Magnetron Co-sputtering Synthesis of CuI-SnO Alloys: Structural Evolution from p-Type Crystalline to Semi-Insulating Amorphous Films

Amorphous p-type transparent copper iodide (CuI) films offer potential advantages for optoelectronic applications, such as smooth surfaces that overcome the limitations of their crystalline counterparts. However, their synthesis via vapor deposition has been a challenge. This study reports the synthesis of CuI-SnO alloy films with varying Sn content (x = Sn/(Cu + Sn)) using a nonequilibrium magnetron co-sputtering technique. Structural analysis revealed a controlled transition from a polycrystalline zincblende (x < 0.4) to an amorphous phase with increasing Sn content. Notably, scanning transmission electron microscopy showed a mixed-phase region at x = 0.39, where SnO components concentrate in the amorphous matrix, demonstrating the critical role of Sn in promoting amorphization. Optically, an excitonic absorption feature at ∼3.05 eV, attributed to Mahan exciton formation, was observed for films with x ≤ 0.16 but vanished at higher Sn concentrations due to structural disorder. Electrically, films exhibited robust p-type conductivity with hole concentrations in the range of 1018–1020 cm–3 for x ≤ 0.36. Crucially, higher Sn content (x ≥ 0.39) led to semi-insulating behavior. This electrical degradation is attributed to a combination of donor compensation and a significant downward shift of the valence band maximum position, which increases the acceptor ionization energy. These findings provide crucial insights into the fundamental properties and electronic limits of the CuI-SnO alloy system, establishing a viable route for synthesizing materials with structural and electronic properties tunable for future transparent electronics, where amorphous semiconductors are desired.