Indirect optical absorption and radiative recombination in silver bromoiodide

The low-temperature (4.2 K) and room-temperature absorption spectra of several ${\mathrm{AgBr}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{I}}_{\mathrm{x}}$ (x=0–0.25) samples have been obtained. Conventional analysis of these spectra yields the band-gap energy ($E_g$), which decreases with iodide content. The band-gap energies at 4.2 K range from 2.684 eV for pure AgBr to 2.448 eV for ${\mathrm{AgBr}}_{0.75}$ ${\mathrm{I}}_{0.25}$. The more accurate band-gap information is used to estimate the trap depth of the acceptor states observed in the low-temperature emission. Data indicate that both donors and acceptors are shallow traps in ${\mathrm{AgBr}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{I}}_{\mathrm{x}}$ systems.