  Rutile Germanium Dioxide (GeO2) has been recently theoretically identified as
an ultrawide bandgap (UWBG) semiconductor with bandgap 4.68 eV similar to Ga2O3
but having bipolar dopability and ~2x higher electron mobility, Baliga figure
of merit (BFOM) and thermal conductivity than Ga2O3. Bulk crystal growth is
rapidly moving towards making large sized native substrates available. These
outstanding material properties position GeO2 as a highly attractive UWBG
semiconductor for various applications. However, the epitaxial growth in the
most advantageous polymorph (rutile), ensuring controlled phase, pristine
surface/interface quality, precise microstructure, and optimal functional
properties, is still in its infancy. In this work, we explored growth of GeO2
by metal-organic chemical vapor deposition (MOCVD) on both C- and R-plane
sapphire. Utilizing tetramethylgermane (TMGe) as a precursor, we have
investigated the influences of different parameters on the film properties,
including growth temperature, chamber pressure, TMGe flow rate, oxygen flow
rate, shroud gas flow rate, and rotation speed. The total pressure emerged as a
crucial parameter while growth attempts at low total pressure resulted in no
films for a wide range of temperatures, precursor flow rate, argon flow rates,
and susceptor rotation rate. A phase diagram, derived from our experimental
findings, delineates the growth windows for GeO2 films on sapphire substrates.
This study serves as a pioneering guide for the MOCVD growth of GeO2 films.
