Optical anisotropy of a- and m-plane InN grown on free-standing GaN substrates

Jochen Raethel 1Pascal Schley 1Egidijus Sakalauskas 1Gerhard Gobsch 1René Müller 1Thomas A. Klar 1J. Pezoldt 2Ruediger Goldhahn 1Gregor Koblmueller 3James S. Speck 3Matthias Wieneke 4Jürger Bläsing 4Alois Krost 4

1. Technische Universität Ilmenau, Institut für Physik, PF 100565, Ilmenau 98684, Germany
2. Technische Universität Ilmenau, Zentrum für Micro- and Nanotechnologien, Ilmenau 98693, Germany
3. University of California, Santa Barbara, CA 93106, United States
4. Otto-von-Guericke-Universität Magdeburg, Institut für Experimentelle Physik, Universitätsplatz 2, Magdeburg 39106, Germany

Abstract
The optical response of wurtzite nitride semiconductors at the band gap and in the spectral region of high-energy critical points differs appreciably for light polarization perpendicular (ordinary configuration) and parallel (extraordinary) to the c-axis. In order to demonstrate this effect, films (or bulk crystals) are required for which the optical axis lies in the surface plane (a- or m-plane). For InN, the only studies reported so far employed an a-plane layer deposited on r-plane sapphire with an a-plane GaN buffer. Considerable improvement of the film quality was recently achieved by growing a- and m-plane InN directly on free-standing a- and m-plane GaN by plasma-assisted molecular beam epitaxy. In this contribution, we present a comprehensive characterization of these higher-quality non-polar InN materials by polarization-dependent Raman spectroscopy and ellipsometry. The influence of stress on the transition energies is discussed based on the lattice parameters obtained by high-resolution X-ray diffraction.

Raman measurements yielded two strong phonon lines with an intensity ratio strongly dependent on the polarization conditions, but being consistent to the results found for GaN. In comparison to the material grown on sapphire, we observed here a clear shift of the phonon lines to lower wave numbers. No evidence was found for strong phonon-plasmon coupling.

Ellipsometry reveals a clear energy splitting between the ordinary and extraordinary absorption edges for both samples. This splitting amounts to 19 and 39 meV for the m- and a-plane films, while the transition energy at the Fermi wave vectors was found at 680 and 720 meV, respectively, in the ordinary configuration. The different values are a clear indication for the influence of stress on the valence band structure at the Γ-point of the Brillouin zone and for band filling. Finally, the polarization behaviour at the high-energy van Hove singularities was almost identical to the previous findings.

 

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Presentation: Oral at E-MRS Fall Meeting 2009, Symposium A, by Jochen Raethel
See On-line Journal of E-MRS Fall Meeting 2009

Submitted: 2009-05-20 12:35
Revised:   2009-06-07 00:48