Adsorbed states of NH3 and C6H6 on the Si(111)( sqrt 3 x sqrt 3 )R30 degrees-B surface: Thermal-desorption and electron-energy-loss-spectroscopy studies.

Abstract

Adsorbed states of ${\mathrm{NH}}_{3}$ and ${\mathrm{C}}_{6}$${\mathrm{H}}_{6}$ on a Si(111)(\ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 )R30\ifmmode^\circ\else\textdegree\fi{}-B surface have been studied by the use of thermal desorption and high-resolution electron-energy-loss spectroscopy. At 90 K, a bulklike ${\mathrm{NH}}_{3}$ multilayer, ${\mathrm{NH}}_{3}$ hydrogen bonded to a chemisorbed one, and molecularly chemisorbed ${\mathrm{NH}}_{3}$ exist on the (\ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 )R30\ifmmode^\circ\else\textdegree\fi{}-B surface. These desorb at \ensuremath{\sim}115, 140, and 170 K, respectively. Chemisorbed ${\mathrm{NH}}_{3}$ has a very low NH stretching energy of \ensuremath{\sim}370 meV, and its N-H bonds are significantly weakened. A 2.0-eV loss, which corresponds to the electronic transition from an occupied backbond state to an empty dangling-bond state of the Si adatom on the (\ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 )R30\ifmmode^\circ\else\textdegree\fi{}-B surface, is removed upon ${\mathrm{NH}}_{3}$ adsorption. It is proposed that chemisorbed ${\mathrm{NH}}_{3}$ is coordinated to the Si adatom on the surface. ${\mathrm{C}}_{6}$${\mathrm{H}}_{6}$ does not chemisorb on the (\ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 )R30\ifmmode^\circ\else\textdegree\fi{}-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 (\ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 )R30\ifmmode^\circ\else\textdegree\fi{}-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 (\ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 )R30\ifmmode^\circ\else\textdegree\fi{}-B surface has few defects when a large number of B atoms (\ensuremath{\sim}7\ifmmode\times\else\texttimes\fi{}${10}^{19}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$) exists in the subsurface, which was evaluated by analyzing the surface-plasmon loss.