Single axis strain provides a new level of control over optical bandgap

Helium implantation allows for continuous control of optical band gaps in semiconducting films through the resultant strain — an impossibility with traditional strain engineering. This critical advance allows a direct approach to tuning material properties which may significantly impact several alternative energy technologies.

Low-energy helium implantation directly and continuously generates single-axis strain in thin films of the prototypical semiconducting oxide SnO₂, directly modifying the optical band gap. While traditional epitaxy results in multi-directional lattice changes that only allow discrete increases in bandgaps, the present work demonstrates that a downward shift in the bandgap can be linearly dictated as a function of out-of-plane lattice expansion. The experimental observations closely match density functional theory calculations, which demonstrate that uniaxial strain provides a fundamentally different effect on the band structure than traditional epitaxy-induced multi-axes strain effects. Charge density calculations further support these findings and provide evidence that uniaxial strain can be used to drive orbital hybridization inaccessible to traditional strain engineering techniques. Importantly, the use of noble ions may allow for bandgap tuning without inducing charge trap sites in photovoltaic applications.

This ability to minutely control single axis strain allows unprecedented access to new parameters from which to design materials needed for next generation functionalities.

Andreas Herklotz, Stefania F. Rus, and Thomas Zac Ward, “Continuously Controlled Optical Band Gap in Oxide Semiconductor Thin Films,” Nano Letters (2016).   DOI: 10.1021/acs.nanolett.5b04815