Subsurface hydrogen bonds at the polar Zn-terminated ZnO(0001) surface

Abstract

The role of hydrogen and other defects in the stabilization of polar oxide interfaces is a matter of significant fundamental and practical interest. Using experimental (scanning tunneling microscopy, x-ray photoelectron spectroscopy) and theoretical (density functional theory) surface science techniques, we find that the polar Zn-terminated ZnO(0001) surface becomes excessively Zn deficient during high-temperature annealing (780 K) in ultrahigh vacuum (UHV). The Zn vacancies align themselves into rows parallel to the $[10\overline{1}0]$ direction, and the remaining surface Zn ions alternately occupy wurtzite (hcp) and zinc-blende (fcc) lattice positions, giving a characteristic ``striped'' $c(\sqrt{12}\ifmmode\times\else\texttimes\fi{}\sqrt{12})\mathrm{R}30$\ifmmode^\circ\else\textdegree\fi{} surface morphology with three types of rows: wurtzite Zn, zinc-blende Zn, and Zn vacancies. Interstitial H plays a central role in such a reconstruction, as it helps to compensate the excessive Zn deficiency. We propose a model in which hydrogen occupies positions in half of the vacancy rows to form hydroxide ions that can participate in hydrogen bonds in the O subsurface layer as a result of the mixed wurtzite/zinc-blende stacking.