Influence of heteroepitaxy on the width and frequency of the E2 (high)-phonon line in GaN studied by Raman spectroscopy
2001; American Institute of Physics; Volume: 89; Issue: 7 Linguagem: Inglês
10.1063/1.1347406
ISSN1520-8850
AutoresM. Giehler, M. Ramsteiner, Patrick Waltereit, O. Brandt, K. H. Ploog, H. Obloh,
Tópico(s)ZnO doping and properties
ResumoWurtzite GaN layers are commonly grown heteroepitaxially on 6H–SiC or Al2O3 substrates, because of the lack of lattice-matched substrates. We study the influence of these substrates mainly on the E2(high)-phonon Raman line by temperature dependent Raman spectroscopy. We find that the line broadening with sample heating is predominantly caused by intrinsic phonon–phonon scattering in GaN. The small three-phonon contribution as well as the small intrinsic linewidth at low temperature are due to the rather low two-phonon density of states at the E2(high)-phonon energy. Substrates with large lattice mismatch cause inhomogeneous strain and defects in the layers, which lead to a large, temperature independent, line broadening. We show that the temperature shift of the E2(high)-phonon frequency is dominated by the GaN lattice expansion. The lattice of epilayers is strongly modified by the thermal in-plane expansion of the substrate. The degree of relaxation at the growth temperature is reflected by deviation of the E2(high)-line from the intrinsic phonon frequency.
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