Growth of GeO2 on R-Plane and C-Plane Sapphires by MOCVD

Rutile germanium dioxide (GeO2) has been recently theoretically identified as an ultrawide-bandgap (UWBG) semiconductor with bandgap 4.68 eV similar to that of Ga2O3 but having bipolar dopability and ∼2× higher electron mobility, Baliga figure of merit (BFOM), and thermal conductivity compared to Ga2O3. Bulk crystal growth is rapidly moving toward 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 a controlled phase, pristine surface/interface quality, precise microstructure, and optimal functional properties, is still in its infancy. In this work, we explored the growth of GeO2 by metal–organic chemical vapor deposition (MOCVD) on both C- and R-plane sapphires. Utilizing tetramethyl germane (TMGe) as a precursor, we 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.