Epitaxial Growth of Rutile GeO$_2$ via MOCVD

Abstract: Rutile Germanium Dioxide (r-GeO$_2$) has been identified as an ultrawide bandgap (UWBG) semiconductor recently, featuring a bandgap of 4.68 eV, comparable to Ga$_2$O$_3$ but offering bipolar dopability, higher electron mobility, higher thermal conductivity, and higher Baliga's figure of merit (BFOM).These superior properties position GeO$_2$ as a promising material for various semiconductor applications. However, the epitaxial growth of r-GeO$_2$, particularly in its most advantageous rutile polymorph, is still at an early stage. This work explores the growth of r-GeO$_2$ using metal-organic chemical vapor deposition (MOCVD) on an r-TiO$_2$ (001) substrate, utilizing tetraethyl germane (TEGe) as the precursor. Our investigations reveal that higher growth temperatures significantly enhance crystalline quality, achieving a full width at half maximum (FWHM) of 0.181 degree at 925 degree C, compared to 0.54 degree at 840 degree C and amorphous structures at 725 degree C. Additionally, we found that longer growth durations increase surface roughness due to the formation of faceted crystals. Meanwhile, adjusting the susceptor rotation speed from 300 RPM to 170 RPM plays a crucial role in optimizing crystalline quality, effectively reducing surface roughness by approximately 15 times. This study offers a foundational guide for optimizing MOCVD growth conditions of r-GeO$_2$ films, emphasizing the crucial need for precise control over deposition temperature and rotation speed to enhance adatom mobility and effectively minimize the boundary layer thickness.