Adsorbed states of NH3 and C6H6 on the Si(111)(√3 × √3 )R30°-B surface: Thermal-desorption and electron-energy-loss-spectroscopy studies

Abstract

Adsorbed states of ${\mathrm{NH}}_3$ and ${\mathrm{C}}_6$ ${\mathrm{H}}_6$ on a Si(111)(√3 × √3 )R30°-B surface have been studied by the use of thermal desorption and high-resolution electron-energy-loss spectroscopy. At 90 K, a bulklike ${\mathrm{NH}}_3$ multilayer, ${\mathrm{NH}}_3$ hydrogen bonded to a chemisorbed one, and molecularly chemisorbed ${\mathrm{NH}}_3$ exist on the (√3 × √3 )R30°-B surface. These desorb at ∼115, 140, and 170 K, respectively. Chemisorbed ${\mathrm{NH}}_3$ has a very low NH stretching energy of ∼370 meV, and its N-H bonds are significantly weakened. A 2.0-eV loss, which corresponds to the electronic transition from an occupied backbond state to an empty dangling-bond state of the Si adatom on the (√3 × √3 )R30°-B surface, is removed upon ${\mathrm{NH}}_3$ adsorption. It is proposed that chemisorbed ${\mathrm{NH}}_3$ is coordinated to the Si adatom on the surface. ${\mathrm{C}}_6$ ${\mathrm{H}}_6$ does not chemisorb on the (√3 × √3 )R30°-B surface even at 90 K, but only physisorbs at <160 K. The adsorbed states of ${\mathrm{NH}}_3$ and ${\mathrm{C}}_6$ ${\mathrm{H}}_6$ on the (√3 × √3 )R30°-B surface are quite different from those on clean Si surfaces, and the reactivity criterion of the surface is discussed. It is also found that the (√3 × √3 )R30°-B surface has few defects when a large number of B atoms (∼7×${10}^{19}$ ${\mathrm{cm}}^{\mathrm{-}3}$) exists in the subsurface, which was evaluated by analyzing the surface-plasmon loss.