Vacancy-Enhanced $N-N$ Bonding and Deep Level Complex Defect Formation in $\beta-Ga_2O_3$

The formation and electronic properties of nitrogen-related defect complexes in $\beta-Ga_2O_3$ are investigated using first-principles calculations. Starting from the energetically favorable $N_{i9}-N_{OI}$ configuration, nitrogen atoms exhibit a strong tendency toward co-localization, leading to reduced $N-N$ separation. However, analysis of bond lengths and electron localization function shows that these configurations do not fully attain molecular $N_{2}$ character. The role of intrinsic defects is further examined by introducing oxygen and gallium vacancies. Vacancy-assisted configurations enhance local lattice relaxation and further decrease the $N-N$ distance. Formation energy calculations indicate that several vacancy-assisted complexes are thermodynamically favorable, while binding energy analysis confirms their stability against dissociation. Despite this, the density of states analysis reveals that all configurations introduce localized electronic states within the band gap. These states originate primarily from hybridized $N$-$2p$ and $O$-$2p$ orbitals and remain energetically separated from the band edges. Spin density analysis further confirms strong localization. Overall, these defect complexes act as deep trapping centers, limiting carrier transport in $\beta-Ga_2O_3$ and thereby promoting semi-insulating behavior and current blocking characteristics.