Anisotropic strain on phonons in a-plane GaN layers studied by Raman scattering

Abstract

The strain state of a-plane GaN layers grown on r-plane sapphire was studied by Raman spectroscopy. Some of the layers investigated have been uncoalesced stripe structures grown by epitaxial lateral overgrowth (ELOG). Apart from frequency shifts, anisotropic strain within the a-plane leads to a slight splitting of the twofold degenerate E1 and E2 phonon modes. Information about the nature of strain is obtained from plots of the frequency shift of phonons with different symmetry in selected sample regions. The ratio of the frequency shifts is independent of the absolute phonon frequency of unstrained crystals and is sensitive to the strain state, especially in the case of the A1(TO) and \( {\text{E}}^{{\text{h}}}_{2} \) phonons with lattice displacements parallel/perpendicular to the c-axis, respectively. From the Raman data it is concluded that besides biaxial strain in the a-plane due to lattice mismatch and different thermal expansion coefficients of substrate and layer additional hydrostatic components exist originating from the lattice distortion near native defects or impurities. Raman maps of the phonon frequencies and full width at half maximum of the uncoalesced ELOG structures show that the wing above the mask is relaxed and of improved crystalline quality in comparison with the window region.