Giant Strain Tunability in Polycrystalline Ceramic Films via Helium Implantation

TL;DR

Helium implantation enables significant, crack-free strain tuning in polycrystalline ceramic films, expanding lattice parameters up to 3.2% while maintaining ferroelectric properties, thus offering a cost-effective method for strain engineering.

Contribution

This study demonstrates for the first time that helium implantation can induce large, stable strains in polycrystalline films without cracking, extending strain engineering techniques beyond epitaxial monocrystalline materials.

Findings

Achieved up to 3.2% lattice expansion in polycrystalline BiFeO3 films.

Helium implantation did not cause structural cracks in the films.

Ferroelectric properties remained stable up to certain helium doses.

Abstract

Strain engineering is a powerful tool routinely used to control and enhance properties such as ferroelectricity, magnetic ordering, or metal-insulator transitions. Epitaxial strain in thin films allows manipulation of in-plane lattice parameters, achieving strain values generally up to 4%, and above in some specific cases. In polycrystalline films, which are more suitable for functional applications due to their lower fabrication costs, strains above 1% often cause cracking. This poses challenges for functional property tuning by strain engineering. Helium implantation has been shown to induce negative pressure through interstitial implantation, which increases the unit cell volume and allows for continuous strain tuning with the implanted dose in epitaxial monocrystalline films. However, there have been no studies on the transferability of helium implantation as a strain-engineering technique to polycrystalline films. Here, we demonstrate the technique's applicability for strain engineering beyond epitaxial monocrystalline samples. Helium implantation can trigger an unprecedented lattice parameter expansion of up to 3.2% in polycrystalline BiFeO3 films without causing structural cracks. The film maintains stable ferroelectric properties with doses up to 1E15 He/cm2. This finding underscores the potential of helium implantation in strain engineering polycrystalline materials, enabling cost-effective and versatile applications.