Tensile Strain Effect on Thermoelectric Properties in Epitaxial CaMnO₃ Thin Films

A deterministic platform for engineering epitaxial strain in CaMnO3 (CMO) thermoelectric thin films is demonstrated using pulsed laser deposition, enabling precise control of the interplay between strain state and oxygen-vacancy formation. High-quality epitaxial CMO films are grown on four different single-crystalline substrates, which impose fully relaxed, partially relaxed, low-tensile, and high-tensile strain states, respectively. Increasing tensile strain induces a monotonic expansion of the unit-cell volume and a systematic rise in oxygen vacancy concentration. Oxygen vacancies increase carrier concentration but decrease mobility due to enhanced scattering. Reducing tensile strain suppresses vacancy scattering and increases both electrical conductivity (σ) and the Seebeck coefficient (S), mitigating the conventional inverse relationship between S and σ. Fully relaxed films exhibit σ approximately four orders of magnitude higher at room temperature than highly tensile-strained films. These relaxed films also show the highest power factor (PF = S2・σ), exceeding strained films by up to six orders of magnitude. Strain-controlled oxygen vacancies thus provide a direct route to optimize charge transport and maximize the thermoelectric performance of CMO thin films.