Indium-Assisted Gallium Solubility Expansion and Defect Hierarchy Evolution in Thermoelectric ZnO Ceramics

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Abstract
Ga–In co-doped ZnO ceramics were synthesized by a solid-state reaction to elucidate the role of defect chemistry and dopant solubility in governing thermoelectric transport. Compared with pristine ZnO and Ga singly doped ZnO, the In–Ga co-doped sample (IGZO) exhibits a distinct evolution in microstructure and defect configuration. A minor fraction of Ga2O3(ZnO)9 spinel phase is present, contributing to phonon scattering as part of a composite-like microstructure. Structural analyses reveal that In incorporation suppresses the formation of Ga2O3(ZnO)9 spinel clusters, indicating an expansion of Ga solubility in the ZnO lattice. This solubility-assisted effect leads to a transition from secondary-phase-dominated microstructures toward point-defect-dominated configurations. The competition between spinel clusters and point defects plays a key role in determining phonon scattering behavior, where atomic-scale defects become dominant in the IGZO ceramic. At the same time, the enhanced substitution of Ga, assisted by In, modifies the electronic structure through increased carrier concentration and density-of-states effective mass. As a result, the IGZO ceramic exhibits reduced lattice thermal conductivity and modified charge transport behavior under conventional bulk processing conditions, with a ZT of 0.182 at 1073 K. These findings highlight that solubility-assisted defect engineering provides an effective pathway to tune defect hierarchy and transport mechanisms in ZnO-based thermoelectric materials.
Funding Information
  • Vingroup Innovation Foundation (VINIF) (VINIF.2025.STS.27)
  • Vietnam National University, Ho Chi Minh city (VNU-HCM) (B2025-18-06)

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