Two-dimensional conducting layer on SrTiO₃ surface induced by hydrogenation

We found that a surface state induced by hydrogenation on the surface of SrTiO$_{3}$(001) (STO) did not obey the rigid band model, which was confirmed by in situ electrical resistivity measurements in ultrahigh vacuum. With exposure of atomic hydrogen on the STO, a new surface state (H-induced donor state, HDS) appears within the bulk band gap (an in-gap state), which donates electrons thermally activated to the bulk conduction band, resulting in downward bending of the bulk bands beneath the surface. The doped electrons flow through the space-charge layer in two-dimensional manner parallel to the surface. The observed semiconducting behavior in the temperature dependence of electronic transport is explained by the thermal activation of carriers. The HDS and the bulk conduction band are non-rigid in energy position; they come closer with increasing the hydrogen adsorption. Eventually the HDS saturates its position around 88 meV below the bottom of the bulk conduction band. The sheet conductivity, accordingly, also saturates at 1.95$\pm$ 0.02 $\mu$ S/sq. with increasing hydrogen adsorption, corresponding to completion of the hydrogenation of the surface.