Technological progress since the late twentieth century has centred on semiconductor devices, such as transistors, diodes and solar cells1,2,3,4,5,6,7,8. At the heart of these devices is the internal motion of electrons through semiconductor materials due to applied electric fields3,9 or by the excitation of photocarriers2,4,5,8. Imaging the motion of these electrons would provide unprecedented insight into this important phenomenon, but requires high spatial and temporal resolution. Current studies of electron dynamics in semiconductors are generally limited by the spatial resolution of optical probes, or by the temporal resolution of electronic probes. Here, by combining femtosecond pump–probe techniques with spectroscopic photoemission electron microscopy10,11,12,13, we imaged the motion of photoexcited electrons from high-energy to low-energy states in a type-II 2D InSe/GaAs heterostructure. At the instant of photoexcitation, energy-resolved photoelectron images revealed a highly non-equilibrium distribution of photocarriers in space and energy. Thereafter, in response to the out-of-equilibrium photocarriers, we observed the spatial redistribution of charges, thus forming internal electric fields, bending the semiconductor bands, and finally impeding further charge transfer. By assembling images taken at different time-delays, we produced a movie lasting a few trillionths of a second of the electron-transfer process in the photoexcited type-II heterostructure—a fundamental phenomenon in semiconductor devices such as solar cells. Quantitative analysis and theoretical modelling of spatial variations in the movie provide insight into future solar cells, 2D materials and other semiconductor devices.
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The work at Rice University was supported by Function Accelerated nanoMaterial Engineering, one of six centres of STARnet, a Semiconductor Research Corporation programme sponsored by the Microelectronics Advanced Research Corporation and the Defense Advanced Research Projects Agency, and also supported by the Multidisciplinary University Research Initiative Army Research Office programme, grant number W911NF-11-1-0362.
The authors declare no competing financial interests.
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Man, M., Margiolakis, A., Deckoff-Jones, S. et al. Imaging the motion of electrons across semiconductor heterojunctions. Nature Nanotech 12, 36–40 (2017). https://doi.org/10.1038/nnano.2016.183
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