Effect of oxygen vacancy and Al-doping on the electronic and optical properties in SnO2

The electronic and optical properties of Al-doped SnO2 without and with O vacancies are investigated by means of first-principles calculations. Our results indicate that Al-doped SnO2 is of p-type conductivity character and its band gap decreases compared with undoped SnO2. The accompanied oxygen vacancies can introduce localized gap states below the bottom of the conduction band and lead to the conductivity character translating to n-type from p-type in Al-doped SnO2. The localized gap states are contributed by the hybridizing of Sn 5s, 5p, O 2p, and Al 3p states and the density of these states becomes much stronger with oxygen vacancies increasing. Meanwhile, Fermi level shifts upward, showing a stronger n-type conductive character. With respect to the optical properties, Al-doped SnO2 has a weak absorption peak at about 2.73 eV in the visible region, while the presence of oxygen vacancies reinforces the absorption in the visible light region.