We found that a surface state induced by hydrogenation on the surface of SrTiO3(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 surface state (H-induced donor state, HDS) appears within the bulk band gap (an in-gap state), which donates electrons thermally activated to the 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 semiconductor behavior in the temperature dependence of electronic conductivity is explained by the thermal activation of carriers. The HDS and the conduction band are nonrigid in energy position; they come closer with increasing the hydrogen adsorption. Eventually the HDS saturates its position around 60 meV below the conduction-band minimum. The sheet conductivity, accordingly, also saturates at ∼1.0μS□ with increasing hydrogen adsorption, corresponding to completion of the hydrogenation of the surface.
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