Elastic strain modulation of energy bandgap in β-Ga2O3 sheet: Experimental and computational investigations

Elastic strain engineering has proven to be an efficient tool to substantially modify the physical properties of semiconductors. Likewise, modification of properties by elastic strain based on 2D materials has drawn a considerable attention recently. While on ultrawide-bandgap (UWBG) materials, emergent alternatives for next-generation high-power and high-efficient electronic devices, there exist only a few experimental reports related to the strain modification of fundamental physical properties. According to the theoretical prediction, metallization and indirect-to-direct transition can be achieved in diamonds through elastic strain. In previous work, the modulation of bandgap and indirect-to-direct bandgap transition in 2D β-Ga2O3 under uniaxial strains were demonstrated. The present work reports a modified two-point bend method to apply the elastic strain to β-Ga2O3 material continuously without any damage in the crystal quality, and investigates the changes in bandgap caused by the strain-induced deformation. Free-standing β-Ga2O3 sheets exhibit a sizeable reduction in the bandgap of up to 30% (from 4.9 eV to 3.4 eV) under the bending state, as measured by X-ray photoelectron spectroscopy (XPS). Based on the predictions obtained from first-principles calculations, a strong anisotropic character in the Young's modulus of β-Ga2O3 indicates the existence of strain gradient in the free-standing β-Ga2O3 sheets under bending states, which was evinced by the confocal Raman spectroscopic measurements. Moreover, the lattice distortion caused by the strain gradient and elastic strain can be attributed to the dramatic changes in the bandgap. More importantly, in as-bent samples, the variation of bandgap was continuous and recoverable. These findings provide a direct experimental evidence of bandgap modification of β-Ga2O3 by elastic strain, and indicate a much larger and more versatile platform for the application of flexible Ga2O3 based devices to a wide range of application areas.