Correlations of thermal properties with grain structure, morphology, and defect balance in nanoscale polycrystalline ZnO films

Two sets of ZnO thin films grown by atomic layer deposition method were investigated to analyse the correlation between morphology, chemical defects and thermal properties. This analysis became possible by measuring series of similarly thick wurtzite ZnO films synthesized using identical number of deposition cycles at two temperatures -100 degrees C and 200 degrees C. By comparing these sample series, exhibiting the thickness variations from 12 to 118 nm, we observed several prominent correlations for the thermal properties as measured by high-resolution scanning thermal microscopy at room temperature. Specifically, comparing thinnest samples in the series, a lower thermal conductivity was revealed in samples grown at 100 degrees C and these variations were interpreted in terms of changing populations of native donors, affecting the free carrier concentration and, as such, the thermal conductivity. Further, the increase in thickness for the samples grown at 200 degrees C was accompanied with a continuous shift to Zn-rich conditions and the corresponding increase in the thermal conductivity, specifically from 0.28 Wm(-1) K-1 to 2.81 Wm(-1) K-1. As such, we have decoupled the thickness effect on the thermal conductivity from the rest of the phenomena, explaining the thermal transport in terms of the bulk thermal contributions within the grains combined with additional thermal resistance introduced by the grain boundaries.