Professor J. Kanasaki , U. Nagoya, Japan
4 PM, Thursday, Feb. 22, 1996
Room 101, Olin Physical Laboratory
I will review our current investigations of laser-induced atomic emissions from semiconductor surfaces, particularly Si emissions from Si(111)7x7 and Si(100)2x1 surfaces and Ga emission from GaAs(110)1x1 surface, studied by combining high-sensitivity measurements of emitted atoms using resonant ionization spectroscopy (RIS) and observations of laser-induced structural changes by scanning tunneling microscopy (STM).
Characteristic features of desorption induced by electronic transitions (DIET) from semiconductor surfaces are a strong nonlinear yield vs. fluence relationship and a strong photon energy dependent yield[1]. The emission yield can be described by a power function of laser fluence typically with indices of 2-6 below the ablation threshold fluence, and the emission yield is enhanced as the photon energy exceeds the energies capable of exciting electrons from the surface occupied state, indicating that the surface excitation is effective to the emissions. Recently, we have studied tunneling images of Si(111)7x7 surfaces before and after laser irradiations below the ablation threshold fluence and have revealed that the number of adatom missing sites increases by laser pulses. It has been also shown that the number of center-adatom missing sites is much larger than that of corner-adatom missing sites, showing the strong site-dependent yield of laser-induced atomic emission from this surface. In the case of the DIET from Si(100) and GaAs(110) surfaces, the emissions have been ascribed to be originated from several types of defects on the surfaces.
It has been suggested that the DIET is originated from localized excited states possessing an anti-bonding like adiabatic potential energy surface (APES). Therefore, the energy localization inducing a state with an anti-bonding like APES is needed as the primary step, since the excitation energy is initially delocalized on the surfaces. The strong nonlinear yield vs. fluence relationship indicates that multiple excitations are involved in the energy localization inducing the emission. The present results that the yield of laser-induced atomic emission exhibits the strong site dependence indicates that the energy localization depends srtongly on bonding properties.
[1] N. Itoh, J. Kanasaki, A. Okano, and Y. Nakai, Annu. Rev. Mater. Sci. 25, 97(1995).