  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.
