Optical polarization anisotropy in strained A‐plane GaN films on R‐plane sapphire

Abstract

We have used polarized photoreflectance (PR) spectroscopy to study [1120]-oriented A-plane GaN films on R-plane sapphire substrates. For this type of films, the c-axis of GaN lies in the film plane. When the probe beam is polarized perpendicular to the c-axis, we observe a single feature in the PR spectrum, while two features at higher energies are observed for parallel polarization. Comparing the transition energies with the ones obtained by electronic band-structure calculations, we identify the observed PR spectral features as the three possible transitions around the fundamental band gap of GaN. However in comparison to unstrained GaN, their polarization properties and energies have been significantly modified by the inplane anisotropic strain in the film. Finally, we can also determine the individual in-plane anisotropic strain components.

Strain Effects on the Optical Polarization Properties of R-Plane Wurtzite GaN

“…These are similar to the A-and M-planes. [13][14][15] Figure 2(d) displays the energy difference E T2 À E T1 . For isotropy biaxial strains, the energy different E T2 À E T1 is the lowest and remain almost the same as that in the unstrained case, this means the biaxial strains hardly cause the upper two valence-bands degenerating.…”

“…These changes in the symmetry affect the optical polarization properties and have been extensively investigated by Ghosh and co-workers. [13][14][15] The investigation on the anisotropy and optical gain for the semi-polar planes have also been reported. [16][17][18] The strains in R-plane can influence the electronic band structure and need to be determined by detail calculations.…”

Nonpolar a-plane GaN films grown on γ-LiAlO2 (302) substrates by low-pressure MOCVD

“…As the polarization changes to E?c, the T 1 becomes prominent after polarization selection. At room temperature, the T 2 band is much less occupied, which is related to the reduction of the PL intensity [19]. Therefore, the intensity reduced a lot when the polarization changed.…”