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Second-Harmonic Generation (SHG) from Silicon Surfaces

shg_exp4                shg_exp6
Second-harmonic generation (SHG) provides an optical probe of surfaces and interfaces. Within the electric dipole approximation SHG only occurs if the medium lacks inversion symmetry. Therefore the nonlinear susceptibility vanishes in the bulk of centrosymmetric media but has a finite value at the surface where the symmetry is broken.
Microscopically SHG can be described by the excitation of an electron from the ground state |g> via an intermediate |n'> to the excited state |n> by two photons with energy ħω and the subsequent emission of a second-harmonic photon with energy 2ħω. Whenever ħω or 2ħω coincide with real transitions from |g> to |n'> or |n> the process exhibits resonant enhancement.
In the case of silicon surfaces such levels are provided by surface states in the band gap originating from Si dangling-bonds or reconstruction-induced bonds. The band structure of Si(111)7×7 is shown schematically in the figure to the left with occupied (Si) and unoccupied surface states (Ui).

Photon Energy Dependence

In spectroscopic SH experiments of the clean Si(111)7×7 surface the spectra reveal two resonant structures.
The peak at 3.4 eV is a resonance that almost coincides with the E1 transition between the valence and conduction band of bulk silicon. It arises because the bulk electronic structure is distorted at the surface. This resonance exhibits relatively little sensitivity on adsorbed hydrogen because the 7×7 surface structure remains intact for moderate exposures.
The broad feature emerging for photon energies below 1.3 eV beyond the accessible wavelength range completely disappears if hydrogen is adsorbed. Its origin is the resonant enhancement of SHG by the Si dangling bond states which are quenched upon hydrogen adsorption. Around 1.2 eV hydrogen adsorption reduces the nonlinear susceptibility almost by a factor of 10 which leads to a dramatic decrease of the measured signal by a factor of 100.

Coverage Dependence

si111speThe high sensitivity of the nonlinear susceptibility on hydrogen coverage has been the basis for many applications of SHG. The SH signal is dominated by the contribution of the dangling bonds resulting in a nearly linear dependence of the nonlinear susceptibility on the hydrogen coverage for low coverages. The minimum for higher coverages is a result of a phase shift between the two contributions to the nonlinear susceptibility. Besides the resonant part a weak nonresonant background e.g. from adatom backbonds exists.

Experimental setup for spectroscopy
Experimental setup for adsorption measurements

Further Information

U. Höfer
Nonlinear optical investigations of the dynamics of hydrogen interaction with silicon surfaces

Appl. Phys. A 63, 533-47 (1996). Abstract Reprint (PDF) (© Springer)
G. A. Schmitt

Untersuchung der nichtlinearen optischen Eigenschaften von Siliziumoberflächen im nahen Infrarot:
Frequenzabhängigkeit und mikroskopische Mechanismen

(Doctoral thesis, TU München, 1996).

SHG from Semiconductor Surfaces - Worldwide Links

Oleg A. Aktsipetrov Moscow State University, Russia
Mike Downer University of Texas, Austin, USA
Tony F. Heinz Columbia University, New York, USA
Ulrich Höfer Philipps-Universität Marburg, Germany
Dietrich von der Linde Universität Essen, Germany
John F. McGilp Trinity College Dublin, Ireland
Frank Rebentrost MPQ Garching, Germany
Georg Reider TU Wien, Austria
Y. Ron Shen University of California, Berkeley, USA
John E. Sipe University of Toronto, Canada
Takanori Suzuki Riken, Japan
Harry K. Tom University of California, Riverside, USA
Henry M. van Driel University of Toronto, Canada
Arjun G. Yodh University of Pennsylvania, Philadelphia, USA
Helmut Zacharias Universität Münster, Germany

Last modified: 27.05.2015 · armbrusn

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