Abstract
Optical band gap control of semiconducting thin films is critical for the optimization of photoelectronic and photochemical applications. In this work, we demonstrate that the optical band gap of WO3 films can be continuously controlled through uniaxial strain induced by low-energy helium implantation. We show that the implantation of He into epitaxially grown and coherently strained WO3 films can be used to induce single axis out-of-plane lattice expansion of up to 2%. Ellipsometric spectroscopy reveals that this lattice expansion shifts the absorption spectrum to lower energies and effectively reduces the optical band gap by about 0.18 eV per percent expansion of the out-of-plane unit cell length. Density functional calculations show that this response is a direct result of changes in orbital degeneracy driven by changes in the octahedral rotations and tilts.
