In-situ etching using Ga flux in an ultra-high vacuum environment like MBE is introduced as a method to make high aspect ratio 3 dimensional structures in $beta$-Ga2O3. Etching of $beta$-Ga2O3 due to excess Ga adatoms on the epilayer surface had been viewed as non-ideal for epitaxial growth especially since it results in plateauing and lowering of growth rate. In this study, we use this well-known reaction from epitaxial growth to intentionally etch $beta$-Ga2O3. We demonstrate etch rate ranging from 2.9 nm/min to 30 nm/min with the highest reported etch rate being only limited by the highest Ga flux used. Patterned in-situ etching is also demonstrated and used to study the effect of fin orientation on the sidewall profiles and dopant (Si) segregation on the etched surface. Using in-situ Ga etching, we also demonstrate 150 nm wide fins and 200 nm wide nano pillars with high aspect ratio. This new etching method could enable future development of highly scaled vertical and lateral 3D devices in $beta$-Ga2O3.
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