Diffusion of Acceptor Dopants in Monoclinic β-Ga₂O₃

β-Ga₂O₃ is a promising material for next-generation power electronics because of its ultrawide band gap and high breakdown voltage. However, realizing its full potential requires precise control of dopant incorporation and stability. Using first-principles calculations, we systematically assess the diffusion behavior of eight deep-level substitutional acceptors (Au, Ca, Co, Cu, Fe, Mg, Mn, Ni) in β-Ga₂O₃. Two diffusion mechanisms are considered: (i) interstitial diffusion under nonequilibrium conditions relevant to ion implantation and (ii) trap-limited diffusion (TLD) under near-equilibrium annealing. Our results show strong diffusion anisotropy along the b and c axes, governed by competition between diffusion and incorporation (or dissociation) barriers. Under interstitial diffusion, Ca and Mg show the most favorable combination of low migration and incorporation energies, making them efficient dopants. In contrast, Au diffuses readily but has an incorporation barrier exceeding 5 eV, rendering it ineffective. Co shows poor activation but high diffusion barriers, which may suppress unwanted migration. Under TLD, dissociation of dopant-host complexes limits mobility. Mg again emerges as the most mobile, with the lowest dissociation barriers along both axes, whereas Co and Fe exhibit the highest, suggesting improved dopant retention. These findings guide dopant selection by balancing activation and thermal stability, essential for robust semi-insulating substrates.