Thermal crystallization of amorphous InSe thin films: phase evolution and spatial inhomogeneity

Controlled crystallization of amorphous thin InSe films can be considered one of the key steps toward the development of scalable technologies for the production of 2D semiconductors for post-silicon electronics and optoelectronics. However, the optimization of thermal treatment conditions to obtain structurally homogeneous films remains challenging due to the formation of competing crystalline phases in the In–Se system. This work investigates the structural inhomogeneity that can develop during the thermal crystallization of InSe thin films, revealing a multistage phase evolution. X-ray diffraction analysis combined with Raman mapping was employed to investigate the crystallization behavior and spatial phase distribution at different annealing temperatures. As a result, optimal annealing conditions were identified for obtaining a predominantly single-phase β -InSe phase. The most homogeneous structure was obtained within the narrow annealing temperature range of 335 ± 5 °C for film with thickness of ∼250 nm. At lower temperature, the film consisted of β -InSe as the main phase with In 4 Se 3 inclusions as a secondary phase, whereas higher temperatures promoted the formation of γ -In 2 Se 3 inclusions within the main InSe phase. However, thicker films (∼540 nm) exhibited a different phase distribution under similar annealing conditions, particularly along the film depth. These findings highlight the complex phase evolution of amorphous InSe films and demonstrate that the optimal annealing conditions alone do not guarantee uniform crystallization throughout the films thickness.