Fabrication of fully undercut ZnO-based photonic crystal membranes with 3D optical confinement

• A fabrication method for highly adaptable, fully undercut ZnO-based photonic crystal membranes is demonstrated.
• The entire process is realized on a silicon substrate, with the MBE-grown ZnO film sandwiched between thin films of SiO2.
• Silicon is utilized as a sacrificial material, enabling selective etching between ZnO and the substrate.
• PL measurements show photonic resonances within the photonic band gap, demonstrating tailored 3D optical confinement.

For studying nonlinear photonics, a highly controllable emission of photons with specific properties is essential. Two-dimensional photonic crystals (PhCs) have proven to be an excellent candidate for manipulating photon emission due to resonator-based effects. Additionally, zinc oxide (ZnO) has high susceptibility coefficients and therefore shows pronounced nonlinear effects. However, in order to fabricate such a cavity, a fully undercut ZnO membrane is required, which is a challenging problem due to poor selectivity of the known etching chemistry for typical substrates such as sapphire or ZnO. The aim of this paper is to demonstrate and characterize fully undercut photonic crystal membranes based on a thin ZnO film sandwiched between two layers of silicon dioxide (SiO2) on silicon substrates, from the initial growth of the heterostructure throughout the entire fabrication process. This process leads to a fully undercut ZnO-based membrane with adjustable optical confinement in all three dimensions. Finally, photonic resonances within the tailored photonic band gap are achieved due to optimized PhC-design (in-plane) and total internal reflection in the z-direction. The presented approach enables a variety of photon based resonator structures in the UV regime for studying nonlinear effects, including photon-exciton coupling and all-optical switching.