Optical polarization anisotrop in nonpolar GaN thin films due to crystal symmetry and anisotropic strain
In this work, we focus on the optical response of GaN thin films grown along various orientations. The optical properties of strained M- and A- and unstrained C-plane GaN thin films are investigated, and the results are explained with help of band-structure calculations. We calculate the strain-induced band-structure modification using the k.p perturbation approach. The valence-band (VB) states are modified affecting both the transition energies as well as the oscillator strengths. We observe that C-plane GaN does not show any in-plane polarization anisotropy, when an isotropic in-plane strain is applied. For the case of M- and A-plane GaN, one expects to see an in-plane polarization anisotropy even for the unstrained case. Additionally, the in-plane strain significantly changes the band structure and the symmetry of the VB states. The three transitions, involving electrons in the conduction band (CB) and holes in the top three VBs, will exhibit a very different polarization characteristic than the ones for C-plane GaN. These transitions are predominantly x, z, and y polarized, respectively, for a certain range of in-plane strain values, present in our samples. For M-plane GaN thin films, two fundamental transitions can be identified, which occur when the electric field vector E is perpendicular (x-polarization) and parallel c (z-polarization). These transitions give rise to a transmittance spectrum separated by 50 meV at room temperature with respect to each other. This result in a polarization filtering of an incident linearly polarized light beam after transmission, because the electric field component with x-polarization is more strongly absorbed than with z-polarization. This filtering manifests as a rotation of the polarization vector toward the c axis and can be as large as 40 degrees for an initial angle of 60 degrees, for our samples.
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School:Humboldt-Universität zu Berlin
Source Type:Master's Thesis
nonpolar planes of GaN
Date of Publication:02/14/2006