Changes in VO2 band structure induced by charge localization and surface segregation

Vanadium vacancies introduce acceptor doping with hole localization, while oxygen vacancies cause electron localization and donor doping. As deposition temperature increases, donor concentration stays constant, whereas acceptor concentration significantly increases, leading to enhanced (011) lattice-plane compression and surface segregation. Localized charges result in shifts of O 1s and V4+ 2p core levels toward higher binding energies, and O 2p and V4+ 3d valence bands toward the Fermi level, but e(g)(pi) bands lifting and a(1g) bands splitting energies are both insensitive to charge localization. Particularly, band-gap energy decreases with increasing V-V pair distance, and is significantly reduced by band tailing.