Evaporation‐Rate and Substrate‐Temperature Dependence of Direct Exciton Transitions in BiI 3 Thin Films Formed by Hot‐Wall Technique on Al 2 O 3 Substrates

Using a hot‐wall technique, BiI 3 thin films are deposited on α ‐Al 2 O 3 substrates at different evaporation rates and substrate temperatures in order to optimize these conditions. From X‐ray diffraction (XRD) data, it is confirmed that BiI 3 layers are regularly stacked. Absorption due to direct excitons in BiI 3 is observed in the deposited thin films. To study the translational symmetry and homogeneity of the films, changes in direct exciton transitions are examined. Direct excitons can be split into inner X Inner and interface X Inter excitons due to a collapse of translational symmetry along the stacking direction for a finite thickness, and the mean valueE ˆand the energy difference Δ E for the transition energies for these excitons are obtained. To evaluate the sample quality, the dependence ofE ˆand Δ E on the deposition conditions is investigated based on a tight‐binding model for flake‐like crystals consisting of a finite number n of BiI 3 layers. In this model, a site shift energy δ is introduced for the interfaced BiI 3 layers, which represents the difference of the on‐site energy for the excitons. From the magnitude of Δ E , the BiI 3 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) BiI 3 layers. From the variations of δ , it is found that an evaporation rate of about 0.8 Å s −1 and a substrate temperature of 75 °C are the best conditions for BiI 3 thin film deposition on α ‐Al 2 O 3 substrates.