Structure-derived electronic and optical properties of transparent conducting oxides

Using the first-principles method, we have studied the stability and electronic band structures of the transparent conducting oxides SnZn2O4, SnCd2O4, and CdIn2O4, Our calculated lowest-energy phases of these compounds are similar to those found experimentally. However, we find an orthorhombic structure of SnZn2O4, which is close in energy to the inverse spinel structure, and a new inverse orthorhombic structure for CdIn2O4, with an energy close to that of the inverse spinel structure. The stability of these compounds can be explained by the Coulomb energy, atomic size, and chemical character of the constituent elements. We analyze the chemical character of the band edges and explain the general trend observed in the fundamental band gap and energy difference between the first and second conduction bands. The latter is found to be large for the thermodynamically stable structures, which explains the transparency of these n-type conducting oxides. Based on these analyses, we derive general rules for designing more efficient transparent conducting oxides. We have also calculated the Moss-Burstein electron effective masses and the optical transition matrix elements of these compounds. We find that transitions between the valence band maximum and the conduction band minimum are forbidden by symmetry, and that the optical gaps are about 1 eV larger than the corresponding fundamental band gaps. The same forbidden transitions are found between the first and second conduction bands. Our calculated dependence of the absorption spectrum on the carrier concentration reveals new features for n-type doped transparent conductor oxides. At very high doping concentration, we find a possible inverse Moss-Burstein shift, where the apparent band gap decreases with increasing carrier concentration.