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Germanium is a material widely used in various applications, especially in optoelectronic devices, due to its excellent optical and electrical properties. In these devices, germanium is typically deposited in the form of thin films. Because of the mismatch in thermal expansion coefficients between the Ge film and the substrate, the material can develop either compressive or tensile stress that affects the reliability and durability of the devices. Therefore, studying stress in such materials is essential and it plays a critical role in its fabrication. Ion irradiation offers a method for modifying the stress state of thin films. In this work, we investigate the effect of ion irradiation on the stress state of Ge films and its correlation with the optical and structural properties. Germanium thin films were deposited by sputtering onto fused silica substrates at room temperature, followed by thermal annealing at 600 °C for 1 hour. The samples were then irradiated with 1.8 MeV Au+ ions with ion fluences ranging from 5e11 to 1.5e15 ions/cm². In situ stress measurements were performed during irradiation using a laser reflection technique. The results show that the samples before irradiation were tensile stressed and it decreases with increasing ion fluence up to a critical fluence of ~5e12 ions/cm². Beyond this point, the tensile stress begins to increase again. Post-irradiation characterization using X-ray diffraction revealed a structural transition from a polycrystalline to an amorphous phase with increasing fluence. Optical measurements showed a red shift in absorption, leading to a decrease in the band gap, and a decrease in reflectance as the irradiation fluence increased. These findings demonstrate that ion irradiation can effectively modify the mechanical, optical, and structural properties of germanium thin films which could be beneficial for device applications.
