Thermally activated increase of the average grain size as the origin of resistivity enhancement in NbO2 films grown by pulsed-laser deposition

We investigated $\mathrm{Nb}{\mathrm{O}}_{2}$ thin films grown by pulsed-laser deposition on ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$(0001) substrates. Increasing the growth temperature from 700 to $900{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$, a clear improvement of the structural quality and the interface abruptness has been observed for the epitaxial films, together with an increase in the average grain size from tens of nanometers to above 100 nm. For achieving high film resistivities, increasing the average grain size is found to be crucial. For a more detailed investigation of the carrier transport characteristics, Raman spectroscopy is demonstrated as a powerful tool. In the case of relatively large grain sizes, we reveal that the film resistivities directly correlate with the carrier concentration in the $\mathrm{Nb}{\mathrm{O}}_{2}$ grains. The results obtained for comparatively small grain sizes can be explained by carrier transport via a percolation mechanism along metallic grain boundaries in accordance with a previously reported model.