Characterisation of nanostructured GaSb : Comparison between large-area   optical and local direct microscopic techniques
  Low energy ion-beam sputtering of GaSb results in self-organized nanostructures, with the potential of structuring large surface areas. Characterisation of such nanostructures by optical methods is studied and compared to direct (local) microscopic methods. The samples consist of densely packed GaSb cones on bulk GaSb, approximately 30, 50 and 300 nm in height, prepared by sputtering at normal incidence. The optical properties are studied by spectroscopic ellipsometry, in the range 0.6-6.5 eV, and with Mueller matrix ellipsometry in the visible range, 1.46-2.88 eV. The optical measurements are compared to direct topography measurements obtained by Scanning Electron Microscopy (SEM), High Resolution Transmission Electron Microscopy (HR-TEM), and Atomic Force Microscopy (AFM). Good agreement is achieved between the two classes of methods when the experimental optical response of the short cones (<55 nm) is inverted with respect to topological surface information via a graded anisotropic effective medium model. The main topological parameter measured was the average cone height, but estimates of typical cone shape and density (in terms of volume fractions) were also obtained. The graded anisotropic effective medium model treats the cones as a stack of concentric cylinders (discs) of non-increasing radii. Optical methods are shown to represent a valuable characterization tool for nanostructured surfaces, in particular when large coverage area is desirable. Due to the fast and non-destructive properties of optical techniques, they may readily be adapted to in-situ configurations. 
I.S. Nerbo, M. Kildemo, S. Leroy, I. Simonsen, E. S{\o}nderg{\aa}rd,   L. Holt and J.C. Walmsley