Polarization anisotropy and filtering for M ‐plane GaN films

Abstract

PACS 42.25.Ja, 78.20.Ek, 78.20.Fm, 78.66.Fd The polarization anisotropy and filtering are investigated for strained M-plane GaN films as a function of the angle of the in-plane polarization vector. 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 toward the c axis, which can be as large as 75• . 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. Consequently, the rotation can be very well approximated by the product of the film thickness and the difference of the absorption coefficients for the two polarization directions. Temperature-and polarization-dependent photoluminescence (PL) spectroscopy was also used to study the polarization anisotropy. For an arbitrary initial polarization angle, the total PL intensity can be decomposed into two contributions due to the PL intensity for E c and E ⊥ c.1 Introduction For C-plane GaN films, the growth is along the [0001] direction, which is parallel to the c axis of the wurtzite (WZ) crystal structure. While isotropic strain in the C plane preserves the crystal symmetry in the x-y plane, anisotropic strain can result in an in-plane polarization anisotropy [1]. For M-plane GaN films, the growth direction is along [1100] so that the c axis lies in the growth plane. Therefore, an in-plane polarization anisotropy will occur even in the absence of in-plane strain [2]. Furthermore, anisotropic in-plane strain can drastically enhance the in-plane polarization anisotropy. For a certain range of in-plane strain values, the valence-band (VB) structure changes in such a way that the transitions between the three uppermost VBs and the conduction band labelled T 1 , T 2 , and T 3 become fully polarized in the x-, z-, and y-direction, respectively [3,4].In this paper, we present a theoretical and experimental investigation of the in-plane optical polarization properties of M-plane GaN films grown on γ-LiAlO 2 (100). In addition to linear birefringence, the film also exhibits linear dichroism, resulting in a very effective polarization filtering. We have included both effects to describe this polarization filtering, which can be detected as a rotation of the in-plane polarization vector. The rotation angle can be approximately described by the film thickness times the difference in the absorption coefficients for the two polarization directions. Although the outgoing light is generally elliptically polarized, it turns out that, in the energy range where the polarization filtering is most effective, the linear birefringence almost vanishes. As mentioned above, the polarization filtering is a manifestation of the modified VB structure, and the energy separation between the two uppermost VB's corresponding to the transitions T 1 and T 2 becomes more than 40 meV at 3...

Anisotropic characteristics of a-plane GaN films grown on γ-LiAlO2 (302) substrates by MOCVD

“…The transitions between the three uppermost valencebands (VB) and the conduction band labeled T1, T2 and T3 become fully polarized in the x-, z-and y-direction, respectively [11]. The reduction of the PL intensity is because the energy states of the second VB are much less occupied at 300 K [15]. The emission peak at 370 nm is clearly observed both in the spectrum for E⊥c and E||c, which is due to excitons bound to stacking faults [16].…”