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Title: Super-diffusion of excited carriers in semiconductors

Abstract

The ultrafast spatial and temporal dynamics of excited carriers are important to understanding the response of materials to laser pulses. Here we use scanning ultrafast electron microscopy to image the dynamics of electrons and holes in silicon after excitation with a short laser pulse. We find that the carriers exhibit a diffusive dynamics at times shorter than 200 ps, with a transient diffusivity up to 1,000 times higher than the room temperature value, D 0≈30 cm 2s –1. The diffusivity then decreases rapidly, reaching a value of D 0 roughly 500 ps after the excitation pulse. We attribute the transient super-diffusive behaviour to the rapid expansion of the excited carrier gas, which equilibrates with the environment in 100–150 ps. Numerical solution of the diffusion equation, as well as ab initio calculations, support our interpretation. Lastly, our findings provide new insight into the ultrafast spatial dynamics of excited carriers in materials.

Authors:
 [1];  [2];  [1];  [3]
  1. California Inst. of Technology (CalTech), Pasadena, CA (United States). Physical Biology Center for Ultrafast Science and Technology, Arthur Noyes Lab. of Chemical Physics
  2. California Inst. of Technology (CalTech), Pasadena, CA (United States). Dept. of Applied Physics and Materials Science and Dept. of Physics
  3. California Inst. of Technology (CalTech), Pasadena, CA (United States). Dept. of Applied Physics and Materials Science
Publication Date:
Research Org.:
Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). National Energy Research Scientific Computing Center
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
1489371
Grant/Contract Number:  
AC02-05CH11231
Resource Type:
Accepted Manuscript
Journal Name:
Nature Communications
Additional Journal Information:
Journal Volume: 8; Journal ID: ISSN 2041-1723
Publisher:
Nature Publishing Group
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE

Citation Formats

Najafi, Ebrahim, Ivanov, Vsevolod, Zewail, Ahmed, and Bernardi, Marco. Super-diffusion of excited carriers in semiconductors. United States: N. p., 2017. Web. doi:10.1038/ncomms15177.
Najafi, Ebrahim, Ivanov, Vsevolod, Zewail, Ahmed, & Bernardi, Marco. Super-diffusion of excited carriers in semiconductors. United States. doi:10.1038/ncomms15177.
Najafi, Ebrahim, Ivanov, Vsevolod, Zewail, Ahmed, and Bernardi, Marco. Thu . "Super-diffusion of excited carriers in semiconductors". United States. doi:10.1038/ncomms15177. https://www.osti.gov/servlets/purl/1489371.
@article{osti_1489371,
title = {Super-diffusion of excited carriers in semiconductors},
author = {Najafi, Ebrahim and Ivanov, Vsevolod and Zewail, Ahmed and Bernardi, Marco},
abstractNote = {The ultrafast spatial and temporal dynamics of excited carriers are important to understanding the response of materials to laser pulses. Here we use scanning ultrafast electron microscopy to image the dynamics of electrons and holes in silicon after excitation with a short laser pulse. We find that the carriers exhibit a diffusive dynamics at times shorter than 200 ps, with a transient diffusivity up to 1,000 times higher than the room temperature value, D0≈30 cm2s–1. The diffusivity then decreases rapidly, reaching a value of D0 roughly 500 ps after the excitation pulse. We attribute the transient super-diffusive behaviour to the rapid expansion of the excited carrier gas, which equilibrates with the environment in 100–150 ps. Numerical solution of the diffusion equation, as well as ab initio calculations, support our interpretation. Lastly, our findings provide new insight into the ultrafast spatial dynamics of excited carriers in materials.},
doi = {10.1038/ncomms15177},
journal = {Nature Communications},
number = ,
volume = 8,
place = {United States},
year = {2017},
month = {5}
}

Journal Article:
Free Publicly Available Full Text
Publisher's Version of Record

Citation Metrics:
Cited by: 10 works
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Figures / Tables:

Figure 1 Figure 1: Schematic of scanning ultrafast electron microscopy (SUEM) imaging. In our SUEM measurements, the optical pump pulse excites electron-hole pairs in silicon, which undergo super-diffusion until they equilibrate with the environment. Electron probe pulses with a known delay raster-scan the surface, resulting in the emission of secondary electrons (SEs)more » employed to image the carrier dynamics. The right panel schematically shows carrier dynamical processes mapped onto the bandstructure of Si. Shown are the carrier optical excitation (green arrow), as well as energy gain and recombination (upward and downward oriented red arrows, respectively) due to carrier scattering processes. The SEs preferentially observed by SUEM (blue dot) are emitted from the high-energy tail of the carrier distribution.« less

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