AVS 45th International Symposium
    Surface Science Division Wednesday Sessions
       Session SS2-WeM

Paper SS2-WeM10
Nonthermal Effects of Photon Illumination on Surface Diffusion

Wednesday, November 4, 1998, 11:20 am, Room 309

Session: Gas-Surface Dynamics
Presenter: D. Llera-Rodriguez, University of Illinois, Urbana
Authors: E.G. Seebauer, University of Illinois, Urbana
D. Llera-Rodriguez, University of Illinois, Urbana
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The presence of an energy bandgap in semiconductors together with their ability to support significant separations in charge offer the potential to observe surface phenomena not seen on metals. For example, several years ago this laboratory predicted the possibility of photon-influenced surface diffusion on semiconducors by nonthermal mechanisms. The present work reports the first experimental confirmation of this prediction for Ge, In, and Sb diffusion on Si(111), using second harmonic microscopy as the measurement probe. At modest illumination intensities of 2 W/cm@super 2@ or less, we have observed substantial changes in diffusional activation energy E@sub diff@ and prefactor D@sub o@. For all the adsorbates on p-type Si, illumination increases E@sub diff@ by about 0.3 eV (out of 2.4) and increases D@sub o@ by nearly two orders of magnitude. However, use of n-type substrates causes decreases of the same magnitude. All effects remain independent of illumination wavelength as long as the photon energy remains higher than the substrate bandgap energy. We can explain most of these effects by extending our adatom-vacancy model for surface diffusion at the experimental temperatures (400 to 1000 C) to incorporate charging of the surface vacancies, in direct analogy with vacancy behavior in bulk semiconductors. Charging of the vacancies requires electron transfer to or from surface quasi-fermi levels, whose positions are modified by illumination as in surface photovoltage spectroscopy. Our results have significant implications for semiconductor manufacturing steps performed by the lamp illumination of rapid thermal processing.