Some optical properties of single crystals of hexagonal silver iodide

Abstract

Transmission measurements on nominally pure single crystals of β-AgI have enabled the wavelength dependence (λ=437-750 nm) of the absorption constant and refractive index of this material to be determined in the region of the absorption tail. From an analysis of the absorption data, evidence is put forward in support of a phononassisted indirect transition between the valence and conduction bands. Experimentally it is found that the absorption edge shifts to longer wavelengths with increasing temperature, while calculations indicate that nearly all of the observed shift can be attributed to broadening of the energy bands brought about by lattice vibrations. Experimental evidence is presented to show that absorption in the long-wavelength tail (λ=650 nm) is related to the density of Frenkel defects, although the exact mechanism by which this absorption occurs is not known.The variation of photoconductivity with light intensity suggests that recombination is not by a simple monomolecular process. This conclusion is further supported by the lack of a simple exponential decay time for free carriers after the cessation of excitation.A measurement is described which enables the drift mobility of photo-excited electrons to be determined even when considerable trapping of the carriers occurs. A value of (31 ± 3) × 10−4 m2 V−1 s−1 is calculated for the drift mobility at 248 K.