Photo-excited hot carrier dynamics in hydrogenated amorphous silicon imaged by 4D electron microscopy
Charge carrier dynamics in amorphous semiconductors has been a topic of intense research that has been propelled by modern applications in thin-film solar cells, transistors and optical sensors. Charge transport in these materials differs fundamentally from that in crystalline semiconductors 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-flight and ultrafast optical measurements, 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). We observe an unexpected regime of fast diffusion immediately after photoexcitation, together with spontaneous electron–hole separation and charge trapping 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.
Additional Information© 2017 Macmillan Publishers Limited, part of Springer Nature. Received 21 October 2016; Accepted 22 May 2017; Published online 03 July 2017. 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. Author 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. The authors declare no competing financial interests.
Submitted - 1610.03030.pdf
Supplemental Material - nnano.2017.124-s1.pdf