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 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.