Giant Apparent Flexoelectricity in Semiconductors Driven by Insulator-to-metal Transition

TL;DR

This paper reveals a giant flexoelectric effect in semiconductors like Si and Ge caused by high strain gradients inducing an insulator-to-metal transition, leading to significant polarization and potential applications.

Contribution

It uncovers a novel flexoelectric mechanism driven by insulator-to-metal transition in bent semiconductor thin films, with high flexoelectric coefficients and quadratic thickness dependence.

Findings

High flexoelectric coefficients observed in bent Si and Ge films.

Insulator-to-metal transition causes electron transfer and polarization.

Effect extendable to other semiconductors with moderate gaps.

Abstract

We elucidate the flexoelectricity of materials in the high strain gradient regime, of which the underlying mechanism is less understood. By using the generalized Bloch theorem, we uncover a strong flexoelectric-like effect in bent thinfilms of Si and Ge due to a high strain gradient-induced insulator-to-metal transition. We show that an unusual type-II band alignment is formed between the compressed and elongated sides of the bent film, resulting in a spatial separation of electron and hole. Therefore, upon the insulator-to-metal transition, electrons transfer from the compressed side to the elongated side to reach the thermodynamic equilibrium, leading to pronounced polarization along the film thickness dimension. The obtained transverse flexoelectric coefficients are unexpectedly high, with a quadratic dependence on the film thickness. This new mechanism is extendable to other semiconductor materials with moderate energy gaps. Our findings have important implications for the future applications of flexoelectricity in semiconductor materials.