Interfacial thermal and electrical transport properties of pristine and nanometer-scale ZnS modified grain boundary in ZnO polycrystals

Abstract

Grain boundary plays an important role in energy carrier transport. By choosing ZnO as a model system of technological importance, and by measuring the thermal and electrical transport properties of ZnO polycrystals with a wide range of grain boundary spacing, we determine the interfacial thermal (Kapitza) resistance of the ZnO grain boundary Rk=4.0±0.7×10−9 m2 K W−1, which is relatively independent of grain size. The effective electron potential barrier height and the depletion width of the grain boundary generally increase with spacing, but they collapse below ∼100 nm and become almost invariant above ∼1 μm. When the grain boundary is locally modified by nanometer-thick ZnS thin film, the Kapitza resistance increases by more than three times, up to about 12.9×10−9 m2 K W−1, and the depletion region expands more than twice. The charge carrier concentration is influenced by the effective potential barrier height due to the grain boundary energy filtering effect whereas the electron mobility is related to the depletion width. Our investigations demonstrate the significance of grain boundary characteristics for interfacial and effective bulk transport properties. The findings and the approach are broadly important for polycrystalline materials of which the functional performance can be adjusted via grain boundary engineering.