Growth Phase Map of α and \b{eta} Tantalum as a Function of Growth Temperature and Oxygen Partial Pressure
Tantalum thin films are used in low-loss superconducting quantum circuits, yet the growth conditions governing selection of $α$-Ta(110), $α$-Ta(111), and metastable $β$-Ta on c-plane sapphire remain uncertain. We deposited 100 nm Ta films by dc magnetron sputtering on Al$_2$O$_3$(0001) at 300-900 $^\circ$C while varying the O$_2$ partial pressure from a $5.5\times10^{-11}$ Torr ultrahigh-vacuum baseline to $2.23\times10^{-4}$ Torr. Absorption-corrected X-ray diffraction texture coefficients were used to construct a ternary phase-orientation growth map in terms of temperature and the oxygen-to-Ta impingement flux ratio ($2Φ_{O_2}/Φ_{Ta}\leq4.8\times10^{-6}$ to 19). Without added oxygen, $α$-Ta(110) dominated at every temperature. At an O$_2$ pressure of only $2.5\times10^{-9}$ Torr ($2Φ_{O_2}/Φ_{Ta}=2.2\times10^{-4}$), $α$-Ta(111) accounted for 98-99% of the normalized texture weight from 300 to 700 $^\circ$C, decreasing to 65% at 900 $^\circ$C. Substantial $β$-Ta formed only at the highest oxygen dose, dominating at 300 $^\circ$C and disappearing at 900 $^\circ$C. The apparent $β$(001)/$α$(110) selection energy of $1.07\pm0.16$ eV is comparable to the activation energy for oxygen diffusion in Ta, consistent with oxygen transport influencing phase selection. Oxygen also produced orientation-dependent changes in crystallite size and microstrain. These findings establish oxygen-controlled processing windows for phase- and orientation-selective growth of Ta films for superconducting devices.