Mechanical Stress Evolution in Polycrystalline Ge Thin Films Under MeV Ion Irradiation

ABSTRACT Ion irradiation is a powerful tool for modifying the electrical and optical properties of device‐relevant heterostructures, but it also induces mechanical stress that can degrade or—if controlled—enhance performance. We investigated in situ the stress evolution in polycrystalline Ge films irradiated with 1.8 MeV Au ions across wide fluences. Three distinct regimes emerge. At low ion fluences, defect‐induced lattice expansion compensates initial deposition‐induced tensile stress, bringing films to near‐zero stress. The defects can migrate toward grain boundaries, which act as effective sinks, promoting recombination and enhancing irradiation resistance, thereby delaying amorphization compared to single‐crystalline Ge. At intermediate fluences, defect accumulation drives crystalline‐to‐amorphous transition with increasing tensile stress, confirmed by post‐irradiation X‐ray diffraction and Raman spectroscopy. At high fluences, stress partially relaxes due to swelling, plastic deformation or interface effects. Our results demonstrate controlled ion‐irradiation induced stress tuning in poly‐Ge/fused‐silica systems, indicating the key role of microstructure, and thus yield criteria for stress engineering in Ge‐based optoelectronic and CMOS‐based devices.