Photon Factory Activity Report 2018 #36 (2019)

BL-2A/2016G621

High-density 2D electron system induced by oxygen vacancies in ZnO
T. C. Rödel1, 2, J. Dai1, F. Fortuna1, E. Frantzeskakis1, P. Le Fèvre2, F. Bertran2, M. Kobayashi3,R. Yukawa3, T. Mitsuhashi3, M. Kitamura3, K. Horiba3, H. Kumigashira3, and A. F. Santander-Syro1*
12

CSNSM, Univ. Paris-Sud, CNRS/IN2P3, Université Paris-Saclay, 91405 Orsay Cedex, France
Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin-BP48, 91192 Gif-sur-Yvette, France3
Photon Factory, 1-1 Oho, Tsukuba, 305-0801, Japan

1 Introduction
The transparent semiconductor ZnO stands out for its
numerous applications, notably due to its optoelectronic
properties, which are also determined by electron densityand dimensionality [1, 2]. The realization of a highly
doped 2D electron system (2DES) in ZnO would behighly desirable for, e.g., high-power applications, as
ZnO is cheaper, easier to process, and non-toxic
compared to other transparent semiconductors such as
In2O3. However, so far, only moderate electron densitiesof 2x1013 cm2 had been achieved in ZnO, while thecontribution of oxygen vacancies to its n-typeconductivity remained controversial. This work reports
the creation of a high-density 2DES in ZnO.

Fig. 1: (a) Fabrication of a highly doped 2DES in ZnO. Pure Al isevaporated at the surface of ZnO. The redox reaction produces a 2DES
at the AlOx/ZnO interface. (b) ARPES energy-momentum map of the2DES. The red curve is the momentum distribution curve at EF. The
black vertical bars show the Fermi momenta kFi and kFo of the inner andouter subbands, respectively. Bands are coupled to phonons with a
Debye energy ωD shown by the dashed black line. (c, d) Experimental
real and imaginary parts of the electron self-energy (red circles), andtheir Kramers–Kronig transforms (black crosses), for the outer subbandof the 2DES. The dark blue curves are simultaneous fits to Σ1 and Σ2
using a model of a 2D Fermi-liquid (filled gray) dressed by Debyephonons (filled light blue). Adapted from Ref. [3].

2 Experiment
ARPES experiments were performed at BL-2A ofKEK-PF and at synchrotron SOLEIL (France). The
AlOx/ZnO interface was obtained by in situ surface
preparation as described in Ref. [3]. The sample
temperature during measurements was 20 K. Variation of
the photon energy in the range 30-110 eV revealed nochanges
intheenergy-momentum
dispersion,demonstrating the 2D character of the band structure. The
pressure was below 10-11 mbar during measurements, andno degradation of the spectra quality was observed.
3 Results and Discussion
We observed (Fig. 1a) that the evaporation in ultrahigh vacuum of an atomic layer of aluminum on ZnO
creates a 2DES with electron densities up to 100 times
higher than in previous studies. The 2DES results fromoxidation of the Al layer and concomitant doping withoxygen vacancies of the underlying ZnO surface. The2DES is composed of two subbands (Fig. 1b) withdifferent effective masses. The self-energy of the 2DES
(Figs. 1c-d) can be accurately described using a 2D Fermi
liquid coupled to a Debye distribution of phonons. Thus,
the inner band is wholly renormalized due to theproximity of its bottom with the phonon cutoff energy,
whereas the outer band, dispersing deeper in energy,
shows only a kink due to electron-phonon interaction.
References[1] C. F. Klingshirn et al., Zinc Oxide: From
Fundamental Properties Towards Novel Applications,
Springer Series in Materials Science 120, SpringerVerlag (2010).
[2] H. Morko and Ü. Özgür, Zinc Oxide: Fundamentals,Materials and Device Technology, Wiley-VCH
Verlag GmbH & Co. KGaA (2009).
[3] T. C. Rödel et al., Phys. Rev. Materials 2, 051601(R)(2018).
Research Achievements1. Our paper [3] was selected as an “Editors’Suggestion” in Phys. Rev. Materials.
* andres.santander@csnsm.in2p3.fr