Microstructure-dependent thermoelectric properties of polycrystalline InGaO₃(ZnO)₂ superlattice films

The authors fabricated polycrystalline InGaO₃(ZnO)_m superlattices with different degrees of c-axis-preferred orientation and grain sizes using atomic layer deposition (ALD)-grown zinc oxide (ZnO) buffer layers to introduce nanometer-scale grains and modulate the thermoelectric properties. The ALD-grown ZnO buffer layer facilitates crystallization of solution-processed amorphous indium gallium zinc oxide (InGaZnO) films into an InGaO₃(ZnO)₂ superlattice film, acting as a preferential seed layer to reduce the lattice mismatch between InGaO₃(ZnO)₂ nuclei and the sapphire substrate. Thus, the preferential orientation of the ZnO buffer layer dramatically influenced the final microstructure of the polycrystalline InGaO₃(ZnO)_m superlattice films. The c-axis-preferred orientation and grain size in the ALD-grown polycrystalline ZnO buffer layer can be easily controlled by varying the growth temperature. The ZnO buffer layer with a superior c-axis-preferred orientation produced a polycrystalline InGaO₃(ZnO)₂ film consisting of InGaO₃(ZnO)₂ grains with a strong c-axis-preferred orientation. Interestingly, it showed dramatically reduced thermal conductivity (0.61 W/m K) compared to randomly oriented poly- and single-crystalline InGaO₃(ZnO)₂ films (>1 W/m K) owing to effective phonon-interface and phonon-grain boundary scattering by the well-ordered alternating stacking structure and introduced grain boundaries.