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Nanostrukturen und Grenzflächen Poster: Do., 13:00–15:30 D-P270
GaN and InN core level excitations studied by synchrotron ellipsometry
Christoph Cobet 1 , Munise Rakel 1,3 , Rüdiger Goldhahn 2 , Norbert Esser 1,3
1 ISAS - Institute for Analytical Sciences, Department Berlin, Albert-Einstein-Str. 9,
D-12489 Berlin – 2 Institut für Physik, TU Ilmenau, Weimarer Str. 32, D-98684 Ilmenau
– 3 Institut für Festkörperphysik, TU Berlin, Hardenbergstr. 36, D-10623 Berlin
An ellipsometric study on wurtzite InN, wurtzite GaN and zincblende GaN in a spectral
region between 12 and 30eV using synchrotron radiation is presented. Especially
for InN, relatively few experimental studies of the general optical/electronic properties
have been carried out although the recently found small band gap value of 0.7 eV
increases the number of possible applications for III-nitride alloys. In the VUV spectral
range of 12-30eV the dielectric function is usually determined by electron energy
loss rather than ellipsometry. But spectroscopic ellipsometry, however, has a superior
spectral resolution and a Kramers-Kronig analysis requiring extrapolation to infinite
energies is avoided. The direct determination of the complex dielectric function (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. In the spectral region beyond the plasmon frequency
the DF of InN and GaN is dominated by optical transitions between the In4d/Ga3dcore
level and the p-like unoccupied (conduction) electron states. Thus the measured
structures in the imaginary part give a characteristic view of the p-like conduction
bands. The comparison between the experimental spectra and the calculated partial
DOS by DFT-LDA shows an overall good agreement. By using linear polarized light in
an ellipsometric measurement, the dielectric tensor components corresponding to the
electric field vectors along the crystallographic axes, were separately examined. This
allows a separation of direction dependent electronic contributions and thus a study of
the anisotropy of the chemical bonds in hexagonal crystal structures. With an a-plane
InN(1120) and a m-plane GaN(1100) film we observed a considerable polarisation dependence
in the DF of hexagonal InN and GaN. That emphasizes the assumption of an
anisotropy in the conduction band induced by direction dependent mixing of unoccupied
p-orbitals in wurtzite crystals, yielding a pz-DOS different from the px+py-DOS.
Low temperature measurements are used in order to determine the electron phonon
coupling strength in GaN separately for the p-like conduction bands and the highest
valence bands. At low temperature, finally, we could detect also the spin-orbit splitting
of the d-bands in InN and GaN by means of synchrotron ellipsometry.