Growth Phase Map of α and {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 Al₂O₃(0001) at 300-900 ^∘C while varying the O₂ partial pressure from a 5.5×10⁻¹¹ Torr ultrahigh-vacuum baseline to 2.23×10⁻⁴ 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 (2ΦO₂/ΦTa≤4.8×10⁻⁶ to 19). Without added oxygen, $α$-Ta(110) dominated at every temperature. At an O₂ pressure of only 2.5×10⁻⁹ Torr (2ΦO₂/ΦTa=2.2×10⁻⁴), $α$-Ta(111) accounted for 98-99% of the normalized texture weight from 300 to 700 ^∘C, decreasing to 65% at 900 ^∘C. Substantial $β$-Ta formed only at the highest oxygen dose, dominating at 300 ^∘C and disappearing at 900 ^∘C. The apparent $β$(001)/$α$(110) selection energy of 1.07±0.16 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.