Surface relaxation and rumpling of Sn-doped β-Ga2O3(010)

We have studied the surface structure of single-crystal, wide-gap semiconductor beta-Ga2O3 (010) using x-ray photoelectron diffraction (XPD), low-energy electron diffraction (LEED), and x-ray photoelectron spectroscopy (XPS). The XPS measurements show typical spectra for stoichiometric Ga2O3(010). Annealing in vacuum produced a sharp (1 x 1) LEED pattern, characteristic of the monoclinic structure. The XPD angular anisotropies were collected for the Ga 2p(3/2 )and O 1s is core levels. Surface interlayer relaxation up to 8% of the bulk interplanar distance and 0.11-0.14 angstrom rumpling are observed at the beta-Ga2O3(010) surface. At the surface, the oxygen atoms shift toward the vacuum with respect to the gallium atoms. The rumpling decreases to zero and and the interplanar distance reaches the bulk value of 1.52 angstrom by the sixth atomic layer. The surface structure agrees with that predicted by first-principles density functional theory calculations which, in addition, suggest a significant band gap narrowing of approximate to 1 eV in the surface layer, due to surface states spatially localized on surface oxygen atoms of O-Pi type.