Manganese 3×3 and3×3-R30∘structures and structural phase transition onw-GaN(0001¯) studied by scanning tunneling microscopy and first-principles theory

Abstract

Manganese deposited on the N-polar face of wurtzite gallium nitride [GaN (000$\overline{1}$)] results in two unique surface reconstructions, depending on the deposition temperature. At lower temperature (less than 105${\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}$C), it is found that a metastable $3\ifmmode\times\else\texttimes\fi{}3$ structure forms. Mild annealing of this Mn $3\ifmmode\times\else\texttimes\fi{}3$ structure leads to an irreversible phase transition to a different, much more stable $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}\ensuremath{-}R{30}^{\ensuremath{\circ}}$ structure which can withstand high-temperature annealing. Scanning tunneling microscopy (STM) and reflection high-energy electron diffraction data are compared with results from first-principles theoretical calculations. Theory finds a lowest-energy model for the $3\ifmmode\times\else\texttimes\fi{}3$ structure consisting of Mn trimers bonded to the Ga adlayer atoms but not with N atoms. The lowest-energy model for the more stable $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}\ensuremath{-}R{30}^{\ensuremath{\circ}}$ structure involves Mn atoms substituting for Ga within the Ga adlayer and thus bonding with N atoms. Tersoff-Hamman simulations of the resulting lowest-energy structural models are found to be in very good agreement with the experimental STM images.