Calcium doping-induced metal–insulator transition in SrTiO3-based two-dimensional electron gas

Abstract

Among oxygen-deficient ABO3 perovskite titanates, SrTiO3 (STO) surfaces and interfaces exhibit a high-mobility two-dimensional electron gas (2DEG), generating significant research interest. To understand the origin of this 2DEG, we investigated the 2DEG at Ca-doped STO surfaces using combined experimental and theoretical approaches. Sr1−xCaxTiO3 (x = 0.01–0.5) thin films were epitaxially grown via pulsed laser deposition, with oxygen vacancies introduced by Ar+ bombardment to generate the 2DEG. Electrical transport measurements reveal composition-dependent suppression of conductivity and magnetoresistance, accompanied by a low-temperature metal–insulator transition for x > 0. Density functional theory calculations demonstrate that Ca doping induces a downward shift of the Fermi level, the (0/2+) transition level [ε(0/2+)] of VO, and the energy level of the neutral oxygen vacancy defect (VO0) relative to the conduction band minimum (CBM). Oxygen vacancies in SrTiO3−δ always act as shallow donors; however, they exhibit bipolarity in CaTiO3−δ and even become deep acceptors when the Fermi level approaches the CBM. The Fermi level is pinned below the CBM in CaTiO3−δ, thereby inhibiting conductivity. The work establishes the relationship between oxygen-vacancy energy levels and electronic transport in STO-based oxygen 2DEG systems, offering a strategy for tuning 2DEG properties through cation doping.