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

TL;DR

This paper demonstrates a simple method to create a high-density two-dimensional electron system in ZnO using aluminum deposition, revealing strong many-body interactions and multiple subbands.

Contribution

It introduces a novel, straightforward approach to induce a high-density 2DES in ZnO via aluminum oxidation and oxygen vacancies, with detailed electronic structure analysis.

Findings

Achieved a 2DES with electron density up to 100 times higher than in heterostructures.

Observed two s-type subbands with signatures of many-body interactions.

Modeled the system as a 2D Fermi liquid coupled with phonons.

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.