Growth and characterization of radio frequency magnetron sputter-deposited zinc stannate, Zn2SnO4, thin films

Abstract

Single-phase, spinel zinc stannate $({\mathrm{Zn}}_2{\mathrm{SnO}}_4)$ thin films were grown by rf magnetron sputtering onto glass substrates. Uniaxially oriented films with resistivities of ${10}^{\text{−2}}{\text{–10}}^{\text{−3}}\mathrm{\hspace{0.17em}\Omega }\mathrm{cm},$ mobilities of 16–26 cm²/V s, and n-type carrier concentrations in the low ${10}^{19}\hspace{0.17em}{\mathrm{cm}}^{\text{−3}}$ range were achieved. X-ray diffraction peak intensity studies established the films to be in the inverse spinel configuration. ${}^{\mathrm{119}}\mathrm{Sn}$ Mössbauer studies identified two octahedral Sn sites, each with a unique quadrupole splitting, but with a common isomer shift consistent with ${\mathrm{Sn}}^{\text{+4}}.$ A pronounced Burstein–Moss shift moved the optical band gap from 3.35 to as high as 3.89 eV. Density-of-states effective mass, relaxation time, mobility, Fermi energy level, and a scattering parameter were calculated from resistivity, Hall, Seebeck, and Nernst coefficient transport data. Effective-mass values increased with carrier concentration from 0.16 to 0.26 $m_e$ as the Fermi energy increased from 0.2 to 0.9 eV above the conduction-band minimum. A bottom-of-the-band effective-mass value of 0.15 $m_e$ is in good agreement with local density approximation calculations. Temperature-dependent transport measurements and calculated scattering parameters correlated well with ionized impurity scattering with screening by free electrons for highly degenerate films.