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

Abstract

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.