Work function and affinity changes associated with the structure of hydrogen-terminated diamond (100) surfaces

Abstract

The positron and electron work functions and affinities of diamond (100) surfaces were measured using positron reemission and Kelvin probe techniques to reveal changes in the chemical potential, surface dipole, and band bending. The positron affinity ${\ensuremath{\chi}}_{+}$ is negative; at temperatures between 20 and 100 \ifmmode^\circ\else\textdegree\fi{}C we find ${\ensuremath{\chi}}_{+}=\ensuremath{-}4.20\ifmmode\pm\else\textpm\fi{}0.04\mathrm{eV}$ for the $(2\ifmmode\times\else\texttimes\fi{}1)$ reconstructed, hydrogen-free surface, $\ensuremath{-}3.76\ifmmode\pm\else\textpm\fi{}0.04\mathrm{eV}$ for the monohydride surface, and $\ensuremath{-}3.03\ifmmode\pm\else\textpm\fi{}0.04\mathrm{eV}$ for a fully hydrogenated surface. Similar magnitude, but opposite sign changes are observed for the electron work function ${\ensuremath{\varphi}}_{\ensuremath{-}}.$ The increases in the room-temperature values of ${\ensuremath{\chi}}_{+}$ and the decreases in ${\ensuremath{\varphi}}_{\ensuremath{-}}$ as hydrogen is added are in agreement with theoretical results. The positron affinity of the $(2\ifmmode\times\else\texttimes\fi{}1)$ surface is nearly temperature independent, indicative of a surface that is nearly hydrogen-free and consisting of \ensuremath{\pi}-bonded dimers. As the temperature is raised, the positron affinity of the monohydride surface decreases to a minimum of -4.08 eV at $(575\ifmmode\pm\else\textpm\fi{}50)\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ and returns to the room-temperature value by 800 \ifmmode^\circ\else\textdegree\fi{}C. We speculate that the complex temperature dependence is caused by fluctuations or phase transitions of the two-dimensional array of hydrogen atoms. Contrary to assumptions that band bending is the major contribution to changes in the contact potential of diamond, little band bending is evident at room temperature for variously prepared undoped diamond surfaces. Nevertheless, substantial changes in the positron yield at elevated temperatures are consistent with the formation of a positive electric field due to $3\ifmmode\times\else\texttimes\fi{}{10}^{10}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ surface electrons occupying states associated with absorbed oxygen or structural defects.