ANALYSIS OF MULTIPLE-ANGLE ELLIPSOMETRY OF UNIAXIAL ULTRATHIN ORGANIC FILMS AT THE AIR-WATER-INTERFACE AND DETERMINATION OF THE REFRACTIVE-INDEXES OF BEHENIC ACID MONOLAYERS

Multiple-angle ellipsometry of uniaxial ultrathin films is discussed analytically and by computer analysis. The various relations between angle of incidence, layer thickness, and refractive indices are presented. It is shown that multiple-angle ellipsometry conveys sets of experimental data well suited for computer analysis with high redundancy by exposing systematic experimental errors. The ellipsometric data allow no unique characterization of ultrathin, uniaxial, nonabsorbing films and it is demonstrated that the derived layer thickness depends strongly on assumptions of the layer anisotropy. For typical experimental results the allowed range of combinations of the refractive index values as a function of an assumed layer thickness is presented. Calculations show that certain combinations of refractive indices will result in an ellipsometric angle delta-DELTA = 0 for all angles of incidence. Some of these combinations are physically realistic and might in fact be experimentally accessible. The potential of ellipsometric experiments to characterize ultrathin films is demonstrated by combining the method with data known from X-ray diffraction. Thus we have determined the refractive indices for a behenic acid monolayer in the S and CS phase, respectively, to n(x)S = 1.47, n(z)S = 1.54 and n(x)CS = 1.48, n(z)CS = 1.56. These values are deduced from the measured ellipsometric angles, a Lorentz-Lorenz approximation for the refractive indices, and structural data known from X-ray measurements.