Hydrogen-induced metallicity of ${\text{SrTiO}}_3$ (001) surfaces: A density functional theory study

The effect of atomic hydrogen adsorption on ${\text{TiO}}_2$-terminated and SrO-terminated ${\text{SrTiO}}_3$ (001) surfaces is studied using density functional theory calculations. Several adsorption coverages (1/12, 1/6, 1/3, 1/2, 2/3, and 1 monolayer) are considered. It is found that the hydrogen adsorption shows site selectivity and causes remarkable surface distortion. Surface metallicity induced by the hydrogen adsorption is observed and revealed to be caused by the electron donation from hydrogen to the surface. Our results suggest a mechanism of hydrogen-induced degradation and hydrogen-sensitive $I\text{−}V$ characteristics of ${\text{SrTiO}}_3$-based devices.

Figure 1

(Color online) The $1\times 1$ supercells of the slab model with (a) ${\text{TiO}}_2$-terminated surface and (b) SrO-terminated surface. In (a), ${\text{Ti}}_1$, ${\text{O}}_{1a}$, and ${\text{O}}_{1b}$ (${\text{Sr}}_2$ and ${\text{O}}_2$) represent the Ti and O atoms of the surface layer (the Sr and O atoms of the subsurface layer), respectively. In (b), ${\text{Sr}}_1$ and ${\text{O}}_1$ (${\text{Ti}}_2$, ${\text{O}}_{2a}$, and ${\text{O}}_{2b}$) represent the Sr and O atoms of the surface layer (the Ti and O atoms of the subsurface layer), respectively. Green, gray, and red balls represent Sr, Ti, and O atoms, respectively.

Figure 2

(Color online) Optimized structures of hydrogen-adsorbed [(a)–(d)] ${\text{TiO}}_2$-terminated and (e) SrO-terminated ${\text{SrTiO}}_3$ surfaces, which are calculated using the $1\times 1$ supercell. Four surface unit cells are shown here. (a), (d), and (e) correspond to the adsorption coverages of 1/3, 1, and 1/2 ML, respectively. (b) and (c) correspond to two different cases of 2/3 ML coverage, O-2/3 and Ti-2/3 ML. Green, gray, red, and white balls represent Sr, Ti, O, and H atoms, respectively.

Figure 3

(Color online) Top view of the optimized structures of hydrogen-adsorbed ${\text{TiO}}_2$-terminated ${\text{SrTiO}}_3$ surfaces, which are calculated using the $2\times 2$ supercell. (a), (b), and (c) correspond to the adsorption coverages of 1/3, 1/12, and 1/6 ML, respectively. Note that there exist several different adsorption configurations for 1/3 and 1/6 ML coverages, while only the most energetically favorable ones are given here. Green, gray, red, and white balls represent Sr, Ti, O, and H atoms, respectively.

Figure 4

Band structures of (a) clean and (b)–(e) hydrogen-adsorbed ${\text{TiO}}_2$-terminated ${\text{SrTiO}}_3$ surface. Herein, (b) and (c), respectively, correspond to the adsorption coverages of 1/12 and 1/6 ML, which are calculated using $2\times 2$ supercell, while, (a), (d), and (e), respectively, correspond to the clean surface, Ti-2/3 ML adsorption, and 1 ML adsorption, which are calculated using $1\times 1$ supercell. The Fermi level is denoted by the dashed line. Surface-state bands (SSBs) are denoted by arrows.

Figure 5

(Color online) Electron density differences (side view) between (a) the ${\text{TiO}}_2$-terminated ${\text{SrTiO}}_3$ surface with the 1/12 ML hydrogen adsorption and the clean ${\text{TiO}}_2$-terminated surface plus individual H atoms, (b) the ${\text{TiO}}_2$-terminated surface with the Ti-2/3 ML hydrogen adsorption and that with the 1/3 ML coverage plus individual H atoms, and (c) the SrO-terminated ${\text{SrTiO}}_3$ surface with the 1/2 ML hydrogen adsorption and the clean SrO-terminated surface plus individual H atoms. The isosurfaces at the values of $0.06e/{\text{Å}}^3$ and $-0.06e/{\text{Å}}^3$ are depicted by the blue and yellow contours, respectively. Green, gray, red, and white balls represent Sr, Ti, O, and H atoms, respectively.

Figure 6

APDOS of the ${\text{TiO}}_2$-terminated ${\text{SrTiO}}_3$ surface with the Ti-2/3 ML hydrogen adsorption. The Fermi level is denoted by the dashed line.

Figure 7

Band structures of the (a) clean and (b) hydrogen-adsorbed (1/2 ML coverage) SrO-terminated ${\text{SrTiO}}_3$ surface. The Fermi level is denoted by the dashed line. SSB is denoted by the arrow.