous semiconductors that can be revealed by direct imaging based on SUEM.

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Figure 1: SUEM images at different delay times after photo-excitation.
Figure 2: Analysis of image intensity along a centre-cut line.
Figure 3: Quantitative analysis of carrier diffusion and trapping processes.
Figure 4: Monte Carlo simulation of the carrier dynamics.

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Acknowledgements

The authors thank Y. Zhu for providing the sample and X. Fu for discussions. This work is supported by the National Science Foundation (DMR-0964886) and the Air Force Office of Scientific Research (FA9550-11-1-0055) in the Gordon and Betty Moore Centre for Physical Biology at the California Institute of Technology. B.L. acknowledges financial support from the KNI Prize Postdoctoral Fellowship in Nanoscience at the Kavli Nanoscience Institute of the California Institute of Technology.

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Author notes

  1. Ahmed H. Zewail: Deceased.

Authors and Affiliations

  1. Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, 91125, California, USA

    Bolin Liao, Ebrahim Najafi, Heng Li & Ahmed H. Zewail

  2. Kavli Nanoscience Institute, California Institute of Technology, Pasadena, 91125, California, USA

    Bolin Liao, Austin J. Minnich & Ahmed H. Zewail

  3. Division of Engineering and Applied Science, California Institute of Technology, Pasadena, 91125, California, USA

    Austin J. Minnich

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  1. Bolin Liao
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  4. Austin J. Minnich
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Contributions

B.L. and A.H.Z. conceived the project. B.L., E.N. and H.L. conducted the experiment and analysed the results. B.L. wrote the paper. A.J.M. proofread and commented on the manuscript, and advised on the modelling work. A.H.Z. supervised the research.

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Correspondence to Austin J. Minnich.

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Liao, B., Najafi, E., Li, H. et al. Photo-excited hot carrier dynamics in hydrogenated amorphous silicon imaged by 4D electron microscopy. Nature Nanotech 12, 871–876 (2017). https://doi.org/10.1038/nnano.2017.124

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