Hydrogen-induced metallization on the ZnO(0001) surface

The formation of hydrogen overlayers on the Zn-terminated $\mathrm{ZnO}\left(0001\right)$ surface has been reexamined by angle-resolved photoemission spectroscopy (ARPES). While low-energy electron diffraction patterns display the same (1 × 1) symmetry for different surface preparations, the electronic structure feature close to the Fermi level shows the formation of electron pockets, compatible with hydrogen-induced metallic states. Using ARPES and density functional theory (DFT) calculations, we show that hydrogen adspecies can also lead to metallization of this zinc-oxide surface in a similar manner as observed previously on $\mathrm{ZnO}\left(10\overline{1}0\right)$ and O-terminated $\mathrm{ZnO}\left(000\overline{1}\right)$. Importantly, our DFT calculations indicate that these electron pockets are formed by $sp$ hybridized states and therefore the angular distribution of the emitted photoelectron is significantly suppressed at the normal emission.

Figure 1

(a) XPS survey spectra of a $\mathrm{ZnO}\left(0001\right)$ single-crystal surface after hydrochloric acid etching followed by annealing in UHV at 850 K. The relevant photoemission peaks are indicated. (b) Typical (1 × 1) LEED pattern and (c) topographic atomic force microscopy image obtained for the clean surface after the etching and annealing procedure. (d) O $1s$ core-level spectra for different sample preparations indicating the presence of hydrogen on the surface. The spectra are offset for clarity.

Figure 2

(a)–(d) ARPES intensity plots for $k_x$ and $k_y$ scans in the $\overline{\mathrm{\Gamma }}\text{−}\overline{M}\text{−}\overline{K}$ planes at the first ($\overline{\mathrm{\Gamma }}$) and second (${\overline{\mathrm{\Gamma }}}_1$) surface Brillouin zones of $\mathrm{ZnO}\left(0001\right)$. The scan directions along $\overline{\mathrm{\Gamma }}\overline{K}$ and $\overline{\mathrm{\Gamma }}\overline{M}$ are shown in (a) and (c); (b) and (d), respectively. (e),(f) Constant-energy contour plots for photoemission intensity in the $\overline{\mathrm{\Gamma }}\text{−}\overline{M}\text{−}\overline{K}$ planes for 4.6 and 0.2 eV, respectively. (e) indicates the corresponding Brillouin zone, as well as the high-symmetry points and directions calculated using the crystal bulk lattice values.

Figure 3

(a),(b) ARPES intensity plots of the dispersive features close to the $E_F$ of the $\mathrm{ZnO}\left(0001\right)$ surface along two high-symmetry axes, ${\overline{\mathrm{\Gamma }}}_1\overline{K}$ and ${\overline{\mathrm{\Gamma }}}_1\overline{M}$, respectively. The dashed lines through the plots indicate fitted parabolas assuming parabolic energy-dispersion bands. (c) Fermi surface map centered at $k_x=0$ and $k_{y}2.26{\AA }^{-1}$ at 40 meV binding energy.

Figure 4

(a) Unit cell employed in the DFT calculations with O atoms in red, Zn atoms in pink, and pseudo-H atoms in gray. Adsorbed H atoms (in blue) for (b) 0.25, (c) 0.5, and (d) 1.0 ML on top sites and (e) for hydrogen adsorbed on the fcc hollow site.

Figure 5

$k$-resolved band structure calculations for the total density of states and each surface atom orbital contribution along the $\mathrm{\Gamma }K$ and $\mathrm{\Gamma }M$ directions for (a) clean (1 × 1) ZnO (0001) surface; (b) hydrogen adsorbed on zinc and oxygen topmost atoms; (c) hydrogen adsorbed on oxygen subsurface atoms; (d) hydrogen adsorbed on zinc topmost atoms; and (e) hydrogen adsorbed on fcc hollow sites. The hydrogen adsorption sites employed in the DFT calculations are shown in the right column with O atoms in red, Zn atoms in pink, and H atoms in blue.