Influence of Bi doping on electronic structure and optical behavior of congruent LiNbO3: Insights from density functional theory

Lithium niobate (LN), a synthetic crystal with unique properties, has played a significant role in optoelectronic technologies over the past two decades. In this study, we employ large supercells to model realistic defect and impurity concentrations in order to investigate the electronic structure and optical properties of congruent LN, both in its pure form and with Bi doping, using density functional theory (DFT) calculations. First, a congruent system containing Nb anti-site ( Nb Li 4 + ) and Li vacancy ( V Li − ) defects is examined. Then, Bi dopants are introduced into the system in two charge-compensated defect configurations: Bi Li 4 + + Nb Li 4 + + 8 V Li − and Bi Li 4 + + 4 V Li − . Our results indicate that the coexistence of bismuth anti-site Bi Li 4 + and niobium anti-site Nb Li 4 + leads to the largest reduction in the effective band gap (about 17%) compared to pristine LN. Furthermore, the bismuth anti-site 6 s state creates deep levels within the gap, which act as intermediary electronic bridges for electronic transitions from valence to the conduction bands. Consequently, this doping configuration exhibits enhanced optical properties near the absorption edge, with its absorption coefficient increasing by 56% compared to pristine LN in the visible spectrum. Such improvements make this structure highly promising for applications in optoelectronic devices and optical sensors in advanced technologies. • 540-atom supercells used to model realistic defect/dopant concentrations. • Most stable configurations of intrinsic defect and Bi-dopant are determined. • The electronic structure of Bi-doped congruent LiNbO 3 (CLN) is then found. • Optical properties are found and compared in pristine and Bi-doped CLN. • Bi-doping is found to enhance light absorption by 56% in the visible range.