[MOS] 4/13 - Modern Optics and Spectroscopy Seminar with Julia Stahler (Humboldt University of Berlin)

[Christine Brooks]

There will be a virtual Modern Optics and Spectroscopy Seminar held next Tuesday, April 13 at 12pm. A Q&A segment will immediately follow the conclusion of the seminar.

Zoom link: https://mit.zoom.us/j/91246100504?pwd=YVR0eUp4OEdueHhVZk05eVlWdS9mdz09
Password: 199137
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Julia Stähler
Humboldt University of Berlin, Fritz Haber Institute

“ZnO: Ultrafast generation and decay of a surface metal”

Band bending (BB) at semiconductor surfaces or interfaces plays a pivotal role in technology, ranging from field effect transistors to nanoscale devices for quantum technologies. The control of BB via chemical doping or electric fields can create metallic surfaces with properties not found in the bulk, such as high electron mobility, magnetism or superconductivity. Optical generation of metallic surfaces via BB on ultrafast timescales would facilitate a drastic manipulation of the conduction, magnetic and optical properties of semiconductors for novel high-speed electronics. We demonstrate the ultrafast (20 fs) generation of a metal at the (10‑10) surface of ZnO upon photoexcitation. This semiconductor is widely used in optoelectronics due to its transparency for visible light and its ease of nanostructuring. Compared to hitherto known ultrafast photoinduced semiconductor-to-metal transitions (SMTs) that occur in the bulk of inorganic semiconductors, the SMT at the ZnO surface is launched by 3-4 orders of magnitude lower photon fluxes; also, the back-transition to the semiconducting state is at least one order of magnitude faster than in previous studies of other materials. Using time- and angle-resolved photoelectron spectroscopy, we show that the SMT is caused by the photoexcitation of deep surface defects. The resulting positive surface charges lead to downward BB toward the surface. Above a critical excitation density, a metallic band below the equilibrium Fermi level is formed. This process is in analogy to chemical doping of semiconductor surfaces. Hence, it is not material-specific and presents a general route for controlling metallicity confined to semiconductor interfaces on ultrafast timescales.

L. Gierster1, S. Vempati1,2, and J. Stähler1,3
1Fritz-Haber-Institut der Max-Planck-Gesellschaft, Abt. Physikalische Chemie, Faradayweg 4-6, 14195 Berlin, Germany
2Present address: Department of Physics, Indian Institute of Technology Bhilai, Raipur-492015, India
3Humbolt-Universität zu Berlin, Institut für Chemie, Brook-Taylor-Str. 2, 12489 Berlin, Germany

Christine Brooks
Administrative Assistant
Massachusetts Institute of Technology
Department of Chemistry
77 Massachusetts Ave, 6-333
Cambridge, MA 02139
p: 617.253.7239
e: cbrooks at mit.edu<mailto:cbrooks at mit.edu>

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