Control of temperature dependence of exciton energies in CuI-CuBr alloy thin films grown by vacuum deposition

We report the temperature dependence of exciton energies in CuI-CuBr (CuI_1-xBr_x) alloy thin films with the thickness of 50 nm grown on Al₂O₃ substrates by vacuum deposition. From X-ray-diffraction measurements, we have confirmed that crystalline alloy thin films are preferentially oriented along the <111> crystal axis. The Z_1,2-exciton energy exhibits a negative bowing as a function of the alloy composition, while the Z₃-exciton energy changes smoothly. The novel finding in the present work is a drastic change of the temperature dependence of the exciton energies in the alloy thin films. The temperature dependence in the CuI film exhibits the negative shift with the increase of temperature, while that in the CuBr film shows the positive one. We have found that the temperature dependence of the CuI_1-xBr_x thin film changes from the negative shift to the positive one around the alloy composition of x = 0.1. We qualitatively discuss the above finding from the aspect of electron-phonon interactions.