A member of p-type TCO family: Sn₂Ta_xNb_2-xO₇ with a tunable band gap and controllable hole mobility

Via density functional theory, we considered a new member of p-type transparent conducting oxides (TCOs) family, Sn₂Ta_xNb_2-xO₇ (x = 0, 0.5, 1, 1.5, 2), to modify the carrier mobility, band gaps, optical properties by Ta⁵⁺ substitution at Nb⁵⁺ sites. The calculated effective masses indicate that the carrier mobility is expected to be controlled by varying the ratio of Ta and Nb elements in Sn₂Ta_xNb_2-xO₇ compounds. Nb 4d orbital considerably exists in the valence band maximum (VBM) of Sn₂Ta_xNb_2-xO₇ (x = 0.5, 1, 1.5), which impedes the overlap between Sn 5s and O 2p orbitals. It offers reliable evidence that non (n-1) d¹⁰ns² cations in the VBM would be detrimental to the hole transport in TCOs. The introduction of Ta 5d orbitals promotes the conduction band edge toward a higher level, thus broadening the band gaps of Sn₂Ta_xNb_2-xO₇. A structural distortion has been observed when Ta⁵⁺ ions replaced Nb⁵⁺ sites since the two cations have different bindings with the surrounding oxygen atoms.