The complex dielectric function (DF) of wurtzite InN and GaN in a spectral region between 12 and 30eV was determined by spectroscopic ellipsometry with synchrotron radiation. Using a-plane (11-20) InN and M-plane (1-100) GaN, measurements were performed with the electric field vector parallel and perpendicular to the c-axis of the crystal. The direct determination of the DF with ellipsometry provides full access on the linear optical response and gives valuable information about the electronic structure since the imaginary part is associated with the joint density of states. Comparing to published EELS and XAS measurements ellipsometry yields more detailed information and has a better accuracy.

Above the plasmon frequency the DF is dominated by the In4d/Ga3d core level transitions to the unoccupied states. The main contribution for those transitions are described by the nondispersive (well localized) part of the cation d-states, thus the imaginary part gives a characteristic view of the conduction band states with p-symmetry. The comparison to calculated DOS by DFT-LDA of the conduction band agrees well in line shape and energetic positions. A considerable polarisation dependence of the DF, indicating the anisotropy of the conduction band, was observed. This anisotropy is induced by a direction dependent mixing of the empty p-states and can be discussed in terms of symmetry consideration in hexagonal crystals. Finally, the detected spin-orbit splitting of the In4d- and the Ga3d-band was determined to be 0.82eV and 0.41eV.