High-density two-dimensional electron system induced by oxygen vacancies in ZnO

Abstract

We realize a two-dimensional electron system (2DES) in ZnO by simply depositing pure aluminum on its surface in ultra-high vacuum, and characterize its electronic structure using angle-resolved photoemission spectroscopy. The aluminum oxidizes into alumina by creating oxygen vacancies that dope the bulk conduction band of ZnO and confine the electrons near its surface. The electron density of the 2DES is up to two orders of magnitude higher than those obtained in ZnO heterostructures. The 2DES shows two s-type subbands, that we compare to the d-like 2DESs in titanates, with clear signatures of many-body interactions that we analyze through a self-consistent extraction of the system self-energy and a modeling as a coupling of a 2D Fermi liquid with a Debye distribution of phonons. PACS numbers: 79.60.-iZnO is a transparent, easy to fabricate, oxide semiconductor with a direct band gap E g = 3.3 eV. Its many uses include window layers in photovoltaic devices, varistors for voltage surge protection, UV absorbers, gas sensors, and catalytic devices [1,2]. ZnO is also a candidate for novel applications, such as transparent field effect transistors, UV laser diodes, memristors, or hightemperature/high-field electronics [1][2][3][4][5][6]. In fact, ZnO can be seen as a link between the classical group-IV or III-V semiconductors, e.g. Si or GaAs, and transition metal oxides (TMOs), such as SrTiO 3 . Due to their valence d-orbitals, the latter show a rich variety of collective electronic phenomena, like magnetism or high-T c superconductivity [7,8]. Moreover, the controlled fabrication of a two-dimensional electron system (2DES) in ZnO can result in extremely high electron mobilities, even competing with the ones of GaAs-based heterostructures, and showing the quantum hall effects [9,10].2DES in TMOs have also improved in mobility over the last decade, but what makes them really unique is the control of superconductivity, magnetism and spinorbit coupling by varying the electron density using a gate voltage [11][12][13][14][15][16][17][18][19][20][21][22]. Additionally, for many insulating TMOs, recent works demonstrated that oxygen vacancies near the surface provide a simple and efficient mechanism to produce a 2DES, with electron densities as high as n 2D ∼ 3 × 10 14 cm −2 [23-33], and showing magnetic states linked to the presence of such vacancies [34].Here we show, using angle-resolved photoemission spectroscopy (ARPES), that the simple evaporation in ultra-high vacuum (UHV) of an atomic layer of pure aluminum on ZnO creates a 2DES with electron densities up to two orders of magnitude higher than in previous studies. We demonstrate that the 2DES results from oxidation of the Al layer and concomitant doping with oxygen vacancies of the underlying ZnO surface. The 2DES is composed of two subbands with different effective masses, as the mass of the inner band is wholly renormalized due to the energetic proximity of its band bottom with a phonon frequency, whereas the outer band, dispersing deeper in energy, shows only a ...

Quantum interference effects of out-of-plane confinement on two-dimensional electron systems in oxides

“…Since the initial reports, a great deal of progress has been made to investigate the origin of the intriguing metallic states appearing at the bare oxide surfaces, along with their main properties, and on how to improve their quality. Experimental evidence strongly suggests that they are created by the presence of oxygen vacancies (V O ) at or near the surface [9,10,[12][13][14][15][16][17][18][19][20][21]. Such vacancies seem to have a double key role:…”

“…In this case, the reducing agent is oxidized by capturing oxygen ions from the STO, leaving on its (buried) surface the vacancies that realize the metallic state, and simultaneously creating a protective capping layer. This procedure demonstrated its significance by forming an electron gas even in oxide materials where the oxygen vacancies by themselves are not stable or believed to be unable to dope the conduction band [23], such as the surface of ZnO [21], or by realizing a magnetically tunable capping layer, such as EuO/SrTiO 3 [20]. Moreover, this procedure allowed the measurement of a gate-tunable superconducting state on the 2DEG state at the STO surface [24].…”

Ultrafast generation and decay of a surface metal

“…Semiconductor-to-metal transitions (SMTs) also occur in two dimensions at semiconductor surfaces leading to the formation of two-dimensional electron gases (2DEGs) 2 . In the case of oxide surfaces, 2DEG formation is often caused by surface doping with shallow donor defects, such as oxygen vacancies 3 , 4 or adsorbates as, for instance, hydrogen 5 , 6 . The positively charged impurity sites modify the surface potential, causing downward surface band bending (BB) of the conduction and valence band (CB and VB, respectively).…”

Imaging the itinerant-to-localized transmutation of electrons across the metal-to-insulator transition in V 2 O 3

“…The surface of the V 2 O 3 films was cleaned in situ using protocols previously developed for the investigation of two-dimensional (2D) electron gases in oxides (26)(27)(28)(29)(30). The cleaned surfaces showed welldefined low-energy electron diffraction (LEED) patterns (the Supplementary Materials).…”