AVS 56th International Symposium & Exhibition
    Applied Surface Science Tuesday Sessions
       Session AS+EM+MS+TF-TuM

Paper AS+EM+MS+TF-TuM6
VUV Optical Properties of III-Nitrides in the Thin Film Limit

Tuesday, November 10, 2009, 9:40 am, Room C2

Session: Spectroscopic Ellipsometry III
Presenter: C. Cobet, Institute for Analytical Sciences, Germany
Authors: C. Cobet, Institute for Analytical Sciences, Germany
M. Röppischer, Institute for Analytical Sciences, Germany
C. Werner, Institute for Analytical Sciences, Germany
R. Goldhahn, Ilmenau University of Technology, Germany
N. Esser, Institute for Analytical Sciences, Germany
Correspondent: Click to Email
In the last few years no other class of semiconductors has attracted so much scientific and simultaneously commercial attention like the group III-nitrides. The extraordinary physical properties have stimulated many new developments of (opto-)electronic devices. Prominent examples are the short wavelength laser diodes, which take advantage of the wide band gap of GaN (3.42eV). Such devices consist of three compounds: InN, GaN, AlN and their ternary or quaternary alloys. The respective band gap shifts from the near infrared for pure InN (0.68eV) to the ultraviolet for AlN (6.03eV), while the lattice constant variation is relatively small and allows the growth of heterostructures. It is probably surprising that several fundamental physical properties are still under discussion, although the III-nitrides are already widely used. Uncertainties concern, for example, the question whether zinc blende AlN has an indirect band gap or the impact of electric fields and strain on the electronic band structure. The latter effects are significant in particular for thin films, quantum wells, and super lattice structures. Furthermore, quantum size effectsalter the optical properties of such structures. But, also the knowledge about the bulk optical properties above the fundamental band gap is still incomplete.

We apply broad band spectroscopic ellipsometry from the visible to the vacuum ultraviolet. It is an excellent method in order to study electronic band structure peculiarities by means of dipole transition features in the dielectric function. On the other hand, it also allows a very precise determination of the dielectric function itself (refractive index and absorption). Device performance critically depends on the optical properties around e.g. the emission wavelength. However, this spectral region is strongly influenced by all higher inter band transitions according to the Lyddane-Sachs-Teller relation. For our investigations on binary GaN and AlN, as well as on ternary AlxGa1-xN, we have mainly used a home made synchrotron ellipsometer. The extraordinary properties of the synchrotron light source allow measurements with very high spectral and spatial resolution in a very broad spectral range. By taking advantage of the polarization sensitivity, we could determine the independent ordinary and extraordinary dielectric function in the hexagonal materials. In a comprehensive discussion of the dielectric functions for the hexagonal and cubic crystal structure, we could identify band gap related excitons and all higher interband transitions. This knowledge is used in order to study effects of composition and strain, as well as quantum size effects in more detail.