Thermochromic behavior of Ta-doped VO2 thin films: Influence of configuration, pulsed oxygen and dopant ratio

• Co-deposition of V x Ta 1−x O y thin films by combining GLAD and RGPP approaches. • Monolayer configuration shows better optics; multilayers offer doping control. • Longer O 2 pulses effectively reduce critical temperature (T c ). • Unlike W, Ta doping does not harm solar modulation and visible transparency. • Remarkable T c drops: monolayers 45 °C (0.9 at.% Ta); multilayers 46 °C (1.67 at.% Ta). This study focuses on the optimization of the optical performance of thermochromic vanadium dioxide (VO 2 ) films doped with tantalum. V x Ta 1−x O 2 layers of nominal thickness 50 nm were deposited on glass by co-sputtering of V and Ta in glancing-angle geometry, and subsequent fast annealing in air. The impact of the deposition configuration, either a V 0.973 Ta 0.027 O y monolayer or alternating VO y /V 0.916 Ta 0.084 O y multilayers, and the optional coupling of reactive oxygen pulses (t ON = 0, 4, 8 s) was studied for different Ta/V atomic ratios (1, 2.7, 3, and 5Ta). The longest oxygen injection times allow to achieve films with lower transition temperature (T c ). Although monolayers generally provided superior optical performance, multilayers deposited at higher Ta target currents enabled more effective and reproducible control of the doping level. The best performance for monolayers was achieved with 0.9 at.% Ta, yielding a T c of 49.9 °C at heating, a luminous transmittance (T lum ) near 50%, and solar modulation (ΔT sol ) and infrared solar modulation (ΔT IR ) capacities of 4.4% and 9.4%, respectively. For multilayers, the sample with 1.67 at.% Ta has T c = 51 °C, T lum = 47.3%, ΔT sol = 4.3%, and ΔT IR = 8.2%. With T c values up to 16 °C lower than those of undoped films produced under comparable conditions, these results indicate that Ta doping presents a promising alternative to existing dopants, enabling significant T c reduction without substantially compromising the properties required for VO 2 -based smart-window applications.