Właściwości optyczne cienkich warstw krzemionkowych wytwarzanych metodą zol-żel

Abstract

The main aim of this study was to determine the optical properties of thin silica films with varying degrees of porosity produced by the sol-gel method on glass and silicon substrates, and to determine the optical parameters of the films. The studies were carried out using spectrophotometry and spectroscopic ellipsometry, and the very important element of the work was to compare the results obtained by both methods. These studies have been completed by structural studies, including scanning electron microscopy (SEM), atomic force microscopy (AFM), and interference contrast (Nomarski) microscopy that enables three-dimensional imaging surface. The main results of this work can be summarized as follows: 1. Structural studies have shown the presence of macropores (the size of 90 – 1300 nm) and their contribution to the total porosity of the silica films has been estimated to be approximately 5%. 2. Ellipsometric and spectrophotometric investigations of the glass substrate have allowed to determine its absorption edge [λcut-off = 280 nm (4.43 eV)], Urbach energy (EU = 0.34 eV), which is a measure of disorder in amorphous materials, and dispersion relations of the refractive index that can be described by the Sellmeier formula. 3. Based on the spectrophotometric studies of reflection and transmission for the films on glass substrates and the reflectivity of films on silicon it has been found that silica films act as antireflective layers greatly reducing the average reflection, and thereby increasing an average transmission. 4. The joint analysis of the elipsometric and spectrophotometric results obtained for the silica films on glass allowed to determine their thickness, and the refractive index dispersions that can be described by the Sellmeier formula. It has been found that the differences between the values of optical paths determined by both methods are small and do not exceed 4%. The values of the refractive indices have been found to be much lower than the value of this coefficient for thermal silica that is caused by the porosity of films lying in the range 17 – 39%. 5. Spectrophotometric studies of the silicon p-type doped (Si:B) substrate have allowed to determine the dispersion dependence of the absorption coefficient that indicates a mechanism of energy absorption of electromagnetic radiation by the free carriers (holes) in the energy range below the energy gap of silicon (Eg = 1.1 eV). 6. The joint analysis of the elipsometric and spectrophotometric studies obtained for silica films on silicon has allowed to determine their thickness, and the refractive index dispersions that can be described by the Sellmeier formula. It has been found that the differences between the values of optical paths determined by both methods, below 4 eV, do not exceed 4%, however, above this energy the difference increases with the photon energy which may follow from the difference between the optical constants of Si:B used in this work and those of intrinsic silicon used in the elipsometric data analysis. Similarly as for the silica films on glass, the refractive indices for the silica films on silicon have been found to be significantly lower than the value of this coefficient for thermal silica, due to the presence of pores in the volume of the investigated films, the porosity of which lies in the range of 33 – 41%. 7. The results have allowed to establish a correlation between the conditions of technological, and geometric and optical parameters of the films studied. On the one hand, the refractive index is, to a good approximation, a linear function of porosity, and on the other hand, the refractive index decreases with increasing film thickness, approaching a constant value (n ≈ 1.25) for thicker films (above 800 nm). 8. Comparison of the total porosity determined from optical studies (17 – 41%) with the porosity estimated from structural studies (approx. 5%) indicates that a large contribution to the porosity of the silica films is due to the presence of pores with sizes less than 100 nm.

The performed measurements, analysis and interpretation of experimental data made it possible to determine the properties, and geometric and optical parameters, as well as their correlation with the preparation conditions of thin silica films by sol-gel method and thus fully achieve the objective of the present work. This work has shown that effective and cheap sol-gel method of preparation of porous silica thin films enables significant and controlled reduction of the refractive index, and thus gives the possibility of their use as antireflective layers in various types of optical systems.