Extremely Large Refractive Indexes in Anodic Tantalum Pentoxide

Abstract

We have investigated optical properties of tantalum pentoxide films obtained by anodization of thin tantalum layers that were deposited on BK7 glass in an e-beam system. Ellipsometric studies were conducted in the wavelength range 246nm to 1679nm. The ellipsometric complex reflection coefficient versus λ was optimized using the same routine for all samples and provided values of n, k, and α. Some extreme n (up to 1000) and large n (up to 23) and α values were obtained for some samples in the wavelength range 455nm to 531nm. These extreme values probably result from small polycrystalline clusters dispersed in amorphous tantalum pentoxide. Such clusters usually are not charge neutral; therefore, they can interact with electric fields, such as incident light. As for the large n values these are probably resulting from possible presence of some impurity molecules.