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The polarization characteristics and intrinsic photoluminescence of M-plane GaN films on LiAl[O.sub.2] substrates were studied. The results have shown that the strain relaxation can occur in an anisotropic manner allowing for a maximum of the valence band splitting at a certain film thickness.

Polarized photoreflectance (PR) spectroscopy is used to investigate a [1120]- oriented A-plane GaN film on R-plane sapphire, where the c axis of GaN lies in the film plane. The results indicate that the observed transition energies and polarization properties of the three transitions can be used to describe the presence of an overall compressive, anisotropic in-plane strain in the film.

Polarized photoluminescence spectra of M-plane GaN films grown on LiAlO2 substrates demonstrate the existence of a large polarization anisotropy in the near-band-gap excitonic transitions, which is consistent with the reduction of the in-plane crystal symmetry and the anisotropic strain generated by the lattice mismatch between GaN and LiAlO2. Band structure calculations based on the k·p formalism support the observed experimental results and allow us to explain the temperature dependence of the photoluminescence energy. From the results of these calculations and the measured excitonic transition energies, we accurately determined the in-plane strain. The strain relaxation as a function of temperature and the role of the film thickness in the in-plane strain anisotropy are also discussed. [ABSTRACT FROM AUTHOR]

The anisotropic optical properties and polarization filtering are investigated for strained M-plane GaN films as a function of the angle of the in-plane polarization vector. Since the c axis of the underlying wurtzite crystal structure lies in the film plane, its symmetry is reduced in comparison to conventional C-plane films, giving rise to an in-plane polarization anisotropy. This anisotropy is further enhanced by the anisotropic compressive in-plane strain, which occurs due to the lattice mismatch between the GaN film and the substrate LiAlO2. In addition to linear birefringence, the film also exhibits linear dichroism, resulting in a polarization filtering of an incident, linearly polarized light beam after transmission. This filtering manifests itself as a rotation of the polarization vector towards the c axis and can be as large as 80°. We compare the measured polarization rotation with calculated values taking into account the birefringence of the GaN film. In the energy range where the filtering is most effective, the birefringence almost disappears and can therefore be neglected for the determination of the rotation angle. Finally, the rotation, which is determined by the transmittance for the two orthogonal polarization directions, can be very well approximated by the product of the film thickness and the difference of the absorption coefficients for the two polarization directions. [ABSTRACT FROM AUTHOR]

The anisotropic optical properties and polarization filtering are investigated for strained M-plane GaN films as a function of the angle of the in-plane polarization vector. Since the c axis of the underlying wurtzite crystal structure lies in the film plane, its symmetry is reduced in comparison to conventional C-plane films, giving rise to an in-plane polarization anisotropy. This anisotropy is further enhanced by the anisotropic compressive in-plane strain, which occurs due to the lattice mismatch between the GaN film and the substrate LiAlO2. In addition to linear birefringence, the film also exhibits linear dichroism, resulting in a polarization filtering of an incident, linearly polarized light beam after transmission. This filtering manifests itself as a rotation of the polarization vector towards the c axis and can be as large as 80°. We compare the measured polarization rotation with calculated values taking into account the birefringence of the GaN film. In the energy range where the filtering is most effective, the birefringence almost disappears and can therefore be neglected for the determination of the rotation angle. Finally, the rotation, which is determined by the transmittance for the two orthogonal polarization directions, can be very well approximated by the product of the film thickness and the difference of the absorption coefficients for the two polarization directions. [ABSTRACT FROM AUTHOR]

Measured polarization rotation is compared with calculated values taking into account the birefringence almost of the GaN film. The anisotropic optical properties and polarization filtering are investigated for strained M-plane GaN films as a function of the angle of the in-plane polarization vector.

We investigate a strongly anisotropically strained M-plane GaN film by photoluminescence spectroscopy. The spectra are dominated by the lowest-energy free-exciton transition, but the second lowest one is also detectable even at low temperatures. Rotating the linear polarization of the excitation results in a systematic change of the intensity ratio between these two transitions. We analyze this observation with the help of 6 × 6 k·p calculations and associated rate equation models. This analysis shows that the pronounced excitation polarization anisotropy for this M-plane GaN film originates from a competition between hole relaxation and exciton recombination. [ABSTRACT FROM AUTHOR]