Photochromism and influence of point defect charge states on optical absorption in aluminum nitride (AlN)

In this work, we study the absorption properties of AlN in the range of 1.5 - 5.5 eV, as well as the meta-stable change in absorption induced by ultraviolet (UV) irradiation (photochromism), and the restoration of the initial state under the action of irradiation of 2 - 4 eV or elevated temperatures. UV irradiation results in a decrease of the absorption coefficient from 110 cm$^{-1}$ to 55 cm$^{-1}$ at 4.7 eV while in the visible range the absorption coefficient increases from values below 5 cm$^{-1}$ to ~35 cm$^{-1}$. Measurements with two linear polarizations, parallel and perpendicular to the optical axis, provide determination of several different absorption bands at 2.6, 2.8, 3.4, 4.0, 4.5, and 4.8 eV. The bands at 2.6 eV and 3.4 eV identify the defect levels near to the valence band, while the band peaking at 2.8 eV is related to the conduction band. The photochromism allows controlling the absorption of light in two related spectral ranges because the decrease of UV absorption and increase of the visible absorption are related to switching the charge state of the same defects.