Photon Factory Activity Report 2018 #36 (2019)

BL-13A/2009S2-007, 2010G044

Metallization of the ZnO Surface by Ethanethiol
Kenichi OZAWA1,* and Kasuhiko MASE2,3
Department of Chemistry, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan
2
Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK),Tsukuba, Ibaraki 305-0801, Japan
3
SOKENDAI (The Graduate University for Advanced Studies), Tsukuba, Ibaraki 305-0801, Japan
1

1 Introduction
ZnO is one of metal-oxide semiconductors whichdevelop a two-dimensional electron gas (2DEG) at theirsurfaces [1]. The 2DEG formation results in the surfaces ofmetal oxides having a metallic conductivity. This property
is crucial for realization of all-oxide electronics.
The single crystal surfaces of ZnO are known to bemetallized by illumination of the ultraviolet light [2],hydrogen atom adsorption [1, 3], and methanol and water
adsorption [4]. However, the metallic nature of thesesurfaces is easily removed when the surfaces are exposed
to atmosphere. Thus, a more stable system is desired.
In the present study, surface electronic properties of Znterminated ZnO(0001) and O-terminated ZnO(000-1)
surfaces modified by ethanethiol (C2H5SH) by angleresolved photoelectron spectroscopy (ARPES) and X-ray
photoelectron spectroscopy (XPS).

substrate. On the other hand, an adsorption-induced shift is
negligible on the Zn-terminated surface. Thus, dissociated
H is not so effective to donate electrons on the Znterminated surface.
Fig. 1 shows an ARPES intensity plot of the ethanethiolsaturated O-terminated ZnO surface. A slightly brightregion is seen at just below the Fermi level around thecenter of the surface Brillouin zone (a wave number of 0nm−1). This proves the metallic nature of this surface. On
the other hand, no such state is observed on the Znterminated surface after ethanethiol adsorption. Thus,
ethanethiol is effective for metallization of the Oterminated ZnO surface. This result may pave the way forthiolate-protected ZnO-based optoelectric devices whose
thioate-ZnO interface is metallic in nature.

2 Experimental
The ARPES and XPS measurements were carried out ata beam line 13A of the Photon Factory [5]. The Zn- and Oterminated ZnO surfaces were cleaned in ultrahigh vacuum
conditions (UHV) by cycles of Ar+ sputtering andannealing at 1000 K in 1 × 10−4 Pa O2. The clean surfaces
were confirmed to be semiconductors in nature because ofabsence of a density of states at the Fermi level.
A vapor of ethanethiol was introduced to the loadlock
chamber, in which the clean ZnO samples were stored andethanethiol was allowed to be adsorbed on the surfaces.
The adsorption temperature was 300 K.
3 Results and Discussion
The 2DEG at the oxide surfaces always accompanies aformation of a strong accumulation layer. Thus, anecessary factor for the metallic oxide surfaces is electron
dopants on the surface. Hydrogen is known to be a very
efficient dopants on the ZnO surfaces [1, 3]. Water andmethanol also act as electron dopants [4]. These molecules
adsorb dissociatively on the ZnO surfaces to give electrondonating hydrogen atoms. Ethanethiol is found in thepresent XPS study to dissociate into ethanethiolate(C2H5S) and H on both Zn- and O-terminated ZnO surfacesat room temperature. Thus, the H adatoms are expected todonate the electrons to ZnO. This is true on the Oterminated surface but is not on the Zn-terminated surface.
Upon adsorption of the saturation amount of ethanethiol on
the O-terminated surface, the energy levels of ZnO shift by
0.85 eV to a high binding energy side. A sufficient numberof the electrons are transferred from adsorbates to theFig. 1 ARPES intensity map of the ethanethiolsaturated O-terminated ZnO surface.

Acknowledgements
This work was financially supported by a Grant-in-Aidfor Scientific Research (Grant No. 21560695) and Asahi
Glass Foundation.
References[1] R. Yukawa et al., Phys. Rev. B 94, 165313 (2016).
[2] L. F. J. Piper et al., Phys. Rev. B 81, 233305 (2010).
[3] K. Ozawa et al., Phys. Rev. B 83, 125406 (2011).
[4] K. Ozawa et al., Phys. Rev. B 81, 205322 (2010).
[5] A. Toyoshima et al., J. Phys.: Conf. Ser. 425, 152019(2013).
* ozawa.k.ab@m.titech.ac.jp