Selective Undercut of Undoped Optical Membranes for Spin-Active Color Centers in 4H- Silicon Carbide

Silicon carbide (SiC) is a semiconductor used in quantum information processing, microelectromechanical systems, photonics, power electronics, and harsh environment sensors. However, its high-temperature stability, high breakdown voltage, wide bandgap, and high mechanical strength are accompanied by a chemical inertness, which makes complex micromachining difficult. Photoelectrochemical (PEC) etching is a simple, rapid means of wet processing SiC, including the use of dopant-selective etch stops that take advantage of the mature SiC homoepitaxy. However, dopant-selective PEC etching typically relies on highly doped material, which poses challenges for device applications such as quantum defects and photonics that benefit from low doping to produce robust emitter properties and high optical transparency. In this work, we develop a selective PEC process that relies not on high doping but on the electrical depletion of a fabricated diode structure, allowing the selective etching of an n-doped substrate wafer versus an undoped epitaxial (carrier density of 1(10)¹⁴ cm^-3) device layer. We characterize the photoresponse and PEC behavior of the diode under bias and use those insights to suspend large (100 × 100 μm) undoped membranes of SiC. We further characterize the compatibility of membranes with quantum emitters, performing comparative spin spectroscopy between undoped and highly doped membrane structures, finding the use of undoped material improves ensemble spin lifetime by >5×. This work enables the fabrication of high-purity suspended thin films suitable for scalable photonics, mechanics, and quantum technologies in SiC.