p-type conductivity mechanism and defect structure of nitrogen-doped LiNbO3 from first-principle calculation
Most metal-doped lithium niobate (LiNbO3, LN) exhibit n-type conductivity. The absence of LiNbO3 with p-type conductivity sets a limit to application. Based on the finding that p-type conductivity LiNbO3 can be realized by doping with non-metallic element N, we investigate the most stable defect configurations and formation energies of LiNbO3 doped with non-metal nitrogen (LN:N) by first-principle calculation. Nitrogen substitution, interstitial and quasi-substitution point defects in different sites and their effect were explored. The results show that N prefers to occupy the oxygen site with only little lattice distortion. Ab initio molecular dynamic (AIMD) simulation confirms the structure stability of an N ion occupying the O site. The charge-state transition level ε (0/-1) slightly above the valence band maximum (VBM) illustrates that N point defects would contribute to p-type conductivity of LiNbO3. The analysis of the band structure reveals that the partially filled impurity levels can accommodate electrons that jump from valence bands and result in holes to become the main charge carriers. The calculation not only explains the occurrence of p-type conductivity in LN:N but also provides a simple and efficient way to discover p-type conductivity candidates in numerous doped LiNbO3 crystals.