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

Abstract

In this work, we study the absorption properties of AlN in the range of 1.5-5.5 eV, as well as the metastable change in absorption induced by ultraviolet (UV) irradiation (photochromism). We also study the restoration of the initial state under the action of the irradiation of 2-4 eV or elevated temperatures. UV irradiation results in a decrease of the absorption coefficient from 110 to 55 cm-1 at 4.7 eV, while in the visible range, the absorption coefficient increases from values below 5 to ∼35 cm-1. Measurements with two linear polarizations, E â1 c and E âŠ1 c, provide the 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 and 3.4 eV identify the defect levels near the valence band, while the band peaking at 2.8 eV is related to the conduction band. Photochromism allows for controlling the absorption of light in two related spectral ranges, because the decrease of UV absorption and increase of visible absorption are related to switching the charge state of the same defects.