The atomic layer deposition (ALD) growth of Er_xGa_2−xO₃ (0 ≤ x ≤ 2) thin films was demonstrated using two precursor systems: Er(thd)₃, Ga(acac)₃, and ozone and Er(C₅H₄Me)₃, Ga₂(NMe₂)₆, and water at substrate temperatures of 350 and 250 °C, respectively. Both processes provided uniform films and exhibited surface-limited ALD growth. The value of x in Er_xGa_2−xO₃ was easily varied by selecting a pulse sequence with an appropriate erbium to gallium precursor ratio. The Er(thd)₃, Ga(acac)₃, and ozone precursor system provided stoichiometric Er_xGa_2−xO₃ films with carbon, hydrogen, nitrogen, and fluorine levels of <0.2, <0.2, <0.3, and 0.6–2.2 atomic percent, respectively, as determined by Rutherford backscattering spectrometry (RBS) and time of flight-elastic recoil detection analysis (TOF-ERDA). The film growth rate was between 0.25 and 0.28 Å cycle⁻¹. The effective permittivity of representative samples was between 10.8 and 11.3. The Er(C₅H₄Me)₃, Ga₂(NMe₂)₆, and water precursor system provided stoichiometric Er_xGa_2−xO₃ films with carbon, hydrogen, nitrogen, and fluorine levels of 2.0–6.1, 5.0–10.3, <0.3–0.7, and ≤0.1 atom percent, respectively, as determined by RBS and TOF-ERDA. The film growth rate was between 1.0 and 1.5 Å cycle⁻¹ and varied as a function of the Er(C₅H₄Me)₃ to Ga₂(NMe₂)₆ pulse ratio. The effective permittivity of representative samples was between 9.2 and 10.4. The as-deposited films of both precursor systems were amorphous, but crystallized either to Er₃Ga₅O₁₂ or to a mixture of β-Ga₂O₃ and Er₃Ga₅O₁₂ upon annealing between 900 and 1000 °C under a nitrogen atmosphere. Atomic force microscopy showed root mean square surface roughnesses of <1.0 nm for typical films regardless of precursor system or film composition.