Band alignment at metal/ferroelectric interfaces: insights and artifacts from first principles

TL;DR

This paper develops a first-principles model for band alignment at metal/ferroelectric interfaces, highlighting potential artifacts due to DFT limitations and providing criteria to identify them.

Contribution

It introduces a general model for band offsets, discusses artifacts caused by DFT band gap underestimation, and offers practical criteria to detect these issues in ferroelectric systems.

Findings

Identification of a pathological regime where insulators appear metallic

DFT underestimation of band gaps can cause simulation artifacts

Criteria to distinguish real effects from computational artifacts

Abstract

Based on recent advances in first-principles theory, we develop a general model of the band offset at metal/ferroelectric interfaces. We show that, depending on the polarization of the film, a pathological regime might occur where the metallic carriers populate the energy bands of the insulator, making it metallic. As the most common approximations of density functional theory are affected by a systematic underestimation of the fundamental band gap of insulators, this scenario is likely to be an artifact of the simulation. We provide a number of rigorous criteria, together with extensive practical examples, to systematically identify this problematic situation in the calculated electronic and structural properties of ferroelectric systems. We discuss our findings in the context of earlier literature studies, where the issues described in this work have often been overlooked. We also discuss formal analogies to the physics of polarity compensation at LaAlO3/SrTiO3 interfaces, and suggest promising avenues for future research.