Comparison of the surface electronic structures of H-adsorbed ZnO surfaces: An angle-resolved photoelectron spectroscopy study

Synchrotron-radiation angle-resolved photoelectron spectroscopy has been utilized to study the interaction of atomic H with the nonpolar ZnO($10\overline{1}0$) surface and polar ZnO($000\overline{1}$) and (0001) surfaces. H adsorption leads to the semiconductor-to-metal transition on the ZnO($10\overline{1}0$) and ($000\overline{1}$) surfaces. Metallization is a consequence of the formation of a single metallic band within the potential well between the surface/vacuum interface barrier and the edge of the conduction band, which is bent downwardly at the surface. The electrons confined in the potential well exhibit a free-electron-like behavior along the surface parallel, realizing a two-dimensional electron gas system. For the H/ZnO(0001) system, on the other hand, no feature associated with the metallic band is observed. Higher reactivity of the ZnO(0001) surface toward H than the other two ZnO surfaces is responsible for the different behavior for the modification of the surface electronic structure by H adsorption.

Figure 1

Plots of relative intensities of the O 2$s$ emission peaks from the OH component to the total O 2$s$ peak intensities on ZnO($10\overline{1}0$), ZnO($000\overline{1}$), and ZnO(0001) against H exposure. The insets show O 2s core-level spectra ($h\nu$ $=$ 65 eV) for the clean and 200-L H-dosed surfaces. The spectra were measured at the detection angle of ${60}^{°}$ from the surface normal direction to enhance the surface sensitivity. The spectra shown here are those after subtracting polynomial background curves. The O 2$s$ components are reproduced by Gaussian functions.

Figure 2

Angle-integrated valence-band photoemission spectra of the ZnO($10\overline{1}0$), ($000\overline{1}$), and (0001) surfaces exposed to various amounts of H. The photon energy used was 65 eV.

Figure 3

(Upper) Changes of the VBM positions as a function of H exposure. The VBM positions are determined by extrapolating the leading edge of the valence-band emission to the baseline in the case of the clean surfaces, and the Zn 3$d$ peak shifts are used to reproduce the H-induced shift of the VBM. Assuming the band-gap energy of ZnO (3.37 eV,[5]) the CBM positions can be estimated from the VBM positions. The horizontal dotted line in each panel is drawn at 3.37 eV on the left axis and at 0.0 eV on the right axis. (Lower) Schematic of the energy band structures of the clean and 200-L H-dosed surfaces.

Figure 4

Hydrogen exposure dependences of the magnitude of band bending ($e\Delta V_{\mathrm{s}}$), the work function change ($\Delta \Phi$) and the change in the ionization energy ($\Delta I$). $e\Delta V_{\mathrm{s}}$ is positive (negative) for downward (upward) band bending. $\Delta I=e\Delta V_{\mathrm{s}}+\Delta \Phi$.

Figure 5

Photoemission spectra in the vicinity of $E_{\mathrm{F}}$ of three ZnO surfaces at various H exposures. The photon energy was 65 eV. Each spectrum is constructed by integrating the ARPES spectra with detection angles between $-{4.5}^{°}$ and ${4.5}^{°}$ along the high symmetry axes of the SBZ indicated in each panel. Dotted lines in the right panel are the backgrounds. Essentially the same sets of spectra are obtained when integrating along other high-symmetry axes.

Figure 6

Detection-angle-dependent spectra of the H-induced states of the 200-L H-exposed surfaces of ZnO($10\overline{1}0$) and ZnO($000\overline{1}$) along the high symmetry axes of the SBZ. The detection angle (${\theta }_{\mathrm{d}}$) is measured from the surface normal direction. The photon energy used was 65 eV.

Figure 7

(a) Plots of the raw emission intensity of the ARPES spectra (upper panels) and those divided by a GFD function (lower panels) along two high symmetry axes for the 200-L H-dosed ZnO($10\overline{1}0$) surface. The bright regions correspond to the high-emission-intensity regions. (b) Band structure of the H-induced state formed on ZnO($10\overline{1}0$) by H exposure (200 L). The plotted points correspond to the peak positions in the GFD-function-divided ARPES spectra. The size of the symbols represents the emission intensity, and the different symbols are the results of independent measurements. A solid line is a least-square-fitted result by a parabolic function.

Figure 8

Same as Fig. 7 but for the 200-L H-dosed ZnO($000\overline{1}$) system.

Figure 9

Schematic of the reactions induced by H exposure on (a) Zn-polar ZnO(0001) and (b) O-polar ZnO($000\overline{1}$), explaining the different etching behavior on these surfaces.