Enhanced thermoelectric properties of atomic-layer-deposited ZnO-Based superlattice thin films by tuning the composition and structure of interlayers

Abstract

Superlattice thin films composed of a lightly Hf-doped ZnO matrix and various periodically inserted interlayers—including TiO2, ZrO2, HfO2, and their combinations, with various thicknesses—were prepared by atomic layer deposition (ALD) and characterized in terms of thermoelectric properties. The effects of interlayer type and thickness on the comprehensive thermoelectric properties of the superlattice films were determined, including electron mobility (μe), electron concentration (ne), electrical conductivity (σ), Seebeck coefficient (S), power factor (PF), thermal conductivity (κ), and ZT value. The TiO2, ZrO2, and HfO2 interlayers provided different potential barriers (HfO2 > ZrO2 ≫ TiO2) and atomic-mass mismatches (HfO2 > ZrO2 > TiO2) with the matrix, resulting in differing trade-off’s among energy filtering (raising S), carrier blocking (lowering σ), and phonon scattering (reducing κ) effects. Favorable balances of the effects were obtained by combining the single-content interlayers into composite interlayers, where the ALD preparation method enabled precise tuning of the interlayer composition and structure for optimal thermoelectric properties of the superlattice films. Specifically, superlattice films with a composite interlayer consisting of an alternating 1 cycle TiO2/1 cycle HfO2 structure at 1.5 nm thickness achieved a maximum of approximately 17-fold increase in ZT over that of bulk ZnO films. The results presented a quantitative guide for designing interlayer structures in superlattice films for enhanced thermoelectric properties.