Structural, electronic, and optical properties of N-doped SnO2

Structural, electronic, and optical properties of various N-doped SnO2 were investigated using first-principles calculations. The calculated formation energies show that both the substitutional and the interstitial N atoms are preferentially occupied in anion sites, while the N defect formation energies in the O-rich conditions are much lower than that in the Sn-rich ones. The electronic structures demonstrate that three mechanisms are possible with regard to the red-shift of photoluminescence. The first is that the band gap width reduces because of N2p repulsing O2p states and raising up the top of valence band (EEV) with N substituting for Sn; the second is that some N2p gap states are induced by N substituting for 0 resulting in the band gap reducing; and the third is N2p states lowering the bottom of the conduction band (E-C) leading to the reduction of band gap by introducing a interstitial N. On the basis of the calculated formation energy and experimental results, the red-shift phenomenon should not be the transition from band to band but the band to gap states. The red-shift mechanism should be N2p gap states to band transition.