Strain engineering and strain measurement by spring tethers on suspended epitaxial GaN-on-Si photonic crystal devices

Suspended epitaxial gallium nitride (GaN) on silicon (Si) photonic crystal (PhC) devices suffer from large residual tensile strain, especially for long waveguides, because fine structures tend to crack due to large stress. By introducing spring-like tethers, designed by the combination of a spring network model and finite element method (FEM) simulations, the stress at critical locations was mitigated and the cracking issue was solved. Meanwhile, the tethered-beam structure was found to be potentially a powerful method for high-precision strain measurement in tensile thin films, and in this case, a strain of $2.27(\pm0.01)\times10^{-3}$ was measured in 350 nm epitaxial GaN-on-Si.