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Imaging the motion of electrons across semiconductor heterojunctions

Abstract

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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Figure 1: Schematic of the TR-PEEM set-up and the band alignment of the InSe/GaAs heterostructure.
Figure 2: Photoexcited electrons residing at higher energy in GaAs compared with InSe at the instant of photoexcitation due to type-II band alignment.
Figure 3: Electron transport over time in the InSe/GaAs heterostructure showing the initial accumulation (red) and eventual recombination (blue) after photoexcitation.
Figure 4: Quantitative model reproducing the salient features of charge separation and transfer, formation of internal fields impeding further charge flow and eventual electron–hole recombination.

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Acknowledgements

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.

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Authors

Contributions

M.K.L.M. performed all experiments. A.M., J.M. and M.B.M.K. built the laser pump–probe set-up. S.D.-J., T.H. and A.W. fabricated and characterized the 2D InSe/GaAs samples. S.L., R.V. and P.M.A. grew the bulk InSe crystals. E.W. and M.K.L.M. analysed the data. K.M.D. supervised the project. All authors contributed to the discussions and manuscript preparation.

Corresponding author

Correspondence to Keshav M. Dani.

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The authors declare no competing financial interests.

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Supplementary Movie 1

Supplementary Movie 1 (MP4 6035 kb)

Supplementary Movie 2

Supplementary Movie 2 (MP4 6430 kb)

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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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