Photo-Induced Structural Changes on Semiconductor Surfaces

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Structural changes and desorption of constituent atoms induced by irradiations of Si (111) - (7x7) and InP (l10) surfaces have been studied by means of scanning tunneling microscopy (STM) and high-sensitivity measurements of desorbed species using resonant- and non-resonant ionization spectroscopy. For Si (111) - (7x7) surfaces, it has been shown that excitation with laser pulses with low intensity induces vacancies selectively at surface adatom-sites, via electronic bond-breaking, accompanied with desorption of Si atoms of electronic ground state. Statistical investigation of spatial distribution of the laser-induced vacancies have revealed strong site-dependence of the bond-breaking efficiency. The efficiency stays constant with increasing the dose of excitation laser pulses, meaning that the site-dependence is not due to any accumulative effects but a characteristic feature of bond-breaking induced within a single laser pulse. The present study also shows that the electronic bond-br … More eaking exhibits strong photon-energy and fluence dependent efficiency, and that the translational energy distribution of desorbed Si atoms independent of wavelength, fluence and time duration of excitation laser lights. Experimental results obtained in the present study have shown that non-linear localization of surface electronic excitations selectively at surface adatom sites plays an important role on the photo-induced bond-breaking on the Si (111) - (7x7) surface. Based on the detail analyses of the results, we have proposed a desorption mechanism including two-hole localization on a surface atomic site followed by bond-breaking via phonon-kick process.Similarly, excitation between surface electronic bands on InP (110) - (lxl) induces an electronic bond-breaking with a strong site-dependent efficiency : laser irradiation produces vacancies selectively at P sub-lattice. In addition, it has been also shown that vacancy-strings elongated selectively along In-P chains are formed efficiently with increasing excitation dose, meaning preferential bond-breaking at atomic sites neighboring to vacancies. The total number density of vacancy string with respective sizes increases linearly for low dose range, and then saturates at a few percent of monolayer. Since the laser-induced change of the number of perfect lattice sites is little, the saturation of number density of vacancies suggests strongly the decrease of bond-breaking efficiency. The efficiency of electronic bond-breaking at perfect lattice sites shows non-linear increase with increasing dose, suggesting that non-linear process are included in the bond-breaking. Based on analyses of the non-linear efficiency, we suggest that two-hole localization is included as a primary step of the electronic bond-breaking. Experimentally observed saturation of the efficiency may be explained by efficient transfer and trapping of photo-generated surface free holes, which are extended quasi-one dimensionally along the In-P chain.The present study have demonstrated that the surface electronic excited states on semiconductor surfaces are localized non-linearly with a strong site-selective efficiency, leading to breaking of local atomic bonds. Observed difference in geometric feature and efficiency of structural changes on the two surfaces may indicate that the electronic bond breaking depends strongly on structural and electronic properties of the surfaces. Less