Hf_xZr_1-xO₂ thin films have excellent complementary metal–oxide semiconductor compatibility and scalability compared to other ferroelectric materials. This makes them a promising candidate for non-volatile memory applications. However, the polymorphism of the materials presents a challenge in stabilizing the ferroelectric properties. Since the wake-up free non-volatile memory applications require the presence of ferroelectric properties in the pristine state of the films without additional electric field cycling, it is necessary to understand how to promote the ferroelectric orthorhombic phase formation. In this work, the interaction between in-plane tensile strain and phase formation of atomic layer deposition grown Hf_xZr_1-xO₂ thin films with different thicknesses and different compositions is demonstrated. By combining the biaxial in-plane tensile strain with the electric switching field and remanent polarization, it is observed that the best ferroelectric properties correlated with an in-plane tensile strain range of 0.4–0.6%. Moreover, the observed correlation between strain and phase formation indicates that strain exerts an influence on phase formation in the pristine state, and that phase formation, in turn, affects strain during electrical field cycling. This work is expected to be helpful to improve the ferroelectric properties in Hf_xZr_1-xO₂ films, which can be processed for different memory devices with specialized requirements.