Evaporation-rate and substrate-temperature dependence of direct exciton transitions in BiI3 thin films formed by hot-wall technique on Al2O3 substrates

Using a hot-wall technique, BiI3 thin films were deposited on alpha-Al2O3 substrates at different evaporation rates and substrate temperatures in order to optimize these conditions. From X-ray diffraction data, it was confirmed that BiI3 layers were regularly stacked. Absorption due to direct excitons in BiI3 was observed in the deposited thin films. To study the translational symmetry and homogeneity of the films, changes in direct exciton transitions were examined. Direct excitons could be split into inner X^Inner and interface X^Inner excitons due to a collapse of translational symmetry along the stacking direction for a finite thickness, and the mean value hat-E and the energy difference Delta-E for the transition energies for these excitons were obtained. To evaluate the sample quality, the dependence of hat-E and Delta-E on the deposition conditions was investigated based on a tight-binding model for flake-like crystals consisting of a finite number n of BiI3 layers. In this model, a site shift energy delta was introduced for the interfaced BiI3 layers, which represents the difference of the on-site energy for the excitons. From the magnitude of Delta-E, the BiI3 thin films are considered to consist of packed flake-like crystals, whose translational symmetry will be maintained within four or five (n=4 or 5) BiI3 layers. From the variations of delta, it was found that an evaporation rate of about 0.8 AA/s and a substrate temperature of 75C are the best conditions for BiI3 thin film deposition on alpha-Al2O3 substrates