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Optical absorption of AlN thin films shows a ’’knee’’ structure at 6.2 eV and an intense band at 7.8 eV. The structure at 6.2 eV is interpreted as excitonic absorptions due to transitions across the direct energy gap of about 6.2 eV. Dichroism observed at the absorption edge indicates that the transition Γ1v–Γ1c (E∥c) is of lower energy than the transition Γ6v–Γ1c (E⊥c). Strong dichroism in the 7–8-eV region is thought to cause the birefringence of AlN.
Yamashita et al. (Thu,) studied this question.