Hydrogen-Induced Surface Metallization of ${\mathrm{SrTiO}}_3(001)$

Surface metallization of ${\mathrm{SrTiO}}_3(001)$ by hydrogen adsorption is experimentally confirmed for the first time by photoemission spectroscopy and surface conductivity measurements. The metallic state is assigned to a quantized state in the space-charge layer induced by electron doping from hydrogen atoms. The measured two-dimensional (2D) conductivity is well above the 2D Ioffe-Regel limit indicating that the system is in a metallic conduction regime. The mean free path of the surface electron is estimated to be several nanometers at room temperature.

Figure 1

Normal-emission angle-integrated photoemission spectra of the bare, H-covered (intermediate coverage) and H-saturated ${\mathrm{SrTiO}}_3(001)$ surfaces (a) at the Fermi level ($E_F$) and (b) at the O $2p$ band. Spectra were taken at $h\nu =81\mathrm{eV}$ and at room temperature. MS indicates the position of the metallic state and IGS of the in-gap state.

Figure 2

(a) Wave-number-resolved photoemission spectra of the H-saturated (1350 L) ${\mathrm{SrTiO}}_3(001)$ surface along the $\overline{\Gamma }\mathrm{\text{−}}\overline{X}$ direction, taken at $h\nu =81\mathrm{eV}$ and at 20 K. (b) Color-scale image of the photoemission intensity. (c) Second derivative of the photoemission intensity. MS denotes the metallic state position. The red line indicates the parabolic fit for the metallic state. Color scale: Black (minimum intensity) to white (maximum of intensity).

Figure 3

$\mathrm{Ti}2p$ core-level photoemission spectra of the clean and H-saturated ${\mathrm{SrTiO}}_3(001)$ surfaces at room temperature. The photon energy is $h\nu =605\mathrm{eV}$.

Figure 4

Schematic drawing of the band-bending change induced by surface metallization during hydrogen adsorption on the ${\mathrm{SrTiO}}_3(001)$ surface. The Debye length is estimated to 6 nm.

Figure 5

(a) Schematic of four-terminal method transport measurements on the $\mathrm{H}/{\mathrm{SrTiO}}_3(001)$ surface. (b) Conductivity change $\Delta \sigma$ with H exposure, referred to the conductivity of the clean surface. Measurements were performed at room temperature.