The atomic and electronic structure of nonstoichiometric amorphous tantalum oxide (TaO x ) films of different composition has been investigated by means of electron microscopy, x-ray photoelectron spectroscopy, Raman and infrared spectroscopy. The dispersion of the absorption coefficient and refraction index has been studied by spectral ellipsometry. The optical spectra were interpreted using the results of a quantum-chemical simulation for crystalline orthorhombic TaO x . It was found that the presence of oxygen vacancies in the oxygen-deficient TaO x film show an optical absorption peak at 4.6 eV. It has been established that TaO x consists of stoichiometric Ta2O5, metallic Ta clusters less than 20 nm in size, and tantalum suboxides TaO y (y < 2.5). The model of nanoscale potential fluctuations of TaO x bandgap in the range of 0-4.2 eV is proposed and justified. The design of the flash memory element based on the effect of localization of electrons and holes in Ta metallic nanoclusters in the TaO x matrix is proposed.