Indirect Optical Absorption of AgCl-AgBr Alloys

Abstract

High-resolution measurements of the optical absorption edge of pure crystals of AgBr and AgCl are in good agreement with the theory of phonon-assisted indirect transitions. The indirect optical absorption of AgCl-AgBr mixed crystals has been studied between 4.2 and 77\ifmmode^\circ\else\textdegree\fi{}K. At 4.2\ifmmode^\circ\else\textdegree\fi{}K, the addition of AgBr to AgCl causes the absorption edge to shift continuously from the pure AgCl threshold at 3.252 eV to the pure AgBr threshold at 2.691 eV. Above 5.7 mole% AgBr in AgCl, the absorption coefficient increases steeply to values considerably greater than that of pure AgBr at the same wavelength. The effect of the alloying on the pure-crystal absorption spectra is somewhat different for the two extremes of mixing. In AgBr-rich mixed crystals (>90 mole% AgBr in AgCl), a temperature-independent component with a sharp threshold is observed at an energy precisely halfway between the indirect phonon-emission and phonon-absorption thresholds (which still appear with phonon energy 0.0082 eV as in pure AgBr). This suggests that nonvertical transitions can occur in the alloy without the aid of phonons to conserve crystal momentum. The zero-phonon threshold is not so evident for dilute concentrations of AgBr in AgCl, where a weak shoulder and tail extends approximately 0.07 eV to longer wavelengths. Some comparison with theory is given, but present understanding of the band structure of alloys is rather incomplete.