Thermally activated increase of the average grain size as the origin of resistivity enhancement inNbO₂ films grown by pulsed-laser deposition

We investigated NbO2 thin films grown by pulsed-laser deposition on Al2O3(0001) substrates. Increasingthe growth temperature from 700 to 900 degrees C, a clear improvement of the structural quality and the interfaceabruptness has been observed for the epitaxial films, together with an increase in the average grain size from tensof nanometers to above 100 nm. For achieving high film resistivities, increasing the average grain size is foundto be crucial. For a more detailed investigation of the carrier transport characteristics, Raman spectroscopy isdemonstrated as a powerful tool. In the case of relatively large grain sizes, we reveal that the film resistivitiesdirectly correlate with the carrier concentration in the NbO2 grains. The results obtained for comparatively smallgrain sizes can be explained by carrier transport via a percolation mechanism along metallic grain boundaries inaccordance with a previously reported model