Direct evidence of metallicity at $\text{ZnO}\left(000\overline{1}\right)\text{−}\left(1\times 1\right)$ surfaces from angle-resolved photoemission spectroscopy

We have investigated the stability of O-polar $\text{ZnO}\left(000\overline{1}\right)\text{−}\left(1\times 1\right)$ surfaces with photoemission spectroscopy and low-energy electron diffraction. We present direct evidence of quasi-two-dimensional electron gas (2DEG) formation $\left(N_{2D}\le 2\times {10}^{13}{\text{cm}}^{-2}\right)$ at the O-face from our angle-resolved photoemission spectroscopy studies. These findings are discussed in terms of hydrogenation of the O-face by chemisorptions process and the energetic location of the charge-neutrality level of ZnO. Our results reveal insight into the role of hydrogen in forming the quasi-2DEG and stabilizing the O-polar nonreconstructed surface.

Figure 1

(Color online) XPS of the (a) $\text{O}1s$ and (b) valence band regions for both ZnO(0001)-Zn or $\text{ZnO-}\left(000\overline{1}\right)\text{−}\text{O}$ surfaces following in-situ preparation. The inset in (a) displays a typical LEED pattern from the $\left(000\overline{1}\right)$-O surface, with an incident electron energy of 91 eV.

Figure 2

(Color online) (a) Normal emission ARPES spectra from the $\left(000\overline{1}\right)$-O surface, displaying the variation of the semi-core $\text{Zn}3d$ and $\text{O}2p$ derived valence band region. The inset displays the spectral weight at the Fermi level. Weak features at the Fermi level are observed for 32 and 88 eV, which corresponds to the $\Gamma$-point in the $k_{\perp }$ direction. (b) ARPES photocurrent intensity map of the near-Fermi level emission for $h\nu =90\text{eV}$ along the $\overline{\Gamma }\to \overline{K}$ direction in $k_{\parallel }$-space, recorded at 77 K.