Microstructure and the boson peak in thermally treated InxO films

We report on the correlation between the boson peak and the structural changes associated with thermally treating amorphous indium-oxide films. In this process, the resistance of a given sample may decrease by a considerable margin whereas its amorphous structure is preserved. In the present paper, we focus on the changes that result from the heat treatment by employing electron-microscopy, x-ray, and Raman spectroscopy. These techniques were used on films with different stoichiometry and, thus, different carrier concentrations. The main effect of heat treatment is material densification, which presumably results from elimination of microvoids. The densified system presents better wave-function overlap and more efficient connectivity for the current flow. X-ray and electron-beam diffraction experiments indicate that the heat-treated samples show significantly less spatial heterogeneity with only a moderate change in the radial-distribution function metrics. These results are consistent with the changes that occur in the boson-peak characteristics due to annealing as observed in their Raman spectra.