Optical ellipsometry (polarization) measurements on alkyl ammonium metal complexes

Abstract

Spectroscopic ellipsometry techniques have been used to determine the refractive indices and thicknesses of thin-film Langmuir–Blodgett (LB) films. By using a spectrometer incorporating a charge-coupled detector (Ocean Optics Inc., S2000), extensive and rapid collection of data is possible. The data for each film are obtained by analysis of the ellipsometric reflection coefficient measured as a function of angle of the incident light. The experimental arrangement consisted of an intense white source (incorporating a projection lamp) providing a collimated beam of light approximately 1 mm in diameter that passed through a rotatable polarizer onto the film at an incident angle φ. The reflected beam was then picked up using an optic fiber and transmitted to the spectrometer. Thus, the plane of polarization was variable, as was the angle of incidence, and reflection intensities were scanned over the wavelength range 330–1010 nm using the maximum available instrumental collection (integration) time of 2552 ms. Measurements were made on alkyl ammonium metal complexes previously investigated for potential soft Xray mirrors [1, 2]. These materials deposit with the metal atoms aligned in layers between the alkyl ammonium chains, the appropriate length of which, in principle, can be manipulated. Whereas most of the work on depositing LB films had been carried out on glass substrates, the films themselves are highly transparent with refractive indices differing insufficiently from glass. Hence, films were deposited on silicon substrates (1 0 0 surface) 625μm in thickness supplied by WackerChemitronic. These were cut to 40 mm by 8 mm, an appropriate size for LB deposition. Although deposition of the LB films was more difficult on silicon than on glass, good deposition was obtained, particularly for the complex (C22H45NH3)2CuCl2(OH)2 and for (C18H37NH3)2ZnCl2(OH)2. These compounds are analogous to the well-known materials (CnH2n+1NH3)2MCl4 where M=Mn, Fe, Cu and Cd (see, for instance, [3]). Ellipsometry measurements have been reported previously on LB films (for example [4–6]) but not the technique of a charge-coupled detector to scan as a function of wavelength and angle and of curve fitting as used here. Samples were mounted in the vertical plane on a goniometer rotatable about a vertical axis. Measurements were usually made at 10◦ intervals of the angle of incidence, φ. A second rotation arm tracked the reflected