Origin and quantification of the ultimate carrier concentration limits in In₂O₃ and Sn-doped In₂O₃

The ultimate limits of the carrier concentrations in In₂O₃ and Sn-doped In₂O₃ are derived from operando photoelectron spectroscopy of a solid oxide electrochemical cell with Y-doped ZrO₂ as the oxygen electrolyte. It is demonstrated that the limits are determined by the transition of the oxygen vacancy to the neutral state and to the reduction of Sn⁴⁺ donors to Sn²⁺ electron traps, respectively. Maximum Fermi energies of 3.85 and 3.35eV above the valence band maximum are identified for ITO and In₂⁢O₃. The ultimate carrier concentrations achievable by Sn doping and by oxygen vacancies are estimated to be 1.8–1.9×10²¹ cm⁻³ and 6–7×10²⁰ cm⁻³.