Berlin 2008 – scientific programme
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HL: Fachverband Halbleiterphysik
HL 55: GaN: preparation and characterization II
HL 55.11: Talk
Friday, February 29, 2008, 13:15–13:30, ER 164
Influence of anisotropic strain on excitonic transitions in a-plane GaN films — Marcus Röppischer1, •Carsten Buchheim1, Rüdiger Goldhahn1, Gerhard Gobsch1, Armin Dadgar2, Matthias Wieneke2, Jürgen Bläsing2, and Alois Krost2 — 1Institute of Physics, Technical University Ilmenau, PF100565, 98684 Ilmenau, Germany — 2Institute of Experimental Physics, Otto-von-Guericke-University Magdeburg, Universitätsplatz 2, 39016 Magdeburg, Germany
GaN films on non- or semipolar substrate surfaces are important for light emitters in the ultraviolet spectral region, because on these substrates the polarisation induced electric fields are minimized. With the optical axis in the sample plane, one has to account for both, the ordinary and the extraordinary dielectric function (DF) for the design of devices. Different thermal expansion coefficients and lattice mismatch for the two in-plane directions result in anisotropic in-plane stress. As a consequence the transition energies of the A-, B- and C-exciton are unequally shifted and the oscillatory strengths are changed. In this work the ordinary and extraordinary components of the DF of a-plane GaN films on r-plane sapphire are determined by spectroscopic ellipsometry. The optical selection rules are verified. The results are confirmed by polarisation dependent photoreflectance measurements in which the excitonic transition energies were determined for differently strained samples. Different oscillatory strengths are observed for the incident light polarized parallel and perpendicular to the optical axis. The experiments are supported by 6×6 k→ p→ band structure calculations.