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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]

We have investigated a [1120]-oriented A-plane GaN film on R-plane sapphire, where the c axis of GaN lies in the film plane, by polarized photoreflectance (PR) spectroscopy. Near the fundamental energy gap of GaN, the PR spectrum with the probe light polarized perpendicular to the c axis exhibits one feature corresponding to a single transition labeled T1. For a polarization parallel to c axis, two different features labeled T2 and T3 are observed at higher energies than the transition T1. In order to explain the origin of these three features, we compare the measured energies with calculations of the transition energies and oscillator strengths of the three band-to-band transitions of GaN near its fundamental gap for an anisotropic in-plane strain in the A plane. The analysis shows that the observed transition energies and polarization properties of the three transitions can be explained by the presence of an overall compressive, anisotropic in-plane strain in the film. [ABSTRACT FROM AUTHOR]

Direct imaging of minority electron transport via the spatially resolved recombination luminescence signature has been used to determine carrier diffusion lengths in GaInP as a function of doping. Minority electron mobility values are determined by performing time resolved photoluminescence measurements of carrier lifetime on the same samples. Values at 300K vary from 2000 to 400 cm²/Vs and decrease with increasing doping. Anisotropic diffusion lengths and strongly polarized photoluminescence are observed, resulting from lateral composition modulation along the [110] direction. We report anisotropic mobility values associated with carrier transport parallel and perpendicular to the modulation direction. [ABSTRACT FROM AUTHOR]

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]

Using reflectance spectroscopy, the in-plane polarization behavior of unstrained C- and A-plane GaN films is experimentally investigated. While no in-plane polarization anisotropy is observed for all three band-gap related excitons (A, B, and C) in unstrained C-plane GaN films, the A exciton is completely linearly polarized perpendicular to the c axis in unstrained A-plane GaN films. However, the B and C excitons are only partially polarized. This observation is in excellent agreement with results based on band-structure calculations using the Bir-Pikus Hamiltonian for the wurtzite crystal structure. [ABSTRACT FROM AUTHOR]

We use photoreflectance (PR) spectroscopy to study the electronic band structure modification of GaN films grown along different nonpolar orientations due to biaxial, anisotropic in-plane strain. The exciton transition energies of an unstrained, high-quality C-plane GaN film are used to accurately determine the crystal-field and spin-orbit splitting energies. For films with a nonpolar orientation, the resonant features observed in the PR spectra exhibit a strong in-plane polarization anisotropy and different transition energies from the ones measured in the C-plane GaN film. The deformation potential D5 is accurately determined from four GaN films with a nonpolar orientation using the measured energies together with the polarization properties and out-of-plane strain. [ABSTRACT FROM AUTHOR]

We investigate the dependence of the transmittance spectra of strained M-plane GaN films on the angle of the linear in-plane polarization, with the c axis of the underlying wurtzite crystal structure in the film plane. Because of the reduced in-plane symmetry and the large in-plane anisotropic strain, the transmission spectra for an arbitrary in-plane polarization angle can be directly derived from two transitions with polarizations parallel and perpendicular to the c axis. Since the energy gap for these two polarizations is shifted by more than 50 meV, the M-plane GaN film becomes dichroic, resulting in a polarization rotation after transmission for energies between the two gaps for an initial angle of 45° by as much as 40° towards the c axis. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]