Ab-initio density functional approach is employed to investigate the structural, optical, and electronic properties of twelve undoped (non)stoichiometric multicomponent oxides with layered structure RAMO₄ [R³⁺ =In or Sc, A³⁺ =Al or Ga, and M²⁺ - Ca, Cd, Mg, or Zn], as candidates for novel transparent conducting oxides. The compositional complexity of RAMO₄ leads to a wide range of band gaps varying from 2.45eV for InGaCdO₄ to 6.29 eV for ScAlMgO₄. We find that despite the different band gaps in the constituent binary oxides, namely, 2-4 eV in CdO, In₂O₃, or AnO; 5-6 eV for Ga₂O₃ or Sc₂O₃; and 7-9 eV in CaO, MgO, or Al₂O₃, the states of all cations contribute to the bottom of the conduction band of RAMO₄. We show that this hybrid nature of the conduction band originates from the unusual fivefold atomic coordination of A³⁺ and M²⁺ cations and suggests that both structurally and chemically distinct layers of RAMO₄ are expected to participate in carrier transport. This is consistent with the obtained isotropic electron effective mass of 0.3-0.5 me. Next, in order to understand the carrier generation mechanism in RAMO₄, we have systematically investigated the formation of native point defects in three representative InAMO₄ oxides. We find that the donor antisite defect in InGaZnO₄ and InAlZnO₄ occur in higher concentrations than oxygen vacancies which are major donors in binary oxides. Also in contrast to the binary TCOs, the formation energy of cation vacancies is significantly lower in InAMO₄ owing to a large structural relaxation around the defect. As a result, the equilibrium Fermi level is pushed away from the conduction band and deeper into the band gap. The results agree well with the observed dependence of the conductivity on the oxygen partial pressure in InGaZnO₄. These systematic investigations provide a significant insight into the role of chemical composition and structural complexity of RAMO₄ materials on the carrier generation mechanisms and the resulting properties --Abstract, page iv
