Electronic state characterization of SiOx thin films prepared by evaporation

Abstract

SiOx thin films with different stoichiometries from SiO1.3 to SiO1.8 have been prepared by evaporation of silicon monoxide in vacuum or under well-controlled partial pressures of oxygen (P<10–6 Torr). These thin films have been characterized by x-ray photoemission and x-ray-absorption spectroscopies, this latter at the Si K and L2,3 absorption edges. It has been found that the films prepared in vacuum consists of a mixture of Si3+ and Si+ species that progressively convert into Si4+ as the partial pressure of oxygen during preparation increases. From this spectroscopic analysis, information has been gained about the energy distribution of both the full and empty states of, respectively, the valence and conduction bands of SiOx as a function of the O/Si ratio. The characterization of these films by reflection electron energy-loss spectroscopy (REELS) has provided further evidences about their electronic structure (band gap and electronic states) as a function of the oxygen content. The determination of the plasmon energies by REELS has also shown that the films prepared by evaporation in vacuum consist of a single phase which is characterized by a density (1.7 g cm–3) lower than that of SiO2 (i.e., 2.2 g cm–3) or Si (i.e., 2.4 g cm–3). The optical properties (n and k) of the films as a function of the O/Si content have been deduced from the analysis of REELS spectra in the energy range from 4 to 20 eV. It has been also shown that the O/Si ratio in the films and several spectroscopic parameters such as the Auger parameter or the energy of bulk plasmons present a linear relationship and that this linear dependence can be used for a rapid characterization of SiOx materials. By contrast, the band-gap energy changes differently with the O/Si ratio, following a smooth linear increase from about 3.8 eV for SiO1.3 to ca. 5.0 eV for SiO1.7 and a jump up to 8.7 eV for SiO2. These results indicate that the random-bonding model does not apply to thin films prepared by evaporation under our experimental conditions. Other distributions of Sin+ states can be induced if the films are excited with an external source such as heat or photon irradiation. In this case the electronic properties vary and the previous linear correlations as a function of the oxygen content do not hold any longer.