Investigation on the Thermal Expansion Property of <i>ε</i> ‐Ga ₂ O ₃ Thin Films Grown by Heteroepitaxy

The rapid developments of novel functional materials and devices have led to intensified material growth challenges. Notably, stress engineering impacts the entire materials processes from growth, fabrication, to packaging. This article explores the residual stress of ε ‐Ga 2 O 3 thin film, an emerging wide‐bandgap semiconductor lacking a bulk crystal. Herein, temperature‐dependent high‐resolution X‐ray diffraction ω ‐scan is employed for the curvature measurement, and high tensile stress is observed in all the ε ‐Ga 2 O 3 samples. Combining Stoney's Equation, the thermal expansion coefficients of ε ‐Ga 2 O 3 are extracted for the first time to be 10.8 – 11.2 × 10 −6 °C −1 parallel to the (001) plane and 5.03–7.66 × 10 −6 °C −1 perpendicular to (001) plane. Owing to stark differences in thermal mismatch (209% with Si, 54% with sapphire), 700‐nm‐thick Si‐based ε ‐Ga 2 O 3 exhibits cracking, whereas 1800‐nm‐thick sapphire‐based sample remains crack‐free. This work provides new insights into the stress engineering of ε ‐Ga 2 O 3 and provides a universal thermal expansion measurement method applicable to heteroepitaxial systems of non‐bulk functional materials.