Creating Two-Dimensional Electron Gas in Nonpolar Oxide Interface via Polarization Discontinuity: First-Principles Analysis of CaZrO3/SrTiO3 Heterostructure

TL;DR

This study demonstrates how strain-induced polarization in CaZrO3/SrTiO3 heterostructures can induce two-dimensional electron and hole gases at interfaces, revealing a new method to engineer conductive states in nonpolar oxide interfaces.

Contribution

First-principles calculations reveal polarization-driven formation of 2DEG and 2DHG in CaZrO3/SrTiO3 heterostructures, highlighting the role of surface termination and strain.

Findings

Polarization induces insulator-to-metal transition with increasing CZO thickness.

Strong polarization drives charge transfer forming 2DEG at the interface.

ZrO2 surface termination leads to co-existing 2DEG and 2DHG due to polarization domains.

Abstract

We studied strain-induced polarization and resulting conductivity in the nonpolar/nonpolar CaZrO3/SrTiO3 (CZO/STO) heterostructure (HS) system by means of first-principles electronic structure calculations. By modeling four types of CZO/STO HS-based slab systems, i.e., TiO2/CaO and SrO/ZrO2 interface models with CaO and ZrO2 surface terminations in each model separately, we found that the lattice-mismatch-induced compressive strain leads to a strong polarization in the CZO film, and as the CZO film thickness increases, there exist an insulator-to-metal transition. The polarization direction and critical thickness of the CZO film for forming interfacial metallic states depend on the surface termination of CZO film in both types of interface models. In the TiO2/CaO and SrO/ZrO2 interface models with CaO surface termination, the strong polarization drives the charge transfer from the CZO film to the first few TiO2 layers in the STO substrate, leading to the formation of Two-Dimensional Electron Gas (2DEG) at the interface. In the HS models with ZrO2 surface termination, two polarization domains with opposite directions were formed in the CZO film, which results in the charge transfer from the middle CZO layer to the interface and surface, respectively, leading to the co-existence of the 2DEG on the interface and the Two-Dimensional Hole Gas (2DHG) at the middle CZO layer. These findings open a new avenue to achieve 2DEG (2DHG) in perovskite-based HS systems via polarization discontinuity.