We study the in-plane polarization of wurtzite GaN/(Al, Ga)N multiple quantum wells. Identical M-plane (11̄00) and C-plane (0001) structures are grown by plasma-assisted molecular-beam epitaxy on γ-LiAlO2(100) and 6H–SiC(0001), respectively. While the emission from the conventional [0001] oriented wells is isotropic within the growth plane, we observe a strong polarization anisotropy of over 90% for the M-plane sample. The luminescence is polarized normal to [0001] and shows no spectral shift with polarization angle, i.e., it originates solely from A excitons (px and py valence band states). The deviation of the polarization degree from unity is attributed to the mixing with pz valence band states due to quantum confinement.

Extended x-ray absorption fine-structure measurements have been performed on ZnSe crystals doped with Er by an addition of ErF 3 during growth. The results show that Er exists in these samples in an orthorhombic configuration with nine nearest F neighbours similar to that in orthorhombic ErF 3 . The energy level splittings observed in optical absorption and photoluminescence investigations in the region of the 4 I 15/2 ↔ 4 I 13/2 transitions at 1.54 µm are fully consistent with the orthorhombic symmetry of the Er centre.

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