Abstract Single crystal (−201) β-Ga₂O₃ substrates doped with Si and Sn have been thermally annealed in N₂ and O₂ atmospheres. Structural and electrical properties evaluation was performed via a number of experimental methods in order to quantify the effects of the doping and annealing ambient on the properties of these samples. All samples annealed in O₂ exhibited significantly lower carrier concentration, as determined by capacitance–voltage measurements. Schottky barrier diodes exhibited excellent rectification when the Ga₂O₃ was annealed in N₂, and significantly lower forward current using O₂-annealed Ga₂O₃ substrates. Deep level transient spectroscopy revealed four deep trap levels with activation energies in the range of 0.40–1.07 eV. Electron spin resonance showed a decrease in shallow donor concentration, and cathodoluminescence spectroscopy revealed nearly two orders of magnitude lower emission intensity in O₂-annealed Ga₂O₃ samples. Raman spectroscopy revealed a carrier concentration dependent Raman mode around 254 cm⁻¹ observed only when the final anneal of (−201) β-Ga₂O₃ was not done in O₂. Secondary ion mass spectroscopy measurements revealed diffusion of unintentional Fe towards the surface of the (−201) Ga₂O₃ samples after annealing in O₂. Depth resolved positron annihilation spectroscopy showed an increased density of vacancy defects in the bulk region of O₂-annealed Ga₂O₃ substrates.