  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 etching is a simple, rapid means of wet processing SiC,
including the use of dopant selective etch stops that take advantage of mature
SiC homoepitaxy. However, dopant selective photoelectrochemical 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 new, selective photoelectrochemical etching 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 ($N_a=1(10)^{14}cm^{-3}$) device layer. We characterize
the photo-response and photoelectrochemical etching behavior of the diode under
bias and use those insights to suspend large ($>100\mu m^2$) 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 $>3x$. This work enables the fabrication of
high-purity suspended thin films suitable for scalable photonics, mechanics,
and quantum technologies in SiC.
